OSNR ( Optical Signal to Noise Ratio) is a critical and important parameter in high speed DWDM links. If OSNR is not good it will seriously affect the distance reachability in DWDM systems before regeneration is required. OSNR testing is typically done with OSA (Optical Spectrum Analyzer). There are standard test sets available to test OSNR of 10G lambdas; However when it comes to testing on high speed links at 40G and 100G based on Pol Mux-QPSK modulation techniques, OSNR testing in field leaves a lot to be desired. In fact there is no automated tool available today that can do OSNR testing for Pol Mux modulation.
A POLMUX-QPSK transmitter consists of two quadrature (e.g. QPSK) modulators and a polarization beam splitter (PBS) to multiplex the two outputs on orthogonal projections. This is significantly different than conventional modulation schemes based on single polarized signal. An OSA is simply not designed to test OSNR on such dual polarized signals. There have been ways around methods suggested by leading T&M ( Test and Measurement) vendors using manual methods of testing on existing OSA. But these methods are less accurate, complex and not repeatable. They are simply not aimed for field engineers and are very prone to human mistakes.
The T&M vendors have plans to bring out OSA to test OSNR on Pol Mux signals; they are working on developing those solutions. However the progress has been very slow. It’s been a while now that we have been hearing of these plans. It is need of the hour to expedite these solutions since the industry need them badly today and without them accurate testing of OSNR would not be possible.