基于非线性光纤环形镜的少模脉冲幅度调制再生器

Research on few-mode PAM regenerator based on nonlinear optical fiber loop mirror

随着网络带宽需求的快速增加,波分复用系统的容量已接近非线性香农极限.为了适应未来网络的发展,空分复用技术引起了越来越多的关注.本文首次提出基于少模非线性光纤环形镜(FM-NOLM)的脉冲幅度调制(PAM)全光再生器,描述了其工作原理和具体设计过程.采用COMSOL软件对组成FM-NOLM的硫化物高非线性光纤进行了模式特性仿真.以LP01,LP11,LP21三个光纤模式为例,确定了再生器的参数,计算出每个模式的功率转移函数曲线.仿真分析了该少模PAM-4全光再生器的噪声抑制(NRR)性能,并与单模情形进行了比较.研究表明,1)对于每个空间模式的PAM信号,所有再生电平具有一致的功率转移性能;2)当输入信噪比(SNR)约大于20 dB时,三种模式的噪声抑制比均可超过3 dB,并随着输入信噪比线性增加,其斜率约为1.2;3)在相同输入SNR条件下,三种模式的噪声抑制比相差不大,不超过1.1 dB.为了说明再生器的再生性能,当输入SNR为25 dB时,我们还给出了再生前后PAM-4信号的功率分布直方图.与现有的再生方案相比,本文方案的均匀多电平再生转移性能,使其更适合高频谱效率的长距空分复用系统和任意电平数的PAM信号再生.此外,该方案也能够扩展到波长域,有效提高光通信系统的传输容量.

In recent years,the demand for network bandwidth has increased significantly,and the capacity of wave division multiplexing(WDM)systems has reached the nonlinear Shannon limit.In order to adapt to the development of future networks,space division multiplexing(SDM)technology attracts more and more attention.In this paper,we put forward a novel structure of pulse amplitude modulation(PAM)regenerator based on few-mode nonlinear optical fiber loop mirror(FM-NOLM)for the first time,and theoretically analyze the working principle for few-mode reshaping.It can regenerate degraded PAM signals and improve transmission performance in SDM system.The detailed design steps of the regenerator are given,in which the sulfide highly nonlinear fiber and multimode coupler are used to build up the FM-NOLM and their mode characteristics are simulated by COMSOL software.The parameters of the regenerator are determined by taking the few-mode optical fiber supporting LP01,LP11,and LP21 modes as an example,and then the power transfer function(PTF)curve of each mode for PAM signals is calculated.We simulate and analyze the noise reduction ratio(NRR)performance of the few-mode regenerator for PAM-4 signals,and compare with the case of single mode fiber.Our simulation shows that:(1)for each spatial mode of PAM signal,all regenerative levels have the same consistent power transfer performance;(2)for the input signal-to-noise ratio(SNR)greater than20 dB,the NRR for each mode can exceed 3 dB,and increase with the input SNR at the slope of about 1.2;(3)the NRR difference between the three modes is less than 1.1 dB for the same input SNR.In order to illustrate the reshaping function of the regenerator,we also present the power distribution histograms for PAM-4 signals before and after regeneration when the input SNR is 25 dB.This scheme proposed here has some advantages over the existing regenerators in the applicability for the long-haul SDM system with high spectral efficiency and regeneration of any level number of PAM signals in theory due to its uniform multi-level regeneration function,but also is capable of being extended to the wavelength domain for higher transmission capacity.