| Since the initial application of optical fiber communication commercially, the technology has been advanced significantly in the past four decades. Extensive studies demonstratd that, in communication technology, optical fiber posses many applicable advantages, including properties of wide frequency bands, significant transmitting capacity, lower attenuation, anti-electromagnetism interference, light mass quantity, and as well as easy acquiring of the raw materials for manufacturing of optical fiber . Possessing those special properties, optical fiber has been used in numerous fields of wired communications, such as radio communications, telecommunications, electric power communications and, as well as military communications. In the 21st century, optical fiber communication still has great potential for further development and commercial application. In China, copper wires have been gradually phased out in commercial telecommunication system. Instead optical fiber communication is being extensively used for individual users and homes in both urban and rural areas. As a new concept, the"three in one network"requires the communication network has comprehensive ability in transmitting voice, pictures and diagrams. Such new commercial applications require optical fiber communication towards to even higher speed, longer range distance and less attenuation during transmission.Nevertheless, certain physical properties of optical fiber may slow down its future development and applications. These properties include attenuation, nonlinearity and dispersion during signal transmission. To overcome these problems, Raman fiber amplifier has provided a new option to minimize the limit of the attenuation in long range distance transmission. The availability of various new types of optical fiber also surmounts various nonlinearity effects. Unfortunately, polarization mode dispersion (PMD) in optical fiber communication, in particular in lower speed communication condition, has become the bottleneck in optical fiber communication. Based on the literature available, it is unlikely there are any effective options available to deal with the bottleneck caused by PMD, especially for the high order of PMD. According to the above background, we carried out the study on high order of PMD in high speed optical fiber communication using an adaptive compensation method.Assuming the cross section of a optical fiber is perfect round, under an ideal condition, a given light that enters into an optical fiber from an optimistic angle would result in a total reflection of the light signal which forms the base of optical fiber communication. However, in practical, the cross section of an optical fiber may be ellipse due to internal stress or external bending force. Also, the efficiency of signal transmission in optical fiber can be affected by the ambient temperature, magnetic field and vibration. All those factors would contribute into PMD in real time optical fiber communication. It is important to highlight here the randomized characteristics of PMD in optical fiber caused mainly by external factors and the variations of technical processing (e.g the uneven density of each optical fiber). To understand the randomized characteristics of PMD, we believe it is essential to gather all information related to PMD using a statistical approach to analyze its overall trend, relationships and interactions between all factors as the first step. Based on a polarization-maintaining fiber model, the properties of a third-order PMD (TOPMD) vector are studied in our laboratory by using the Jones Transmission Matrix method. Interesting results on TOPMD and its component's histograms of their probability density. It is concluded that the component in direction plays the most important role to the TOPMD among all the components.It is understood that a polarization mode dispersion emulator (PMDE) should be built to emulate all orders of PMD characters prior to the establishment of an adaptive compensation system. However, because of the difficulties in building a PMDE based on the PMF cascading model, we adopted a"PC+PMF"model to establish the PMDE using the results of our statistical analyses. In the paper, on the basis of the first order of PMDE established previously, we improved the second order of PMDE and then modified one's design to maximize the outcome in demonstrating the theoretic characters. In addition, the third order of PMDE designed in this study formed the foundation for a higher order of PMD compensator.According to the histograms of the PMD, the statistical properties indicated that the first order of PMD vector obeys the Maxwell distribution; the second order of PMD vector is divided into two components, showing one in the p) direction and the other one in the direction (i.e. PSP). Amongst the two directions, the p)ωdirection component has the major impact on the whole PMD vector. The third order of PMD vector can be divided into three directions, including the p) direction, the direction and the p)ωωdirection. Among the three components, the direction has the primary impact on the whole vector. Given that there are three ways in compensating the PMD, i.e. optical domain compensation, electrical domain compensation and optical-electrical domain compensation, this paper focuses on the optical domain compensation to design a three-stage PMD compensator. As expected this design can compensate all the first order of PMD and part of the higher order of PMD, including the direction component of the second order of PMD and the p)ωωdirection component of the third order of PMD. If a compensator for all of the first and second oders of PMD and the direction component of the third order of PMD is required, a four-stage compensator could then be applied.Because of the randomized characters of PMD in optical fiber communication, it would be critical to find the solution on how to compensate the PMD in real-time on the linkages of optical fiber. In comparison with many existing studies based on searching algorithm, we found that the PSO algorithm can search and located the full picture of the feedback signals at their maximum values quickly and accurately. Such action can also minimize the possibility of the local maximum values, which is especially suitable for multi variant situations. The PSO algorithm method used in this study provides a suitable technique in searching and track the online detection and feedback signal-DOP, resulting in a optimal compensation system with high adaptability.It is reported in the literature that, in actual optical fiber systems, the requirement for an effective communication is met provided that the compensation system of PMD can respond within millisecond order. Yet in the experimental system based on a computer+board, the searching time is among hundreds milliseconds order and the tracking time is about tens of milliseconds order. The improved logic control unit based on DSP may have reduced the compensation time to a certain extent, but it is still too hard to achieve an optimal outcome in the real optical fiber communication. This is because that DSP has to receive and deal with large quantity of orders from A/D and D/A, while the speed of calculations is capped within a relative slow mode.In this paper, we use a DSP+GPGA model in the logic control unit in order to enhance the compensation speed and reduce the responding time for the compensation. Given that there are thousands of searching tables and trigger mechnisms in FPGA, the FPGA site can produce much fast processing speed than the general DSP with relatively less financial cost. For example, in our study using a FPGA with two millions gates, the transmitting speed could reach 128 billion MAC per second, which is significantly quicker than the fastest DSP currently available. Using FPGA to deal with orders from A/D and D/A in our experimental system, the amount of processed orders by the DSP chip has been reduce by 89%, showing more capacity for calculations. Our data indicate that the average responding time for compensation is 13.6ms, which accounts only for 45% of the responding time for compensation under the PMD system. |
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