Two nonlinear directional couplers at two outer-arms of the structure are used as all-optical phase shifters to achieve all switching states and to control the switching states.. The aim
Trang 1Contents lists available atScienceDirect
Optics Communications journal homepage:www.elsevier.com/locate/optcom
High bandwidth all-optical 3×3 switch based on multimode interference
structures
Duy-Tien Lea, Cao-Dung Truongb, Trung-Thanh Lec,⁎
a Posts and Telecommunications Institute of Technology (PTIT) and Finance-Banking University, Hanoi, Vietnam
b VNPT Technology, Hanoi, Vietnam
c International School, Vietnam National University (VNU), Hanoi, Vietnam
A R T I C L E I N F O
Keywords:
All-optical switches
Wavelength selective switches
MMI coupler
Nonlinear directional coupler
Nonlinear phase shifters
A B S T R A C T
A high bandwidth all-optical 3×3 switch based on general interference multimode interference (GI-MMI) structure is proposed in this study Two 3×3 multimode interference couplers are cascaded to realize an all-optical switch operating at both wavelengths of 1550 nm and 1310 nm Two nonlinear directional couplers at two outer-arms of the structure are used as all-optical phase shifters to achieve all switching states and to control the switching states Analytical expressions for switching operation using the transfer matrix method are presented The beam propagation method (BPM) is used to design and optimize the whole structure The optimal design of the all-optical phase shifters and 3×3 MMI couplers are carried out to reduce the switching power and loss
1 Introduction
All-optical devices have been rapidly growing in recent years
Optical switch is a key component and plays a very important role in
optical communication systems There are some different types of
commercialized switches One is thin-film based switch (expensive for
packaging and difficult to integrate with other devices) Two is liquid
crystal based switch[1] Three isfiber couplers based switch[2] The
other type is based on planar lightwave circuits (PLCs) and is more
promising due to its advantages such as the small size, high reliability,
and possibility for large scale production[3] Some novel PLC-based
optical switches have been reported and the total size is about several
millimeters Some compact optical switches are designed by using the
decoupling performance of directional couplers based on planar
waveguides[4]
In recent years, multimode interference couplers (MMI) are
attractive for PLCs based optical switches[5], due to their advantages
of low loss, ultra compact size, high stability, large bandwidth and
fabrication tolerance In addition, two wavelengths 1310 nm and
1550 nm are commonly used in optical communication networks,
respectively In the literature, the proposal of the switching devices
operating at two wavelengths 1550 nm and 1310 nm has not been
presented
The aim of this study is to propose a novel structure of all- optical
switch operating at both wavelengths of 1550 and 1310 nm, based on
two 3×3 MMI couplers using nonlinear directional couplers as phase shifters Nonlinear directional couplers at two outage arms in the inter-stage of two 3×3 MMI couplers play the role of phase shifters In order
to realize the phase shifters using nonlinear directional couplers, the control signal is at an arm of the nonlinear directional coupler, and the information signal is at the other arm The nonlinear directional couplers are carefully designed so that the control signal must be separated from input signals and enters the switching structure from a different single-mode access waveguide after the switching operation The aim is to reduce the powers transferring between control wave-guides and information signal wavewave-guides Numerical simulations using the BPM then are used to verify the operating principle of the proposed all-optical switch
2 Device design and analysis
Fig 1shows the proposed device structure in our study It consists
of two 3×3 MMI couplers with the same size cascaded to form the switching structure, where two nonlinear directional couplers are placed at inter-stage of two 3×3 MMI couplers to obtain all-optical phase shifters
The operation of the proposed switch is based on 3×3 MMI couplers The operation of an MMI coupler is based on the self-imaging theory Self-imaging is a property of a multimode waveguide by which input field is reproduced in single or multiple images at periodic
http://dx.doi.org/10.1016/j.optcom.2016.11.034
Received 5 August 2016; Received in revised form 15 November 2016; Accepted 15 November 2016
⁎ Corresponding author.
E-mail address: thanh.le@vnu.edu.vn (T.-T Le).
0030-4018/ © 2016 Elsevier B.V All rights reserved.
crossmark
Trang 2intervals along propagation direction of the waveguide [6] An MMI
coupler can be characterized by the transfer matrix theory, where the
relationship between the input vector and output vector can be
obtained To achieve the required transfer matrix, the positions of
the input and output ports of the MMI coupler must be set exactly The
beat length of two lowest-order modes and can be written as
L π≈ 4n W r e2/3λ
0, whereλ0is operation wavelength, Weis effective width
of the MMI and it can be determined by W e≈W MMI+ (λ π n0/ )( r2−n c2 −0.5)
for TE mode (nrand nc are refractive indices of core and cladding
layers, respectively) In this design, three input ports and three output
ports are located at positions x i= (2 + 1)i W e/6(i=0, 1, 2)
In this study, we use the chalcogenide glass As2S3for designing the
whole device The material used in core layer of the proposed optical
switching structure is chalcogenide glass As2S3with refractive index
nr=2.45 [7] The silica material SiO2 used in cladding layer has refractive index nc=1.46 As2S3(arsenic trisulfide) is a direct band-gap, amorphous semiconductor By using a highly controlled deposi-tion process, a photo-polymerizablefilm of As2S3can be deposited on standard silica glass substrates Chalcogenide As2S3is chosen due to its advantages For example, it is attractive for high rate photonics integrated circuits, especially attractive for all optical switches in recent years because of the fast response time associated with the near-instantaneous third order nonlinearity allowsflexible ultrafast signal processing [8] In- addition, the chalcogenide glass supports the operation of wavelengths range in the windows of 1550 nm and
As2S3material has a high refractive index contrast to allow for a high confinement of light also ultra-compact size[9] Therefore, it is useful and important for large scale integrated circuits The chalcogenide glass As2S3has a high nonlinear coefficient n2about 2.92×10−6μm2/
W This would be better for operation of the proposed switch because a very high intensity of the control beam will overwhelm the signal Moreover, since the control beam intensity is much higher than the signal beam one, the nonlinear directional coupler needs an extreme high isolation; so that it is difficult to design and optimize the proposed structure Silicon dioxide SiO2is used in cladding layer because of high refractive index difference between core and cladding layers that allows for a high confinement of light and also supports a larger mode numbers in MMI region In addition, both As2S3and SiO2materials are available and cheap also they can implement in the practical fabrication Recently, these materials are very attractive for ultrahigh bit-rate signal processing applications
The device used in our designs is shown onFig 2 Here, we use the
TE (Transverse Electric) polarization and both operating wavelengths
of 1550 and 1310 nm for analyses and simulations If the uniformity of the time harmonic of TE-polarized waves can be assumed along the x direction ofFig 1, the simulation can be done assuming it as a 2D structure In order to reduce time consuming but still have accuracy results a 3D device structure is converted to a 2D structure using the
effective index method (EIM) first, then the 2D-BPM method is used for simulations[10] By using the BPM simulations, the width of each 3×3 MMI couplers WMMI is 18 µm with the height hco of the waveguide is 0.95 µm, the width Waof the access waveguides is 3 µm for single-mode operation
In order for the proposed switch operating at both wavelengths
λ1=1550 nm,λ2=1310 nm, the length LMMIof the multimode region is
chosen to satisfy the condition as follows: L MMI≈mL π( ) ≈λ1 nL π( )λ2, where m, n are positive integers andλ1=1550 nm,λ2=1310 nm The purpose of this requirement is that the wavelengthsλ1andλ2can be switched selectively and optically to any output ports from any input ports By using Sell Meier model, we can obtain that the refractive index difference for chalcogenide glasses at the two wavelengths (λ1 andλ2) isΔn=0.02
Firstly, we calculate the beat lengths at two wavelengths with the proposed design parameters using analytical analysis We have found that the optimum length of the multimode region is LMMI=14335 µm
≈20Lπ(λ1)≈17Lπ(λ2) At this length, the first MMI will operate as a splitter and the second MMI will operate as a combiner at both wavelengths To optimize the operation of the MMI regions in the role
of the splitter and combiner, linear taper waveguides at access waveguides are used By using the BPM, the width and length of the linear tapers are calculated to be 4.8 µm and la=130 µm, respectively Fabrication of two phase shift control waveguides including directional coupling waveguides that are symmetrically through the center line of the MMI region as shown inFig 1 InFig 1, sine-shape waveguides with a length of 1300 µm are used to connect straight waveguides with the coupling waveguides By using the BPM, the two parallel wave-guides at the outer-arm of the structure can be viewed as a directional coupler with a gap of d=80 nm and coupling length of Lc=360 µm The aim of this design is to reduce the power coupling between the control
Fig 1 A proposed optical switch based on 3×3 MMI couplers using directional couplers
as phase shifters.
Fig 2 Rib waveguide used in our design.
Table 1
Phase shifter states and optimal control fied intensities for operation of the proposed
switch.
Wave-length
(nm)
Input port Output port φ1 φ2 I 1 (GW/
cm 2 )
I 2 (GW/
cm 2 )
Trang 3waveguide and signal waveguide.
As mentioned above, the proposed switch requires two nonlinear
directional couplers as phase shifters at two outage arms of the device
Originally, the nonlinear directional coupler includes two waveguides
that have small distance and full coupling takes place between them in
one coupling length, provided that one or both of them have non-linear
behavior This non-linear behavior can be guaranteed with high
intensity control field which changes the nonlinear refractive index
When the distance of two nonlinear directional couplers is very small
and modefield amplitudes vary slowly in the z- propagation direction,
the interaction of electrical fields in nonlinear directional couplers
complies with coupled mode equations[11,12]
i dA
dz κB γ A B A
− = + 1( 2+ 2 2)
(1)
i dB
dz κA γ B A B
− = + 2( 2+ 2 2)
(2) whereκis the linear coupling coefficient,κ=π/2L c; A and B arefield
amplitudes of the control waveguide and signal waveguide,
respec-tively, γ1 and γ2 are nonlinear coefficients describing the self-phase
modulation (SPM) and cross-phase modulation (XPM) effects
Nonlinear coefficient is determined by γ=2πn2/λ0Aeff, where n2 is
nonlinear refractive index of the waveguide; Aeffis the effective modal
cross–section area Under influencing of self-phase modulation in the
nonlinear directional coupler, the change of phase in directional
coupler will be proportional to the intensity of input of electricalfields
of waveguides
Let φ1 and φ2 are relative phase shifts of outage arms in
comparison with the phase of the center access waveguide which
linking between two 3×3 MMI regions We also assume that, the
intensity of the signal introduced into control waveguide is I and the
intensity introduced into data or signal waveguide of the switch is
always set as I0=1 GW/cm2 As presented, when applying a
high-intensity control field to nonlinear waveguide, its refractive index is
changed and therefore it causes a change in phase shift at outage arm The phase shift varies proportionally with intensity offield
Due to multimode interference principle, self-imaging is formed and mirrored on a periodic cycle that is an even and odd integer times
of 3Lπrespectively Therefore, when the proposed structure is operated
at wavelength λ1=1550 nm, the outputs of the imaging at L=20Lπ equivalent to the length 2Lπ
When the proposed structure is operated at wavelength
λ2=1310 nm, the outputs of the imaging at L=17Lπequivalent to the length 2Lπ and mirrored symmetry through the center line of the proposed structure At length 2Lπ, the transfer matrix of the MMI coupler is determined by
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
M= 1 3
− 1
j π j π
− 2 /3 − 2 /3
− 2 /3 − 2 /3
Hence, the transfer matrices at length LMMIof the MMI couplers at wavelengthsλ1=1550 nm andλ2=1310 nm for 3×3 MMI coupler are
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
M
M
=
−1
,
=
−1
j π j π
j π j π
1 13
− 2 /3 − 2 /3
− 2 /3 − 2 /3
− 2 /3 − 2 /3
2 13
− 2 /3 − 2 /3
− 2 /3 − 2 /3
By using analytical expressions of the MMI coupler, at wavelength
1550 nm, if (φ1,φ2)=(-π/3, π) then the signal is at output port 1; if (φ1,
φ2)=(π, -π/3) the the signal is at output port 3 and if (φ1,φ2)=(π/3, π/ 3) then the signal is at output port 2 As an example, we investigate the switching mechanism for the case input signal at port B and output signal at port 2 First, we need find the intensity I1 introduced to control waveguide 1 (also seeFig 1) by varying the intensity slowly We find out that the appropriate value is about 480 GW/cm2to obtain a phase shift of–π/3 in comparison with the center access waveguide
Fig 3 2D BPM simulation of electric field pattern in the switch when (a) λ 1 =1550 nm; (b) λ 2 =1310 nm.
150
Trang 4Then we change the the intensity I2introduced into control waveguide
2 to find out the its value We find out that the intensity is about
318 GW/cm2to make a phase shift ofπ in comparison with the center
access waveguide As a result, we reproduce the simulations by varying
I1 and I2 slowly around these values again, we have obtained the
optimal values I1=479 GW/cm2and I2=320 GW/cm2 At these values,
the minimum of the insertion loss and crosstalk is achieved By using
the similar analysis, we simulation carry out the operation of the switch
at both wavelengths and the results are presented inTable 1
3 Simulation results and discussions
Due to symmetric nature of the proposed structure, the role of input
ports A and B inFig 1are equivalent Without loss of generality, we
carry out simulations for the following cases: The signal is at input port
A port or at input port B port of the proposed structure and the signal is
switched to output port 1 or output port 2
By using the 2D BPM, thefield propagation in the whole device is
shown inFig 3 The simulation results show that the operation of the
switch has a good agreement with our theoretical analysis The output
powers at different output ports (normalized to the input power) are
shown inTable 2
The BPM simulation results have shown that a high outputfield
intensity can be achieved As a result, high performance of the switch
can be obtained (Table 2) Calculation formulas for insertion loss (I L.)
and extinction ratio (Ex R.) as follows[13]
⎛
⎝
⎞
⎠
P
( ) = 10 log out
in
10
(5)
⎛
⎝
⎞
⎠
P
Ex ( ) = 10 log high
l
10
where Poutand Pinare the output and input power of the switch in
Table 2
Insertion loss, extinction ratio and crosstalk of the proposed switch.
Wave
length
(nm)
Input port Output port Insertion
Loss (dB)
Crosstalk (dB) Extinction
Ratio (dB)
−38.34
−35.25
−41.32
−29.74
−40.68
−25.88
−37.43
−29.74
−40.68
−35.25
−41.32
−38.34
Fig 4 Insertion loss at the wavelengths (a) 1550 nm (b) 1310 nm and (c) Crosstalk at wavelength 1550 nm.
Trang 5operation state, Phighand Ploware output power levels in ON and OFF
states of input port respectively
The results presented in Table 2show that almost all important
parameters of the proposed structure such as insertion loss, cross-talk,
extinction ratio, etc can be obtained Crosstalk is the ratio between the
power at a specific output port from the desired input port and the
output powers from all other input ports[13]
By using the BPM, we calculate the insertion loss and cross-talk of
the switch at two wavelengths 1550 nm and 1310 nm as shown in
Fig 4 The−2 dB bandwidths of the spectral responses at output ports
from input ports of insertion loss are 5 nm and 6 nm; the crosstalk in
these cases are from−10 dB to −15 dB, respectively
Fig 5shows the spectral responses of the extinction ratios of the
switch It can be seen that the extinction ratio of the proposed structure
are quite good The extinction ratios at two wavelengths 1550 nm
(Fig 5a) and 1310 nm (Fig 5b) are−25 dB and −35 dB, respectively
The proposed switch has an ability to switch non-blocking from any
input ports to any output ports In comparison with an existing 3×3
optical switch using a 3×3fiber coupler[2], we can see that the 3×3
fiber coupler cannot switch non-blocking between input and output
ports despite having phase shift in each input port Compared with the
existing approach structure in the literature which used the 3×3 Mach
Zehnder interferometer structure and electro-optic effect[14,15], our
proposed structure has a better insertion loss and can work in
all-optical domain
4 Conclusion
A novel high bandwidth all-optical 3×3 switch working in both
1550 nm and 1310 nm regions has been presented in this study By using two non-linear directional couplers as phase shifters, a 3×3 all-optical non-blocking switch based on 3×3 MMI structures is realized The proposed device structure are analyzed and designed by using analytical expressions and then the beam propagation method is used for verifying the working principle and theory The simulation results have shown that a good performance of the proposed switching device can be obtained As a result, the proposed structure can be useful for applications in all-optical networks
Acknowledgements
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number “103.02–2013.72" and Vietnam National University, Hanoi (VNU) under project number QG.15.30
References
[1] X Hu, O Hadaler, H.J Coles, High optical contrast liquid crystal switch and analogue response Attenuator at 1550 nm, IEEE Photonics Technol Lett 23 (2011) 1655–1657
[2] D.O Culverhouse, T.A Birks, S.G Farwell, et al., 3×3 all-fiber routing switch, IEEE Photonics Technol Lett 9 (1997) 333–335
[3] Y Shi, S Anand, S He, et al., Design of a polarization insensitive Triplexer using directional couplers based on submicron silicon rib waveguides, J Light Technol.
27 (2009) 1443–1447 [4] N Yamamoto, T Ogawa, K Komori, Photonic crystal directional coupler switch with small switching length and wide bandwidth, Opt Express 14 (2006) 1223 [5] Álvaro Rosa, Ana Gutiérrez, Antoine Brimont, et al., High performace silicon 2×2 optical switch based on a thermo-optically tunable multimode interference coupler and e fficient electrodes, Opt Express 24 (2016) 191–198
[6] L.W Cahill, The synthesis of generalised Mach-Zehnder optical switches based on multimode interference (MMI) couplers, Opt Quantum Electron 35 (2003)
465 –473 [7] Christopher G Poulton, Ravi Pant, Adam Byrnes, et al., Design for broadband on-chip isolator using stimulated Brillouin scattering in dispersion-engineered chal-cogenide waveguides, Opt Express 20 (2012) 21235–21246
[8] M.D Pelusi, F Luan, S Madden, et al., Wavelength conversion of high-speed phase and intensity modulated signals using a Highly nonlinear chalcogenide glass chip, IEEE Photonics Technol Lett 22 (2010) 3–5
[9] Juejun Hu, Ning-Ning Feng, Nathan Carlie, et al., Optical loss reduction in high-index-contrast chalcogenide glass waveguides via thermal reflow, Opt Express 18 (2010) 1469
[10] Shuo-Yen Tseng, Canek Fuentes-Hernandez, Daniel Owens, et al., Variable splitting ratio 2×2 MMI couplers using multimode waveguide holograms, Opt Express 15 (2007) 9015–9021
[11] Cao-Dung Truong, Trung-Thanh Le, Duc-Han Tran, All-optical switches based on 3×3 multimode interference couplers using nonlinear directional couplers, Appl Phys Res 5 (2013) 58–69
[12] R Ghayour, A.N Taheri, M.T Fathi, Integrated Mach-Zehnder-based 2×2 all-optical switch using nonlinear two-mode interference waveguide, Appl Opt 47 (2008) 632–638
[13] G.I Papadimitriou, C Papazoglou, A.S Pomportsis, Optical switching: switch fabrics , techniques , and architectures, J Light Technol 21 (2003) 384 –405 [14] M Syuhaimi, A Rahman, K.M Shaktur et al., Analytical and simulation of new electro-optic 3x 3 switch using Ti: LiNbO3 as a wave guide medium, in: Proceedings
of the International Conference on Photonics, 2010, pp 4–8 [15] Sixuan Mu, Ke Liu, Shuang Wang, et al., Compact InGaAsP/InP 3×3 multimode-interference coupler-based electro-optic switch, Appl Opt 55 (2016) 1795–1802
1549 1549.5 1550 1550.5 1551 1551.5 1552 1552.5 1553 1553.5 1554
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Wavelength (nm)
a)
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Wavelength (nm)
b)
Fig 5 Extinction ratio at the three output ports as the wavelength band: (a) 1550 nm
and (b) 1310 nm.
152