We demonstrate a simple all-fiber photonic phase detector that can measure the phase (timing) difference between an optical pulse train and a microwave signal with subfemtosecond resolution and $-60\mathrm{dB}$-level amplitude-to-phase conversion coefficient. It is based on passive phase biasing of a Sagnac loop by the intrinsic phase shift of a symmetric $3\times 3$ fiber coupler. By eliminating the necessity of magneto-optic components or complex radio frequency (RF) electronics for phase biasing of the Sagnac loop, this phase detector has potential to be implemented as an integrated photonic device as well. When using this device for synchronization between a 250 MHz mode-locked Er-fiber laser and an 8 GHz microwave oscillator, the minimum residual phase noise floor reaches $<-154\mathrm{dBc}/\mathrm{Hz}$ (at 8 GHz carrier) with integrated root mean square (rms) timing jitter of 0.97 fs [1 Hz–1 MHz]. The long-term rms timing drift and frequency instability are 0.92 fs (over 5000 s) and $4\times {10}^{-19}$ (at 10,000 s averaging time), respectively.

中文翻译：

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结构简单，亚飞秒分辨率的光学微波相位检测器

我们演示了一种简单的全光纤光子相位检测器，该检测器可以以亚飞秒分辨率和 $-60\mathrm{D\; b}$电平幅度到相位的转换系数。它基于通过对称的固有相移对Sagnac环路进​​行无源相位偏置$3\times 3$光纤耦合器。通过消除对Sagnac环路进​​行相位偏置的磁光组件或复杂的射频（RF）电子设备的必要性，该鉴相器也有可能被实现为集成光子器件。当使用此设备在250 MHz锁模Er光纤激光器和8 GHz微波振荡器之间进行同步时，最小残留相位本底噪声达到$<-154\mathrm{分贝}/\mathrm{赫兹}$（在8 GHz载波上），积分均方根（rms）时序抖动为0.97 fs [1 Hz-1 MHz]。长期均方根时序漂移和频率不稳定性为0.92 fs（超过5000 s），并且$4\times {10}^{-19}$ （平均10,000秒）。