The OSNR is defined as the ratio between the transmitted optical power and the received optical power over 1 km of fiber, including both signal and optical noise.

The OSNR is the ratio between the optical output signal power and the optical input signal power of the device being analyzed.

The OSNR is defined as the ratio between the average optical signal power and the average optical noise power over a specific spectral bandwidth.

The OSNR is defined as the ratio between the optical signal power (including noise) and the optical noise power over a specific spectral bandwidth.