Research On Polarization Compensation Mechanism In High-Speed Coherent Optical Communication System

Recently,with the rise and development of artificial intelligence technologies such as ChatGPT,virtual reality,and cloud computing,global data traffic has shown exponential growth.In the face of the huge demand for data transmission,establishing low-cost,high-capacity and high-speed optical fiber communication has become the common goal of communication transmission.The related research on optical fiber communication has gradually become a hot topic concerned by researchers at domestic and abroad.Due to the limitations of device bandwidth,cost,sensitivity and power budget,the traditional intensity modulation direct detection(IM-DD)technology is difficult to support high-speed and high-capacity transmission.The coherent detection technology combined with high-order modulation and digital signal processing(DSP)technology has high sensitivity and can significantly improve the single-channel transmission capacity.In addition,polarization multiplexing(PM)technology improves the spectral efficiency of coherent systems and doubles the data rate of the system smoothly.A typical link distance of ≤2 km of data center internal traffic accounts for more than 80%of the total Internet traffic and is growing rapidly,and it has become a generally accepted trend in the industry for coherent technologies to sink to medium and short distances.However,with the continuous improvement of the transmission rate and modulation format of optical fiber communication systems,and the gradual sinking of coherent technology,the complexity and power consumption of DSP have become the main problems in short-reach applications.Among them,reducing the power consumption of the adaptive equalization(AEQ)algorithm is the key to reducing the total power consumption in DSP.In order to solve this problem,this paper studies the compensation method of polarization impairment from the point of view of the electrical and optical domains.The main research work of this paper is summarized as follows:1.Firstly,the composition of a high-speed PM coherent optical communication system is introduced.Nextly,the channel impairment is described.In the DSP module,the compensation algorithms for different impairments are studied,which provides a theoretical basis for the research and simplification of the polarization impairment compensation module.2.In the electrical domain,a simplified AEQ algorithm for joint compensation of phase noise is proposed in this paper.The improved constant modulus algorithm(MCMA)and parallel decision-directed least mean square(DD-LMS)cascade are used to simultaneously realize polarization demultiplexing,adaptive equalization and carrier phase recovery(CPR).Compared with the traditional constant modulus algorithm(CMA)and blind phase search(BPS)algorithm to compensate for polarization impairment and phase noise,the multiplier resource of the proposed algorithm is reduced by about 32%.In addition,the bandwidth of the optoelectronic device is limited,and the limited bandwidth output result is simulated.On this basis,in order to further simplify the electrical domain AEQ algorithm,this paper proposes a simplified AEQ algorithm based on the 1-N structure.It has been proved that the CMA-CMA algorithm based on the 1-N structure reduces the multiplier resources by 40%compared with the traditional butterfly CMA,but requires additional CPR algorithm.The MCMA+DD-LMS algorithm based on the 1-N structure reduces the multiplier resources by 57.9%compared with the above MCMA+parallel DD-LMS algorithm,and no additional CPR algorithm is needed.The implementation of a low-complexity polarization compensation algorithm further reduces the power consumption of DSP.3.In the optical domain,in order to reduce the complexity of the AEQ algorithm,a new compensation method is proposed,which uses optical domain optimization to simplify the complexity of the electrical domain and realize the joint compensation mechanism of optic and electricity.Aiming at the short-reach scene,a new type of coherent optical system,a self-homodyne coherent system,is designed in this paper.The system can greatly improve the tolerance to phase noise and frequency offset,and uses a large linewidth laser as optical carrier.The requirement for the laser is reduced.A transmission system working at 1550nm wavelength can eliminate dispersion compensation and nonlinear effect compensation et al.,which greatly reduces the complexity of DSP.With the aid of an automatic polarization controller,the polarization state of the local laser can be stabilized and polarization fading can be avoided.The AEQ of multiple-input multiple-output(MIMO)-free can be realized without butterfly FIR in the electrical domain,which simplifies the electrical AEQ algorithm.For the link mismatch,the proposed simplified phase compensation mechanism is used to replace the complex BPS algorithm.It is proved that the simplified phase compensation scheme can further reduce the electrical domain complexity.Finally,the effects of laser linewidth and frequency offset on the coherent system are discussed.The results show that compared with the traditional homodyne system,the self-homodyne system greatly reduces the requirements of laser linewidth and frequency offset,which reduces the communication cost.