Optical PMD Compensation Research Articles

High-speed polarization mode dispersion compensation in a 43-Gb/s RZ-DQPSK transmission system over 1200 km of standard single-mode fibre

This paper reports that the designed optical polarization mode dispersion compensator shows a good performance under the real-time variation of differential group delay, state of polarization and principal state of polarization in a (40×43)-Gb/s dense-wavelength-multiplexing, 1200-km enhanced return-to-zero differential-quadrature-phase-shift-keying (RZ-DQPSK) system. The polarization mode dispersion tolerance of the system is improved by 26 ps using the optical polarization mode dispersion compensator. The short and long time stabilities are tested with the bit error ratio recorded.

Optical polarization mode dispersion compensators for high-speed optical communication systems

Optical polarization mode dispersion compensators for high-speed optical communication systems

An experiment of PMD compensation in 40-Gb/s PSBT transmission system

An adaptive polarization mode dispersion (PMD) compensation experiment is reported in a 40-Gb/s phase shaped binary transmission (PSBT) communication system, with the use of a new digital signal processor (DSP)-based optical PMD compensator. PMD tolerance is found to be enhanced by 8 ps after PMD compensation with 1-dB optical signal-to-noise ratio (OSNR) penalty. Under the condition of fast change of states of polarization up to 85 rad/s in the fiber link, the performance of our PMD compensator undergoes the bit error ratio (BER) test for as long as 10 h.

Design, estimation and experimental validation of optical Polarization Mode Dispersion Compensator in 40 Gbit/s NRZ and RZ optical systems

Polarization Mode Dispersion (PMD) compensation has been a matter of investigation of several papers in literature. The proposed solutions belong basically to two large families: electronic compensators and optical compensators. Both PMD compensator schemes have advantages and disadvantages: electronic PMD compensators are usually simple to include in line-terminal, potentially low-cost, very fast, and FEC compatible but their development is strongly dependent on IC technology capability that at this time allows device developments up to the 10 Gbit/s bit rates area. Furthermore, they are strongly dependent on modulation formats, and they can operate only on a single channel. Conversely, optical compensators are independent of bit rate and modulation format and potentially they can compensate more channels simultaneously; their major drawbacks are the longer response time and the complexity in the feedback signal process within the control algorithm. In this paper we consider an optical Polarization Mode Dispersion Compensator (PMDC) that is simple to realize and easy to include at limited costs in each EDFA module (distributed compensation) as well as a single-stage front-end compensator. Numerical analysis of the PMDC and experimental results confirm the utility of the PMDC proposed and its capability in compensating DGD larger than 20 ps in NRZ and RZ 40 Gbit/s optical systems.

PMD Compensation/mitigation techniques for high-speed optical transport

In high-speed long-haul optical transport systems with channel data rates at 40Gb/s and beyond, polarization mode dispersion (PMD) is one of the major obstacles that limits system performance. It is desired or in most cases required to increase the system tolerance to PMD through PMD compensation and mitigation. In this paper, we review the techniques of PMD compensation and mitigation, including per-channel based optical PMD compensators (PMDCs), analog and digital electronic PMDCs, and all-channel PMD mitigation using distributed fast polarization scrambling and forward error correction (FEC). With the emergence of digital coherent detection, the signal optical field becomes digitally available, enabling powerful digital PMDC techniques to be applied. Digital PMDC techniques in optical single carrier coherent systems and orthogonal frequency division multiplexing (OFDM) systems are briefly reviewed.

A Change of Perspective on Single- and Double-Stage Optical PMD Compensation

We present a rigorous investigation on how to optimize the degrees of freedom of optical polarization mode dispersion (PMD) compensators composed of differential group delay sections and polarization controllers, up to two stages. The analytical treatment relies on the extracted Jones matrices of the transmission and compensation fibers. The analysis of a single-stage compensator with two degrees of freedom (fixed DGD) is based on the maximization of the eye opening, as provided by the generalized Chen formula. The outage probability is quantified through a fast semi-analytical technique. It is shown how the benefits of single-stage compensation are strongly reduced and can lead to outage events, when certain critical input states of polarization are launched into transmission fibers with strong eigenmodes depolarization (i.e., strong higher order PMD). Focusing on such transmission fibers and input configurations, a novel algorithm is introduced for controlling a double-stage compensator with five degrees of freedom. The algorithm is based on an ideal equalization of the transmission fiber at half the bit-rate, realized resorting to spherical geometry. To this aim, we show that the first compensator stage must be a PMF fiber with very large DGD, equal to the bit period, in order to compensate the most critical configurations associated with outage events.

Distributed PMD Compensator in Lithium–Niobate–Tantalate: Performance Modeling Toward Highest Bit Rates

Polarization mode dispersion (PMD) is caused by noncircular fiber cores and poses a serious threat for transmitting 10-Gb/s optical signals over older fibers and 40-Gb/s optical signals over any type of fiber. We study the architecture of a PMD compensator (PMDC) capable of 40-Gb/s operation in X-cut Y-propagation lithium niobate (LiNbO3) based on cascaded integrated optical TE-TM mode converters with endlessly adjustable coupling phases and propose several improvements in its architecture to tailor its performance toward highest bit rates. The performance of such distributed PMDCs can be pushed toward highest bit rates of 160 and 640 Gb/s if they are implemented in mixed ferroelectric crystals such as lithium niobate tantalate (LiNb1-y/Tay/O3) or lithium tantalate (LiTaO3) crystals, respectively. A tantalum (Ta) content y of up to 0.5 is good to realize a distributed PMDC for about 160-Gb/s operation. Two- and three-phase TE-TM mode converters for integrated optical PMD compensation are compared, and the latter are found to have slightly better electrooptic efficiency. For Z-cut lithium tantalate, four-phase electrodes which need only two independent operation voltages are found to be more attractive.

Higher-order optical polarization mode dispersion compensator evaluation using Markov chain Monte Carlo simulation

We describe how to use Markov chain Monte Carlo simulation (MCMC) to accurately estimate the outage probability of the optical communication system compensated by higher-order optical polarization mode dispersion (PMD) compensators. Markov chain fiber channel samples can drastically reduce the number of simulations required to estimate the outage probability of the compensated systems to 10 –6 or less. Using MCMC, the efficiency of our proposed higher-order optical PMD compensator is proved further, and the accuracy of MCMC is affirmed.

New optical compensator for higher-order polarization mode dispersion in optical fiber channels

We propose a new optical compensator for both first-order and higher-order polarization mode dispersion (PMD) effects in an optical fiber channel. The compensation scheme needs just one more polarization controller and one more variable differential group delay (DGD) element than first-order optical compensators with adjustable DGD elements. And the control algorithm for this compensator is easier and more stable than that for the typical higher-order optical polarization mode dispersion compensators. By an advised control method, the proposed compensator can always have better performance than first-order optical compensators. Numerical results are given to prove the efficiency of the proposed compensation scheme.

Extended Jones matrix for first-order polarization mode dispersion

I present numerical simulations of the average transfer function of polarization mode dispersion (PMD) in optical fibers conditioned on various given values of the differential group delay (DGD). I find that even fibers with relatively small mean DGD can exhibit significant coupling between the two principal states of polarization. The average frequency dependence of this coupling can be approximated by a generic analytic function that deviates substantially from the quadratic frequency dependence that is often assumed in second-order PMD models. Finally, I define an extended transfer matrix for first-order PMD that describes the average frequency dependence of all PMD-induced distortions as a function of the DGD and show that this matrix is much better suited for optical PMD compensation than that of a conventional first- and second-order PMD model.

Improved Phase Delays for Optical Compensation of Polarization-Mode Dispersion

We present numerical simulations of the average frequency dependence of the differential phase delays (DPDs) that are introduced by polarization-mode dispersion (PMD) between the signal components transmitted in the two principal states of polarization of the fiber. Our study confirms that the first-order approximation of frequency-independent DPDs is valid for fibers that normally do not require PMD compensation. However, for fibers with larger mean differential group delays (DGDs), the first-order approximation tends to overestimate the mean DPDs for any given DGD. Based on our numerical results, we derive closed-form approximations for the frequency and group-delay dependence of the mean DPD conditioned on a given DGD and show that these more accurate DPDs may significantly improve the performance of optical PMD compensators.

Polarization mode dispersion compensation in optical and electrical domain and their limitations

Polarization mode dispersion (PMD) compensation is one of the challenges to deployment of ultra-high-bit-rate systems. PMD impact on system performance and outage probability is explained. Then both optical and electrical PMD compensation techniques are reviewed and important design considerations are discussed. Issues regarding PMD-induced distortion detection, feedback and control algorithms in the presence of local minima, and interaction between PMD and other degrading effects [such as chromatic dispersion, polarization-dependent loss (PDL), and fiber nonlinearities] are discussed.

Practical Considerations for Optical Polarization-Mode Dispersion Compensators

For transmission systems at 10 Gb/s and beyond, polarization-mode dispersion (PMD) is one of the limiting factors. Optical PMD compensators aim at increasing the PMD value tolerated by the system; however, they do not cancel out its effects. Therefore, the performance of a PMD compensator is assessed statistically. Requirements for optical PMD compensators include a response time in the range of 1 ms in order to follow polarization fluctuations over the line. The system design should account for the interaction of other transmission impairments with the PMD compensator operation. For instance, transmitter chirp and residual chromatic dispersion have a deleterious impact on the compensator performance. While self-phase modulation is harmless, cross-phase modulation greatly reduces the compensator efficiency. System design rules have been applied to a one-year field trial, showing the compensator's efficiency and reliability. However, reducing their cost is the next challenge that will bring optical PMD compensators to be used in installed systems.

PMD in High-Bit-Rate Transmission and Means for Its Mitigation

Polarization-mode dispersion (PMD) prevents the cost-effective upgrading of fiber networks to 40 and sometimes even to 10 Gbit/s. This paper reviews recent progress in its mitigation and compensation and points out where more research is needed. Electronic PMD mitigation is preferable at 10 Gbit/s, due to its low cost, even though it is accompanied by a considerable residual penalty. A lot of work takes place in the field of optical PMD compensation. Among the numerous detection methods for first-order PMD, we prefer a purely electronic, hence low-cost, arrival time detection method, with a linear readout and ps-sensitivity. Surprisingly, the most easily detectable higher order of PMD is the third order, indicated by a slope steepness difference. Both methods rely on a polarization scrambler at the transmitter side, which can be shared. Regarding PMD compensators, LiNbO/sub 3/ devices are probably needed to guarantee a sufficient speed. A distributed PMD compensator allows to integrate a number of polarization transformers and differential group delay sections on one chip, thereby exactly emulating the way how the fiber accumulates, but in reverse order and orientation. We report on progress in using these devices, including their use for PMD compensation in a 40-Gbit/s carrier-suppressed return-to-zero differential phase-shift keying experiment. More work is needed to perfection the device, and to implement a fast endless polarization control. The theory of the distributed PMD compensator lends itself to a new definition of higher order PMD by a Fourier expansion of mode conversion, as an alternative to the familiar Taylor expansion of the PMD vector.

Effect of cross-phase-modulation-induced polarization scattering on optical polarization mode dispersion compensation in wavelength-division-multiplexed systems.

Interchannel cross-phase-modulation-induced polarization scattering (XPMIPS) and its effect on the performance of optical polarization mode dispersion (PMD) compensation in wavelength-division-multiplexed (WDM) systems are studied. The level of XPMIPS in long-haul WDM transmission systems is theoretically quantified, and its effect on optical PMD compensation is evaluated with numerical simulations. We show that in 10-Gbit/s ultra-long-haul dense WDM systems XPMIPS could reduce the PMD compensation efficiency by 50%, whereas for 40-Gbit/s systems the effect of XPMIPS is smaller.

A tunable interferometrically stable three-section higher order PMD emulator

We have developed a technique for emulating higher order polarization mode dispersion (PMD) that utilizes a three-section interferometrically stable configuration. Using this approach, we are able to study the impact of higher order PMD on 10-Gb/s nonreturn-to-zero and carrier-suppressed return-to-zero signals when first-order PMD is zero, which is a typical condition that occurs in one-stage optical PMD compensators.

Effects of nonlinear crosstalk in optical PMD compensation

We report on the effects of nonlinear crosstalk on the optical polarization-mode dispersion (PMD) compensation in wavelength-division-multiplexing (WDM) systems. An analytical model is presented to describe the effect of nonlinear crosstalk on the first-order PMD compensation. We also experimentally show that the performance of WDM systems could be degraded by nonlinear birefringence, even when the first-order PMD is completely compensated.

Polarization mode dispersion compensator field trial and field fiber characterization

Two high-PMD long distance routes were characterized and used to test an optical polarization mode dispersion compensator (PMDC) under field conditions. For this trial, 110 km routes with mean PMD values of 25 and 26.5 ps were provisioned with commercial WDM transport equipment and tested for several weeks. The route was comprised of three spans of characterized fiber that followed railroad tracks. We show the temporal variation of the output polarization state and the evolution of first- and second-order PMD spectra over 7 days. The deployment of a variablelength PMDC on these links allowed error-free transmission of an OC-192 signal. Splitting the output to receivers with and without PMDC demonstrated specific PMD events that caused errors in the absence of a PMDC.

Analytical calculation of pulse broadening in optical higher order PMD compensation

The system performances in second- and third-order polarization mode dispersion (PMD) compensations have been calculated analytically and a proper approach for the higher order PMD compensation has been investigated. The investigation has revealed that the frequency averaged higher order terms of the fiber link's PMD vector should be used as the coefficient vectors in emulating the frequency dependent compensation vector for higher order PMD compensation.

Performance limits of first-order PMD compensators using fixed and variable DGD elements

In this letter, we analyze optical polarization-mode dispersion compensators using a single differential-group-delay (DGD) element that is either fixed or variable. We evaluate the upper-bound performance limits, and quantify the compensator performance under dynamic operations. We show that the variable-delay compensator can reduce the risk of feedback loops trapping a locally optimized state. Moreover, the variable-delay approach provides superior performance as compared to a fixed-delay compensator by increasing the optimal system tolerance for a 10-Gb/s nonreturn-to-zero signal from 28 to 33 ps of average link DGD.