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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
This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
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
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