Series of ultra-low loss and ultra-compact multichannel silicon waveguide crossing.

Ultra-compact waveguide crossing (UC-WC) is a basic component in optoelectronic fusion chip solutions, as its footprint is smaller in the orders of magnitude than that of traditional photonic integrated circuits (PICs). However, a large loss of UC-WC (decibel level) becomes a barrier to scaling and practicality. Here, we propose a series of ultra-low loss UC-WC silicon devices using an advanced hybrid design that combines the adjoint method with the direct binary search (DBS) algorithm. Simulation results show that our 2 × 2 UC-WC has an insertion loss as low as 0.04 dB at 1550 nm, which is about ten times lower than the previous UC-WC results. In the valuable C-band (1530-1565 nm), the insertion loss of UC-WC is lower than -0.05 dB, and the channel crosstalk is lower than -34 dB. Furthermore, for the 3 × 3 UC-WC device, the highest insertion loss in the entire C-band is approximately -0.07 dB, and the highest channel crosstalk is lower than -33 dB. Additionally, the 4 × 4 and more complex 8 × 8 UC-WC devices were also analyzed. The highest insertion loss for 4 × 4 and 8 × 8 UC-WC in the C-band is only -0.19 dB and -0.20 dB, respectively, and the highest channel crosstalk is approximately -22dB and -28 dB, respectively. These results confirm that the designed devices possess two attractive features simultaneously: ultra-compactness and ultra-low insertion loss, which may be of great value in future large-scale optoelectronic fusion chips.