Optics Communications 387 (2017) 148–152 Contents lists available at ScienceDirect 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 b c crossmark ⁎ Posts and Telecommunications Institute of Technology (PTIT) and Finance-Banking University, Hanoi, Vietnam VNPT Technology, Hanoi, Vietnam International School, Vietnam National University (VNU), Hanoi, Vietnam A R T I C L E I N F O A BS T RAC T Keywords: All-optical switches Wavelength selective switches MMI coupler Nonlinear directional coupler Nonlinear phase shifters 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 alloptical 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 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 is fiber 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 interstage 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 waveguides and information signal waveguides Numerical simulations using the BPM then are used to verify the operating principle of the proposed all-optical switch Device design and analysis Fig shows 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 Corresponding author E-mail address: thanh.le@vnu.edu.vn (T.-T Le) http://dx.doi.org/10.1016/j.optcom.2016.11.034 Received August 2016; Received in revised form 15 November 2016; Accepted 15 November 2016 0030-4018/ © 2016 Elsevier B.V All rights reserved Optics Communications 387 (2017) 148–152 D.-T Le et al switching structure is chalcogenide glass As2S3 with 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 bandgap, amorphous semiconductor By using a highly controlled deposition process, a photo-polymerizable film of As2S3 can be deposited on standard silica glass substrates Chalcogenide As2S3 is 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 nearinstantaneous third order nonlinearity allows flexible ultrafast signal processing [8] In- addition, the chalcogenide glass supports the operation of wavelengths range in the windows of 1550 nm and As2S3 material 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 As2S3 has a high nonlinear coefficient n2 about 2.92×10−6 μm2/ 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 SiO2 is 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 As2S3 and SiO2 materials 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 on Fig 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 of Fig 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 Wa of the access waveguides is µm for single-mode operation In order for the proposed switch operating at both wavelengths λ1=1550 nm, λ2=1310 nm, the length LMMI of the multimode region is chosen to satisfy the condition as follows: LMMI ≈ mLπ (λ1) ≈ nLπ (λ2 ), where m, n are positive integers and λ1=1550 nm, λ2=1310 nm The purpose of this requirement is that the wavelengths λ1 and λ2 can 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 and λ2) is Δn=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 in Fig In Fig 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 waveguides 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 A proposed optical switch based on 3×3 MMI couplers using directional couplers as phase shifters Fig Rib waveguide used in our design Table Phase shifter states and optimal control fied intensities for operation of the proposed switch Wave-length (nm) Input port Output port φ1 φ2 I1 (GW/ cm2) I2 (GW/ cm2) 1550 1310 A -π/3 π π -π/3 310 568 479 360 1550 1310 A π/3 π/3 π/3 π/3 478 365 322.4 571 1550 1310 A π -π/3 -π/3 π 533 906.7 1031.5 897.5 1550 1310 B -π/3 π π -π/3 480 370.6 320 580.6 1550 1310 B π/3 π/3 π/3 π/3 543.9 916 543.9 916 1550 1310 B π -π/3 -π/3 π 480 370.6 320 580.6 1550 1310 C -π/3 π π -π/3 533 906.7 1031.5 897.5 1550 1310 C π/3 π/3 π/3 π/3 478 365 322.4 571 1550 1310 C π -π/3 -π/3 π 310 568 479 360 intervals 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 Lπ ≈ 4nr We2 /3λ , where λ0 is operation wavelength, We is effective width of the MMI and it can be determined by We ≈ WMMI + (λ / π )(nr2 − nc2 )−0.5 for TE mode (nr and nc are refractive indices of core and cladding layers, respectively) In this design, three input ports and three output ports are located at positions xi = (2i + 1) We /6 (i=0, 1, 2) In this study, we use the chalcogenide glass As2S3 for designing the whole device The material used in core layer of the proposed optical 149 Optics Communications 387 (2017) 148–152 D.-T Le et al Fig 2D BPM simulation of electric field pattern in the switch when (a) λ1=1550 nm; (b) λ2=1310 nm changed and therefore it causes a change in phase shift at outage arm The phase shift varies proportionally with intensity of field Due to multimode interference principle, self-imaging is formed and mirrored on a periodic cycle that is an even and odd integer times of 3Lπ respectively 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=17Lπ equivalent 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 waveguide 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 mode field 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 = κB + γ1 ( A + B ) A dz −i dB = κA + γ2 ( B + A ) B dz (1) M= (2) ⎛− e−j 2π /3 e−j 2π /3 −1 ⎞ ⎜ −j 2π /3 ⎟ − e j 2π /3 ⎟ −1 ⎜ e j π j π − /3 − /3 ⎝ −1 ⎠ e −e (3) Hence, the transfer matrices at length LMMI of the MMI couplers at wavelengths λ1=1550 nm and λ2=1310 nm for 3×3 MMI coupler are where κ is the linear coupling coefficient, κ = π /2Lc ; A and B are field amplitudes of the control waveguide and signal waveguide, respectively, 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; Aeff is 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 electrical fields 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 highintensity control field to nonlinear waveguide, its refractive index is ⎛−e−j 2π /3 ⎜ −j 2π /3 ⎜e ⎝−1 ⎛−1 ⎜ −j 2π /3 M2 = e ⎜ ⎝−e−j 2π /3 M1 = ⎞ e−j 2π /3 −1 ⎟ −1 e−j 2π /3 ⎟ , e−j 2π /3 −e−j 2π /3 ⎠ e−j 2π /3 −e−j 2π /3⎞ ⎟ −1 e−j 2π /3 ⎟ j π − /3 ⎠ e −1 (4) 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 and if (φ1, φ2)=(π/3, π/ 3) then the signal is at output port As an example, we investigate the switching mechanism for the case input signal at port B and output signal at port First, we need find the intensity I1 introduced to control waveguide (also see Fig 1) by varying the intensity slowly We find out that the appropriate value is about 480 GW/cm2 to obtain a phase shift of –π/3 in comparison with the center access waveguide 150 Optics Communications 387 (2017) 148–152 D.-T Le et al Table 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) 1550 1310 A -0.46−0.46 -0.42−0.42 28 27 -25.7−25.7 -36−36 1550 1310 A -0.57−0.57 -0.42−0.42 29 28 -30.1−30.1 -38.34 −38.34 1550 A -0.44−0.44 28 -0.3−0.3 27 -35.25 −35.25 -41.32 −41.32 -0.29−0.29 29 -0.46−0.46 28 -0.27−0.27 28 -0.44−0.44 27 -0.29−0.29 29 -0.46−0.46 28 -0.44−0.44 28 -0.3−0.3 27 1310 1550 B 1310 1550 B 1310 1550 B 1310 1550 C 1310 -29.74 −29.74 -40.68 −40.68 -25.88 −25.88 -37.43 −37.43 -29.74 −29.74 -40.68 −40.68 -35.25 −35.25 -41.32 −41.32 1550 1310 C -0.57−0.57 -0.42−0.42 29 28 -30.1−30.1 -38.34 −38.34 1550 1310 C -0.46−0.46 -0.42−0.42 28 27 -25.7−25.7 -36−36 Then we change the the intensity I2 introduced into control waveguide to find out the its value We find out that the intensity is about 318 GW/cm2 to make a phase shift of π 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/cm2 and 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 in Table Simulation results and discussions Due to symmetric nature of the proposed structure, the role of input ports A and B in Fig are 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 or output port By using the 2D BPM, the field propagation in the whole device is shown in Fig 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 in Table The BPM simulation results have shown that a high output field 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] Fig Insertion loss at the wavelengths (a) 1550 nm (b) 1310 nm and (c) Crosstalk at wavelength 1550 nm ⎛P ⎞ I L (dB ) = 10 log10 ⎜ out ⎟ ⎝ Pin ⎠ (5) ⎛ Phigh ⎞ Ex R (dB ) = 10 log10 ⎜ ⎟ ⎝ Pl ow ⎠ (6) where Pout and Pin are the output and input power of the switch in 151 Optics Communications 387 (2017) 148–152 D.-T Le et al -24 optical domain Extinction ratio (dB) -26 Conclusion -28 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 alloptical 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 -30 -32 -34 -36 1549 1549.5 1550 1550.5 1551 1551.5 1552 1552.5 1553 1553.5 1554 Wavelength (nm) a) -34 Acknowledgements Extinction loss (dB) -36 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 -38 -40 -42 -44 -46 References -48 [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 (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 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wavelength band: (a) 1550 nm and (b) 1310 nm operation state, Phigh and Plow are output power levels in ON and OFF states of input port respectively The results presented in Table show 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 The −2 dB bandwidths of the spectral responses at output ports from input ports of insertion loss are nm and nm; the crosstalk in these cases are from −10 dB to −15 dB, respectively Fig shows 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 3×3 fiber 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- 152 ... power of the switch in 151 Optics Communications 387 (2017) 148–152 D.-T Le et al -24 optical domain Extinction ratio (dB) -26 Conclusion -28 A novel high bandwidth all- optical 3×3 switch working... nm and 1310 nm regions has been presented in this study By using two non-linear directional couplers as phase shifters, a 3×3 alloptical non-blocking switch based on 3×3 MMI structures is realized... [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 efficient electrodes,