Novel differential-phase shift keying optical receivers are taught based on multi-symbol differential phase shift keying detection (DPSK) aided by decision feedback (DF) from the decision bits in earlier symbol intervals. In accordance with the invention, the DF is directed to an optical front-end comprising multiple Delay Interferometers (DIs) with multiple delays T, 2T, . . . , where T is the DPSK symbol duration. In one embodiment, the DF bitstream is applied to electronically switch the polarity of DI outputs prior to additive combination and hard detection. In other embodiments the DF is applied to active phase-shifting electrodes incorporated in the DIs. In additional embodiments the DF is applied to modified DI devices which not of the conventional Mach-Zehnder asymmetric two-arms type, but rather comprise either three or more arms with appropriate couplings, or two arms, one of which comprises a recirculating delay line with delay T. These embodiments comprise pairs of active phase-shifting electrodes to be activated by the DF. In other embodiments the teachings of this invention for DPSK with DF are combined with the amplitude-shift keying (ASK) modulation format, yielding improved Differential Phase Amplitude Shift Keying (DPASK) systems with decision feedback. The resulting receiver structures exhibit improved performance trade-offs between error-rate, transmission distance and bitrate, compared with conventional DPSK systems, yet are simpler to realize than prior art multi-symbol and / or DF-aided optical DPSK systems.