An orthogonal heterodyne in-band optical signal-to-noise-ratio (OSNR)
monitoring method and apparatus that use the polarization characteristics
of two narrow bandwidth filtered optical spectral components which is not
only insensitive to the effects of polarization mode dispersion (PMD) and
inter-channel cross-phase modulation (XPM) induced nonlinear polarization
scattering, but also independent of bit rate and modulation format. The
monitoring method is demonstrated for both linear and nonlinear systems.