有源光学点对点稳频传输
Point-to-Point Stabilised Optical Frequency Transfer with Active Optics
摘要：
在地对空和地面自由空间激光链路上对光学原子钟进行时标比较，将对基础科学和应用科学产生巨大的好处，从测量基本常数和寻找暗物质到物理学和环境监测。然而，大气中的湍流会在激光信号上产生相位噪声，大大降低测量精度，并且还会引起闪烁和光束漂移，从而导致周期性的深衰落和信号丢失。我们演示了相位稳定的光频率传输在265米的水平点到点的自由空间链路之间的光学终端与主动倾斜倾斜镜，以抑制光束漂移，在一个紧凑的，人类便携式的设置。两个终端之间的相位稳定的715米地下光纤链路用于测量自由空间链路的性能。有源光学终端能够在长达一小时的时间内实现连续、相干的传输。我们实现了80 dB的大气相位噪声抑制$3\times10^{-6}$ rad $^{2}$ Hz $^{-1}$在1 Hz，和最终的分数频率稳定度$1.6\times10^{-19}$后40 s的集成。在高频下，这种性能受到补偿后残留大气噪声和通过自由空间和光纤链路的不等延迟看到的激光器频率噪声的限制。我们的长期稳定性受到相稳定系统的热屏蔽的限制。我们实现了最好的光学原子钟以下的剩余不稳定性，确保在动荡的自由空间链路的时钟限制频率比较。
英文摘要：
Timescale comparison between optical atomic clocks over ground-to-space and terrestrial free-space laser links will have enormous benefits for fundamental and applied science, from measurements of fundamental constants and searches for dark matter, to geophysics and environmental monitoring. However, turbulence in the atmosphere creates phase noise on the laser signal, greatly degrading the precision of the measurements, and also induces scintillation and beam wander which cause periodic deep fades and loss of signal. We demonstrate phase stabilized optical frequency transfer over a 265 m horizontal point-to-point free-space link between optical terminals with active tip-tilt mirrors to suppress beam wander, in a compact, human-portable set-up. A phase stabilized 715 m underground optical fiber link between the two terminals is used to measure the performance of the free-space link. The active optics terminals enabled continuous, coherent transmission over periods of up to an hour. We achieve an 80 dB suppression of atmospheric phase noise to $3\times10^{-6}$ rad$^{2}$Hz$^{-1}$ at 1 Hz, and an ultimate fractional frequency stability of $1.6\times10^{-19}$ after 40 s of integration. At high frequency this performance is limited by the residual atmospheric noise after compensation and the frequency noise of the laser seen through the unequal delays of the free space and fiber links. Our long term stability is limited by the thermal shielding of the phase stabilization system. We achieve residual instabilities below those of the best optical atomic clocks, ensuring clock-limited frequency comparison over turbulent free-space links.
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