Improved off-center narrow filter method to monitor in-band optical signal-to-noise ratio

In the long haul optical fiber link, the spectrum of the accumulated amplified spontaneous emission noise is not flat because of filtering effects of the devices along the fiber link. The unflatness of amplified spontaneous emission (ASE) noise and its impacts imposed on the performance of the off-center narrow filter polarization-nulling method when monitoring the in-band optical signal-to-noise ratio (OSNR) of dense wavelength division multiplexing (DWDM) system are analyzed, and a modified method by introducing an unflat factor in the calculation of OSNR is proposed. The simulated results show that the OSNR could be measured accurately with the modified method.