At present, polarization mode dispersion (PMD) has become an important limiting factor for the performance of long distance transmission ,high rate data and analog communication systems. This paper introduces the principle and characteristics of PMD, and tells the causes and effects of it. We also report the standardized techniques for measurement of PMD according to the ITU-T Recommendation G.650.At the end ,we show some experimental result in our project.
To learn the surrounding conditions in the fiber link and its effect on PMD, and to provide the first-hand design basis, we have carried out the data observation of PMD in a fiber link for a long time. We have tested the first-order and second-order PMD. The fiber tested is the G652 fiber produced by Corning Co. of USA, and the testing distance is 1000km; n segments of same fibers are linked into one, and n equals to 40, that is to say, the length of every segment is 25km; for the requirement of dispersion compensation in the high-speed and long distance fiber optical communication system, one fiber grating dispersion compensator is added in the place of every 200km, and there are five compensators; one EDFA is added in the place of every 100km, and there are eleven EDFA. The result suggests that, with the increase of length of fiber link, the distribution of PMD intends to be stable, that is, with the number n increasing, the relative error of PMD becomes less.
The testing methods are the Jones matrix eigenanalysis technique and interference technique. HP8509B fiber polarization analyzer of Agilent in USA is used for measuring instrument of the Jones matrix eigenanalysis technique; FPMD-5600 Femtosecond PMD Analyzer of EXFO in Canada is used for measuring instrument of interference technique. The difference between these two testing methods is analyzed.
With the Jones matrix eigenanalysis technique, fibers of 1000km are inspected through 48 hours, and the result suggests that, at nine o'clock in the morning, PMD reaches the maximum, at nine o'clock in the evening, it reaches the minimum, during other time, its change is very little. So it can be concluded that, PMD in the long distance fiber link is affected by temperature of the lab. Stress testing is carried in the ultra-short fiber (less than one meter). PMD has no obvious change in the range of stress which can be endured by the fiber.
In this paper, the optimization algorithms are introduced in adaptive PMD compensation in 10Gb/s optical communication system. The PMD monitoring technique based on degree of polarization (DOP) is adopted. DOP can be a good indicator of PMD with monotonically deceasing of DOP as differential group delay (DGD) increasing. In order to use DOP as PMD monitoring feedback signal, it is required to emulate the state of DGD in the transmission circuitry. A PMD emulator is designed. A polarization controller (PC) is used in fiber multiplexer to adjust the polarization state of optical signal, and at the output of the fiber multiplexer a polarizer is used. After the feedback signal reach the control computer, the optimization program run to search the global optimization spot and through the PC to control the PMD. Several popular modern nonlinear optimization algorithms (Tabu Search, Simulated Annealing, Genetic Algorithm, Artificial Neural Networks, Ant Colony Optimization etc.) are discussed and the comparisons among them are made to choose the best optimization algorithm. Every algorithm has its advantage and disadvantage, but in this circs the Genetic Algorithm (GA) may be the best. It eliminates the worsen spots constantly and lets them have no chance to enter the circulation. So it has the quicker convergence velocity and less time. The PMD can be compensated in very few steps by using this algorithm. As a result, the maximum compensation ability of the one-stage PMD and two-stage PMD can be made in very short time, and the dynamic compensation time is no more than 10ms.
EDFAs are broadband optical amplifiers, which are increasingly replacing conventional optoelectronic regenerators. In high-speed data transmission they have to be designed for low polarization dependence to prevent a rise in the bit error rate. Long links like undersea or transcontinental cables contain many EDFAs, this way multiplying their polarization dependencies. Therefore, EDFAs have to be characterized carefully for polarization mode dispersion (PMD) and polarization dependent gain (PDG). In this paper, the PMD measurement is achieved with a modified Stokes vector method. In this method, the polarization controller is used to generate a number of input polarization states, all of which arelocated on an unit circle on the the North Pole of Poincare sphere. For each state, a small wavelength step is applied. Using the Polarization Analyzer, the arc on the Poincare sphere, which is caused by changing the wavelength, is determined. The arc is known to vary sinusoidally with the input polarization state. The widest arc is the measure for the PDG. The measuring system includes HP8164A Tunable Laser, HP8509B Polarization Analyzer, HP 8169A polarization controller and HP8153A Power Meter etc. Four dedicated polarization states (0° lin., 45° lin., 90° lin., circular) are set and the power without device-under-test (DUT) is measured. Then the EDFA is inserted and the power is measured again. This delivers the first row of the Mueller-Matrix from which the PDG can be calculated.