A system for monitoring optical signal-to-noise ratio (OSNR) is provided. In some specific examples, the system may use a pilot tone power of a signal modulated with pilot tone to derive the pure signal power and the variance of the whole electric field to derive the total power (pure signal power plus amplified spontaneous emission (ASE) power of the signal). The ASE power can be obtained by subtracting the pure signal power from the total power (ASE+pure signal). Once the ASE power and the pure signal power are known, the OSNR can be calculated.