Dual Polarization Full-Field Arbitrary Waveform Reconstruction Using Intensity Only Measurements for Coherent Communications

Conventional optical coherent receivers capture the full electrical field, both amplitude and phase, of a signal waveform by measuring its interference against a stable continuous-wave local oscillator (LO). In optical coherent communications, powerful digital signal processing (DSP) techniques that operate on the full electrical field can undo transmission impairments such as chromatic dispersion (CD), and polarization mixing. Simpler direct detection techniques cannot measure the full electrical field and lack the ability to compensate for these impairments. We present a full-field measurement technique using only direct detection that does not require any beating with a strong carrier (LO). Rather, a phase retrieval algorithm based on alternating projection employing dispersive elements recovers the optical phase from intensity only measurements. In this demonstration, the phase retrieval algorithm is a modified Gerchberg?Saxton (GS) algorithm that achieves a simulated optical signal-to-noise ratio (OSNR) penalty less than 4~dB compared to theory at a bit-error-rate of ${2times 10^{-2}}$. With the phase retrieval scheme, we experimentally demonstrate polarization-multiplexed 30-Gbaud QPSK signal detection and equalization using standard 2X2 multiple-input-multiple-output (MIMO) after 520-km standard single-mode fiber (SMF) transmission.