基于阿秒抖动光纤锁模激光器的时钟同步

Timing synchronization based on mode-locked fiber lasers with attosecond timing jitter

光纤锁模激光器结构简单,运转稳定,且输出的超短脉冲序列具有极高的时钟稳定性,在抽运探测、脉冲相干合成等要求高精度时钟同步的前沿领域有着广阔的应用前景.本文通过激光器腔内的电光调制器进行反馈控制,实现了两台光纤锁模激光器之间的紧密时钟信号同步;并且通过平衡光学互相关方法,对残余的时钟误差信号进行了测量,分辨率达到了13 as.通过优化激光器的腔内动力学过程及反馈环路的参数,在[1 Hz,10 MHz]的积分区间内得到了109 as的残余时钟误差,对应单台激光器的平均时间抖动为77 as.

Mode-locked fiber lasers output ultra-short pulse trains with extremely high temporal stability, showing great potential in systems that require precise timing synchronization, such as pump-probe experiments, high-speed analog-to- digital conversion, large-scale timing distribution and coherent combination. Fiber lasers are usually simpler, less costly, more efficient and more robust to the environment than solid state lasers, making them a better option for real-world applications. With the attosecond temporal resolution of the balanced optical cross-correlation （BOC） method, timing jitter of mode-locked fiber lasers has been carefully measured and optimized over the last decade. However, due to the inherently large amplified spontaneous emission noise in the long gain fiber and broad pulse width inside the laser cavity, the quantum-noise-limited timing jitter of mode-locked fiber lasers is still much higher than that of the solid state lasers. In order to further optimize the timing synchronization of mode-locked fiber lasers, larger locking bandwidth is required to suppress the low-frequency timing jitter, which contributes significantly to the total amount of residual timing jitter. In this work, tight timing synchronization between two mode-locked Yb-fiber lasers is achieved via a feedback loop built on an intra-cavity electro-optic phase modulator. Both lasers work in the stretched-pulse regime, which has been proven to support the lowest quantum-noise-limited timing jitter of mode-locked fiber laser. The output of the BOC system provides a timing error discriminator of 40 mV/fs, corresponding to 13 as resolution within the integration bandwidth. When the pulse trains from both lasers are successfully synchronized, the residual timing jitter can be measured with the same signal as that used for timing synchronization. Based on the residual timing jitter measurement, the intra-cavity dynamics of the laser and the locking parameters of the feedback loop can be further optimized and a tight synchronization with 400 kHz locking bandwidth is finally achieved. When integrated from 1 Hz to 10 MHz, the residual timing error is as low as 109 as, corresponding to 77 as averaged timing jitter of each laser. A parallel out-of-loop single-arm cross-correlation measurement is also performed to test the validity of the in-loop results, and both measurements agree with each other.