Novel Optical 90$^{\circ}$ Hybrid Consisting of a Paired Interference Based 2$\,\times\,$ 4 MMI Coupler, a Phase Shifter and a 2 $\,\times\,$2 MMI Coupler

We propose a novel optical 90° hybrid consisting of a paired-interference based 2 × 4 multimode interference (MMI) coupler, a phase shifter and a 2 × 2 MMI coupler. We theoretically clarified that the proposed device shows superior performances to a conventional 4 × 4 multimode interference coupler from the viewpoint of a wavelength sensitive loss and an operating bandwidth. We fabricated the proposed 90° hybrid using a deep-ridge waveguide with a GalnAsP/InP material system, and experimentally demonstrated quadrature phase response with a low interchannel imbalance, a high common-mode rejection ratio (> 20 dB) and a low phase deviation (< 5°) over 94-nm-wide spectral range.