Liu Yi, Jiang Kai, Liu Sha, Fang Xinyue, Wang Linyi, You Yajun, He Wenjun, Chou Xiujian
Opt Express. 2023 Jul 3;31(14):22457-22469. doi: 10.1364/OE.496768.
A dual-ring parity-time (PT) symmetric Brillouin fiber laser (BFL) with an unbalanced polarization Mach-Zehnder interferometer (UP-MZI) is proposed and experimentally investigated. An UP-MZI consisting of optical coupler, polarization beam combiner (PBC) and two asymmetric length arms with 10 km and 100 m single-mode fiber, is used to achieve Vernier effect and PT symmetry. Due to the orthogonally polarized lights created in the PBC, the dual-ring PT symmetry BFL with an UP-MZI implements two unbalanced length feedback rings that are connected to one another, one long length ring with a Brillouin gain and the other short length ring with a loss of the same magnitude, to break a PT symmetric and maintain the Vernier effect. By contrast with existing PT symmetry BFL studies, this design does not require same lengths of the gain and loss loops, but can manipulate freely PT symmetry status in accordance with a rational scaling factor between them. Experimental results reveal that the 3-dB linewidth of dual-ring PT symmetry BFL with an UP-MZI is about 4.85 Hz with the threshold input power of 9.5 mW, in accordance with the 97 Hz measured linewidth at the -20 dB power point. Within 60 mins of the stability experiment, the power and frequency stability fluctuation are ±0.02 dB and ±0.137 kHz, respectively. Thanks to the two asymmetric ring lengths, the sidemode suppression ratio (SMSR) is optimized by 54 dB compared to that with the only long ring structure, 26 dB when using only the Vernier effect or 12 dB for existing PT symmetry BFL. This BFL design with single longitudinal mode and high SMSR output can be applied to high coherent communication and Brillouin-based microwave photonics systems with low phase noise.
提出并实验研究了一种具有不平衡偏振马赫-曾德尔干涉仪（UP-MZI）的双环奇偶时间（PT）对称布里渊光纤激光器（BFL）。一个由光耦合器、偏振光束合成器（PBC）以及两个分别具有10 km和100 m单模光纤的非对称长度臂组成的UP-MZI，用于实现游标效应和PT对称性。由于在PBC中产生的正交偏振光，具有UP-MZI的双环PT对称BFL实现了两个相互连接的不平衡长度反馈环，一个是具有布里渊增益的长长度环，另一个是具有相同量级损耗的短长度环，以打破PT对称性并保持游标效应。与现有的PT对称BFL研究相比，该设计不需要增益和损耗环具有相同的长度，而是可以根据它们之间的合理缩放因子自由控制PT对称状态。实验结果表明，具有UP-MZI的双环PT对称BFL的3 dB线宽约为4.85 Hz，阈值输入功率为9.5 mW，与在-20 dB功率点处测得的97 Hz线宽一致。在60分钟的稳定性实验中，功率和频率稳定性波动分别为±0.02 dB和±0.137 kHz。得益于两个非对称环长度，与仅具有长环结构相比，边模抑制比（SMSR）优化了54 dB，仅使用游标效应时为26 dB，对于现有的PT对称BFL为12 dB。这种具有单纵模和高SMSR输出的BFL设计可应用于高相干通信和低相位噪声的基于布里渊的微波光子学系统。
