Silicon Mode (de)Multiplexer Based on Cascaded Particle-Swarm-Optimized Counter-Tapered Couplers

In this paper, a design of a silicon mode (de)multiplexer based on cascaded counter-tapered couplers is proposed and investigated. By using the particle swarm optimization algorithm and finite difference time domain method, structural parameters of each counter-tapered coupler in our proposed mode (de)multiplexer are optimized so that high conversion efficiencies can be obtained and coupling lengths can be significantly shortened. The coupling lengths of counter-tapered couplers in the proposed silicon four-mode (de)multiplexer operating in transverse-electric polarization are 8 mu m, 9 mu m, and 10 mu m. The corresponding conversion efficiencies are respectively 98.98%, 99.98%, and 98.68%. Experimental results show that, the fabricated device exhibits a demultiplexed crosstalk of -32.76 dB, -27.10 dB, -32.45 dB, or -33.29 dB at 1550 nm wavelength, when the fundamental transverse-electric mode is respectively launched into the ports Input(0), Input(1), Input(2), or Input(3). The measured demultiplexed crosstalk is less than -15.15 dB from 1525 to 1596 nm and lower than -18.12 dB from 1525 to 1561 nm.