In The High-speed Optical Fiber Communication Systems, Polarization Mode Dispersion Dynamic Compensation

In high-speed optical fiber systems, polarization mode dispersion (PMD) can spread optical signal, and is acknowledged as one of the serious degradation factors limiting the bit rate and the transmission distance, since it is a statistical quantity and leads to random broadening of optical transmitting signal. As the only economical solution to the severe effect of PMD on system performance, dynamic PMD compensator has an irreplaceable wide market. Accordingly, the related PMD compensating thedry and technology have been the significant topics in recent researches related to high-speed and long-haul optical communication systems.Combining our research on the National Key Technologies R&D Program in the 10th Five-year-Plan of China, and the National "973" plan of China, we have investigated many problems in this dissertation, such as the research and development of commercial dynamic PMD compensator, the degree of polarization (DOP) property of optical transmitting signal in different modulating formats and different pulse forms, the effect of chromatic dispersion (CD), signal initial chirp and polarization dependence loss (PDL) on the DOP-feedback PMD compensation, as well as the multi-channel PMD compensation techniques of fiber links. The main innovations of this dissertation are as follows:A commercial dynamic DOP-feedback PMD compensator is successfully researched and developed; the DOP properties of arbitrary signal when it suffers from PMD in non-return-to-zero (NRZ) and return-to-zero (RZ) modulated systems is presented; and the actual polarization control scheme of the PMD compensator is designed. According to the theoretical and experimental analyses of the signal's DOP properties, it can be found that the DOP sensitivity increases with, while the DOP monitoring range decreases with the signal's duty circle; and the DOP monitoring range which has been normalized by system bit time is not more than the signal's duty circle. The widest DOP monitoring range but the lowest DOP sensitivity is gotten in NRZ modulated systems, whose duty circle is two times of that in the general RZ case. In practice, the signal's duty circle could be adjusted properly in order to achieving an optimization between the effective monitoring range and a detecting sensitivity. Simultaneously, the performance of the PMD compensator using was validated in a single channel of an actual 40 Gb/s optical fiber system. It is shown that this PMD compensator can mitigate a PMD value as large as 23 ps, while its response time is not more than 20 ms. As far as we know, this is the first internal dynamic PMD compensator which directly has a commercial value, and therefore it has an important significance for the national industry. The impact of CD and initial chirp on the DOP feedback signal in PMD compensation is analyzed and demonstrated experimentally or numerically in actual 2.5, 10 and 40 Gb/s non-return-to-zero modulated systems. Initial chirp affects DOP only in the presence of CD. Signal suffering from PMD and CD only shows a larger DOP than that in the only PMDcase; while signal DOP suffering from PMD, CD and initial chirp maybe smaller/larger than DOP in the only PMD case, in accordance with the signal's narrowing/broadening caused by CD and initial chirp. Moreover, these DOP distortions caused by CD and initial chirp increase not only with the system bit rate, but also with the power difference and DGD between the two orthogonal principle states of polarization (PSP). Because the DOP distortion increases with the system bit rate and fiber CD values, CD caused by even the shortest 40 km single mode fiber span could endanger PMD compensation significantly at 40 Gb/s, which differs from the general opinion that CD doesn't affect the performance of PMD compensation. As a result, we suggest that the effect of CD on DOP should be reappraised and CD compensation should be done before PMD compensation in practice.? The impact of PDL on the DOP feedback signal in PMD compensation is analyzed andnumerically simulated in actual 40 Gb/s NRZ systems. Our analyses have provided thealways absent theory of DOP distortion in the presence of PMD and PDL although someexperimental and simulated results have been reported about the destructive effect of PDLon DOP-feedback PMD compensation, and further explained the difference about the PDLminimum endangering PMD compensation among these experiments and simulations. Wefind that PDL affects DOP only in the presence of PMD; and in the presence of PMD andPDL, DOP relates not only to both the PMD and the PDL vectors, but also to the principalstates of polarization components of the output signal; on the other hand, it adds newfrequency dependence, and is no longer independent of system bit rate. Since the PMDcompensating principle is actually to optimize the polarization splitting ratio and/or theDGD of the signal passing through the PMD compensator, if DOP still acts as the feedbacksignal in PMD compensation, and if the DOP fluctuation caused by PMD and PDL matchesthat caused by the PMD compensation algorithm, the new DOP properties in the presenceof PDL would disturb DOP and thus lead to a failure of PMD compensation. On one hand,since the magnitude of DOP fluctuation caused by PMD and PDL, i.e., two randomvariables in practice, is unpredictable, the PMD compensation algorithm would be difficultto choose its step size and be entangled by the many sub-maximums caused by PMD andPDL. Besides, the PDL minimum endangering PMD compensation is determined by thestep size of the PMD compensation algorithm, and the DGD value in optical fiber systems.DOP could no longer act as the feedback signal in PMD compensation unless PDL in thefiber system has been effectively eliminated before PMD compensation, such as takingsome PDL compensation measures, and so on.+ A multi-channel PMD compensation scheme basing on the signal's depolarization in time domain is designed. In this PMD compensation scheme, all signals of the multiplexed channels are firstly depolarized by a high-speed time-domain depolarizer, in order to achieve each signal's power equalization between two PSP axes after they transmit through optical fiber of arbitrary length; then the signals are de-multiplexed and each removed one PSP components by additional dynamic polarizer in its corresponding de-multiplexedchannel. The significance of the signal's depolarization is to avoid the random power fluctuation of each channel after its PMD compensation. In comparison with the multi-stage PMD compensation scheme which has too many freedoms and sub-optimums to control, and the worst-PMD-channel-seiective scheme which only suits some larger mean PMD values (>10ps), this scheme not only is easy to control, cost-effective, and independent of system bit rate, but also can mitigate PDL as well as multi-channel PMD. Moreover, a novel frame of the high-speed depolarizer used in this PMD compensation scheme is presented.