Chinese Optics Letters, 2024, 22 (1): 011301, Published Online: Jan. 2, 2024  

Experimental demonstration of a flexible-grid 1 × (2 × 3) mode- and wavelength-selective switch using silicon microring resonators and counter-tapered couplers

Author Affiliations
1 Faculty of Electrical Engineering and Computer Science, Ningbo University, Ningbo 315211, China
2 College of Electrical and Electronic Engineering, Wenzhou University, Wenzhou 325035, China
3 College of Science and Technology, Ningbo University, Ningbo 315300, China
4 Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China
5 Department of Information Science and Electronics Engineering and Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China
Abstract
A flexible-grid 1×(2×3) mode- and wavelength-selective switch which comprises counter-tapered couplers and silicon microring resonators has been proposed, optimized, and demonstrated experimentally in this work. By carefully thermally tuning phase shifters and silicon microring resonators, mode and wavelength signals can be independently and flexibly conveyed to any one of the output ports, and different bandwidths can be generated as desired. The particle swarm optimization algorithm and finite difference time-domain method are employed to optimize structural parameters of the two-mode (de)multiplexer and crossing waveguide. The bandwidth-tunable wavelength-selective optical router composed of 12 microring resonators is studied by taking advantage of the transfer matrix method. Measurement results show that, for the fabricated module, cross talk less than -10.18 dB, an extinction ratio larger than 17.41 dB, an in-band ripple lower than 0.79 dB, and a 3-dB bandwidth changing from 0.38 to 1.05 nm are obtained, as the wavelength-channel spacing is 0.40 nm. The corresponding response time is measured to be 13.64 µs.

1. Introduction

Because of the explosive growth of bandwidth (BW)-intensive applications, network traffic is boosting dramatically[1,2]. To handle the ever-increasing network traffic demands, novel technologies are desired to update optical networks to improve spectrum efficiency and transmission capacity. With the characteristic of fine-grained BW allocation, elastic optical networks (EONs) allow efficient spectrum utilization[3,4]. In addition, space division multiplexing (SDM) is a technology that exploits the core or mode as an independent data channel to further expand capacity[5,6]. Hence, combining EONs with SDM can offer a promising solution for tackling the capacity limitation[7,8].

In SDM-EONs, the flexible-grid mode- and wavelength-selective switch (MWSS) in which spatial and spectral resources can be allocated independently and flexibly is considered one of the crucial elements[9,10]. Due to its CMOS-compatible property, MWSSs in silicon photonics platforms have aroused increased research interest. To date, a number of MWSSs in different configurations have been presented. A 1×2 switch comprising mode (de)multiplexers and switching modules based on silicon microring resonators (MRRs) has been experimentally demonstrated[11]. This device can support four data channels, which contain two transverse-electric (TE) modes at two wavelengths. An MWSS possessing an add/drop function that is composed of silicon MRRs and Mach–Zehnder interferometers has been reported[12]. The proposed MWSS also can have four data channels, including two wavelength channels and two mode channels. Although the above MWSSs can have good performance, they do not possess BW-tunable properties. In order to attain efficient spectrum utilization, flexible-grid MWSSs, which are suitable for SDM-EONs, are highly desired. In our previous work, a design of silicon flexible-grid MWSS based on Y-junctions and double-series coupled MRRs has been proposed and analyzed[13]. However, to the best of our knowledge, the experimental demonstration of a flexible-grid 1×(2×3) MWSS has never before been discussed.

In this Letter, a flexible-grid 1×(2×3) MWSS consisting of seven two-mode (de)multiplexers using counter-tapered couplers and two BW-tunable wavelength-selective optical routers (BTWSORs) utilizing reconfigurable add-drop silicon MRRs is designed, fabricated, and characterized in detail. To achieve a relatively compact size, a small insertion loss (IL), and a low cross talk (CT), the particle swarm optimization (PSO) algorithm and the finite difference time-domain (FDTD) method are used for optimizing structural parameters of the two-mode (de)multiplexer and crossing waveguide. The transfer matrix method is utilized to numerically study behaviors and properties of the BTWSOR. The optimized flexible-grid 1×(2×3) MWSS is demonstrated experimentally on a silicon-on-insulator (SOI) platform. For the fabricated module, an extinction ratio (ER)>17.41dB, a CT<10.18dB, an in-band ripple (IBR)<0.79dB, and the 3-dB BW changing from 0.38 to 1.05 nm are measured as the wavelength-channel spacing Δλ is 0.40 nm. The corresponding response time of 13.64 µs is obtained.

2. Principle and Design

Figure 1(a) shows the schematic diagram of the mentioned flexible-grid 1×(2×3) MWSS, which is composed of a two-mode demultiplexer based on a counter-tapered coupler marked as TDI0, six two-mode multiplexers based on counter-tapered couplers labeled as TDOi (i=1,2,,6), and two BW-tunable wavelength-selective optical routers containing 24 reconfigurable add-drop silicon MRRs denoted by RUp and RDp (p=1,2,,12). Figures 1(b) and 1(c) show the detailed drawing and corresponding cross-sectional view of the two-mode (de)multiplexer. The schematic diagram and cross-sectional view of the reconfigurable add-drop silicon MRR are illustrated in Figs. 1(d) and 1(e). Figure 1(f) depicts the structure of the adopted crossing waveguide. For RUp and RDp, the corresponding resonance wavelengths are named λUp and λDp. To adjust the path phase differences ΔφU1, ΔφU2, ΔφD1, and ΔφD2, phase shifters are designed in straight waveguides. In Fig. 1(a), the presented device supports 2×3 data channels, comprising two TE modes at three wavelengths. By employing the two-mode demultiplexer, all mode channels are converted into the fundamental mode. After then, for each mode channel, by carefully adjusting the resonance of silicon MRRs and path phase differences, the signal is wavelength-demultiplexed and then is routed flexibly with the BW-adjustable property. Finally, mode multiplexers are utilized to guarantee mode and wavelength signals can be conveyed to any one of the output ports, and different BWs are generated as required. Table 1 lists the typical states in the worst situation.

Fig. 1. (a) Schematic diagram of the flexible-grid 1 × (2 × 3) MWSS; (b) detailed drawing and (c) cross-sectional view of the proposed two-mode (de)multiplexer; (d) schematic diagram and (e) cross-sectional view of the mentioned silicon MRR; (f) structure of the proposed crossing waveguide.

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Table 1. Typical States of the Presented Flexible-grid 1 × (2 × 3) MWSS in the Worst Situation

StatesOutput Ports
O1O2O3O4O5O6
1TE0-λ2TE0-λ3TE0-λ1TE1-λ2TE1-λ3TE1-λ1
2TE0-λ3TE0-λ2TE0-λ1TE1-λ3TE1-λ2TE1-λ1
3TE1-λ3TE0-λ2TE0-λ1TE0-λ3TE1-λ2TE1-λ1
4TE0-λ2TE1-λ3TE1-λ1TE1-λ2TE0-λ3TE0-λ1
5TE1-λ3TE0-λ2TE1-λ1TE1-λ2TE0-λ3TE0-λ1
6TE1-λ2TE0-λ3TE1-λ1TE1-λ3TE0-λ2TE0-λ1
7TE0-λ1TE1-λ2TE1-λ3TE1-λ1TE0-λ2TE0-λ3
8TE1-λ1TE1-λ2TE1-λ3TE0-λ1TE0-λ2TE0-λ3
9TE0-λ1λ3TE1-λ1λ3
10TE0-λ2, λ3TE0-λ1TE1-λ2, λ3TE1-λ1
11TE0-λ1, λ3TE0-λ2TE1-λ1, λ3TE1-λ2
12TE0-λ1, λ2TE0-λ3TE1-λ1, λ2TE1-λ3
13TE0-λ3TE0-λ1, λ2TE1-λ3TE1-λ1, λ2
14TE0-λ2TE0-λ1, λ3TE1-λ2TE1-λ1, λ3
15TE0-λ1TE0-λ2, λ3TE1-λ1TE1-λ2, λ3
16TE0-λ1λ3TE1-λ1λ3

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To investigate the behaviors and properties of the presented device, the two-mode (de)multiplexer is first designed. As seen in Fig. 1(b), if the effective refractive index of TE1 mode in the bus waveguide is matched with that of TE0 mode in the neighboring tapered waveguide, the conversion between the TE1 mode in the bus waveguide and the TE0 mode in the adjacent taper waveguide will be achieved. In this work, the PSO algorithm and FDTD method are adopted to optimize the structural parameters of the two-mode (de)multiplexer involved to realize compact size and good performance.

As seen in Fig. 1(b), the bus waveguide is separated into n (n1) equilong segments. The segments’ length and width are marked as L0 and Wm (m=0,1,2,,n). In the simulation, the optimization figure of merit (FOM) is defined as FOM1=PCross_TE1_TE0 in the case of 1mn3, while in the case of n2mn, the definition of the FOM is described as FOM2=min(PCross_TE1_TE0/PBar_TE1,PBar_TE0/PCross_TE0_TE0). Here, PCross_TE1_TE0 is the optical power of the TE0 mode obtained from the port OM2, and PBar_TE1(TE0) is the optical power of the TE1 (TE0) mode obtained from the port OM1, as the TE1 (TE0) mode is input into the port IM. The segment’s width and the corresponding variation range are considered as the particle’s position and velocity, which is renewed by employing the following equations[14,15]: vel+1=wT×vel+r1×γ1×(iblpsl)+r2×γ2×(gblpsl),psl+1=psl+vel,where vel (l=1,2,) and psl represent the particle’s velocity and position, wT is the inertial weight, ibl and gbl are the individual and global best positions, the cognitive and social rates are denoted by r1 and r2, and γ1 and γ2 are the random numbers distributed uniformly between 0 and 1, respectively.

Table 2 summarizes the optimal widths of segments. In the optimization, the length L0 is chosen to be 1 µm, the thickness H0 is 220 nm, the width Wa is selected as Wa=300nm, the gap G0 is 200 nm, r1 and r2 are set to be 2, and wT is selected to be 1.

Table 2. Optimized Segments’ Widths for the Bus Waveguide in the Coupling Region

SymbolValue/µmSymbolValue/µmSymbolValue/µm
W00.85W10.85W20.74
W30.75W40.78W50.71
W60.62W70.62W80.58
W90.59W100.45

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For the mentioned BTWSOR, each wavelength signal from the IU1 (ID1) port could be transmitted to any of the ports TU1 (TD1), TU2 (TD2), TU3 (TD3), DU1 (DD1), DU2 (DD2), and DU3 (DD3). Furthermore, via RU1 (RD1), RU2 (RD2), and RU3 (RD3) in the first row, the wavelength signals from the port IU1 (ID1) are demultiplexed; then, via RU4 (RD4), RU5 (RD5), and RU6 (RD6) in the second row, RU7 (RD7), RU8 (RD8), and RU9 (RD9) in the third row, or RU10 (RD10), RU11 (RD11), and RU12 (RD12) in the fourth row, the wavelength-demultiplexed signals from neighboring channels going through equal-length optical paths are combined, and the corresponding transmission spectra with tunable BWs are implemented on demand at the ports DU1 (DD1), DU2 (DD2), and DU3 (DD3). In order to study the mentioned BTWSOR, the transfer matrix method is utilized. The transfer functions of the optical router at the ports DU1 (DD1), DU2 (DD2), DU3 (DD3), TU1 (TD1), TU2 (TD2), and TU3 (TD3) are described below, DU(D)1=DrU(D)1·DrU(D)4·ξ·ThU(D)5·ξ·ThU(D)6+ThU(D)1·ξ·ejΔφU(D)1·DrU(D)2·DrU(D)5·ξ·ThU(D)6+ThU(D)1·ξ·ejΔφU(D)1·ThU(D)2·ξ·ejΔφU(D)2·DrU(D)3·DrU(D)6,DU(D)2=DrU(D)1·ThU(D)4·ξ·DrU(D)7·ξ·ThU(D)8·ξ·ThU(D)9+ThU(D)1·ξ·ejΔφU(D)1·DrU(D)2·ThU(D)5·ξ·DrU(D)8·ξ·ThU(D)9+ThU(D)1·ξ·ejΔφU(D)1·ThU(D)2·ξ·ejΔφU(D)2·DrU(D)3·ξ·ThU(D)6·DrU(D)9,DU(D)3=DrU(D)1·ThU(D)4·ξ·ThU(D)7·ξ·DrU(D)10·ξ·ThU(D)11·ξ·ThU(D)12+ThU(D)1·ξ·ejΔφU(D)1·DrU(D)2·ξ·ThU(D)5·ξ·ThU(D)8·DrU(D)11·ξ·ThU(D)12+ThU(D)1·ξ·ejΔφU(D)1·ThU(D)2·ξ·ejΔφU(D)2·DrU(D)3·ξ·ThU(D)6·ξ·ThU(D)9·DrU(D)12,TU(D)1=DrU(D)1·ξ·ThU(D)4·ξ·ThU(D)7·ξ·ThU(D)10,TU(D)2=ThU(D)1·ξ·ejΔφU(D)1·DrU(D)2·ξ·ThU(D)5·ξ·ThU(D)8·ξ·ThU(D)11,TU(D)3=ThU(D)1·ξ·ejΔφU(D)1·ThU(D)2·ξ·ejΔφU(D)2·DrU(D)3·ξ·ThU(D)6·ξ·ThU(D)9·ξ·ThU(D)12,ThU(D)i=1k2(1αejθU(D)i)1α(1k2)ejθU(D)i,DrU(D)i=α14k2ejθU(D)i41α(1k2)ejθU(D)i,where DrU(D)p and ThU(D)p represent the drop and through transmission of RU(D)p, the scattering loss at each crossing is denoted by ξ, k is the field coupling coefficient of RU(D)p, α is the corresponding field transmission coefficient, and θU(D)p is the round-trip phase shift of RU(D)p.

To decrease the scattering loss ξ, the PSO algorithm and FDTD method are utilized to optimize structural parameters of the crossing waveguide involved. As seen in Fig. 1(f), the crossing waveguide involved consists of two identical multimode waveguides, which are orthogonal. For each multimode waveguide, the input and output tapers are mirror-symmetrical. The taper is separated into 15 equilong segments. The segments’ length and width are denoted as L1 and WCs (s=0,1,2,,15). Here, the definition of the FOM is written as FOM3=PCBar_TE0, where PCBar_TE0 represents the optical power of the TE0 mode obtained from the port OC1 with the TE0 mode input into the port IC. The segment’s width and the corresponding variation range are regarded as the particle’s position and velocity, which are renewed utilizing Eqs. (1) and (2). Table 3 lists the optimized segments’ widths. In the optimization, the length L1 is chosen to be 0.3 µm, and the slab thickness H1 is 90 nm. Thus, ξ is calculated as ξ=0.08dB/crossing.

Table 3. Optimized Segments’ Widths for the Taper in the Crossing Waveguide

SymbolValue/µmSymbolValue/µmSymbolValue/µm
WC00.45WC10.70WC20.86
WC30.88WC40.90WC50.90
WC60.96WC71.04WC81.16
WC91.16WC101.16WC111.10
WC121.06WC131.06WC140.90
WC151.00

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Figure 2 illustrates the working principle of forming the tunable BW. Take the spectrum with large BW at the port DU3 as an example. As shown in Figs. 1(a) and 2, optical signals with wavelengths of λU1, λU2, and λU3 are separated when the light beam passes through the wavelength demultiplexer. Then, as RU1, RU2, RU3, RU10, RU11, and RU12 are turned on, the separated signals, which pass equal-length optical paths, are coherently combined so that the spectrum with large BW can emerge from the port DU3. In the simulation, the field transmission coefficient α is 0.9989 (a propagation loss of 1.52 dB/cm), the group refractive index is 3.8, and the radii of silicon MRRs are 10 µm, making the bend-related loss negligible[16,17]. The channel spacing in wavelength Δλ is set as Δλ=λU1λU2=λU2λU3=0.4nm. To form flat-top spectra, the path phase differences ΔφU1 and ΔφU2 are optimized to be 0.5π. The gap G1 is 195 nm, and thus the field coupling coefficient is 0.465.

Fig. 2. Working principle of forming the tunable BW.

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Figure 3 shows the transmission spectra of the designed flexible-grid 1×(2×3) MWSS in the typical states, which is shown in Table 1. As described in Fig. 3, it is found that, the 3-dB BW ranges from 0.38 to 1.09 nm, the IL is smaller than 5.67 dB, and the IBR is lower than 0.68 dB, as Δλ is set as 0.40 nm. For the input TE0 mode, an ER>18.96dB and a CT<10.98dB are achieved; while for the input TE1 mode, the ER is greater than 18.96 dB, and the corresponding CT is less than 10.83dB.

Fig. 3. Simulated transmission spectra of the designed silicon-based flexible-grid 1 × (2 × 3) MWSS in the typical states.

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The simulated maximum IL, worst CT, and minimum ER of the designed flexible-grid 1×(2×3) MWSS or the designed flexible-grid 1×(2×3) MWSS cascaded with a mode multiplexer and six mode demultiplexers as a function of the waveguide width variation ΔW are shown in Fig. 4. As the value of ΔW deviates from ΔW=0nm, the IL changes significantly and grows larger, while the CT and ER change slightly and get worse. Note that in Fig. 4(a), in the case of the designed flexible-grid 1×(2×3) MWSS, when ΔW ranges from 20nm to 20 nm, the IL increases from 5.64 to 11.25 dB, the ER changes from 17.13 to 18.96 dB, and the CT varies from 9.98 to 10.83dB. From Fig. 4(b), in the case of the designed flexible-grid 1×(2×3) MWSS cascaded with a mode multiplexer and six mode demultiplexers, the corresponding IL increases from 6.22 to 13.38 dB, the ER changes from 17.11 to 18.96 dB, and the CT ranges from 9.97 to 10.79dB when ΔW increases from 20 to 20 nm. From Figs. 4(a) and 4(b), it also can be found that, as the value of ΔW gradually stays away from 0, the IL of the cascaded mode (de)multiplexer would grow larger, while the ER and CT for the two cases above are basically unchanged.

Fig. 4. Simulated maximum IL, worst CT, and minimum ER of (a) the designed flexible-grid 1 × (2 × 3) MWSS or (b) the designed flexible-grid 1 × (2 × 3) MWSS cascaded with a mode multiplexer and six mode demultiplexers changing with ΔW.

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3. Fabrication and Characterization

The designed flexible-grid 1×(2×3) MWSS was fabricated on an 8-inch (20.3 cm) SOI wafer via commercial CMOS-compatible technologies at the Institute of Microelectronics, Singapore. Figure 5 shows the microscope image of the fabricated module comprising a flexible-grid 1×(2×3) MWSS cascaded with a mode multiplexer and six mode demultiplexers.

Fig. 5. Microscope image of the fabricated module.

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A broadband light source and an optical spectrum analyzer are utilized for characterizing the fabricated devices first. A TE-type grating coupler is employed to couple the light beam into or out of the fabricated devices. By thermally tuning phase shifters and silicon MRRs, the corresponding transmission spectra can be recorded. By subtracting the transmission of the nearby straight waveguide, the measured optical power transmission of the fabricated module is normalized. Figure 6 shows the measured transmission spectra of the fabricated module. As described in Fig. 6, for the fabricated module, a CT<10.18dB, an ER>17.41dB, a maximum IL of 11.55 dB, a 3-dB BW ranging from 0.38 to 1.05 nm, and an IBR<0.79dB are measured. The maximum power consumption (PC) is 301.21 mW. As seen in Figs. 4(b) and 6, it is found that the actual width of the fabricated waveguide deviates from the optimal value owing to the process deviation. Under this situation, ΔW is extrapolated as ΔW=15nm, and thus the maximum IL of the fabricated MWSS is about 10 dB. The measured dynamic response is characterized by employing the tunable laser, photodiode detector, oscilloscope, and signal generator. State 16 is taken as an example to explain the measurement. As described in Fig. 7, the dynamic response at the port O3 would be recorded, when a 5-kHz square-wave signal in which the high-level and low-level voltages are 8 V and 0 V is loaded onto the heaters of RU1, RU2, RU3, RU10, RU11, and RU12. Note that in Fig. 7, the 10%–90% rise time of 13.64 µs and 90%–10% fall time of 8.87 µs can be measured. In future work, the IL can be reduced by using high-quality fabrication processes with a finer minimum feature size. Additionally, to further improve the CT and ER, double-series coupled MRRs could be used for enhancing the roll-off characteristic and decreasing the optical output power of unwanted wavelengths. The plasma dispersion effect can be adopted to raise the operation speed.

Fig. 6. Measured transmission spectra of the fabricated module.

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Fig. 7. Measured dynamic response of the fabricated device.

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4. Conclusion

In conclusion, a flexible-grid 1×(2×3) MWSS consisting of seven two-mode (de)multiplexers using counter-tapered couplers and two BTWSORs using silicon MRRs has been designed and experimentally demonstrated. By using the FDTD method and the PSO algorithm, we optimize the structural parameters of the two-mode (de)multiplexer and crossing waveguide to achieve relatively compact size, small IL, and low CT. The behaviors and properties of the BTWSOR are investigated by using the transfer matrix method. The optimized flexible-grid 1×(2×3) MWSS is fabricated on an SOI platform to verify its feasibility. As Δλ is 0.40 nm, a CT of <10.18dB, an ER>17.41dB, an IBR<0.79dB, a 3-dB BW ranging from 0.38 to 1.05 nm, and a maximum PC of 301.21 mW are measured. The corresponding response time is 13.64 µs. With these characteristics, the mentioned flexible-grid 1×(2×3) MWSS can be an attractive candidate for switching applications in SDM-EONs.

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Dejun Kong, Hao Lu, Pengjun Wang, Qiang Fu, Shixun Dai, Weiwei Chen, Yuefeng Wang, Bohao Zhang, Lingxiao Ma, Jun Li, Tingge Dai, Jianyi Yang. Experimental demonstration of a flexible-grid 1 × (2 × 3) mode- and wavelength-selective switch using silicon microring resonators and counter-tapered couplers[J]. Chinese Optics Letters, 2024, 22(1): 011301.

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