Off-axis integrated cavity output spectroscopy and its application

Based upon the property of optical cavity and the principle of signals and systems, the measural theory and signal expression of off-axis integrated cavity output spectroscopy (Off-axis ICOS) have been analyzed and deduced. A compact and low-cost Off-axis ICOS apparatus, which used a special circuit to integrate the cavity output signal, was then described and experimented. The absorption spectrum of water vapor in the spectral range of 6587∼6595.5 cm⁻¹ was measured with this Off-axis ICOS system, and the measured spectral parameters were compared with the results of continuous-wave cavity ring down spectroscopy (CW-CRDS) and HITRAN2004 database. The detectable sensitivity of the system was achieved as ∼2.8 × 10⁻⁹ cm⁻¹.     

The study of wavelength modulation off-axis integrated cavity output spectroscopy in the case of Lorentzian absorption profile

According to the property of optical cavity and the principle of signals and systems, the mathematical derivations for wavelength modulation off-axis integrated cavity output spectroscopy (WM-OA-ICOS) are presented. And based on the analytical method, the function expression of WM-OA-ICOS signal from Lorentzian absorption profile is obtained. To validate the analyses and deductions, some special cases in 2f-harmonic detection of WM-OA-ICOS are studied and experimented with a built compact WM-OA-ICOS apparatus. It is found that the measured results can coincide with the deduced conclusions. The works can provide some references for the application of WM-OA-ICOS in the case of Lorentzian absorption profile.     

Multi-mode absorption spectroscopy, MUMAS, using wavelength modulation and cavity enhancement techniques

Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band $b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-}Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band b¹\varSigma _g⁺? X³\varSigma _g^-b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-} of molecular oxygen at 760 nm. Direct MUMAS and WMS-MUMAS signals were recorded using a White cell for air and pure oxygen for pressures in the range 0 to 1 bar. CEAS-MUMAS signals were recorded with and without WMS in an open enhancement cavity containing laboratory air. Enhancement of the signal-to-noise ratio has been obtained demonstrating the potential for increased detection sensitivity for gas-sensing applications of MUMAS.     

Multi-mode absorption spectroscopy of oxygen for measurement of concentration, temperature and pressure

The use of multi-mode absorption spectroscopy (MUMAS) to detect multiple transitions in the A-band b¹Σ_g ⁺-X³Σ_g ^- of molecular oxygen is reported. The modelling of MUMAS signatures and the procedure for fitting such model signatures to experimental data obtained using a multi-mode diode laser is described. The technique is shown to allow accurate and precise measurement of concentration, temperature over the range 300 to 500 K and of pressure over the range 0.2 to 1 bar. Extension of the technique to other ranges of temperature and pressure and to other species is discussed. PACS 42.62.Fi; 33.20.Kf     

Transformation of nonparaxial Gaussian beams

Based on the vector theory of nonparaxial Gaussian beams, the paper presents a theory about the transformation of nonparaxial Gaussian beams after lens. The collimating characteristic and propagation properties are discussed in detail, and the collimating field after lens for the vector and scalar theory respectively is compared, and the quantitative difference is also derived.     

Simple design flow injection PMMA acrylic sample cell for nitrite determination

A polymethyl-methacrylate(PMMA)acrylic sample cell using flow injection is developed in this research for the determination of nitrite in an aqueous media.The research focuses on exhibiting direct absorbance spectrophotometry of nitrite using concentration of samples ranging from 0.1078 to 1.725 ppm.Nitrite determination is done colorimetrically using the Greiss reagent method.This method is based on the reaction of nitrite with sulphanilamide acid and N-1-napthylamine(NED)utilizing diazo coupling,and a syringe is used to administer the nitrite solution.The sample cell being used possesses a diameter of 1mm with an overall size of 7.35×22 mm2.To gauge the direct absorbance,a wavelength range from 400to 650 nm has been selected for the testing,and the maximum absorbance is found to be at 545 nm.The validity of the proposed cell is explained in this letter.     

Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorption. The sensitivity of measurements of intracavity absorption grows with the laser pulse duration. The ultimate sensitivity obtained with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dynamics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70000 km effective length of the absorption path. The ultimate spectral resolution is determined by the spectral width of mode emission, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers. Received: 10 May 1999 / Published online: 29 July 1999     

Reduced-background gas-phase absorption spectroscopy

We propose and demonstrate a new method for single-shot multiplex absorption spectroscopy that permits enhanced sensitivity in the simultaneous measurement of multiple spectral lines in rapidly changing gas-phase media, such as turbulent flames. It uses an ultrashort laser pulse that propagates through the absorbing medium, for which the relevant absorption information resides in the free-induction decay that is trailing behind the transmitted pulse. Time gating out most of the transmitted pulse, but not the free-induction decay, enhances the relative fraction of light that contains absorption information when the spectrum is measured. This procedure reduces the background associated with the input light, thus enhancing detection sensitivity.     

超光谱成像差分吸收光谱技术研究

基于散射光的被动差分吸收(DOAS)技术利用气体的特性吸收谱线可实现对不同大气污染气体的定量测量,介绍了一种基于成像光谱仪的光学遥感系统,该系统运用被动DOAS原理实现了对大气污染气体的二维成像测量,并报道了该系统对实验室样品池一维测量与城市道路上方NO₂组分的成像测量实验. 基于成像光谱仪的被动DOAS系统利用太阳散射光可获取垂直方向一维的光谱信息,结合扫描装置,便可实现对污染气体的二维成像解析. 关键词： 被动差分吸收光谱 成像光谱仪 污染气体 二维成像     

Artifacts in femtosecond transient absorption spectroscopy

The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed. Received: 29 May 2001 / Final version: 15 October 2001 / Published online: 29 November 2001     

Lineshape effects in atomic absorption spectroscopy

Deviations from Beer's Law caused by the relative lineshape functions of source and absorber in atomic absorption experiments are considered. The validity of using a modified form of the law incorporating a correction factor γ, which is particularly convenient in time-resolved atomic absorption experiments, to account for these deviations, has been critically examined for a wide range of source and absorber lineshapes using numerical evaluation of the transmission integrals involved. It is concluded that there is, in general, no theoretical justification for the use of such a γ factor, except in the case of large source linewidth/absorber linewidth ratios when the line broadening in the absorber involves a significant homogeneous (Lorentzian) contribution. The use of empirically determined γ factors, much different from unity to analyse experimental data, should be viewed with suspicion unless direct evidence is presented to show that under the experimental conditions γ happens to be constant or slowly varying.     

X-ray absorption spectroscopy and high pressure

Abstract

The difficulties of the adaptation of high pressure to x-ray absorption are presented. The advantages of the energy-dispersive geometry are discussed as well as the future improvements expected with the new synchrotron radiation sources.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.     

Improved absorption spectroscopy with pulsed lasers

A simple formalism is given to allow for intensity correction and background suppression in absorption spectroscopy with pulsed lasers. By experimental examples the online application with a microcomputer is demonstrated, and the computer is also used for analyzing the correlation between signal and reference channels. The optimization of the correlation and the intensity correction yields a gain of a factor of about 30 in the signal-tonoise ratio of the recorded spectra compared to conventional absorption experiments. Spectral lines were observed, which would not be detectable by fluorescence technique.     

Ultraviolet-visible absorption spectroscopy of carbon onions

The optical properties of spherical and polyhedral carbon onions were studied in relation to the strong hump centered at 217.5 nm (4.6 μm⁻¹) in the interstellar-dust extinction curve. The ultraviolet-visible absorption spectra of onions prepared by thermal annealing of diamond nanoparticles were measured. Theoretical calculations for the spherical and polyhedral carbon onions were also carried out to explain the experimental spectra. From Fizika Tverdogo Tela, Vol. 44, No. 3, 2002, pp. 433–436. Original English Text Copyright ? 2002 by Tomita, Hayashi, Tsukuda, Fujii. This article was submitted by the authors in English.