Accepted Manuscript

Title: 4-Port Reciprocal Optical Circulators EmployingPhotonic Crystals for Integrated Photonics Circuits

Authors: M. Djavid, M.H.T. Dastjerdi, M.R. Philip, D.D.Choudhary, A. Khreishah, H.P.T. Nguyen

PII: S0030-4026(17)30791-XDOI: http://dx.doi.org/doi:10.1016/j.ijleo.2017.06.115Reference: IJLEO 59378

To appear in:

Received date: 23-10-2016Revised date: 29-4-2017Accepted date: 28-6-2017

Please cite this article as: M.Djavid, M.H.T.Dastjerdi, M.R.Philip, D.D.Choudhary,A.Khreishah, H.P.T.Nguyen, 4-Port Reciprocal Optical Circulators EmployingPhotonic Crystals for Integrated Photonics Circuits, Optik - International Journal forLight and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.06.115

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4-Port Reciprocal Optical Circulators Employing Photonic

Crystals for Integrated Photonics Circuits

M. Djavid1, M. H. T. Dastjerdi2, M. R. Philip1, D. D. Choudhary1, A. Khreishah1, and H.

P. T. Nguyen1‡

1Department of Electrical and Computer Engineering, New Jersey Institute of Technology,

Newark, New Jersey 07102

2Department of Engineering Physics, McMaster University, Hamilton, Ontario L8S 4L7,

Canada

‡: Email: hieu.p.nguyen@njit.edu; Phone: 1 973 596 3523

Abstract: We present the design of a 4-port photonic crystal-based optical circulator

employing ring resonator cross connect filters, suitable for photonic integrated circuits

schemes. This unique design allows the operation in both clockwise as well as

counterclockwise directions and shows a calculated normalized transmission of over 80%.

Since the spectra ranges cover the whole third communication window, any wavelength in

these ranges can be circulated through the proposed photonic crystal-based optical

circulator even different wavelengths at the same time.

1. Introduction

In order to address the future telecommunication network requirements for fast, efficient

and low cost information transfer, the currently dominant electrical interconnects should

be replaced by their optical alternatives due to their limited bandwidth and power hungry

characteristics. According to the International Technology Roadmap for Semiconductors

(ITRS), the device energy budget should reach ∼ 2–10 fJ/bit for on-chip interconnects by

2022 [1, 2] which is far apart from what can be practically achieved using electrical

interconnects. Although fiber optic communication technology [3-6] has been employed

for long distance communication networks, it should also be applied to much shorter

distance communication networks including chip-level networks [2] as well, in order to

keep up with the rapid reduction of component sizes as stated by Moore’s law [7-10].

Optical circulators are one of the key components for a highly functional photonic circuits.

However, most of the currently available optical circulators are bulky components which

contain separate optics such as polarization beam splitters, Faraday rotators, and half-wave

plate. Therefore, they are not suitable for integrated circuit applications. Over the past few

years a variety of integrated optical circuit designs have been reported to overcome this

problem. Among them are an optical circulator based on Mach-Zehnder interferometer

fabricated in a silicon nanowire waveguide [11-13] and optical circulators by stimulated

Brillouin scattering induced non-reciprocal phase shift [14]. Additionally, photonic

crystals have been intensively studied for the compact optical circulators with several new

geometries [15-20]. The two-dimensional (2D) photonic crystals include periodic arrays of

nanowires with certain photonic bandgaps which can prevent light from propagating

through the device structure. Utilizing photonic crystal structures, several optical devices

such as channel drop filters [21, 22] and wavelength division demultiplexers [23] have

been proposed and developed. These optical devices have various key properties such as

flexible mode design, efficient coupling, high optical confinement, and frequency selective

dropping [24].

In this paper, we propose the design of a 4-port photonic crystal optical circulator which

can be used in either clockwise (CW) or counter clockwise (CCW) directions. Moreover,

such optical circulator exhibits high normalized transmission over the operation range at

telecom region. Since the operating spectra cover the whole third communication window,

any wavelength in these ranges can be circulated through this photonic crystal circulator

(PCC), even different wavelengths at the same time. The PCC simulation results shows a

normalized transmission of 80% and the normalized transmission of the cross connect filter

(XCF) is more than ~ 70% which covers the whole third communication window.

Additionally, the reciprocal characteristic of the device is a great advantage over the

common magnetic non-reciprocal optical circulators by allowing both CW and CCW

operation modes. Such small-size optical circulators are perfectly suited for enabling

bidirectional operation in advanced optical interconnects [25-27] and interferometric

optical sensors[28]

Actual realization of the proposed optical circulator design can be achieved by Molecular

Beam Epitaxy (MBE) growth [29, 30] of III-V semiconductor nanowire structures on a

silicon platform via Selective-Area-Growth (SAG) technique [31-33]. Defect-free III-V

nanowires can be grown on silicon substrate thanks to lateral strain relaxation [34-36]

associated with vertical nanowire structure growth. The proposed design and realization

approach are suitable for integration of the optical circulators in the chip-level silicon

photonics circuits.

2. Photonic crystal optical circulators

Various theoretical analysis techniques such as coupled mode theory [37], and particle

swarm optimization theory [38] have been previously utilized to calculate the optical

propagation in the photonic crystal structures. In this study, to analyze the proposed

photonic crystal switches, we have used the well-known Finite Difference Time Domain

(FDTD) technique, which is the most popular numerical approach in electromagnetics. The

essence of the FDTD method consists in solving the Maxwell’s equations discretized to

stepping formulas in time and two dimensional mesh within the x–y coordinate system for

the E-polarization. The index n denotes the discrete time step, indices i and j denote the

discretized grid point in the x–y planes respectively. Equations 1, 2, and 3 show the

discretized formulas for the 2D E-polarization.

y

EEtHH

n

ji

n

jin

ji

n

ji

zz

xx

,1,21

21,

21

21,

0 (1)

x

EEtHH

n

ji

n

jin

ji

n

ji

zz

yy

,,121

,2

1

21

,2

1

0 (2)

y

HH

x

HHtEE

n

ji

n

ji

n

ji

n

jin

ji

n

ji

xxyy

ji

zz

21

21,

21

21,

21

,2

1

21

,2

1

,

1

,

, (3)

In these equations εi,j is permittivity of the material which is position dependent. In two

dimensions, the fields can be decoupled into two transversely polarized modes, the TM and

TE modes. These equations are discretized in the space and time domain using the

principles of Yee algorithm [39].

Perfectly matched layers (PML) are located around the whole structure as absorbing

boundary condition and acts as free space [40]. An adequately broad band and modulated

Gaussian pulse is launched into the input port, and then we placed some detectors which

measure the time varying electric and magnetic fields at the output ports. Using the Fast

Fourier Transform (FFT) of the fields calculated by FDTD, the Poynting vector over the

detectors is integrated and power transmission spectra are computed.

The 2D schematic of the XCF and its associated electric mode profile are shown in Figs.

1(a) and (b). The proposed XCF includes the 2D array of semiconductor nanowires with

radius of r=100 nm and pitch size of 540 nm (center to center), shown in Fig. 2(a).

Illustrated in Fig. 2(b), to create the rectangular waveguides, one row of nanowires was

removed. The XCF ring resonators were created by removing a ring-shape of nanowire

columns at desired positions. The MBE growth of such nanowire structures has been

described elsewhere [18-19]. Moreover, the precise size and positioning of the nanowire

structures can be achieved by SAG technique [20-22].

Fig. 1 (a) Two-dimensional schematic of the proposed ring resonator cross connect filter using a ring resonator laterally

coupled waveguide crossing. (b) Electric filed intensity of the cross connect filter.

Fig. 2 (a) Two-dimensional schematic of the 4-port optical circulator (b) Two-dimensional schematic of photonic crystal

optical circulator.

To realize the 4-port photonic crystal based optical circulator as shown in Fig. 2(a), four

XCF are considered and connect together through their ports. The output port of one XCF

is connected to the input port of sided XCF. The light can thoroughly propagate via

different XCFs.

Ain Bout

Utilizing FDTD, the power spectra of the XCF structure was calculated and is shown in

Fig. 3. The output power is normalized to the input power spectra. The normalized

transmission of the XCF structure covers over the whole third communication window of

more than ~70%.

Fig. 3 Power spectra of the XCF structure calculated using FDTD over third communication window.

In this FDTD simulations, the wavelength of 1590 nm was calculated with the normalized

transmission of above 80%. The time-domain simulation of the 4-port optical circulator is

shown in Fig. 4. Due to the high optical transmission of the XCF elements the isolation

between the different ports in this optical simulator is high. This is clearly visible from the

electric mode profile, shown in Fig. 4, where the concentration of the optical signal is much

higher – i.e. the profile is much brighter- in the waveguides and the ring resonator parts of

the XCF structures than that of the connecting waveguides.

Fig. 4 The electric mode profile of photonic crystal optical circulator with the inputs and outputs.

It is noticed that the important point to design the photonic crystal optical circulator is the

distance between ring resonators that should be located so far away to avoid light

interference affecting different inputs and outputs. In addition if the ring resonators are

located too close together, it can affect their resonant wavelength. The distance of 12

nanowires is found to obtain the device without any interference utilizing optimization

process. However, the designed optical circulator is optimized to have no interference,

some parts light can propagate through the waveguide and reach to other ports. The

crosstalk information for different outputs at the wavelength of 1590 nm is presented in

table 1.

Conventional optical circulators are non-reciprocal devices in which the optical signal

entering any port is solely transmitted to the next port in rotation. It is due to the fact that

the symmetry of such systems are broken by an external magnetic field as governed by

Faraday effect. However, due to the symmetric structure of the current design, the “in” and

“out” channels of each port can be used for either input or output of that certain port.

Therefore it is possible to operate the circulator in CW and CCW modes depending on the

application. The possibility of operating this photonic crystal circulator in both CW and

CCW directions together with its high isolation between different ports makes this design

a promising candidate for applications in chip-level photonic integrated circuits.

4. Conclusion

We have presented the design of a 4-port photonic crystal-based optical circulator

employing ring resonator cross connect filters with more than 80% normalized

transmission and the possibility of operating in clockwise and counter clock wise

directions. This novel approach can be implemented for future silicon-based chip-level

photonic integrated circuits. The proposed optical circulator constructed by top-down

process will be fabricated and studied in the near future.

Acknowledgment

This work was supported by New Jersey Institute of Technology (NJIT) and the National

Science Foundation grant EEC-1560131.

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Aout Bout Cout Dout

Normalized Output (%) 5 82 6 4

Table 1: Crosstalk information for different outputs at the wavelength of 1590 nm