The Study Of 1542 Nm Optical Fiber Transmission Link Based On Comparison Of Sr Atomic Optical Clock

The high-precision atomic clock not only improves the measurement accuracy of the time and frequency,but also provides an important method for related basic physics research.In recent years,the researches on the optical clock have made significant progresses,and the precision of the atomic optical clock can reach a very high level.The "second" of the future is very likely to be reproduced by the optical clock,replacing the existing fountain clock of the Cesium atom.The comparison of the two optical clocks at a long distance requires to transmit the narrow linewidth laser signal of the optical clock over a long distance with high precision.And the most effective transmission medium is the optical fiber currently.However,in the transmission process of a narrow linewidth laser,the complex external environment will have a significant influence on the characteristics(refractive index,loss,etc.)of the fiber,which may cause a random change in the phase of the transmitted optical frequency signal.This is equivalent to produce the phase noise.Therefore,the optical frequency signal transmitted along the fiber link loses the original stability and accuracy.This paper mainly studies the 1542 nm fiber transmission link based on the strontium(Sr)atomic optic clock.First,we introduce the system and the device for the comparison of Sr atomic optic clocks and the principle of comparison.Secondly,we introduce the mechanism of phase noise generation in fiber transmission link and the influence of fiber phase noise on narrow linewidth laser.At the same time,the method and principle of suppressing phase noise in optical fiber transmission link by active method used in our experimental scheme is described.Then,according to the requirements of the optical fiber transmission link in the Sr atomic optic clock comparison system,we selected various optoelectronic components and designed an all-fiber phase noise suppression system,in order to effectively suppress the phase noise and ensure the transmission stability of the transmission link.Experimentally,we built an all-fiber type transmission link phase noise-eliminating system which is based on the Michelson interferometer.The ultra-narrow and ultrastable 1542 nm laser(with a linewidth of 0.6 Hz)is transmitted round trip along the fiber link,and then beats with the local reference signal.A phase difference between the reference arm and the measuring arm can detect the phase noise introduced by the optical fiber transmission link,and finally we adopt a precision phase-locked control system to actively compensate the phase noise by using an acousto-optic modulator.Firstly,the background of the fiber link noise cancellation system was observed experimentally.And then,the system was optimized to reduce the background noise and improve the ability to suppress noise.Secondly,we measured the 50 km,100 km,300 km,500 km laboratory-wound fiber link,compared and analyzed the Allan variance before and after the phase noise suppression.Finally,the system is used for the Sr atomic optical clock comparison between Changping Campus and Hepingli campus of China Institute of Metrology with the urban commercial fiber link of 108 km.The experimental results show that the all-fiber phase noise cancellation system can effectively suppress the phase noise introduced by the fiber link and meet the requirements of optical clock comparison.