Optical Field Recovery in Stokes Space

Coherent detection empowers optical receivers the capability of recovering the optical field propagating through the fiber. Field recovery not only increases the degree of freedom for optical modulations, but also enables the digital compensation of fiber dispersion that avoids the sophisticated optical dispersion management. Compared with coherent detection, direct detection (DD) owns a natural advantage—the simplicity, but lacks the capability of field recovery. To increase the modulation dimension for DD, the recent development of Stokes vector receiver takes the advantage of polarization diversity and extends the modulation dimension up to three-dimension with reference to the four-dimensional coherent detection (i.e., dual-polarization intensity and phase). However, a Stokes vector receiver is not inherently capable of field recovery because the second-order signal representation in Stokes space nonlinearizes the first-order linear field information, making it infeasible to digitally compensate the chromatic dispersion. In this paper, we provide a comprehensive review of Stokes-space field recovery (SSFR) that completely or partially recovers the optical field by DD. We analyze the degree of freedom for a variety of Stokes-space modulations compatible with SSFR, and reveal their tradeoff between optical spectral efficiency (that determines fiber capacity) and electrical spectral efficiency (that determines transceiver cost per bit). We review the Stokes-space polarization recovery methods and generalize a novel concept as analog polarization identification to simplify the DSP of SSFR. Furthermore, we compare the OSNR sensitivity among various common direct detection schemes to provide reliable predictions of their transmission performance. With its high spectral efficiency and capability of digital dispersion compensation, SSFR is very promising for future high-capacity short-reach applications over single- or multispan fiber transmission.