Nonlinear spectroscopy with a vertical-cavity surface-emitting laser (VCSEL)

We have evaluated the suitability of a vertical-cavity surface-emitting laser diode (VCSEL) for spectroscopic applications. Despite its low output power it is possible to observe narrow resonances in a saturated absorption spectroscopy experiment on the cesium D ₂ transition at 852 nm, limited in width by the laser linewidth of several tens of MHz. High modulation efficiency of the VCSEL allows us to create modulation sidebands at 9.2 GHz frequency via direct modulation of the laser injection current. Using the carrier and either one of the sidebands coherent population trapping (CPT) resonances in a buffered cesium vapor can be prepared with linewidths below 130 Hz. With this very compact setup we have studied the dependence of CPT resonance position and linewidth as a function of optical detuning and find evidence of the influence of the excited state hyperfine structure. Received: 30 April 1999 / Revised version: 25 June 1999 / Published online: 30 November 1999     

混沌半导体激光器的弛豫振荡频率对随机序列速率的影响

利用光反馈半导体激光器产生的混沌激光作为随机数发生器的物理熵源, 分析了混沌信号自相关系数与随机序列游程数之间的联系, 在此基础上研究了激光器的弛豫振荡频率f_r与随机序列速率f_n在不同比值时, 序列游程数的变化情况. 研究发现, 当f_r/f_n=(2k+1)/4时, 随机序列的游程数容易满足NIST SP800-22随机数测试标准的要求. 当k=1时, 得到随机序列最大速率为f_n=4f_r/3.     

Absolute frequency measurement of the iodine-stabilized He-Ne laser at 633 nm

The absolute frequency of an iodine-stabilized He-Ne laser at 633 nm stabilized on the i (or a₁₃) component of the 11-5 R(127) hyperfine transition of the ¹²⁷I₂ molecule is measured using a femtosecond optical comb generator and an iodine-stabilized Nd:YAG laser standard at 1064 nm. We link the measured absolute frequency to the current internationally adopted value via frequency intercomparison between JILA and the Bureau International des Poids et Mesures (BIPM), leading to the determination of the absolute frequency of the BIPM-4 standard laser. The resulting absolute frequency f_i(BIPM) of the BIPM-4 standard laser is f_i(BIPM)=473612214711.9±2.0 kHz, which is 6.9 kHz higher than the value adopted by the Comité International des Poids et Mesures (CIPM) in 1997. Received: 29 May 2000 / Revised version: 13 September 2000 / Published online: 22 November 2000     

Efficient long-term laser excitation of a collimated beam of potassium atoms on the D2 line

With the aim to provide a reliable scheme for efficient laser excitation of the potassium D2 line over long periods of time, we have developed a robust stabilization of a single mode laser of frequency f ₀ = (f ₁₂ + f ₂₃)/2 onto a crossover peak in the saturation spectrum of the ³⁹K(D2) line (4s ²S P . The two hyperfine transitions (frequency f ₁₂) and (frequency f ₂₃) are simultaneously excited by the first order sidebands of the laser beam (f ₀), generated by its electro-optical modulation at the frequency f _EOM = (f ₁₂-f ₂₃)/2. In this way stable excitation of the two transitions on their proper frequencies is achieved and hyperfine pumping compensated.Received: 10 March 2004, Published online: 29 June 2004PACS: 39.30. + w Spectroscopic techniques - 32.80.Bx Level crossing and optical pumping - 42.60.Fc Modulation, tuning, and mode locking     

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.     

基于振幅调制的超冷铯原子高分辨光缔合光谱的实验研究

基于振幅调制的超冷铯原子高分辨光谱的实验研究，用相对于铯分子6S_1/2+6P_3/2离解限红失谐的光缔合激光作用于磁光阱中超冷铯原子，观察到通过光缔合产生的激发态超冷分子.在实验中，为了得到高信号-噪声比的光缔合光谱，利用声光调制器对俘获光进行振幅调制，将探测到的超冷铯原子的荧光信号利用lock-in技术解调.同时利用密度矩阵方程系统地分析了实验结果.     

Absolute frequency measurement and high resolution spectroscopy of In 5sS0-5s5p P0 narrowline transition

We measure the frequency of the 5s²¹S₀-5s5p ³P₀ narrowline clock transition at 236.5 nm, for a single, trapped and laser cooled ¹¹⁵In⁺ ion. In the experiment, an ultra-narrow linewidth laser (<1.34 Hz at 3 s integration time) is used to interrogate the clock transition for high resolution spectroscopy. A linewidth of 43 Hz of the clock transition is observed. The uncertainty of the line centroid is 18 Hz, leading to a fractional uncertainty of 1.4×10^-14. The frequency is measured by using an optical frequency comb referenced to a cesium clock. The transition frequency is found to be 1, 267, 402, 452, 901.265 (256) kHz, averaged over 13 days of separate measurement. The accuracy of 2.35×10^-13 is due to the reference cesium clock calibrated against UTC time. We discuss ways for further improvements.     

Pulsed CO2 laser induced multiphoton decomposition of cyclobutanone: A comparison of predictions for three models of decomposition probability vs. fluence

The multiphoton decomposition (MPD) probability (f) for photolysis of cyclobutanone (pressure: 0.333 kPa) with a pulsed (pulse width: 40 ns) CO₂ laser tuned to the 9P(20) line, was studied,in a collimated beam geometry, as a function of fluence (=0.75 to 11.7 J/cm²). The values off () were fitted to three suggested 1- or 2-parameter models. Models with two parameters correlated the experimental values equally well, whereas a 1-parameter model was less accurate for low fluences. The MPD yield for two focussed beam geometries was measured and compared to predicted yields obtained by integrating the three models forf () over the focussed beam geometries. All three models predicted yields which agreed well with the measured values.Issued as AECL Contribution Number 8943AECL National Program Summer (1983)     

铯原子调制转移光谱在激光稳频中的应用

利用铯原子D₂线的调制转移光谱的鉴频特性,实现了光栅外腔式主振荡器-功率放大型(MOPA)半导体激光器相对于铯原子6²S_1/2F=4→6²P_3/2 F’=5超精细跃迁的偏频锁定,在100 ms内激光频率的相对起伏小于±280 kHz.相对于一般的饱和吸收锁频方法而言,调制转移光谱在锁频中的应用不仅彻底消除了Doppler背景对锁频的影响,而且还避免了对激光器直接进行频率扰动而带来的附加频率噪音.     

Femtosecond laser pulse train effect on Doppler profile of cesium resonance lines

We present direct observation of the velocity-selective optical pumping of the Cs ground state hyperfine levels induced by the femtosecond (fs) laser oscillator centered at either D2 (6 ²S_1/2↦6 ²P_3/2, 852 nm) or D1 (6 P_1/2, 894 nm) cesium line. We utilized previously developed modified direct frequency comb spectroscopy (DFCS) which uses a fixed frequency comb for the excitation and a weak cw scanning probe laser centered at the ¹³³Cs 6 ²S_1/2↦6 ²P_3/2 transition (D2 line) for ground levels population monitoring. The frequency comb excitation changes the usual Doppler absorption profile into a specific periodic, comblike structure. The mechanism of the velocity selective population transfer between the Cs ground state hyperfine levels induced by fs pulse train excitation is verified in a theoretical treatment of the multilevel atomic system subjected to a pulse train resonant field interaction.     

光纤飞秒光学频率梳的研制及绝对光学频率测量

实验利用商品光纤飞秒激光器,自行构建了一套完整的光学频率梳系统,并获得了约30 dB信噪比的系统频移(f_ceo)信号.实现了光频梳重复频率(f_rep)信号及系统频移(f_ceo)信号的高稳定度锁定,并通过实验验证了光频梳锁定的跟踪精度.基于此稳定光频梳完成了对1064 nm碘稳频Nd:YAG固体激光器的绝对频率测量.实验结果表明,f_rep的跟踪精度在100 s取样时间时优于3.7×10^-14,测量得到的1064 nm激光器绝对频率为:281630111757362 Hz.这一测量结果与国际计量委员会(CIPM)给出的国际推荐值符合到不确定度之内. 关键词： 光纤光频梳 稳频 锁相技术 光学频率计量     

掺Er光纤飞秒激光器中高信噪比载波包络位相偏移频率获取的实验研究

在实验上分析了掺Er光纤光学频率梳中激光器与一级放大器之间光纤长度、 光纤扰动以及放大器抽运功率对倍频程光谱和载波包络位相偏移频率(f₀)信噪比的影响. 通过系统优化, 实现了40 dB信噪比的f₀输出, 为f₀的长期锁定和光纤光学频率梳的实现提供了技术保障.     

Selective laser pumping of magnetic sublevels in the hyperfine structure of the cesium atom

The evolution of the populations of the magnetic sublevels of the cesium atom (¹³³Cs isotope) in resonant laser fields with linear polarization is analyzed using the equations for the density matrix. Analytic expressions are derived for stationary populations resulting from laser-induced optical transitions on the hyperfine structure components F _g = 3 ? F _e = 2, 3 and F _f = 4 ? F _e = 3, 4 of lines D ₁ (6² S _1/2 → 6² P _1/2) and D ₂ (6² S _1/2 → 6² P _3/2) depending on the initial values of the populations. The numerical solution of the evolution equations gives the characteristic times of stabilization of the steady regime as functions of laser field intensities and detuning from optical resonance. We determine the sequences of optical transitions increasing (by more than an order of magnitude) the population of the lower sublevel 6² S _1/2 F _g = 3 M = 0 of the “clock” microwave transition F _g = 3 M = 0 ? F _f = 4 M = 0 in the cesium frequency standard, which increases the signal intensity in the recording system by the same proportion.     

Very low frequency optical source for spectroscopy: Difference-frequency mixing of CO2 laser radiations

In order to develop a coherent optical source intended for very low frequency laser spectroscopy, we have studied, manufactured and then tested (calibrated) a device for generating submillimetric waves by frequency difference in a non linear crystal.The mounting is made of two single mode and single line T E A CO₂ lasers which, by means of an AsGa crystal, allows the difference f₃=f₁–f₂ of the two near frequencies f₁ and f₂ emitted by each laser to be obtained.We present a system using two CO₂ atmospheric lasers with a simple and efficient pre-ionization and an original treatment of the electrodes.Manufacturing characteristics of the primary sources are detailed and then the obtained Far Infra Red (F I R) emission is studied.     

Optical frequency measurement of the 1S–3S two-photon transition in hydrogen

This article reports the first optical frequency measurement of the 1S–3S transition in hydrogen. The excitation of this transition occurs at a wavelength of 205 nm which is obtained with two frequency doubling stages of a titanium sapphire laser at 820 nm. Its frequency is measured with an optical frequency comb. The second-order Doppler effect is evaluated from the observation of the motional Stark effect due to a transverse magnetic field perpendicular to the atomic beam. The measured value of the 1S_1/21\mathrm{S}_{1/2}(F = 1)-3S_1/2(F = 1) frequency splitting is 2 922 742 936.729(13) MHz with a relative uncertainty of 4.5 × 10^-12. After the measurement of the 1S–2S frequency, this result is the most precise of the optical frequencies in hydrogen.     

High-accuracy frequency atlas of 13C2H2 in the 1.5 μm region

A pair of 1.5 μm semiconductor laser frequency standards have been developed for optical telecommunications use, stabilised to Doppler-free transitions of the ν₁ + ν₃ and ν₁ + ν₂ + ν₄ + ν₅ combination bands of ¹³C₂H₂. The Allan deviation σ/f for a laser locked to line P(10) of the former band follows a slope of 1.6 × 10⁻¹²τ^−1/2, reaching a minimum of 5.7 × 10⁻¹⁴ at τ = 4000 s. The absolute frequencies of 61 lines of the ν₁ + ν₃ band and 43 lines of the ν₁ + ν₂ + ν₄ + ν₅ band, covering the spectral region 1520 nm to 1552 nm, have been measured by use of a combined frequency chain and femtosecond comb, together with a passive optical frequency comb generator. The mean uncertainties for the line frequencies within each band are 1.4 kHz for the ν₁ + ν₃ band and 1.9 kHz for the ν₁ + ν₂ + ν₄ + ν₅ band, representing improvements on the precision of previously published data by factors of 100 and 10⁴, respectively. Improved values of the rotational constant B″ and centrifugal distortion coefficients D″, H″ and L″ of the vibrational ground state are presented.This article is published with the permission of the Controller of HMSO and the Queen’s Printer of Scotland     

Double-resonance optical-pumping effect and ladder-type electromagnetically induced transparency signal without Doppler background in cesium atomic vapour cell

In a Doppler-broadened ladder-type cesium atomic system (6S_1/2-6P_3/2-8S_1/2), this paper characterizes electromagnetically induced transparency (EIT) in two different experimental arrangements, and investigates the influence of the double-resonance optical-pumping (DROP) effect on EIT in both arrangements. When the probe laser is weak, DROP is explicitly suppressed. When the probe laser is moderate, population of the intermediate level (6P_3/2 F'=5) is remarkable, therefore DROP is mixed with EIT. An interesting bimodal spectrum with the broad component due to DROP and the narrow part due to EIT has been clearly observed in cesium 6S_1/2 F=4-6P_3/2 F'=5-8S_1/2 F"=4 transitions.     

Temporal coherent control induced by wave packet interferences in one and two photon atomic transitions

The interaction of a sequence of two identical ultrashort laser pulses with an atomic system results in quantum interferences as in Ramsey fringes experiments. These interferences allow achievement of temporal coherent control of the excitation probability. We present the results of a temporal coherent control experiment on two different atomic systems: one-photon absorption in K (4s-4p) and two-photon absorption in Cs (6s-7d). In K, the quantum interferences between the two excitation paths associated with the laser pulses are revealed through rapid oscillations of the excitation probability as a function of the time delay between the two pulses. These oscillations take place at the transition frequency (period T = 2.56 fs). The interferences are modulated by beats (at about 580 fs) resulting from the doublet structure of the excited state (4p (² P _1/2 , ² P _3/2 )). Three complementary interpretations of this experiment are presented: in terms of beats of quantum interferences, of variation in the spectrum intensity, and of wave packet interferences. Whenever the two laser pulses are temporally overlapped, optical interferences are superimposed on to the quantum interferences. The distinction between these two types of interference is clearly revealed in the two-photon excitation scheme performed on Cs (6s-7d (² D _3/2 , ² D _5/2 )) because quantum interferences occur at twice the frequency of the optical interferences. Received: 30 December 1997 / Revised: 28 February 1998 / Accepted: 4 March 1998     

Optical actuation of a macroscopic mechanical oscillator

An intensity-modulated HeNe-laser beam was utilized to optically actuate the mechanical resonance of a macroscopic torsional silicon oscillator (f₀ = 67 700 Hz, Q = 42 100 at p = 1 mbar and T = 300 K). Both radiation pressure and photothermal effects may cause optical actuation of a mechanical device. Both excitation effects were studied. In actuation through radiation pressure, the actuating laser beam was focused on the high-reflectivity-coated oscillator surface. In the case where the intensity-modulated laser beam was incident on the uncoated silicon surface the photothermal effect was shown to be the dominating excitation factor. Oscillation amplitudes due to the actuation through radiation pressure and photothermal effects were Δ x_rad = 1.4 pm and Δ x_ph = 4.3 pm with the same optical power of 1.5 mW. The measured resonance frequency and quality value were not changed when purely mechanical and radiation pressure actuation mechanisms were compared. With photothermal actuation the absorbed optical power heats the oscillator, introducing a slight decrease in the resonance frequency. Our experiments demonstrate that optical actuation combined with sensitive optical interferometric measurements can be utilized to perform dynamic vibration analysis of micromechanical components. Prospects of using micromechanical devices for observing extremely weak external forces are discussed.     

Quality assessments of electrochromic devices: the possible use of 1/<Emphasis Type="Italic">f</Emphasis> current noise

Electrochromic (EC) devices, capable of modulating their optical transmittance by charge insertion/extraction, were produced by laminating films comprised of nanoporous W oxide and Ni–V oxide by a polymer electrolyte and having this three-layer stack between transparent conducting In₂O₃:Sn films backed by polyester foils. 1/f noise in the current (I) was observed when the charged (colored) EC device was discharged via a resistor. The power spectral density S _i at fixed frequency scaled as S _i  ∼ I ². Extended color/bleach cycling degraded the optical quality and homogeneity of the device and concomitantly increased the 1/f noise intensity. These initial data indicate that 1/f noise has a potential to serve as a quality measure for EC devices.