Time-resolved performance analysis of a second-order PMD compensator

Design, test, and performance requirement and analysis for a polarization-mode-dispersion compensator (PMDC) with four degrees of freedom is presented. The performance is analyzed on the basis of time-integrated and time-resolved bit-error ratio (BER) measurements. Signal impairments are generated by both, first- and higher-order emulators. The probability distributions of bit errors measured over many one second intervals exhibit very long tails. Therefore even a PMDC with a good average BER performance may result in a significant total outage time for a given system.     

Robustness of polarization-mode-dispersion compensation in the presence of polarization-dependent loss

The robustness of polarization-mode dispersion (PMD) compensation utilizing radio-frequency feedback in the presence of polarization-dependent loss (PDL) is investigated using a closed-loop dynamic model. Simulations using mean PMD of 30% of the bit period show that mean PDL of up to 2 dB increases the Q-penalty after PMD compensation by less than 0.1 dB. We find that the compensator can become trapped in secondary optima at high PDL, incurring significant additional penalty.     

Three-stage polarization mode dispersion compensator capable of compensating second-order polarization mode dispersion

The impacts of polarization-dependent dispersion on systems are investigated briefly, based on which a three-stage polarization mode dispersion compensator has been proposed to compensate polarization-dependent chromatic dispersion and the principal states of polarization rotation rate at the same time. The two possible operating points of this compensator are also proposed. Numerical results show that the maximum tolerable polarization mode dispersion value after three-stage compensation has been improved by 17% per bit slot compared with using a two-stage compensator.     

High-Rate Uncorrelated Bit Extraction for Shared Secret Key Generation from Channel Measurements

Secret keys can be generated and shared between two wireless nodes by measuring and encoding radio channel characteristics without ever revealing the secret key to an eavesdropper at a third location. This paper addresses bit extraction, i.e., the extraction of secret key bits from noisy radio channel measurements at two nodes such that the two secret keys reliably agree. Problems include 1) nonsimultaneous directional measurements, 2) correlated bit streams, and 3) low bit rate of secret key generation. This paper introduces high-rate uncorrelated bit extraction (HRUBE), a framework for interpolating, transforming for decorrelation, and encoding channel measurements using a multibit adaptive quantization scheme which allows multiple bits per component. We present an analysis of the probability of bit disagreement in generated secret keys, and we use experimental data to demonstrate the HRUBE scheme and to quantify its experimental performance. As two examples, the implemented HRUBE system can achieve 22 bits per second at a bit disagreement rate of 2.2 percent, or 10 bits per second at a bit disagreement rate of 0.54 percent.     

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.      

160-gb/s OTDM signal source with 3R function utilizing ultrafast mode-locked laser diodes and modified NOLM

High-quality 160-Gb/s optical time-division-multiplexing (OTDM) signal source with 3R functions was demonstrated using ultrafast mode-locked laser diodes (MLLDs) and a nonlinear optical fiber loop mirror (NOLM) modified with inline and external polarizers and an inline optical phase-bias compensator. Pulse quality for each OTDM channel was successfully improved and equalized owing to the clock extraction at the true data bit rate using a 160-GHz MLLD and also to the improvement of the switching performance of the NOLM.     

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.     

Comparison of different feedback signals for one-stage polarization-mode dispersion compensators

The performance of a one-stage polarization-mode dispersion (PMD) compensator with different feedback signals is studied and compared with that of a pure first-order PMD compensator. We show that the performance of the one-stage PMD compensator has a strong dependence on feedback signals, and that the efficiency of some feedback signals depends on modulation formats. With proper choice of feedback signals, the one-stage PMD compensator can achieve better performance than the first-order PMD compensator.     

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 (LiNb_1-y/Ta_y/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.     

Near Pseudolinear Regime in 10-Gb/s Single Sideband-Alternate Mark Inversion Systems Using Electrical Dispersion Precompensation

A 10-Gb/s optical single sideband (OSSB) system using alternate mark inversion return-to-zero and ideal electrical precompensation of dispersion is optimized numerically by means of an optical dispersion compensator at the receiver side. The transmission regime observed in the optimized system resembles the pseudolinear regime previously described for systems with bit rates of 40 Gb/s and above. Considering multichannel transmission, the OSSB system has a Q -factor penalty of 2 dB compared to an intensity modulated optical double sideband system with optimized optical dispersion map.     

Performance of non-fibre based dispersion compensation for long-haul 10.7 Gbit/s DWDM transmission

Non-fibre based in-line dispersion compensation is employed to achieve long-haul transmission of 80 channels at 10.7 Gbit/s bit rate over 1200 km of LEAF fibre. Average system margins of more than 4 dB and small transmission are demonstrated. The additional penalty incurred by the periodic nature of the dispersion compensator is assessed.     

Bit-error-rate saturation due to mode-partition noise induced by optical feedback in 1.5-µm single longitudinal-mode C3and DFB semiconductor lasers

We report first measurements of the effect of optical feedback on the bit error rate (BER) as a function of received power of single longitudinal mode cleaved-coupled cavity (C³) and distributed-feedback (DFB) semiconductor lasers. Mode-partition noise, induced by the optical feedback, can cause a bit error-rate floor for large optical feedbacks, but it is shown that high-performance C³lasers and especially DFB lasers can withstand very large optical feedbacks without system degradation.     

Estimated performance of 2.488 Gb/s CPFSK optical non-repeateredtransmission system employing high-output power EDFA boosters

The configuration of a practical nonrepeatered coherent optical transmission system and its performance are reported. The practicability of combining continuous-phase frequency-shift keying (CPFSK) with erbium-doped fiber amplifier (EDFA) boosters is verified by laboratory and field experiments. A system gain of 60.8 dB is achieved at a BER at 10^-11; the EDFA's optical output power is +18 dBm and the receiver sensitivity is -42.8 dBm. The stimulated Brillouin scattering (SBS) effect is examined to estimate the dependence of error rate characteristics on the bit sequence length. No power penalty is observed for a pseudorandom bit sequence (PRBS) of more than 2⁵-1 or STM-16 patterns containing a 30-byte block of consecutive identical digits. The power penalty of 1.3 dB caused by the 310-km non-dispersion-shifted transmission fiber is successfully compensated by installing a chromatic dispersion compensator in each orthogonal polarization branch. During a four month field experiment, error-free operation was observed over a 30 day period, and the long-term error rate is under 6×10^-16     

Performance limits of first-order PMD compensators using fixed andvariable 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     

Unrepeated transmission of 20-Gb/s optical quadrature phase-shift-keying signal over 200-km standard single-mode fiber based on digital processing of homodyne-detected signal for Group-velocity dispersion compensation

We demonstrate unrepeated optical transmission of 20-Gb/s quadrature phase-shift-keying (QPSK) signals over a 200-km-long standard single-mode fiber (SMF) without using any optical dispersion compensator. By employing optical homodyne detection, which can restore the entire information of the complex amplitude of the transmitted signal, group-velocity dispersion (GVD) of the SMF can be compensated electrically by a linear equalizer at the receiver. From off-line bit-error-rate measurements, we find that a simple transversal filter implemented in digital signal processing circuits after homodyne detection can effectively cancel the fiber GVD of up to 4000 ps/nm, enabling successful 20-Gb/s QPSK transmission.     

高速光纤通信系统中高阶PMD补偿技术研究

随着光放大器和色度色散补偿技术的不断提高，光纤的偏振模色散（PMD）已经成为超高速、超长距离光纤通信系统发展的主要障碍，在40Gbit/s或更高速率的光纤通信系统中，PMD的影响已不可忽略，必须考虑PMD的补偿问题，从高阶PMD对40Gbit/s NRZ系统影响的数值模拟发现，当光纤中PMD高阶效应比较明显时，将严重劣化一阶PMD补偿的效果，另外，通过对两种高阶PMD补偿器的比较介绍，认为两段级联的高队PMD补偿系统是一种比较现实的补偿方法。     

44-ns Continuously Tunable Dispersionless Optical Delay Element Using a PPLN Waveguide With Two-Pump Configuration, DCF, and a Dispersion Compensator

This letter presents an optically controlled, continuously tunable, dispersionless optical delay element in fiber based on a selective wavelength converter using a periodically poled lithium-niobate waveguide with two-pump configuration, dispersion-compensated fiber, and a dispersion compensator. Optical phase information is preserved in this technique, and there is no fundamental limitation on pulsewidth or bit rate. A Continuous optical delay up to 44-ns is shown for 10-Gb/s nonreturn-to-zero applications, which is equal to a 440-bit slot     

A Standalone Receiver With Multiple Access Interference Rejection, Clock and Data Recovery, and FEC for 2-D$lambda -t$OCDMA

We demonstrate a standalone (no global clock) receiver for two-dimensional wavelength-time optical code-division multiple-access. The receiver provides the following functions: quantization (to eliminate multiple access interference), clock and data recovery, return-to-zero to nonreturn-to-zero conversion (for optical code-division multiple-access compatibility with digital logic), framing (for byte synchronization), and forward-error correction (FEC) using a (255, 239) Reed–Solomon decoder. The receiver more than doubles the number of supported users at a bit-error rate$≪ 10 ^-10$. The receiver supports an information rate of 156.25 Mb/s. We performed the measurements at a bit rate of 167.4 Mb/s and a chip rate of 1.339 Gb/s (eight chips per bit) to account for FEC overhead.     

Optimum length of one-stage polarization-mode dispersion compensators with a fixed delay line

We investigate the performance of the one-stage polarization-mode dispersion (PMD) compensator with a fixed delay line. We find that there exists an optimum delay line length for the PMD compensator and the optimum delay line length decreases with the increase of signal bandwidths. We show that such PMD compensators, when optimized, can achieve almost the same performance as that achieved by those with a variable delay line.     

Analysis of feedback for higher order polarization-mode dispersion mitigation of NRZ 40-Gb/s optical transmission

First-order polarization-mode dispersion (PMD) compensation by means of a polarization controller and a differential delay line is not sufficient to guarantee error-free transmission for 40-Gb/s channels when higher order effects severely increase signal distortion. Higher order mitigation is possible by cascading more than one first-order block. However, only two-stage or three-stage devices remain simple enough to be actually controlled. The performance of such higher order PMD compensators is evaluated by means of numerical simulations. Two different feedback signals have been used, demonstrating that first-order and higher order PMD distortion of nonreturn-to-zero (NRZ) pulses at 40 Gb/s can be strongly mitigated for instantaneous values of the differential group delay (DGD) up to the bit slot, when the compensator is properly controlled.