Study On Optical Performance Monitoring And Equalization Techniques In High Speed Fiber-optic Communication Systems

Today the human society is placed in the information age. During the information age, the everyday activities of the human produce high requirements on massive information delivery. As one of the basic infrastructure of modern society, fiber-optic communication systems undertake the majority of the task of the global information delivery.During the past decades, the delivery of information such as voice and IP data, has obtained fast development. At present the fast development of new services and application scenarios as diverse as mobile Internet,online high definition video, Internet of things, Industry 4.0, visual reality,produce explosive growth of the global IP data traffic furtherly. To fulfill the ever-increasing demand for massive information delivery from the human society, fiber-optic communication systems have achieved fast evolution towards high information rate after its birth. In the evolutionary history of the high speed fiber-optic communication systems, the latest success story is 100G digital coherent optical communication systems.The two key technologies in 100G digital coherent optical communication systems are coherent detection and digital signal processing(DSP). Based on coherent detection, the information carried by the optical signal can be down converted linearly to the electrical domain,which enables high order modulation formats and electrical compensation of optical fiber channel impairments. Based on DSP technology, various channel impairments experienced by the optical signal can be compensated by DSP algorithms. Due to the benefits of coherent detection and DSP technology, digital coherent optical communication systems also become the primary technical solution of the future 400G/1T high speed fiber-optic communication systems. Therefore, it is no doubt on the importance of the digital coherent optical communication systems.Related studies on the high speed digital coherent optical communication systems are of great significance.Optical performance monitoring and equalization techniques are two important contents in high speed digital coherent optical communication systems. Optical performance monitoring aims at the monitoring of a group of parameters which can reflect the quality of the optical signal.The usual optical performance monitoring parameters contain the optical signal to noise ratio(OSNR), chromatic dispersion(CD) and polarization mode dispersion(PMD). The goal of optical performance monitoring is to guarantee the reliability of high speed fiber-optic communication systems.Equalization techniques focus on various DSP algorithms used for the compensation of the impairments in digital coherent optical communication systems. As stated above, in digital coherent optical communication systems, the compensation of various channel impairments relies mainly on equalization techniques. Therefore,equalization techniques are necessary in digital coherent optical communication systems. Along with the introduction of numerous new material and technologies, high speed digital coherent optical communication systems always make progressive development. New system parameters and structure emerge continuously. Therefore, optical performance monitoring and equalization techniques for high speed fiber-optic communication systems are valuable research problems.According to the topic of study on optical performance monitoring and equalization techniques in high speed fiber optic communication systems, researches on optical performance monitoring and equalization techniques using digital signal processing technology are carried out in this thesis. The main research work in this thesis is as follows:(1) Periodic training sequence aided in-band OSNR monitoring. In this thesis, we propose a new data-aided in-band OSNR monitoring method, which employs the periodic property of a specially designed short training sequence to differentiate the signal power from noise and is proved to be insensitive to the first order PMD. This method is verified in a typical 112 Gb/s polarization multiplexed-quadrature phase shift keying(PM-QPSK) and a 224 Gb/s polarization multiplexed-16 quadrature amplitude modulation (PM-16QAM) coherent optical transmission system. The results show that the method is highly accurate with the monitoring error less than 1 dB in the OSNR range of 7?30dB under different linewidth and has good robustness to a large amount of residual accumulated CD and first order PMD.(2) Digital pilot aided carrier frequency offset estimation. In this thesis, we present a digital pilot aided carrier frequency offset estimation(FOE) method for coherent optical transmission systems. Unlike the conventional pilot tone insertion scheme, the pilot of the proposed method is generated in a digital manner and can serve as a good FOE indicator. Aided by this kind of digital pilot, the FOE is implemented by determining the location of the digital pilot in the spectrum. Theoretical analysis and numerical simulations show that the proposed method has the advantages in wide range, high accuracy, modulation formats independent, no need to remove the modulation, and high tolerance to the residual CD and PMD.(3) Study on the equalization techniques in digital coherent optical communication systems. In this thesis, we study the typical DSP algorithms in digital coherent optical systems, including the algorithms of the time domain and frequency domain compensation of CD, the algorithms of the polarization demultiplexing based on the constant modulus algorithm, the algorithms of differential phase based carrier frequency offset estimation and the algorithms of Viterbi-Viterbi phase estimation based carrier phase recovery. On the basis, using simulation and experiments, we verify the typical DSP algorithms and obtain good results.