Researches On Several Key Technologies For High-Speed Coherent Optical Communication Systems

Recent years, the ever-growing demand for telecommunications makes it indispensable to increase the capacity of communication systems. The large-scale commercialization of100G technology has started. With the development of high-speed analog-to-digital converters (ADCs), digital signal processing (DSP) and manufacturing technology of optical components, revival of coherent optical communications research was ignited and it became possible to commercialize coherent optical communication systems. For100G technology, digital coherent optical communications is the vital part and inevitably applied in future. Combining coherent detection and DSP, digital coherent optical communications allow carrier phase recovery and polarization-tracking to be realized electronically which breaks through the bottlenecks of traditional analog coherent optical communications.Laser, transmitter and receiver are three important parts in coherent optical communication systems. The linewidth of laser should be narrow and the frequency of laser should be stable. Most commercial tunable lasers could meet the requirements. Unfortunately, it is hard to measure the linewidth of a narrow linewidth laser and integrate the measurement system. Generally the external modulation is performed in transmitters using IQ optical modulator based on Mach-Zehnder modulator (MZM). It is necessary to lock the bias point of a MZM to gain the best performance due to bias drifts. Technologies, such as balance detection and DSP, are used in receiver to fulfill functions including demodulation and fiber impairment compensation. Moreover, forward error correction (FEC) technique is an effective way to suppress channel impairment and widely used in coherent optical communication systems. Correspondingly, high-speed reliable encoders and decoders are necessary. It is worthy to focus on several key technologies because they would solve different problems and meet respective requirements for laser, transmitter and receiver.This dissertation’s topic is researches on several key technologies for high-speed coherent optical communication systems. Researches mainly focus on laser measurement and key technologies used in transmitter. And the researches on related technologies are thorough. Moreover some novel schemes were proposed. Besides a bunch of hardware system demonstrations and experiments were performed. Highlighted work includes:1. An improved delayed self-heterodyne measurement system was proposed and implemented. It was based on delayed self-heterodyne method. Optical circulator and Faraday rotator mirror was deployed to reduce the required length of delay fiber by half and make the measurement system more integrated. An experiment was performed to measure a laser’s linewidth and the linewidth of the test laser was about500kHz. Lasers of which the linewidth was above20kHz could be measured by the proposed measurement system. If the length of delay fiber used in the measurement system was increased, it would be able to measure lasers of which the linewidth were narrow than20kHz.2. A novel auto bias control technique for a Lithium Niobate (LiNbO3) MZM was proposed. It was based on that the bias drift would change the bias point and result in varying the output optical average power of MZM and its first and second derivatives. The ratio of the first to the second derivative of the output optical average power was used in the proposed technique as the key parameter. Theoretical analysis implied that the proposed technique could lock a MZM at any bias point along the power transmission curve. It was also a modulation format free technique and not related to with the bit rate of the optical modulated signal.3. Based on the proposed auto bias control technique, a feedback system for controlling the bias point of a MZM was implemented. An experimental system was built to test the performance of the feedback system. The experimental results showed that the output optical average power of LiNbO3MZM hardly changed at the desired bias point, and the maximum deviation of output optical average power was less than±4%. In the back-to-back test, NRZ-OOK modulation format was generated and the modulation speed was10Gbit/s. The bit error rate was about10-9and changed to10-2without the auto bias control feedback system. And the bit error rate was about10-9and hardly changed with the auto bias control feedback system.4. Theoretical analysis of differential phasor monitor was accomplished and it showed that a differential phasor monitor could perform real-time phase monitoring as long as the modulation format was related to phase modulation. Based on differential phasor monitor, an auto bias control technique based on real-time phase monitoring for IQ modulator consisted of several MZMs was proposed. Bias drift would affect the phase of optical modulated signals and phase error could be calculated to analyze the bias drift. And then a feedback system could be used to control the bias point of IQ modulator.5. An integrated IQ transmitter was designed. Integrated meant that IQ transmitter combined the modulation and control effectively and the hardware and software cooperated with each other. Its hardware was consisted of several components, modules and circuits. Modularized design made it integrated. The designs of gain control module, auto bias control module and control processing unit were the vital parts. Software was designed to set the parameters and monitor the integrated IQ transmitter. It could generate complex modulation formats and the modulation speed could be up to22.5GBaud. The integrated IQ transmitter had a friendly and flexible interface. It also deployed auto bias control module to realize auto bias control for IQ modulator.6. A prototype of the integrated IQ transmitter was built. All the work, such as designing the hardware and software, trouble shooting the problems, were finished alone by the writer. An experimental platform was built. Both signal analyzer and optical modulation analyzer were used to perform time domain tests. The modulation format was QPSK and back-to-back system test was accomplished by optical modulation analyzer. The test results showed that the prototype successfully generated10GBaud and22.5GBaud QPSK modulated signals. And EVM was about9%and11%respectively. The effect of the signal impairments was quite acceptable.7. A simple and effective searching algorithm was proposed and verified. The searching algorithm could calculate the number of girth-4in parity check matrix of π-rotation low-density parity-check (LDPC) codes. Under the help of the searching algorithm, a couple of keys were defined and these keys could generate parity check matrix of π-rotation LDPC codes without girth-4. And hardware encoder was implemented successfully.8. A study of decoding algorithms of LDPC codes was carried out. And hardware decoders based on DSP chip were successfully implemented and experimentally demonstrated. Experimental results showed that the LLR BP decoder had better performance than BF decoder.