一种基于3×3光纤耦合器的短距离光纤频率传递的噪声抑制方法

Noise Suppression of Frequency Transfer on Short-distance Optical Fiber Link Based on 3×3 Fiber Coupler

提出了一种噪声抑制方法,设计了基于3×3光纤耦合器迈克尔逊干涉仪的频率传递系统,使用嵌入式系统进行控制,通过调整光纤长度,实时补偿由温度变化等环境因素引起的时延变化,并进行了实验验证。启用时延补偿后,实验用的30 m长传输光纤在环境温度变化21℃条件下长度变化量小于±1μm,对应时间延迟变化量小于10 fs,所传输的光梳重频信号的频率稳定度没有明显变化。本文工作有望为空间条件下的光钟信号向比对设备的传输路径噪声抑制提供有效的解决方法。

Optical clocks have reached an instability level of 10^(-19). Remote clock comparisons require more stable frequency transfer systems. Remote time and frequency transfer are currently performed using RF link or laser link. The signal pathway between the clock and RF/laser transceiver also requires high stability. In some applications, ambient temperature change is the main noise source. However, it is difficult to control optical fiber temperature precisely under certain circumstances such as on a space station.A noise suppression method is developed and a frequency transfer system is designed using Michelson interferometer based on a 3×3 fiber coupler. A 3×3 fiber coupler rather than a 2×2 fiber coupler is chosen to build a Michelson interferometer because the direction of the fiber length change can be judged effectively when a 3×3 fiber coupler is used. An optical comb is used as a frequency source, with a wavelength of 1550 nm, an output power of 40 mW, and f_(r)(repetition frequency) at 200 MHz. A 1 310 nm CW-laser is used as a measurement scale. The two laser beams are input into the same optical fiber via a WDM.Another WDM split the two laser beams at the end of the optical fiber. The optical comb laser beam can be directly input into the target device. The measurement laser beam is reflected by a Faraday rotation mirror back to the fiber coupler. The optical fiber is the measuring arm of the Michelson interferometer.Photodiodes detect the interference fringe.An embedded system is employed in counting the interference fringe shift amount, calculating the compensation value, tuning the fiber length, and compensating the time delay variation in real time. A compensation device was built, and experiments were carried out.The experimental system is mainly composed of a compensation device, 30 m optical fiber, wavelength division multiplexers, Faraday rotation mirrors, and other auxiliary devices. The temperature of the 30 m optical fiber is controlled by an external TEC. For each test, the temperature of 30 m optical fiber is decreased from 26 ℃ to 5 ℃ and then increased to 18 ℃. The temperature variation range is 21 ℃.When the compensation is off, the time delay changes for approximately 20 ps for each 10 ℃ change.When the compensation is on, the time delay has mostly no change. The change is more remarkable than 2 fs at only a few points, which is still lesser than 6 fs.In another experiment, the optical comb laser is split into two beams and input into two optical fibers.Each laser beam is detected by a photodiode. The frequency signals from photodiodes are filtered by bandpass filters and input to a 5 125 A phase noise test set as input signal and reference signal respectively.The frequency instability of f_(r) is measured in the following three cases. In the first case, two short optical fibers of a similar length are used. The frequency instability characterizes the background noise of the test system. In the second case, one optical fiber is a short fiber, and the other is a 30 m fiber. The compensation device is disabled and connected in front of the 30 m fiber. The frequency instability of fr increases by about an order of magnitude compared with the first case. In the third case, one optical fiber is a short fiber and the other is a 30 m fiber. The compensation device is connected in front of the 30 m fiber and enabled. There is no obvious change in the frequency instability of f_(r) between the first and the third case.This work is expected to provide an effective solution for the noise suppression of the transmission pathway from the optical clock to the RF/laser transceiver under space conditions.