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PROJECT FINAL REPORT Final Publishable Summary Report Grant Agreement number: 249012 Project acronym: COPERNICUS Project title: Compact Otdm/wdm oPtical rEceiveRs based on photoNic crystal Integrated CircUitS Funding Scheme: EC 7 th Framework Programme, Theme FP7-ICT-2009.3.8b Disruptive Photonics Technologies Period covered: from 1 January 2010 to 30 June 2013 Name of the scientific representative of the project's co-ordinator 1 , Title and Organisation: Dr. Alfredo de Rossi Thales Research and Technology Tel: +33 1 69 41 57 52 Fax: +33 1 69 41 55 52 E-mail: [email protected] Project website address: http://www.copernicusproject.eu/ 1 Usually the contact person of the coordinator as specified in Art. 8.1. of the Grant Agreement.

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Page 1: PROJECT FINAL REPORT - CORDIS · 2017-04-20 · COPERNICUS targeted advances in the physics, technology, modelling, and integration of photonic crystal devices. Key devices included

PROJECT FINAL REPORT

Final Publishable Summary Report

Grant Agreement number: 249012

Project acronym: COPERNICUS

Project title: Compact Otdm/wdm oPtical rEceiveRs based on photoNic crystal Integrated CircUitS

Funding Scheme: EC 7th Framework Programme, Theme FP7-ICT-2009.3.8b – Disruptive Photonics Technologies

Period covered: from 1 January 2010 to 30 June 2013

Name of the scientific representative of the project's co-ordinator1, Title and Organisation:

Dr. Alfredo de Rossi

Thales Research and Technology

Tel: +33 1 69 41 57 52

Fax: +33 1 69 41 55 52

E-mail: [email protected]

Project website address: http://www.copernicusproject.eu/

1 Usually the contact person of the coordinator as specified in Art. 8.1. of the Grant Agreement.

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Final publishable summary report

1. Executive summary

In recent years, the world of communications and computing has been transformed with rapid advances in

science and technology. These advances and the evolution of the Internet have impacted all aspects of our daily

lives. The demands of consumers for higher speeds and greater capacities (at acceptable cost, of course) appear

unending. Photonic chips are set to play an important role in the development of future communication and

information technologies, analogous to the impact of electronic integrated circuits. Photons (the carriers of

optical signals) have important fundamental properties that make them superior to electrons (the carriers of

electrical signals) for representing and transmitting data. In particular, very high bandwidth signals can be

transmitted with very low loss over long distances using optical fibre. As the bandwidth increases, the

advantages of optics even holds for very short transmission distances, such as between high-speed processors in

a computer, and ultimately, across individual computer chips.

Although, electronics continues to remain unbeaten for performing high-level data processing (such as that

performed by modern computers), the growth in performance is today falling short of earlier predictions.

Analysis has shown that electrical interconnects are a key factor limiting the speed and integration level of

electronic circuits. Faster switching is possible with current transistor technology, but it takes too much power

and generates too much heat to send information across the chip at higher data rates. Photonics, therefore, has a

crucial role to play in global interconnects, but this is a difficult challenge as the size and power consumption of

traditional photonic devices must be reduced by more than two orders of magnitude. The technologies developed

in the COPERNICUS project address these issues and provide devices capable of meeting these challenges.

The overall aim of COPERNICUS was to develop a photonic integrated circuit (PIC) platform based on photonic

crystals (PhC) and demonstrate this platform with compact demultiplexing receivers for 100 Gb/s optical time

division multiplexed (OTDM) and wavelength division multiplexed (WDM) signals, based on photonic crystal

technology. There is a pressing need for these devices for ultra-high bandwidth data links in server farms, optical

storage networks and on-board internet/entertainment systems, where demand is driving the data bandwidth and

technology integration level rapidly upwards. Next generation telecom systems will also benefit from these

devices. Photonic crystals are a nascent and newly emerging technology, where most of the activities reported

have focused on isolated devices. The purpose of COPERNICUS was not only to develop PhC materials and

devices with state of the art performance, but also to develop the associated photonic circuitry and necessary

design and simulation tools for the realisation of practical, high-performance photonic integrated circuits.

The COPERNICUS Consortium achieved many successes in the development of individual elements as well as

in its efforts to bring all of the individual elements together to have a fully "joined up" technology platform, from

which future devices/PICs can be developed.

New material technologies and PhC technologies were developed and optimised for high-speed, low-power all-

optical devices. Using these new materials and technologies, the Consortium conceived and demonstrated

entirely new functional devices with exciting performance, including a photodetector, new filters, a wavelength

demultiplexing PIC and an exciting variety of new all-optical gates (AOGs). (AOGs offer similar all-optical

functionality to the diode or transistor in electronics, but operate on signals entirely in the optical domain.) The

Consortium performed several successful all-optical signal processing demonstrations with these AOGs.

The Consortium also developed and optimised all of the optical circuitry (couplers, bends, splitters and

waveguides) for the different PhC technologies. As these passive devices are also essential for coupling the

functional devices to each other and to the outside world, their performance is also critical for the success of the

technology platform. The new passive devices designed by the Consortium achieved excellent performance by

minimising insertion losses and reflections while maximising their optical bandwidth.

Photonic crystal technology depends upon extreme optical confinement and greatly enhanced light-matter

interactions. This makes it difficult to design even simple functional devices without the aid of accurate design

tools. These simulation and design tools must consider the impact of strong nonlinear optical phenomena as well

as strong coupling with the spatio-temporal dynamics of the electrons and holes in the semiconductor materials.

As conventional simulation tools are not adequate for accurately describing these processes, powerful simulation

and design tools were developed in COPERNICUS and extensively used for the development of this platform.

By pursuing new levels of photonic crystal integration to combine these devices, the Consortium has achieved

devices with complex all-optical functions attractive to both medium- and long-term markets. The objective of

establishing a "joined up" technology platform has been successful and the COPERNICUS Consortium was

ultimately able to show that all of the different elements can be successfully combined to achieve the higher

functionality expected of PICs.

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2. Summary description of project context and objectives

2.1 Project aims

COPERNICUS aimed to develop compact demultiplexing receivers for 100 Gb/s optical time division

multiplexed (OTDM) and wavelength division multiplexed (WDM) signals, based on photonic crystal

technology. There is a pressing need for these devices for ultra-high bandwidth data links in server farms, optical

storage networks and on-board internet/entertainment systems, where demand is driving the data bandwidth and

technology integration level rapidly upwards. Next generation telecom systems will also benefit from these

devices for OTDM and optical packet switching. Their high-speed and bandwidth, together with their ultra-low

power consumption and extreme compactness, also make them a very promising technology for seamless cross-

chip and off-chip data links for CMOS electronics. This approach has all the hallmarks of a highly disruptive

technology with the potential to place Europe at the forefront of photonics.

COPERNICUS targeted advances in the physics, technology, modelling, and integration of photonic crystal

devices. Key devices included high-speed all-optical gates, low-crosstalk wavelength drop filters, and high-speed

integrated photodetectors. These devices rely on very strong light-matter interactions arising from the large,

ultrafast nonlinear optical response of III-V semiconductors and the strong resonant field enhancement in

photonic crystals. This is ideal for filters and all-optical gates, enabling a dramatic reduction in size and

switching energy. Their switching energy*delay product is two orders of magnitude smaller than that of

competing technologies (see Fig. 2.1.1). Modelling considered carrier plasma (spectral and spatial) contributions

to the nonlinear optical response and develop a robust optical, thermal and electronic design tool for photonic

crystal devices. New levels of photonic crystal integration were pursued to combine these devices and achieve

complex all-optical functions attractive to both medium- and long-term markets.

Fig. 2.1.1: State of the art in optical switching in resonant devices at the start of the COPERNICUS project,

showing the energy per pulse required for all-optical switching –vs– switching recovery time.

Participants:

No. Participant Short Name Country

1 Thales Research & Technology TRT France

2 University of Nottingham UNott UK

3a CNRS Laboratory for Photonics and Nanostructures CNRS-LPN France

3b CNRS Fonctions Optiques pour les Technologies de l’Information CNRS-FOTON France

4 Technical University of Denmark DTU Denmark

5 University of Ferrara UniFe Italy

6 u2t photonics AG U2T Germany

7 Thales Systemes Aeroportes S.A. TSA France

Coordinator: Technical Manager:

Dr. Alfredo de Rossi Prof. Eric Larkins

Thales Research and Technology University of Nottingham

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2.2 Motivation

The need COPERNICUS addressed was the development of a specific technological platform for high density

integration of basic optical functions, including filtering, wavelength routing, detection and switching.

COPERNICUS also targeted key integration issues, including optical coupling, timing and dispersion

management, and optical routing issues (waveguide crossings). The underlying technology is III-V

semiconductor membrane photonic crystals, which provide an extremely compact platform that is ideally suited

to both wavelength filtering/routing and to high-speed/low-power optical switching.

The motivation for this work was the pressing need for integrated all-optical functionality with high-bandwidth,

low power consumption and ultra-compact size (footprint). COPERNICUS targeted the development of specific

technologies (all-optical gates, two-level optical integration) for long-term applications (e.g. optical signal

processing/routing, high-speed all-optical logic, photonic/CMOS integration) as well as technologies

(wavelength demultiplexer, integrated photodetector) intended to give photonic crystals early market entry for

important high-growth applications (e.g. high-speed data links, on-board entertainment networks).

Photonic chips are set to play an important role in the development of future communication and information

technologies, analogous to the impact of electronic integrated circuits. Photons have important fundamental

properties that make them superior to electrons for representing and transmitting data. In particular, very high

bandwidth signals can be transmitted with very low loss over long distances using optical fibre. As the

bandwidth increases, the advantages of optics even holds for very short transmission distances, such as between

high-speed processors in a computer. Indeed, fundamental arguments were already put forward 20 years ago,

showing why optics is preferable for low-energy communications2.

Electronics continues to remain unbeaten when it comes to performing high-level data processing, such as

needed in a computer. However, the growth in performance is falling short of Moore's law, as seen by the

saturation in the clock frequency of modern PCs. A critical analysis of the international technology roadmap for

semiconductors (ITRS) showed that the global (i.e. cross-chip) electrical interconnect is a key factor limiting the

speed and integration level of VLSI circuits2,3

. CMOS transistors can switch much faster (< 1 ps), but it takes too

much power and generates too much heat to send the information across the chip at higher data rates. As feature

sizes shrink, the problem gets worse, since the RC time constant of the interconnect increases hyperbolically.

Photonics has a crucial role to play in global interconnects, but this is a difficult challenge. To be compatible

with CMOS technology, the size and power consumption of traditional photonic devices must be reduced by 1-2

orders of magnitude. The photonic crystal devices developed by COPERNICUS can meet these challenges.

2.3 Project objectives (and extent they were achieved)

The COPERNICUS project was funded under the EC 7th

Framework Programme, Theme FP7-ICT-2009.3.8b

– Disruptive Photonics Technologies. The objective was to develop PhC technology and key devices for

integrated photonic crystal (PhC) receiver chips supporting 100 Gb/s data rates. Photonic crystals are a nascent

and newly emerging technology, where most of the activities reported have focused on isolated devices. The

purpose of COPERNICUS was not only to develop PhC materials and devices with state of the art performance,

but also to develop the associated photonic circuitry and necessary design and simulation tools for the realisation

of practical, high-performance photonic integrated circuits (PICs). This was an ambitious objective, which

required that all of the technologies be developed in a “joined up” fashion. It also required a wide range of skills

and close cooperation within the Consortium in order to develop the essential skills and “know how” in a manner

going beyond the independent development of individual tools, technology and devices.

The key functions to be developed in COPERNICUS, all of them on a PhC chip, were the following:

1) PhC based wavelength drop filters matching the requirements for WDM in terms of cross-talk rejection,

bandwidth and frequency spacing.

2) All Optical Gates. Features: Low power consumption (< 1 pJ per pulse) and ultra-fast (< 10 ps) response

for demultiplexing at 100 GB/s

3) Photodetectors with large bandwidth (28 GHz) and high responsivity (0.5 A/W) integrated on the PhC chip

4) Single mode optical “circuitry” integrated on the chip to deliver pulses to the different sub-devices with

well controlled timing

The first chip was to be a wavelength division multiplexed (WDM) receiver and the second chip was to be an

optical time division multiplexed (OTDM) receiver, as shown in Fig. 2.3.1.

2 D.A.B. Miller, Opt. Lett. 14, 146 (1989).

3 R.G. Beausoleil, et al., Proc. IEEE, 96(2), 2008.

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Signal

Input

Signal

Output 1

Control Signal

Output 2

Bit streams

1, 2, 3, 4

4 3 2 1

Optical control

c

bit 0

Optical control

c

bit 2

Optical control

c

bit 1

Optical control

c

bit 3

Bit stream

s

s s s s

Fig. 2.3.1: Schematic representation of the targeted wavelength division multiplexed (WDM) PhC receiver (left)

and optical time division multiplexed (OTDM) receiver (right).

The WDM receiver targets medium term applications and will demonstrate the wavelength drop filter and

integrated photodetector technologies. The WDM receiver will demultiplex four optical channels with bit rates

of 25 Gb/s and a channel (wavelength) spacing of 5 nm. Ultimately, the WDM receiver will be packaged with a

quad transimpedance amplifier (TIA), as illustrated in Fig. 2.3.2, and its performance assessed.

CMOS

electronics

4 3 2 1

Bit streams

1, 2, 3, 4

CMOS

electronics

444 33 222 11

Bit streams

1, 2, 3, 4

Bit streams

1, 2, 3, 4

Fig. 2.3.2: Schematic of hybrid integrated WDM PhC receiver and CMOS preamplifier

The (unpackaged) OTDM receiver will target long term applications and will demonstrate the all-optical gate

and key technologies for the integration of photonic crystal circuits (timing, dispersion management, routing).

The AOG will be assessed both as a single gate and in cascaded configurations with other gates.

All-Optical Gate

The main features of the proposed all-optical gate, illustrated in Fig. 2.3.3, include an ultra-small footprint and

the possibility of ultrafast response (~ 10 ps).

Fig. 2.3.3: Schematic of all-optical gate based on PhC technology.

Integrated Photodetector

A photodetector will be developed specifically for integration with membrane photonic crystal circuits. Initially,

two photodetector geometries were considered for air-bridge and BCB-encapsulated membrane PhCs. The final

photodetector must have a bandwidth of f3dB ≥ 28 GHz and a reasonable responsivity (e.g. > 0.5 A/W).

Wavelength Drop Filter and WDM Demultiplexer

The wavelength drop filters are needed to perform a channel drop function (i.e. selective removal of one optical

channel, while allowing the remaining channels to pass unhindered). For the envisaged WDM receiver, a 4-

channel WDM demultiplexer is required, with a channel spacing of 5 nm and crosstalk suppression of > 10 dB.

With this performance, subsequent scaling (after the project) to 10 or 16 channels will be straight forward.

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Photonic Crystal Circuitry

In order to realize useful photonic integrated circuits (PICs), photonic crystal circuitry must be developed to

connect the key functional devices – both with each other and with the outside world. This requires the

development of passive optical components, such as optical couplers, waveguides, bends and splitters – all of

which require low insertion losses and low reflections across a sufficiently wide optical bandwidth. Furthermore,

the design of the circuitry is strongly dependent upon the PhC technology and/or technologies chosen. Finally,

the design layout of the final photonic integrated circuits must take both the PIC performance and the packaging

requirements into account.

Materials Engineering

As indicated above, photonic crystal technology is in an early stage of development and different groups are

pursuing different materials and PhC structures/technologies. COPERNICUS has chosen to focus on the use of

III-V semiconductors to realize the different components of the chip: waveguides, AOGs and detectors. Two

main options were pursued for the materials technology: GaAs and InP. A key advantage of GaAs is its short

carrier lifetime, which is attractive for AOGs. A key advantage of InP is that it is compatible with active telecom

devices (e.g. InGaAs photodetectors and lasers/amplifiers). At the same time, two main options were also

pursued for the planar PhC technology: air-bridge PhC membranes and BCB-encapsulated PhC membranes.

Specific materials engineering objectives were:

To engineer the base materials to achieve ultrashort carrier lifetimes, suitable for 100Gb/s AOGs

To fabricate photonic crystal devices with different materials and PhC technologies

To integrate monolithically grown photodetectors into the PhC chip

To develop a two-level optical integration platform

Simulation and Design

The photonic functions and photonic crystal devices developed in COPERNICUS are inherently complex and

must satisfy challenging specifications to ensure the required performance. Accurate theoretical models and

simulation tools are even required for the design of passive PhC structures. The design of active PhC devices

(e.g. photodetectors, AOGs, wavelength converters) is even more challenging. Consequently, COPERNICUS

needed to develop a range of steady-state and time-domain simulation/design tools for membrane PhC AOGs

and photodetectors, including optical, electronic and thermal solvers. The challenges of these tools were to

achieve a combination of physical accuracy (i.e. the ability to predictively design photonic crystal structures and

devices) with computational efficiency. Although finite difference time domain (FDTD) electromagnetic

simulations are established tools for accurately simulating the linear response of photonic crystal structures,

COPERNICUS addresses the challenge of applying them to nonlinear optical problems and of coupling them

with electrical/thermal simulations. Further challenges are the computational complexity and the computational

resources required for the multi-physics simulation of photonic crystal devices. The objective here was to

advance the state of the art in PhC simulation/design tools and to guide/support the experimental work.

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3. Main Scientific and Technological results

This chapter describes the main scientific and technological achievements within the project. The seven sections

reflect the organisation of the Technical Workplan into seven Workpackages. At the same time, the presentation

seeks to convey the same awareness of the relevance of each of the activities and results to the global project

objectives, which was maintained throughout the life of the project.

3.1 Modelling and Design

WP Leader: UNott

Contributors: DTU, TRT, UniFe

This section describes the COPERNICUS approach to the design and the realisation of key photonic crystal

devices. These devices included simple passive devices, such as waveguides, couplers, bends and splitters, but

also included functional devices, such as filters, AOGs and photodetectors. Photonic integrated circuits, such as

the WDM demultiplexer, clearly combine several passive and active devices together. It is less obvious,

however, that even single devices use multiple passive elements, such as couplers, waveguides and bends.

This activity also included the development and implementation of numerical methods for the modelling and

simulation of different nonlinear optical processes (Kerr effect, plasma effect, two-photon absorption, etc.). It

also included the development of state of the art coupled simulation tools for the self-consistent modelling of the

highly coupled optical and electronic processes. Several partners (DTU, TRT, UniFe, UNott) were responsible

for various aspects of the design of these components and a range of modelling techniques were employed.

3.1.1 Nonlinear Response of PhC AOGs

Two-photon absorption (TPA), free-carrier dispersion (FCD), free-carrier absorption (FCA) and carrier diffusion

are the main nonlinear effects in semiconductor PhC devices that can be profitably used to manipulate light.

A model based on a Coupled Mode Theory (CMT) approach, suitable for a first qualitative investigation of these

phenomena, is introduced first. This tool is fundamental for understanding how these phenomena can be used to

exploit nonlinear effects in practical devices. In COPERNICUS, a full-wave FDTD simulator has been

developed as a tool for the design of optimized optical devices exploiting these nonlinearities. Numerical FDTD

simulations performed by UniFe have been validated by comparison with CMT results and with experimental

data provided by TRT. The good agreement between numerical outcomes and both, theoretical and experimental

results, obtained by these comparisons, allows us to conclude that the FDTD code is a useful tool for describing

complex optical phenomena in semiconductors as well as a capable design tool for new optical devices.

The development of advanced tools for the simulation of PhC-based devices is described in section 3.1.3, where

the challenging issues in the coupling of models with different computational requirements is discussed. In order

to minimise computational effort, a computationally efficient method of representing the polarisation in the time

domain in order to couple the different simulation tools has been proposed and assessed.

3.1.2 Structural Design and Optimisation of PhC Structures

Photonic Crystal Circuitry

The functional components (AOGs, filters and photodetectors) of each receiver must be properly connected by

means of single mode optical “circuitry” integrated on the same chip. To optimise the layout, DTU and UniFe

have established a design procedure based on 3D-FDTD simulations and topology optimisation (TO). This

procedure has been successfully applied to the design of a Y-junction and a double 60° bend, whose optimised

layout is illustrated in Fig. 3.1.1. Fig. 3.1.1 also shows the transmission (S21) and the reflection (S11) coefficients

for the optimised (solid lines) and the unoptimised (dashed lines) configurations – the impressive positive effects

of the TO are clearly evident. In the C-band, the reflection coefficient was reduced from S11 > -5 dB (and in

places, nearly 0 dB) to S11 < -10.49 dB across a bandwidth of > 100 nm. The transmission coefficient increased

from S21 = -15 to -4 dB to S21 > -1.01 dB across the same bandwidth (~ 0.6 dB loss for a double bend at 1550

nm).

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UniFe: Initial Design (3D-FDTD)

DTU: TopologicalOptimisation (2D-FDFD)

UniFe: Final Check (3D-FDTD)

Fig. 3.1.1: Optimisation of a double 60° bend, showing the initial and final layouts. The transmission (S21) and

reflection (S11) coefficients for the optimised (solid lines) and unoptimised (dashed lines) configurations (right).

Excellent performance was also achieved for optimised Y-junctions. Fig. 3.1.2 shows an optimised layout for a

Y-junction (3 dB optical splitter) and the flat response that is obtained.

Fig. 3.1.2: Transmission and reflection of an optimised Y-junction (3 dB splitter).

Coupling and Packaging

The major breakthrough brought by photonic integrated circuit devices for future optical communication systems

relies on their ability to be miniaturised. Many of the costs for miniaturised components are generated by the

required precision for the alignment of individual components. Thus, packaging can often account for 40-70% of

the costs for components, using classical micro-lenses and optical fibre couplers. In order to drastically reduce

costs for complex IT- or data-transmission devices, key developments are focussed on integrating the maximum

number of functions and reducing the number of connections while maximising information transfer.

Within COPERNICUS, inverse tapers are used to couple lensed fibres fabricated by FOTON with the PhC

circuits. To optimise the coupling and relax the constraints due to misalignments, a proper design is needed. The

collaboration between FOTON, TRT and UniFe on this topic has allowed the design of tapers and lensed fibres

which present losses of about 4 dB per coupler. The inverse tapers were optimised by 3D-FDTD simulations. In

Fig. 3.1.3 (left), a top view of the beam intensity is plotted for two different taper lengths. The membrane

material is GaAs and the wavelength is 1550 nm. Fig. 3.1.3 (right) shows the reflection coefficient for a W1 PhC

waveguide terminated with inverse tapers of different lengths (7 periods, 10 periods and 20 periods). The

reflection is always less than 10 dB compared to the 5 dB obtained when the taper is not considered.

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Fig. 3.1.3: Beam intensity profiles for two inverse tapered couplers with different taper lengths (left). Coupling

reflections between a lensed fibre for a butt-coupled PhC and inverse couplers with different lengths (right).

Filters and WDM Demultiplexer

The second key device developed within COPERNICUS is a four channel WDM receiver operating at 25 Gb/s

per channel. At the core of this device are wavelength drop filters that must match the performance

specifications for WDM in terms of crosstalk rejection, bandwidth and frequency spacing.

Many different filter topologies have been considered, with the aim of obtaining high drop efficiencies and

suitable linewidths. Filters based on a mirror cavity configuration were initially investigated and are discussed

here to illustrate the design problem. In this configuration, the first cavity acts as resonant tunnelling-based

channel drop filter, whereas the second resonator is used to realise a wavelength-selective reflection feedback in

the bus waveguide. Detailed CMT analysis and intensive 3D-FDTD simulations have allowed the design and the

optimisation of different topologies based on this concept. Fig. 3.1.4 (left) illustrates one topology of a filter used

for the design in both C- and O-bands. In the same image, the reflection coefficient at the input port and the

transmission curves at the output ports are plotted as a function of wavelength (O-band operation). For this

configuration, the resonance is around 1315 nm, the efficiency is close to 80% and the linewidth is ~ 2 nm. TRT

fabricated high-performance C-band filters based on this design using air-bridge InP membranes. Filters based

on a single H0 cavity had a drop efficiency of 50% and an extinction ratio of 30 dB (Fig. 3.1.4 (centre & right)).

Fig. 3.1.4: Transmission curves of a filter operating in the O-band (Red curve: reflection coefficient (S11); Blue

dotted curve: transmission for the bus waveguide (S21); Solid blue curve: transmission to the drop port (S31)) –

the inset shows the top view of the 3-port filter based on a mirror cavity configuration (left); SEM image of a

single cavity drop filter made at TRT (centre) and measured transmission as a function of wavelength (right).

All-Optical Gates (AOGs)

Design and modelling of the AOG is also vital. AOGs can be considered as filters (see previous section)

operating in the nonlinear optical regime. The key tools for the modelling of the AOG are coupled mode theory

(CMT) in the time domain and FDTD (both linear and nonlinear). Since the modelling and operation of AOGs

are discussed extensively in sections 3.1.1, 3.1.3 and 3.5, they are not discussed further here.

Photodetectors

In both receivers, the electronic output is provided by a compact integrated photodetector. The key requirements

for this device are a fast response (>28 GHz) and a high responsivity (>0.5 A/W). The realisation of this

component presents a number of design challenges. For example, a vertical p-i-n structure with an absorbing

layer must be integrated with the PhC waveguide. To meet the bandwidth specification, devices with a small RC

time constant are required, so the junction area must be kept small. For high responsivity, the optical field must

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be transmitted from a single-mode PhC waveguide into the small photodetector, requiring careful optical

matching and efficient lateral optical confinement.

Optical and electrical simulations were performed using FDTD (UniFe) and bipolar electrical/thermal models

(UNott), respectively. The simulated small-signal frequency response of the device is shown in Fig. 3.1.5.

0 2 4 6 8 10

10

15

20

25

30

35

40

Reverse bias (V)

3 d

B b

an

dw

idth

(G

Hz)

dInGaAs (nm)

500 600 700 800

Fig 3.1.5: Equivalent circuit for external bias circuit (left). Simulated small-signal modulation response with

bias circuit. The targeted performance (2 V, ≥ 28 GHz) is indicated by the cross-hairs (right).

The large-signal response of the photodetector was also simulated by UNott using a 32-bit long, 30 Gbps NRZ

optical input (5 mW) with an extinction ratio of about 5. To produce a realistic input, the simulated output from a

10 GBit/s laser diode was scaled in time to produce a 30 GBit/s signal. Fig. 3.1.6 shows the input signal (top left)

and its corresponding eye diagram (top right). Impairments are already observed in the input eye diagram, due to

the non-perfect input signal. Fig. 3.1.6 also shows the output signal from the photodetector (bottom left) and

corresponding output eye diagram (bottom right). The additional degradation of the output eye is not significant.

The finite rise- and fall-times of the photocurrent is attributed to the finite transit time for the carriers from the

generation region. The PhC photodetectors were fabricated and characterised by DTU. As seen in section 3.4.2, a

PhC photodetector was successfully demonstrated with a 3 dB bandwidth > 30 GHz at a reverse bias of 2 V.

Fig. 3.1.6: Bit pattern and eye-diagrams for a noisy 5 mW peak amplitude 30 Gbps optical input signals with an

extinction ratio ~5 (top). Large-signal photocurrent response and eye diagrams for -2 V bias voltage (bottom).

3.1.3 Robust Simulation Tools for PhC Devices

UNott, UniFe and DTU developed several tools for the design and simulation of functional PhC devices. These

tools included:

coupled mode theory (CMT) optical model with multiple rate equations for the carrier dynamics (DTU);

CMT optical model and 1D spherical dynamic bipolar electrical model with self-consistent surface

recombination/charging (UNott);

CMT optical model with a dynamic 2D dynamic bipolar electrical/thermal model, including air-holes and

self-consistent surface recombination/charging (UNott);

2D FDTD solver with nonlinear optical response and 2D dynamic unipolar carrier diffusion model (UniFe);

3D FDTD solver with nonlinear optical response with a dynamic 2D dynamic bipolar electrical/thermal

model, including air-holes and self-consistent surface recombination/charging (UniFe, UNott).

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Due to the range of physical effects included and the different computational resources required, each of these

models produced useful results for the project. These tools will also serve as a software platform for the

simulation of other active nanophotonic devices and PICs in the future.

The most advanced model couples UNott’s time-domain 2D bipolar electrical/thermal model with UniFe’s 2D

and 3D FDTD optical model (see Fig. 3.1.7 left) for application to both PhC photodetectors and AOGs.

Unidirectional coupling can be assumed for the photodetector, since the photogenerated carriers are swept away

by the applied electric field and do not affect the material polarisation.

For the AOG, however, strong bidirectional coupling is essential, since rapid carrier-induced index changes are

directly responsible for the operation of the AOG. This coupling is challenging for two reasons. Firstly, the

physical processes (and models) have different spatial and temporal meshing requirements. Both models push

the limits of existing computational platforms, so the mesh definition is critical. Secondly, the dielectric

polarisation is calculated in the frequency domain, while the optical and electrical simulations are both

performed the time domain.

UNott also developed a simplified version of this model, whereby the dynamic 2D electrical/thermal solver is

coupled to the CMT solver from the spherically-symmetric AOG model, as illustrated in Fig. 3.1.7 (right). The

CMT optical model is computationally more efficient than an FDTD solver, so that longer simulations can be

performed. This model was used to explore different applications of the PhC AOG.

Carrier Generation

Rate Distribution Dielectric Polarisation

Distribution

Optical

Inputs /Outputs

Optical

Inputs /Outputs

Optical Model

Electrical Model

Energy in the cavity

|a(t)|2

Averaged ∆n(t); carrier density in cavity cavity

Optical

Inputs /Outputs

Optical

Inputs /Outputs

Electrical Model

Coupled Mode Theory (CMT) Model

Fig. 3.1.7: Block diagram of the coupling of the electrical-thermal model and the FDTD model (left). A

simplified, computationally efficient model was developed by replacing the FDTD solver with CMT (right).

3.2 Materials Engineering

WP Leader: CNRS-LPN

Contributors: DTU, TRT, UNott

This workpackage dealt with the technological developments required to achieve the main goals of the project,

i.e. WDM receivers, AOGs and photodetectors. There were three main activities within this work package:

1. material growth and engineering, to achieve low-power, high-speed AOGs and PIN photodiodes;

2. photonic crystal processing; and

3. custom processing (e.g. wafer bonding for heat management, focussed ion beam (FIB) processing).

3.2.1 Material growth and engineering for AOG and detectors

With a view to obtaining fast AOG’s, CNRS-LPN manipulated the carrier lifetime in InP-based materials

through growth and process engineering. Lifetime measurements were performed in laterally patterned mesas of

varying radii containing 4 InGaAs QWs emitting at 1.55 µm and explored through time resolved

photoluminescence. The lifetime (Fig. 3.2.1 right) showed a significant reduction from 1 ns to < 200 ps for

mesas with diameters 500 nm. This diameter is close to the dimensions of the cavities to be used for AOGs.

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Fig. 3.2.1 Carrier life time of 4 InGaAs/InP QWs as a function of the mesa diameter (left). InP-based

heterostructure with 1 QW at the surface (right).

Samples were then fabricated with the InP-based heterostructure positioned at the surface (Fig. 3.2.1 left)

through adhesive bonding on top of a SiO2 layer in order to ensure good optical confinement as well as better

heat dissipation. Photoluminescence measurements were performed and showed a decrease of a factor of 10 in

the PL intensity, which attests to the carrier lifetime reduction. By combining the two approaches within one

structure, it was then possible to demonstrate ultrafast all optical gating using PhC cavities. An SEM picture of

the fabricated structure as well as schematic of the structure with the constituent layers is shown in Fig. 3.2.2.

Fig. 3.2.2: SEM picture of the PhC cavity (left); Layer stack of the fabricated sample (right).

The PhC cavity consisted of 1 row of holes drilled in a single InP wire waveguide (600 nm x 260 nm), where a

single quantum well was placed at each of its horizontal surfaces. The absorption edges of the QWs were chosen

to be near 1.55 µm. The cavity was designed to exhibit a high Q resonance around 1.55 µm destined for the

ultrafast switching application. The cavity was fabricated on top of a SOI passive wire waveguide ensuring good

evanescent coupling to the PhC cavity from the lower level. Finally, the sample was fully encapsulated in SiO2

in order to ensure sufficient thermal heat sinking.

The linear optical response was characterised by measuring the transmission spectrum using a tuneable laser. On

the sample of interest, a clear dip in the transmission was obtained at 1549 nm, as shown in Fig. 3.2.3. The

measured Q factor is 2500. These results are in accordance with the values expected from numerical simulations.

To demonstrate the capability of this system as a fast and efficient optical gate, degenerate pump probe

experiments were performed where the probe signal is detected as a function of probe delay using heterodyne

detection. The transmission decay was fit with a single exponential decay function (see Fig. 3.2.4). The decay

time was 12 ps. Using this sample with PhC cavities, ultrafast all optical gating was demonstrated (reported in

section 3.2.5).

1,50 1,52 1,54 1,56 1,58 1,60

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

Tra

nsm

itte

d s

ign

al (a

rb.

un

its)

wavelength (µm)

400nm SOI wire

Fig. 3.2.3: Linear transmission of the sample. Fig. 3.2.4: Probe transmission vs pump-probe delay.

Photodetectors

The photodetector structure was grown at DTU. Since operation at 1.3 m was initially needed for the final

WDM receiver, the doped contact layer was recalibrated to PQ(1.25):Zn to minimize parasitic absorption. Later

InP+ surface quantum wells

Si

SiO2

BCB

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in the project, the WDM receiver specification was shifted to the C-band due to increased interest in applications

at these wavelengths. The final photodetector was tested with both C- and O-band signals.

3.2.2 Photonic crystal processing

Initially, effort was devoted to improving the control of the optical response of the PhC structures for the WDM

receiver. TRT showed that, by coating with a thin layer of silicon nitride, it is possible to precisely tune the

wavelength of a PhC cavity resonance (see Fig. 3.2.5). Uniformity of the processing over a large area of the

wafer was tested by fabricating PhC cavities and monitoring their resonant wavelength. Good results were

obtained by finely controlling the focus of the electron beam and by optimising the writing conditions.

Fig. 3.2.5: PhC cavity spectrum as a function of silicon nitride deposition.

TRT developed an automatic layout generator which could generate the complex layout required for the multi-

channel WDM filter using design parameters as inputs. A 4 channel WDM filter is represented in Fig. 3.2.6. This

tool was then deployed systematically for generating any layout for the fabrication of WDM filters based on both

air-bridge and BCB encapsulated membranes.

Fig. 3.2.6: This WDM demultiplexer PIC layout was generated systematically by the layouts of the individual

sections. This PIC comprises two AOGs, with the input signal from the right and the output signals from the bus,

drop port 1 and drop port 2 on the left.

TRT also implemented electrical tuning of the cavities, via the thermo-optical effect (see Fig. 3.2.7). The

electrical power consumption is in the mW range, per cavity and per nm.

Fig. 3.2.7: Photonic crystal with integrated heater: Modelling of the heat diffusion from the heater (left);

Measurement of the spectral shift of the cavity resonance as a function of bias voltage, where the maximum

dissipated power is about 1 mW (right); Screenshot from the oscilloscope showing tuning at 80 kHz (middle).

At DTU, particular attention was paid as to how electron beam resolution is limited by effects, such as: electron

scattering in the resist/substrate stack; resist material; and development process effects. As the electron beam

strikes the resist layer, the forward scattering and backward scattering of the electrons in the resist and substrate

gives rise to the exposure of an undesired region. This is known as the e-beam proximity effect. The proximity

effect and resist profiles have a significant influence on the pattern quality, and therefore the optical properties of

the devices. A detailed analysis showed that, for the use of the HSQ resist, the proximity effect correction (PEC)

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is mandatory for fabricating PhCs that fit the designed parameters. Secondary electron contributions and beam

blur also played a non-negligible role and were taken into consideration.

Fig. 3.2.8: Cross-sectional SEM images of proximity corrected structures including mid-range effects and beam

blur (left) and standard proximity correction (right).

Fig. 3.2.8 shows two cross-sectional SEM pictures of the InP wafer after the PhC holes were transferred from the

HSQ mask to the semiconductor using dry etching (the HSQ mask is still in place). In the left image, the

secondary electron and beam blur contributions were taken into account in the PEC. The image shows that all the

holes, except the one adjacent the WG which is intentionally smaller than the others, have the same size. On the

right, is a SEM image of a PhC fabricated using a PEC that only takes backscattering effects into account. In this

case, not all of the holes are fully developed, indicating that short and middle range effects cannot be ignored

when PhC structures are fabricated in HSQ. This work improved the fabrication of the filters and photodetectors.

Final improvement in the fabrication of the photodetectors at DTU

One of the goals DTU achieved while developing the fabrication process for the photodetectors, was the

capability to fabricate photonic crystal holes whose dimensions fit the designed ones. For this, the entire mask

was divided in two parts and exposed separately. In Fig. 3.2.9, the final e-beam mask is shown. First, the green

area is exposed and developed. Next, the HSQ resist is spun again and the blue area, aligned to the already

patterned PhC WG, is exposed and developed. Of course, the use of this mask adds further steps to the PD

fabrication process, but it allowed good PhCs to be obtained, as seen in the SEM pictures shown in Fig. 3.2.9.

Fig. 3.2.9: PhC WG mask layout divided in two areas to be exposed separately. Cross-sectional pictures taken in

a SEM of the PD PhC WG obtained using the double e-beam exposure approach. In the right hand SEM picture,

the transition between the rib and the PhC WG is shown.

Improvement in the fabrication of the hybrid SOI/InP all-optical gates at CNRS-LPN

CNRS-LPN worked on the fabrication of 2-port and 3-port hybrid SOI/InP AOGs. To simplify and improve the

processing of the structures, efforts were devoted to the electron beam lithography and plasma etching steps. For

the electron beam lithography, it was decided to use the negative tone electron beam resist HSQ, which allowed

the definition of a mask for the InP material with extremely smooth sidewalls. This decision simplified the

processing as the nanobeam PhC cavities used for the AOGs were now defined using only one lithographic level.

A top view of the HSQ mask can be seen in Fig. 3.2.10. The smoothness of the HSQ mask can also be seen in

Fig. 3.2.10, which shows a SEM picture of the sample taken right after the plasma etching.

Fig. 3.2.10: Left: Top view of the HSQ mask. Centre: Side view of the structure after plasma etching. The HSQ

mask is visible on top of the InP-based material. Right: InP-based nanobeam cavity after HSQ mask removal.

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Fig. 3.2.11: Fabricated 2-port and 3-port hybrid AOG.

The plasma etching of the InP-based material was also improved by using Cl2/H2 chemistry to obtain vertical and

smoothly etched sidewalls. The results can also be seen in Fig. 3.2.10. This new process flow was used to

fabricate 2-port and 3-port AOGs. Fig. 3.2.11 shows an SEM picture of a 2-port AOG.

3.2.3 Custom processing

Focused Ion Beam Processing

The ability to fabricate new holes and modify existing holes in a free-standing membrane is important for

making local geometry modifications to improve the performance of fabricated devices. Examples where this is

employed include tuning the cut-off wavelength of waveguides; creating polarisation converters (using tilted

holes); and tuning the performance of components (delay lines, bends, splitters...). Details of the intended holes

diameters, FIB process settings, actual hole diameters and the resultant errors show that the FIB process settings

affect the accuracy of the fabricated holes. UNott demonstrated the ability to drill high-quality tilted holes in InP

using FIB (see Fig. 3.2.12) as they are required to address the issue of polarisation control.

Fig. 3.2.12: SEM images of tilted holes fabricated in an InP membrane. Image with sample perpendicular to the

SEM column (left) showing 6 holes angled at 45 degrees to the surface, with 3 pointing towards the top of the

image and 3 pointing towards the bottom of the image. Image of the same sample titled to 45 degrees with

respect to the SEM column (right), showing a clear view through 3 of the angled holes.

The FIB was also used to investigate the possibility of repairing inverse tapered couplers (e.g. when damaged

during fibre coupling). UNott showed that the FIB could be used to fabricate new couplers in air-bridge

membrane PhCs. The results are shown in Fig. 3.2.13.

Fig. 3.2.13: SEM images of a FIB-fabricated tapered coupler (length ~ 7a, fabrication time ~ 30 minutes);

broken membrane edge before FIB milling (left); fabricated coupler (centre); magnified view (right).

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Bonding of two-level system: PhC on SOI waveguide

The heterogeneous integration of the III-V semiconductor photonic crystal (PhC) nanocavity and the silicon on

insulator (SOI) wire waveguide was done using a BCB planarising polymer, which enabled the bonding of a flat

surface (InP membrane) to a substrate already patterned with circuitry (SOI). The SOI circuitry is used for

channelling both the pump and the signal. The structure is composed of 2 optical levels, where one level is a

single mode SOI wire waveguide and the other is an InP-based PhC nanocavity with embedded quantum wells.

The 2 levels are separated by a low index layer (silica + benzocyclobutene), which preserves the vertical optical

confinement within the SOI waveguide and the PhC cavity. Optical coupling between the two levels is ensured

by the penetration the evanescent tail of the optical modes into the other level. The hybrid structure is

schematically represented in Fig. 3.2.14.

1,50 1,51 1,52 1,53 1,54 1,55 1,56 1,57 1,58 1,59 1,60

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

Tra

nsm

issio

n (

arb

. units)

wavelength (µm)

Fig. 3.2.14: Schematic representation of the hybrid structure (left). Linear transmission of the sample (right).

On the fabricated sample, the parameters of the PhC cavities (holes radius and lattice constant) were varied in

order to tune the resonant wavelength. The width of the SOI wire was also varied to control the amount of

coupling between the two levels. An example of the transmission spectrum is shown in Fig. 3.2.14, where two

dips are clearly observed. This is evidence of the coupling of the PhC resonant modes at these wavelengths to the

SOI wire. Both of these modes are used, one for the pump to drive the AOG and one for the signal to be gated. In

order to tune the coupling strength, the same cavity was used, but was positioned above SOI wires with different

widths. The maximum coupling was obtained when the SOI wire was 400 nm wide. This system was used for

the demonstration of all optical gates, as it simplifies the guiding of the signal and the pump. This has been

implemented for the high bit rate demonstration of an AOG.

3.3 Photonic Crystal Circuitry

WP Leader: UniFe

Contributors: CNRS-FOTON, CNRS-LPN, DTU, TRT, UNott

This activity focused on the design and optimization of the optical circuitry needed for the integration of the key

devices (WDM Receiver, OTDM demultiplexer) developed in COPERNICUS into a single semiconductor chip.

Optical circuitry is needed to suitably connect the key components of the different devices (filter stages,

photodetectors, cavities and cavity systems). Since different technologies were explored for the realization of

these key devices, several solutions for the circuitry were also investigated, optimized and fabricated. The

problem of optimizing the coupling among integrated devices and fibres at the input/output ports was also

tackled. Different solutions were also proposed and examined, depending on the fabrication technology used.

The three different technologies used in COPERNICUS are:

1. Air suspended PhC membranes, which were used at the beginning of the Project for the fabrication of

both WDM filters and All Optical Gates (AOGs). The Consortium designed circuitry for

interconnections, such as Y-junctions and S-bends, as well as inverse tapers for coupling with lensed

fibres at the Input/Output interfaces of the PhC structure;

2. BCB-embedded PhC membranes, whose introduction was mainly determined by the technological

process required to fabricate photodetectors. The use of this technology necessitated a strong revision of

some of the concepts previously adopted for the design of PhC circuitry, due to the need to fabricate

waveguides with low loss and high transmission bandwidth in materials with a lower index contrast;

3. Hybrid III-V/SOI technology, for the fabrication of high-performance AOG devices.

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The devices realized in the framework of this workpackage are summarized in the following table, where the

different technologies in which these components have been used are also indicated.

Device Description Technology Built Y-junction Topology Optimized Y-junction Air suspended PhC membranes No

S-bend Topology Optimized S-bend Air suspended PhC membranes No

S-bend Simplified design S-bend Air suspended and BCB-embedded PhC membranes Yes

S-bend Photonic wire S-bend BCB-embedded membranes & Hybrid III-V/SOI PhCs Yes

Mode adapter Vertical Coupling Hybrid III-V/SOI technology Yes

Transition Photonic wire to PhC transition BCB-embedded PhC membranes Yes

Taper Taper for PhC to fibre coupling Air suspended PhC membranes Yes

Lensed Fibres Lensed Fibres for coupling with PhC devices Air suspended and BCB-embedded PhC membranes Yes

Grating Gratings for vertical coupling with I/O fibres Hybrid III-V/SOI technology Yes

Circuitry for air-suspended PhC membranes was developed initially: of the different components investigated, it

is worth mentioning Y-junctions and S-bends designed using the topological optimization process developed by

DTU and UniFe. Although effective, this design is very complex to implement. However, it inspired a simplified

design for S-bends (Fig. 3.3.1), that was also successfully employed for BCB-embedded devices.

Fig. 3.3.1: Comparison of double S-bend designed by topological optimization (left) and one designed with the

simplified approach (right). Although the performance of the simplified S-bend is not as good as that of the TO

S-bend, it was successfully fabricated and used for both air-bridge and BCB-embedded membranes.

The photodetector technology used made the use of BCB-embedded membranes unavoidable. This introduced

some issues in the design of cavities and waveguides, due to the reduced index contrast of the membrane and the

surrounding material. In particular, waveguides exhibited high losses and reduced transmission bandwidth. To

cope with these problems, the Consortium made the decision to change its approach to the design of PhC devices

by using photonic wire waveguides to link the key components (such as cavities and photodetectors), still based

on PhC membranes. This made it possible to build the required connections with a very simple high-performing

component, where the losses and distortion determined by group velocity dispersion can be easily controlled.

This approach was then extended to air-bridge membrane devices, where the realization of long PhC waveguides

with low losses (caused by stitching errors or other imperfections in the fabrication process) can be a serious

issue for the currently available technology. Optimized transitions between photonic wires and PhC waveguides

were investigated and fabricated.

The use of hybrid III-V/SOI technology was introduced to fabricate high-performance AOG devices. From the

circuitry point of view, this technique required the design of vertical couplers between SOI and InP waveguides

and the development of a technique based on grating couplers for vertical coupling with optical fibres at the I/O

sections. The details of this technology, with results for coupling with optical fibres, are illustrated in Fig. 3.3.2.

Bonding layerPhC AOG

CavitySOI waveguide

Grating

Fig. 3.3.2: Left: Sketch of the 1D-PhC AOG built with hybrid III-V/SOI technology. Gratings are used at the

ends to couple to the I/O optical fibres. Right: Transmission profiles of the hybrid III-V/SOI sample for different

fibre coupling angles - Intermediate coupling (red), L-band coupling (green), and C-band coupling (blue).

At the same time, the Consortium developed a technique for a non-destructive loss measurement, a design

method to allow tailoring of properties of BCB-embedded PhC waveguides and a layout generator for the mask

design of complex structures. This tool proved to be fundamental for the design and fabrication of the complex

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devices structures developed in the Project. Fig. 3.2.6 (see section 3.2) illustrates the layout of a 2-stage WDM

filter.

In conclusion, these activities related to photonic crystal circuitry successfully provided the Consortium with the

circuitry needed to support and couple the different technologies developed in COPERNICUS. Moreover, it also

provided fundamental insights into which technologies were best suited for the fabrication of different devices.

The device shown in Fig. 3.3.3 is an effective demonstration of the success of this work and of the Consortium’s

capabilities for designing and assembling complex structures. In fact this device, which refers to a 4-channel

WDM filter, integrates several of the components described in this section (simplified S-bends, tapers at the I/O

sections of the PhC structure, tailored PhC waveguides, etc.).

Fig. 3.3.3: Integration of four filter stages for the realization of a 4-channel WDM filter. This device shows the

capabilities of the Consortium to integrate different components on the same membrane. Filters, S-bends,

tailored waveguides to optimize the coupling with the resonators and maintain the correct propagation

characteristics and tapers for coupling with suitable developed I/O fibres are all included in this device.

3.4 Linear Devices

WP Leader: DTU

Contributors: CNRS-FOTON, TRT, U2T, UniFe, UNott

3.4.1 Filters

No penalty for 40 Gbit/s RZ signal through a 3 port filter at 1550 nm

Measurements performed by FOTON on a GaAs-based PhC membrane filter made by TRT, had a 50% drop

efficiency, 72.4 GHz bandwidth and > 20 dB isolation. The filter transmission characteristics are shown in Fig.

3.4.1 (left). BER measurements and eye diagrams of a 40 Gbit/s RZ signal through the Bus and Drop channels

are shown in Fig. 3.4.1 (right), along with a comparison to Back to Back transmission. No penalty was observed.

Fig. 3.4.1: Optical transmission spectrum (left) and 40 Gbit/s BER data transmission (right) of 3 port drop filter.

Demonstration of wavelength drop filter tuning using integrated micro-heaters

TRT demonstrated the tunability of the air-bridge membrane filters using integrated heaters. A 1 nm resonance

shift required 0.7 mW of electrical power. Fig. 3.4.2 shows that a tuning range of 10 nm can be achieved.

1 µm

Low-loss Bend Low-loss Fiber Adapter

Lensed fiber

Input port Output ports [bus + 4 drop channels]

Cavity

1 µm

Drop Filter

1 µm

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Fig. 3.4.2: Thermal tuning of an air-bridge membrane PhC wavelength drop filter.

High drop efficiency PhC filter for advanced telecom modulation formats

Fig. 3.4.3 (left) shows a 2D PhC 3-port filter with 47% drop efficiency and insertion losses of 6 dB (bus channel)

and 10.5 dB (drop channel). Fig. 3.4.3 (right) shows error free filtering with low penalty (< 0.5 dB at BER=10-9

)

for 28 Gbit/s OOK and 56 Gbit/s Differential Quadrature Phase Shift Keying (DQPSK) modulation formats.

Fig. 3.4.3: Filter transmission and drop spectra (left). Low-penalty, error-free filtering of 28 Gbit/s OOK and 56

Gbit/s DQPSK signals (right).

3.4.2 Photodetectors

A high-speed photodetector (PD) compatible with photonic crystal technology was developed and fabricated at

DTU (see Fig. 3.4.4 (right)). Small signal dynamic measurements (0–40 GHz) were performed to evaluate the

PD bandwidth. The S21curves in Fig. 3.4.4 (left) show that our PD has a 3 dB bandwidth > 40 GHz at -2 V bias.

The experimental results are in good agreement with the simulated performance.

Fig. 3.4.4: High-speed, ultra-compact photonic crystal photodetector (right). For an applied bias of -2 V, the

frequency response is > 40 GHz - comfortably exceeding the COPERNICUS requirements.

3.4.3 WDM Demultiplexer

A four channel wavelength demultiplexer was successfully fabricated by TRT using a device based on resonant

structures implemented in a GaInP membrane photonic crystal. Using this demultiplexer, FOTON demonstrated

the transmission of a 100 Gbit/s NRZ signal at 25 Gbit/s per channel. The measured device characteristics

showed good transmission performance for all channels and open eye diagrams after demultiplexing.

Fig. 3.4.5 (top) shows an image of the 1.3 mm long WDM demultiplexer. The single input port and 4 output

ports are clearly seen. The transmission through each of the channels of the PhC filter (the input power is

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normalised to 0 dBm) is also seen in Fig. 3.4.5 (bottom). The eye diagrams of the signals at the outputs of each

port are shown after the wavelength demultiplexing of the 100 Gbit/s NRZ signal into 4 channels at 25 Gbit/s.

Fig. 3.4.5: Image of the 4-channel WDM demultiplexer (top). Transmission spectra of each of the drop ports

(bottom left) and eye diagrams of 100 Gbit/s signal after demultiplexing into 4x25 Gbit/s signals (bottom right).

A second WDM demultiplexer PIC was successfully demonstrated. This chip was based the use on BCB-

encapsulated PhC wires in order to be compatible with the PhC photodetector. The design was made by UniFe,

the mask layout and device characterization were performed at TRT, while the processing was done at DTU in a

reduced version of the process used for the photodetectors. The multi-stage filter has a common input bus and

two drop channels (drop 1 and drop 2). Fig. 3.4.6 illustrates the filtering behaviour using a top IR view of the

device under input illumination. (Note: The V-shaped silhouette to the left of the Drop 1 port is a lensed fibre.)

Fig. 3.4.6: BCB encapsulated filter with two drop ports. IR camera images under input illumination at different

wavelengths. Scattering at the outputs and cavities are seen illustrating the filtering behaviour of the device.

Fig. 3.4.7 shows the transmission spectra for the different ports (Bus, Drop 1 and Drop 2). Strong Fabry-Perot

modulation of the spectra is observed, since the output waveguides are abruptly terminated by a cleaved facet.

Fig. 3.4.7: Spectral dependence of the different ports from the device shown in Fig. 3.4.6.

Cavity 1 Cavity 2

Bus

Drop 1

Drop 2

Bus

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The combined efforts of UniFe and LPN also allowed the design and fabrication of a 3-port AOG in Hybrid III-

V Semiconductor/SOI technology. FDTD simulations give a resonance at = 1558 nm with a linewidth of

2.48 nm (Q = 630) and a drop efficiency close to the maximum theoretical value of 50%. This device can also be

used as a filter, and hence as a demultiplexer. A fully integrated receiver has not been realized yet, since the

successful BCB encapsulated cascaded filter technology (i.e. 2 channel WDM demultiplexer PIC) was realized

shortly before this report. A contingency plan was also pursued using out of plane emission from cavities into

commercial surface pin detectors. Single cavity structures worked well, but the multi-cavity device was less

successful. Finally, as described above, the 3-port AOG recently developed by LPN and UniFe on the hybrid III-

V/SOI platform can also be used as a filter, and hence as a demultiplexer, but this technology could not be

merged with the photodetector process prior to the end of the project. In conclusion, the elements for the WDM

receiver are now all in place, but the combined fabrication has not been completed at the time of this report.

3.5 All-optical Gates

WP Leader: TRT

Contributors: LPN, DTU, FOTON, UniFe

3.5.1 Air-suspended membrane AOG technology

At the beginning of the project, all-optical modulation (modulation of the transmission of an optical signal using

another optical signal) was demonstrated using an air-suspended photonic crystal membrane based on Gallium

Arsenide. The pump-probe measurement reported a recovery time of only 6 ps, which is remarkably fast for a

semiconductor device. In fact, it was much faster than any other nanophotonic device demonstrated at that time,

as illustrated by Fig. 2.1.1 in section 2.1

The basic principle of the AOG is the optically induced spectral shift of a Photonic Crystal microcavity, which is

configured to operate as a band-pass filter. As a consequence, the transmission spectra near the cavity resonance

will be controlled dynamically. The underlying physical mechanism enabling the detuning is the dependence the

refractive index on the density of free carriers. Here, free carriers are generated by two-photon absorption of the

optical "control" signal. Importantly, two-photon absorption is localized only in the area of the device, where

intensity of the optical field is large. This is designed to occur in the AOG cavity, when resonantly excited by the

pump. The material is virtually transparent elsewhere. This enables a remarkably simple design based on a single

semiconductor material, e.g. no need for active-passive technologies or mixing of different materials.

GaAs AOGs, while extremely interesting in terms of speed, were found to be of limited practical use because of

their very low switching contrast, namely about 1 dB. Thus, PhC cavity technology based on a different material,

InP, was developed in COPERNICUS by TRT and also by DTU. As the surface recombination in InP is much

weaker than in GaAs, the carrier lifetime tends to be longer, increasing the switching recovery time. On the other

hand, pump-probe experiments on these devices have revealed a very large switching contrast, easily exceeding

10 dB (as shown in Fig. 5.3.1 a), which is acceptable for all-optical signal processing applications.

Another limitation of the device demonstrated at the beginning of the project is the fact that the same resonance

of the cavity is common to the optical "signal" and "control". Therefore, these two optical signals tend to be very

close in frequency, which is impractical for applications.

Fig. 5.3.1: (a) Device layout of two H0 (single-mode) cavities coupled together to form a "molecule". (b)

Transmission spectra revealing two well-separated resonances (dashed line is the predicted transmission using

the coupled mode model). Pump and probe are tuned as indicated by the arrow.

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The device developed here is based on two coupled cavities, as shown in Fig. 5.3.1 a, such that two peaks appear

in the transmission spectra (Fig. 5.3.1 b). This enables a substantial (10 nm) spectral separation of the two optical

signals, thereby eliminating any risk of cross-talk.

The device consists of an InP membrane (the epitaxy is from the group of Prof. H.-P. Reithmaier at University of

Kassel) patterned with a hexagonal PhC lattice. The static contrast (minimum and maximum transmission in the

linear regime) is 30dB. The oscillations in the transmission spectra are due to imperfections in the waveguides

and are not related to the nonlinear dynamics of the cavities. The device was designed in collaboration between

the UniFe and TRT. The fabrication and characterization were performed at TRT.

The dynamical response was measured at TRT using a non-degenerate heterodyne pump-probe technique using

2 ps long pulses obtained by spectrally slicing a 100 fs mode-locked fibre laser. This enabled the measurement of

the key parameters of the device (Fig. 5.3.2 a). The switching contrast, defined as the power ratio between the

"on" and "off" states, was as large as 10 dB for the maximum pump power of 40 W (average power at the input

for a 36 MHz repetition rate) or about 200 fJ per pulse coupled into the waveguide. The recovery time reveals a

fast (12 ps) and a slow (200 ps) time constant.

Fig. 5.3.2: (a) non-degenerate pump-probe measurement of the nonlinear dynamics in the "photonic-molecule"

AOG. The transmission of the probe, tuned as indicated in Fig. 5.3.1a, is plotted as a function of the delay

relative to the pump. The switching contrast and recovery time are measured. Frequency conversion is

demonstrated by showing the all-optically modulated probe (input as a CW signal) at the oscilloscope (b) with

the pump (10 GHz mode-locked diode laser). The pump is filtered out using a band-pass filter. The response to a

single short pulse is seen at the optical oscilloscope (c).

The high-speed operation capability of the device has been demonstrated using a mode-locked laser diode

operating at 10.098 GHz (fabricated by Alcatel-Lucent-Thales III-V Lab). The pump was obtained by spectrally

slicing the laser output with a 1.2 nm wide bandpass filter, while the probe was generated by a CW tunable laser.

The oscilloscope trace of the modulated signal (Fig. 5.3.2 b) demonstrates that the AOG can follow a clock at 10

GHz. These results have been reported at a recent conference [1] and submitted for publication [2].

DTU has also demonstrated similar results on a single-cavity switch based on InP [3]. In particular, the

wavelength conversion of signals has been demonstrated [4].

References

[1] S. Combrié, G. Lehoucq, S. Malaguti, G. Bellanca, J. Reithmaier, S. Trillo, and A. De Rossi, “Two-color switching and

wavelength conversion at 10 GHz using a photonic crystal molecule” in Proc. Conf. Lasers Electro-Optics, 2013, CM4D.5.

[2] Sylvain Combrié, Galle Lehoucq, Alexandra Junay, Stefania Malaguti, Gaetano Bellanca, Stefano Trillo, Loic Menager,

Johan-Peter Reithmaier, and Alfredo De Rossi, “All-optical signal processing at

10 GHz using a Photonic Crystal molecule”, submitted to Applied Physics Letters.

[3] Y. Yu, E. Palushani, M. Heuck, S. Ek, N. Kuznetsova, P. Colman, D. Vukovic, C. Peucheret, L. K. Oxenløwe, K. Yvind,

and J. Mørk, “Ultra-fast low energy switching using an InP photonic crystal H0 nanocavity” in Proc. Conf. Lasers Electro-

Optics Pacific Rim, 2013, paper MI1-4.

[4] Dragana Vukovic, Yi Yu, Mikkel Heuck, Sara Ek, Nadezda Kuznetsova, Pierre Colman, Evarist Palushani, Jing Xu,

Kresten Yvind, Leif Katsuo Oxenløwe, Jesper Mørk, and Christophe Peucheret, "Wavelength Conversion of a 9.35 Gb/s

RZ#OOK Signal in an InP Photonic Crystal Nanocavity", submitted at Photonic Technology Letters

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3.5.2 Two-Port AOG based on Hybrid III-V/Silicon Technology

The two-port sample consists of a hybrid III-V photonic crystal nanobeam cavity on a silicon on insulator (SOI)

waveguide, as shown in Fig. 3.5.3. Quantum wells are positioned at the surface of the cavity in order to enhance

the surface recombination of the carriers and obtain fast operation. The sample is also fully encapsulated in silica

in order to increase heat sinking, which enables high bit rate measurements to be performed.

Fig. 3.5.3: SEM image (left) and schematic view (right) of the fabricated two port hybrid III-V/SOI structure.

Demonstration of fast switching

Following fabrication, degenerate pump-probe experiments were performed to validate the fast nonlinear

response of the structure. As seen in Fig. 3.5.4, the typical switching times were ~ 12 ps. Here, the switching

energy was 40 fJ. This result gave the first proof that these structures were capable of high bit rate operation.

Fig. 3.5.4: Probe signal transmission -vs- pump-probe delay for a pump pulse energy of 40 fJ.

Demonstration of wavelength conversion

The ability of the hybrid III-V/SOI switch to perform wavelength conversion was assessed at both 10 and 20

Gbit/s. In addition, the possibility of using both resonances of the device for pumping and probing was explored.

A 10 Gbit/s Pseudo Random Bit Sequence (PRBS) was used as a pump signal at one resonance and a CW signal

as a probe signal at the other resonance of the device. At the maximum available pump power (corresponding to

a coupled peak power of 6 mW), a clearly open converted eye diagram was obtained, as shown in Fig. 3.5.5

(left). Bit Error Rate (BER) measurements show error free operation on the converted signal (grey circles).

However, a 4 dB penalty is measured at a BER of 10-9

. In order to investigate the origin of this penalty, the BER

of the back-to-back (B2B) coupled fibres was measured with a degraded extinction ratio of 3.6 dB (full

symbols). This data is very close to that of the converted signal, demonstrating that the penalty is a result of the

low extinction ratio of the converted signal which arises from a limitation of the switching contrast.

-32 -30 -28 -26 -24 -22 -20

1e-9

1e-7

1e-5

1e-3

Receiver input power (dBm)

BE

R

Back to back (ref)

Converted signal

B2B for ER = 3.6 dB

4 dB

Fig. 3.5.5: Demonstration of 10 Gbit/s (left) and 20 Gbit/s (right) wavelength conversion.

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A similar experiment was performed at 20 Gbit/s with a coupled peak power of 9 mW. The eye diagram is still

quite open, as shown in Fig. 3.5.5 (right). Error free operation can be obtained, but with a consequent increase of

the penalties. As in the previous experiment, the back-to-back curve with a degraded extinction ratio shows that

the limiting factor at this bit rate is still the extinction ratio of the converted signal.

Demonstration of noise limiting function

The two-port switch was also tested as a power limiter at 10 Gbit/s and its noise reduction capabilities assessed.

Amplitude fluctuations were generated by two different methods, as shown in Fig. 3.5.6.

In the first experiment, intensity noise was generated using a CW laser modulated through an external modulator

driven by a 1 GHz bandwidth noise diode. The relative intensity noise (RIN) of this signal was varied by

adjusting the noise diode voltage. This noisy optical source was then modulated at 10 Gbit/s with a sequence

length of 231

-1 bits into a second modulator and injected into the component at a slightly blue shifted wavelength

(with respect to the cavity resonance).

Polarisation

controller

DUT

EDFA

Noisy

Tx

Noisy

TxRxRx

1st experiment

Variable

attenuator

Modulator

PRBS 231-1

Pattern generator

10 Gbit/s

Tunable

Laser

Thermal Noise

Generator

Modulator

CLK1 = 10,66 GHz

PRBS 231-1

CLK2 = 10,675 GHz

2nd experiment

Pattern generator

10 Gbit/sPattern generator

10 Gbit/s

S STunable

Laser

Fig. 3.5.6: Two setups for demonstrating the noise limiting function of the two-port device.

The eye diagrams in Fig. 3.5.5 (left) clearly show the amplitude noise reduction at the device output. The output

SNR as a function of the input SNR curve clearly shows noise reduction.

0 50 100 150 200 2500

100

200

300

400

SNRin

SN

Rout

Experiment

Linear transmission

IN

OUT

-35 -30 -25 -20

10-10

10-8

10-6

10-4

Receiver input power (dBm)

BE

R

Back to back (ref)

Degraded signal

Regenerated signal

4.5 dB

Degraded

Regenerated

Fig. 3.5.7: Output-vs-input SNR with eye diagrams at SNRin = 34 (left). BER curves of the noise limiter (right).

In the second experiment, to increase the noise bandwidth, the noise diode was replaced with a second 10 Gbit/s

pattern generator (27-1 PRBS sequence). There was no synchronisation with the pattern generator used for the

BER measurements owing to a 15 MHz frequency shift. Fig. 3.5.7 (right) shows a BER measurement with a

back-to-back reference signal (crosses), the degraded signal (grey circles) and the regenerated signal (black

circles). This shows a penalty reduction of 4.5 dB at 10-9

BER with a coupled peak power as low as 1 mW.

3.5.3 Three-Port AOG based on Hybrid III-V/Silicon Technology

The hybrid III-V/SOI technology was also employed to realise a three-port AOG. Again, surface quantum wells

and silica encapsulation were used to enable high bit rate operation. Here, the III-V level consisted of a

nanobeam cavity coupled to a wire waveguide, terminated on one side by a high reflectivity mirror. The other

side was positioned above a second SOI waveguide in order to couple light evanescently.

Demonstration of fast switching

Non-degenerate pump-probe experiments were performed to show the fast switching operation of the three-port

device. A 150 fs probe pulse was applied to the SOI bus waveguide, near the resonant wavelength of the cavity.

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A 150 fs pulse at 810 nm was then used to excite carriers in the cavity and shift its resonant wavelength. The

dynamics of the nonlinearity were then followed by exploring this wavelength shift as a function of the pump-

probe delay. The results are plotted in Fig. 3.5.6, where the intrinsic response time of this gate is around 16 ps.

200 250 3001557

1558

1559

1560

1561

Delay (ps)

Wa

ve

len

gth

(n

m)

200 220 240 260 280 300 320

1558.4

1558.6

1558.8

1559

1559.2

1559.4

1559.6

delay (ps)

lam

bda m

ax (

nm

)

untitled fit 1

lambda_max vs. delay

Fig. 3.5.8: Transmission spectrum (left) and resonant wavelength (right) -vs- pump-probe delay.

High bit rate experiments

Following the demonstration of fast switching, the three-port AOG based on hybrid III-V/SOI technology was

tested with telecommunication signals. The cavity that gave the best switching contrast was selected and the

measured transmissions through the drop and bus channels are plotted in Fig. 3.5.7 (left). A 690 MHz pump-

probe experiment with 100 ps pump pulses was performed and achieved a 6.2 dB switching contrast with an

estimated 18 mW of coupled pump peak power into the SOI waveguide (1.2 mW average power), as shown in

Fig. 3.5.9 (right). Unfortunately, this contrast value is too low for 10 Gbit/s wavelength conversion or OTDM

demultiplexing, as the instantaneous power decreases with the increase of the repetition rate. The switching

contrast at 10 Gbit/s would hence be below 1 dB.

1556 1558 1560 1562 1564

-50

-40

-30

-20

-10

Wavelength (nm)

Tra

nsm

issio

n (

dB

)

bus

drop

119 120 121 122 123 124 1251.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

2.3x 10

-4

Po

we

r (W

)

Time (ns)

Fig. 3.5.9: Measured transmission characteristics (left) and modulated probe signal through the three-port

device (right).

For the two-port AOG made using the same technology, the switching contrast at this repetition rate reached 11

dB. It might seem surprising that the contrast was worse for the three-port device, since the quality factor of the

resonance was increased (leading to a higher switching contrast for the same resonance shift). However, in the

three-port sample, a wavelength shift of the resonance (caused by a process issue) meant that only one resonance

was reachable by the source laser available. Consequently, in the two-port component, the pump and probe

signals were positioned at two different resonances and pumping right into the resonance was efficient. In the

case of the three-port component, the pump and probe had to be positioned around a single resonance. The pump

cannot be located right into the resonance in the case of a non-degenerate pump-probe and this results in a less

efficient pump process. Future three-port devices that better fit the telecom band should give access to both

resonances and allow the switching contrast to be improved and higher bit rate operation to be demonstrated.

3.6 Receiver Integration and Packaging

WP Leader: U2T

Contributors: CNRS-FOTON, TRT

The packaging requirements for PhC based receiver were also addressed. Several objectives were identified

before the project and finished within COPERNICUS. The objectives were:

Specification of the receiver parameters

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Design of receiver building block elements and their assembling processes

Assess the usability of the PhC technologies in terms of reliability

Assembling of WDM receiver demonstrator

Packaging of discrete PhC devices for evaluation and experimentation

3.6.1 Specification of the receiver parameters

The first step was the specification of the receiver parameters for 100 Gbps Ethernet and 4-channel optical duo-

binary applications. From these receiver parameters, the electrical and optical specification for the

photodetectors and WDM filters were derived, discussed and finalized with the partners.

3.6.2 Design of receiver building block elements and their assembling processes

The next step was the preparation of a packaging concept for the WDM receiver. Since the PhC chips have a

very small coupling window, an optimum coupling process had to be developed. U2T and FOTON worked on

the PhC to fibre coupling process to resolve problems related to instability and performance degradation due to

possible misalignments in the final packaging. The near field profiles at the tapered output of a photonic crystal

(PhC) waveguide were simulated by UniFe and measured by FOTON, as seen in Fig. 3.6.1. The coupling

between lensed fibres and inverse tapers fabricated at the end of a PhC waveguide was established and the total

loss (P1 – P0) at a wavelength of 1520 nm was measured to be 3.5 dB (TM mode) and 7.2 dB (TE mode).

Fig. 3.6.1: Schematic diagram of setup to measure fibre-coupling losses (left). Transverse mode profile of

inverse tapered coupler (right).

Together with experienced partners of the Consortium, a PhC coupling station was set-up and used for PhC

coupling. The coupling station consists of a near IR camera for pre-alignment of the lensed fibres with XYZ-

tables. Optimization of the fibre alignment was done by maximizing the photocurrent of the photodetectors.

Photos of the coupling station and the fibre holder are shown in Fig. 3.6.2.

Fig. 3.6.2: COPERNICUS coupling station (left). Submount with two fibre holders to align fibres on both sides

of the chip (right).

Once the PhC coupling station was established, coupling process and fibre gluing processes were developed for

the COPERNICUS PhC chips. The coupling process was evaluated by packaging a PhC photodetector. For

minimum coupling losses, the fibre was fixed with UV-glue. UV-glue curing was done in various steps. During

curing, the fibre position was re-optimized as shown in Fig. 3.6.3 (left). Reliability testing with shock and

vibration cycling, pursuant to industrial standards, was performed on the test package.

Next the design of the receiver building blocks was started. Simply speaking, the COPERNICUS receiver

consists of a hermetic housing with four electrical channels, RF-connection between the transimpedance

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amplifier (TIA), the COPERNICUS opto-chip with integrated photodetectors and a mechanical fixture for the

lensed fibre. The fibre was provided by FOTON and was attached on a Kovar ferrule, which was soldered into

the fibre pipe of the receiver. The soldered Kovar ferrule is a must for hermetic receiver housings which is

mandatory for packaging air-bridge membrane PhCs.

In addition to the hermetic fibre assembly, the RF-path was also designed. The RF-part is required to fan-out the

four differential channels from the 4-channel TIA with a pitch of 250 m to a receiver channel pitch of 2 mm. A

ceramic RF-fan-out board was designed, simulated and fabricated, which is suitable for transmission of signals

with data rates > 25 Gbps. In addition to the design of the receiver building blocks, a suitable 4-channel 25 Gbps

TIA array was purchased from the market.

Fig. 3.6.3: Coupling loss of a packaged COPERNICUS photodetector during glue curing (left). Photo of the

assembled receiver with receiver housing, RF-board, TIA and PD array (right).

3.6.3 Assembling of WDM receiver demonstrator

Once all building blocks were available, packaging of a receiver prototype started. The receiver building blocks

were tested with commercially available waveguide photodiodes. For the packaging, standard receiver packaging

processes were adapted to the COPERNICUS receiver. Once the receiver was pre-assembled, the fibre was

coupled to the waveguide diodes and the Kovar ferrule was soldered into the housing. Finally, the lid was

welded onto the receiver housing. For testing, eye diagrams of the electrical signal as well as a BER curve were

measured. A photograph of the receiver, an eye diagram and a BER curve are shown in Fig. 3.6.4.

Fig. 3.6.4: Packaged COPERNICUS receiver, eye diagram and 25 Gbps BER curve of the receiver.

3.6.4 Assess the usability of the PhC technologies in terms of reliability

The reliability of the PhC structures was also evaluated. For assessing the reliability, pre-qualification shock- and

vibration tests were conducted according to MIL-STD-883(E). All packaged waveguide samples passed the tests

after visual inspection. Moreover, the COPERNICUS technology also passed the bond wire pull and PhC shear

tests. We conclude that future products based on the COPERNICUS technologies can also be qualified against

TELCORDIA.

3.6.5 Packaging of discrete PhC devices for evaluation and experimentation

The final objective was the packaging of the discrete PhC for evaluation. As described above, the developed

coupling process required an electrical feedback signal from the photodetectors for fine alignment of the lensed

fibre. Unfortunately, although the Consortium is continuing to pursue this beyond the end of the project, the final

PhC WDM demultiplexer with integrated photodetectors was not yet available by the end of the project. Hence,

only one PhC photodetector was packaged during the coupling process development.

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3.7 Specification, Evaluation, Dissemination and Exploitation

3.7.1 Specification

This activity precisely determined the component specifications based on system requirements established by the

industrial partners. This activity was delayed due to the uncertainties of the situation with one of the original

industrial partners (involved with component fabrication) at the start of the project. Once this situation was

resolved and u2t photonics acceded to the contract, initial specifications of the components were detailed:

for various passive components: wavelength drop filters, splitters, waveguides & input/output couplers;

for the active components: all-optical gates and photodetectors; and

for the final devices: WDM and OTDM receivers.

The following testing phases, defined for all those previous passive and active components and devices, helped

to describe the experimental setups that would be used to test each component / device. The components and

functions that had to be realized were divided into three sets, each step having an increasing complexity in

comparison with the previous one:

Set 1: Development of passive building blocks (waveguides, couplers, bends, splitters). Their control is

mandatory for further function realisation. Strong performance targets were established for these devices.

Set 2: Development of active building blocks (filter, WDM demultiplexer, AOGs, photodetectors).

Set 3: Development of the PICs: packaged WDM receiver operating in O-band and (unpackaged) OTDM

receiver operating in C-Band.

Demonstrations of excellence would be achieved through the development and assessment of both the WDM

receiver and the OTDM receiver functions.

Plans were also developed for assessing the usability of the PhC technologies in terms of reliability. Two

system-oriented analyses were also carried out on the OTDM receiver (at 25 Gbps) and WDM Receiver (for the

future 100G Ethernet standard transceivers) and provided valuable insights.

The documents were then used as working documents to track the progress of the project. The specifications for

each component and device were discussed in detail between the partners and refined and expanded. Hence, the

Consortium decided to expand the initial OTDM Receiver target to a broader objective on AOG Photonic

Integrated Circuits, in order to explore other applications like: optical sampling, wavelength conversion, optical

limiter or all-optical monitoring. Similarly, for the WDM Receiver, when the industrial partner shifted the focus

from the short-reach markets to the metro and long-haul markets, the Consortium was able to shift its technical

developments to new specifications (wavelength shift to C-band, 10-Gbps and 40-Gbps ODB format, …), while

strengthening the PhC capabilities over those updated performances. The related analyses were discussed and

reported to the European Commission in dedicated reports.

3.7.2 Demonstration of Excellence

This activity was devoted to the assessment of the integrated functions developed in the project through simple

and convincing system validations. On the way to the demonstrations of target devices, several intermediate

demonstrations were performed involving COPERNICUS components. This allowed the Consortium to compare

the performance of elementary components with the targeted specifications before their integration into the final

demonstrators. This also allowed the Consortium to propose innovative and complementary (or alternative)

functions (e.g. wavelength conversion, noise limiters, etc.), beyond those foreseen in the original proposal.

One key component was the WDM Receiver. The demonstrations were based on the elementary component

high-speed PhC photodetector, using DTU technology. In parallel, two WDM filters were developed. One air-

bridge membrane PhC WDM demultiplexer PIC was developed by TRT, and the other BCB-encapsulated PhC

wire WDM demultiplexer PIC was developed by DTU, TRT and UniFe.

Unfortunately, no integrated PD-WDM chip was available for packaging at the end of the project. Apart from the

final integrated PD-WDM chip, however, all of the building blocks for receiver were available and

tested/validated – all using compatible BCB-embedded PhC technologies. Reliable coupling and packaging

processes were developed and reported: U2T and TRT worked on the WDM-Receiver integration and validated

it in a prototype. The performance of all these blocks looks promising. The demonstration showed the

appropriate compliance for 25-Gbps signal demultiplexing in long-reach applications.

Three different PhC AOGs technologies were experimentally investigated. On the elementary stage, the first

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three-port AOGs were fabricated by LPN (characterized by FOTON) and by TRT (characterized by DTU) for

switching applications. Despite very fast switching speeds, the low contrasts on the output signals prevented the

demonstration of their operation with Telecom data signals.

Using some special rearrangements of the previous AOG realisations, several new advanced concepts were

proposed and demonstrated. These AOGs are of interest for signal processing functions:

The AOG switches have shown very fast switching speeds, around 10 ps. Those results were achieved with

either two-port AOGs or three-port AOGs. The all-optical routing capabilities of such switches are

controlled with very low power commands.

The AOG switches were tested by FOTON as power limiters at 10 and 20 Gbps for noise reduction

applications. A power penalty reduction of 4.5 dB at 10-9

BER was successfully demonstrated with only 1

mW of coupled peak power.

By inserting an AOG in an analogue microwave-photonic link and coupling it with a 10 Gbps pulsed optical

source, TSA brought out the capacity of all-optical sampling of high-frequency optical signals. This

technique of mixing process also demonstrated the transfer of a microwave signal (at 10 GHz, only limited

by test equipment) from one input signal to a dropped signal. Based on this capability, several advanced

concepts for optical processing were derived, and could be evaluated with cascaded AOGs.

AOG cavities can also greatly enhance non-linear optical effects. FOTON demonstrated high bit rate

wavelength conversion in 2- and 3-port AOGs.

Using the Four-Wave Mixing effect in a PhC waveguide, FOTON characterized the capability of

wavelength conversion at 10 Gbps with a NRZ signal. The promising result has received favourable

feedback from the scientific community. Wavelength conversion is an alternative option to OTDM for node

network management.

Using Second Harmonic Generation, FOTON also demonstrated the all-optical monitoring of signal-to-

noise ratio and chromatic dispersion of a high bit rate signal (42.5 Gbps RZ), with high efficiency inside the

nanosize of the PhC waveguide.

Even if the initial projected demonstrations were not fully performed, the main goals of the project were

achieved and experimentally demonstrated, while additional advanced devices and functions were brought out.

3.7.3 Dissemination of Project Results

(This is described in section 4.2, below, and in the publically available version of Deliverable D7.10)

3.7.4 Exploitation

The general applications areas in which COPERNICUS technology could be used have been identified and

Deliverable D7.8 (exploitation plans) has been prepared in both public and confidential versions.

(The exploitation potential and the Consortium’s exploitation plans are discussed in section 4.3, below.)

3.8 Summary and Future Outlook

The COPERNICUS project focussed on a very new kind of technology (with a high risk, but exciting payback)

and set some very ambitious targets. Starting from a very nascent stage of development, not only did the

Consortium set out to demonstrate a few exciting devices (already a challenging target) – it set out to establish an

entire PhC photonic integrated circuit technology platform based on these new devices. The COPERNICUS

Consortium has achieved many successes in the development of individual elements as well as in their efforts to

bring all of the individual elements together to have a fully "joined up" technology platform, from which future

devices/PICs can be developed.

At the outset of the project, the Consortium began by developing and critically assessing new material

technologies (starting from two different materials, GaAs and InP), successfully developing new materials

technologies to address the challenges of degradation (GaAs) and long carrier lifetimes (InP). Using these

materials, fabrication processes for a number of very different PhC technologies (air-bridge membrane, BCB

encapsulated membrane, hybrid InP/SOI) were developed and optimised. With these new materials and

optimised photonic crystal technologies, the project partners developed entirely new functional devices with

exciting performance, including a photodetector, several new filters, WDM demux and several new AOGs. At

the same time, the Consortium developed all of the passive optical circuitry needed to couple elements to each

other and to the outside world. A number of innovations were needed for these passive circuit elements to

achieve the required performance in terms of bandwidth, insertion loss and reflections, leading to new designs

for waveguides, optical couplers, bends and splitters. Finally, new and powerful simulation and design tools

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were developed and actively used to support all parts of this exciting, yet challenging new technology platform.

These tools needed to be developed alongside the experimental technology, since the strong light-matter

interactions, enhanced nonlinear optical effects and strong coupling to the spatio-temporal carrier dynamics play

an essential role in photonic crystal devices.

Through the conscious pursuit of establishing a "joined up" technology platform, the COPERNICUS Consortium

was ultimately able to show that all of these different elements can be successfully combined to achieve the

higher functionality expected of a PIC. Finally, the Consortium performed a wide range of successful

experimental demonstrations, which verified the performance and the potential of both the PhC devices and their

suitability for integration into photonic integrated circuits with higher levels of functionality. The Consortium

also took the first steps towards demonstrating process control/reproducibility for the fabrication of the PhC

devices, as well as their suitability in terms of packaging and reliability.

The table below provides a summary of the wide range of successful experiments using the COPERNICUS

devices and technology. A further global summary of the breadth of the COPERNICUS technology platform is

given at the end of Section 4.1.1.

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COPERNICUS Experiment Summary

Experiment Device / technology used Key results obtained Partner(s) involved

Wavelength Conversion

(10 Gbit/s, NRZ)

Switch in cross modulation configuration

/ Hybrid (IIIV-SOI) technology

~ 15 ps switching recovery time

Error free operation with 3 mW coupled signal

4 dB extinction ratio, 4 dB penalty (BER = 10-9)

LPN - Foton

Wavelength Conversion

(20 Gbit/s, NRZ)

Switch in cross modulation configuration

/ Hybrid (IIIV-SOI) technology

~ 15 ps switching recovery time

Error free operation with 4 mW coupled signal

4 dB extinction ratio, 15 dB penalty (BER = 10-9)

LPN - Foton

Power limiting function for

noise reduction (10 Gbit/s,

NRZ)

Switch in self modulation configuration /

Hybrid (IIIV-SOI) technology

Error floor suppression with 0.5 mW coupled signal

Up to 10 dB penalty reduction for very noisy signal LPN - Foton

WDM demultiplexing 6-ports filter / Air-suspended membrane

(IIIV)

NRZ WDM demultiplexing : 100 Gbit/s 4 output wavelengths at 25 Gbit/s

Error free operation, no penalty TRT - Foton

Phase modulation format

wavelength extraction

3-ports filter / Air-suspended membrane

(IIIV)

56 Gbit/s DQPSK (28 Gbaud) wavelength extraction:

Error free operation on I and Q components, no penalty TRT - Foton

Tunability 3-ports filter / Air-suspended membrane

(IIIV)

40 Gbit/s NRZ wavelength extraction with 25 dB channel isolation

Error free operation, no penalty

Tunability assessment using heaters : 2 nm tunability demonstrated (1.4 nm/mW)

TRT - Foton

Filtering cross talk assessment 3-ports filter / Air-suspended membrane

(IIIV)

28 Gbit/s NRZ wavelength extraction with adjacent channel impact:

No penalty for channel spacing down to 300 GHz, 1 dB penalty (BER = 10-9) at 235

GHz channel spacing

TRT - Foton

Four wave mixing

measurements – slow light

improvement

Waveguide / Air-suspended membrane

(IIIV)

- 22 dB of FWM efficiency in engineered waveguides group index (including

coupling)

7 dB FWM efficiency improvement thanks to higher group index

10 GHz conversion

TRT - Foton

Optical performance

monitoring at 40 Gbit/s using

2nd harmonic generation (SHG)

Waveguide / Air-suspended membrane

(IIIV)

Broadband and efficient surface emission SHG : 45 µW SH (peak) power, 2.10-4 W-1

conversion efficiency, 10 nm bandwidth (3 dB)

OSNR and chromatic dispersion signal performance monitoring over the entire C-

band.

TRT - Foton

High speed photodetector on

photonic crystal platform

Vertical pin structure integrated with a

photonic crystal waveguide

> 40 GHz (3 dB) bandwidth at – 2V

Error free operation at 10 Gbit/s DTU - Foton

Eye diagram measurements and

BER test

Single channel receiver with U2T

photodetector and TIA

Excellent open eyes for optical input power of -12 dBm. Performance of receiver

package and RF path was evaluated. Receiver sensitivity -11dBm for a BER of 1E-12.

Rx performance is comparable with commercial available receivers.

U2T

Two-colour switching and

wavelength conversion at 10

GHz

Cavity (AOG) Air-suspended membrane

(III-V)

Demonstration of a two-colour operation using a doublet of cavities.

Signal mixing (sum and subtraction) at 10GHz TRT - TSA

2-channels demultiplexing

based on 1D PhC filters

Cavity (1D PhC) III-V encapsulated with

BCB

Characterisation of a 2-channels demultiplexer (CW only). Note: process fully

compatible with integrated photodetectors. TRT - DTU

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4. Potential Impact

4.1 Socio-economic impact

4.1.1 Basic Contributions to Science and Technology

COPERNICUS targeted the development of integrated photonic crystal devices, which are based upon

resonantly enhanced nonlinear light-matter interactions in nanophotonic resonators whose size approaches the

diffraction limit in the material (/2n)3. The strength of nonlinear optical effects scales as Q

2/V, where Q is a

measure of the resonant field enhancement. As Q in these resonators can be made as large as 106, strong

nonlinear optical effects can be achieved at very low power levels. These nonlinear light-matter interactions are

further enhanced by the use of III-V semiconductor materials, whose electronic bandstructure can be tailored

(e.g. compositionally and through quantum confinement) to optimise the nonlinear optical response (e.g.

switching energy, power dissipation, recovery time). This has made it possible to make III-V photonic crystal

all-optical switches (see Fig. 2.1.1), whose switching energy*time product (~5x10-26

Js) is smallest of all all-

optical switching technologies and two orders of magnitude smaller than has been achieved with silicon

photonics (2-20x10-24

Js). Currently, the best silicon photonic AOGs are ring resonators, whose footprint (~80

m2) is ~3-4 times larger (see Fig. 2.1.1). In addition to all-optical switching, photonic crystals provide other

compact functionality, such as high-quality filters (including wavelength drop filters) with a high side-mode

suppression ratio. By addressing a disruptive technology of Photonic Crystals, where light-matter interactions are

hugely enhanced, COPERNICUS made strong contributions to science and technology. These contributions

included the development of new types of functional nanophotonic device (e.g. AOGs, PhC photodetectors) and

the investigation of new functionalities based on their enhanced nonlinear optical response. At the same time,

new materials and fabrication technologies were developed for their realisation. Finally, a range of simulation

and design tools were also developed and validated, which advanced the state of the art. The table below

attempts to capture a global summary of the breadth of the COPERNICUS technology platform.

COPERNICUS Technology Platform Summary

Type Device / PhC

Technology Air-Bridge Membrane BCB Encapsulated III-V/SOI Hybrid

Pa

ssiv

e D

evic

es Waveguides

Bends

Splitters

Couplers

Act

ive

Co

mp

on

ents

2-port Filters ()

3-port Filters

4-port Filters

All-Optical Gates

Photodetectors

Wavelength

Converters

PIC

s Wavelength

Demultiplexer

Su

pp

ort

ing

Tec

hn

olo

gie

s Packaging

(Preliminary)

Reliability

Assessment

Modelling Tools

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4.1.2 Photonic/CMOS Convergence

Photonics is not likely to replace CMOS in the foreseeable future, but continued performance growth of CMOS

will depend strongly upon photonics. The clock speed of VLSI CMOS electronics has hardly increased during

the past 5 years, largely due to the limitations of electrical interconnects. In fact, in 2007, the International

Technology Roadmap for Silicon (ITRS) was forced to revise its clock frequency target for 2010 downwards

from 15.1 GHz to just 5.875 GHz. Researchers at Hewlett Packard labs in California have shown that photonic

crystal based all-optical gates appear best matched to CMOS technology (in terms of size, speed and power

dissipation) for optical interconnects in CMOS3. The introduction of optical interconnects in CMOS will have a

wide impact on technology and markets. Clock frequencies will increase 3-5 times and there will be no

technological difference between cross-chip global interconnects, inter-chip interconnects and links to fibre-optic

networks. The logical expectation is that performance issues and market forces will ultimately drive systems

engineers to package CMOS onto photonic interconnect planes - and not the other way around! (This was the

potential paradigm that inspired the project name, COPERNICUS.) This will have will have wider

consequences. It will allow large chips to be split into several smaller chips with fewer transistors connected by

photonic links. This will improve both thermal management and process yield as CMOS feature sizes decrease

and packing densities increase. Furthermore, if personal computers of the future have clock rates >10 Gb/s and

inherent all-optical connectivity – how wide will the impact on the telecoms and internet industries be?

4.1.3 Training and Structuring of the European Research Area

The COPERNICUS Consortium sought to ensure that key skills in photonics are developed at and beyond the

doctoral level, bringing together researchers across Europe, thereby developing a strong network of interpersonal

connections that will last well beyond the lifetime of this project. In the Photonics21 Strategic Research Agenda,

the European photonics industry has predicted a shortage of qualified workforce (at all levels) in the near future.

Many more researchers with photonics expertise are needed, but if they are trained in isolated pockets, Europe is

not likely to compete well. The additional edge acquired through the international collaborations and

intersectorial connectedness facilitated by a project like this is vital to the future of the European Photonics

industry that needs to proceed in an integrated fashion. Should the industry assume such a coherent shape, it will

thrive, bringing manufacturing in the high technology sector back to Europe.

COPERNICUS contributed European leadership in RTD in photonics from components to systems, securing the

necessary human resources and knowledge to design, produce and use new generations of photonic components.

A key aspect of the COPERNICUS was its “joined up” approach to the development of a new, potentially

disruptive technology for ultra-compact, high-speed photonic integrated circuits – photonic crystals. This

approach ensured that the impact of each part of the underlying technology (design tools, materials, devices,

PICs) on the other contributing technologies (and applications) were considered when making in all design and

technology development decisions.

4.2 Dissemination activities

From the outset of the project, the Consortium ensured that opportunities for dissemination were considered at

all stages. Thus, throughout the project, all partners were actively engaged in promoting the project and

disseminating the results of their work. The key dissemination activities / routes employed are discussed below

and evidence of all of these can be seen on the dedicated public project website, www.copernicusproject.eu. All

dissemination materials have been compiled into a dissemination kit, which is available on the project website.

At the start of the project, the Consortium collaborated on the design and choice of a project logo as it was felt

that this was one of the simplest ways in which to give the project a common brand identity thereby promoting

the project and making documents from COPERNICUS instantly recognisable. The chosen logo was used on all

project materials. Also at the start of the project, a dedicated EU domain, www.copernicusproject.eu, was

acquired for use by the COPERNICUS Consortium. A public project website was subsequently designed and the

pages populated with initial content. These pages gave details of the project background, aims and objectives.

Further pages were dedicated to the Consortium as whole and the individual partners. Throughout the project,

the website was regularly updated with details of results obtained within the project; details of publications

resulting from the project and also with items which could be downloaded by visitors (see further details given

below). The project website was publicised in a variety of ways including links from partner websites and other

project websites, through advertisement in project e-Newsletters and press releases and in the acknowledgements

of scientific publications. The project website will remain available online beyond the end of the project.

Once the project was more established, the Consortium engaged in further dissemination activities, namely a

project press release and the preparation and publication of the first in a series of three e-Newsletters. The press

release was prepared towards the end of 2010, which gave details of the project, its objectives and target

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applications and the partners involved. The document was translated from English into the other languages of the

partners in the Consortium (namely, Danish, French, German and Italian). This initial press release was quite

technically-focussed and following further review it was felt that the text was perhaps too technical for a general

audience. Therefore, a second more accessible version of the press release was prepared and once again

translated into the languages of all partners in the Consortium. The project press releases were distributed to the

press and a range of scientific / technical publications and websites (around 50 outlets in total) with good

coverage being obtained. All press release documents are available to download from the project website.

The series of e-Newsletters produced by the Consortium aimed to raise awareness of the project and its results,

within both the scientific community and industry. The e-Newsletters were published around the end of Year 1,

around the end of Year 2 and following the end of the project. Each edition was distributed to more than 750

people outside of the Consortium. This distribution list was compiled from lists of partners’ contacts and through

web searches for appropriate recipients (e.g. scientific magazine / trade journal editors, photonics companies,

photonics research groups / labs, managers of EC and national research funding agencies, etc.). Recipients could

choose to unsubscribe at any time by emailing the e-Newsletter editor. Conversely, visitors to the project website

could also sign up to receive future editions. Each edition followed a standard format in order to maintain a

consistent and recognisable project identity. The contents of each edition included a welcome from the project

coordinator and editor, a summary of latest project results, partner profiles (with all partners being profiled once

in the series of three e-Newsletters) and an extended article on a key project topic. Additionally, the first edition

gave more general details of the project aims and objectives. Copies of each e-Newsletter can be downloaded

from the project website.

In addition to the major activities described above, the Consortium were invited to and participated in the EC

Photonics Concertation meeting held in Brussels in October 2010. The project coordinator gave a presentation

on behalf of the Consortium outlining the vision and objectives of the project. This presentation is publically

available on the project website and also on the European Commission’s FP7 ICT web pages for Photonics and

Organic Electronics. During the final year of the project, a number of tutorial presentations and public lectures

were given by members of the Consortium. These included an invited tutorial at CLEO 2012, a keynote talk to

the Academy of Finland and two series of lectures given during exchange visits between academics from the

University of Nottingham and the University of Ferrara.

Finally, the Consortium, of course, disseminated the results of its work through the traditional route of scientific

publications. During the project, the Consortium produced a total of 71 such scientific publications (papers /

conferences). The full COPERNICUS project publication list is available on the project website where links to

full text articles are given where available. Please note that a subscription may be required to access the full text

of some publications. A significant number of further publications based on the work of COPERNICUS are

expected beyond the conclusion of the project.

4.3 Exploitation of results

Photonic Crystal (PhC) technology has a high potential as a disruptive technology in several markets using

optoelectronic technology. Indeed, this technology offers several benefits such as the size, the power

consumption, and ultimately the cost, and will be one of the technologies of interest for realising advanced

Photonic Integrated Circuits (PICs). Targeted applications for this technology include optical interconnects from

evolving datacom and synchronous optical networks to the future chip-to-chip platforms through the growing

board-to-board market; signal routing functions like reconfigurable wavelength division multiplexing and

demultiplexing (WDM), add-drop components, optical filters; signal processing functions for network controls

or for enhanced microwave circuits (like ultrafast analogue-to-digital converters, frequency up- and down-

conversion, optical time-delay matrix or (electro-) optical logical processors), and also sensor technologies like

infrared spectrometers, photonic Laboratory-On-a-Chip (LOC) to probe chemical and biological agents or for

environmental monitoring.

The following table summarizes some possible applications for PhC technologies.

Applications Examples Data rates and optical specificity

Telecom Metro (1-100 km) & long-haul applic. (50-1000 km) 10G, 40G, 100G, 400Gbps

Datacom Data centres (1m - 1 km) & campus applic. (1-10 km) 10G, 25G, 40G, 100Gbps

Interconnections between systems

High-performance

computing & data centres

A “supercomputer” could consume 10’s thousands of

active optical cables and 100’s thousands of on-board

modules

Up to 100Gbps

Up to Tb for future on-chip platforms

Consumers Connecting PC and HD TV devices

Radio-over-fibre 5G, 50Gbps

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Signal processing Network controls

Ultrafast A-to-D converters, optical time-delay matrix

ps switching time

40 GHz

Sensors and bio-sensor Measurements of time, temperature, strain

Medical and chemical

Low data rate but using integrated (or

large-scale integration) photonic sensors

Scientific Scientific instrument High

Automotive Sensors and interconnections Low data rate, reliable, harsh environ.

4.3.1 Telecom applications

For telecom markets, Internet applications have been seeing explosive growth, driven by the demand for new

types of video and Internet services. This is driving a dramatic transformation in the bandwidth requirements and

design of optical networks. In addition, optical interconnects are replacing electrical solutions at a rapid rate.

Increasing data volumes require higher frequency bands to be exploited. Therefore, higher integration levels and

advanced optical processing functions are required. In others words, novel photonic devices will lead to

improved overall performance of photonic systems and will lower their cost.

For Access Networks, photonic integrated circuits can penetrate to the optical network units (ONU) at the user

side. In particular, low-cost 10 Gbps ONU transceivers can be realized with photonics. Such a transceiver chip

consists of a laser, an avalanche photodiode operating at 1550 nm, a wavelength multiplexer and a mode adapter

for easing the coupling with an optical fibre.

Among the devices that could be of interest for future systems, one can mention WDM devices similar to the 4

channel 100Gbps WDM ROSA developed in COPERNICUS. For the metro market, the COPERNICUS WDM

ROSA is an excellent fit for 100 Gbps, using the Optical Dual Binary (ODB) modulation format. This format is

widely used in 10 and 40 Gbps long-haul optical communication systems. The receiver is similar to a standard

on-off keying receiver. The binary signal can be amplified by standard TIAs. The COPERNICUS WDM ROSA

is a very promising candidate for next generation 100 Gbps metropolitan networks with link lengths exceeding

100 km. In the future, the WDM Receiver has the potential to be extended to a larger number of channels, more

complex data formats and higher bit rates.

4.3.2 Datacom/Optical communication networks

The intensive growth in data traffic obliges better use of the huge bandwidth of optical fibre networks. The

datacom product roadmaps focus on this challenge:

The current AOCs, mainly based on VCSEL solutions, but recently the first devices based on integrated

photonics (namely Silicon Photonics) were released to the market. The data rate is moving from current 10G

to 25G. Their main application is HPC supercomputer and Ethernet data centres (see below).

Transceivers for switching and routing – expected for 2015 after the widespread released of 100G products.

High-speed switches for servers.

In June 2010, the IEEE ratified various 40 Gbps and 100 Gbps Ethernet standards (IEEE 802.3ba) to

establish compatible sources of low-cost, low-power, pluggable 100G optical transceivers based on 10

optical lanes at 10G. The 10x10 MSA is intended as a lower cost alternative to 100GBase-LR4 for

applications. In order to match in priority the request of lowering costs, the IEEE study group is currently

targeting a non-WDM solution for the short-reach applications (IEEE 802.3bm), making the expected

COPERNICUS outputs inefficient for that application.

To manage the signal over networks, photonic solutions must address different kinds of devices such as:

Wavelength Division Multiplexing (WDM): The main performance requirements for WDM multiplexers

include up to 80 channels for Dense WDM and up to 8 channels for Coarse WDM, with channel spacings of

0.4 nm (200 GHz) for DWDM and 20 nm for CWDM, and an insertion loss per channel of < 6 dB.

Optical Add-Drop Multiplexers (OADM): Even if it is still possible to add more wavelengths, electrical-to-

optical and optical-to-electrical signal conversions are very expensive. Another alternative based on the use

of optical add/drop multiplexers (OADM) has been found. In order to have more flexibility and functionality

in the network management, another level of freedom has been created by using reconfigurable OADM

(ROADM), a tuneable version of OADM in which any wavelength can be added and dropped at any node.

Typical ROADMs include more than 32 wavelength channels and more than 8 optical add/drop lines with

configuration times of several μs (burst switching transfer mode) or far below a μs (packet switching

transfer mode).

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Optical Time Division Multiplexing (OTDM): A further increase in the channel speed to 160 Gb/s or higher

will lead to a cost reduction as the number of required light sources is reduced and network management is

simplified when compared to a WDM system.

Optical Cross-Connect (OXC): In addition to the capability of dynamically changing the wavelengths added

and dropped at each node, optical networks also require pure optical cross-connects (OXC). Typical future

OXC nodes must be compliant with the possibility to convert signals to different wavelengths in a few μs

(burst switching transfer mode) or far below a μs (packet switching transfer mode) and with transparent bit

rate per wavelength (or 40 Gbps when equipped with wavelength converters).

Additionally, some key functions are also required in these networks, like data format conversion, OTDM

demultiplexing or signal regeneration. Despite the shift of COPERNICUS devices toward metro and long-haul

applications, a reduced version of the 25-Gbps transceiver (based on QSFP-like form factor) would remain

feasible using COPERNICUS technology.

In that perspective, the module would be intended for use with standard single-mode fibre with ten 10-Gbps

wavelengths. Each signal would be multiplexed into a waveguide (optical AWG or electrical MMIC for the

electronics driver) to deliver N*10 Gbps optical signals into a single optical output. For the receiver chip, the

level of integration must be applied with a multiple PIN or APD 10 Gbps array with an associated

transimpedance amplifier (TIA) circuit array aligned in front of an optical demultiplexer to receive N*10 Gbps

optical signals also from a single optical input.

For the network management, passive solutions have already been developed for these applications, but their size

is quite important and there are still several issues to address due to the large number of optical connections. In

the future, PICs, thanks to their integration capability, could realise a very compact processing core with only the

requested input and output optical fibre interfaces.

Concerning OXC with wavelength conversion, one key function is required - the wavelength converter. Its

realisation with PICs requires the use of a non-linear optical material able to change the wavelength of an

incoming optical signal, but also to provide it with some optical amplification.

There are multiple applications of the All-Optical-Gates (AOGs) designed and developed by the COPERNICUS

Consortium. Key applications are add-drop devices, optical limiters, wavelength conversion and time-domain

demultiplexing, optical header recognition:

Add-drop device: can typically consist of a multi-port device with two ports for the input and transmission

output, one port for the dropping terminal, and (possibly) a way of command (here optically). Based on the

TRT technology, namely 2D planar AOG on InP, DTU managed the combination of dual frequency

resonance to tailor the non-linear response and outputs dropping signal in response to control signal (with

low crosstalk).

An add-drop device was also realised with a 3-port AOG by LPN, with 20% drop efficiency for the first

realisation. These components are compatible with SOI integration (for Silicon Photonics).

Optical limiter: a two-layer, two-port 1D PhC AOG was used as a noise limiter by FOTON, showing a

power penalty reduction of 4.5 dB at BER=10-9

, with a coupled peak power of less than 1 mW.

Wavelength conversion: FOTON has also demonstrated the use of a two-layer, two-port 1D PhC AOG for

the wavelength conversion of 10 Gbps and 20 Gbps NRZ signals. There was no limitation by the response

time (>20 Gbps), but only by extinction ratio (to be improved).

Time-domain demultiplexing was depicted on a theoretical basis: a sampling gate is realised with an

interferometric waveguide, with two complementary output ports. The gate is illuminated by a pulsed laser.

On one extracted output port, a photodiode (at low rate, 10 Gbps for example) converts the optically

sampled pulses. Since the complementary output from the gate is obtained when there is no laser pulse

applied to the control port, the input signal is left unaffected during the “off” time slots, allowing other

demultiplexing to be performed on the remaining data (at 100 Gbps for example)

Optical header recognition was depicted on a theoretical basis: The OTDM concept previously proposed for

the receive mode can be used for optical routing, in order to get free of any electronic control. In this case,

the digital input signal must be considered to be composed of the payload bits plus header address. The 1x2

routing stage is achieved through two cascaded “switching gates”, (the first gate is used to extract the header

from the incoming stream and the optical bits are sent as the gating signal to the control port of the second

gate, configured as the routing switch).

The success of PhC devices in these areas will strongly depend upon improvements to the switching time and

contrast. It will be improved as disruptive technology thanks to the capability to ensure all-optical drop and

cascade operations. Furthermore, the technology developed at LPN is already compatible with silicon photonics.

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It is very likely that this will become the much needed missing element in photonic integrated circuits which,

once combined with modulators, splitters/multiplexers and photoreceivers will enable the full integration of

applications. A strong advantage of PhC integration is the ultra-compact footprint and very low power

consumption. This technology is expected to have a significant future potential for the realisation of photonic-

electronic convergence.

Computing & Data centres

The high performance computing (HPC) systems roadmap predicts the integration of electrical circuitry with

photonic functions as a means to increase the functionality, improve the power efficiency and to reduce costs.

The continuing rise in the speed of computer chips means that copper tracks on printed circuit boards could soon

be unable to keep up. Over the next decade, the bandwidth of interconnects inside a computer is expected to

increase by an order of magnitude - from around 1 GHz to 10 GHz - thanks to developments such as the PCI

Express bus. However, this increase will require a shift in technologies from the electrical to the optical domain.

Optical interconnections of electronic processors is already a standard inside large electronic computers, high

speed switches and data centres. High-speed signals travelling more than five meters are converted into optical

data streams and then converted back into electronic signals. Connections are made through ribbons of a dozen

parallel fibres, each carrying one data stream. Total data rates of almost 30 Gbps have been obtained with several

2.5 Gbps links.

With the telecommunication market well established, and already pre-empted by Silicon Photonics like the 50-

Gbps optical fibre link released by Intel in early 2013, the next generation of optical interconnects with a total

capacity of 100 Gbps may go inside high performance computers. If the costs of electronic conversion are

decreased, high-speed, highly parallel optical interconnects will find a market in shorter links, such as those

between modules on printed circuit boards and between boards in a single equipment shelf. Thus, cheap and

highly integrated optical coupling solutions into waveguides and optical signal distribution could be realised

with photonic crystal technology, based on optical splitting or WDM.

One key challenge for data centres and supercomputers is their power consumption. Current optical

interconnects combine emitter, mux/de-mux and photoreceiver. Since the laser emission is a major energy

consumer, it is necessary to reduce the emitted power, which implies improving photodetection to very low

levels, pushing the necessity for very high sensitivity (above the typical 0.7 A/W for Ge in Si Photonics) and

very low dark current. Finally, the Mux/DeMux devices will remain the main energy consuming (accounting for

4 pJ/bit, compared to 1.5 pJ/bit for both laser and photoreceiver). The critical issues for Si Photonics WDM are

the centre wavelength accuracy and the accurate channel spacing, due to manufacturing tolerances (layer

thickness variation, waveguide width variation, residual stress) and temperature sensitivity. Thus, an integrated

tuning resistor is still required to adjust the wavelength channel, causing a dramatic power consumption increase.

At this stage, the PhC could find a way to complete Si Photonics solutions, thanks to their low power

consumption (in case of evolved active routing) and reliable manufacturing tolerance. For example, the

COPERNICUS Consortium demonstrated that the switching dynamics of an AOG is about 10 ps, which is fast

enough for processing signals with 100 GHz bandwidth, with an energy budget that is still well below 100 fJ/bit.

4.3.3 Other possible fields of applications

Beyond the initial target of OTDM applications, the current studies of the performance of the All-Optical Gates

(AOGs) have shown that the exploitation potential for AOGs would also benefit from exploring several other

alternatives.

The following applications are addressed:

optical sampling (to achieve high-speed analogue-to-digital conversion);

optical monitoring (for optical networks, handling either analogue or digital information);

optical up- and down- conversion, enabling mixing functions;

all-optical serial-to-parallel 100 Gbps demultiplexing (OTDM in receive mode, see section 4.3.2); and

header recognition (mainly for telecom applications, but also compatible with analogue signals, see section

4.3.2).

Optical Sensors refers to devices which convert a physical measurement into a signal that produces a modulation

of light in a photonic system. Those modulations can affect light transmission, dispersion or wavelength change

(among the main used effects).

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Sensors of this type have advantages (over conventional electrical solutions) in that they can be monitored

remotely, tolerate harsh environments and have immunity to electro-magnetic interference. With PhC

technology, additional and significant advantages are their compactness and light weight.

Two types of sensors can be realised with optics, either intrinsic or extrinsic sensors:

Extrinsic sensor means the optical propagation media is only used to deliver the light to the sensor that is not

the optical media itself, but which is external to it (an external Fabry Perot cavity, for instance).

In intrinsic sensors, the optical media is used as a sensor (for instance, the temperature modifies the

transmission properties of an optical waveguide).

In both cases, several sensors can be put in a network and addressed separately. Once again, the network can be

created using many different configurations: unidirectional or bi-directional ring, star, tree or hybrid

combinations of these structures. Optical sensors can be used for almost all possible measurements: temperature,

strain, pressure, refractive index, chemicals, biological agents, electric fields, etc.

The commercial potential for miniaturized optical sensors is quite large, since it gives the possibility to measure

physical parameters in a system during its realization phase of devices or systems.

Optical sensing is essential for the development of smart systems in areas such as homeland security; civil

infrastructures; and biomedical fields.

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5. Project public website and contact details

Project website: http://www.copernicusproject.eu/

Coordinator: Technical Manager:

Dr. Alfredo de Rossi Prof. Eric Larkins

Thales Research and Technology University of Nottingham

Tel: +33 1 69 41 57 52 Tel: +44 115 951 5534

Email: [email protected] Email: [email protected]