Coherently bridging across wide spectral regions in the optical domain with ultra-high precision will enable progress in optical spectroscopy and time and frequency metrology. We report optical frequency synthesis at the ${10}^{-18}$ instability level based on a novel technique actively canceling the differential phase noise of a dual-branch Er-fiber based optical frequency comb spanning 500–2200 nm. We validate the method with an out-of-loop measurement demonstrating the stability transfer from 1560 nm to 780 nm with an instability level at $3\times {10}^{-18}$ at 1 s averaging time and $1\times {10}^{-19}$ at 1000 s (approximately averaging on $1\times {10}^{-17}/\sqrt{\tau }$), limited mainly by uncompensated optical interferometers used to detect optical beatnotes. To confirm this limitation, we propose an experimental setup that eliminates the excess noise induced by uncompensated optical paths and show that the proposed method exhibits an ultimate unprecedented instability level at $6\times {10}^{-19}$ at 1 s averaging time when two optical signals can share a common photodetector. The system realized becomes a powerful and universal tool for optical frequency comparison and spectral purity transfer across the visible and infrared domains.

中文翻译：

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具有10 ^-18不稳定性的超宽带双分支光学频率梳

以超高精度在光学域中的宽光谱区域进行相干桥接将使光学光谱学以及时间和频率计量学取得进展。我们在${10}^{-\mathrm{18岁}}$基于一种新技术的不稳定性水平，该技术可主动消除基于双分支Er光纤的跨频500–2200 nm的光学频率梳的差分相位噪声。我们通过闭环测量验证了该方法，证明了从1560 nm到780 nm的稳定性转移，其不稳定性水平为$3\times {10}^{-\mathrm{18岁}}$ 在1 s的平均时间内 $\mathrm{1个}\times {10}^{-19}$ 在1000 s（大约是 $\mathrm{1个}\times {10}^{-17}/\sqrt{\tau }$），主要受用于检测光学节拍的无补偿光学干涉仪的限制。为了确认这一局限性，我们提出了一种实验装置，该装置可以消除由未补偿的光路引起的多余噪声，并表明所提出的方法在1200℃时表现出最终的前所未有的不稳定性水平。$6\times {10}^{-19}$当两个光信号可以共享一个公共光电探测器时，平均时间为1 s。所实现的系统已成为功能强大的通用工具，可用于在可见光和红外域进行光频率比较和光谱纯度传递。