As the transmission speed and distance are continued to be increasing in modern lightwave systems, the effect of polarization, such as polarization mode dispersion (PMD) and polarization dependent loss (PDL), imposes serious limitations on their performances. In particular, PMD has been identified as one of the most critical limiting factors for the high-speed optical transmission system. To overcome this problem, there have been substantial efforts to understand the nature of PMD and develop an efficient method to mitigate its effect. As a result, various techniques for the emulation, monitoring, and compensation of PMD have been proposed and demonstrated, as well as the theoretical models describing the PMD dynamics and statistics.
PMD represents the temporal difference of group delay between two orthogonal polarization modes. It arises from the slight geometrical asymmetry of optical fiber caused by the imperfect manufacturing process or the external stress occurred in cabling and installation processes. The most important characteristics of PMD should be attributed to the random nature. It varies randomly with ambient temperature and external stresses. Thus, unlike chromatic dispersion, the effect of PMD is not deterministic. As a result, the effect of PMD is often described by the outage probability instead of power penalty. The typical range of outage probabilities is about 10$^{-4}$ in optical transmission systems.
The temporal drift of PMD caused by the temperature changes in an embedded fiber is slow. It has been reported that the typical correlation time of PMD drift is in the order of several hours. Thus, it could take extremely long time to measure the outage probability of about 10$^{-4}$ in a real transmission system. To overcome this problem, a PMD emulator is often used to generate the realistic PMD rapidly in laboratory. Previously, several techniques have been proposed to emulate PMD by using multiple sections of high-birefringent fiber or birefringent crystals. However, these PMD emulators are difficult to implement and control in practice, as they require a large number (>15) of sections to achieve a good distribution. In addition, these emulators are inconvenient since their configurations should be changed to vary the statistics of the emulated PMD. Furthermore, even if these emulators are implemented by using a large number of DGD sections, the emulated PMD is typically much smaller than the value expected by Maxwellian distribution at the tails. Thus, when these emulators are used, the system's outage probabilities could be significantly underestimated in the rare differential group delay (DGD) events. In this dissertation, a novel PMD emulator capable of emulating the PMD of real fiber including higher-order PMD accurately is proposed to overcome these problems. The proposed emulator was implemented by using variable DGD elements, polarization controllers, and a microprocessor. The results show that, by using only five sections of DGD elements, an excellent statistical distribution not only in the first-order but also in the second-order can be obtained with reasonable autocorrelation characteristics (< 30%). In addition, unlike the previously proposed PMD emulators, this emulator can generate various PMD statistics without changing its physical configuration.
For the efficient PMD compensation, an accurate PMD monitoring is essential. It has been well known that the intensity of the RF signal measured at the receiver is strongly correlated with the PMD experienced by the optical signal. Accordingly, there have been many efforts to utilize this correlation for the PMD monitoring. For example, it has been reported that the RF spectrum of the received signal was filtered at the sub-harmonic frequency, such as 0.5/T or 0.25/T, where T is the bit interval, by using a narrow electrical band-pass filter (BPF), and then used for the PMD monitoring. However, these reports considered the correlation between the intensity of the RF signal and the first-order PMD only. Thus, the effect of the higher-order PMD on the intensity of RF signal is yet to be clarified. Since the higher-order PMD induces the frequency-dependent variation of PMD vector, the correlation between the intensity of RF signal and higher-order PMD can be seriously affected by the monitoring bandwidth (i.e., the bandwidth of electrical BPF). In this dissertation, the effects of the monitoring bandwidth on the performance of PMD monitoring technique based on RF spectrum analysis under the influence of higher-order PMD is analyzed. For this analysis, the monitoring sensitivity and the BER improvement achievable by the PMD compensator are investigated as a function of the monitoring bandwidth. The results show that the monitoring bandwidth should be reduced to improve the monitoring sensitivity, while it should be increased for the BER improvement. Thus, considering both monitoring sensitivity and BER improvement, the monitoring bandwidth should be about 10 % of the signal's bit rate.
To overcome the limitations imposed by PMD, various techniques have been proposed and demonstrated. However, most of these techniques have been demonstrated in single-wavelength system rather than in WDM systems. In addition, these techniques did not consider the effects of fiber nonlinearities, although these are the most important sources of crosstalk in WDM systems. Recently, it has been reported that the nonlinear birefringence originated from optical Kerr effect (OKE) can generate the nonlinear crosstalk and impair the system's performance when it interacts with PDL. Nonlinear crosstalk can also be generated by the nonlinear birefringence interacting with PMD. As a result, this nonlinear birefringence could deteriorate the performances of the first-order PMD compensators in WDM systems, However, such a phenomenon has been shown by numerical simulation only. In this dissertation, the effect of nonlinear crosstalk on the PMD compensation is investigated theoretically and experimentally. An analytical model was developed to describe the effects of nonlinear crosstalk on the first-order PMD compensation. The validity of this model was confirmed by numerical simulation and two-channel WDM transmission experiment. The effect of nonlinear crosstalk on PMD compensation was evaluated by measuring the performance degradation after completely compensating the first-order PMD, The results show that, in the presence of the OKE-DGD crosstalk, the first-order PMD compensation could degrade the performance of WDM system rather than improve it.
The principal state-of-polarization (PSP) transmission is one of the common techniques used to avoid the PMD-induced distortion. This technique utilizes a polarization controller at the transmitter's site to align the state-of-polarization (SOP) of the input optical signal to the fiber's PSP. Thus, this technique can prevent the signal's distortion caused by PMD, However, it should be noted that this technique can only compensate the first-order PMD. As a result, the performance of this technique can be affected by the variation of the PSP caused by higher-order PMD. In this dissertation, a simple technique is proposed to improve the performance of the PSP transmission technique by modulating the SOP of the input optical signal at the clock frequency. The results show that, when DGD was smaller than 80 ps, the proposed technique could improve the receiver sensitivity up to 1 dB for 10-Gb/s signal by transforming the NRZ signal into a RZ signal. This was in contrast with the previously proposed PMD compensation technique, which was merely capable of reducing the degradation of the receiver sensitivity rather than improving it. The proposed technique also improved the performance of the PSP transmission in the presence of the higher-order PMD. With a simple modification, this technique can also be used to prevent the impairments of various polarization effects such as PMD, PDL, and polarization-dependent gain (PDG) simultaneously. It has been reported that the degree-of-polarization (DOP) of the propagating light should be decreased as low as possible to negate the effects of PDG and PDL. Using the proposed technique, DOP could be reduced to <25 %, regardless of the DOD values. As a result, the proposed technique could minimize the performance degradation caused by PDL or PDG in long-haul WDM transmission systems. The proposed technique also improved the receiver sensitivity of 10-Gb/s signal up to 1.3 dB, when DGD was less than 100 ps.
The optical signal-to-noise ratio (OSNR) has been used widely to estimate the performance of WDM system. However, it has been recently reported that such performance estimation could become inaccurate if the ASE noise is partially polarized by PDL. Since there are numerous PDL elements along the optical transmission link, the partially polarized ASE noise could also cause large measurement errors in the OSNR monitoring technique based on polarization-nulling. In this dissertation, an exact analysis of the DOP statistics of ASE noise has been studied. The result shows that the probability density function of the DOP of ASE noise in the transmission link with a large number of spans has a Maxwellian shape.
