Coherent Receivers With Advanced Modulation Optical Signal

In recent years, with the development of data services with high speed and highquality such as HDTV, cloud computing, online video conferencing and so on, therequirement of communication bandwidth is increasing rapidly. The backbonenetworks using optical communication technology need for high speed、large capacityand long haul in the single channel. With this background, intensity modulation/directdetection(IM/DD) cannot meet the requirements while coherent communications issuitable and becoming the hot point in the currently research, for the reason thatcoherent communications have the properties of high data rate, high sensitivity, goodwavelength selectivity, long repeater distance, advanced modulation format, highspectral effciency and so on.The paper begins with the introduction of the structures of optical coherentcommunication system and receiver. Then the effect of optical pre-amplifier oninterferometric coherent receivers is analyzed. The factors influencing theperformance of erbium-doped fiber amplifier are also discussed in detail. Consideringthe trade-off between the high gain and low noise figure in optical amplifier, atwo-stage amplifier which has the structure to suppress the forward andbackward-travelling amplified spontaneous emission noise based on Erbium-Dopedfiber is set up. A complete experimental platform is constructed to test the gain andnoise figure and some other parameters of the amplifier. The experimental resultsshow that the saturated gain of the EDFA in the experiment is40dB and the noisefigure is3.8dB which is0.8dB above3.0dB quantum-limit. The aim of high gainand low noise figure is achieved. The development of this EDFA has greatsignificance of improving the performance of optical communication system.The digital optical coherent receiver and digital signal processing algorithm arealso introduced in this paper, mainly focusing on the estimation algorithms forfrequency offset and carrier phase. Firstly, to solving the problem that it is difficult toachieve accurate estimation existing in the frequency offset estimation algorithm, afrequency estimator based on fast Fourier transform and multiple signal classificationis presented and the numerical simulations to demonstrate the algorithm is carried out.The simulation results show that the max and mean estimation error of the proposedalgorithm is below0.8MHz and0.1MHz through the whole estimation range (-2.5 GHz,2.5GHz). In addition, this estimator has excellent performance comparing tothe common algorithm when the data length is short and Signal-Noise Rate is above10dB. Then, to solving the problem of low tolerance to the initial phase of lasersexisting in the carrier phase estimation algorithm, a novel feedforward phaseestimation method with wavelet transform are proposed for the first time. Numericalsimulations are carried out to demonstrate the algorithm. The results show that theproposed algorithm has higher tolerance for the initial phase of laser than any otheralgorithms. For the other phase noise, the performance of our algorithm is close to orbetter than any other algorithms.