高速相干光通信系统中关键技术的研究

发布时间：2018-08-13 20:54

【摘要】：近些年,随着信息业务容量持续增长,进而通信系统的容量需求也不断提高。100G规模商用的进程已经开始。随着模数转换(ADC)、数字信号处理(DSP)、光器件制造等技术的发展,相干光通信重新成为研究热点。数字相干光通信是100G技术的重要支撑,也是未来光通信的必然发展道路。通过结合相干检测和DSP技术,数字相干光通信可将载波相位恢复、偏振跟踪等工作在电域中实现,突破了传统模拟相干光通信的障碍。相干光通信系统中,光源、发送端和接收端是三个重要组成部分。对于光源,要求光谱线宽窄、频率稳定度高,当前器件技术已经很好地达到该要求,但如何测量窄线宽激光器的线宽并使测量系统集成化是有待解决的问题。对于发送端,可通过外调制技术完成调制工作,广泛使用基于马赫-曾德尔调制器(MZM)的IQ调制器来实现高阶调制格式,但MZM在使用过程中需要稳定控制偏置点以保持良好的调制效果。对于接收端,可采用平衡接收技术、DSP技术等完成解调、补偿传输损伤等功能。此外,前向纠错编码(FEC)技术可以很好地抑制信道损伤,因此已被广泛应用在相干光通信系统中,相应的需要高速可靠的编译码器,在接收端有译码器。光源、发送端、接收端这三部分有各自需要达到的要求及瓶颈问题,因此每一项解决问题的关键技术都具有重要的研究意义。本论文围绕高速相干光通信系统中的关键技术,特别是高速相干光通信系统中发射端的关键技术开展了深入系统的理论和实验研究。本论文的主要创新点和研究成果包括： 1.提出并实现了改进的延时自外差窄线宽半导体激光器线宽测量系统。该测量系统以单激光器延时自外差法为基础,通过引入光学环形器和法拉第旋转反射镜,使得测量所需的延时光纤长度减半,有效地减小了系统所占体积,提高了系统集成度。利用该测量系统成功地对500kHz线宽的半导体激光器进行了测量。该测量系统适用于测量线宽大于20kHz的半导体激光器,调整系统中延时光纤长度可测量线宽更窄的半导体激光器。 2.提出了一种新型的适用于铌酸锂MZM的偏置点稳定控制技术。该技术中通过监测MZM输出光信号的平均光功率对偏置点漂移进行初步判断,再根据MZM输出光信号平均光功率的一阶与二阶导数之间的比值确定漂移的真实性,在漂移确实发生后判断出漂移方向,进而可以相应地调整偏置电压,达到对MZM偏置点稳定控制的目的。理论研究表明,该技术可以将MZM的偏置点稳定控制在传输响应曲线上任一位置,可适用于多种调制格式,与调制速率无关。 3.基于所提出的新型MZM偏置点稳定控制技术,实现了MZM偏置点稳定控制反馈系统。搭建了实验测试系统,通过测试对反馈系统进行了功能验证和性能评估。实验测试结果表明,利用所提出的新型技术,能够正确地实现对MZM的偏置点进行稳定控制,偏置点可为传输响应曲线上任一位置,并且MZM输出光信号平均光功率的波动被控制在±4%之内。在调制速率为10Gbit/s,调制格式为非归零开关键控(NRZ-OOK)的背靠背系统测试中,若不包含偏置点稳定控制反馈系统,误码率会从10-9级别恶化到10-2级别,而包含偏置点稳定控制反馈系统后,误码率可以保持在10-9级别,很好地保持了系统性能。 4.理论分析了差分相位监测器的工作原理,结果表明差分相位监测器可用于对调制光信号进行实时相位监测,且适用于各种进行相位调制的调制格式。基于差分相位监测器,提出了适用于IQ调制器的基于相位监测的偏置点稳定控制技术。理论研究表明,偏置点漂移现象会影响调制光信号的相位,利用差分相位监测器来进行实时相位监测,便可根据相位误差来分析偏置点漂移程度,进而可以通过反馈系统对IQ调制器的偏置点进行稳定控制。 5.设计了集成IQ发射机。集成IQ发射机采用了一体化解决方案,使调制与控制有效结合,硬件与软件有效集成。在实现方案中,硬件部分集成了调制与控制需要的各器件、模块、电路,并进行了模块化的设计,重点设计了增益控制模块、偏置控制模块和控制处理单元等部分；软件部分研发了配套的上层控制软件,可设置控制参量并监控集成IQ发射机的工作状态。该集成IQ发射机采用外调制方式,支持高阶调制格式,调制速率最高可达22.5GBaud,并拥有丰富的、友好的、便捷的人机操控接口,具备自动进行偏置点稳定控制的功能。 6.独立完成了集成IQ发射机样机的研制工作,包括硬件设计与调试,软件编写等。搭建了实验测试平台,利用信号分析仪和基于数字相干技术的光调制信号分析仪,对样机进行了时域测试,验证了其功能并评估了其性能。测试时的调制格式为QPSK,并利用光调制信号分析仪得到了背靠背系统的指标。当调制速率为10GBaud时,测试结果显示,该样机正确地调制出了10GBaud的QPSK信号,EVM为9%,背靠背系统无误码。当调制速率提高到22.5GBaud时,测试结果显示,该样机则正确地调制出了22.5GBaud的QPSK信号,EVM为1l%,背靠背系统也无误码。测试结果还显示,样机的信号损伤较小。 7.提出并验证了一种简单有效的四环搜索算法,可计算π旋转低密度奇偶校验(LDPC)码校验矩阵中四环(Girth-4)的个数。利用该算法,经过分析比较,给出了几组索引(Key),使得利用这几组索引构造的π旋转LDPC码,其校验矩阵中不会存在四环,进而提升该π旋转LDPC码的性能。此外,基于对π旋转LDPC码编码器硬件实现方式的研究分析,在硬件上实现了π旋转LDPC码编码器。测试结果显示,该硬件编码器正确地完成了π旋转LDPC码编码功能。 8.研究分析了LDPC码的译码算法,利用DSP芯片实现了基于硬判决的BF算法译码器和基于软判决的LLR BP算法译码器,并比较了两种译码器的性能。测试结果表明,LLR BP算法译码器性能优于BF算法译码器,该结论与理论分析结果一致。
[Abstract]:In recent years, with the continuous growth of information service capacity, the capacity requirement of communication system is also increasing. 100G commercial process has begun. With the development of analog-to-digital conversion (ADC), digital signal processing (DSP), optical device manufacturing and other technologies, coherent optical communication has become a research hotspot again. Digital coherent optical communication is an important 100G technology. By combining coherent detection with DSP technology, digital coherent optical communication can realize carrier phase recovery and polarization tracking in the domain, breaking through the obstacles of traditional analog coherent optical communication.
In coherent optical communication system, light source, transmitter and receiver are three important components. For light source, narrow spectral linewidth and high frequency stability are required. Current device technology has achieved this requirement very well, but how to measure the linewidth of narrow linewidth laser and integrate the measurement system is a problem to be solved. External modulation technology is used to complete the modulation work, and IQ modulator based on Mach-Zehnder modulator (MZM) is widely used to achieve high-order modulation format. However, MZM needs to stabilize the bias point to maintain good modulation effect. For the receiver, balanced reception technology, DSP technology can be used to complete demodulation to compensate transmission damage. In addition, forward error correction coding (FEC) technology can effectively suppress channel damage, so it has been widely used in coherent optical communication systems, the corresponding need for high-speed and reliable encoder and decoder at the receiver, light source, transmitter, receiver these three parts have their own needs and bottlenecks, so each of them has its own requirements and bottlenecks. The key technologies to solve problems have important research significance.
In this paper, the key technologies of high-speed coherent optical communication system, especially the key technologies of transmitter in high-speed coherent optical communication system, have been studied systematically and theoretically.
1. An improved delay self-heterodyne narrow linewidth semiconductor laser linewidth measurement system is proposed and implemented. The system is based on the single laser delay self-heterodyne method. By introducing an optical ring and a Faraday rotating mirror, the required delay fiber length is reduced by half, the volume of the system is reduced effectively, and the system is improved. A 500 kHz linewidth semiconductor laser has been successfully measured with this measuring system. The measuring system is suitable for measuring semiconductor lasers with linewidth greater than 20 kHz and adjusting the length of the delay fiber in the system to measure narrower linewidth.
2. A novel offset point stabilization control technique for lithium niobate MZM is proposed. The offset point drift is estimated by monitoring the average optical power of the MZM output signal, and then the drift is determined by the ratio of the first-order to the second-order derivative of the average optical power of the MZM output signal. After birth, the drift direction can be judged, and then the bias voltage can be adjusted accordingly to achieve the purpose of stabilizing the MZM bias point. Theoretical studies show that this technique can stably control the MZM bias point at any position on the transmission response curve, and can be applied to a variety of modulation formats, independent of modulation rate.
3. Based on the proposed new MZM bias stabilization control technology, the MZM bias stabilization control feedback system is realized. An experimental test system is built, and the feedback system is verified and evaluated by testing. The experimental results show that the proposed new technology can correctly stabilize the bias of MZM. With fixed control, the offset point can be any position on the transmission response curve, and the fluctuation of the average optical power of the MZM output signal is controlled within (+) 4%. At the modulation rate of 10 Gbit/s and the modulation format of NRZ-OOOK back-to-back system test, the bit error rate will be from 10-9 level if the offset point stabilization control feedback system is not included. Don't deteriorate to 10-2 level, but the BER can be maintained at 10-9 level with the bias stabilization control feedback system, and the system performance is well maintained.
4. The working principle of the differential phase monitor is analyzed theoretically. The results show that the differential phase monitor can be used for real-time phase monitoring of modulated optical signals and can be used for various modulation formats. Based on the differential phase monitor, a phase-based bias stabilization control technique for IQ modulator is proposed. Theoretical research shows that the phase of modulated optical signal is affected by the offset drift. The phase error can be used to analyze the offset drift, and then the offset of IQ modulator can be stably controlled by the feedback system.
5. The integrated IQ transmitter is designed. The integrated IQ transmitter adopts an integrated solution, which effectively combines modulation and control, and effectively integrates hardware and software. In the implementation scheme, the hardware part integrates all the components, modules and circuits needed for modulation and control, and carries out modular design. The gain control module and bias control module are designed in particular. The control module and the control processing unit, etc. The software part develops the matching upper control software, which can set the control parameters and monitor the working state of the integrated IQ transmitter. The control interface has the function of automatic bias point stability control.
6. The prototype of the integrated IQ transmitter has been developed independently, including hardware design and debugging, software compilation, etc. The experimental test platform has been set up. The time domain test of the prototype has been carried out by using signal analyzer and optical modulation signal analyzer based on digital coherence technology. The function of the prototype has been verified and its performance has been evaluated. When the modulation rate is 10 GBaud, the test results show that the prototype correctly modulates the QPSK signal of 10 GBaud, EVM is 9%, and the back-to-back system has no error code. When the modulation rate is increased to 22.5 GBaud, the test results show that the prototype correctly modulates 22 GBaud. 5 GBaud QPSK signal, EVM is 1l%, back-to-back system is also error-free. The test results also show that the signal damage of the prototype is small.
7. A simple and effective four-loop search algorithm is proposed and verified to calculate the number of Girth-4 rings in the check matrix of pion-rotating low-density parity-check (LDPC) codes. Furthermore, based on the research and analysis of the hardware implementation of the pion-rotating LDPC coder, the pion-rotating LDPC coder is implemented on the hardware. The test results show that the hardware encoder performs the function of the pion-rotating LDPC coder correctly.
8. The decoding algorithm of LDPC is studied and analyzed. The hard-decision-based BF algorithm decoder and the soft-decision-based LR BP algorithm decoder are implemented with DSP chip. The performance of the two decoders is compared. The test results show that the performance of the LLR BP algorithm decoder is better than that of the BF algorithm decoder.
【学位授予单位】：北京邮电大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TN929.1

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