In-service OTDR limitations in CWDM systems caused by spontaneous Stokes and anti-Stokes Raman scattering

We demonstrate that traffic-bearing signals in coarse wavelength-division-multiplexing systems can limit in-service optical time-domain reflectometry sensitivity on presumably "dark" channels being monitored. This effect is due to spontaneous Stokes and anti-Stokes Raman scattering into the wide spectral bands.