Pulsed, single-mode cavity ringdown spectroscopy

We discuss the use of single-mode cavity ringdown spectroscopy with pulsed lasers for quantitative gas density and line strength measurements. The single-mode approach to cavity ringdown spectroscopy gives single exponential decay signals without mode beating, which allows measurements with uncertainties near the shot-noise limit. The technique is demonstrated with a 10-cm-long ringdown cavity and a pulsed, frequency-stabilized optical parametric oscillator as the light source. A noise-equivalent absorption coefficient of 5 x 10(-10) cm(-1) Hz(-1/2) is demonstrated, and the relative standard deviation in the ringdown time (varsigma(tau)/tau) extracted from a fit to an individual ringdown curve is found to be the same as that for an ensemble of hundreds of independent measurements. Repeated measurement of a line strength is shown to have a standard deviation <0.3%. The effects of normally distributed noise on quantities measured using cavity ringdown spectroscopy are discussed, formulas for the relative standard deviation in the ringdown time are given in the shot- and technical-noise limits, and the noise-equivalent absorption coefficient in these limits are compared for pulsed and continuous-wave light sources.     

Uncertainty analysis of absolute concentration measurement with continuous-wave cavity ringdown spectroscopy

To evaluate the uncertainty of concentration measurement using cavity ringdown spectroscopy, we analytically derived expressions for uncertainty for parameters, such as temperature, laser frequency, and ringdown time deviation, from the model equation. The uncertainties that are due to systematic errors in a practical cavity ringdown system were assessed through an experimental study of the PQ(35) transition in an A band of molecular oxygen. We found that, except for the line strength that is regarded as a reference value independent of the measurement, the laser frequency jitter is the largest uncertainty source in the system. Some practical requirements for minimizing the uncertainty in concentration measurements are discussed. We also demonstrated determination of the line strength of the PQ(35) transition line of oxygen to be 8.63(3) x 10(-27) cm(-1) with a relative uncertainty of less than 0.4%.     

Spatial characterization of laser-induced plasmas by deconvolution of spatially resolved spectra

The spatial characterization of laser-induced plasmas, including their temperature, electron density, and relative atom density, has been carried out by emission spectroscopy. The plasmas were generated with iron samples in air and argon at atmospheric pressure. An imaging spectrometer equipped with an intensified CCD detector procured spectra with spatial resolution. The plasma characterization was made at three temporal gates (2-3, 5-6, and 9-11 micros) to permit the plasma's evolution to be studied. A deconvolution procedure was developed to transform the measured intensity, integrated along the line of sight, into the radial distribution of emissivity. Temperature and electron density distributions were obtained under the assumption of local thermodynamic equilibrium and Stark broadening of the emission lines. The relative atom density distributions in the plasma of the Fe atoms arising from the sample and of the Ar atoms arising from the ambient gas were determined and evidenced an important interaction between the plasma and the surrounding atmosphere.     

Measurement and analysis of OH emission spectra following laser-induced optical breakdown in air

The measured emission spectra of the OH radical subsequent to laser-induced optical breakdown in air are analyzed to infer spectroscopic temperature and species number density. Emissions from the UV A2sigma+ --> X2IIi transition dominate the spectra in the wavelength range of 306-322 nm and for time delays from the optical breakdown of 30-300 micros. Contributions from other species to the recorded OH emission spectra were also investigated for spectroscopic temperature measurements in the range of 2000-6000 K and for OH number densities in the range of 10(14) - 2 x 10(16) cm(-3). Monte Carlo simulations are applied to estimate errors in the analysis of the hydroxyl spectra.     

Cavity ringdown spectroscopy of thin films in the mid-infrared

To demonstrate the potential of the cavity ringdown technique in mid-infrared spectroscopy of thin film samples, we measured absorption losses in a C60 film on a BaF2 substrate using a tunable optical parametric amplifier source. With a Brewster angle sample geometry, we achieved a fractional loss sensitivity as small as 1.3 x 10(-7) with 1.5 cm(-1) resolution, an improvement in sensitivity of 2 orders of magnitude compared to standard Fourier transform infrared methods. At an absorption sensitivity of 5 x 10(-7), spectra of several C60 overtone lines were recorded.     

Measurements of cavity ringdown spectroscopy of acetone in the ultraviolet and near-infrared spectral regions: potential for development of a breath analyzer

We report a study on the cavity ringdown spectroscopy of acetone in both the ultraviolet (UV) and the near-infrared (NIR) spectral regions to explore the potential for development of a breath analyzer for disease diagnostics. The ringdown spectrum of acetone in the UV (282.4-285.0 nm) region is recorded and the spectrum is in good agreement with those obtained by other spectral techniques reported in the literature. The absorption cross-section of the C-H stretching overtone of acetone in the NIR (1632.7-1672.2 nm) is reported for the first time and the maximum absorption cross-section located at 1666.7 nm is 1.2 x 10(-21) cm(2). A novel, compact, atmospheric cavity with a cavity length of 10 cm has been constructed and implemented to investigate the technical feasibility of the potential instrument size, optical configuration, and detection sensitivity. The detection limit of such a mini cavity employing ringdown mirrors of reflectivity of 99.85% at 266 nm, where acetone has the strongest absorption, is approximately 1.5 ppmv based on the standard 3 criteria. No real breath gas samples are used in the present study. Discussions on the detection sensitivity and background spectral interferences for the instrument development are presented. This study demonstrates the potential of developing a portable, sensitive breath analyzer for medical applications using the cavity ringdown spectral technique.     

Periodically locked continuous-wave cavity ringdown spectroscopy

We demonstrate a simple periodically locked cw cavity ringdown spectroscopy technique that enables a very large number of ringdown events to be rapidly acquired. An external cavity diode laser is locked to a high-finesse cavity, and as many as 16,000 ringdown events per second are obtained by periodically switching off the light entering the high-finesse cavity. Following each ringdown event, the light to the cavity is switched back on and cavity lock is rapidly reacquired. Limited only by our relatively modest digitization rate, we obtained a minimum detectable absorption loss of 4.7 x 10(-9) cm(-1), but we show that faster digitization could provide a sensitivity of 5.9 x 10(-10) cm(-1) Hz(-1/2).     

Excitation temperature and electron number density behavior of atmospheric pressure D.C. argon plasma jet during spheroidization of nickel

The excitation temperature and electron number density of the atmospheric pressure D.C. argon plasma jet during spheroidization of nickel have been measured at 5 mm from the nozzle exit by optical emission spectroscopic technique. Emission spectra of the argon plasma, argon plasma with carrier gas and carrier gas with nickel powder were recorded in 400-450 nm wavelength interval. The effect of carrier gas and powder loading on the excitation temperature and electron number density of the plasma jet were determined using atomic Boltzmann plot method and stark broadening of the Ar I (430.010 nm) line respectively. The experiment was done at 6.5, 7.9, 11.4 and 12.1 kW input power levels. Argon was used as plasma gas and also carrier gas. Nickel powders in the size ranging from 40 to 100 μm were processed. On introduction of carrier gas and nickel powder loading, the excitation temperature and electron density of the jet were found to decrease. From the results, the degree of ionization of the plasma jet was calculated by using the measured excitation temperature and electron density values. The spheroidzed nickel powder was characterized by SEM, optical photographs and XRD.     

Multiplexed continuous-wave diode-laser cavity ringdown measurements of multiple species

Rapid cavity ringdown measurements of multiple broadband absorbing species (methanol and isopropanol) in gas mixtures have been recorded with two multiplexed continuous-wave distributed-feedback diode lasers operating near 1.4 mum. A measurement sensitivity of 2.4 x 10(-9) cm(-1) for a 4.3-s averaging time was achieved in a 39.5-cm-long static cell with 99.94% reflectivity mirrors. This corresponds to a water-vapor detection limit of less than 2 ppb (parts in 10(9)) for the strong H(2)O lines near 1.4 mum. The shot-to-shot noise of the decay time constant tau was approximately 0.3-0.7%, and ringdown acquisition rates as great as 900 Hz were achieved.     

Intensity ratios of H lines: departures from the ideal conditions in the range of laser-induced breakdown spectroscopy experiments

In the present paper we analyze the behavior of H line intensity ratios with electron density and electron temperature in intermediate-density plasmas. We analyze the influence on the line intensity ratios of (1) the departures from local thermodynamic equilibrium (LTE) of the level population ratios, (2) the plasma opacity, and (3) the lowering of the ionization potential. We look, particularly, at the lines H(alpha), H(beta), H(gamma), and H(delta) and the energy levels involved in the corresponding atomic transitions for their use as diagnostics in laser-induced breakdown spectroscopy (LIBS) experiments. One important conclusion is that, for typical values of the plasma dimension and the electron temperature taking place in LIBS, i.e., L = 1 mm and T(e) = 10 000 K, respectively, the intensity ratios H(beta)/H(alpha), H(gamma)/H(alpha), and H(delta)/H(alpha) depart from the ideal values by less than 10% in the interval 0.65 x 10(14) part/cm(3) 相似文献   

Laser-induced carbon plasma emission spectroscopic measurements on solid targets and in gas-phase optical breakdown

We report measurements of time- and spatially averaged spontaneous-emission spectra following laser-induced breakdown on a solid graphite/ambient gas interface and on solid graphite in vacuum, and also emission spectra from gas-phase optical breakdown in allene C3H4 and helium, and in CO2 and helium mixtures. These emission spectra were dominated by CII (singly ionized carbon), CIII (doubly ionized carbon), hydrogen Balmer beta (Hbeta), and Swan C2 band features. Using the local thermodynamic equilibrium and thin plasma assumptions, we derived electron number density and electron temperature estimates. The former was in the 10(16) cm(-3) range, while the latter was found to be near 20000 K. In addition, the vibration-rotation temperature of the Swan bands of the C2 radical was determined to be between 4500 and 7000 K, using an exact theoretical model for simulating diatomic emission spectra. This temperature range is probably caused by the spatial inhomogeneity of the laser-induced plasma plume. Differences are pointed out in the role of ambient CO2 in a solid graphite target and in gas-phase breakdown plasma.     

OH absorption spectroscopy in a flame using spatial heterodyne spectroscopy

We demonstrate measurements of OH absorption spectra in the post-flame zone of a McKenna burner using spatial heterodyne spectroscopy (SHS). SHS permits high-resolution, high-throughput measurements. In this case the spectra span approximately 308-310 nm with a resolution of 0.03 nm, even though an extended source (extent of approximately 2x10(-7) m(2) rad(2)) was used. The high spectral resolution is important for interpreting spectra when multiple absorbers are present for inferring accurate gas temperatures from measured spectra and for monitoring weak absorbers. The present measurement paves the way for absorption spectroscopy by SHS in practical combustion devices, such as reciprocating and gas-turbine engines.     

Cavity ringdown spectroscopic detection of nitric oxide with a continuous-wave quantum-cascade laser

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N(2) with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath.     

Diode-Laser Wavelength-Modulation Absorption Spectroscopy for Quantitative in situ Measurements of Temperature and OH Radical Concentration in Combustion Gases

The in situ quantitative profiles of temperature and OH radical concentration in a postflame region of methane-air premixed counterflow flames were measured by wavelength modulation spectroscopy with a 1.5-mum external cavity diode laser. The second harmonic (2f) signal was generated from absorption by overtone vibrational-rotational transitions of OH: the ?(3/2) (v?, v?) = (2, 0) P11.5e (nu(0) = 6421.35 cm(-1)) or the ?(3/2) (v?, v?) = (3, 1) P5.5f (nu(0) = 6434.61 cm(-1)) transitions. The absorption occurred in the postflame region between methane-air premixed twin flames stabilized in a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The temperature and OH concentration profiles at an equivalence ratio of phi = 0.85 were determined by least-squares fitting of theoretical 2f line shapes to the experimental counterparts and by calculation of the ratio of the line intensities of the two different OH transitions (two-line thermometry). The measured temperature and OH concentration profiles were cross checked by Rayleigh scattering thermometry, thermocouple measurements, and two-dimensional numerical prediction of premixed combustion by use of a detailed chemical kinetic mechanism. The measurements and the prediction showed reasonable agreement.     

Measurements of aluminum and hydrogen microplasma

We report time-averaged and time-resolved emission spectra subsequent to laser-induced optical breakdown of aluminum in laboratory air and in hydrogen gas. The microplasma generated by nominal 10 ns IR laser radiation shows Stark-broadened and shifted atomic lines. An analysis of the H(alpha) and H(beta) Balmer series lines and selected aluminum lines allows one to determine electron number density in the range of 0.01 - 10 x 10(18) cm(-3) early in the plasma decay. Atomic and molecular features are investigated for diagnostic applications in laser material processing.     

Fiber loop ringdown for physical sensor development: pressure sensor

A new method of developing optical fiber pressure sensors by use of a fiber loop ringdown scheme is described. The fiber loop ringdown system is characterized in terms of the ringdown baseline stability, fiber transmission loss, and fiber refractive index. The overall sensor performance is demonstrated by use of sensing forces applied to the sensor head. The current device can sense pressures in the range of 0 to 9.8 x 10(6) Pa, converted approximately from the applied forces. The sensor's linear response, repeatability, detection sensitivity, measuring dynamic range, and temperature tolerance are explored.     

Absorption cross-sections of the C-h overtone of volatile organic compounds: 2 methyl-1,3-butadiene (isoprene), 1,3-butadiene, and 2,3-dimethyl-1,3-butadiene

Many molecules or transient radicals have well-documented absorption cross-sections in the ultraviolet (UV) region, but their absorption cross-sections in the near-infrared (NIR) region are much less often known and are difficult to measure. We propose a method to determine the unknown NIR absorption cross-sections using the known absorption cross-sections in the UV region, in which single-path UV absorption spectroscopy and NIR continuous wave cavity ringdown spectroscopy (cw-CRDS) are employed in a cross-arm reaction chamber for simultaneous measurements. Without knowing the actual sample partial pressures (or concentrations), the NIR absorption cross-sections can be accurately determined through the two sets of measurements. The method is demonstrated by measuring the NIR absorption cross-section of the first overtone of the asymmetric C-H stretch of 2-methyl-1,3-butadiene (isoprene) (3.24 (+/-0.16) x 10(-22) cm(2) molecule(-1)) at 1651.52 nm using the known value of the absorption cross-section at 220 nm. The diode laser wavelength was calibrated by atmospheric cavity ringdown spectra of CH(4), CO(2), and H(2)O. By comparison with sample pressure measurements, this method can also be used as a pressure calibration means for the reaction chamber, and this has been demonstrated with two additional measurements of the absorption cross-sections of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene (2.50 (+/- 0.08) x 10(-22) and 2.82 (+/-0.16) x 10(-22) cm(2) molecule(-1), respectively) at 1651.52 nm. The applicability of the method to determining absorption cross-sections using the simultaneous measurements of cw-CRDS and single-path absorption spectroscopy is discussed.     

Fiber-optic cavity sensing of hydrogen diffusion

A novel type of fiber-optic cavity sensor for hydrogen diffusion into and out of fibers is presented. The sensor is an implementation of a cavity ringdown scheme in a silica-based single-mode fiber that has been exposed to gaseous hydrogen at normal pressure. The measured ringdown times during the H2 diffusion show good agreement with a theoretical diffusion model. This model allows the determination of the diffusion coefficient of hydrogen in silica, resulting in D = (3.02 +/- 0.07) x 10(-15) m2/s at 30 degrees C.     

Measurements of OH radical concentration in combustion environments by wavelength-modulation spectroscopy with a 1.55-microm distributed-feedback diode laser

Wavelength-modulation spectroscopy with a standard commercial 1.55-microm distributed-feedback diode laser was applied to in situ quantitative measurements of OH radical concentration in combustion environments. The second-harmonic (2f) signal was generated from absorption by the P11.5 (nu', nu") = (2, 0) overtone vibrational transition of OH at 6421.354 cm(-1). The absorption occurred in the postflame region of a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The postflame region lies between propane-air premixed twin flames stabilized in the Tsuji burner at various equivalence ratios (phi = 0.65-1.0). The OH concentrations were determined by least-squares fitting of theoretical f line shapes to the experimental counterparts. The measured OH concentrations were in general agreement with adiabatic chemical equilibrium predictions. The lower limit of OH detectivity by multiline deconvolution was limited by ubiquitous unidentified high-temperature H(2)O transitions.     

Cavity ringdown detection of losses in thin films in the telecommunication wavelength window

The method of cavity ringdown spectroscopy (when a tunable pulsed optical parametric oscillator was used) was extended for the loss evaluation in thin films (2-20-mum thickness). The technique was applied in two key telecommunication wavelength ranges of 1260-1330 and 1480-1650 nm. The measurement sensitivity was determined to be 50 ppm (5 x 10(-5)). The results for polymer films are in close correlation with conventional spectrophotometric data and propagation loss for planar waveguides. Films of greater thickness and better optical quality are expected to provide an even higher loss resolution.