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.     

Applications of cavity ring-down spectroscopy to high precision isotope ratio measurement of 13C/12C in carbon dioxide

Recent measurements of carbon isotopes in carbon dioxide using near-infrared, diode-laser-based cavity ring-down spectroscopy (CRDS) are presented. The CRDS system achieved good precision, often better than 0.2 per thousand, for 4% CO2 concentrations, and also achieved 0.15-0.25 per thousand precision in a 78 min measurement time with cryotrap-based pre-concentration of ambient CO2 concentrations (360 ppmv). These results were obtained with a CRDS system possessing a data rate of 40 ring-downs per second and a loss measurement of 4.0 x 10(-11) cm(-1) Hz(-1/2). Subsequently, the measurement time has been reduced to under 10 min. This standard of performance would enable a variety of high concentration (3-10%) isotopic measurements, such as medical human breath analysis or animal breath experiments. The extension of this ring-down to the 2 microm region would enable isotopic analysis at ambient concentrations, which, combined with the small size, robust design, and potential for frequent measurements at a remote site, make CRDS technology attractive for remote atmospheric measurement applications.     

Quantification of stable minor species in confined flames by cavity ring-down spectroscopy: application to NO

Cavity ring-down spectroscopy (CRDS) is used to measure the NO mole fraction formed in the burnt gases of low-pressure premixed flames. It is shown that the line-of-sight absorption is greatly increased by the contribution of the NO molecules surrounding the burner. This contribution has been quantified by developing a mathematical procedure taking into account the spatial and spectral features of the CRDS measurement. Calculations have been undertaken in the general case of a stable species not consumed in the flame. The most sensitive parameter is the temperature both in the flame and outside the flame. Simulations allow the selection of the best spectroscopic transitions for a given flame (i.e. a given temperature profile), ensuring the weakest influence of the inaccuracy affecting the temperature determination. High quantum states belonging to the A–X (0–1) band of NO have been found to be the most valuable and have led to a NO mole fraction determination with an accuracy of ±13%. NO absorption in the flame was completely masked using the A–X (0–0) band. Finally, the prompt-NO mole fraction formed in a methane/air flame stabilized at 33 Torr is obtained by combining CRDS and laser induced fluorescence techniques. Received: 12 October / Revised version: 1 February 2002 / Published online: 14 March 2002     

强流离子源光腔衰荡光谱应用研究

强流离子源是托卡马克中性束注入器的核心部件,为了满足未来对高能量离子束中性化效率的要求,负离子源成为中性束注入系统的首选。光腔衰荡光谱(cavity ring-down spectroscopy,CRDS)是一种超高灵敏探测吸收光谱技术。在强流负离子源中,利用氢负离子的光致剥离过程,CRDS可以用来测量氢负离子的绝对积分密度。与激光光致剥离法与光学发射光谱法相比,CRDS具有不受电磁干扰、不依赖等离子体参数、测量精度高等优点。强流离子源负离子密度测量所用CRDS系统由激光器、光学谐振腔、光电探测器和数据采集系统四部分组成。本文根据CRDS测量氢负离子密度的原理,详细推导了氢负离子密度的计算方法,给出了氢负离子密度测算表达式;然后,结合强流离子源实验室应用的具体情况,分析了各部分装置的选择原则与注意事项;最后,介绍了CRDS技术在德国马克斯-普朗克等离子体物理研究所、日本国立聚变科学研究所、意大利Consorzio RFX研究所强流负离子源研究中的应用情况。实验结果表明,源腔气压、源功率等源参数会影响氢负离子密度;铯的注入可以将氢负离子密度从1016 m-3量级提高到1017 m-3量级;同时,日本NIFS的实验结果证明氢负离子密度与引出电流呈线性关系。     

Two-color laser-induced incandescence and cavity ring-down spectroscopy for sensitive and quantitative imaging of soot and PAHs in flames

Laser-induced incandescence is a technique which enables the measurement of soot volume fractions. However, the laser-induced soot emission might be affected by a fluorescence background generally ascribed to the polycyclic aromatic hydrocarbon compounds (PAHs) present at the soot location. In this paper, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames. The original method developed here consists in comparing the emission distributions induced by two different laser wavelengths: (1) at 1064 nm emission signals are exempt from PAH fluorescence and (2) at 532 nm both soot incandescence and PAH emission contribute to the total signal. In addition, the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy (CRDS) at 1064 nm and 532 nm. The proposed method can provide highly sensitive 2D imaging of PAHs and soot using the fundamental and the second-harmonic frequencies of a single YAG laser. Finally, 2D distributions of PAH absorbance and soot volume fraction calibrated by CRDS are obtained in two diffusion flames, particularly in a very low-sooting flame exhibiting a maximum PAH absorbance of 6×10^-4 cm^-1 and a maximum soot volume fraction of 3 ppb only. The respective spatial distributions of PAHs and soot are shown to vary with the initial C/O ratio. PACS 33.20.Lg; 42.62.Fi; 44.40.+a     

C2H2 Overtones Near 12300 cm-1 Revisited with a Very Sensitive Cavity Ring-down Spectrometer

利用单模连续的钛宝石激光器, 构建了一台光腔衰荡光谱仪, 其可探测的最小吸收可达1.8×10^-10/cm. 该光谱仪被用来记录C₂H₂分子在12240～12350 cm^-1的泛频光谱. 与在同一波段测量的已报到的CRDS和激光腔内吸收光谱仪结果比较,本测量同时具有更好的灵敏度和精度. 由此,获得了乙炔分子在12290.12、12311.82和12350.61 cm^-1附近高泛频谱带更准确的振转参数     

Determination of the ratio of soot refractive index function E(m) at the two wavelengths 532 and 1064 nm by laser induced incandescence

A new method is proposed to measure the ratio of the refractive index function of soot particles E(m) at the two fixed wavelengths: 532 and 1064 nm. Using a non-intrusive, in-situ laser based technique, the ratio E(m,1064 nm)/E(m,532 nm) can be determined by comparing laser induced incandescence (LII) intensities at 532 and 1064 nm excitation wavelengths. The method consists of selecting laser energies that insure the equality of the LII signals in the low fluence regime under given conditions. Such equality is consistent with the fact that the soot particle will have reached the same temperature independently of the laser wavelength, i.e. the soot particle has absorbed the same energy. As the absorbed energy is proportional to the laser irradiance times E(m), the measurement of the laser energies required to insure perfect concordance of the LII intensities (spatially and temporally) serves to deduce the ratio E(m,1064 nm)/E(m,532 nm). The method is demonstrated in an acetylene/air flame, validated against extinction measurements performed by cavity ring-down spectroscopy (CRDS) by using laser radiations at 532 nm and 1064 nm and finally applied to different flame conditions. PACS 78.20.Ci; 78.90.+t; 81.05.Uw; 42.62.-b     

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。     

Theory and experiment of CW dye laser injection locking and its application to second harmonic generation

High-power single-mode operation of a cw ring dye laser has been obtained by injection of cw single-mode radiation. The intracavity power of this system was used to generate tunable single-mode uv radiation by frequency doubling in a temperature phase-matched ADA crystal: up to 45 W intracavity fundamental power and up to 70 mW extracted uv power have been observed. A theoretical treatment of the injection locked cw ring dye laser system is given for the stationary state. Expressions for the intracavity intensities as a function of the small-signal gain and the saturation intensity are derived.     

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.     

Problems for the elimination of atomic variables and the single-mode dye laser equations

The stochastic generalization of the adiabatic approximation has been developed earlier by Wunderlin and Haken [6] and recently by Schöner and Haken [7] and the authors [8]. Using these theories, a complex field amplitude equation for the single-mode dye laser is derived based on a set of semiclassical single-mode laser equations. An effective Fokker-Planck equation in which the laser intensity is decoupled from the phase variable is obtained. The relations between our theory and the phenomenological single-mode dye laser equation are discussed.     

Measurement of multi-exponential optical decay processes by?phase-shift cavity ring-down

Multi-exponential decay waveforms are common occurrences in cavity ring-down spectroscopy and the respective ring-down times are typically obtained by fitting the ring-down waveform to the sum of exponential decay functions. In phase-shift cavity ring-down (CRD) spectroscopy the measurement of a single phase angle will not provide sufficient information and needs to be complemented by either intensity measurements or phase angle measurements at different modulation frequencies. Here, a formalism analogous to that developed for fluorescence lifetime spectroscopy is adapted to the phase-shift CRD technique and is tested for two types of waveguide CRD systems: (1) a single-mode fiber cavity in which light is confined by two identical Fiber Bragg Gratings and (2) a multimode fiber loop. By measuring the phase angle at different modulation frequencies, lifetimes for up to three different decay processes were obtained.     

Detection of chemical warfare agents based on quantum cascade laser cavity ring-down spectroscopy

Chemical warfare agents(CWAs) are recognized as serious threats of terrorist acts against the civilian population.Minimizing the impact of these threats requires early detection of the presence of CWAs.Cavity ring-down spectroscopy(CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species.In this letter,the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate(DMMP).A limit of DMMP detection of approximately 77 ppb is achieved.The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2×10-7cm 1.     

Cavity ring-down spectroscopy of molecularly thin iodine layers

Cavity ring-down spectroscopy (CRDS) has so far mostly been used for measurements in the gas phase. Only in 1999 was a first spectrum of condensed phase published. This spectrum was measured by using a coated plate between the cavity mirrors. Rather than using this method, our measurements were made using the cavity mirrors as a substrate. This way, the scattering losses could be reduced by approximately a factor of 100. In our measurements we investigated molecularly thin layers of iodine. The iodine spectra were taken in the frequency range from 16200 to 17200 cm^-1 using pulsed CRDS. Received: 14 April 2000 / Revised version: 26 July 2000 / Published online: 22 November 2000     

Absolute radical concentration measurements in low-pressure H2/O2 flames during the combustion of graphite

Absolute CN and CH radical concentrations were determined in situ during the combustion of a graphite substrate in premixed, laminar, low-pressure, H₂/O₂ flames for two different equivalence ratios, = 1.0 and = 1.5. For CN measurements, a small amount of NO (1.8%) was added. The concentration of CN was measured by cavity ring-down spectroscopy (CRDS) probing the absorption of the P_1,2 (13) in the B–X (0, 0) band at 388.1 nm, and the concentration of CH was measured by linear unsaturated laser-induced fluorescence (LIF) exciting the fluorescence of the R₁ (4) in the B–X (0, 0) band at 387.4 nm. Temperature measurements were done based on LIF excitation spectra of OH in the A–X (0, 0) band. It was found that the graphite substrate reduces the flame temperature in the vicinity of its surface. The CN concentrations were found to be three times higher for the rich flame than for the stoichiometric flame. CH concentrations were slightly higher for the stoichiometric flame than for the rich flame. The observed CH/CN concentration ratio is substantially lower compared to NO-doped low-pressure CH₄/O₂ flames. The obtained quantitative information can serve as a first calibration point for detailed numerical simulations of the burning solid graphite, which are based on the concept of surface elementary reactions.     

光腔衰荡光谱法中衰荡时间的优化提取

采用列文伯格 马夸尔特算法（L M）和最小二乘法（LS）对带有高斯白噪音的两种模拟衰荡信号进行了拟合分析.结果表明:L M相对于LS所得结果更准确,相对误差更小,抗噪音能力更强,且重复性更好.搭建了两种光腔衰荡光谱法系统,分别得到连续和脉冲衰荡信号,依据理论分析结果对实验数据进行了处理,所得结论与模拟结果一致.     

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.     

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.     

Colored noise in a two-dimensional nonlinear system

A two-dimensional decoupling theory is developed when colored noise is included in a nonlinear dynamical system. By a functional analysis, the colored noise is transformed to an effective noise that includes the noise correlation time, the mean dynamical variable, and the original noise strength. When the two-dimensional decoupling theory is applied to single-mode and two-mode dye laser systems, the mean, variance, and effective eigenvalue of laser intensity are calculated. Excellent agreement between theoretical analysis, numerical simulations, and experimental measurements are obtained. It is seen that the increase of noise correlation time can reduce the fluctuations in the laser system. It is also shown that there is relatively large fluctuation in the phase when the laser undergoes from thermal light to coherent light when the theory is applied to a single mode dye laser. Received 20 August 2001 and Received in final form 4 December 2001     

Trace Moisture Measurement with 5. 2 mu m Quantum Cascade Laser Based Continuous-Wave Cavity Ring-Down SpectroscopyEI北大核心CSCD

Trace moisture concentration in high-purity gases is an important parameter in semiconductor manufacturing because many manufacturing processes are sensitive to moisture even on the level of parts per billion by volume (ppbv). Detection of trace moisture in mid-infrared spectral region is beneficial due to more abundant and stronger spectral lines in this region. Recently, Quantum cascade lasers (QCLs) with high output power, narrow line-width, and high reliability have been developing rapidly and have become promising light sources for sensitive spectroscopic measurements. By employing a 5. 2 mu m external-cavity tunable quantum cascade laser, a continuous-wave cavity ring-down spectroscopy (CRDS) experimental setup is established and applied to detect trace moisture in high-purity nitrogen gas. In the experiment, the CRDS signal is averaged to improve the detection sensitivity, and the optimal averaging number is determined by Allan variance calculation to be 602. For trace moisture detection, the absorption cross-section of H2O in the spectral range between 1 905 and 1 925 cm(-1) is simulated according to the HITRAN database and the optimal detection spectral line is chosen. Detected at 1 918 cm(-1) absorption line at 296 K temperature and 1 atm pressure, the measured moisture concentration is in good agreement with the nominal value, and the minimum detectable moisture concentration of 24. 8 ppbv is achieved when cavity mirrors with reflectance of 99.93% are used. The experimental results show that mid-infrared cavity ring-down spectroscopy technique has great potential in a wide variety of applications, such as industrial production control, environmental monitoring and health diagnosis, etc.