Ultra-compact and low loss onchip higher order mode pass filter based on topology optimization

We propose and experimentally demonstrate a silicon based ultra-compact and low loss higher order mode bandpass filter (HMF), which was inversely designed by a direct-binary search topology optimization algorithm. The proposed HMF is a directional coupler with optimally positioned etched circular air holes in the coupling region which footprint is only 1.8 x 3 mu m(2). The experimental results show that the HMF has low insertion loss of <1 dB and low crosstalk of <-15 dB in the wavelength range of 1520 similar to 1580 nm. It can be extended to selectively pass other higher order modes.