Ivan T. Lima

Time and frequency domain characteristics of polarization-mode dispersion emulators

We investigate both experimentally and theoretically a new technique to realistically emulate polarization-mode dispersion (PMD). We propose and demonstrate a PMD emulator using rotatable connectors between sections of polarization-maintaining fibers that generates an ensemble of high PMD fiber realizations by randomly rotating the connectors. It is shown that: (1) the DGD of this emulator is Maxwellian-distributed over an ensemble of fiber realizations at any fixed optical frequency; and (2) the frequency autocorrelation function of the PMD emulator resembles that in a real fiber when averaged over an ensemble of fiber realizations. A realistic autocorrelation function is required for proper emulation of higher order PMD and indicates the feasibility of using this emulator for wavelength-division-multiplexing (WDM) systems.

Comparison of polarization mode dispersion emulators

We analyze polarization mode dispersion (PMD) emulators comprised of a small number of sections of polarization-maintaining fibers with polarization scattering at the beginning of each section. Unlike previously studied devices, these emulators allow the emulation of a whole ensemble of fibers. We derive an analytical expressions and determine two main criteria that characterize the quality of PMD emulation. The experimental results are in good agreement with the theoretical predictions.

Study of system performance in a 107-km dispersion-managed recirculating loop due to polarization effects

We investigate the polarization evolution for both signal and noise in two 107-km recirculating loops with polarization-dependent loss per round-trip of 0.35 dB and less than 0.1 dB, respectively. When the system is optimized, in the first case, both signal and noise are polarized, while in the second case, the signal tends to depolarize due to the noise. We experimentally measured and theoretically simulated the Q factor distribution, which is far from what is expected for straight-line systems, after 5000 km in the second case. We also suggest a simple method for obtaining the same Q distribution in recirculating loop experiments as expected in straight-line experiments.

Analysis of PMD compensators with fixed DGD using importance sampling

In this letter, we use importance sampling to analyze polarization-mode dispersion compensators with a constant differential group delay (DGD) element. We optimize the value of the fixed DGD element of the compensator with respect to the outage probability. We show that the optimum value of the fixed DGD element of the compensator can reduce the outage probability by several orders of magnitude, even though it does not provide a substantial reduction of the average penalty due to polarization-mode dispersion in the cases that we studied. By contrast, choosing the fixed DGD element to maximally reduce the average penalty may lead to an outage probability that is orders of magnitude larger than the optimal choice.

A novel polarization diversity receiver for PMD mitigation

We present a novel polarization diversity receiver for mitigating polarization-mode dispersion (PMD). We show that with simple fixed optics and electronics we can significantly decrease the outage probability due to PMD and can achieve better performance than an electronic equalization solution using no diversity. By incorporating equalization into the diversity receiver structure, further performance improvements can be achieved and the intersymbol interference due to higher order PMD distortions can be reduced. We demonstrate the performance advantages of the proposed diversity receiver using importance sampling to accurately calculate the outage probability due to PMD.

Error estimation in multicanonical Monte Carlo Simulations with applications to polarization-mode-dispersion emulators

This paper shows how to estimate errors in multicanonical Monte Carlo (MMC) simulations using a transition-matrix method. MMC is a biasing Monte Carlo technique that allows one to compute the probability of rare events, such as the outage probability in optical-fiber communication systems. Since MMC is a Monte Carlo technique, it is subject to statistical errors, and it is essential to determine their magnitude. Since MMC is a highly nonlinear iterative method, linearized error-propagation techniques and standard error analyses do not work, and a more sophisticated method is needed. The proposed method is based on bootstrap techniques. This method was applied to efficiently estimate the error in the probability density function (pdf) of the differential group delay (DGD) of polarization-mode-dispersion (PMD) emulators that has been calculated using MMC. The method was validated by comparison to the results obtained using a large ensemble of MMC simulations.

A comparative study of single-section polarization-mode dispersion compensators

This paper shows how to use multiple importance sampling to study the performance of polarization-mode dispersion (PMD) compensators with a single differential group delay (DGD) element. We compute the eye opening penalty margin for compensated and uncompensated systems with outage probabilities of 10/sup -5/ or less with a fraction of the computational cost required by standard Monte Carlo methods. This paper shows that the performance of an optimized compensator with a fixed DGD element is comparable to that of a compensator with a variable DGD element. It also shows that the optimal value of the DGD compensator is two to three times larger than the mean DGD of the transmission line averaged over fiber realizations. This technique can be applied to the optimization of any PMD compensator whose dominant sources of residual penalty are both the DGD and the length of the frequency derivative of the polarization-dispersion vector.

Statistical analysis of the performance of PMD compensators using multiple importance sampling

In this letter, we evaluate the performance of fixed and variable differential group delay (DGD) polarization-mode dispersion (PMD) compensators as the first- and second-order PMD varies using multiple importance sampling. We show that importance sampling yields estimates of the average penalty with low variance over the entire region of interest of first- and second-order PMD. We also show that there is little advantage in using a compensator with a variable-DGD element and that the performance of a compensator that minimizes the residual DGD at the central frequency of the channel is considerably worse than a compensator that maximizes the eye opening.

Comparison of penalties resulting from first-order and all-order polarization mode dispersion distortions in optical fiber transmission systems

We compare the eye-opening penalty from a first-order polarization mode dispersion (PMD) model with that from an all-order PMD model in optical fiber transmission systems. Evaluating the performance by taking into account only first-order PMD produces a good approximation of the true eye-opening penalty of uncompensated systems when the penalty is low. However, when the penalties are high, this model overestimates the penalty for outage probabilities in the range of interest for systems designers, which is typically approximately 10(-5) to 10(-6).

Statistics of the system performance in a scrambled recirculating loop with PDL and PDG

We have demonstrated that with the loop-synchronous scrambling technique, the Q distribution of a recirculating loop closely resembles that of a straight-line system. By carefully choosing the scrambling rate, we show that slow scrambling at the transmitter improves the system performance and reduces the performance variation. We investigate the system performance for different polarization-dependent loss (PDL) levels and obtain excellent agreement between the experimental and simulation results. Our results show, for the first time, that the repolarization of the noise due to significant PDL causes an asymmetric Q-factor distribution.