The optical spectrum analyzer (OSA) is the device typically used to measure OSNR. Signal and noise measurements are made over a specific spectral bandwidth Br , which is referred to as the OSA’s resolution bandwidth (RBW). The RBW filter acts as a bandpass filter allowing only the set amount of light spectrum to strike the OSA’s photodetector. The photodetector measures the average optical power in the spectral width. It cannot discriminate between two separate signals in the RBW spectrum. If there is more than one signal in RBW, it will treat and display them as one. Therefore, the ability of an OSA to display two closely spaced signals as two distinct signals is determined by the RBW setting. Typically, an OSA’s RBW range is adjustable between 10 and 0.01 nm with common settings of 1.0, 0.5, 0.1, and 0.05 nm.