Recovery of single-polarization waveforms with dual-polarization coherent receivers at low SNRs

Abstract

The leap to 100 Gbps data transmission rates has relied on coherent communication technology that use dual-polarization modulation formats. While several complex modulation formats use polarization to increase data rate, it can be an unwanted degree of freedom in free space links that baseline single-polarization modulation formats. In links that are signal-to-noise ratio (SNR) limited; have receivers with limited processing resources; or rely on polarization for duplex through a shared aperture; single polarization links may be preferable. Often times, a system of polarization-maintaining (PM) fibers and PM amplifiers preserve single-polarization signals from degradation as they propagate; however, these systems can be challenging to implement due to tight tolerances on components and PMfiber splices. In this paper we present a method for recovering single-polarization signals from arbitrary polarization received signals using integrated dual-polarization coherent receivers. This removes the reliance on PM fiber components while maintaining single polarization receiver performance. The algorithm uses the received signal on both polarization channels to reconstruct the initial single-polarization coherent waveform. This is accomplished by implementing a polarization rotation and polarizing filter in digital signal processing (DSP). A feature of this method is it combines the signal energy in each of the receiver’s polarization channels while rejecting the noise energy in the polarization that is orthogonal to the signal polarization. This preserves SNR while simplifying subsequent DSP steps by eliminating the unwanted polarization mode. Perhaps most importantly, our algorithm is deterministic and can be added to established DSP processes without requiring significant processing.