The standard method of measuring optical signal-to-noise ratio (OSNR), as described in the IEC 61280-2-9 test procedure, is based on averaging noise power to the left and right of the channel of interest. This gives what is often referred to as the “out-of-band” OSNR value. In reconfigurable optical add/drop multiplexer (ROADM) based networks, each channel passes through a number of paths, amplifiers and filters. As a result, the noise at the channel wavelength is not the same as the noise to the left and right of the channel. In this case, relying on a standard, out-of-band OSNR measurement can give misleading ideas about network performance. To accurately determine the OSNR, you should calculate the “in-band” OSNR using actual noise levels at the channel wavelength. A number of approaches are suggested in the literature.
This issue of the RSoft enewsletter describes how OptSim™ users can implement two of the commonly used approaches: (1) independent treatment of signal and noise; and (2) polarization splitting.
Approach 1: Treating Signal and Noise Independently
Figure 1 depicts the schematic layout for a DPSK system used for in-band OSNR measurement. A single-stage ROADM is modeled by a super-Gaussian filter of order 2. Multiple spans represent cascaded stages of amplifiers and ROADMs. At the end of 3, 6, and 9 spans, optical power meters measure power over a 0.1 nm bandwidth. During the first simulation run, the optical noise is turned off, keeping only the signal of interest. During the second simulation run, the signal of interest is turned off (by setting the “Source Status” parameter in the laser to 0) and only the optical noise is modeled.