Abstract
Low-noise microwave signals are of vital importance in fields such as cold atomic optical clocks, photon radars, and remote synchronization at large facilities. Here, we report a compact all-optical-fiber method to generate a low noise microwave signal, in which the fiber loop optical-microwave phase detector is used to coherently transfer the frequency stability of the ultra-stable laser to the microwave. Combining a narrow linewidth optical frequency comb and a fiber loop optical-microwave phase discriminator, a tight phase-lock between 7 GHz dielectric oscillator and optical frequency comb is achieved, the remaining phase noise of the synchronized optical pulse sequence and the microwave signal is –100 dBc/Hz@1 Hz, and the timing jitter is 8.6 fs (1 Hz—1.5 MHz); by building two sets of low-noise microwave generation systems, the measured residual phase noise of the 7 GHz microwave is –90 dBc/Hz@1 Hz, and the corresponding frequency stability is 4.8 × 10<sup>–15</sup>@1 s. These results provide a novel idea for generating the low-noise microwaves based on optical coherent frequency division.
Highlights
We report a compact all-optical-fiber method to generate a low noise microwave signal, in which the fiber loop optical-microwave phase detector is used to coherently transfer the frequency stability of the ultra-stable laser to the microwave
These results provide a novel idea for generating the low-noise microwaves based on optical coherent frequency division
39 1577 [25] Lu X, Zhang S, Jeon C G, Kang C S, Kim J, Shi K 2018 Opt. Lett
Summary
图 1 FLOM-PD 原 理 图 . 其 中 , Circulator 为保偏光纤环 形器, PM EOM 为保偏光纤电光调制器, QWP 为 1/4 波片, FR 为法拉第旋光镜, HWP 为 1/2 波片, 3 dB coupler 为 2 × 2 的 3 dB 保偏光纤耦合器, BPD 为平衡光电探测器. 图 2(a) 表示超稳激光系统, 两台低噪声集成外腔半导体激光器 (RIO, ORION 1550 nm laser module) 分别输出波长为 1542.18 nm 和 1563.47 nm 的连续光, 采用 PDH 方法, 锁定到 同一个高 Q 值光学参考腔上. 参考腔购买自 SLS (Stable Laser System) 公司, 长度为 50 mm, 精细 度 > 5 × 105, 为立方体型, 腔内使用单层控温结 构, 零膨胀温度为 (59 ± 1) °C, 对应的真空度为 5 × 10–7 Pa, 整个系统安放在一个被动隔振平台上, 并 置于隔声箱内. 性偏振旋转 (NPR) 技术实现自启动锁模, 重复频 率约为 200 MHz, 可依靠腔内的电动位移台和压 电位移器 (PZT) 分别实现大范围 (1—2 MHz) 和 精细 (1—2 kHz) 调节 [27]. 锁相环 I用于光频梳与 1542 nm 超稳激光的锁定, 超稳 激光与光频梳拍频产生误差信号, 经 PID 控制器 (Vescent, D2-125) 反馈腔外声光调制器 (AOM), 用于快速伺服光频梳的载波包络偏移频率, 同时通 过控制器辅助输出端口, 缓慢调控激光器腔内 PZT 驱动电压, 抑制重复频率的漂移; 锁相环II用 于光频梳与 1563 nm 超稳激光的锁定, 结构与锁 相环I大致相同, 拍频形成的误差信号经高速伺服 控制器 (New Focus, LB1005) 反馈至泵浦激光的 驱动电流, 进而控制光频梳的重复频率和载波包络 偏移频率, 最终实现光频梳相位稳定 性偏振旋转 (NPR) 技术实现自启动锁模, 重复频 率约为 200 MHz, 可依靠腔内的电动位移台和压 电位移器 (PZT) 分别实现大范围 (1—2 MHz) 和 精细 (1—2 kHz) 调节 [27]. 实验建立两套锁相环路 来精密控制光学频率梳的两个自由度 [28]. 锁相环 I用于光频梳与 1542 nm 超稳激光的锁定, 超稳 激光与光频梳拍频产生误差信号, 经 PID 控制器 (Vescent, D2-125) 反馈腔外声光调制器 (AOM), 用于快速伺服光频梳的载波包络偏移频率, 同时通 过控制器辅助输出端口, 缓慢调控激光器腔内 PZT 驱动电压, 抑制重复频率的漂移; 锁相环II用 于光频梳与 1563 nm 超稳激光的锁定, 结构与锁 相环I大致相同, 拍频形成的误差信号经高速伺服 控制器 (New Focus, LB1005) 反馈至泵浦激光的 驱动电流, 进而控制光频梳的重复频率和载波包络 偏移频率, 最终实现光频梳相位稳定
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