We demonstrate a stable narrow linewidth single-frequency erbium-doped fiber laser (EDFL) operating at 1.6 µm. A Fabry–Perot fiber Bragg grating and two cascaded subrings are incorporated in the main ring cavity to achieve single-frequency operation. The experimentally measured optical signal-to-noise ratio is greater than 73 dB. Furthermore, the linewidth of the EDFL is measured to be about 480 Hz by the self-built short-delayed self-heterodyne interferometry device. The laser shows superior stability, with no mode-hopping during the 60-min observation period. The proposed EDFL provides a new experimental idea for realizing a single-frequency fiber laser in the L-band.

An approach for frequency division of an optical pulse train (OPT) based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. When the OPT is injected into the OEO, a microwave signal with a frequency equaling fractional multiples of the repetition rate of the OPT is generated. This signal is then fed back to the OEO, maintaining its oscillation, while simultaneously serving as the control signal of a Mach–Zehnder modulator (MZM) in the OEO. The MZM acts as an optical switch, permitting specific pulses to pass through while blocking others. As a result, the repetition rate of the OPT is manipulated. A proof-of-concept experiment is carried out. Frequency division factors of 2 and 3 are successfully achieved. The phase noises of the OPT before and after the frequency division are investigated. Compared to previously reported systems, no external microwave source and sophisticated synchronization structure are needed.

To ensure the frequency accuracy of a heterodyne laser source in the ambient temperature range of -20°C to 40°C, a dual-longitudinal-mode thermally stabilized He–Ne laser based on non-equilibrium power locking was designed. The ambient adaptive preheating temperature setting scheme ensured the laser could operate normally in the range of -20°C to 40°C. The non-equilibrium power-locked frequency stabilization scheme compensated for the frequency drift caused by different stabilization temperatures. The experimental results indicated that the frequency accuracy of the laser designed in this study could reach 5.2 × 10^-9 in the range of -20°C to 40°C.

Sensors based on optical resonators often have their measurement range limited by their cavity linewidth, particularly in the measurement of time-varying signals. This paper introduces a method for optical frequency shift detection using multiple harmonics to expand the dynamic range of sensors based on optical resonators. The proposed method expands the measurement range of optical frequency shift beyond the cavity linewidth while maintaining measurement accuracy. The theoretical derivation of this method is carried out based on the equation of motion for an optical resonator and the recursive relationship of the Bessel function. Experimental results show that the dynamic range is expanded to 4 times greater than the conventional first harmonic method while still maintaining accuracy. Furthermore, we present an objective analysis of the correlation between the expansion factor of the method and the linewidth and free spectrum of the optical resonator.

针对时域脉冲信号样本的射频指纹提取与深度学习模型的可解释性，提出了一种基于Grad-CAM的重要区域可视化呈现方法，并通过重要区域的多次掩码测试，来分析重要区域对射频指纹识别结果的影响。基于10个辐射源的信号样本，对比了层数不同的两种ResNet模型的测试结果。测试发现该方法能够区分不同类型信号并呈现个体差异。分析表明，该方法能够发现不同辐射源发送相同信号时的重要区域定位差异，能可视化反映辐射源个体特征的空间距离，以及不同模型的特征表征与指纹定位准确度差异；同时发现对重要区域的掩码更容易产生误预测，证明特定信号存在与时频特征相关的射频指纹，并可辅助可视化定位影响射频指纹样本识别的关键点。

针对复杂电磁对抗环境中电子信息装备面临的强电磁环境威胁以及防护加固需求，对强电磁防护技术的发展现状进行了综述，并提出了强电磁系统级综合防护的发展展望。强电磁防护是为了保证电子信息装备在强电磁环境中免受损伤或损毁的技术手段，从电磁波的耦合途径分析了当前电磁防护的重点和难点，然后分别以限幅器技术、频率滤波技术、能量选择防护技术三个方面对前门防护技术的发展现状进行了分析总结，最后从新型屏蔽材料和防护器件两个方面对新型电磁防护技术进行了展望，并从前后门一体防护、场路一体防护、多域联合防护三个角度对系统级电磁防护进行了总结，为电子信息装备在复杂电磁环境下的电磁防护加固设计提供了支撑。

针对光频梳下变频信号接收过程中存在的频率信息丢失问题，提出了一种基于频率-相位映射的信号频率恢复方法，该方法使用可调光纤延迟线在两路光频梳变频链路之间产生一组固定已知的时延，时延在信号原始频率与下变频信号相位差之间建立映射关系，利用该映射关系可以从测得的相位差计算出信号的原始频率。分析了时延值等参数对频率恢复的影响，估计了该方法对相位测量不确定度的限值要求，最后给出了该方法具体实施方案中关键参数的设置策略。所有下变频信号的相位差可以通过快速傅里叶变换等数据处理一次性得出，因此该方法的时间代价和计算成本几乎不随着信号个数增加而增加。在不考虑下变频信号混叠的情况下，本文所提出的方法在理论上对处理信号的数量没有限制，因此相比于已有的光频梳下变频信号频率恢复方法，在多信号频率恢复方面更具有优势。

针对航空发动机电子控制器（EEC）易受高强辐射场（HIRF）干扰问题，以ZF-M600型EEC为研究对象，通过三维电磁仿真软件CST对EEC建模并进行平面波辐照仿真，模拟HIRF对EEC的干扰效应，仿真结果表明，HIRF可以通过孔缝耦合进入EEC内部并在腔体内部产生谐振，谐振频率处电场强度显著增加。依据RTCA DO-160G在400 MHz~4 GHz开展EEC辐射敏感度试验，试验结果表明：EEC失效频点为2.40 GHz和3.48 GHz，敏感模块为模拟量输入输出模块，敏感现象为模拟量数据丢失。EEC失效频点与谐振频率接近, 说明EEC失效与腔体谐振有关，在EEC内部贴装吸波材料并进行仿真，分析吸波材料的放置方式和厚度对EEC谐振电磁干扰强度的影响，仿真结果表明，贴装吸波材料可以有效抑制谐振电磁干扰。