Research On High-precision Optical Frequency Transfer Via Fiber Link

In recent years,with the development of high-precision optical frequency standards,the uncertainty and instability of the optical frequency standards,such as the strontium optical clock,have reached 10-19 level,providing a high precision tool for many scientific fields,such as the detection of foundermental constants of physics,geodesy,dark matter detection,satellite navigation,and general relativity verification,etc.However,these applications need to transfer and compare the optical frequency standards in different areas.Hence,a transmission method with precision much higher than that of the optical frequency standards is required.Fiber link is reliable and has a low loss for optical signal at wavelength of 1.5 ?m.Moreover,a worldwide fiber network infrastructure already exists.In addition,optical frequency transfer via fiber link shows a transmission precision superior than traditional satellite-based transmission methods(such as two-way satellite frequency transfer)more than four orders of magnitude.Hence,fiber link is a promising method for optical frequency transfer,providing technical support for many scientific applications.In China,many scientific groups located in Xi'an,Beijing,Wuhan and other places have researched high-precision optical frequency standards and obtained certain achievements.In order to compare these optical frequency standards and apply them for scientific research in China,thousands-kilometers high-precision optical frequency transmission method is essential.At present,single-span transfer and cascaded transfer are two main optical frequency transfer schemes.This paper researches on the two transmission methods.First,single-span optical frequency transfer via fiber links with different lengths is demonstrated,and the aspects limited the dissemination distance are analyzed.An optical frequency transmission system is built and the environmental factors that affect the noise floor of the system are researched,such as temperature fluctuations and acoustic noise.By adopting acoustic insulation and temperature control to the transfer system,the noise floor reaches 3 × 10-17 at 1 s.Using an ultra-stable cavity-stabilized laser as the transfer laser,the 246 km optical frequency transmission via urban fiber link is demonstrated.The phase noise introduced by the link is detected by an analog mixer and actively suppressed by an acousto-optic modulator.The phase noise suppression is closed to the theoretical limitation.Transfer instability of 3 × 10-20 at 2000 s is reached.Then,480 km optical frequency transmission is researched.In order to accurately detect the phase noise that increases with the extendance of transmission distance,the digital phase detection technology with adjustable dynamic range is used in the 480 km link,and a transmission instability of 3 × 10-20 at 4000 s is achieved.As we know,it is currently the longest distance of optical frequency transfer via fiber link in China with a precision able to transfer the most precise optical standards.Finally,the optical frequency transmission distance is extended to 687 km,and transfer instability of 3.9 × 10-19 at 100 seconds is achieved.However,the deterioration of the signal-to-noise ratio restricted the continuous running of 687 km link and the further expansion of the transmission distance.In order to further extend the optical frequency transmission distance and improve the transfer precision,a cascaded transmission technology based on regenerative amplification is studied.The phase noise suppression limit of cascaded transfer and the advantages of cascade transfer over single-span transfer in precision are analyzed for the first time in the paper,which provides a theoretical basis for estimation of cascaded transfer precision.By locking the phase of the regenerative laser to the input phase,optical regenerative amplification is achieved with an amplification gain of 62 d B and a bandwidth of only a few tens of k Hz.Hence,the regenerative amplifier can filter out-of-band noise within a certain range and improve the signal-to-noise ratio.Using a regenerative amplifier,112 km + 112 km cascaded optical frequency transfer over urban fiber is demonstrated.The phase of each cascaded is controlled by each phase locking loop in the sender site.The phase control bandwidth of the 224 km link increases by one time compared with single-span transmission.300 km + 200 km cascaded optical frequency transmission is demonstrated,as well as 500 km single-span transfer for comparison.The noise suppression range at 1 Hz is improved by 5 d B,and the transmission instability is 0.5 times higher than that of single-span transfer,which is basically consistent with the theory,verifying the advantages of cascaded transmission over single transmission in transfer accuracy and distance.It is necessary to automatically lock and remotely control the existing high-gain optical amplifiers(such as fiber Brillion amplifier)to ensure the continuous operation,so the device is complex and costly.This paper proposes an amplification technology with simple structure,low cost and high reliability — low noise two-stage Erbium-doped fiber amplifier(EDFA).The low-noise two-stage EDFA can achieve a gain of more than 45 d B by connecting two-stage unidirectional EDFA in series,and realizes low-noise optical amplification by detecting and suppressing the phase noise introduced by the two-stage EDFA.The phase noise of the two-stage EDFA is much smaller than that of the ultra-stable laser,so it can be used for cascaded amplification.Compared with the longest continuous operation time of the regenerative amplifier of only a few days,the continuous locking time of the low-noise two-stage EDFA can be as long as several months,so this amplification scheme greatly improves the reliability of the cascaded station.Using low-noise two-stage EDFA,205 km + 205 km optical frequency transmission via fiber is demonstrated.The phase noise suppression reaches the theoretical limit,and the transmission instability obtains 7.1×10-18 at 2500 s.The cascaded transmission is compared with single-span 410 km transfer and the instability is improved by one time.This amplification scheme provides technical support for building a high-precision thousands-kilometers optical frequency transfer fiber network in China.