Integrating angled multimode interferometer with Bragg grating filters for coarse wavelength division (de)multiplexing with optimized shape factor

We propose and experimentally demonstrate an angled multimode interferometer (AMMI) integrated with cascaded phase-shifted Bragg gratings (PSBGs) for coarse wavelength division multiplexing, based on a 400 nm-thick silicon nitride waveguide platform. Due to the design constraints, the spectral response of a standard AMMI filter suffers from the drawbacks of limited optical bandwidth, high inter-channel crosstalk, and non-ideal shape factor. Two techniques were used to improve performance. Firstly, it was found that increasing the input waveguide width of the AMMI is beneficial for the optical bandwidth while maintaining good insertion loss and crosstalk performance. Experimental results show that the 1 dB bandwidth increases from 7.6 nm to 9.7 nm. To achieve an improved shape factor and decreased crosstalk, the PSBG filters were utilized as the second-stage filters by integrating them at the output waveguides of the AMMI. Simulation results demonstrate an average crosstalk reduction of approximately 15.1 dB at the center wavelengths of the four channels and shape factor improvement by about 0.29. However, measurement results show an average crosstalk decrease of only about 5.5 dB, which is possibly due to the fabrication imperfection of the PSBGs and the unsatisfactory film quality of the SiN wafers used. Despite deviations between measured and simulated performance, the shape factor is increased from 0.26 to 0.74 using these proposed techniques, showing the feasibility of such an integration method.