Phase-diversity method using phase-shifting interference algorithms for digital coherent receivers

We describe phase-diversity optical coherent receivers (CRx) using the phase-shifting-interference (PSI) framework. The goals of the analysis are several-fold. First, we show that coherent detection can be realized with optical hybrids that have an arbitrary number of branches and phase shifts using a closed-form solution of the inphase-quadrature mapping. Second, we show that CRx with 2×4 90° hybrids using balanced detection (BD) and CRx with 2×3 120° hybrid using single-ended detection (SED) perform optimally compared to alternative configurations. A proof-of-concept WDM colorless 10×132-Gb/s PDM-QPSK transmission experiment is conducted. We demonstrate that an example of arbitrary phase diversity CRx with a 2×3 90° hybrid SED operates with a 0.3 dB signal-to-noise-ratio (SNR) penalty relative to conventional CRx at 6400 km and 4480 km for bit-error-rates below the threshold of 2×10−2 and the threshold of 3.8×10−3 respectively. The sensitivity degradation of the 2×3 90° hybrid SED with respect to the 2×4 90° hybrid BD in the shot noise limited regime at a distance of 4480 km is 3 dB, which matches well with the predicted penalty from the PSI analytical model. To the best of our knowledge, this paper is the first attempt to model a CRx using the PSI model.