Wavelength- and Polarization-Independent Operation of Differential $M$ -ary PSK Receiver Based on Single Delay Interferometer Using 120$^{\circ}$ Optical Hybrid

In this letter, we demonstrate the wavelength- and polarization-independent operation of a receiver for differential quadrature phase-shift-keying and 8-ary phase-shift-keying signals, which utilizes a single delay interferometer made of a 3 times 3 optical coupler. We show that the phase distortion, which occurred in the proposed receiver, can be reduced to less than 2deg without any adjustments. This feature enabled us to achieve nearly quantum-limited sensitivities.