Optical Systems With High-Order DPSK and Star QAM Modulation Based on Interferometric Direct Detection

Increased interest in novel modulation formats for optical transmission has come up in recent years. Receivers with interferometric direct detection can be used to detect arbitrary modulation formats with differentially encoded phases as differential phase-shift keying (DPSK) and differentially phase-encoded star-shaped quadrature amplitude modulation (Star QAM). In this paper, two novel 16-ary modulation formats, which are the 16 DPSK and the Star 16 QAM (ASK-8 DPSK), are characterized for optical transmission for the first time. To be able to identify clear performance tendencies for high-order optical modulation, the novel formats are compared to a wide range of already investigated formats by conducting comprehensive calculations in a uniform simulation environment. The influence of different transmitter structures and decision schemes is considered, and all the systems are characterized with respect to the optical signal-to-noise ratio requirements, dispersion tolerance, and self-phase-modulation (SPM) performance for nonreturn-to-zero (NRZ) and RZ pulse shapes. Moreover, an inherent problem of Star QAM transmission concerning SPM is illustrated, and compensation techniques are examined. The results give a substantial insight into the properties of high-order optical modulation formats.     

强度调制高速光通信系统传输性能研究

码型调制格式是提高通信传输质量的一项关键技术,针对高速光传输系统中码型调制格式及其传输性能进行理论分析,重点研究了强度调制格式在高速光传输系统中的性能;通过光通信仿真软件搭建强度调制格式的产生装置,得到各种调制格式的光谱,对强度调制格式在系统中抗色散能力和抗非线性能力进行了研究和分析。     

新型光接收系统架构：应用于超大容量低成本的短距光互连

大规模数据中心等新兴网络基础设施的部署急需超大容量低成本的短距光互连系统。传统的直调直检系统拥有简单的接收机结构,但只能检测信号的强度信息,这限制了其系统容量的进一步扩展。经典的相干传输系统能够调制解调高阶信号进而实现大容量传输,但收/发端都需要昂贵的窄线宽激光器和高复杂度的数字信号处理,这阻碍了其在短距光互连中的广泛应用。新型的直接检测光接收系统旨在结合直接检测和相干检测两者的优点,弥补二者之间的研究空白。因此,新型的光接收系统架构主要基于自相干检测。介绍了新型单偏振、双偏振、少模光接收系统架构。该类新型的光接收系统不需要本振激光器且能通过直接检测恢复光场信号,实现超大容量低成本的短距光互连。     

40/100Gb/s偏振复用相干检测光模块的技术分析

利用偏振复用四相相移键控(PM-QPSK)调制技术和相干检测技术,实现40/100 Gb/s光模块的长距离传输。基于相干检测的偏振复用四相相移键控调制格式是40/100 Gb/s长距离传输的主要研究方向,与直接检测系统相比,相干检测具有更高的频谱效率。现有的相干检测是基于高速模数转换器(ADC)的快速发展,ADC对混频后的基带信号采样,通过数字信号处理(DSP)芯片数字均衡的方式完成相干接收。数字信号处理芯片数字均衡的方式可以实现定时恢复、信号恢复、偏振和偏振模色散(PMD)跟踪,以及色散补偿。     

光双二进制技术在城域光网络中的应用

对一种新型光部分响应数字传输技术即光双二进制技术的特点与发展趋势进行了深入研究和介绍。在详细阐述光双二进制系统的实现原理及主要实现方式,并介绍该技术的主要性能优越性以及近来研究成果的基础上,认为由于其与非归零等传统调制方式相比所具有的色散容忍度大、频谱窄、频带利用率高等诸多优点,使其在城域光纤网中的广泛应用正在日益成为可能。     

Joint Effect of MLSE and Receiver Filters Optimization on Dispersion Robustness of IMDD, DPSK, DQPSK, and Duobinary Modulation

We analyze the effectiveness of a 32-state maximum-likelihood sequence-estimation (MLSE) receiver on chromatic dispersion robustness of optical transmission based on several binary modulation formats: intensity modulation direct detection, differential phase-shift keying, and duobinary line coding. Multilevel differential quadrature phase-shift keying modulation is also analyzed using a four-state 2-bit/symbol joint MLSE processor. For all modulation formats, receiver filters are optimized together with the use of the MLSE technique.     

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.     

Fundamental Limits of Electronic Signal Processing in Direct-Detection Optical Communications

Electronic signal processing is becoming very attractive to overcome various impairments that affect optical communications, and electronic dispersion compensation (EDC) represents a typical application in the currently designed systems. However, the inherent limits in performance achievable by electronically processing the signal at the output of a nonlinear photodetector have not received the attention they deserve. In this paper, we investigate the information-theoretic limits of electronic signal processing in transmission systems employing direct photodetection and two possible modulation formats: 1 on-off keying (OOK) with nonreturn-to-zero pulses; and 2 optical duobinary modulation (ODBM). The analysis is based on the computation of the information rate, i.e., the maximum achievable data transfer rate, and accounts for the modulation format as well as relevant parameters of the transmission scheme. In particular, we investigate the impact of sampling rate, uncompensated chromatic dispersion (CD), and quantization resolution of the electrical signal at the output of a direct photodetector. For OOK systems, the obtained results show that the optical signal-to-noise ratio penalty entailed by EDC can be limited to about 2 dB at most values of CD of interest in current applications. Moreover, ODBM systems at high values of CD can almost perform as OOK systems at zero CD. For all the considered modulation formats, the obtained results show that the received electrical signal can be sampled at a rate of two samples per bit interval and quantized with a precision of 3 bits per sample to practically achieve the ultimate performance limits.     

System Performance of High-Order Optical DPSK and Star QAM Modulation for Direct Detection Analyzed by Semi-Analytical BER Estimation

Recently, higher order modulation formats are intensively investigated to further increase spectral efficiency for building next generation optical transport networks. Direct detection receivers are capable of detecting arbitrary modulation formats with differentially encoded phases such as differentially phase shift keying (DPSK) and star shaped quadrature amplitude modulation (Star QAM) formats. In an earlier publication of the authors (M. Seimetz , “Optical systems with high-order DPSK and star QAM modulation based on interferometric direct detection,” J. Lightw. Technol. , vol. 25, no. 6, pp. 1515–1530, Jun. 2007) a system analysis was performed where performance was mainly characterized by eye opening penalties. Here, these investigations are extended. A tool for semi-analytical BER estimation is developed allowing to calculate the BER down to small values such as $10^{-9}$ for a wide range of modulation formats, as well as for different receiver structures. Using this module, the back-to-back OSNR requirements are calculated. CD and SPM tolerances are characterized by optical signal-to-noise ratio (OSNR) penalties at ${hbox {BER}} = 10^{-9}$ . As far as the authors know, this is the first analysis of the transmission characteristics of optical 16DPSK and Star 16QAM based on the BER. Moreover, further novel aspects such as optimization of the optical and electrical receiver filter bandwidths are investigated.      

Transmission Performance Comparison of Direction Detection‐Based 100‐Gb/s Modulation Formats for Metro Area Optical Networks

Transmission performances of direct detection‐based 100‐Gb/s modulation formats are investigated and compared for metro area optical networks. The effects of optical signal‐to‐noise ratio sensitivity, chromatic dispersion, cross‐channel nonlinearity, and transmission distance on the performance of differential 8‐ary phase‐shift keying (D8PSK), differential phase‐shift keying plus three‐level amplitude‐shift keying (DPSK+3ASK), and dual‐carrier differential quaternary phase‐shift keying (DC‐DQPSK) are evaluated. The performance of coherent dual‐polarization quadrature phase‐shift keying (DP‐QPSK) with block phase estimation and coherent DP‐QPSK with digital differential detection are also presented for reference. According to our analysis, all three direct detection modulation formats could transmit a 100‐Gb/s signal over several hundred kilometers of a single‐mode fiber link. The results also show that DC‐DQPSK outperforms D8PSK and DPSK+3ASK, and the performance of DC‐DQPSK is comparable to that of coherent DP‐QPSK with digital differential detection. The maximum transmission distance of DC‐DQPSK is over 1,000 km, which is enough distance for metro applications.     

光传输网（OTN）的技术演进和标准化进展

简单回顾了光纤通信系统的发展历程,指出建设下一代每信道超过100 Gbit/s的高速相干光纤通信系统的必要性和可用技术,列出使用不同的调制和检测技术,每信道100 Gbit/s的WDM系统实现的不同传输容量和频谱效率,介绍了光传输网（OTN）的技术演进、标准化进展和一种可能的OTN复用映射结构。     

High-capacity coherent lightwave systems

Recent theoretical work on coherent optical detection systems is reviewed and experimental results in high-speed coherent transmission are summarized. The theoretical advantages and limitations of the various modulation and detection formats are discussed and experimental progress towards the implementation of these systems is reviewed. The most significant obstacles to the attainment of quantum-noise limited detection at higher speeds are seen to be the requirement of uniform frequency response from electronic components and the local oscillator laser power requirement, which increases as the square of the bit rate. To make full use of the single-mode fiber bandwidth, frequency-division multiplexing of many moderate-rate channels is a very promising technique for local systems. For long-distance applications, frequency multiplexing is still possible but is limited by the need for optical amplifiers or wavelength-selective multiplexers     

Single and Dual-Level Minimum Shift Keying Optical Transmission Systems

We present optical transmission systems employing minimum shift keying modulation formats of single and dual-amplitude level under linear, weakly nonlinear, strongly nonlinear variation of the lightwave carrier within a bit-period depending on whether the phase variation within a symbol period is linear or nonlinear. These formats are externally modulated, incoherently and differentially detected based on the Mach-Zehnder delay interferometric optical balanced receiver. Transmission performance of these optical transmission systems is evaluated in terms of receiver sensitivity, amplification stimulated emission noise loading, dispersion tolerances. These performance characteristics are compared with return-to-zero (RZ) differential phase shift keying (DPSK) and carrier-suppressed RZ on-off keying modulation formats. Accurate bit-error ratios are obtained and confirmed by different statistical techniques: Monte Carlo, single-Gaussian or multiple Gaussian distributions and generalized Pareto distribution statistical methods, especially when the eye diagrams are distorted. Among the three minimum shift keying (MSK) types, the weakly nonlinear optical MSK is found to be the most promising because of its robust transmission performance and more importantly, its reduced-complexity of the electrical driving signals for transmitter in modulating the lightwave carrier as compared to the linear MSK counterpart. Transmission performance of dual-level MSK optical transmission systems depends on the intensity-splitting ratio of the two levels. The performance of three ratios: 0.7/0.3,0.8/0.2, and 0.9/0.1 are demonstrated. The spectral attributes of 80 Gb/s dual-level MSK optical signals for these three ratios are similar to each other and compatible with that of 40 Gb/s optical MSK, but narrower than that of 40 Gb/s optical nonreturn-to-zero DPSK, hence high spectral efficiency of the dual-level MSK.     

On the use of NRZ, RZ, and CSRZ modulation at 40 Gb/s with narrow DWDM channel spacing

The use of nonreturn-to-zero (NRZ), return-to-zero (RZ), and carrier-suppressed return-to-zero (CSRZ) modulation formats in an ultradense wavelength-division multiplexing (UDWDM) scenario at 40 Gb/s is investigated. The results of a simulative analysis on back-to-back setups are presented. Narrow optical filtering at the receiver and at the transmitter, as well as the orthogonal polarization launch of adjacent channels, is studied in order to improve spectral efficiency. We demonstrate that standard setups do not allow acceptable performances with 50-GHz channel spacing for all the considered modulation formats, while the use of a transmission optical filter may dramatically improve the performance of RZ and CSRZ modulation formats that become suitable for the use in UDWDM systems. We show that, due to the narrow transmission filtering, the RZ pulse becomes NRZ-like, and the CSRZ modulation is duobinary coded. Furthermore, we demonstrate that NRZ modulation does not benefit from the introduction of a transmission optical filter, while it takes advantage of the orthogonal polarization launch of adjacent channels, but its performance is still worse than the RZ and CSRZ performance in a UDWDM scenario.     

Polarization modulated direct detection optical transmissionsystems

A novel polarization modulated direct detection (PM-DD) system suitable both for binary and multilevel transmission is presented. At the transmitter the optical field is polarization modulated by a standard modulator. The receiver is based on the estimation of the Stokes parameters of the received optical field by means of a direct-detection optical front end and baseband electrical processing. The Poincare sphere rotation induced by the fiber is compensated by means of a purely electronic algorithm and the decision is performed in the Stokes space. The system performance is evaluated by an analytical model when the only relevant noise source is the receiver thermal noise and when erbium-doped optical amplifiers introduce amplified spontaneous emission (ASE) noise. The system is completely compatible with a direct-detection-based optical network, and it is possible to implement efficient multilevel modulation formats     

An efficient multilevel coherent optical system: M-4Q-QAM

A coherent multilevel transmission system based on the modulation of all the optical field degrees of freedom (the four quadratures of the optical field) is presented. The transmitted symbols are represented in the space of the field quadratures by a square lattice of points, so that both the transmitter and the decision devices can be easily implemented. The proposed modulation formats allow high spectral efficiency systems to be designed without paying a great sensitivity penalty with respect to binary systems. For this reason the proposed modulation format seems promising for application in densely spaced very-high-capacity FISM networks and in high-speed data transmission in parallel computers     

Prechirp technique as a linear dispersion compensation forultrahigh-speed long-span intensity modulation directed detectionoptical communication systems

Prechirp technique, as a linear dispersion compensation for intensity modulation direct detection (IM-DD) optical transmission systems, has been investigated theoretically and experimentally. This technique is based on a predistortion technique in an optical transmitter. Implementation to the ordinary IM-DD optical transmitter, which uses an external intensity modulator, is easily realized merely by adding a small injection current modulation to a semiconductor laser diode, allowable optical transmission fiber dispersion will be more than doubled with this technique, modified prechirp technique, which utilizes a time division superimposing prechirped bit streams, has also been investigated to achieve greater dispersion compensation capability     

高速密集波分复用系统中的二级调制格式研究

论述了强度调制直接检测(IM-DD)高速密集波分复用(DWDM)系统的二级调制原理，并从马赫-曾德尔调制器的传递特性出发推导了四种调制格式[全频率调制归零码(FFMRZ)、半频率调制归零码(HFMRZ)、单边带调制归零码(SSBRZ)、载波抑制归零码(CSRZ)]。根据信号光眼图及归一化频谱描述了各种码型的时频特征。在此基础上分别对各种码型的色散容限、非线性容限进行了数值仿真，通过比较眼图张开度损伤(EOP)发现对于单波长系统载波抑制归零码传输性能最优。最后利用全面的密集波分复用系统模型计算了各种码型的Q因子，发现载波抑制归零码的性能仍然保持最优，这种优势在考虑偏振模色散(PMD)的情况下更为突出。因此载波抑制归零码是强度调制直接检测系统较好的选择。     

Advanced Optical Modulation Formats

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity.     

Theoretical investigation of optical wavelength conversion techniques for DPSK Modulation formats using FWM in SOAs and frequency comb in 10 gb/s transmission systems

We have investigated the wavelength conversion techniques for differential phase-shift keying (DPSK) modulation formats in 10 Gb/s transmission systems, compared with the non-return-to-zero (NRZ) modulation format. For the wavelength conversion of DPSK modulation formats, we employed the wavelength converters based on the four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) and the frequency comb generated by phase modulation. The power penalty at 10/sup -9/ bit error rate was used as a measure of the system performance degraded by the wavelength conversion. Our simulation results show that the DPSK modulation formats have a smaller power penalty than the NRZ modulation format for the wavelength conversion using the FWM effect in an SOA due to a much lower pattern effect. However, as the wavelength conversion uses the frequency comb generated by phase modulation, it has a similar power penalty compared with the NRZ modulation format. It is also shown that the DPSK modulation formats are possible to obtain the power penalty less than 0.4 dB for both wavelength conversion techniques.