A design method for high fabrication tolerance integrated optical mode multiplexer

The tapered asymmetric directional coupler has the potential to realize high fabrication tolerance and high transmission efficiency on-chip mode multiplexer. However, the geometry parameter selection of tapered structure remains empirical. In this paper, we propose a design method for the tapered structure based on genetic algorithm. Combined with the adjusted coupling equations and interpolation method, lowtime- cost optimization can be realized. Three mode multiplexers (TE0&TE1, TE0&TE2 and TE0&TE5) are designed by our method. According to simulation results, the insertion loss of the designed devices is lower than 1.8 dB and the crosstalk is lower than -15 dB when the fabrication error is within required range (+/- 10 nm for TE0&TE2 and TE0&TE5, and +/- 20 nm for TE0&TE1) in the bandwidth of 1.5-1.6 mu m. In addition, the entire optimization process takes only 2 h for each device, which is around the time cost of a single 3D simulation.