Digital Signal Processing (DSP) And Waveform Generation In High Speed Optical Communication Systems

As the internet becomes more popular and multi-mideia sevice increases, the bandwidth requirement of communication networks increases rapidly. To meet the huge bandwidth requirement, the optical transmission backbone networks will adopt coherent optical communication technology, which utilizes coherent optical detection with digital signal processing (DSP), and realizes high-speed and large capacity information transmission. In such systems, DSP is the key technology for signal equalization. This dissertation studies the DSP algorithms and their performance in high-speed coherent optical communication systems, focusing on fiber nonlinearity compensation and carrier phase recovery for high-order quadrature amplitude modulation (QAM) signals. Besides, waveform generation is very important for high speed optical communications, and optical wireless integration techniques, etc. The research on optical waveform generation in this dissertation includes photonic generation of ultra-wide band (UWB) signal and waveform generation based on an optical frequency comb.The main innovations of this dissertation are as follows.1. In the next generation100Gb/s per-channel polarization division multiplexed quadrature phase shift keying (PDM-QPSK) system, linear impairments during fiber transmission can be well compensated. Fiber nonlinearity compensation is a critical factor to further improve the system performance. Fiber nonlinearity compensation based on DSP technologies is fully investigated in this dissertation, and the innovative works are:1) Pilot-aided fiber nonlinearity compensation is proposed to compensate the nonlinear impairments in dispersion compensated WDM system. Compared with the digital back propagation (DBP) algorithm, pilot-aided method can compensate both intra-and inter-channel fiber nonlinearities simultaneously. Simulation results from112Gb/s per-channel PDM-QPSK system show that, pilot-aided fiber nonlinearity compensation has better performance than DBP algorithm in dispersion compensated WDM transmission systems. The system Q-factor using pilot-aided method is higher than DBP by1dB in4000km transmission system. Also, the maximum transmission distance can be improved by7%when pilot-aided method is used instead of DBP. Since earlier fibier links usually use in-line dispersion compensation, and only the transmitter and receiver are upgraded when the system capacity needs to be enlarged. Pilot-aided fiber nonlinearity compensation will play a significant roll in improving the system performance and reducing the cost in dispersion compensated WDM systems.2) Anti-fiber-nonlinearity transmission system using offset QPSK (OQPSK) and minimum shift keying (MSK) formats are proposed. Compared with QPSK format, OQPSK and MSK signals have less power variation, and they are more resistant to fiber nonlinarities than QPSK. The performance of112Gb/s per-channel PDM-QPSK, PDM-OQPSK and PDM-MSK system is investigated through simulations. Results show that MSK fromat has the best fiber nonlinear tolerance and OQPSK is more resistant to fiber nonlinearity than QPSK. In WDM condition, the maximum transmission distance of MSK system is larger than QPSK system by14.8%. When DBP method is used for intra-channel fiber nonlinearity compensation, performance is improved and MSK system still performs the best.2. Carrier phase recovery for high-order QAM is a crucial technique for400Gb/s or1Tb/s transmission system. The following innovations are achieved for carrier phase recovery of high-order QAM signals.1) QPSK partitioning with multi-stage maximum likelihood (ML) algorithm is proposed for carrier phase recovery of high-order QAM signals. Simulation results from16-QAM and64-QAM systems prove that, this algorithm has better laser phase noise tolerance than QPSK partitioning algorithm and performs close to blind phase search (BPS) algorithm. However, its computational complexity is far less than that of BPS algorithm. In16-QAM and64-QAM systems, the complexity of the proposed method is less than1/4and1/6of the BPS complexity, respectively.2) An improved pilot-aided carrier phase recovery method is proposed. By using average processing, amplified spontaneous emission (ASE) noise filtered out together with the pilot is suppressed. Simulation results prove that this method achieves better laser phase noise tolerance than original pilot-aided method:the laser phase noise tolerance is improved to six and four times of the original tolerance in16-QAM and64-QAM systems, respectively. On this basis, we propose the method using improved pilot-aided method with ML algorithm, and the tolerance to laser phase noise is further doubled both in16-QAM and64-QAM systems.3. The innovations on optical waveform generation:1) Two novel schemes for photonic generation of UWB signals are proposed and experimentally demonstrated. First, an impulse radio UWB (IR-UWB) transmitter using a phase modulator (PM) together with a fiber delay interferometer is proposed. Compared with the previous reported schemes, the advantage of our method is that, an electrical baseband non-return-to-zero (NRZ) signals can be used as the system input, and IR-UWB pulse generation and pulse amplitude or polarity modulation are reliazed simultaneously. Second, a technique for multi-channel IR-UWB and millimeter-wave (MMW) UWB signal genratation based on four-wave mixing effect in highly nonlinear photonic crystal fiber is proposed, which is the first report on multi-channel and multi-band UWB signal generation. It will find good applications in UWB over fiber systems where WDM technique is used to transmit multi-channel signals simultaneously.2) A flat optical frequency comb generation scheme using electroabsorption modulator (EAM) and two cascaded PMs is proposed and experimentally realized. The spectral power variation near the optical carrier is0.9dB. Compared with the comb generated using MZM and PM, which has a power variation of4dB, the optical frequency comb generated by our scheme has much better flatness. Using the generated flat optical frequency comb, we propose the waveform generation methods based on heterodyne mixing and Fourier transmission. High repeatition rate (160GHz and320GHz) optical cosine pulse, optical pulse burst, as well as optical square pulse, optical triangle pulse and optical parabolic pulse with40GHz repeatition rate are successfully generated in the experiment.