2.5- and 5-Gbps time-delay self-homodyne interference differential phase-shift keying optical receiver for space-to-ground communication link

The availability and reliability of the high data rate space-to-ground optical coherent communication links are critically challenged because the wave front of the signal beam is distorted and impaired when propagating through atmospheric turbulence. Based on the free-space interference of two successive data bits delayed by an unequal-arm-length Mach–Zehnder interferometer (MZI), a pupil-matching time-delay self-homodyne interference differential phase-shift keying (DPSK) optical receiver with 2 × 4 90-deg optical hybrid is designed for the high data rate space-to-ground optical communication links due to its immunity from wave front impairment. The delayed optical path difference (OPD) in the unequal-arm-length MZI corresponds to the duration of one bit and can be stabilized to below 1000th of the wavelength by the closed-loop feedback control. The maximum system insertion loss is