To monitor the performance of wavelength-division-multiplexed (WDM) networks, it is essential to measure the optical-to-noise ratio (OSNR) of each channel. Recently, several techniques have been proposed to monitor the OSNR in WDM networks. However, none of these techniques could be used in the realistic WDM network environment due to various limitations.
In this thesis, a simple technique has been proposed and demonstrated for monitoring the OSNRs of WDM signals. This technique, based on the polarization-nulling method, was implemented by using a rotating quarter-wave plate and rotating linear polarizer. Therefore, the OSNR could be monitored without using the complex polarization control loop. However, the performance of this technique could be affected by nonlinear birefringence and polarization-mode dispersion (PMD). When the transmission rate is lower than 10-Gb/s, the dominant source of the measurement error for the proposed OSNR monitoring technique is the effect of nonlinear birefringence. However, as the transmission rate increases beyond 10-Gb/s, PMD becomes increasingly important while the effect of nonlinear birefringence is reduced gradually. Therefore, this technique is suitable for monitoring the OSNRs of high-speed (＞ 10-Gb/s) WDM signals transported over the fiber link with low PMD.
To negate the deleterious effects of PMD and nonlinear birefringence, it has been also proposed to modify the polarization-nulling method by using an additional optical bandpass filter. The results show that the modified technique could monitor the OSNR with accuracy better than 1 dB, regardless of the bit-rates, even when the transmission link had large PMD and low dispersion (thus, large nonlinear birefringence).
