Application of the light-shift effect to laser frequency stabilization with reference to a microwave frequency standard

A new method allowing laser frequency stabilization with reference to a microwave oscillator, independently of the laser intensity, is described. The method makes use of the dependence of the ground-state hfs transition frequency on the optical radiation frequency in alkali atoms irradiated by quasi-resonant light. Preliminary experimental investigations are reported in the case of a cw GaAs diode laser tuned to the D₂ absorption line in a cesium gas cell. The absolute laser frequency exhibited variations of 1.4 MHz r.m.s. around an average value determined to within 2 parts in 10¹⁰ for a period of 5 minutes. The possibility of defining a cesium beam, reference wavelength connected with the time standard is discussed.     

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.     

A Simple Frequency Lock of Green YAG Laser to Dispersion Line of the Linear Absorption Spectrum of Iodine Molecules Utilizing an Acousto-Optic Frequency Shifter

Frequency of a 532 nm emission line of a diode-laser-pumped Nd:YAG laser is stabilized by lock of the output line to the linear absorption line of iodine molecules. The stabilization method is very simple utilizing the frequency shift caused with an acousto-optic modulator. The laser frequency was stabilized at the zero-crossing point of the second-derivative of the dispersion curve. Instability obtained by the error signal is affected by the phase-characteristics of detectors, which results in 3^*10^-8.     

A laser system for the spectroscopy of highly charged bismuth ions

We present and characterize a laser system for the spectroscopy on highly charged ²⁰⁹Bi⁸²⁺ ions at a wavelength of 243.87?nm. For absolute frequency stabilization, the laser system is locked to a near-infra-red laser stabilized to a rubidium transition line using a transfer cavity based locking scheme. Tuning of the output frequency with high precision is achieved via a tunable rf offset lock. A?sample-and-hold technique gives an extended tuning range of several THz in the UV. This scheme is universally applicable to the stabilization of laser systems at wavelengths not directly accessible to atomic or molecular resonances. We determine the frequency accuracy of the laser system using Doppler-free absorption spectroscopy of Te₂ vapor at 488?nm. Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty of σ _{244 nm}=6.14?MHz (one standard deviation) over six days of operation.     

一种内腔式He-Ne激光器频率稳定方法的实验研究

实现了用微型风扇冷却的方法替代常见的加热方式完成对He-Ne内腔激光器的频率稳定.研究了微型风扇驱动电压与转速的响应特性和内腔式He-Ne激光器的热膨胀特性.采用风冷方式对激光器的腔长进行调节和控制,并通过双纵模功率平衡原理完成了激光频率的稳定.稳定后的激光器管壁平均温度低于50℃.与高精度碘稳定激光的拍频实验结果表明,其频率在20h内的波动范围小于1.4 MHz (τ=1 s),4个月内激光频率的相对标准不确定度为U=4.7×10-9.     

Frequency stabilisation of the He-Ne laser with an external Ne-absorption tube in an alternating magnetic field

A single mode He-Ne laser has been frequency stabilised with a relative frequency stability of 10^–10 and an absolute accuracy of 10^–9. The reference frequency is derived from a Ne-absorption cell in an alternating magnetic field. Theoretical analysis and experiments show that this high frequency stability can be achieved only with certain time-dependent functions of the alternating field and with the proper choice of the operating parameters of the laser and absorption tube.     

Frequency stabilization of an internal mirror HeNe laser at 543.5 nm to I2-Saturated absorptions

An internal mirror green HeNe (GRENE) laser at 543.5 nm has been frequency stabilized to I₂-Doppler free absorption signals with a frequency stability of few parts in 10¹⁰. The saturated absorption signals were detected using doubly-differential saturated absorption which eliminates the Doppler background and allows first-derivative locking methods. Two absorption lines R(12)26-0 and R(106)28-0 of ¹²⁷I² were investigated and the hyperfine structure was resolved. Random polarization flips of the GRENE were suppressed by applying a transverse magnetic field to the laser tube near the anode.Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-3300 Braunschweig, Fed. Rep. Germany     

Stabilization of a CO2 pump laser using external CH3OH stark-cell modulation

In order to stabilize the 118.8 μm CH₃OH laser output, frequency stabilization of the CW 9P(36) CO₂ laser pump is carried out using external CH₃OH Stark-cell modulation without internal frequency modulation. The CO₂ laser output power around the CH₃OH absorption center is stabilized to within ±1.0% for 60 min. The frequency stability of the CO₂ laser is estimated to be within ±1.4 MHz for 60 min, assuming the output fluctuation to be caused by this frequency fluctuation. As a result of the CO₂ pump laser stabilization, an output power stability of about ± 1.0% for 30 min is obtained for the 118.8 μm CH₃OH laser.     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.     

基于二次谐波特性的DFB激光器稳频新方法研究

窄线宽稳频激光器在工业生产控制中具有广泛的应用,但自由运转的半导体激光器的频率漂移限制了激光器的使用。为稳定半导体激光器的频率,提出了一种基于二次谐波吸收特性来实现窄线宽二极管激光器的稳频新方法,利用1.396 μm的DFB二极管激光器测量水汽的二次谐波信号来实现激光的稳频,实验结果表明在100 h内激光器输出波长漂移有效的抑制在±0.16 pm范围内,激光稳频后,其吸收峰的位置不随环境温度的变化而漂移。该方法具有简单、可靠等优点,对二极管激光频率的稳定具有广阔的应用前景。     

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.     

Doppler-free frequency-modulation spectroscopy of atomic erbium in a hollow-cathode discharge cell

The erbium atomic system is a promising candidate for an atomic Bose–Einstein condensate of atoms with a non-vanishing orbital angular momentum (L≠0) of the electronic ground state. In this paper we report on the frequency stabilization of a blue external cavity diode laser system on the 400.91 nm laser cooling transition of atomic erbium. Doppler-free saturation spectroscopy is applied within a hollow-cathode discharge tube to the corresponding electronic transition of several of the erbium isotopes. Using the technique of frequency-modulation spectroscopy, a zero-crossing error signal is produced to lock the diode laser frequency on the atomic erbium resonance. The latter is taken as a reference laser to which a second main laser system, used for laser cooling of atomic erbium, is frequency stabilized.     

Frequency-stabilized seed laser system for dial applications around 0.94 μm

Absolute frequency stabilization of an extended-cavity diode laser at 0.94 μm is reported. The diode laser was frequency locked against rovibrational absorption lines of water vapour by using the frequency modulation spectroscopy technique. The stabilized oscillator shows a short-term frequency stability level of 40 kHz for integration times of 1 s and a long-term frequency drift lower than 10 MHz for observation times longer than 10³ s. The frequency-stabilized oscillator system is mounted on a compact breadboard (75 cm×50 cm) and constitutes the seed laser system for the injection of a high-energy DIAL laser transmitter operating in the 0.94-μm spectral region.     

Frequency stabilization and measurements of 543 nm HeNe lasers

In this paper we report our investigations on the frequency stabilization and frequency measurements of 543 nm HeNe laser. It contains following four different works. (1) Using a metal laser tube we have built an iodine-stabilized 543 nm HeNe laser by the Frequency-Modulation (FM) spectroscopy. The signal-to-noise ratio of the hyperfine spectrum reached 2 × 10^–12 at 1 s sampling time. (2) We have built a compact iodine-stabilized 543 nm HeNe laser system using the third-harmonic locking technique. Stability better than 1 × 10^–12 for sampling time >1 s is obtained. We also suggest the b₁₀ line for the future recommendation. (3) We constructed the Lamb-dip stabilized He-²⁰Ne and He-²²Ne lasers and measured their frequency stability, reproducibility, and absolute frequencies. The results suggest that the Lamb-dip stabilized lasers are appropriate for secondary wavelength standards. We have also deduced the isotope shift of Ne atom at 543 nm. (4) We have developed two two-mode stabilized 543 nm HeNe lasers using the bang-bang control method. The Allan variance is 1 × 10^–11 at 1 s sampling time.     

Compact all-solid-state continuous-wave single-frequency UV source with frequency stabilization for laser cooling of Be<Superscript>+</Superscript> ions

A compact setup for generation, frequency stabilization, and precision tuning of UV laser radiation at 313 nm was developed. The source is based on frequency quintupling of a C-band telecom laser at 1565 nm, amplified in a fiber amplifier. The maximum output power of the source at 313 nm is 100 mW. An additional feature of the source is the high-power output at the fundamental and the intermediate second- and third-harmonic wavelengths. The source was tested by demonstration of laser cooling of Be⁺ ions in an ion-trap apparatus. The output of the source at the third-harmonic wavelength (522 nm) was used for stabilization of the laser frequency to molecular iodine transitions. Sub-Doppler spectroscopy and frequency measurements of hyperfine transitions in molecular iodine were carried out in the range relevant for the Be⁺ laser cooling application.     

Quantum cascade laser-based carbon monoxide detection on a second time scale from human breath

We present three different detection schemes for measuring carbon monoxide (CO) in direct absorption using a thermoelectrically cooled, distributed-feedback pulsed quantum cascade (qc) laser operating between 2176 and 2183 cm^-1. The laser emission has overlap with the strong R(8)1 ro-vibrational transition in CO at 2176.2835 cm^-1. Firstly, by utilizing the frequency chirp of the qc-laser with long laser pulses, a minimal detectable absorption of 1.2×10^-5 cm^-1 is achieved at an acquisition rate of 3 Hz. Additionally, with short laser pulses and slow frequency scanning a minimal detectable absorption 8.2×10^-7 cm^-1 is reported, with an acquisition time of 60 s. Finally, a novel amplitude modulation technique is developed to facilitate real-time measurement of CO in exhaled air. The application of this detector to detection of CO in a single breath as a potential non-invasive diagnostic tool is shown. PACS 07.07.Df; 42.62.Be     

基于匹配算法的脉冲差分吸收CO2激光雷达的稳频研究

利用差分吸收激光雷达探测大气CO₂, 可以获得其浓度的垂直分布, 对于研究碳源、碳汇的过程有重要意义. 设计了一套种子注入的脉冲差频激光器系统, 作为差分吸收激光雷达的激光光源. 针对脉冲差分吸收CO₂激光雷达on波长的高精度稳频的研究空白, 本文提出一种基于匹配的on波长的连续稳频算法. 其基本思想是采用分子饱和吸收法, 测量通过双路吸收池后的差分信号, 计算其光学厚度值(optical depth, OD), 获得实测的伪吸收谱, 当监测到on波长发漂移后, 进行连续的波长调节, 获取其OD值, 最后基于一维的图像匹配算法, 将OD值作为灰度值, 利用图像匹配原理, 进行OD值匹配, 确定当前输出波长在伪吸收谱中的位置, 进而调节至on波长, 实现on波长的连续、稳定输出. 实验结果表明, 提出的稳频算法能够很好的满足高精度的稳频要求, 同时差平方和法在该应用中是最优的, 稳频精度可达到0.3 pm.     

Wavelength stabilization of an Ar+ laser with an external129I2 absorption cell

Wavelength stabilization of the green Ar⁺ laser line at 514.5 nm by¹²⁹I₂ absorption was found to be more favourable than by¹²⁷I₂ absorption because the maximum of¹²⁹I₂ absorption is closer to the center of the 514.5 nm gain curve. A simple method for stabilizing a short air cooled laser with only one servo loop is given, yielding a stability of 10⁻⁸ λ.     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

High resolution emission spectra of laser-excited molecular iodine as the excitation frequency is tuned through and away from resonance

We have studied the ΔJ = 2 rotationally shifted emission lines in the region of the strong absorption of molecular iodine which occurs within the 5145 Å argon ion laser line. We used an etalon tuned, single frequency argon ion laser with a linewidth of 20 MHz to excite the iodine emission and recorded the spectra of the rotational lines with Fabry-Perot spectrometers having resolutions up to 70 MHz (0.0023 cm^-1). To overcome Doppler linewidth limitations we took spectra of the emission at small angles to the exciting beam and found the lines to have widths less than our instrumental resolution and frequencies which accurately tuned with the incident laser frequency. We recorded the emission lines for laser frequencies in the absorption line center and out into the absorption wing. Our spectra show that the intensity of the emission lines follows the absorption line profile while the frequency of the emission lines is determined by the laser frequency; the intensity is maximum at the absorption line center, falling by 10⁴ as the laser frequency is moved off the line center, while the line position maintains a constant frequency shift from the laser frequency.