MLSE Equalizers for Frequency Discrimination Receiver of MSK Optical Transmission System

We propose maximum-likelihood sequence estimator (MLSE) equalizers based on either Viterbi algorithm or template matching temple matching (TM) for the equalization of impairments imposed on the minimum shift keying (MSK) modulation formats in long haul transmission without optical dispersion compensation. The TM-MLSE equalizer is proposed as a simplified alternative for the Viterbi-MLSE equalizer. It is verified that the Viterbi-MLSE equalizer can operate optimally when noise approaches a Gaussian distribution. Simulation results of the performances of the two MLSE equalizers for optical frequency discrimination receiver-based optical MSK systems are described. The transmission performance is evaluated in terms of: (1) the chromatic dispersion (CD) tolerance for both Viterbi-MLSE and TM-MLSE equalizers; (2) transmission distance limits of Viterbi-MLSE equalizers with various number of states; (3)the robustness to fiber polarization mode dispersion (PMD) of Viterbi-MLSE equalizers; and (4) performance improvements for Viterbi-MLSE equalizers when utilizing sampling schemes with two and four samples per bit over the conventional single sample per bit. With a small number of states (64 states), the non-compensating optical link can equivalently reach up to approximately 928 km SSMF for 10 Gb/s transmission or 58 km SSMF for 40 Gb/s. The performance of 16-state Viterbi-MLSE equalizers for optical frequency discrimination receiver (OFDR)-based optical MSK transmission systems for PMD mitigation is also numerically investigated. The performance of Viterbi-MLSE equalizers can be further improved by using the sampling schemes with multiple samples per bit compared to the conventional single sample bit. The equalizer also offers high robustness to fiber PMD impairment.     

基于10 Gb/s传输链路的40 Gb/s光传输实验研究

基于中国自然科学基金网(NSFCNet)的400 km×10 Gb/s光传输链路实现了40 Gb/s光传输,没有出现误码率(BER)平台,说明在常规的中短距离10 Gb/s系统可以直接升级至40 Gb/s系统,而不需要升级传输链路。但是,由于相对10 Gb/s系统而言40 Gb/s系统的色散容限非常小,在升级时必须精确补偿原有链路的色散,在接收机前一般需要加可调色散补偿单元。同时,还分析了光纤注入功率对系统性能的影响,结果表明在设计这种由10 Gb/s向40 Gb/s升级的系统时,不仅要考虑信号带宽增加带来信噪比要求的提高,而且必须充分考虑光纤非线性的影响。     

Planar lightwave circuit dispersion equalizer

The authors report an integrated-optic dispersion equalizer fabricated on a planar lightwave circuit (PLC). This PLC dispersion equalizer is composed of several asymmetrical Mach-Zehnder interferometers cascaded in series. The dispersion equalizer has a high degree of design flexibility and can compensate for both normal and anomalous fiber dispersions at any center wavelength. Moreover, the equalizer can be applied to WDM transmission systems. This equalizer employing five asymmetrical interferometers is fabricated and its measured dispersion values are +834 and -1006 ps/nm. The effectiveness of the equalizer is demonstrated in a 2.5 Gb/s transmission experiment with a 1.3 μm zero-dispersion fiber at 1.55 μm. Also, its performance is evaluated theoretically     

Dispersion penalty reduction using an optical modulator with adjustable chirp

Using a unique Ti:LiNbO/sub 3/ modulator, the value of the modulation chirp parameter that minimizes the transmission power penalty caused by fiber chromatic dispersion was experimentally identified. System experiments at 5 Gb/s using nonreturn-to-zero (NRZ) amplitude-shift-keyed (ASK) transmission with direct detection reception are discussed, and the optimum values of the modulation chirp parameter versus distance for transmission at 1.5 mu m wavelength over fibre having zero dispersion at 1.3 mu m are identified. 5 Gb/s NRZ transmission was achieved through distances of 128, 192, and 256 km of conventional fiber while incurring dispersion penalties of -0.5, 0.1, and 1.1 dB respectively, by operating at the quantum chirp value.<>     

Theoretical performance of multigigabit-per-second lightwavesystems using injection-locked semiconductor lasers

The locking conditions for multigigabit-per-second modulation are examined, and the dependence of the receiver sensitivity on the fiber dispersion coefficient-length product is investigated. With a 4.8-Gb/s NRZ (nonreturn-to-zero) modulation, a 1-dB penalty in receiver sensitivity occurs for a transmission distance of 68 km. The injected power is 0.4 mW and the frequency detuning is -35 GHz. With 10-Gb/s NRZ modulation, the allowable transmission distance is 12.5 km for an injected power of 1.0 mW and a frequency detuning of -35 GHz. These results represent increases in the transmission distances obtained with a solitary laser by factors of 3.7 at 4.8 Gb/s and 2.7 at 10 Gb/s     

Transmission limitations due to self-phase modulation in opticalPSK heterodyne detection systems employing chromatic dispersionequalization

Even if the amplitude of a phase-modulated optical signal is constant before transmission, amplitude modulation is caused by fiber chromatic dispersion. As a result, self-phase modulation (SPM) is induced. In optical heterodyne detection, SPM cannot be compensated for by the delay equalizer (electrical domain) used to compensate fiber chromatic dispersion. However, the transmission distance limitation of multi-repeatered coherent transmission systems has not been investigated in the presence of SPM. This paper theoretically and experimentally investigates the transmission distance achievable with a phase-shift-keying (PSK) heterodyne detection system employing in-line optical amplifiers and delay equalization. The calculated results show that equalization is effective when γP₀/2B²|β₂|<10 in the normal dispersion regime, and γP₀/2B²|β ₂|<15 in the anomalous dispersion regime. Furthermore, the increase in transmission distance achieved by using equalization is experimentally shown in an 8 Gb/s PSK heterodyne transmission experiment using a conventional single-mode (SM) fiber and in-line fiber amplifiers     

Chromatic dispersion compensation in coherent opticalcommunications

Chromatic dispersion compensation techniques in coherent transmission systems are reviewed and discussed for potential feasibility. The key compensation device is the wideband delay equalizer. It is shown that stripline-type delay equalizers have the potential for compensating distortion up to 10 Gb/s using a conventional 1.3-μm zero-dispersion single-mode fiber at 1.5 μm. Chromatic dispersion is successfully compensated with a stripline delay for CPFSK transmission at 4 and 6 Gb/s over 200 km of 1.3-μm zero-dispersion single-mode fiber at 1.55 μm. The bandwidth requirement of the compensation techniques for heterodyne detection is more than 10 GHz. However, it is difficult to realize such broadband receivers. Therefore, phase diversity detection with dispersion compensation is a promising scheme     

All-Raman ultralong-haul single-wideband DWDM transmission systems with OADM capability

In this paper, we present a comprehensive experimental investigation of an all-Raman ultrawide single-band transmission system for both 10 and 40 Gb/s line rates. Enabling technologies include forward-Raman pumping of the transmission fiber, counter-Raman pumping of the fiber spans and dispersion compensation modules, wideband dispersion, and dispersion-slope compensation, and modulation formats resistant to both linear and nonlinear impairments. Ultralong-haul (ULH) 128/spl times/10 Gb/s return-to-zero (RZ) and ultrahigh-capacity (UHC) 64/spl times/40 Gb/s carrier-suppressed (CS) RZ transmission are demonstrated for commercially deployed fiber types, including both standard single-mode fiber (SSMF) and nonzero dispersion shifted fibers (NZDSF). The span losses of 23 dB (NZDSF) and 20 dB (SSMF) are consistent with those encountered in terrestrial networks. The optical reaches for 10 Gb/s rate are 4000 km (NZDSF) and 3200 km (SSMF). Using the same distributed Raman amplification (DRA) scheme, UHC over 2.5 Tb/s at a 40-Gb/s per channel rate is also demonstrated for all of the tested fiber types and for optical reaches exceeding 1300 km. We then study the impact of including optical add/drop modules (OADMs) in the transmission system for both 10 and 40 Gb/s channel rates. System performance is characterized by the system margin and the transmission penalty. For all of the experiments shown in this paper, industrial margins and small transmission penalties consistent with operation in commercially deployable networks are demonstrated, showing the feasibility of practical implementation of all-Raman amplified systems for ULH and UHC optical backbones. Attractive features of single-wideband transmission enabled by DRA include simplicity of design, flexible gain and gain-ripple control, good noise performance, and a small system footprint.     

Experimental evaluation of chromatic dispersion distortion inoptical CPFSK transmission systems

Receiver sensitivity degradations due to fiber chromatic dispersion without chromatic dispersion compensation in multigigabit optical continuous-phase frequency-shift-keying (CPFSK) transmission systems are evaluated experimentally and theoretically. Transmission distance limited by the chromatic dispersion of 15 ps/nm/km is estimated to be 130 km at 5 Gb/s and 33 km at 10 Gb/s     

Optimum fiber dispersion in high-speed TDM systems

Single-channel transmission at 40 Gb/s and above is investigated by numerical simulations with respect to optimal fiber dispersion. Since optimum dispersion depends on the bit rate nonzero dispersion shifted fiber is recommended for 40-Gb/s transmission and standard single-mode fiber for 160-Gb/s transmission     

Self-phase modulation and dispersion in high data rate fiber-optictransmission systems

The theoretical transmission limits imposed by the interaction of first- and second-order group velocity dispersion and intensity-dependent self-phase modulation (SPM) effects for a range of wavelengths around the zero dispersion wavelength (λ₀) for fibers in which polarization dispersion is negligible are investigated. It is found that increasing the peak input power to 30 mW reduces the transmission distance for data rates greater than 50 Gb/s, if operating at wavelengths shorter than λ₀. Operating at wavelengths longer than λ₀ improves the performance due to the cancellation of first-order dispersion by self-phase modulation. For example, at 50 Gb/s and 30 mW peak input power, the maximum transmission distance is 255 and 162 km, if operating at wavelengths 1 nm longer or shorter than λ₀, respectively. Above 100 Gb/s, higher-order dispersion limits the transmission distance even at wavelengths equal to, or longer than, λ₀. Linear dispersion compensation using a grating-telescope combination can significantly improve system performance for wavelengths where first-order dispersion dominates     

Optimal 40 Gb/s modulation formats for spectrally efficient long-haul DWDM systems

Three modulation formats are compared by numerical simulation of highly dense (75-GHz-spaced for 40 Gb/s channel), long-haul (600-1800 km) wavelength division multiplexed systems with three fiber types. Nonreturn-to-zero (NRZ) format, being the most spectrally compact and the simplest in transmitter and receiver configuration of the three, seems to be capable enough at shorter transmission distances than 1000 km regardless of fiber type. Carrier-suppressed return-to-zero (RZ) format, being the most tolerant to the self-phase modulation effect, showed better performance with fibers having larger chromatic dispersion. However, its transmission distance with low dispersion fibers is severely limited by the four-wave mixing effect. Bit-synchronous intensity modulated differential phase shift keying (IM-DPSK) format seems to be the best choice for a transmission distance beyond 1000 km because of its superior tolerance to optical noise and fiber nonlinear effects regardless of fiber types, despite slightly more complex transmitter and receiver configurations.     

直接检测的光正交频分复用传输系统最小均方自适应算法

为了减少直接检测的光正交频分复用(DD-OOFDM)传输系统中色散对系统的影响,传输系统使用了基于频域的最小均方(LMS)自适应均衡技术,由于基于频域的LMS估计方法计算复杂度低且便于信号块处理,相比最小平方(LS)估计方法,可更有效地追踪信道变化,减小相位噪声对传输系统的影响。实验结果表明,经背靠背(BTB)和100km标准单模光纤(SSMF)传输后,使用频域LMS估计方法的信号比使用频域LS估计方法的信号系统接收功率代价在误码率为10×10-2.5和10×10-2.0时分别降低了2dB及2.5dB,频域LMS估计方法比频域LS估计方法对传输系统具有更好的色散补偿效果。     

100 km fiber span in 292 km, 2.38 Tb/s (16×160Gb/s) WDM DQPSK polarization division multiplex transmission experiment without Raman amplification

We investigate the performance of 160 Gb/s WDM transmission with up to 100 km long fiber spans. Using differential quadrature phase-shift keying (DQPSK) and polarization division multiplexing (PolDM), a 160 Gb/s capacity per wavelength is realized at 40 Gsymbol/s rate. We demonstrate that in a relatively conventional EDFA-supported transmission line with an average span loss of 22 dB, a total distance of 292 km is reached with only three spans. Even without proper dispersion management and Raman amplification, we still observe clear eye openings for all channels. In this spectrally highly efficient system, we have achieved a bitrate × span distance product of 16 Tb/s km per wavelength which is, to our knowledge, the highest figure reported so far for 160 Gb/s WDM systems.     

Comparison between NRZ and RZ signal formats for in-line amplifiertransmission in the zero-dispersion regime

Nonreturn-to-zero (NRZ) and return-to-zero (RZ) signal formats are experimentally and numerically compared for single-channel long-distance transmission in an in-line amplifier system with dispersion management providing average zero dispersion and local nonzero dispersion at an interval equal to the in-line amplifier spacing. Among a 20-ps RZ signal, a 40-ps RZ signal, and an NRZ signal transmitted in 10 Gb/s straight-line experiments, the last signal achieves the longest transmission distance of 6000 km while the others are limited to 4400 km. Numerical simulations explain these results well and show that, along with linear amplified spontaneous emission (ASE) accumulation, signal waveform distortion due to the combined effect of higher order group-velocity dispersion (GVD) and self-phase modulation (SPM) dominates the performance. Nonlinear optical noise enhancement is not obvious because of the fiber dispersion arrangement. Signals with large pulse widths are less affected by the combined effect, while small-width signals yield superior initial signal-to-noise ratio (SNR) as determined by optical noise. A detailed simulation indicates that a pulse width of about 60 ps is optimum for long distance transmission under the fiber dispersion arranged in this paper     

40-Gb/s RZ transmission over a transoceanic distance in a dispersion managed standard fiber using a modified inline synchronous modulation method

This paper analyzes in detail numerically a 40-Gb/s return-to-zero (RZ) transmission system over a transoceanic distance in a strongly dispersion managed line composed of standard single-mode fiber (SMF) and dispersion compensation fiber (DCF). We derived a periodically steady-state pulse (a DM soliton) in a DM line. Since the pulse width of a steady-state pulse is too broad for a 40 Gb/s system, the conventional in-line synchronous modulation technique cannot greatly improve the transmission quality. However, we found that the modified inline synchronous modulation technique, which is reported as the black-box optical regenerator, can effectively extend the transmission distance even in such a strongly DM line. We discuss the mechanism of the modified synchronous modulation technique with respect to a steady-state pulse in a transmission line, and show that a 40-Gb/s RZ signal can be transmitted over 20 000 km.     

Transmission properties of 1.3-/spl mu/m InGaAlAs MQW FP lasers in 10-gb/s uncooled operation

Limitations on transmission by an uncooled InGaAlAs Fabry-Perot (FP) laser in 10-Gb/s operation are experimentally and theoretically investigated. The InGaAlAs laser has both high relaxation-oscillation frequency and superior light-current characteristics over a wide temperature range, making it suitable for uncooled operation at 10 Gb/s. Over most of the temperature range used in transmission testing with standard-dispersion fiber, a small power penalty of 1 dB for a bit error rate (BER) of 10/sup -12/ was obtained in transmission over 4-7 km at the measured temperature range, whereas an error floor was seen in transmission over greater distance, despite the negative chromatic dispersion between the fiber and laser light. Theoretical analysis of these results indicates that mode-partition noise (MPN) imposes the major limitation on transmission distance. Moreover, the minimum transmission distance was estimated as 2.1 km, even when the production tolerance of lasing characteristics and zero-dispersion range of installed fiber are taken into account.     

Compensation for chromatic dispersion and nonlinear effect in highdispersive coherent optical repeater transmission system

A design for a chromatic dispersion equalizer that provides 4.4 times higher efficiency in the dispersion compensation characteristics, compared with a conventional equalizer, is proposed. In addition, the amplitude response slope in the frequency domain is less than half of the conventional characteristic. This extends the compensation limit for chromatic dispersion up to 82500 ps/nm for a 2.5-Gb/s heterodyne system, which corresponds to a 4900-km normal dispersion fiber transmission system. A compensation method for modulational instability is also proposed. The method was confirmed by a 2.5-Gb/s continuous-phase frequency-shift-keying (CPFSK) 764-km normal dispersion fiber transmission experiment, with the abovementioned chromatic dispersion equalizer. Employing computer simulations, an over-1000-km normal dispersion fiber optical repeater transmission system with 2.5-Gb/s CPFSK heterodyne detection was shown to be feasible     

10-Gb/s upgrade of bidirectional CWDM systems using electronic equalization and FEC

We discuss options for upgrading coarse wavelength-division multiplexed (CWDM) optical access links over standard single-mode fiber (SSMF) by increasing per-channel data rates from 2.5 to 10 Gb/s. We identify electronic equalization and forward error correction (FEC) as the enabling technologies to overcome the dispersion limit of SSMF. In addition, we show how FEC enhances the tolerance to in-band crosstalk, and paves the way toward fully bidirectional CWDM transmission. Due to the lack of CWDM sources rated for 10-Gb/s operation, we demonstrate full-spectrum (1310 to 1610 nm) 10-Gb/s CWDM transmission over standard-dispersion fiber using uncooled, directly modulated lasers specified for 2.5 Gb/s. All 16 CWDM channels could be transmitted over more than 40 km, yielding a capacity-times-distance product of 6.4 Tb/s/km. The longest transmission distance (80 km) was achieved at 1610 nm, equivalent to 1600 ps/nm of chromatic dispersion.     

Design of a transversal equalizer for electronic dispersion compensation in optical communication links

A programmable transversal equalizer for electronic dispersion compensation(EDC) in optical fiber communication systems is developed.Based on the SiGe technology with a cut-off frequency of 80 GHz,the equalizer consists of 6 seriesparallel amplifiers as delay units and 7 Gilbert variable gain amplifiers as taps,which ensure that the equalizer can work at the bit rate of 10 Gb/s.With different tap gains,the forward voltage gain of the transversal equalizer varies,which demonstrates that the equalizer has various filtering characteristics such as low pass filtering,band pass filtering,band reject filtering,and notch filtering,so it can effectively simulate the inverse transfer function of dispersive channels in optical communications,and can be used for compensating the inter-symbol interference and other nonlinear problems caused by dispersion.The equalizer(including pads) occupies an area of 0.40 mm × 1.08 mm,and its total power dissipation is 400 mW with 3.3 V power supply.