Patent ID: 7149428

Claim:
An apparatus for monitoring optical signal to noise ratio (OSNR) of wavelength division multiplexed optical signals having center wavelengths of λ 1 , . . . ,λ i , . . . ,λ n , (i=1,2, . . . ,n) in a wavelength division multiplexing optical transmission system, comprising: a polarization controller for controlling polarization states of the wavelength division multiplexed optical signals; a tunable optical bandpass filter for varying its center wavelength λ v and filtering an optical signal having a center wavelength of λ i for which the OSNR is to be measured among the wavelength division multiplexed optical signals; an optical signal separating means for separating the filtered optical signal into a first optical signal and a second optical signal; a linear polarizer for linearly polarizing the second optical signal; and an OSNR measuring means for obtaining, when λ v conforms to λ i , power (P 1 ) of the first optical signal and power (P 2 ) which is the minimum power of the second optical signal passing through the linear polarizer; and when λ v is shifted from λ i by a predetermined distance, power (P 3 ) of the first optical signal and power (P 4 ) which is the minimum power of the second optical signal passing through the linear polarizer; and obtaining the OSNR using the powers of P 1 , P 2 , P 3 and P 4 , wherein the power of the second optical signal passing through the linear polarizer is minimized when a polarization state of the second optical signal is orthogonal to the polarization state of the linear polarizer and the powers of P 2 and P 4 are obtained while scanning the polarization state of the optical signal using the polarization controller.