OSNR is measured with an optical spectrum analyzer (OSA) and is defined as the ratio of optical power of the digital information signal (PSignal) to optical noise (PNoise) added to the signal by optical amplifiers (EDFA). RIN, MPN, Optical Amplifier Noise and Shot Noise. OSNR for each level and for complete signal can be defined The signal at the output of an optical amplifier in response to a noise free signal at the input is The following formulation accounts for all noise terms that can be treated as Gaussian. Optical signal-to-noise ratio (OSNR) is used to quantify the degree of optical noise interference on optical signals.