基于微波相位补偿的56 km高精度光纤微波频率传递

报道了实验室内56 km光纤微波频率传递的实验研究,在56 km的传递距离上实现了1.8×10^-15/s,4×10^-18/10⁴s的传递稳定度。系统通过环回法比较往返传递的微波信号相位获得链路上的相位扰动量,并实时控制本地发射端的微波发射信号相位实现预补偿。在环回往返传递的不同方向上,系统方案采用不同频率的微波调制信号,这种方法极大避免了光寄生反射效应的影响,同时利用色散补偿光纤改善探测信号相噪等措施,提高了系统的传递稳定度。     

用于光频传递的通信波段窄线宽激光器研制及应用

通信波段窄线宽激光器在基于光纤的光学频率传递中有着重要应用. 本文报道了1550 nm超窄线宽光纤激光器的研制及其在光学频率传递中的初步应用结果. 利用一台激光光源, 分别锁定到两个参考腔上(精细度分别为344000和296000), 锁定后经拍频比对测得单台激光线宽优于1.9 Hz, 秒级频率稳定度为1.7×10^-14, 优于国内同类报道. 将研制的超窄线宽激光器用于光纤光学频率传递, 在50 km光纤盘上实现了 7.5×10^-17/s的传递稳定度, 较采用商用光纤激光器提高了3.2倍.     

用于长距离光学频率传递链路的双向掺铒光纤放大器的优化设计

光纤链路中高精度光学频率传递对光钟比对有重要意义,双向掺铒光纤放大器(EDFA)有助于在长距离光学频率传递中对信号进行损耗补偿和高精度传输。基于铒粒子受激放大的基本原理,设计了可用于光纤光学频率传递链路中的低噪声、高增益双向EDFA,并对其参数进行了仿真优化。实验结果表明,该双向EDFA的噪声指数为3.86dB,增益为20.14dB,引入的相位噪声在频率为1Hz处仅为0.1rad2/Hz。将该双向EDFA作为放大补偿器件应用于200km光纤光学频率传递链路中,获得了3.8×10-16/s的秒级频率稳定度及2.8×10-19/(104 s)的万秒级频率稳定度,在频率信号传递和光钟比对领域有着广阔的应用前景。     

相位自校准的光纤微波频率绝对相位传递

提出一种相位自校准的光纤微波频率相位传递方案。该方案使用声表面波滤波器的冲激响应的窄带信号作为时间信号，使其与频率信号可同时使用同一波长进行传输。为实现稳定的可重复相位差，利用时间信号的往返传输时延来确定频率信号的整数个周期，并在多次重启的情况下验证了系统相位的稳定性。在60 km实验室平台上对所提方案进行验证，频率传递的稳定度优于4×10^-14@1 s,5×10^-17@10 000 s。系统多次重启的情况下，所获得的平均相位差最大不一致的峰峰值为0.008 rad，对应于整个周期的0.15%，可保证较高的相位一致性。     

冷原子气体的时频测量——光晶格原子钟

基于冷原子气体的时频测量在近20年里快速发展,引起了人们的广泛关注,其典型代表是基于大量中性原子的光晶格原子钟。利用超稳钟激光同时探测囚禁在光晶格里成千上万个冷原子的钟跃迁信号,光晶格原子钟已实现10^-18量级的频率准确度和10^-17量级的秒级稳定度,大幅度提高了时频测量的精度。文章概述了光晶格原子钟的发展历史、工作原理、性能评估及应用前景。     

冷原子气体的时频测量——光晶格原子钟

基于冷原子气体的时频测量在近20年里快速发展,引起了人们的广泛关注,其典型代表是基于大量中性原子的光晶格原子钟。利用超稳钟激光同时探测囚禁在光晶格里成千上万个冷原子的钟跃迁信号,光晶格原子钟已实现10^-18量级的频率准确度和10^-17量级的秒级稳定度,大幅度提高了时频测量的精度。文章概述了光晶格原子钟的发展历史、工作原理、性能评估及应用前景。     

基于光学时延补偿的50 km光纤微波频率传递

报道了利用50 km光纤实现4.38×10~(-15)@1 s和2.80×10~(-18)@65.5×10~3s稳定度的微波频率传递的实验研究.实验系统采用多普勒噪声消除技术,通过在本地端探测往返传递的微波信号相位获得链路上的相位变化信息,并实时控制光延迟调整机构进行补偿.光延迟控制采用压电陶瓷的快速拉伸和慢速光纤温控联合方式,可实现5 ns和千赫兹带宽的光延迟控制,能够实现光纤噪声的长期高精度补偿.与电相位补偿相比,光延迟补偿受微波泄露的影响相对较小,而微波泄露影响在类似系统难以避免,因此这种方式更利于获得高稳定度的频率传递.此外,系统采用变频往返传递消除光寄生反射效应,以及色散补偿光纤减小因色散引起的调制信号衰减等措施,提高了系统的技术指标.     

基于210km实地通信链路的高稳定性光学频率信号传递

利用西安和咸阳之间的电信省级骨干光纤网构建了210km的光学频率信号传递测试链路,链路损耗为0.23dB/km。实验中采用可搬运、基于光纤干涉仪、线宽约为200 Hz的激光器作为光源,利用两台低噪声双向掺铒光纤放大器(EDFA)补偿光纤链路损耗和增加光信号的传输距离,放大器平均增益控制在15dB左右,以防止激射。通过测量和分析不同情况下光纤链路的附加相位噪声,可观测到铁路震动引起的规律性干扰。当噪声抑制系统在锁定状态时,链路的相位噪声被抑制了23dB,在剔除铁路干扰时段数据后,获得的210km实地通信链路的秒级频率稳定度达到了1.51×10-14,万秒频率稳定度达到了5×10-17。利用210km通信链路进行了光学频率信号的远程传递测试,分析了限制频率稳定度的主要影响因素,并针对现行光纤布设方式提出了补充要求。该研究为基于通信链路的高精度光学频率信号的传递与比对提供理论支撑。     

散弹噪声极限稳定度优于1×10-13τ-1/2的铷频标物理系统

近年来,铷原子频标研究取得长足进展,频率稳定度达到10^-13τ^-1/2量级.为进一步改善铷频标稳定度性能,本文设计了一种高信噪比物理系统.物理系统中的腔泡组件采用微波场磁力线与量子化轴方向高度平行的开槽管式微波腔,滤光泡和吸收泡独立控温.抽运光源采用了光学滤光和同位素滤光双重滤光方案.本文实测了背景光电流I₀和鉴频斜率K_d,结果分别为95 μA和7.7 nA/Hz,在此基础上计算物理系统的散弹噪声极限稳定度为7.5×10^-14τ^-1/2.研究结果表明,只要锁频环路的电子学噪声得到有效控制,铷频标的频率稳定度突破1×10^-13τ^-1/2,进入10^-14τ^-1/2量级是完全可能的.     

一种基于3×3光纤耦合器的短距离光纤频率传递的噪声抑制方法

提出了一种噪声抑制方法，设计了基于3×3光纤耦合器迈克尔逊干涉仪的频率传递系统，使用嵌入式系统进行控制，通过调整光纤长度，实时补偿由温度变化等环境因素引起的时延变化，并进行了实验验证。启用时延补偿后，实验用的30 m长传输光纤在环境温度变化21℃条件下长度变化量小于±1μm，对应时间延迟变化量小于10 fs，所传输的光梳重频信号的频率稳定度没有明显变化。本文工作有望为空间条件下的光钟信号向比对设备的传输路径噪声抑制提供有效的解决方法。     

All polarization-maintaining Er:fiber-based optical frequency comb for frequency comparison of optical clocks

We demonstrate an optical frequency comb (OFC) based on a turnkey mode-locked laser with a figure-9-shape structure and polarization-maintaining fibers, for the comparison of frequency among optical clocks with wavelengths of 698 nm, 729 nm, 1068 nm, and 1156 nm. We adopt a multi-branch approach in order to produce high power OFC signals at these specific wavelengths, enabling the signal-to-noise ratio of the beatnotes between the OFC and the clock lasers to exceed 30 dB at a resolution bandwidth of 300 kHz. This approach makes the supercontinuum spectra much easier to be generated than a single branch OFC. However, more out-of-loop fibers degrade the long-term frequency instability due to thermal drift. To minimize the thermal drift effect, we set the fiber lengths of different branches to be similar, and we stabilize the temperature as well. The out-of-loop frequency instability of the OFC due to the incoherence of the multi-branch is about 5.5×10^-19 for 4000 s, while the in-loop frequency instability of f_ceo and that of f_beat are 7.5×10^-18 for 1 s and 8.5×10^-18 for 1 s, respectively. The turnkey OFC meets the requirement for the comparison of frequency between the best optical clocks.     

Suppression of servo error uncertainty to 10~(-18) level using double integrator algorithm in ion optical clock

A universal locking model for single ion optical clocks was built based on a simple integrator and a double integrator.Different integrator algorithm parameters have been analyzed in both numerical simulations and experiments. The frequency variation measured by the comparison of two optical clocks coincides well with the simulation results for different second integrator parameters. According to the experimental results, the sensitivity of the servo error influenced by laser frequency drift with the addition of a double integrator was suppressed by a factor of 107. In a week-long comparison of optical clocks, the relative uncertainty of the servo error is determined to be 1.9 × 10~(-18), which is meaningful for the systematic uncertainty of the transportable single ~(40)Ca~+ ion optical clock entering the 10~(-18) level.     

Joint transfer of time and frequency signals and multi-point synchronization via fiber network

A system of jointly transferring time signals with a rate of 1 pulse per second(PPS) and frequency signals of 10 MHz via a dense wavelength division multiplex-based(DWDM) fiber is demonstrated in this paper.The noises of the fiber links are suppressed and compensated for by a controlled fiber delay line.A method of calibrating and characterizing time is described.The 1PPS is synchronized by feed-forward calibrating the fiber delays precisely.The system is experimentally examined via a 110 km spooled fiber in laboratory.The frequency stabilities of the user end with compensation are1.8×10~(-14) at 1 s and 2.0×10~(-17) at 10~4 s average time.The calculated uncertainty of time synchronization is 13.1 ps,whereas the direct measurement of the uncertainty is 12 ps.Next,the frequency and 1PPS are transferred via a metropolitan area optical fiber network from one central site to two remote sites with distances of 14 km and 110 km.The frequency stabilities of 14 km link reach 3.0×10~(-14) averaged in 1 s and 1.4×10~(-17) in 10~4 s respectively;and the stabilities of 110 km link are 8.3×10~(-14) and 1.7×10~(-17),respectively.The accuracies of synchronization are estimated to be 12.3 ps for the14 km link and 13.1 ps for the 110 km link,respectively.     

200 km沙漠链路高精度光纤时频传递关键技术研究

针对沙漠环境实地链路存在的温度变化大、室外风力、地表振动等多种复杂噪声来源,通过对系统反馈补偿带宽、反馈补偿强度、光功率等时频传递系统关键参数的优化配置,研究了不同反馈补偿参数下复杂链路噪声的有效抑制技术.全链路的频率传递稳定度8×10~(-14)@1s,1×10~(-16)@1000 s,千秒尺度下时间信号传递的时间方差仅为1.2 ps.实现了氢钟信号在200 km量级沙漠环境实地链路的无损传输.该验证实验在基于短基线干涉测量的卫星测轨系统中发挥了重要作用.     

Modeling of cascaded high isolation bidirectional amplification in long-distance fiber-optic time and frequency synchronization system

We propose a physical model of estimating noise and asymmetry brought by high isolation Bi-directional erbiumdoped fiber amplifiers(Bi-EDFAs), no spontaneous lasing even with high gain, in longdistance fiber-optic time and frequency(T/F) synchronization system. It is found that the Rayleigh scattering noise can be suppressed due to the high isolation design, but the amplified spontaneous emission(ASE) noise generated by the high isolation Bi-EDFA and the bidirectional asymmetry of the transmission link caused by the high isolation Bi-EDFA will deteriorate the stability of the system.The calculated results show that under the influence of ASE noise, the frequency instability of a 1200 km system composed of 15 high isolation Bi-EDFAs is 1.773 × 10-13/1 s. And the instability caused by asymmetry is 2.6064 × 10-16/30000–35000 s if the total asymmetric length of the bidirectional link length is 30 m. The intensity noises originating from the laser and detector, the transfer delay fluctuations caused by the variation in ambient temperature and the jitter in laser output wavelength are also studied. The experiment composed of three high isolation Bi-EDFAs is done to confirm the theoretical analysis. In summary, the paper shows that the short-term instability of the T/F synchronization system composed of high isolation Bi-EDFAs is limited by the accumulation of ASE noise of amplifiers and the laser frequency drift, while the long-term instability is limited by the periodic variation in ambient temperature and the asymmetry of the amplifiers.The research results are useful for pointing out the direction to improve the stability of the fiber-optic T/F synchronization system.     

Fiber-based multiple-access frequency synchronization via 1f–2f dissemination

Considering the reference frequency dissemination requirements of the Square Kilometre Array telescope(SKA)project,on the basis of the 1f–2f precision frequency synchronization scheme,we propose and demonstrate a fiber-based multiple-access frequency synchronization scheme.The dissemination reference frequency can be recovered at arbitrary nodes along the entire fiber link.It can be applied to antennas close proximity to the SKA central station,and will lead to a better SKA frequency synchronization network.As a performance test,we recover the disseminated 100-MHz reference frequency at an arbitrary node chosen as being 5 km away from the transmitting site.Relative frequency stabilities of2.0×10~(-14)/s and 1.6×10~(-16)/10~4 s are obtained.We also experimentally verify the feasibility of a frequency dissemination link with three access points.     

高准确度的钙离子光频标

近年来,冷原子技术和激光技术促进了高精度光频标的发展,有望在建立时间基准、推动基础研究和满足国家需求等方面发挥重要的作用.本文介绍了中国科学院武汉物理与数学研究所近年来在高准确度钙离子(~(40)Ca~+)光频标研究方面的进展:采用新的ULE腔系统,实现了729 nm钟跃迁激光器1—100 s的频率稳定度均优于2×10~(-15),通过对外场和环境效应的控制及克服,特别是囚禁离子运动效应的抑制,获得单个钙离子光频标的不确定度优于5.5×10~(-17);通过两台光频标的比对,测得20000 s的稳定度也进入10~(-17)量级;基于高精度钙离子光频标平台,进行了相关精密测量的工作,包括:基于全球定位系统的超高精度远程光频绝对值测量方案,第二次测量了钙离子的光频跃迁绝对值,该测量结果再次被国际时间频率咨询委员会采纳,更新了钙离子的频率推荐值;精确测量了钙离子的钟跃迁魔幻波长,由此提出新型的全光囚禁离子光频标的方法;精密测量了钙离子的亚稳态寿命等参数.以上工作推动了基于冷原子的精密测量工作.     

利用传输腔技术实现镱原子光钟光晶格场的频率稳定

为了获得高稳定度和高精确度的原子光晶格钟,光晶格场的频率必须得到锁定,线宽必须控制到特定水平用来消除交流斯塔克频移.本文提出利用传输腔技术来实现对镱原子光钟的光晶格场的频率锁定和抑制频率长期漂移的锁定方案.首先,将一个殷钢材料的传输腔锁定在基于调制转移谱技术锁定的780 nm激光场上,再将759 nm的光晶格光场锁定在传输腔上.实验结果表明,光晶格光场的线宽可以锁定和控制在1 MHz以下.光晶格光场与锁定于氢钟的光梳拍频结果显示,光晶格光场的长期频率稳定度优于3.6×10~(-10),可以确保实现镱原子光钟的不确定度进入10~(-17).