Electrical Equalization Research Articles

Electrical Joint Equalization for PMD-Induced Performance Degradation in Direct-Detection Polarization-Multiplexed Transmission Systems

Polarization-division-multiplexed (PDM) systems are sensitive to polarization-mode dispersion (PMD), since PMD induces not only intersymbol distortion but also interchannel crosstalk. Electrical equalization for individual channels is not sufficient for the lack of information from another channel. In this letter, electrical joint equalization including both the decision-feedback equalizer and Viterbi equalizer for PMD compensation in direct-detection PDM systems is investigated. Simulation shows that joint equalizers can effectively combat PMD-induced performance degradation and lower the requirement for dynamic polarization control.

DWDM VSB modulation-format optical transmission: Effects of optical filtering and electrical equalization

The transmission of 40 Gb/s wavelength multiplexed channels under vestigial single side band modulation format is transmitted over long haul optically amplified fiber systems. Bit-error-rate (BER) of 10 −12 or better can be achieved across all channels. Optical filters are designed with asymmetric roll-off bands. Simulations of the transmission performance, BER versus receiver sensitivity are demonstrated with wavelength channel spacing of 20–40 GHz. An optical filter, whose passband is 28 GHz and 20 dB cut-off band, performs best for 40 Gb/s bit rate due to optimum filtering and minimum noise contribution. Furthermore the single-sideband property of VSB format can assist linear equalization by electronic processing. The transmission performance is accurately evaluated based on the eye opening using a fast statistical method based on an equivalent Gaussian probability density distribution (pdf) which is derived from multiple peaks pdf of distorted eye diagram.

Electrical Mitigation of Penalties Caused by Group Delay Ripples for Different Modulation Formats

Electrical equalizers are proposed to mitigate the distortions caused by group delay ripples (GDRs) of cascaded optical filters (e.g., fiber Bragg gratings for chromatic dispersion (CD) compensation). Different electrical equalizers including feedforward equalizer (FFE), decision-feedback equalizer (DFE), nonlinear FFE-DFE, and maximum-likelihood sequence estimator are investigated and compared comprehensively for four different modulation formats: ON-OFF keying, optical duobinary, optical single sideband, and differential phase-shift keying. Especially, we find that the relationship of electrical equalization performance on the modulation format is very similar for the mitigation of GDR and CD.

Nonlinear Electrical Equalization for Different Modulation Formats With Optical Filtering

Based on bit-error-ratio simulations, we investigate electrical-dispersion-compensation performance by using a nonlinear electrical equalizer based on nonlinear Volterra theory for different modulation formats. This nonlinear equalizer is compared to conventional decision-feedback equalizer as well as to maximum-likelihood sequence estimation. Especially, we show that nonlinear equalizers, in conjunction with narrowband optical filtering of the light signal, result in improved performance. First of all, the system can benefit from the noise reduction due to narrowband filtering. Second, interacting with chromatic dispersion, nonlinear equalizers benefit from the improved dispersion tolerance through spectrum reshaping by narrowband optical filtering. Finally, nonlinear equalizers can efficiently mitigate the distortion resulting from strong optical filtering

Electrical equalization for advanced optical communication systems

We investigate equalizers for electronic dispersion compensation (EDC) of dispersion limited optical fibre communication links in combination with different modulation formats. We show that the performance of conventional equalizers including feedforward equalizer (FFE) and decision feedback equalizer (DFE) are fundamentally limited by the nonlinearity of square-law detection of the photodiode in direct detection systems. Advanced modulation formats such as differential phase shift keying (DPSK) and optical duobinary further enhance this kind of nonlinearity and degrade further FFE/DFE performance. However, nonlinear FFE–DFE and maximum likelihood sequence estimation (MLSE) take into account the mitigation of nonlinear inter symbol interference (ISI) and hence can achieve much better performance. We show that in contrast to other modulation formats, optical single sideband modulation results in approximately linear distortions after detection and thus a simple linear FFE equalizer can achieve good compensation.

Minimum BER criterion for electrical equalizer in optical communication systems

The electrical equalizer (EE) for dispersion compensation is a key and cost-effective element in optical communication OOK systems in the presence of a chromatic dispersion The most common but suboptimal optimization criterion for the EE weights is based on minimizing the mean square error (mse). Here, the authors examine the optimal optimization criterion, minimum bit error rate (MBER) for a feedforward equalizer (FFE) weights in optical communication systems. A BER performance comparison analysis shows that the MBER optimization criterion introduces improvements over the conventional minimum mse optimization criterion. In addition, the authors introduce a new simplified method for computing the FFE weights using the concept of decision boundary.

Limits of optical dispersion compensation using linear electrical equalizers

This letter considers optical dispersion compensation in the electrical domain. It is shown that after electrical detection the spectrum of the optical signal suffers a frequency cutoff. The maximum frequency, which can be detected by the square-law detector, is inversely proportional to dispersion in the optical line. This puts a limit on the maximum value of optical dispersion, which can be compensated by electrical equalizers

Optical versus electrical dispersion compensation: role of timing jitter

This paper calculates timing jitter in intensity-modulated/direct detection lightwave systems in which chromatic dispersion is compensated in the optical domain and in systems where the effects of chromatic dispersion are mitigated by means of electrical equalization at the receiver. The authors focus on a linear communication channel and derive a new expression for the timing jitter after the photodetector using frequency-domain analysis. It shows that timing jitter depends cubically on link length when dispersion is compensated in the electrical domain. In contrast, when dispersion is fully compensated optically, timing jitter depends only linearly on the link length. A new expression for the optimum timing jitter in the presence of residual dispersion is also presented.

Study on the PMD impairment of optical multilevel DPSK systems and its mitigation methods

We examine the polarization-mode dispersion (PMD) impairment of optical multilevel differential phase-shift keying systems, and also its mitigation method by comparing the electrical equalization technique and the receiver bandwidth optimization. Analysis has been carried out with 40-Gb/s systems experiencing 0/spl sim/60 ps of instantaneous differential group delay, corresponding up to 240%/80% of bit/symbol transmission rate. Results show considerable improvement in the PMD tolerance for return-to-zero format for the optimized receiver bandwidth either with or without the electrical equalization. For nonreturn-to-zero, the bandwidth optimization alone does not significantly improve the system's PMD tolerances.

On the performance of the electrical equalization technique in MMF links for 10-gigabit ethernet

In this paper, the electrical equalization technique utilized in multimode fiber (MMF) links for 10-Gigabit Ethernet is reported. This paper presents and compares three kinds of equalizers: linear equalizer, decision feedback equalizer (DFE), and a nonlinear finite-impulse-response DFE (NL-FIR-DFE) based on the analysis of nonlinear operations of direct modulation and detection in MMF links. Computer simulations reveal that NL-FIR-DFE exhibits superior performance in comparison with normal DFE. The equalization performance for an MMF channel with bandwidth 500 MHz-km as well as 160 MHz-km is demonstrated. It is demonstrated that with direct modulation of cost-effective vertical-cavity surface-emitting laser (VCSEL) or Fabry-Perot laser, the transmission distance for installed MMF with a bandwidth of 500 MHz-km at 10 Gb/s can be extended to 300 m with an appropriate offset restricted mode launch condition by utilizing normal DFE and to more than 650 m for NL-FIR-DFE. Moreover, it is shown that the transmission distance for an MMF with a bandwidth of 160 MHz-km can reach 300 m with more than seven times bandwidth improvement by using NL-FIR-DFE.

Polarization Mode Dispersion in High-Speed Optical Communication Systems

We review the research progress concerning some fundamental issues related to polarization-mode dispersion (PMD) in high-speed fiber-optic transmission systems. We pay particular attention to issues such as the PMD-induced pulse broadening, PMD measurement and emulation, as well as PMD compensation. An electrical equalization technique based on a transversal filter and an optical technique based on a nonlinear chirped fiber Bragg gratings for PMD compensation will bediscussed.

Performance of Electrical Equalizers in Optically Amplified OOK and DPSK Systems

We study the performance of feed-forward equalizers (FFEs) and decision-feedback equalizers (DFEs) in compensating chromatic dispersion and first-order polarization-mode dispersion (PMD) in optically amplified nonreturn-to-zero on-off keying (NRZ-OOK) and NRZ differential phase-shift-keying (DPSK) systems at 10 Gb/s. We use Monte Carlo simulation of the least-mean-square algorithm to estimate equalizer tap weights. Once stationary tap weights are obtained, we compute the bit-error ratio using an analytical method whose accuracy has been verified by simulation. We find that FFE and DFE offer the greatest improvement when combating first-order PMD with NRZ-OOK, reducing power penalties by about 45%. With NRZ-DPSK, the equalizers offer quite limited improvement.

On the performance of electrical equalization in optical fiber transmission systems

A variety of equalizers have been proposed to improve the bit-error rate (BER) of optical fiber communications by reducing the effects of chromatic dispersion (CD), polarization-mode dispersion (PMD), and other fiber impairments. Therefore, it is of interest to establish the ultimate performance of electrical equalizers under different conditions. The results presented here are based on the fact that maximum-likelihood sequence detection (MLSD) obtains the lowest BER of all possible detectors, under mild conditions. Thus, the MLSD performance is a lower bound on the BER of any electrical equalizer. We consider linear channels with PMD (all orders), CD, and the nonlinear effect of the photodetector.

Exploiting diversity in multimode fiber communications links via multisegment detectors and equalization

we propose diversity combination via optical multisegment detectors and electrical equalization techniques to mitigate the effects of intermodal dispersion in multimode fiber (MMF). With no equalization in MMF links, intermodal dispersion produces intersymbol interference (ISI) in the received optical signal that severely limits the achievable data transmission rates and fiber link lengths. Our quasi-simulated performance results (obtained with measured impulse responses) demonstrate that multisegment detection and equalization provide a low-cost and efficient solution to combat ISI and hence, to enhance the performance of MMF communications links.

Superresolution readout system with electrical equalization for optical disks

An electrical equalizer for a superresolution readout system with an optical apodizer is proposed and verified experimentally. This superresolution readout system uses a five-tap transversal filter as the electrical equalizer instead of additional optics to suppress enlarged sidelobes, and it achieves higher resolution than the diffraction-limited system. The transfer function of the electrical equalizer is also derived theoretically. This approach allows fabrication of a readout system with a good signal-to-noise ratio and a compact head.

Requirements for LD FM characteristics in an optical CPFSK system

A procedure for evaluating the influence of a nonideal FM response on the bit error rate characteristics is proposed which explains experimental results fairly well. Criteria for the laser diode (LD) FM response are obtained in order to realize low-penalty operation with a long bit pattern. The position and the shape of the dip (typical for a conventional LD FM response), especially the slope of the higher frequency part of the dip, are the dominant factors which determine the nonideal FM response influence of transmitter devices. The criteria obtained give guidelines for the LD FM response equalization, as well as the device selection. For devices to be applied in practical systems with an FM equalizer, a dip frequency sufficiently below 4*10/sup -4/ of the bit rate should be selected in order to realize less than 0.5-dB sensitivity penalty operation, since the passive electrical equalization is only effective for the frequency region higher than the dip frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Waveform degradation due to dispersion effects in an optical CPFSK system

Waveform degradation due to polarization and chromatic dispersion in a single-mode fiber is calculated for a coherent continuous-phase frequency-shift-keying (CPFSK) signal. Both kinds of dispersion distort the amplitude of the baseband signal and can limit transmission distance and capacity. For instance, polarization dispersion of 5 ps will restrict a bit rate by approximately 60 Gb/s when chromatic dispersion is fully reduced using a dispersion-shifted fiber or applying electrical equalization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Waveform degradation due to dispersion effects in an optical CPFSK system

Waveform degradation due to polarization and chromatic dispersion in a single-mode fiber is calculated for a coherent continuous-phase frequency-shift-keying (CPFSK) signal. Both kinds of dispersion distort the amplitude of the baseband signal and can limit transmission distance and capacity. For instance, polarization dispersion of 5 ps will restrict a bit rate by approximately 60 Gb/s when chromatic dispersion is fully reduced using a dispersion-shifted fiber or applying electrical equalization. >

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S/N and error rate evaluation for an optical FSK-heterodyne detection system using semiconductor lasers

Receiving performance is evaluated for an optical FSK-heterodyne detection system in which semiconductor lasers are used as both an FSK transmitter and an independent local oscillator. Noise and error rate are measured under feedback stabilization of IF signal frequency and electrical equalization of semiconductor laser FM modulation frequency characteristics. The minimum received signal power of -44 dBm, which is about 2 dB better than that in IM direct detection, is achieved at a 10-9error rate for a 200 Mbit/s signal. Excess errors for FSK signals result from frequency broadening of the laser spectrum. Both AM and FM quantum noises in the lasers are primary factors which determine system performance. Error rate characteristics in an optical FSK direct detection system, in which a Michelson interferometer is employed as an optical frequency discriminator, are compared with the above results.