Polarization Mode Dispersion Compensation Research Articles

Tolerance of Duobinary and NRZ-OOK Signals to Second-Order Polarization-Mode Dispersion

The sensitivity of tightly filtered duobinary and conventional nonreturn-to zero on-off-keyed (NRZ-OOK) signals to second-order polarization-mode dispersion (SO-PMD) is assessed through extensive numerical simulations. On average, duobinary signals are significantly more sensitive to SO-PMD than to ordinary fiber chromatic dispersion (CD), whereas NRZ-OOK signals are more tolerant to SO-PMD than to CD. This seemingly contradicting behavior is explained by the difference in the frequency dependence of SO-PMD and CD. In addition, duobinary signals suffer from residual differential group delay, even after first-order PMD compensation, which is introduced between signal components that are transmitted in the launched principal state of polarization (PSP) and those that are coupled from the launched to the orthogonal PSP.

Distributed PMD Compensation Experiment Using Polarizers

We experimentally demonstrated polarization-mode dispersion (PMD) compensation by distributing polarizers with only 1 degree of freedom (DOF) along the transmission line. The average power penalty was measured to be 0.4 dB by inserting four compensators, where average differential group delay was 47% of bit period.

Stabilizing PMD Compensators by Means of Polarization Dithering

Feedback-based single-stage polarization-mode dispersion compensators can suffer from dynamical limitations. These can be prevented by modulating the polarization of the transmitted signal. A high-depth modulation can result in high jitter and partial eye closure. A low-depth modulation (dithering) effectively stabilizes the compensator while it does not introduce significant system penalty.

Demonstration of PMD Compensation by LDPC-Coded Turbo Equalization and Channel Capacity Loss Characterization Due to PMD and Quantization

Polarization-mode dispersion (PMD) compensation by low-density parity-check (LDPC) coded turbo equalization is experimentally demonstrated for a nonreturn-to-zero (NRZ) system operating at 10 Gb/s. This technique provides significant improvement of bit-error rate compared to the optimum threshold receiver. The channel capacity loss due to PMD and quantization is studied as well. It was found that the proposed PMD turbo equalizer is about 1.5 dB away from the theoretical limit.

40-Gb/s Adaptive Electronic Polarization-Mode Dispersion Compensator Using a Hybrid-MIC 3-Tap Linear Equalizer

We report a 40-Gb/s adaptive electronic dispersion compensator (EDC) for mitigation of polarization mode dispersion (PMD). The EDC employs a 3-tap linear equalizer acting as a transversal filter and manufactured on a substrate of duroid 5880 by using hybrid-monolithic-integrated-circuit (MIC) technology. The experimental results showed that for a given 20-ps differential group delay (DGD), the measured eye-opening penalty (EOP) after compensation was improved by 11 dB. This 40-Gb/s adaptive EDC demonstrated the possibility of its use for PMD compensation in a 40-Gb/s long-haul fiber-optic transmission system.

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.

Broadband all-order polarization mode dispersion compensation via wavelength-by-wavelength Jones matrix correction

We demonstrate wideband all-order polarization mode dispersion (PMD) compensation by applying high-speed spectral polarization sensing and ultrafast pulse shaping techniques to characterize and correct the frequency-dependent Jones matrix associated with PMD of optical fibers on a wavelength-by-wavelength basis. We report full compensation of approximately 800 fs pulses distorted to more than 10 ps by a PMD module with approximately 5.5 ps mean differential group delay. The sensing and compensation of Jones matrix take approximately 200 and approximately 500 ms, respectively.

PMD Compensation by LDPC-Coded Turbo Equalization

A turbo equalization scheme suitable for electronic polarization-mode dispersion compensation based on low-density parity-check coding and the Bahl-Cocke-Jelinek-Raviv (BCJR) equalizer is proposed. For reasonable BCJR equalizer complexity, the differential group delay of up to three bit-periods can be compensated.

Digital Equalization of Chromatic Dispersion and Polarization Mode Dispersion

In this paper, we consider a fractionally spaced equalizer (FSE) for electronic compensation of chromatic dispersion (CD) and polarization-mode dispersion (PMD) in a dually polarized (polarization-multiplexed) coherent optical communications system. Our results show that the FSE can compensate any arbitrary amount of CD and first-order PMD distortion, provided that the oversampling rate is at least 3/2 and that a sufficient number of equalizer taps are used. In contrast, the amount of CD and PMD that can be corrected by a symbol-rate equalizer only approaches an asymptotic limit, and increasing the number of taps has no effect on performance due to aliasing that causes signal cancellation and noise enhancement.

Simultaneous Compensation of Polarization Mode Dispersion and Chromatic Dispersion Using Electronic Signal Processing

In this paper, we study the performance of 10.7-Gb/s nonreturn-to-zero (NRZ) and return-to-zero (RZ) on-off keying signals in the joint presence of first-order polarization mode dispersion (PMD) and chromatic dispersion (CD) based on optical signal-to-noise ratio penalty measurements. Our investigations show that the tolerance of RZ to first-order PMD is severely reduced by the presence of typical values of residual CD. Three different receiver strategies are studied: 1. unequalized threshold detection; 2. combined feed-forward and decision-feedback equalization; and 3. maximum-likelihood sequence estimation (MLSE). In all three cases, the presence of CD eliminates the advantage of RZ over NRZ in terms of PMD tolerance. For NRZ, we find that the MLSE improves the tolerance to first-order PMD by 60%-70%, even in the presence of residual CD.

Three-chip differential phase-shift keying maximum likelihood sequence estimation for chromatic-dispersion and polarization-mode-dispersion compensation

We propose a novel three-chip differential phase-shift keying (DPSK) maximum likelihood sequence estimation (MLSE) for chromatic-dispersion (CD) and first-order polarization-mode-dispersion (PMD) compensation to extend the transmission reach of the DPSK signal. Such a technique searches the most probable path through the trellis for DPSK data sequence estimation by exploiting the phase difference between not only the adjacent optical bits but also the bits that are one bit slot apart. The proposed scheme significantly outperforms conventional two-chip DPSK MLSE in CD and PMD compensation. We show that the proposed three-chip DPSK MLSE can enhance the CD tolerance of 10 Gbit/s DPSK signal to 2.5 times of that by using two-chip DPSK MLSE and can bound the penalty for 100 ps differential group delay by 1.4 dB.

Polarization-mode dispersion influenced by polarization-dependent loss and chirp for a Gaussian pulse

A combination of polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) in optical fibers may lead to anomalous pulse broadening. It raises the issue for more complete assessments when studying pulse propagation in the presence of polarization-dependent loss. A mathematical description is put forward in this work, where PMD is evaluated in the presence of XPM, PDL, and chirp. Simulation shows that the delay of a chirped Gaussian pulse depends not only on PDL, but also on the chirp of the pulse itself. To some degree, effective PMD can be controlled by PDL and chirp. The results give guidance for PMD compensation.

Adaptive PMD compensation experiment in 40 Gb/s CSRZ system

In 40 Gb/s systems with low polarization-mode dispersion (PMD) fibers, first-order PMD is the dominate factor with quite small value. An adaptive PMD compensator consisting of an electrical polarization controller (PC), a section of polarization maintaining fiber (PMF), a degree of polarization (DOP) detector and a feedback control module is employed in 40 Gb/s NSFCNet. This compensator has a simple structure and high speed, which can compensate up to 20 ps first-order PMD adaptively and the average searching time is 2 ms.

Improved Numerical Model for the Average Transfer Matrix of First- and Second-Order PMD

An improved model for the average transfer matrix of first- and second-order polarization-mode dispersion (F&SO-PMD) in optical fibers is derived from numerical simulations of the frequency dependence of the differential phase shifts and cross coupling between signal components that are transmitted in two principal states of polarization (PSPs). The mean differential phase shifts and cross-coupling phases are calculated for various given values of the differential group delay (DGD) and the second-order PMD parameter that characterizes the coupling between PSPs. It is found that the mean differential phase delays are equal to the DGDs only over a narrow optical bandwidth, beyond which they decrease with increasing frequency offset and in some cases even reverse sign. Similarly, the mean cross-coupling phases increase less rapidly with frequency than assumed in other popular models and always approach an asymptotic value, at which half of the optical power is coupled from one PSP to the other. Moreover, it is shown that these mean differential phase shifts and cross-coupling phases define a transfer matrix for F&SO-PMD that nicely predicts the average eye-opening penalties in return-to-zero-formatted digital optical signals that are transmitted in one of the two PSPs. These predictions are particularly accurate when the average polarization-dependent chromatic dispersion (PCD) is included in the PSP phases. Additional simulations of F&SO-PMD compensation reveal that the signal impairments caused by PCD, on average, are substantially smaller than those introduced by the cross coupling between PSPs

PMD compensation in fiber-optic communication systems with direct detection using LDPC-coded OFDM

The possibility of polarization-mode dispersion (PMD) compensation in fiber-optic communication systems with direct detection using a simple channel estimation technique and low-density parity-check (LDPC)-coded orthogonal frequency division multiplexing (OFDM) is demonstrated. It is shown that even for differential group delay (DGD) of 4/BW (BW is the OFDM signal bandwidth), the degradation due to the first-order PMD can be completely compensated for. Two classes of LDPC codes designed based on two different combinatorial objects (difference systems and product of combinatorial designs) suitable for use in PMD compensation are introduced.

LDPC-Coded OFDM for Optical Communication Systems with Direct Detection

Low-density parity-check (LDPC) coded orthogonal frequency division multiplexing (OFDM) is proposed as an efficient coded modulation technique suitable for optical communication systems. We show that in long-haul fiber optics communications, LDPC-coded OFDM increases spectral efficiency, and reduces the influence of residual chromatic dispersion and the influence of fiber nonlinearities. LDPC-coded OFDM is as also a promising polarization-mode dispersion (PMD) compensation technique. In free space optical communications, LDPC-coded OFDM improves the immunity to the atmospheric turbulence; while in multimode fiber links, it reduces the influence of intermodal dispersion. In plastic optical fiber links it reduces the influence of dispersion and helps to overcome the bandwidth limitations. Several power efficient OFDM schemes and LDPC codes suitable for use in LDPC-coded OFDM optical communication systems are introduced.

Chromatic dispersion and PMD monitoring and compensation techniques studies in optical communication systems with single channel speed 40Gbit/s and CSRZ format

We study a whole compensation system for chromatic dispersion and polarization mode dispersion, including monitoring subsystems and compensation subsystems in optical communication systems with single channel speed 40Gbit/s and CSRZ format. We employed the spectral shift effect of a semiconductor optical amplifier for chromatic dispersion monitoring, and a non-linearly chirped fiber Bragg grating for chromatic dispersion compensation. The degree of polarization characterizes is used as feedback control signal of polarization mode dispersion monitoring, and a polarization controller and a polarization maintaining fiber are formed a polarization mode dispersion compensator. The transmission experiment demonstrates that the whole compensation system is effective. It is suit for chromatic dispersion management and polarization mode dispersion eliminating in optical communication systems with high single channel speed and CSRZ format.

Non-blocking PMD monitoring in live optical systems

A simple and non-blocking polarisation-mode dispersion (PMD) monitoring technique using coherent detection is demonstrated and applied to a 40 Gbit/s live fibre-optic system. The results demonstrated a clear correlation between the instantaneous differential group delay and the receiver Q-margin. It was also demonstrated that PMD-induced outage can be eliminated by an adaptive PMD compensator.

Dynamic Performance and Speed Requirement of Polarization Mode Dispersion Compensators

To study the dynamic performance of a polarizationmode-dispersion compensator (PMDC), a continuous PMD model and a dynamic PMDC model are presented. The continuous PMD model generates continuous PMD and state of polarization (SOP) variations. The statistics of the SOP variation generated by the continuous PMD model follows the Rayleigh distribution, and the SOP variation amplitude scales with the square root of sampling-time interval. The results are compared with those of a numerical and a commercial polarization scrambler, and the difference of the SOP variation characteristics between the PMD model and the polarization scramblers is explained. The continuous PMD model and the dynamic PMDC model are used to study the dynamic performance and speed requirement of a two-stage PMDC. The speed requirement of the PMDC is quantified in the SOP variation in one PMDC loop cycle. The authors found that when the SOP variation is within 3deg in one PMDC loop cycle, the dynamic PMD and SOP changes virtually does not cause any degradation in the PMDC performance

Practical Solutions to Polarization-Mode-Dispersion Emulation and Compensation

Polarization-mode dispersion (PMD) still remains a challenge for high-data-rate optical-communication systems. Practical solutions are desirable for PMD emulation, monitoring, and compensation. The authors review and compare various techniques for PMD emulation and compensation, with an emphasis on the application of programmable differential-group-delay (DGD) elements for manipulating PMD effects. The authors pay special attention to advanced emulation techniques, such as importance sampling and the hinge model, for practical applications. The tunability of programmable DGD elements proves to be attractive for both system performance evaluation and overall optimization