[1] Predehl K, Grosche G. Raupach S M F, et al. A 920-kilometer optical fiber link for frequency metrology at the 19th decimal place[J]. Science, 336, 441-444(2012).

[2] Riehle F. Optical clock networks[J]. Nature Photonics, 11, 25-31(2017).

[3] Foreman S M, Holman K W, Hudson D D et al. Remote transfer of ultrastable frequency references via fiber networks[J]. Review of Scientific Instruments, 78, 021101(2007).

[4] He Y B. Baldwin K G H, Orr B J, et al. Long-distance telecom-fiber transfer of a radio-frequency reference for radio astronomy[J]. Optica, 5, 138-146(2018).

[5] Zhu X, Wang B, Guo Y C et al. Robust fiber-based frequency synchronization system immune to strong temperature fluctuation[J]. Chinese Optics Letters, 16, 010605(2018). http://www.opticsjournal.net/Articles/Abstract?aid=OJ171226000419QnTpWs

[6] Liang Y F, Xu J N, Wu M et al. Research progress on fiber time-frequency synchronization technology[J]. Laser & Optoelectronics Progress, 57, 050004(2020).

[7] Frank F, Stefani F, Tuckey P et al. A sub-ps stability time transfer method based on optical modems[J]. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 65, 1001-1006(2018).

[8] Kim J, Cox J A, Chen J et al. Drift-free femtosecond timing synchronization of remote optical and microwave sources[J]. Nature Photonics, 2, 733-736(2008).

[9] Deng X, Liu J, Jiao D D et al. Coherent transfer of optical frequency over 112 km with instability at the 10-20 level[J]. Chinese Physics Letters, 33, 114202(2016).

[10] Guillou-Camargo F, Ménoret V, Cantin E et al. First industrial-grade coherent fiber link for optical frequency standard dissemination[J]. Applied Optics, 57, 7203-7210(2018).

[11] Giorgetta F R, Swann W C, Sinclair L C et al. Optical two-way time and frequency transfer over free space[J]. Nature Photonics, 7, 434-438(2013).

[12] Sprenger B, Zhang J, Lu Z et al. Atmospheric transfer of optical and radio frequency clock signals[J]. Optics Letters, 34, 965-967(2009).

[13] Djerroud K, Samain E, Clairon A et al. A coherent optical link through the turbulent atmosphere. [C]∥EFTF-2010 24th European Frequency and Time Forum, April 13-16, 2010, Noordwijk, Netherlands. New York: IEEE, 1-6(2010).

[14] Alatawi A, Gollapalli R P, Duan L Z. Radio-frequency clock delivery via free-space frequency comb transmission[J]. Optics Letters, 34, 3346-3348(2009).

[15] Sinclair L C, Giorgetta F R, Swann W C et al. Optical phase noise from atmospheric fluctuations and its impact on optical time-frequency transfer[J]. Physical Review A, 89, 023805(2014).

[16] Sinclair L C, Swann W C, Bergeron H et al. Synchronization of clocks through 12 km of strongly turbulent air over a City[J]. Applied Physics Letters, 109, 151104(2016).

[17] Sinclair L C, Bergeron H, Swann W C et al. Femtosecond optical two-way time-frequency transfer in the presence of motion[J]. Physical Review A, 99, 023844(2019).

[18] Sinclair L C, Swann W C, Deschênes J D et al. Optical system design for femtosecond-level synchronization of clocks[J]. Proceedings of SPIE, 9763, 976308(2016).

[19] Deschênes J D, Sinclair L C, Giorgetta F R et al. Synchronization of distant optical clocks at the femtosecond level[J]. Physical Review X, 6, 021016(2016).

[20] Miao J. Time and frequency synchronization in free space[D]. Beijing: Tsinghua Univesity(2015).

[21] Hou D, Zhang D N, Sun F Y et al. Research on high-precision free-space time and frequency transfer[J]. Journal of Time and Frequency, 41, 219-227(2018).

[22] Chen S J, Sun F Y, Bai Q S et al. Sub-picosecond timing fluctuation suppression in laser-based atmospheric transfer of microwave signal using electronic phase compensation[J]. Optics Communications, 401, 18-22(2017).

[23] Guo G K, Hou D, Sun F Y et al. Laser-based atmospheric radio-frequency transfer with sub-picosecond timing fluctuation using single phase compensator[J]. Optics Communications, 426, 526-530(2018).

[24] Sun F Y, Hou D, Zhang D N et al. Femtosecond-level timing fluctuation suppression in atmospheric frequency transfer with passive phase conjunction correction[J]. Optics Express, 25, 21312-21320(2017).

[25] Hou D, Zhang D N, Sun F Y et al. Free-space-based multiple-access frequency dissemination with optical frequency comb[J]. Optics Express, 26, 19199-19205(2018).

[26] Yue C L, Li J W, Sun J F et al. Homodyne coherent optical receiver for intersatellite communication[J]. Applied Optics, 57, 7915-7923(2018).

[27] Lu S W, Gao M, Yang Y et al. Inter-satellite laser communication system based on double Risley prisms beam steering[J]. Applied Optics, 58, 7517-7522(2019).

[28] Feng Z T, Zhang X, Wu R et al. High-stability and multithreading phase-coherent receiver for simultaneous transfer of stabilized optical and radio frequencies[J]. Optics Letters, 44, 2418-2421(2019).

[29] Feng Z T, Yang F, Zhang X et al. Ultra-low noise optical injection locking amplifier with AOM-based coherent detection scheme[J]. Scientific Reports, 8, 13135(2018).

[30] Jiang M Y, Chen Y Q, Cheng N et al. Multi-access RF frequency dissemination based on round-trip three-wavelength optical compensation technique over fiber-optic link[J]. IEEE Photonics Journal, 11, 1-8(2019).

[31] Chen Y Q, Cai H W, Jiang M Y et al. Stable radio frequency transfer over free space by passive phase correction[J]. IEEE Photonics Journal, 11, 1-8(2019).

[32] Kang J, Shin J, Kim C et al. Few-femtosecond-resolution characterization and suppression of excess timing jitter and drift in indoor atmospheric frequency comb transfer[J]. Optics Express, 22, 26023-26031(2014).

[33] Meng W D, Zhang H F, Huang P C et al. Design and experiment of onboard laser time transfer in Chinese Beidou navigation satellites[J]. Advances in Space Research, 51, 951-958(2013).

[34] Prochazka I, Yang F M. Photon counting module for laser time transfer via Earth orbiting satellite[J]. Journal of Modern Optics, 56, 253-260(2009).

[35] Fridelance P, Samain E, Veillet C. T2L2 - Time transfer by Laser link: a new optical time transfer generation[J]. Experimental Astronomy, 7, 191-207(1997).

[36] Samain E, Fridelance P. Time transfer by laser link (T2L2) experiment on mir[J]. Metrologia, 35, 151-159(1998).

[37] Exertier P, Samain E, Bonnefond P et al. Status of the T2L2/Jason2 experiment[J]. Advances in Space Research, 46, 1559-1565(2010).

[38] Exertier P, Samain E, Courde C et al. Sub-ns time transfer consistency: a direct comparison between GPS CV and T2L2[J]. Metrologia, 53, 1395-1401(2016).

[39] Samain E, Rovera G D, Torre J M et al. Time transfer by laser link (T2L2) in noncommon view between Europe and China[J]. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 65, 927-933(2018).

[40] Schreiber U, Prochazka I, Lauber P et al. The European laser timing (ELT) experiment on-board ACES. [C]∥2009 IEEE International Frequency Control Symposium Joint With the 22nd European Frequency and Time Forum, April 20-24, 2009, Besancon, France. New York: IEEE, 594-599(2009).

[41] Schreiber K U, Prochazka I, Lauber P et al. Ground-based demonstration of the European Laser Timing (ELT) experiment[J]. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 57, 728-737(2010).

[42] Prochazka I, Kodet J, Blazej J et al. Calibration of system delays in the European Laser Timing to 10 ps accuracy. [C]∥2014 European Frequency and Time Forum (EFTF), June 23-26, 2014, Neuchatel, Switzerland. New York: IEEE, 223-226(2014).