OSNR monitoring in the presence of first-order PMD using polarization diversity and DSP

We analytically study optical signal-to-noise ratio (OSNR) monitoring using polarization diversity and digital signal processing (DSP) in the presence of first-order polarization-mode dispersion (PMD). We derived conditions on the input signal polarization, principle states of polarization (PSP) and differential group delay (DGD) of the fiber, polarization beam splitter (PBS) axes and receiver electrical low-pass filter bandwidth such that the monitored OSNR is least affected by PMD and we propose the use of a polarization scrambler (PS) before the PBS to achieve such conditions in practice. In addition, two special polarization states for which the proposed technique can operate independent of first-order PMD are identified. The theoretical predictions are verified through simulations. The analytical insights developed represent a more general and unified framework of which various OSNR monitoring techniques based on polarization diversity proposed in the literature can be understood as special cases.