Low-loss and broadband polarization-insensitive high-order mode pass filter based on photonic crystal and tapered coupler

In this Letter, a polarization-insensitive high-order mode pass filter is presented, designed, and experimentally demon-strated. When TE0, TM0, TE1, and TM1 modes are injected into the input port, TM0 and TE0 modes are filtered, and TE1 and TM1 modes exit from the output port. To attain compact-ness, broad bandwidth, low insertion loss, excellent extinc-tion ratio, and polarization-insensitive property, the finite difference time domain method and direct-binary-search or particle swarm optimization algorithm are employed for the optimization of structural parameters of the photonic crys-tal region and the coupling region in the tapered coupler. Measurement results reveal that, for the fabricated filter working at TE polarization, the extinction ratio and inser-tion loss are 20.42 and 0.32 dB at 1550 nm. In the case of TM polarization, the corresponding extinction ratio and inser-tion loss are 21.43 and 0.30 dB. Within a bandwidth from 1520 to 1590 nm, insertion loss smaller than 0.86 dB and extinction ratio larger than 16.80 dB are obtained for the fabricated filter working at TE polarization, while in the case of TM polarization, insertion loss lower than 0.79 dB and extinction ratio greater than 17.50 dB are realized. & COPY; 2023 Optica Publishing Group