Novel InP-based optical 45° hybrid for demodulating 8-ary DPSK signal

We proposed a novel optical 45° hybrid using a 2×8 multimode interference (MMI) coupler, three phase shifters and three 2×2 MMI couplers. Since optical demodulation is achieved by using the proposed 45° hybrid with only one delayed interferometer, the 8-ary differential phase shift keyed (DPSK) demodulator has much smaller device dimensions than conventional 8-ary DPSK demodulators consisting of four pairs of delayed interferometers and optical couplers. We fabricated the proposed 45° hybrid with an InP-based deep-ridge waveguide structure, and experimentally demonstrated the octagonal phase response with a relative phase deviation of <±5° over 32-nm-wide spectral range.