Optical frequency transmission has facilitated various research fields in fundamental physics, navigation, time keeping, and geodetic applications. 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. In tackling the above problem, 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. 
    A research team, led by professor Cai Haiwen from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (CAS), have been devoting their 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. Recently, they have proposed a novel optical injection locking amplifier (OILA) with high amplification gain and ultra-low phase noise. The OILA 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. 
    The results, entitled “Ultra-low noise optical injection locking amplifier with AOM-based coherent detection scheme”, were published in Scientific Reports. 
    This work was supported by the Strategic Priority Research Program of the CAS, the Scientific Innovation Fund of the CAS and the National Natural Science Foundation of China.

(a) Phase noise PSD of the optical injection locking amplifer (OILA) .

(b) Allan deviation of the OILA for diferent injection ratios.

(c) Modifed Allan deviation of the OILA for diferent injection ratios.