Introduction
As the development of all-optical communication networks continue to progress and with the advent of all-optical network elements, e.g. all-optical cross-connects, the need for performance monitoring in the optical domain is increasing in order to maintain monitoring of signal quality, ensure quality of service, etc.; The central purpose of optical performance monitoring (OPM) is to measure the optical signal-tol-noise ratio c (OSNR) since it is related to the bit-error rate of the system. It has previously been shown that it is possible to determine the OSNR by utilizing the polarization properties of the light [1], [2]., i.e. the degree of polarization (DOP). The advantage with this method compared to conventional OSNR measurement is primarily that the method provides a true in-channel OSNR measurement. However, in presence of polarization-mode dispersion (PMD) the state of polarization (SOP) may vary within the signal, which means that the signal power can be interpreted as depolarized with an incorrect underestimation of the OSNR as a result. Recently, approximate methods to overcome this problem were presented [3], [4]. Here, we propose a more accurate method including spectral SOP measurements [5], [6] to characterize and separate the underestimation of OSNR due to PMD and for the first time we perform spectral SOP measurements to enable OSNR monitoring. We demonstrate the performance of the measurement technique by measuring on a signal with OSNR = 25 dB, where the, input SOP into three different birefringent fibers is randomized. In addition, the method characterize the PMD-induced depolarization of the system, useful for e.g. PMD compensation.