| In recent years, with the sustained and rapid growth of information demand, the communication system capacity and communication rate need to be improved. Now, the large-scale commercialization of 100G technology has started, the technologies beyond-100G are also under urgent research and development. With the development of optical components manufacture technology, and the development of data acquisition chip and digital processing chip manufacture technology, the coherent optical communication has become a research hotspot again. Digital coherent optical communication is the important support of high-speed optical transmission network of 100G and beyond-100G. After coherent reception, the carrier phase recovery, transmission damage compensation and signal demodulation are implemented in the electric field with digital signal processing. Analog coherent optical communication based on the technology of optical phase-locked loop has become an effective solution in the space optical communication field which has restrict limitation in the system sensitivity, power consumption, system size and cannot use the relay system. Using the optical phase-locked loop technology, the system completes homodyne coherent detection with high sensitivity, and can effectively reduce the power consumption and system size.Transmitter and receiver are two important parts of coherent optical communication system. In the transmitter, the signal modulation is usually finished by Mach-Zehnder modulator (MZM) and IQ modulator based on MZM strcuture, implementing all kinds of high-order modulation schemes. The bias point stability is important to keep good modulation effect, high-order modulation technology is important to improve the system transmission capacity, the simplification and integration of signal modulation transmitter is an important premise to realize commercialization. In the receiver, signal is usually received and demodulated by digital coherent optical receiver based on DSP or analog coherent optical receiver based on optical phase-locked loop technique. These technologies have important research significance because they are key technologies of coherent optical communication. This dissertation’s topic focuses on several key techniques in coherent optical communication system, including bias point control for MZM, high-order format modulation, integrated transmitter development and optical phase-locked loop technology, thorough theoretical and experimental researches were carried out, the main innovation points and researches include:1. Two novel bias control technologies for lithium niobate (LiNbO3) MZM were proposed, which were used for fixed points and any point control respectively. The fixed points stability control technology monitored MZM output average optical power and its slope value to bias the MZM stably at four commonly used working points of the transmission response curve. The any point stability control technology monitored dither signal harmonic component and the average optical power to bias the MZM stably at arbitrary working point of the transmission response curve. Both technologies were applicable to several modulation formats and not related to input optical power fluctuations and bit rate of modulated signal. The control algorithem and feedback control system were designed and implemented based on the proposed technologies. The key parameters used for feedback control and the function of the feedback control system were tested and verified through experiment. And then two control schemes were realized respectively. In the back to back system test with lOGbit/s NRZ-OOK, the system BER gradually deteriorated without bias control, and the BER can be controlled at 10-9 level with feedback control in 10 hours, and the system performance was stable.2. The high-order format modulation technique based on dual drive MZM(DD-MZM) was studied. The method of using DD-MZM to generate arbitrary high-order signal modulation was studied from the perspective of DD-MZM structure and signal constellation characteristic. The modulation schemes based on DD-MZM were designed according to the constellation characteristics of QPSK and 16QAM. We completed three QPSK modulation schemes, and two 16QAM modulation schemes of square and star through simulation, and obtained the modulation signal waveform, eye diagram, spectrum and constellation respectively. All the modulations showed good performance. The study was useful to expand the application range of DD-MZM and simplify the transmitter structure.3. The generation schemes of 64QAM signal were researched systematically. Three evolution routes and six implementation schemes were proposed from the perspective of 64QAM signal constellation. The feasibility of the schemes were verified through simulation, and the optical signal characteristics of different implementation schemes were analyzed according to eye diagrams and constellations. The advantages, disadvantages and transmitter complexity of different schemes were compared and analyzed. The study was valuable to 64QAM modulation and transmitter realization.4. A novel modulator structure was proposed, which was defined as triple drive IQ modulator.64QAM signal can be generated with the proposed modulator just by 2-level driven signals, the requirement for the electrical driven signal was low. The results were compared with traditional modulation scheme from electrical driven signal, eye diagram and constellation. The EVM properties and the effect of modulator splitting and combining ratio were tested. When the OSNR was higher than 23dB, the EVM of signal was controlled below 10%, modulation performance was good. Other modulation formats, such as 16QAM and QPSK, can be generated flexibly by selecting modulation optical path reasonably.5. Three different integrated optical transmitters were designed and completed, including integrated intensity modulation optical transmitter, integrated DPSK optical transmitter, integrated IQ optical transmitter, which were suitable for different rate transmission routes respectively. Overall design and transmitters implementation were finished with integrated solutions. The transmitter control softwares were developed to complete the transmitters control parameter setting and the working condition monitoring. Intensity modulation transmitter and DPSK transmitter were tested at lOGbaud. The formats included OOK, BPSK, DPSK, and the eye diagram, spectrum, constellation and BER performance were tested. IQ transmitter was tested for QPSK modulation at lOGbaud and 22.5Gbaud respectively. The tests showed good eye diagram, constellation and BER performance, and the measured EVM were 9% and 11% respectively. Three transmitters were designed with reasonable structure and convenient operation. Signals were generated correctly with good performance.6. The optical phase-locked loop technique in coherent optical communication system was studied systematically. An improved subcarrier optical phase-locked loop structure was proposed. The scheme added frequency difference capturing circuit to expand the frequency capture range of phase-locked loop. The frequency difference of local oscillator light and signal light can be captured quickly by adjusting the electrical voltage controlled oscillator. According to the improved subcarrier phase-locked loop structure, we completed the key modules selection and circuits design, such as laser, voltage controlled oscillator, frequency divider, data processing module and balanced receiver. The experimental system of homodyne coherent optical beat frequency was set up, and the frequency difference of local oscillator light and the signal light was captured quickly by automatic control, capture time was less than 1s, capture range was 7GHz. The frequency difference of signal light and the local oscillator light can be controlled within 3MHz stably. The system showed good performance of capture time, capture range and stability.
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