The design and error analysis of 90° hybrid based on InP 4×4 MMI

A 90° hybrid based on InGaAsP/InP deep-ridge 4×4 Multi-Mode Interference (MMI) structure is designed. Three-dimensional beam propagation method (3-D BPM) is used in the simulation and error tolerance investigation process. The calculated common-mode rejection ratio (CMRR) and phase deviation are larger than 30 dB and smaller than 1.4° within 1530-1565 nm, respectively. Error tolerance of refractive index, thickness of core layer, waveguide width and length of MMI are analyzed. The designed hybrid is fabricated in the laboratory and measured in the coupling test platform. The test results show that CMRR of the hybrid is almost greater than 10 dB in the wavelength from 1530 to 1565 nm. The estimated phase deviation of the hybrid is less than 12° within 1533-1548 nm and even larger in other wavelengths. The performance degradation is explained well by the error-tolerance analysis.