an optical noise monitor measures noise power within narrow wavelength regions between channel signals of a dense wavelength division multiplexed signal . the noise monitor receives the dwdm signal and cascades the signal along a multipoint travel path where the signal is incident on a series of multi - pass filters . each multi - pass filter selects a narrow , predefined wavelength region between channel signals of the dwdm signal . noise power within the predefined wavelength region is detected and snr is monitored by comparing the detected noise power to the power in the channel signal . according to a first preferred embodiment of the present invention , the noise monitor includes multi - pass filters that each have reflective surfaces to guide optical energy within a predefined wavelength region through multiple passes of an optical filter . according to a second preferred embodiment of the present invention , the noise monitor includes multi - pass filters each having a serial arrangement of multiple optical filters .