In order to transmit an optical signal across a long-distance fiber link, the power loss should be compensated to significantly improve the signal-to-noise ratio of the optical signal detected at both the local and remote end.

An erbium-doped fiber amplifier (EDFA) is the most commonly used amplifier for amplification in an optical fiber link. However, the bidirectional nature of the frequency transfer system limits the gain of the system and deteriorates the stability of transmission.

In a recent study published in Scientific Reports, a research team led by Prof. CAI Haiwen at Shanghai Institute of Optics and Fine Mechanics of Chinese Academy of Sciences proposed a novel optical injection locking amplifier (OILA) with high amplification gain and ultra-low phase noise.

The team has been devoting the efforts to acquire an amplifier with better performance with stability much better than 10^-18 at 1000 s to meet the transmission of the present most stable optical signal.

The OILA developed can provide approximately 59 dB gain and ensures that the input carrier frequency fractional stability can be as good as 10^-20 at 1000 s.

In the 180km transmission experiment, the frequency stability can reach as high as 10^-20 at 20000 s, which is suitable for optical frequency distribution and remote comparison between optical atomic clocks.

Moreover, the OILA can be used in the optical clock network connected via free-space link as the optical injection locking can erase the amplitude fluctuations of the injected signal providing a further benefit.

This work was supported by the Strategic Priority Research Program of Chinese Academy of Sciences, the Scientific Innovation Fund of Chinese Academy of Sciences, and the National Natural Science Foundation of China.

Optical frequency transmission has facilitated various research fields in fundamental physics, navigation, time keeping, and geodetic applications.