OSNR monitoring technique using polarization-nulling method

We report on the simple technique for monitoring the optical signal-to-noise ratios (OSNRs) of wavelength-division-multiplexed (WDM) signals. This technique, based on the polarization-nulling method, was implemented simply by using a rotating quarter-wave plate and rotating linear polarizer. However, the performance of this technique could be affected by nonlinear birefringence and polarization-mode dispersion (PMD). The result shows that this technique is suitable for monitoring the OSNRs of high-speed (>10-Gb/s) WDM signals transported over the fiber link with low PMD.