Abstract

Polarization dependent loss (PDL) in optical systems has the potential of considerably degrading system performance by reducing the signal to noise ratio (SNR).^1,2 It has become particularly important in long terrestrial systems were components are required to be cheaper than in undersea cable systems of a comparable reach, resulting in much higher PDL values. The noise enhancement is generated by two mechanisms, one is related to noise emitted directly into the polarization state of the amplified signal, and the other is related to the noise emitted into an orthogonal polarization state. It is well known that the OSNR penalty caused by the first mechanism can be mitigated by including periodic, channel-by-channel gain equalization along the link. In principle, its effect can be completely eliminated if such equalization is implemented at every amplifier location. The second mechanism, related to the orthogonal noise, is different in several ways. First, the nature of PDL is such that if the transmitted signal experiences larger than average attenuation then the noise that propagates in the orthogonal polarization state is unavoidably experiencing less than the average attenuation, making the signal to orthogonal noise ratio lower.^1,2 However the key feature that increases the importance of the orthogonal noise is related to the fact that in the presence of PDL some of the orthogonal noise is coupled back into the polarization of the signal in the process of propagation. This portion of the orthogonal noise beats with the signal as it impinges upon the photo-detector, giving rise to a potentially significant degradation in performance.

© 2002 Optical Society of America