In situ PMD monitoring using coherent detection and polarization tracking.

摘要

Polarization mode dispersion (PMD) is a major impairment in high bit rate optical communication systems, causing system degradation. Although the random nature of PMD makes it difficult to be characterized, many measurement techniques have been developed to measure PMD and its effects on network reliability. However, the lack of in situ measurement techniques that can measure PMD on traffic carrying fibers has made it difficult for engineers to characterize the effects of PMD on wide bandwidth wavelength division multiplex (WDM) optical systems. The objective of this research is to develop an in situ PMD monitoring technique for long haul fiber optic links and use this technique to characterize the magnitude and distribution of PMD on these links.;Towards this end, a systematic approach was followed to develop a monitoring equipment that can measure PMD on traffic carrying links. First, an earlier implementation of the PMD monitoring equipment based on coherent detection and polarization scrambling [16] was improved in terms of size, speed and accuracy to make it more suitable for field measurements of PMD in traffic carrying fiber optic links. The coherent PMD monitor can measure differential group delay (DGD) values in the range of 0 to 50 ps. Secondly, using theoretical analysis, it was ascertained that the magnitude of PMD, the DGD measured by the PMD monitor, is the apparent DGD of the fiber and not its true DGD. Mathematical analysis was used to derive a relationship between the true DGD and the apparent DGD of the fiber. Also, it was found that the distribution of the apparent DGD is Rayleigh, unlike the true DGD which is Maxwellian.;Thirdly, the hardware and software for implementing a polarization tracking algorithm to measure PMD was developed and tests were conducted to validate the algorithm in terms of speed, accuracy and the characteristics of the measured DGD. The polarization tracking algorithm has a higher measurement speed and lesser memory requirements than polarization scrambling. A number of laboratory experiments and field trials on traffic carrying fibers were conducted for a comparative analysis of polarization scrambling and polarization tracking. Using the polarization tracking algorithm to measure DGD, the measurement speed was found to be 20 times higher and the memory requirements about 80 times less than the memory required for DGD measurements using polarization scrambling. Results of the laboratory experiments and field trials agree with our theoretical analysis and the two algorithms have similar statistics for the measured DGD. Finally, the possibility of a more efficient implementation of polarization tracking was explored to measure PMD in real time. A run time implementation with the existing hardware and software was developed where the advantages of polarization tracking over polarization scrambling was made evident. The use of the in-situ PMD monitoring technique will enable network engineers to monitor the impact of PMD in live traffic carrying links and to select the wavelength bands that are relatively less affected by PMD.