High spectral purity CO2 laser stabilized using amolecular frequency reference

A CO₂ laser has been frequency stabilized to a molecular absorption line with a bandwidth of 10 kHz, yielding a laser noise spectral density reduction below 1 Hz/√ Hz and a relative accuracy of 10^-10. In the time domain, this corresponds to an Allan standard deviation of σ(2, τ)=3·10^-14 τ^-1/2, for τ>100 μs. This source allows experiments of very high resolution spectroscopy as well as the realization of accurate frequency standards in the 10-μm region     

CO2/OsO4 lasers as frequency standards in the29 THz range

The CO₂ lasers, frequency stabilized on OsO₄ transitions around 29 THz, are successfully used as secondary references for visible frequency synthesis and also for high-accuracy spectroscopy in the infrared domain. The BNM-LPTF CO₂/OsO₄ frequency standards exhibit high metrological performances: a long term reproducibility of ±10 Hz (1σ=2×10^-13) and a stability σ_y=6.6×10^-14/√τ with a Flicker plateau of 4×10^-15 (Δν/ν=0.1) for 300 s to 10³ s averaging times. These results and a preliminary study of the main effects which can induce frequency shifts are reported     

Preliminary results of an optically pumped cesium beam frequencystandard at Peking University

The preliminary performance of an optically pumped cesium beam frequency standard is reported. The stability is 1.2×10^-11 /√τ, comparable to the stability of commercial conventional cesium beam frequency standards. The bias-saturated-absorption lock technique is used to lock the probing laser and the pumping laser to the cesium D₂ line. It is a simple, low cost, and reliable locking method with more than 100 h locking time     

Investigation of Environmental Noise in Small Electrical Conductors

Large excess resistance noise has been observed in small conductors surrounded by air. The conductor resistance was found to have a well-defined average power spectral density over the observed frequency range from 10^-4 to 200 Hz and the dependence of the noise was measured as a function of bias current, temperature, temperature coefficient of resistance (TCR), spatial correlation, and state of the surrounding air. In this paper, three different mechanisms were identified that produce the noise. The room-temperature fluctuations were measured and found to have a spectral density nearly proportional to f^-2 over the observed six-order-of-magnitude frequency range. The lowest frequency noise around 10^-4 Hz could be predicted from the measured temperature fluctuations using the TCR. Above 10^-3 Hz and below 1 Hz, enclosing the wire in a box greatly reduces the noise, and placing the wire in a vacuum eliminates the predominant noise. This noise was directly related to the temperature of the conductor, somewhat proportional to the TCR, independent of bias current, and has a correlation length smaller than the specimen size. The highest frequency noise does not depend on the conductor temperature, TCR, or the presence of air. It had a very strong dependence on bias current and had a long spatial correlation. The mechanism that generates this noise is not understood     

A cesium fountain frequency standard: preliminary results

An atomic frequency standard based on a fountain of cold cesium atoms has been in operation in our laboratory for a few months. Ramsey fringes as narrow as 0.7 Hz have been obtained. The short-term frequency stability, measured against an H-maser, is 3 10^-13 τ^-1/2, probably limited by the local oscillator frequency noise. A preliminary evaluation of some systematic effects shows that cold collisions will be the limiting effect for long-term stability and accuracy. Measurements of this shift presented here, using a magnetooptical trap, show that at low density an uncertainty of about 10 ¹⁵ is obtained. Under these conditions, the number of atoms is large enough to allow a potential stability of 10^-13 τ ^-1/2 to be reached. When optical molasses is used, the initial density is lower and collisional effects are reduced by a factor of three     

A multiphase direct-digital-synthesis sinewave generator forhigh-accuracy impedance comparison

In this paper we report on the realization and characterization of a multiphase sinewave generator based on the direct-digital-synthesis technique using the latest Analog Devices direct-digital-synthesizer model AD9853. The generator is intended for use in high-accuracy impedance intercomparison setups; thus, particular attention is paid to the stability and distortion of the sine wave output signal. The sinewave frequency can be tuned in the range from 16 Hz to 16 kHz with a resolution <10 μHz and with a relative frequency short-term stability of 10^-7, whereas the sinewave long-term amplitude stability is better than 10^-6 at full amplitude output. Signal amplitude, frequency, and phase are monitored and set by means of a personal computer     

Development of an ultrastable fiber optic frequency distributionsystem using an optical delay control module [for frequency standard andVLBI]

An ultrastable fiber optic frequency distribution system is described. The ultrastable phase stability is achieved by configuring a closed phase-locked loop with a single-mode optical fiber transmission line in which two optical carrier signals with different wavelengths are transmitted as a forward and a backward signal, and installing an optical delay control module which has no differential dispersion effect between the two optical carrier waves and induces no electrical noise. A phase stabilized optical fiber (PSOF) is used for the signal transmission line. The stabilities of this system are 7.5×10^-17 and 1.1×10^-17 in Allan standard deviation at 1000 and 10000 s averaging time, respectively, while the environmental temperature of the PSOF cable varies as much as the range of 10°C and the rate of 10°C/12 h     

Calculated frequency characteristic of GR1482 inductance standardsbetween 100 Hz and 100 kHz

The Physikalisch-Technische Bundesansalt (PTB) calibrates inductance standards in the frequency range from 100 Hz to 100 kHz and beyond. The relative uncertainties achieved range from less than 5×10^-5 to 5×10^-4 (2σ) depending on the frequency and the value of the standards. Because of the strong frequency dependence of the inductance standards it is of interest to use a suitable equivalent circuit to calculate the most accurate approximation for the frequency variation. A model is presented which allows the values of a GR1482 inductance standard to be calculated over the whole frequency range from 100 Hz to 100 kHz within the measurement uncertainties     

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     

A transformer-ratio bridge for the measurement of small loss angles

A development in the architecture of transformer-ratio bridges for the measurement of the loss angles of low-pass dielectrics is described. This has led to a sensitive and accurate bridge capable of measuring loss angles with a detection limit of 1×10^-8 rad (S /N=1 at a bandwidth of 1 Hz of the detector) and an accuracy of a few percent in the 10^-6 and higher ranges in the frequency range from 200 Hz to 200 kHz. The readout of the bridge is computer controlled and has an automatic capacitive balance. The most important characteristics of the bridge and their influence on the sensitivity and accuracy are discussed. Measurements as a function of frequency are presented as an example. The bridge can be used in numerous applications concerning the characterization of dielectric materials     

Observation of a Sub-10-Hz Linewidth 88Sr+ 2S1/2–2D5/2 Clock Transition at 674 nm

In this paper, a pair of diode lasers at 674 nm, locked to two independent ultralow-expansion high-finesse cavities, have been characterized by monitoring the beat frequency between them. The individual laser linewidth of 1.4 Hz for a 3-s averaging time broadened to 4 Hz at 30 s. The relative frequency stability of the beat at 1s is 2.5 10^-15. One of the lasers has been used to interrogate the ⁸⁸Sr^{+ 2}Si_1/2-²D _5/2 transition at 674 nm, and a transition linewidth of 9 Hz has been observed     

Inductive voltage divider of simple design for 120 V and 50 Hz to30 kHz

An inductive voltage divider (IVD) is presented, which-in spite of its simple design-meets the level of accuracy commonly needed in the field of ac-power measurement. The in-phase errors of about -(20±2) 10^-6 achieved at the power frequency of 60 Hz, -(130±5) 10^-6 at 4 kHz and +(70±35) 10^-6 at 32 kHz can be considered adequate, considering that the determination of the power factor implies greater uncertainties at higher frequencies. The achievement is due to the build-up of the IVD using two cores, a suitable capacitance distribution and a special technique of screening which reduces the stray inductance of the windings     

Ultrastable optical frequency reference at 1.064 μm using a C2HD molecular overtone transition

Saturated molecular overtone transitions in the visible and near infrared can be detected with record-high sensitivities using our cavity-enhanced frequency modulation technique. We have stabilized a Nd:YAG laser onto the P(5) line in (ν₂+3ν₃) overtone band of C₂HD at 1.064 μm and established its absolute frequency. Excellent signal-to-noise ratio produced an Allan variance of 3.4×10^-13 at 1 s averaging, improving to 1×10^-14 after 800 s. Selection of slow molecules with low power and gas pressure gave a linewidth 13 times below the room temperature transit time limit     

GPS在轨卫星原子钟性能评估

卫星钟在轨性能评估对于卫星钟差预报与系统完好性监测具有重要作用。利用国际GNSS服务组织发布的事后GPS精密卫星钟差数据,基于频率准确度、漂移率、万秒稳定度及天稳定度对GPS Block IIR、IIR-M、IIF与IIIA 4类卫星的星载钟性能进行评估。结果表明:1) GPS卫星钟的频率准确度与天漂移率分别在10^-13～10^-12量级与10^-15～10^-14量级;2) 星载铷钟的万秒稳定度与天稳定度分别可达10^-14与10^-15量级,比星载铯钟的同类指标高近一个数量级;3) 新型Block IIIA卫星的星载钟的天稳定度比另外3种类型卫星的星载钟的天稳定度更高,达到(3～5)×10^-15的水平;4) 无论对于不同系列卫星还是同一系列卫星,各星载钟之间均存在一定的性能差异,这种差异与卫星钟在轨运行时间长短无显著关系。     

Ultralow-noise programmable voltage source

To avoid introducing additional noise sources while making low-frequency noise measurements, batteries are normally used instead of electronic power supplies. This paper presents an alternative solution by describing the design, construction, and testing of an ultralow-noise voltage source. Such a power supply can be computer controlled and has a typical noise level two orders of magnitude below that of similar commercial instruments. Some typical values of the spectral density of the voltage fluctuations at its output are: (10^-12, 10^-15, 10^-16) V²/Hz at (0.01, 0.1, and 1) Hz, respectively. These noise performances are almost independent of the supplied current, with a degradation of less than 3 dB up to 400 mA. A special algorithm for digital-to-analog conversion, using passive devices with 1% tolerance, ensures a resolution of 2.5 mV and an accuracy better than ±1.5 mV over the entire output range from 0 to 8 V     

Preparation, Electrical, and Dielectric Characterization of Crosslinked Polyvinyl Alcohol-Phosphotungstic Acid Nanocomposites

The work presents the electrical and dielectric characterization of proton-conducting, chemically-crosslinked nanocomposites of polyvinyl alcohol (PVA) and phosphotungstic acid (PTA). The composite membranes were prepared by in situ crosslinking of the polymer matrix in solution form, containing PTA. The electrical and dielectric properties of the membranes were investigated as a function of blending composition, crosslinking density, and temperature. The conductivity of these membranes shows a temperature dependence of Arrhenius type and highest conductivity of 3.31 × 10^-3 S cm^-1 was obtained. The activation energies for proton conduction were found to be in the range of 15.28-40.62 kJ mol^-1.     

Power stabilized cryogenic sapphire oscillator

Microwave oscillators of exceptional short-term stability have been realized from cryogenic sapphire resonators with loaded Q factors in excess of 10⁹ at 11.9 GHz and 6 K. This has been achieved by a power stabilized loop oscillator with active Pound frequency stabilization. These oscillators have exhibited a fractional frequency stability of 3-4×10^-15 for integration times from 0.3 to 100 s. The relative drift of these two oscillators over one day is a few times 10^-13. To reduce the long-term drift, which is principally due to excessive room temperature sensitivity, we have added cryogenic sensors for the power and frequency stabilization servos to one of these oscillators. We have also implemented a servo to reduce the room temperature sensitivity of our phase modulators. Testing of this oscillator against a Shanghai Observatory H-maser has shown an Allan deviation of 4×10^-15 from 600 to 2000 s     

Very high Q microwave spectroscopy on trapped171Yb+ ions: application as a frequency standard

A microwave frequency standard based on buffer-gas cooled ¹⁷¹ Yb⁺ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional frequency instability characterized by an Allan deviation σ_y (τ)=3.7×10^-13τ^-1/2 for τ<3000 s. Microwave Ramsey fringes with a Q factor of 1.5×10¹³ have been observed     

强流重离子加速器BRing-HFRS超高/极高真空过渡研究

针对强流重离子加速器(HIAF)中增强器(BRing)和放射性次级束流分离器(HFRS)之间的BRing引出线真空系统进行极高真空到超高真空的过渡研究。研究表明,为不使处于极高真空状态下的BRing环真空系统受到来自HFRS和高能束运线的影响,通过烘烤段加非烘烤段的方式以及快关阀的作用,可有效地解决真空系统从极高真空到超高真空的过渡问题,并且能够有效地起到对BRing主真空的保护作用。通过对BRing引出线真空系统进行真空压力分布的模拟计算,从侧面更加验证了真空过渡方案的可行性。     

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。