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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 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.
2
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.
3
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
4
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.
5
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.
6
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.
7
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).
8
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.
9
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
10
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.
12
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
13
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)
14
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.
15
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).
16
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.
17
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
18
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
19
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
20
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
21
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.
22
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
23
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.
24
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.
25
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
26
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
27
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.
28
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
29
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
30
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.
31
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
32
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
33
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
34
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
35
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).
36
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.
37
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).
38
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.
39
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]