Design of ultra-compact and polarization-insensitive multimode interference demultiplexer

An ultra-compact and polarization-insensitive multimode interference (MMI) demultiplexer based on Si3N4/SiO2 platform is proposed to separate the 1310 nm wavelength from 1550 nm wavelength. The polarization insensitivity is obtained by adjusting the refractive index of SiNx in the Si3N4/Si-x/Si3N4 sandwiched structure. The wavelengths are separated by applying the mirrored and quasi-state images, and the size of device is further reduced simultaneously. A cascaded MMI waveguide is introduced to reduce the insertion loss of 1310 nm wavelength. The performance parameters of the demultiplexer are analyzed by three-dimensional finite-difference time-domain (3D-FDTD) method. The results show that the size of the demultiplexer is only 3.4x74 mu m(2), the insertion loss and crosstalk are as low as 0.45 dB and -27.2 dB respectively. Besides, the demultiplexer has a large 3 dB bandwidth up to 190 nm. It has potential application value in the future integrated optical systems.