Investigation of the thermal adaptability for a mobile cold atom gravimeter

The cold atom gravimeter offers the prospect of a new generation of inertial sensors for field applications. We accomplish a mobile atom gravimeter. With the compact and stable system, a sensitivity of 1.4×10~(-7)g·Hz~(-1/2)is achieved.Moreover, a continuous gravity monitoring of 80 h is carried out. However, the harsh outdoor environment is a big challenge for the atom gravimeter when it is for field applications. In this paper, we present the preliminary investigation of the thermal adaptability for our mobile cold atom gravimeter. Here, we focus on the influence of the air temperature on the performance of the atom gravimeter. The responses to different factors(such as laser power, fiber coupling efficiency,etc.) are evaluated when there is a great temperature shift of 10℃. The result is that the performances of all the factors deteriorate to different extent, nevertheless, they can easily recover as the temperature comes back. Finally, we conclude that the variation of air temperature induces the increase of noise and the system error of the atom gravimeter as well, while the process is reversible with the recovery of the temperature.     

Limits to the sensitivity of a low noise compact atomic gravimeter

A detailed analysis of the most relevant sources of phase noise in an atomic interferometer is carried out, both theoretically and experimentally. Even a short interrogation time of 100 ms allows our cold atom gravimeter to reach an excellent short term sensitivity to acceleration of 1.4×10^-8g at 1 s. This result relies on the combination of a low phase noise laser system, efficient detection scheme and good shielding from vibrations. In particular, we describe a simple and robust technique of vibration compensation, which is based on correcting the interferometer signal by using the ac acceleration signal measured by a low noise seismometer. PACS  32.80.Pj; 42.50.Vk; 39.20.+q     

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.     

Relaxation effects and high sensitivity photoacoustic detection of NO<Subscript>2</Subscript> with a blue laser diode

A detection limit of 200 ppt of NO₂ in N₂ at atmospheric pressure was obtained with a photoacoustic detector and a high power blue laser diode. This corresponds to a normalized noise (1σ) equivalent absorption coefficient of 2×10^-9 cm^-1W/Hz^0.5. Measurements at different laser modulation frequencies showed no frequency dependence of the photoacoustic signal, indicating a relaxation time τ < 4 μs. Mixing O₂ into the NO₂ containing gas results in a decrease of the photoacoustic signal. A simple model shows that this effect can be attributed to an increased vibrational-vibrational relaxation of NO₂ to O₂. PACS 31.70.Hq; 34.50.Ez; 42.55.Px     

Frequency comb-referenced narrow line width diode laser system for coherent molecular spectroscopy

We analyze in detail the frequency noise properties of a grating enhanced external cavity diode laser (GEECDL). This system merges two diode laser concepts, the grating stabilized diode laser and the diode laser with resonant optical feedback, thus combining a large tuning range with an excellent short-term frequency stability. We compare the frequency noise spectrum of a GEECDL to that of a grating stabilized diode laser and demonstrate a 10-fold reduction of the frequency noise linear spectral density. The GEECDL is phase locked to a similar laser and to a fs-frequency comb with a servo loop providing an open-loop unity-gain frequency of only 237 kHz, which is a tenth of the bandwidth typically required for grating stabilized diode lasers. We achieve a residual rms phase error as small as 72 mrad (≈ 200 mrad) for stabilization to a similar laser (to the fs-frequency comb). We demonstrate that the novel diode laser can phase-coherently track a stable optical reference with an instability of 1.8×10^-16 at 1 s. This laser system is well suited for applications that require phase locking to a low-power optical reference under noisy conditions. It may also be considered for the implementation of optical clock lasers. PACS 42.55.Px; 42.60.Jf; 42.50.Gy     

Low-energy ion emission from a xenon gas-puff laser-plasma X-ray source

We have measured low-energy ion emission from a gas-puff laser-plasma X-ray source. The ions may cause the degradation of the condenser mirror of the extreme ultra-violet projection lithography system. A 0.7 J in 8 ns Nd:YAG laser at 1.06 μm was focused onto the xenon gas-puff target with an intensity of ∼10¹² W/cm². The silicon (111) plates, placed at a distance of 32 mm from the laser-interaction region, were exposed with the xenon ions. The average ion energy was measured to be less than 50 eV with a Faraday-cup detector placed close to the silicon plates. The xenon deposition occurred in the silicon plates with a depth of less than 40 nm. The deposition density was measured with a quadrupole secondary ion mass spectrometer to be 10²¹ /cm³ after 1500 laser shots. The energy-conversion efficiency from the laser energy into the ions is ∼0.1%/4 π sr/shot. For the lithography system, if we can remove such ion bombardment completely using novel techniques such as electro-magnetic devices or gas flow curtain techniques, the lifetime of the condenser mirror will be extended significantly. Received: 20 November 2000 / Published online: 9 February 2001     

Experimental progress in gravity measurement with an atom interferometer

Precisely determining gravity acceleration g plays an important role on both geophysics and metrology. For gravity measurements and high-precision gravitation experiments, a cold atom gravimeter with the aimed resolution of 10.⁻⁹g/Hz^1/2 (1 g=9.8 m/s²) is being built in our cave laboratory. There will be four steps for our ⁸⁷Rb atom gravimeter, Magneto-Optical Trap (MOT) for cooling and trapping atoms, initial state preparation, π/2-π-π/2 Raman laser pulse interactions with cold atoms, and the final state detection for phase measurement. About 108 atoms have been trapped by our MOT and further cooled by moving molasses, and an atomic fountain has also been observed.      

新型高精度绝对重力仪

精密测量地球表面的重力加速度(g, 常用值9.81 m/s²) 是探测地球重力场的重要途径, 已广泛应用于计量、测绘、地质、地震与资源勘探等领域. 随着我国"2000国家重力基本网"和"中国地壳运动观测网络"的建成, 对高精度绝对重力测量的需求日益增加. 为深入研究现有绝对重力测量技术可能存在的系统误差, 并满足国内多个领域对高精度绝对重力仪的迫切需求, 自主研制T-1型可搬运式高精度绝对重力仪样机, 采用经典的真空自由落体方案, 通过激光干涉测量和数据拟合方法获得重力加速度值. T-1型绝对重力仪主要包括以下几部分: 高真空度自由落体装置、小型化激光干涉测量装置、超低频垂直隔振系统、高速信号采集系统、仪器控制与数据处理系统. 绝对重力测量的长度基准为稳频He-Ne激光器, 时间基准为铷原子钟, 这两项现有基准的测量不确定度都优于1× 10^-9. 测试结果表明, T-1型绝对重力仪在12 h内重力测值的标准差可优于1 μupGal (1 μupGal = 10^-8 m/s²), 测量结果的复现性优于3 μupGal, 可实现微伽量级不确定度的精密重力测量, 有望在我国多个关键领域发挥重要应用.     

Frequency-stabilized Nd:YVO4 thin-disk laser

We have developed a Nd:YVO₄ thin-disk laser at 914 nm with single-frequency operation and active frequency stabilization to a low-finesse reference cavity. The spectral density of laser frequency noise is analysed by means of noise measurements at the error point of the frequency control loop. To address the 3¹S₀→3³P₁ magnesium intercombination line at 457 nm, we use an external frequency doubling stage based on periodically poled KTiOPO₄ for the generation of more than 150-mW output power at 457 nm. Optical beat signal measurements at 457 nm with a frequency-stable dye laser show a short-time line width of the thin-disk laser of less than 100 kHz. PACS 42.55.Xi; 42.60.Lh; 42.62.Fi; 42.65.Ky     

Gain clamping in double-pass L-band EDFA using a broadband FBG

A highly efficient gain-clamped L-band EDFA with improved noise figure characteristic is demonstrated by simply adding a broadband C-band FBG in double-pass system. The combination of the FBG and optical circulator has created laser in the cavity for gain clamping. By adjusting the power combination of pumps 1 and 2, the clamped gain level can be controlled. The amplifier gain is clamped at 28.1 dB from −40 to −25 dBm with a gain variation of less than 0.5 dB by setting the pumps 1 and 2 at 59.5 and 50.6 mW, respectively. The gain is also flat from 1574 nm to 1604 nm with a gain variation of less than 3 dB. The corresponding noise figure varies from 5.6 to 7.6 dB, which is 0.8 to 2.6 dB less than those of unclamped amplifier.     

Design and performance of a high-resolution dual-channel heterodyne laser velocimeter

We describe a dual-channel laser velocimeter based on a single laser source and a single set of carrier wave generation optics. The apparatus is intended for simultaneous vibration measurements on several points of instable objects, such as biological specimens or micro electronic mechanical systems, so that instantaneous phase relationships and amplitude ratios can be determined. Our instrument presently allows measurements on two points of interest which can be arbitrarily chosen. The optical design allows expansion to at least four independent channels. At a maximal velocity amplitude of 52 mm s⁻¹, the velocity resolution and the detection limit equal 2.6 μm s⁻¹ Hz^−1/2. Even with object points less than 0.4 mm apart, channel cross-talk is less than −78 dB at all frequencies.     

Compact and robust laser system for onboard atom interferometry

We propose a compact and robust laser system at 780 nm for onboard atomic inertial sensors based on rubidium atom interferometry. The principle of this system consists in doubling the frequency of a telecom fiber bench at 1560 nm. The same laser source is used to achieve a magneto-optical trap, matter–wave interferences, and the atomic detection. An atomic gravimeter has been realized and the laser system has been validated under hyper- and microgravity.     

Noise performance of a feed-forward scheme for carrier-envelope phase stabilization

The noise performance of a feed-forward scheme for carrier-envelope phase stabilization is discussed. This scheme uses an acousto-optic frequency shifter to directly correct for fluctuations of the carrier-envelope phase in a pulse train emitted by a mode-locked laser without manipulating the intracavity dispersion. Generation of zero-offset frequency combs is demonstrated. Furthermore, it is shown that pump laser noise has only a minor effect on the achievable performance. Limited only by the travel time of the acoustic wave in the shifter, pump laser noise can be corrected up to near-megahertz frequencies, which yields superior noise performance compared to traditional feedback operation. Residual phase jitters down to 45 mrad are experimentally verified.     

A Raman laser system for multi-wavelength satellite laser ranging

~~A Raman laser system for multi-wavelength satellite laser ranging@K.Hamal$Czech Technical University in Prague,Czech @I.Prochazka$Czech Technical University in Prague,Czech @J.Blazej$Czech Technical University in Prague,Czech1. Yang Fumin, Chen Wanzhen, Zhang Zhongping et al., Satellite laser ranging experiment with sub-centimeter single-shot ranging precision at Shanghai Observatory, Science in China, Ser. A, 2002, 32(10): 935-939. 2. Degnan, J. J., Millimeter accurac…     

Stable diode lasers for hydrogen precision spectroscopy

We report on an external cavity diode laser at 972 nmstabilized to a mid-plane mounted Fabry-Perot (FP) resonator with afinesse of 400000. The 0.5 Hz optical beat note line width betweentwo similar lasers (Allan deviation 2 × 10^-15) is limitedby thermal noise properties of two independent FP resonators. Thelong term drift of the FP resonator and mirror substrates made fromUltra-Low-Expansion glass (ULE) is small and can be well predictedon time intervals up to many hours if the resonator is stabilized atthe zero thermal expansion temperature T_c. Using a Peltierelement in a vacuum chamber for temperature stabilization allowsstabilization of the FP cavity to T_c which is usually below theroom temperature. Beat note measurements with a femtosecond opticalfrequency comb referenced to a H-maser during 15 hours have shown awell defined linear drift of the FP resonance frequency of about 60 mHz/s with residual frequency excursions of less than ±20 Hz.     

Limitations to testing the equivalence principle with satellite laser ranging

We consider the possibility of testing the equivalence principle (EP) in the gravitational field of the Earth from the orbits of LAGEOS and LAGEOS II satellites, which are very accurately tracked from ground by laser ranging. The orbital elements that are affected by an EP violation and can be used to measure the corresponding dimensionless parameter η are semimajor axis and argument of pericenter. We show that the best result is obtained from the semimajor axis, and it is limited—with all available ranging data to LAGEOS and LAGEOS II—to η ≃ 2 × 10⁻⁹, more than 3 orders of magnitude worse than experimental results provided by torsion balances. The experiment is limited because of the non uniformity of the gravitational field of the Earth and the error in the measurement of semimajor axis, precisely in the same way as they limit the measurement of the product GM of the Earth. A better use of the pericenter of LAGEOS II can be made if the data are analyzed searching for a new Yukawa-like interaction with a distance scale of one Earth radius. It is found that the pericenter of LAGEOS II is 3 orders of magnitude more sensitive to a composition dependent new interaction with this particular scale than it is to a composition dependent effect expressed by the η parameter only. Nevertheless, the result is still a factor 500 worse than EP tests with torsion balances in the gravitational field of the Earth (i.e. at comparable distance), though a detailed data analysis has yet to be performed. While EP tests with satellite laser ranging are not competitive, laser ranging to the Moon has been able to provide a test of the EP almost 1 order of magnitude better than torsion balances. We show that this is due to the much greater distance of the test masses (the Earth and the Moon) from the primary body (the Sun) and the correspondingly smaller gradients of its gravity field. We therefore consider a similar new experiment involving the orbit of LAGEOS: testing LAGEOS and the Earth for an EP violation in the gravitational field of the Sun. We show that this test may be of interest, though it is a factor 300 less sensitive than in the case of the Moon due to the fact that LAGEOS is closer to the Earth than the Moon and consequently its orbit is less affected by the Sun. The limitations we have pointed out for laser ranging can be overcome by flying in low Earth orbit a spacecraft carrying concentric test masses of different composition with the capability, already demonstrated in ground laboratories, to accurately sense in situ any differential effects between them. An erratum to this article can be found at     

Resonance effects in the nonadiabatic nonlinear quantum dimer

In the presence of 1/f ^β noise, we investigate the logical stochastic resonance (LSR) in an asymmetric bistable model driven by various cycling combinations of two logic inputs. The probability of correct logic outputs is calculated according to true table of logic relationships. Two major results are presented. Firstly, it is shown that the LSR effect can be obtained by changing noise strength. Over entire range of noise variance, white noise can be considered to be better than 1/f noise or 1/f ² noise to obtain clean logic operation. At a smaller noise level, 1/f noise can realize higher output probability than white noise or 1/f ² noise. In the sense, 1/f noise can be considered to be better than white noise or 1/f ². On the other hand, the correct probability can evolves nonmonotonically as noise exponent β increases, and a kind of SR-like effect can be obtained as a result of β. At certain intermediate noise variance, the output probability is able to attain its minimum at β = 1. It is also shown that actually some finite β sometime can be better than β = 0 at small range of noise variance. The study might provide some potential complement to LSR effect in the presence of 1/f ^β noise.     

An adaptive singular value decomposition (SVD) algorithm for analysis of wavelength modulation spectra

Optical interference fringes due to unwanted etalons are often the limiting uncertainty in diode laser spectroscopic trace gas measurements. Temporal variations in the fringe spacings, phases, and amplitudes introduce systematic baseline changes that limit useful signal averaging times to ∼1000 s, and constrain minimum detectable absorbances to between one and three orders of magnitude worse than the fundamental limiting noise sources (shot noise and/or detector thermal noise). We describe an adaptive numerical filtering method based on singular value decomposition (SVD) that shows, for the system studied, a fivefold reduction in baseline drift due to unwanted etalons over a one-week-measurement period. The adaptive algorithm is fast (<1 ms per computation), robust, and uses linear methods. It is computationally equivalent to principal component analysis (PCA). The test system was acetylene detected using a near-infrared telecommunications laser operating at 6541.96 cm^-1. The gas detection limit was 20 ppb (1σ) over the one-week measurement. PACS 07.05.Kf; 07.07.Df; 07.60.-j; 42.60.-v; 42.62.-b; 42.62.Fi     

Optical response of a cold-electron bolometer array

A multielement bolometric receiver system has been developed to measure the power and polarization of radiation at a calculated frequency of 345 GHz. Arrays of ten series-parallel connected cold-electron bolometers have been pairwise integrated into orthogonal ports of a cross-slot antenna. Arrays are connected in parallel in the high-frequency input signal and in series in the output signal, which is measured at a low frequency, and in a dc bias. Such an array makes it possible to increase the output resistance by two orders of magnitude as compared to an individual bolometer under the same conditions of high-frequency matching and to optimize the matching with the JFET amplifier impedance up to dozens of megohms. Parallel connection ensures matching of the input signal to the cross-slot antenna with an impedance of 30 Ω on a massive silicon dielectric lens. At a temperature of 100 mK, a response to the thermal radiation of a thermal radiation source with an emissivity of 0.3, which covers the input aperture of the antenna and is heated to 3 K, is 25 μV/K. Taking into account real noise, the optical fluctuation dc sensitivity is 5 mK, the estimated sensitivity corresponding to the noise of the amplifier is about 10⁻⁴ K/Hz^1/2, and the noise-equivalent power is about (1–5) × 10⁻¹⁷ W/Hz^1/2.     

K<Subscript><Emphasis Type="Italic">α</Emphasis></Subscript> x-ray emission characterization of 100 Hz, 15 mJ femtosecond laser system with high contrast ratio

We report K _α x-ray production with a high energy (110 mJ per pulse at 800 nm before compression/15 mJ at 400 nm after compression), high repetition rate (100 Hz), and high pulse contrast (better than 10⁻⁹ at 400 nm) laser system. To develop laser-based x-ray sources for biomedical imaging requires to use high-energy and high-power ultrafast laser system where compression is achieved under vacuum. Using this type of laser system, we demonstrate long-term stability of the x-ray yield, conversion efficiency higher than 1.5×10⁻⁵ with a Mo target, and the x-ray spot size close to the optical focal spot. This high-repetition K _α x-ray source can be very useful for x-ray phase-contrast imaging. S. Fourmaux and C. Serbanescu contributed equally to this work.