锁相环路对氢原子钟性能影响的实验分析

实验以上海天文台SOHM-4型氢原子钟脉泽信号和新研制的模拟-数字混合型锁相环路为基础,主要分析锁相环路参数对氢原子钟输出信号的短期频率稳定度和单边带相位噪声的影响。针对氢脉泽信号高Q值的特殊性,通过理论分析和实验验证寻求与其相匹配的最佳环路参数。实验结果表明,锁相环路的参数设置直接影响到氢原子钟输出信号的性能;在脉泽信号不变的情况下,改进后的锁相环路和主电子学系统可使氢原子钟的频率稳定度提高至1.7×10^-13/1s, 3.3×10^-14/10s, 9.1×10^-15/100s, 2.9×10^-15/1 000s, 1.4×10^-15/10000s,即较之原有的技术指标,在各取样时间范围内,频率稳定度的测试结果均提高了半个量级。     

Study of the 87Rb maser with both ends pumping

Experiments on the ⁸⁷Rb maser with both ends pumping showed variations of the maximum output power versus cell temperature. The largest output power was 2.0×10^-10 W, and a preliminary measurement gave an improved short-term frequency stability. The results of the numerical calculations based on the density matrix formalism agreed qualitatively with the experiments     

Hg+ trapped ion standard with the superconducting cavitymaser oscillator

The frequency stability of an atomic standard based on ¹⁹⁹ Hg⁺ ions confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition has been measured to be 17 mHz wide, representing a quality factor greater than 2×10¹². A 160-mHz line is used to steer the output of a 5-MHz crystal oscillator to obtain a stability of 2×10^-15 for 24000-s averaging times. In a separate measurement, a 37-mHz line is used to steer the output of the superconducting cavity maser oscillator to reach 1×10^-15 stability at 10000 s     

蓝宝石主动型氢原子频标

研制了采用蓝宝石加载微波腔的蓝宝石主动型氢原子频标,腔体外径18 cm,高度20 cm,重量约2.5 kg。由于微波腔体积的减小,整钟体积较传统大氢钟明显减小。经中国计量科学研究院测试,该蓝宝石主动型氢原子频标的技术指标为3.0×10-13@1 s、3.8×10-15@1 d,是目前国内体积最小的高指标、高可靠主动型氢原子频标,可在实验室、车载等环境下使用。     

The BIPM laser standards at 633 nm and 532 nm simultaneously linked to the SI second using a femtosecond laser in an optical clock configuration

A femtosecond laser comb was used in an optical clock configuration to measure simultaneously the optical frequency of an iodine-stabilized Nd:YAG laser at 532 nm and an iodine-stabilized He-Ne laser at 633 nm at the International Bureau of Weights and Measures (BIPM). The noise characteristics of the data corresponds well to those of the reference standards and the lasers under study. In a second series of measurements during which the comb was phase-locked to a hydrogen maser, laser standards at 532 nm and at 633 nm were measured. A standard deviation of 6/spl times/10/sup -15/ during 2 h of measurements for the Nd:YAG laser illustrates well the excellent stability of these standards and, at the same time, the capabilities of the comb techniques.     

LASER INDUCED CRYSTALLIZATION OF AMORPHOUS Se80Te20 ALLOY THIN FILMS

The present study is concerned with the laser induced crystallization of amorphous thin films of Se₈₀Te₂₀ alloy. The films were prepared on a glass substrate by vacuum evaporation from bulk Se₈₀Te₂₀ alloy. The as-grown films were amorphous. The crystallization induced by an argon ion laser irradiation was studied at different beam intensities ranging from 50 mW to 600 mW and different time durations. The 486 nm (blue green) line was chosen for irradiation. The crystallization and growth processes in the laser irradiated samples were studied in the electron microscope at low temperature (173 K). it was seen that the crystallization was quicker at higher laser beam intensities as expected. The conditions for the onset of nucleation and the progress of crystallization in these films are compared with those observed in the films irradiated by electron beam.     

Frequency of a methane-stabilized helium-neon laser

A phase-locked laser frequency chain was used to measure the absolute frequency of a HeNe/CH₄ laser relative to the primary Cs standard, with measurement uncertainty less than ±1 part in 10¹². The frequency depends on laser parameters; a spread of 8.3×10^-11 was observed. This was attributed to the variation of Lamb dip asymmetry resulting from the gas-lens effect and the accompanying diffraction loss variation near resonance. Means of improving laser frequency reproducibility by reducing cavity diffraction loss variation were studied experimentally. The frequency obtained with a cavity exhibiting the smallest Lamb dip asymmetry was found to be 88 376 181 599.07±0.07 kHz     

一种由NIM5铯喷泉基准驾驭实现的独立时标TA（NIM）

原子时标TA（NIM）是一个独立时标,其频率由NIM5铯喷泉基准驾驭。产生时标的主钟是一台主动型氢原子钟,铯喷泉基准定期对其测量和校准。时标算法通过预估氢钟将来的频率,补偿过去预估频率与校准频率之差,并评估无校准数据期间的氢钟频率,最终尽可能实现TA（NIM）的频率与NIM5铯喷泉基准保持一致。2007年8月,TA（NIM）开始试运行,2008年6月正式运行。1年多来的数据分析表明,TA(NIM)运行连续可靠,与TAI间的时间稳定度(5天)达到1.2 ns,相对频差为2.0×10^-15。     

Investigations on continuous and pulsed interrogation for a CPT atomic clock

We investigated the influence of some critical parameters and operating conditions such as cell temperature, laser intensity, and interrogation technique affecting the performances of a gas cell Cs frequency standard based on coherent population trapping (CPT). Thanks to an original experimental setup, the atoms can be trapped in the dark state and interrogated using continuous wave (CW) or pulsed coherent optical radiations. Using a double-lambda scheme, a signal contrast as high as 52% has been measured in the continuous regime for an optimum cell temperature of 35degC. Compared with the conventional continuous CPT interrogation, the pulsed interrogation technique reduces the light shift by a factor of 300 and allowed it to reach high-frequency stability for higher laser intensities. The frequency stability has been measured to be 9 x 10^-13 for a 1 s integration time. Main noise contributions limiting the short-term and medium-term frequency stability are reviewed and estimated.     

Precision frequency measurement of the 2S1/2 2D5/2 transition of Sr+ with a 674-nmdiode laser locked to an ultrastable cavity

We have measured the frequency of the 5s ²S_1/2 -4d ²D_5/2 clock transition of a single Sr ion confined in a Paul trap. A diode laser locked to an ultrastable Fabry-Perot (F-P) cavity was used to probe the transition with a resolution of 3.5 kHz. The absolute frequency was determined from heterodyne measurements referenced to an iodine stabilized HeNe laser and a CO₂ laser yielding a value for the S-D transition of (444 779 043 963±30) kHz. This work could lead to the development of a new optical frequency standard at 674 nm     

Exchange Collisions, Wall Interactions, and Resettability of the Hydrogen Maser

Experiments were performed in order to verify the resettability of the hydrogen maser. The method consisted of measuring the output frequency of one maser against the hydrogen pressure. It was found that at a given tuning of the cavity no shift larger than 2.1 parts in 1013 was observed for a change of 4 to 1 in pressure. This experiment also showed that the pressure shift due to exchange collisions, predicted by Bender, could not be observed for the field-independent transition in the hydrogen maser. Two masers, having the same storage bulb design and the same wall coating, were tuned by this technique and were found to have a frequency difference of 7.6 parts in 1013. Experiments on the wall coating of the hydrogen maser storage bulb were made. Relaxation and decorrelation times of various materials were measured. The hyperfine splitting of the ground state of hydrogen measured against cesium is also given.     

Influence of processing variables on the structure and properties of ZnO films

Zinc oxide films of high optical quality have been deposited onto both silica and silicon substrates using reactive sputtering, pulsed laser deposition, and an aqueous solution based technique. Films have been characterized with respect to crystalline phase and phase stability, surface morphology, and optical response by means of X-ray diffraction, Raman spectroscopy, atomic force microscopy, optical transmission and ellipsometry measurements. All films studied were of the wurtzite phase, fine-grained, and exhibited varying degrees of c-axis orientation with respect to the substrate normal depending upon deposition conditions. Films showed some degree of residual tensile stress which was inferred from the E₂ Raman line shift relative to the single-crystal frequency. The wurtzite phase was found to be stable to temperatures near 800 °C, but at higher temperatures, reaction with silica led to evolution of Zn₂SiO₄ at the interface. Variations in Raman line intensities upon post-deposition annealing have been correlated with oxidation of excess zinc in the lattice.     

Experimental Study of the Frequency Stability of a Maser Oscillator Operated with an External Feedback Loop

In this paper, we report the results of an experimental study of the short-and mid-term frequency stability for both a hydrogen maser and a rubidium maser operated with an external feedback loop to modify the cavity quality factor. A revised version of the theoretical expression of the frequency stability for this type of maser is given and a numerical solution for various maser parameters is calculated. The predicted frequency stability exhibits an optimum when the cavity Q is varied. The experimental results presented in this study agree with this conclusion.     

Microwave spectrum asymmetry in an optically pumped cesium beamresonator

This paper deals with a theoretical and experimental study that describes the effect of a frequency detuning of the laser used to achieve optical pumping in a small optically pumped cesium beam resonator. The ground state Zeeman sublevels with opposite angular momentum are unequally populated, leading to an asymmetrical microwave spectrum. The relative population asymmetry as a function of the laser frequency has a dispersion-like shape. Its dependence on laser intensity, applied magnetic field, and laser linewidth is demonstrated for a laser at 852 nm and tuned to the ²S_1/2F=3→²P_3/2F'=3 transition of the Cs D₂ line. Finally, the effect of a slight laser detuning on the Rabi pulling frequency shift is discussed     

Low-loss electron beam transport in a high-power, electrostatic free-electron maser

At the FOM Institute for Plasma Physics “Rijnhuizen”, The Netherlands, the commissioning of a high-power, electrostatic free-electron maser is in progress. The design target is the generation of 1 MW microwave power in the frequency range 130–260 GHz. The foreseen application of this kind of device is as a power source for electron cyclotron applications on magnetically confined plasmas.

The device is driven by a high-power electron beam. For long-pulse operation a low loss current is essential. A 3-A electron beam has been accelerated to energies ranging from 1.35 to 1.7 MeV and transported through the undulator at current losses below 0.02%. Further, it was shown that the beam line accepts an electron energy variation of 5% with fixed beam optics. This is essential for rapid tuning of the microwave frequency, over 10%.

Electron beam simulations have shown to be remarkably accurate both for the prediction of the lens settings and for the intercepted current. The operational settings of the beam line which give the highest current transmission are within a few percent of the simulated values.     

SAW velocity measurement of crystals and thin films by the phasevelocity scanning of interference fringes

We present principle and application of a novel noncontact velocity measurement of surface acoustic waves (SAW) on crystals and thin films using laser interference fringes scanned at the phase velocity of SAW. The scanning interference fringes (SIF) are produced by intersecting two laser beams with a frequency difference. The SAW velocity within the laser beam spot is measured as the ratio of observed SAW frequency and predetermined wave number of the SIF. The frequency measurement can be quite precise because of a large number of generated SAW carriers and amplitude enhancement effect. The SAW velocity measurement is free from the water loading effect accompanying the leaky SAW measurements. This principle was successfully applied to evaluate Si ₃N₄ and SiO₂ films deposited on Si (001) surface     

Frequency Stabilization of the He-Ne Maser

A bright and monochromatic radiation from an optical maser can be used as a stable standard of wavelength, when plane mirrors in the maser are automatically controlled so that the oscillation frequency can be kept very close to the center of the atomic line. The separation of mirrors is modulated at a low frequency with a small amplitude. The fundamental-, the second-harmonic, and the third-harmonic components of the modulation frequency in the light output give correction signals for the tilt of mirrors, the power level of excitation, and the separation of mirrors. The photobeat between two independently-stabilized masers of 1.15 microns filled with Ne20 and Ne22, respectively, has been observed. The observed fluctuation of beat frequencies shows a Gaussian distribution, and no systematic frequency drift has been found. It is found that the frequency of each maser stays within several parts in 1010 and the resettability is just as good. Some difficulties with stray magnetic field from magnetostriction coils and with earth magnetic field are found. Preliminary experiments on pressure shift and its effect on the stabilized maser are discussed. The theory has been developed, and the frequency deviations as functions of the gas pressure and of the amplitude of modulation have been calculated.