Optimization of the mode matching in pulsed cavity ringdown spectroscopy by monitoring non-degenerate transverse mode beating

A simple and reliable method is presented for optimizing the mode matching of a laser beam to the high-finesse cavity used in pulsed cavity ringdown spectroscopy (CRDS). The method is based on minimizing the excitation of higher-order transverse cavity modes through monitoring the non-degenerate transverse mode beating which becomes visible with induced cavity asymmetry caused by slight misalignment. No additional instrument is required other than a pinhole aperture, thus this method can be applied for CRDS experiments in the whole wavelength range. Measurements of the CRDS absorption spectrum of acetylene (C₂H₂) near 571 nm demonstrate that the mode-matching optimization improves the sensitivity of pulsed CRDS. Received: 22 October 2001 / Revised version: 16 January 2002 / Published online: 14 March 2002     

Cavity ring-down spectroscopy of CH and CD radicals in a diamond thin film chemical vapor deposition reactor

A mixture of H₂ and CH₄ is passed over a hot-wire tungsten filament in a diamond thin film chemical vapor deposition reactor. The resulting CH radicals are measured in absorption using cavity ring-down spectroscopy (CRDS). The concentration of the CH radicals increases as the filament is approached. The rotational temperature measurements indicate a large temperature discontinuity between the filament and the CH in the gas phase. The pathways for CH production were investigated by replacing H₂ by D₂ in the feed gas mixture, which resulted in the exclusive production of CD. From this observation it is concluded that rapid H/D isotope exchange dominates in the gas phase. Nonperiodic temporal oscillations in the CH concentration are observed when a rhenium filament is used in place of a tungsten filament. The oscillations are attributed to the nonperiodic changes in the amount of carbon at the filament surface. Received: 21 August 2000 / Accepted: 23 August 2000 / Published online: 23 May 2001     

Intracavity laser absorption spectroscopy and cavity ring-down spectroscopy in low-pressure flames

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) have been used for measurement of the NH₂-radical spectrum near 643 nm. NH₂ was obtained in low-pressure methane/air flat flames doped with minor amounts of ammonia (as low as 0.023%). The NH₂ concentration was measured both by CRDS and ICLAS in the same conditions. This enables us to compare the practical sensitivity of the two methods. Both methods were also used for measurements in a sooting acetylene/air flame (ϕ = 2.6). The comparative advantages of the methods and their complementarities are discussed.     

Sensitive birefringence measurement in a high-finesse resonator using diode laser optical self-locking

We demonstrate a new method to measure weak birefringence of dielectric mirrors with excellent spatial resolution and sensitivity (<10^-7 radians). We exploit a well-known optical feedback scheme for line-width narrowing and frequency locking of a diode laser to a high-finesse cavity. Feedback comes from the intracavity field which builds up at resonance, selected by its change in polarization with respect to the incident field. This change, due to the residual birefringence of the cavity mirror coatings, was already exploited for birefringence measurements using an active laser-locking scheme. Here we measure the optical feedback rate as a function of rotation angle of one of the cavity mirrors (around the cavity axis). A stable feedback signal is obtained since the laser, as soon as it locks to a cavity resonance, effectively behaves as a monochromatic source. By fitting the data with a theoretical expression, we determine quantitatively the local birefringence vectors of both mirrors, which are around 10^-6 radians. Our scheme is simple, works with cavities of very high finesse (F∼10⁵), and is promising for measuring birefringence in gases induced by external fields. Received: 18 July 2001 / Final version: 14 March 2002 / Published online: 8 May 2002     

中红外光参变振荡器在高反射率测量中的应用

根据光腔衰荡光谱技术(CRDS)原理,使用中红外光参变振荡器(OPO)为光源建立了直腔与折叠腔相结合的中红外波段3.6 μm 反射率测量实验装置,用于研究中红外波段的高反射膜反射率,测试精度为10-4。使用直型衰荡光腔测试了三对不同薄膜材料设计镀制的高反射腔镜的反射率,并选择了一对腔镜用于实验装置中。采用该装置精确测试了不同薄膜材料镀制的高反射膜的反射率,包括YbF3/ZnS,YbF3 /ZnSe多层膜,以及由银加保护膜镀制的反射镜。研究表明,中红外波段介质膜的反射率可达到R>0.9990,其中由YbF3/ZnSe镀制在硅基底上的多层介质膜3.6 μm反射率可达到99.96％。     

Cavity-ring-down spectroscopic studies of NO2 in the region around 613 nm

Catalytic degradation and diffusion processes of NO₂ were followed by cavity-ring-down spectroscopy (CRDS) at 612.9 nm. The suitability of this absorption method for quasi-continuous, direct quantitative measurements over extended periods of time is demonstrated. The high sensitivity of the method is reflected by the fact that NO₂ concentrations as low as 200 ppb were detected at wavelengths at which the absorption of NO₂ is 12-fold lower than at the absorption maximum at 413 nm. Absorption coefficients of less than 1×10^-7 cm^-1 were measured. Received: 25 February 2000 / Revised version: 4 August 2000 / Published online: 5 October 2000     

Quantitative gas measurements using a versatile OPO-based cavity ringdown spectrometer and the comparison with spectroscopic databases

A versatile OPO-based cavity ringdown spectrometer is reported for quantitative and sensitive gas measurement down to nmol/mol levels. The system is based on cavity ringdown spectroscopy (CRDS) in combination with a continuous wave optical parametric oscillator tunable from 2693 to 3505 nm. Using a single set of CRDS mirrors, spectra were recorded of methane, ethane, benzene, propane, water, acetone and formaldehyde. Gas mixtures were gravimetrically prepared in cylinders or via dynamic generation using diffusion tubes (formaldehyde). Results were compared with data from the Hitran, PNNL and NIST databases. Good agreement was found with PNNL and NIST data for most molecules while agreement with Hitran was less for ethane and formaldehyde.     

光腔衰荡光谱方法探测痕量一氧化碳气体

基于通讯波段的分布式反馈半导体激光器(DFB),搭建了一套光腔衰荡光谱仪(CRDS)。衰荡光腔由一对反射率高于99.997%的高反镜组成,衰荡腔长约为130 cm,空腔衰荡时间约为150 μs。当光谱平均次数达到1 000次时,光谱仪灵敏度(最小可探测吸收系数)达到5×10-12 cm-1。利用热隔绝的方式稳定衰荡腔长,并使用衰荡光腔自身作为光学标准具,来标定光谱的频率：利用反馈式光谱扫描程序步进改变激光器频率,使之与衰荡腔的纵模频率逐一匹配,从而实现所测得光谱的自动标定。通过测量一氧化碳分子在1.565 μm附近的吸收光谱,测定气体中一氧化碳的含量。将光谱测量结果和标准样品中的一氧化碳含量进行对比,对装置的定量精度进行了检验,表明其对一氧化碳的探测极限达4 ppbv。利用该装置对实际大气中一氧化碳的含量进行了实时监测。     

Ultrasensitive optical measurement of thermal and quantum noises

We have developed an experiment of ultrasensitive interferometric measurement of small displacements based on a high-finesse Fabry-Perot cavity. We have observed the internal thermal noise of mirrors and fully characterized their acoustics modes with a sensitivity of 3 × 10⁻²⁰ m/Hz^1/2. This unique sensitivity is a step towards the first observation of the radiation pressure effects and the resulting standard quantum limit in interferometric measurements. Our experiment may become a powerful facility to test quantum noise reduction schemes such as the use of squeezed light or quantum locking of mirrors. As a first result, we present the observation of a cancellation of radiation pressure effects in our cavity. This back-action noise cancellation was first proposed within the framework of gravitational-wave detection with dual resonators, and may drastically improve the sensitivity of such measurements. The text was submitted by the authors in English.     

Frequency doubling of a fibre-amplified 1083 nm DBR laser

A good frequency standard is required at 1083 nm for measurements on the fine structure of helium and of the fine structure constant. Several milliwatts of CW frequency-doubled light offers the prospect of stabilisation to a Doppler-free hyperfine transition in molecular iodine. The 1083 nm emission of an extended-cavity DBR diode laser has been amplified using an ytterbium-doped fibre amplifier, and applied to a type-I phase matched Mg:LiNbO₃ crystal in a high-Q fundamental-resonant cavity for frequency doubling. The amplifier gain at 1083 nm under typical operating conditions was 13.8 dB, with a coherent output power up to 63 mW, limited by the maximum signal input power. The doubling cavity Q was , and about 70% of the incident power was coupled in. The maximum 2nd-harmonic conversion efficiency exceeded 20% and provided an output power of 3.7 mW for making continuous frequency scans of up to 600 MHz in the green. An optical spectrum analyser at 541.5 nm showed fringes of 4.6 MHz full width half maximum, close to the instrumental width. Received: 25 February 1998 / Revised and Accepted: 28 May 1998     

Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy

We report on infrared laser spectroscopic measurements of the isotopic composition of methane (¹²CH₄, ¹³CH₄) in natural air samples with a cavity ring-down technique. A CO overtone sideband laser is utilized to excite a high-finesse cavity which provides an effective optical absorption path length of 3.6 km. We achieved a detection limit of 105 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.9×10^-9 /cm. Rapid determination of the ¹³C/¹²Cisotopic ratio of methane in ambient air without sample preconcentration or gas processing is realized. The present system requires only few minutes for an isotopic ratio measurement with a precision of 11%o . Received: 14 July 2000 / Revised version: 25 October 2000 / Published online: 6 December 2000     

Cavity-enhanced absorption: detection of nitrogen dioxide and iodine monoxide using a violet laser diode

We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO₂) and iodine monoxide (IO) radicals using a violet laser diode at λ=404.278 nm. A noise-equivalent (1σ≡ root-mean-square variation of the signal) fractional absorption for one optical pass of 4.5×10^-8 was demonstrated with a mirror reflectivity of ∼0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8×10¹⁰ molecule cm^-3 and towards the IO radical of 3.3×10⁹ molecule cm^-3 were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO₂+NO→RO+NO₂). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO₂ in the reaction NO₂+O→NO+O₂. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3×10¹⁰ molecule cm^-3 in the discharge-flow-tube experiments. Received: 4 February 2003 / Revised version: 10 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1865/275-410, E-mail: vlk@physchem.ox.ac.uk     

Velocity measurements by cavity ringdown spectroscopy

We demonstrate velocity measurements of gas-phase particles by using cavity ringdown spectroscopy (CRDS). Velocity information is inferred from the Doppler-shift contributions to the measured absorption line shape. Because in CRDS the laser beam propagates back and forth within the optical cavity, a measured absorption feature is both upshifted and downshifted; i.e., it is split by the velocity component parallel to the optical axis. The splitting of the absorption features allows direct velocity measurements to be made without requiring an external frequency reference. The CRDS velocity measurement approach is demonstrated for sputtered molybdenum atoms in a low-pressure (collisionless) environment.     

High-resolution cavity leak-out absorption spectroscopy in the 10-μm region

2 laser. After excitation, the laser power is turned off for a short time and the subsequent decay of the radiation stored in the cavity is observed via detection of the light leaking out through one of the cavity mirrors. Measurement of the decay time allows one to determine the photon losses und thus to detect weakly absorbing species inside the cavity. Since the cavity is frequency-locked to the laser the decay time can be probed with a high repetition rate, basically limited by the sampling rate of the analog-to-digital converter. This approach is closely related to cavity ring-down spectroscopy with pulsed lasers, but exhibits several advantages concerning spectral resolution and detection sensitivity. As a practical example we demonstrate monitoring of trace amounts of ethylene. Using R=99.5% mirrors we achieve a detection limit of 1 ppb ethylene (integration time: 100 s) corresponding to absorption losses of 3×10^-8 /cm. Further improvement is feasible when mirrors with higher reflectivity become available. Received: 2 July 1998     

Development of injection-seeded, pulsed optical parametric generator/oscillator systems for high-resolution spectroscopy

We report the development and application of pulsed optical parametric generator (OPG) and optical parametric oscillator (OPO) systems that are injection seeded with near-infrared distributed feedback diode lasers. The OPG is injection seeded at the idler wavelength without the use of a resonant cavity. Two counter-rotating, beta-barium-borate (β-BBO) crystals are used in the OPG. These crystals are pumped by the third harmonic, 355-nm output of an injection-seeded Nd:YAG laser. An OPO version of the system has also been developed by placing two flat mirrors around the two β-BBO crystals to form a feedback cavity at the signal wavelength. The OPO cavity length is not actively controlled. The output signal beam from the OPG or OPO is amplified using an optical parametric amplifier (OPA) stage with four β-BBO crystals. The frequency bandwidths of the signal and idler laser radiation from OPG/OPA and OPO/OPA systems have been determined to be slightly greater than 200 MHz. The temporal pulses from each system are smooth and near-Gaussian. High-resolution optical absorption measurements of acetylene (C₂H₂) were performed as another check of the frequency spectrum of the idler beam. The frequency-doubled signal output of the OPO/OPA system was used to perform high-resolution, single-photon, laser-induced fluorescence (LIF) spectroscopic studies of the (0,0) vibrational band of the A ²Σ⁺−X ²Π electronic transition of nitric oxide (NO) at low pressure. Excellent agreement was obtained between the theory and the experiment. The signal output of the OPG/OPA system was also used for sub-Doppler, two-photon LIF spectroscopic studies of the same vibration–rotation manifold of NO.This revised version was published online in August 2005 with a corrected cover date.     

单光束多组分温室气体的腔衰荡光谱同步检测

腔衰荡光谱技术(CRDS)作为一种具有高灵敏度高光谱分辨率的检测方法已被广泛用于痕量气体检测。而目前基于CRDS痕量气体检测多针对单一气体进行测量或通过多个激光器产生的多光束进行多种组分气体浓度测量。利用DFB激光器波长可调谐特性,通过强弱吸收峰结合,使用单光束实现了多种温室气体的腔衰荡光谱技术同步检测。由于大气中水汽和二氧化碳浓度较高,为实现同一衰荡系统对三种温室气体的同步测量,在平衡吸收损耗的基础上,选取1 653~1 654 nm内甲烷的强吸收峰与水汽、二氧化碳的弱吸收峰进行测量。通过光谱叠加反演矩阵,分别得到甲烷、水汽、二氧化碳的浓度。在计算测量灵敏度过程中发现,通过去除衰荡过程初期的部分数据点(过滤区间),会对噪声等效吸收系数产生影响。多数情况下,在测量灵敏度计算方面,列文伯格-马夸尔特算法(L-M)会优于离散傅里叶变换法(DFT);但当衰荡曲线的单指数性下降时,上述结论不一定成立。搭建了一个低精细度(F≈6×103)衰荡腔对上述结论进行了实验验证。相较于用于测量温室气体浓度的高精细度衰荡腔(F≈1×105),低精细度衰荡腔的衰荡速率较快,衰荡曲线的单指数性明显低于高精细度衰荡腔。实验表明,在过滤区间长度较短时,采用DFT算法计算得到的噪声等效吸收系数会小于L-M算法得到的结果。当过滤区间长度增加时,L-M算法得到的结果优于DFT算法。在受过滤区间长度影响方面,DFT算法的波动性要明显小于L-M算法。根据Allan方差分析,在512次采样平均(约8 s)下的最小噪声等效吸收系数进行计算,该CRDS装置测量灵敏度为2.4×10-10 cm-1。在25 ℃标准大气压下,对应甲烷、水汽、二氧化碳的测量灵敏度分别为0.64 ppbv,3.5 ppmv和4.0 ppmv。基于该CRDS装置,通过单光束多波长测量方法,利用光谱叠加反演矩阵,测得大气中甲烷、水汽、二氧化碳浓度分别为2.018,3 654和526 ppmv;而采用经典CRDS单波长测量得到的甲烷、水汽、二氧化碳浓度分别为2.037,3 898和630 ppmv。通过与温控调节波长,逐点扫描得到的光谱吸收曲线进行对比,采用多波长测量得到气体浓度进行复合拟合的光谱曲线残差小于单波长测量得到气体浓度进行简单拟合的光谱曲线残差。     

Frequency up-converted radiation from a cavity moving in vacuum

We calculate the photon emission of a high finesse cavity moving in vacuum. The cavity is treated as an open system. The field initially in the vacuum state accumulates a dephasing depending on the mirrors motion when bouncing back and forth inside the cavity. The dephasing is not linearized in our calculation, so that qualitatively new effects like pulse shaping in the time domain and frequency up-conversion in the spectrum are found. Furthermore we predict the existence of a threshold above which the system should show self-sustained oscillations. Received: 10 December 1997 / Received in final form: 27 March 1998 / Accepted: 27 March 1998     

腔衰荡光谱技术测量O2禁戒跃迁绝对吸收截面

腔衰荡光谱技术(CRDS)不仅具有较高的测量灵敏度,还可对样品的绝对吸收进行测量。采用连续激光腔衰荡光谱技术,通过测量O2分子三重禁戒跃迁b1∑g X3∑g-(3,0)带RQ(5)谱线(波数17266.090 cm-1)处,极限真空及不同气压下的衰荡时间,利用逼近法得到空腔寿命为2.9174 ms,由此拟合获得其绝对吸收截面为1.4998(±0.0967)×10-26cm2,与先前的文献估计值一致。由空腔寿命获得的谐振腔高反镜的反射比为99.989(±0.001)%,较通常的测量方法更为精确,该实验条件下的等效吸收程长比几何程长增大了约9090倍。     

基于高精细度光腔锁频激光的分子吸收光谱测量

利用高精细度光腔锁定激光频率,实现了对分子吸收光谱的高精度测量.光腔采用低热膨胀系数的殷钢结构设计和温度控制,实现了腔长度的稳定;通过将激光频率锁定在光腔纵模上,实现了高频率精度和高灵敏度的光腔衰荡光谱测量.利用该装置示范性地测量了二氧化碳分子在6470.42 cm~(-1)附近的光腔衰荡光谱和色散光谱,得到了高精度的谱线参数,并和数据库谱线参数进行了对比.     

Co/C and W/Si multilayers deposited by ion-beam sputtering for the soft X-ray range

We deposited Co/C multilayer mirrors for a wavelength of 4.77 nm and W/Si multilayer mirrors for a wavelength of 1.77 nm by use of ion-beam sputtering. The small-angle diffraction spectrum was used to analyze the structure of the multilayers. With a combination of the experimental diffraction spectra and Apeles’ theory for calculation of the interfacial roughnesses of the multilayers, the interfacial roughnesses of Co/C and W/Si are 0.80 nm and 0.60 nm, respectively, which are lower than that of the substrate. The reflectivity of the Co/C multilayer is measured to be about 20% and that of the W/Si multilayer about 1% at the grazing incidence angle of about 12°. Received: 30 May 2000 / Accepted: 1 August 2000 / Published online: 11 February 2002