Ultrastable cesium atomic clock with a 9.1926-GHz regeneratively mode-locked fiber laser

We describe an ultrastable cesium (Cs) atomic clock with a 9.1926-GHz regeneratively mode-locked fiber laser obtained by use of an optically pumped Cs beam tube. By adopting a 1-m-long Cs beam tube with a linewidth of 110 Hz, we have successfully obtained frequency stabilities of 4.8 x 10(-12) for tau = 1 s and 6.3 x 10(-13) for tau = 50 s for a 9.1926-GHz microwave output signal. This Cs atomic clock can generate an optical pulse train with the same stability as that of the obtained microwave, which allows us to deliver a frequency standard optical signal throughout the world by means of optical fiber networks.     

Mode-locked laser-type optical atomic clock with an optically pumped Cs gas cell

We propose and demonstrate a mode-locked laser-type optical atomic clock with an optically pumped cesium (Cs) gas cell. By adopting the optically pumped Cs gas cell with a double resonance method as a frequency standard, we have successfully demonstrated an ultrastable rack-mount type Cs optical atomic clock with excellent short-term stability. The obtained frequency stabilities reached as high as 1.2 x 10(-12) for tau=1 s and 8.8 x 10(-14) for tau=100 s for a 9.1926 GHz microwave output signal.     

A novel 852-nm tunable fiber laser

We report a novel fiber laser operating at 850-nm band by using semiconductor optical amplifier and fiber grating. The laser system is stable, compact, and the operating wavelength can be tuned continuously from about 851 to 854 nm for Cs atomic clock system by stretching the fiber grating. An output power up to 20 mW is obtained with a signal-to-background ratio beyond 20 dB.     

利用钟跃迁谱线测量超稳光学参考腔的零温漂点

在87Sr光晶格钟实验系统中,通过将自由运转的698 nm激光频率锁定在由超低膨胀系数的玻璃材料构成的超稳光学参考腔上,从而获得短期频率稳定性较好的超稳窄线宽激光.超稳光学参考腔的腔长稳定性决定了最终激光频率的稳定度.为了降低腔长对温度的敏感性,使激光频率具有更好的稳定度和更小的频率漂移,利用锶原子光晶格钟的钟跃迁谱线,测量了698 nm超稳窄线宽激光系统中超稳光学参考腔的零温漂点.通过对钟跃迁谱线中心频率随温度的变化曲线进行二阶多项式拟合,得到698 nm超稳窄线宽激光系统的零温漂点为30.63℃.利用锶原子光晶格钟的闭环锁定,测得零温漂点处698 nm超稳窄线宽激光系统的线性频率漂移率为0.15 Hz/s,频率不稳定度为1.6×10^–15@3.744 s.     

A high-stability semiconductor laser system for a <Superscript>88</Superscript>Sr-based optical lattice clock

We describe a frequency-stabilized diode laser at 698 nm used for high-resolution spectroscopy of the ¹S₀–³P₀ strontium clock transition. For the laser stabilization we use state-of-the-art symmetrically suspended optical cavities optimized for very low thermal noise at room temperature. Two-stage frequency stabilization to high-finesse optical cavities results in measured laser frequency noise about a factor of three above the cavity thermal noise between 2 Hz and 11 Hz. With this system, we demonstrate high-resolution remote spectroscopy on the ⁸⁸Sr clock transition by transferring the laser output over a phase noise-compensated 200-m-long fiber link between two separated laboratories. Our dedicated fiber link ensures a transfer of the optical carrier with frequency stability of 7×10⁻¹⁸ after 100 s integration time, which could enable the observation of the strontium clock transition with an atomic Q of 10¹⁴. Furthermore, with an eye toward the development of transportable optical clocks, we investigate how the complete laser system (laser+optics+cavity) can be influenced by environmental disturbances in terms of both short- and long-term frequency stability.     

飞秒钛宝石光学频率梳的精密锁定

经相位锁定后的飞秒钛宝石光学频率梳已经广泛用于绝对光频的测量，这是光频标领域一个革命性的突破.在自建的90MHz飞秒钛宝石激光器的基础上首先采用光子晶体光纤将其光谱展宽到一个光倍频程，接着利用锁相环技术分别将重复频率和载波包络频移同时高精度地锁定到一台稳定度为6×10^－14的Cs钟上，进而得到了稳定度相同的飞秒光学频率梳.     

AC Stark-shift in CPT-based Cs miniature atomic clocks

We report on studies on the light-shift in caesium miniature atomic clocks based on coherent population trapping (CPT) using a micro-fabricated buffer-gas cell (MEMS cell). The CPT signal is observed on the Cs D1-line by coupling the two hyperfine ground-state Zeeman sublevels involved in the clock transition to a common excited state, using two coherent electromagnetic fields. These light fields are created with a distributed feedback laser and an electro-optical modulator. We study the light-shift phenomena at different cell temperatures and laser wavelengths around 894.6?nm. By adjusting the cell temperature, conditions are identified where a miniature CPT atomic clock can be operated with simultaneously low temperature coefficient and suppressed light-shift. The impact of the light-shift on the clock frequency stability is evaluated. These results are relevant for improving the long-term frequency stability of CPT-based Cs vapour-cell clocks.     

High-resolution spectroscopy with a femtosecond laser frequency comb

The output of a mode-locked femtosecond laser is used for precision single-photon spectroscopy of 133Cs in an atomic beam. By changing the laser's repetition rate, the cesium D1 (6s 2S(1/2)-->6p 2P(1/2)) and D2 (6s 2S(1/2)-->6p 2P(3/2)) transitions are detected and the optical frequencies are measured with accuracy similar to that obtained with a cw laser. Control of the femtosecond laser repetition rate by use of the atomic fluorescence is also implemented, thus realizing a simple cesium optical clock.     

Phase-coherent link from optical to microwave frequencies by means of the broadband continuum from a 1-GHz Ti:sapphire femtosecondoscillator

An optical clockwork is created with a compact 1-GHz repetition-rate laser and three nonlinear crystals. The broadband continuum output of the laser covers sufficient bandwidth to provide direct access to its carrier-envelope offset frequency without the use of a microstructure fiber. We phase lock the femtosecond comb to a Ca optical standard and monitor the stability of the repetition rate, f(r) , at 1 GHz. We demonstrate that the short-term stability of the microwave output of the optical clock is at least as good as that of a high-performance hydrogen maser.     

Phase-locked widely tunable optical single-frequency generator based on a femtosecond comb

We present an arbitrary optical single-frequency generator based on a femtosecond optical frequency comb. The functions of this device are comparable to those of a radio-frequency synthesizer. However, this device operates at hundreds of terahertz. The absolute frequency accuracy of this synthesizer is approximately 1 kHz at a 282 THz carrier frequency. The stability is approximately 2 x 10(-14) at 100 s, and the tuning speed exceeds 30 GHz/s. This source demonstrates the integration of a phase-locked optical comb into a versatile and easy-to-use system for the generation of tunable, absolute optical frequencies. By using downconversion, one could generate tunable terahertz frequencies that are phase locked to a microwave reference, such as a Cs atomic clock, and high-precision interferometry could benefit greatly from the stability and accuracy of this widely tunable source.     

Transferring the stability of iodine-stabilized diode laser at 634 nm to radio frequency by an optical frequency comb

An optical flequency comb phase-locked on an iodine frequency stabilized diode laser at 634 nm is constructed to transfer the accuracy and stability from the optical domain to the radio frequency domain. An external-cavity diode laser is frequency-stabilized on the Doppler-free absorption signals of the hyperfine transition R（80）8-4 using the third-harmonic detection technique. The instability of the ultra-stable optical oscillator is determined to be 7 ×10^-12 by a cesium atomic clock via the optical frequency comb＇s mass frequencv dividing technique.     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

New limits on coupling of fundamental constants to gravity using 87Sr optical lattice clocks

The 1S0-3P0 clock transition frequency nuSr in neutral 87Sr has been measured relative to the Cs standard by three independent laboratories in Boulder, Paris, and Tokyo over the last three years. The agreement on the 1 x 10(-15) level makes nuSr the best agreed-upon optical atomic frequency. We combine periodic variations in the 87Sr clock frequency with 199Hg+ and H-maser data to test local position invariance by obtaining the strongest limits to date on gravitational-coupling coefficients for the fine-structure constant alpha, electron-proton mass ratio mu, and light quark mass. Furthermore, after 199Hg+, 171Yb+, and H, we add 87Sr as the fourth optical atomic clock species to enhance constraints on yearly drifts of alpha and mu.     

Systematic study of the 87Srclock transition in an optical lattice

With ultracold 87Srconfined in a magic wavelength optical lattice, we present the most precise study (2.8 Hz statistical uncertainty) to date of the 1S0-3P0 optical clock transition with a detailed analysis of systematic shifts (19 Hz uncertainty) in the absolute frequency measurement of 429 228 004 229 869 Hz. The high resolution permits an investigation of the optical lattice motional sideband structure. The local oscillator for this optical atomic clock is a stable diode laser with its hertz-level linewidth characterized by an octave-spanning femtosecond frequency comb.     

High-accuracy measurement of atomic polarizability in an optical lattice clock

Presently, the Stark effect contributes the largest source of uncertainty in a ytterbium optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clock's sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels α(clock) = 36.2612(7) kHz?(kV/cm)(-2). The clock's uncertainty due to room temperature blackbody radiation is reduced by an order of magnitude to 3×10(-17).     

Improvement of the short-term stability of atomic fountain clock with state preparation by two-laser optical pumping

To improve the signal to noise ratio(SNR) and the short-term stability of cesium atomic fountain clocks, the work of two-laser optical pumping is presented theoretically and experimentally. The short-term stability of the NIM6 fountain clock has been improved by preparing more cold atoms in the |F = 4, mF= 0〉 clock state with a shortened cycle time.Two π-polarized laser beams overlapped in the horizontal plane have been applied after launching, one is resonant with|F = 4〉→ |F = 4〉 transition and the other is resonant with |F = 3〉→ |F = 4〉 transition. With optical pumping, the population accumulated in the |mF= 0〉 clock state is improved from 11% to 63%, and the detection signal is increased by a factor of 4.2, the SNR of the clock transition probability and the short-term stability are also improved accordingly.     

Interrogation of optical Ramsey spectrum and stability study of an 87Sr optical lattice clock

The optical Ramsey spectrum is experimentally realized in an ⁸⁷Sr optical lattice clock, and the measured linewidth agrees well with theoretical expectation. The coherence time between the clock laser and the atoms, which indicates the maximum free evolution period of using Ramsey detection to measure the atom-laser phase information, is determined as 340(23) ms by measuring the fringe contrasts of the Ramsey spectrum as a function of the free evolution period. Furthermore, with the same clock duty cycle of about 0.1, the clock stability is measured by using the Ramsey and Rabi spectra, respectively. The experimental and theoretical results show approximately the same stability as the two detection methods, which indicates that Ramsey detection cannot obviously improve the clock stability until the clock duty cycle is large enough. Thus, it is of great significance to choose the detection method of a specific clock.     

基于光纤的光学频率传递研究

随着光钟研究的发展, 光钟的稳定度和不确定度均达到10^-18量级. 通过光纤可以实现光钟频率信号的高精度传输, 有望用于未来“秒”定义的复现. 演示了百公里级实验室光纤上的光学频率传递. 对于在实验室70 km光纤盘上实现的光频传递, 光纤相位噪声抑制在1-250 Hz傅里叶频率范围内均接近于光纤延时极限, 对应传输稳定度(Allan偏差)为秒级稳定度1.2×10^-15, 10000 s稳定度为1.4×10^-18. 实验室100 km光纤的光频传递秒级稳定度也达到了5×10^-15. 提出了光纤噪声用户端补偿的方案, 可以简化星形传递网络中心站的复杂度. 在25 km光纤上演示了该传递方案, 实现的传输稳定度接近传统前置补偿传递方案.     

High-accuracy optical clock via three-level coherence in neutral bosonic 88Sr

An optical atomic clock scheme is proposed that utilizes two lasers to establish coherent coupling between the 5s2 1S0 ground state of 88Sr and the first excited state, 5s5p 3P0. The coupling is mediated by the broad 5s5p 1P1 state, exploiting the phenomenon of electromagnetically induced transparency. The effective linewidth of the clock transition can be chosen at will by adjusting the laser intensity. By trapping the 88Sr atoms in an optical lattice, long interaction times with the two lasers are ensured; Doppler and recoil effects are eliminated. Based on a careful analysis of systematic errors, a clock accuracy of better than 2 x 10(-17) is expected.     

Compact, thermal-noise-limited optical cavity for diode laser stabilization at 1x10(-15)

We demonstrate phase and frequency stabilization of a diode laser at the thermal noise limit of a passive optical cavity. The system is compact and exploits a cavity design that reduces vibration sensitivity. The subhertz laser is characterized by comparison with a second independent system with similar fractional frequency stability (1x10(-15) at 1 s). The laser is further characterized by resolving a 2 Hz wide, ultranarrow optical clock transition in ultracold strontium.