We experimentally demonstrate an 80-channel wavelength division multiplexing (WDM) transmission system over a 400 km fiber link. Raman amplification results in a non-flat WDM signal spectrum. Therefore, bit allocation optimization is used to enable different channels to carry different order quadrature amplitude modulation signals according to their optical signal-noise-ratios. A neural network equalizer based on a convolutional neural network (CNN), long short-term memory (LSTM) network, and fully connected (FC) layer structure is adopted in Rx digital signal processing, in which CNN is used for characteristic extraction, LSTM is used for equalization and demodulation, and FC layers are used for output. After transmission, the bit error rate of all channels is below the 25% soft-decision forward error correction threshold, and the line rate reaches 53.76 Tbit/s.We experimentally demonstrate an 80-channel wavelength division multiplexing (WDM) transmission system over a 400 km fiber link. Raman amplification results in a non-flat WDM signal spectrum. Therefore, bit allocation optimization is used to enable different channels to carry different order quadrature amplitude modulation signals according to their optical signal-noise-ratios. A neural network equalizer based on a convolutional neural network (CNN), long short-term memory (LSTM) network, and fully connected (FC) layer structure is adopted in Rx digital signal processing, in which CNN is used for characteristic extraction, LSTM is used for equalization and demodulation, and FC layers are used for output. After transmission, the bit error rate of all channels is below the 25% soft-decision forward error correction threshold, and the line rate reaches 53.76 Tbit/s.

Cascaded random Raman fiber lasers (CRRFLs) have been used as a new platform for designing high power and wavelength-agile laser sources. Recently, CRRFL pumped by ytterbium-doped random fiber laser (YRFL) has shown both high power output and low relative intensity noise (RIN). Here, by using a wavelength- and bandwidth-tunable point reflector in YRFL, we experimentally investigate the impacts of YRFL on the spectral and RIN properties of the CRRFL. We verify that the bandwidth of the point reflector in YRFL determines the bandwidth and temporal stability of YRFL. It is found that with an increase in the bandwidth of the point reflector in YRFL from 0.2 nm to 1.4 nm, CRRFL with higher spectral purity and lower RIN can be achieved due to better temporal stability of YRFL pump. By broadening the point reflector’s bandwidth to 1.4 nm, the lasing power, spectral purity, and RIN of the 4th-order random lasing at 1 349 nm can reach 3.03 W, 96.34%, and –115.19 dB/Hz, respectively. For comparison, the spectral purity and RIN of the 4th-order random lasing with the point reflector’s bandwidth of 0.2 nm are only 91.20% and –107.99 dB/Hz, respectively. Also, we realize a wavelength widely tunable CRRFL pumped by a wavelength-tunable YRFL. This work provides a new platform for the development of ideal distributed Raman amplification pump sources based on CRRFLs with both good temporal stability and wide wavelength tunability, which is of great importance in applications of optical fiber communication and distributed sensing.

针对输电线路等场景的超长距离监测应用问题, 文章在分布式光纤相位敏感光时域反射仪的基础上, 采用拉曼放大和外差检测的方式, 设计了一种光电信号分段探测和采集的光纤传感系统。该系统依据光纤长度将光纤分为两段进行信号采集和处理, 针对不同强度信号适配相应的信号处理和采集参数, 解决了超长距离监测系统的动态范围不足和信噪比差的问题。经试验和分析, 最终实现了超过124 km的单端超长距离分布式光纤监测系统, 系统末端信噪比>10.5 dB。

基于宽带接收的快速布里渊光时域反射技术,通过结合分布式拉曼放大来实现长距离分布式布里渊传感。研究了拉曼放大前后系统的性能差别以及非线性现象,并探究拉曼抽运功率与脉冲功率的最佳组合方式。最终通过实验实现了空间分辨率为50 m、传感距离为100 km左右的快速温度传感,其中,在50 km处获得1.2 ℃的温度测量精度。系统传感距离较无拉曼放大时提高了约50 km,并且能够在10 s左右快速完成一次测量。

晶体拉曼放大器是获得高光束质量、高光谱纯度、高功率拉曼激光的重要途径。通过引入四个归一化综合参量, 推导出了外腔拉曼放大器的归一化输运方程组。通过数值求解该输运方程组得到了描述拉曼放大器运转的一组普适理论曲线; 分析了复合归一化变量对拉曼放大器性能的影响; 研究了晶体拉曼放大器放大率、放大后拉曼脉冲形状、光-光转换效率等参量在泵浦脉冲功率密度、泵浦脉冲与被放大脉冲相对宽度、泵浦脉冲与被放大脉冲时间重叠性等条件的变化规律。用实验数据对归一化理论进行了验证, 结果表明, 理论结果与实验数据相吻合, 证明了文中理论计算的正确性和可行性。

光纤中的随机散射现象凭借其在不同应用场景中的重要影响成为了一个非常热门的研究主题，但到目前为止，还没有理论方法能够准确描述光纤中散射现象的随机性及其影响。文章提出一种能够分析光纤散射现象随时间和散射点位置随机变化的较为准确的数值模型，并用该数值模型分析了分布式拉曼放大系统中的瑞利散射和自发拉曼散射噪声的频谱。之后，将仿真分析结果与现有模型的仿真分析结果和实验测量的结果进行了对比分析。文中模型计算得到的功率密度谱与实验结果误差在5 dBc/Hz以内，而现有模型在高频区的误差往往>10 dBc/Hz，说明文中结果与实际测量结果相比误差更小，证明了模型的准确性。文中模型可作为一种分析优化分布式光纤系统，特别是分析具有分布式光放大的系统的工具，例如具有双向拉曼放大的超长跨距光纤电力通信系统、光纤传感器系统以及光纤随机反馈激光器等。

搭建一个双向拉曼放大超长跨距光纤传输系统，在接收端采用非线性补偿算法来补偿因传输距离长和入纤光功率高所带来的光纤非线性效应,并通过实验成功实现了10 Gbaud/s 16正交幅度调制(QAM)信号的传输，传输距离为300 km。仿真和实验结果表明，通过采用非线性补偿算法，接收端信号的性能相比未使用非线性补偿算法时提高了2 dB。证明了非线性补偿技术在超长跨距光纤传输系统中是非常有效且必要的。

通过实验实现了一个2 000 km的16阶正交振幅调制传输系统，该系统采用后向分布式拉曼放大器对信号进行放大，单段跨距为80 km，并在接收端采用数字背向传输算法补偿光纤传输过程中的非线性效应。采用分布式拉曼放大后，信号在光纤中的功率分布不再是指数衰减，采用的数字背向传输算法对此做了优化。实验结果表明，采用数字背向传输算法后，信号的Q2值提升了2.1 dB。

提出了一种应用于双向拉曼放大超长单跨距光纤传输系统的低复杂度数字背向传输算法，该算法基于在接收端电域中反向求解薛定谔方程，可以在补偿光纤传输系统中色散的同时抑制非线性效应；基于双向拉曼放大系统对算法的功率曲线等进行了优化设计，降低了算法的复杂度；搭建了一个10 Gbaud/s 正交相移键控调制的双向拉曼传输实验系统，传输距离为320 km。实验结果表明，优化后的数字背向传输算法的性能较传统算法提升了0.7～1 dB。