Noise Suppression of Frequency Transfer on Short-distance Optical Fiber Link Based on 3x3 Fiber Coupler

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 3x3 fiber coupler. A 3x3 fiber coupler rather than a 2x2 fiber coupler is chosen to build a Michelson interferometer because the direction of the fiber length change can be judged effectively when a 3x3 fiber coupler is used. An optical comb is used as a frequency source,with a wavelength of 1 550 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 degrees C to 5 degrees C and then increased to 18 degrees C. The temperature variation range is 21 degrees C. When the compensation is off, the time delay changes for approximately 20 ps for each 10 degrees C 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 f(r) 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.