Four-Dimensional Modulation and Stokes Direct Detection of Polarization Division Multiplexed Intensities, Inter Polarization Phase and Inter Polarization Differential Phase

We propose and experimentally demonstrate modulation and Stokes space direct-detection (DD) of four orthogonal dimensions or degrees of freedom (DOF) of a single laser being the intensity on two orthogonal polarizations, the inter-polarization phase and the inter-polarization differential phase. At the transmitter, a dual-polarization in-phase quadrature-phase (DP-IQ) modulator driven by a 4-channel digital-to-analog converter is employed to imprint the transmitted data onto the aforementioned four DOF. Transmitted symbols for each polarization are drawn from multi-ring/multi-phase constellations, e.g., 2-ring/4-ary phase shift keying (PSK) or 2-ring/8PSK, resulting in a polarization division multiplexed (PDM) format with intensity, inter-polarization phase, and inter-polarization differential phase modulation (PDM-IMIPM-IDPM). At the receiver, a novel DD front-end
¹
comprising two 2 × 4 90° hybrids, two single-ended photodiodes (PDs), four balanced PDs, couplers and polarizing optics, provides six photocurrents from which the information on all four modulated DOF is retrieved with the aid of subsequent digital signal processing (DSP) operating in the Stokes space. Adopting the proposed transceiver, we experimentally report transmission of 320 Gb/s over 10 km of single mode fiber (SMF) below hard decision-FEC threshold of 3.8 × 10
^-3
using 40 GBd PDM 2-ring/8PSK modulation delivering 8 bits per symbol. In addition, we also report transmission of 360 Gb/s over 20 km of SMF below soft decision-FEC threshold of 2 × 10
^-2
using 45 GBd PDM 2-ring/8PSK modulation.