Because of the explosive growth of bandwidth (BW)-intensive applications, network traffic is boosting dramatically. 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. In addition, space division multiplexing (SDM) is a technology that exploits the core or mode as an independent data channel to further expand capacity. Hence, combining EONs with SDM can offer a promising solution for tackling the capacity limitation.

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. 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. 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. 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.

The research group led by Prof. Weiwei Chen from Ningbo University, Prof. Pengjun Wang from Wenzhou University, Prof. Jianyi Yang from Zhejiang University proposed, optimized, and experimentally demonstrated a flexible- grid 1×(2×3) mode- and wavelength- selective switch. 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 research results are published in Chinese Optics Letters, Vol. 22, Issue. 1, 2024: Dejun Kong, Hao Lu, Pengjun Wang, Qiang Fu, Shixun Dai, Weiwei Chen, et al. Experimental demonstration of a flexible-grid 1×(2×3) mode- and wavelength- selective switch using silicon microring[J]. Chinese Optics Letters, 2024, 22(1): 011301.

In this work, 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) using silicon MRRs is designed, fabricated, and characterized in detail. As seen in Fig. 1, 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. To achieve relatively compact size, small insertion loss (IL), and low crosstalk (CT), we optimize the structural parameters of the two-mode (de)multiplexer and crossing waveguide by using the FDTD method and PSO algorithm. By using the transfer matrix method, the behaviors and properties of the BTWSOR are numerically studied. The optimized flexible-grid 1×(2×3) MWSS was fabricated on an SOI platform to verify its feasibility. This is an organic combination of theoretical and experimental research, and this work will also inspire us to explore new theoretical mechanisms in the the field of flexible-grid mode- and wavelength- selective switching principles and application, which will produce more effective practical value.

To characterize the fabricated devices, a broadband light source and an optical spectrum analyzer are utilized. 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. For the fabricated module, a CT < −10.18 dB, an ER > 17.41 dB, a maximum IL of 11.55 dB, a 3-dB BW ranging from 0.38 to 1.05 nm, and an IBR < 0.79 dB are measured. The maximum power consumption (PC) is 301.21 mW. The measured dynamic response is characterized by employing the tunable laser, photodiode detector, oscilloscope, and signal generator. The 10%–90% rise time of 13.64 μs and 90%–10% fall time of 8.87 μs can be measured. With these characteristics, the mentioned flexible-grid 1×(2×3) MWSS can be an attractive candidate for switching applications in SDM-EONs.

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.1 Schematic diagram of the presented flexible-grid 1×(2×3) MWSS