Patent ID: 7149407

Claim:
A method for monitoring the signal-to-noise-ratio of an optical signal comprising the steps of: measuring, a power measurement P 0 , of the optical signal; splitting, the optical signal into two branch signals; filtering, one of the branch signals with a first narrow bandwidth optical filter; filtering, the other branch signal with a second narrow bandwidth optical filter, wherein said first and second narrow bandwidth optical filters have different center frequencies; rotating, the polarization of the branch signal filtered by the second narrow bandwidth optical filter to be orthogonally polarized to the other branch signal; measuring, a power measurement P 1 , of the branch signal filtered with the first narrow bandwidth optical filter; measuring, a power measurement P 2 of the polarization scrambled branch signal; combining, the filtered branch signal(s) from each of the branches into a combined branch signal; converting, the combined branch signal into an electrical signal; filtering, the electrical signal with an electrical bandpass filter centered at Δf, where Δf=|f 1 −f 2 |; and f 1 is the center frequency of the first narrow bandwidth optical filter and f 2 is the center frequency of the second narrow bandwidth optical filter; and measuring a minimum power P Δf of the filtered electrical signal.