Bidirectional mode-division multiplexers with antireflection gratings.

A bidirectional mode-division multiplexer (BMDM) with antireflection gratings is designed, and its performance in terms of S-parameters is presented. A BMDM can (de)multiplex three modes with only two waveguides and a Bragg grating. The impact of return losses on the performance of BMDMs is studied and antireflection gratings are designed to reduce their effects. A theoretical analysis of the proposed (de)multiplexer is developed based on the perturbative coupled-mode theory. Analytical expressions for the coupled-mode equations of the proposed device are obtained, taking into account the effects of return losses. Both duty cycle and teeth depths of the antireflection gratings are determined based on optimizing a cost function. In addition, FDTD simulation of the proposed device is performed, and its S-parameters are obtained and studied.