New method of elaboration of the lidar signal

In lidar measurements noise and fluctuations strongly affect the results. The reason is a rapid decrease of the signal-to-noise ratio with an increase of distance. The differential absorption lidar (DIAL) is particularly sensitive to the signal instabilities. In this paper we present a method of the signal acquisition that is suitable for registration of both large light fluxes and single photons. We also present new method of solution of the DIAL equations. Compared to the traditional algorithm used for signal elaboration our procedures are much more stable and they are able to increase the effective range of lidar measurements. Received: 2 February 1999 / Revised version: 30 June 1999 / Published online: 16 September 1999     

Dependence of the lidar signal depolarization on the receiver’s field of view in the sounding of fog and clouds

We report an experimental study of the lidar signal depolarization as a function of the relative contribution of the multiple scattering in case of optically dense objects in the atmospheric planetary boundary layer. Results of the observation of fog and stratus clouds are presented, as well as those obtained by sounding of stratocumulus clouds during a snowfall. The lidar data point to a rise of the depolarization coefficient as the influence of the multiple scattering increases in consequence of both viewing angle enlargement and penetration into the object sounded. The variations of the depolarization coefficient are studied as a function of the field of view. In the case of fog, this dependence is approximated by a three-parameter exponential law; it is found that the depolarization increases steeply when the viewing angle is increased from 9 mrad to 12.5 mrad. The relationships between the approximation parameters and the microphysical characteristics of the scattering medium are considered. The experimentally determined size of the area where multiple scattering occurs is in good agreement with that calculated according to the diffusion model. The results obtained on the multiple scattering effect on the depolarization can also be employed in determining the extinction coefficient profiles in optically dense objects, as well as in evaluating the characteristic size of the scattering particles. Received: 6 September 1999 / Revised version: 7 February 2000 / Published online: 6 September 2000     

Experimental determination of the multiple-scattering effect on the lidar-signal polarization characteristics during liquid- and solid-phase precipitation

We present results of experimental investigations of the signal-polarization characteristics in the case of lidar sounding during precipitation. We show and discuss the lidar signals and the depolarization profiles along the sounding path for liquid- and solid-phase precipitation. In the former case we compare the signal characteristics at different degrees of precipitation rate. In the latter situation, we consider snowfall with particle shape close to that of Chebyshev particles. We also follow the lidar-signal changes depending on the field-of-view of the receiving optics. The experimental data are compared with results of theoretical estimates and models concerning the optical and microphysical characteristics of the rain and snow particles. In the case of liquid-phase precipitation – rain – the observed dependence of the lidar’s signal-polarization structure on the precipitation intensity has two aspects: on the one hand, the change of the raindrops’ shape, and, on the other, the multiple-scattering effects. The lidar data demonstrate that the signal depolarization, and, more specifically, its behavior along the sounding path, can be used as a criterion for the presence of multiple scattering. In the case of a snowfall consisting of Chebyshev particles, the simultaneous role is evident of two factors influencing the lidar-signal depolarization, namely, the non-spherical shape of the particles and the multiple-scattering effects. When the scattering takes place off particles with a large size and a shape strongly differing from spherical, we observed the predominant role of the non-sphericity of the scattering centers in the signal depolarization. Received: 6 December 2000 / Revised version: 11 July 2001 / Published online: 19 September 2001     

Cavity ring-down spectroscopy for atmospheric trace gas detection: application to the nitrate radical (NO3)

Cavity ring-down spectroscopy is a relatively new and quite sensitive technique for the measurement of gas-phase optical extinction. It holds the potential for simple, direct and sensitive measurement of the concentrations of a variety of trace gases in the atmosphere. For example, detection of the nitrate radical, NO₃, and its companion, dinitrogen pentoxide, N₂O₅, has been demonstrated with a sensitivity of 0.25 pptv (1σ). This paper considers several of the requirements for the application of cavity ring-down spectroscopy to concentration measurements of trace gases in ambient air. These include detection sensitivity, measurement of an accurate zero in the presence of competing absorbers, cavity stability and mirror cleanliness, laser line-width effects, saturation effects, Rayleigh scattering, the influence of atmospheric aerosols and sampling issues for reactive species. Examples drawn from our work on NO₃ and N₂O₅ detection in the field illustrate these considerations. Received: 1 April 2002 / Revised version: 5 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-5822, E-mail: sbrown@al.noaa.gov     

Estimation of required parameters for detection of small smoke plumes by lidar at 1.54 μm

Parameters of eyesafe lidar at 1.54 μm for detection of small plumes of smoke from burning wood or oil have been evaluated. It was assumed that a diode-pumped solid-state Er:glass laser at 1.54 μm or a Nd:YAG laser with a Raman cell or optical-parametric oscillator is used as a light source and that detection of backscattered light is performed with an avalanche photodiode. Ash and soot particle size distributions were taken from experiments. A backscattering coefficient at 1.54 μm for various source of smoke was estimated. In computing the laser energy, range between lidar and smoke, receiver optics diameter, fuel mass burned in unit time, fire source radius, laser pulse duration and visibility were varied. Results of the computations enabled estimation of the required laser energy, which ranges from 0.05 to 1400 mJ depending on the parameters. Received: 5 January 2000 / Revised version: 3 March 2000 / Published online: 11 May 2000     

Deconvolution filtering of ground-based LIDAR returns from tropospheric aerosols

A deconvolution filtering model of multiple scattering in ground-based single field of view (SFOV) LIDAR returns is described. It is based on time series deconvolution techniques. The contribution of multiply scattered photons in SFOV LIDAR returns can be numerically modeled by processing LIDAR signals without additional information about aerosol properties and measurement geometry. Deconvolution results are in good agreement with those performed by Monte Carlo calculations, showing that the significance of multiply scattered photons is strongly correlated with aerosol concentration. It is found that, for ground-based LIDAR, the contribution of multiply scattered photons to LIDAR signals is typically below 5% in a clear urban atmosphere, and up to 14% in a very dirty urban atmosphere in Hong Kong during winter seasons. Received: 8 October 2002 / Revised version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/51-8229, E-mail: gao@proteo.gr.jp RID="**" ID="**"Present address: Preteomics Lab., Amakubo 1-16-1, Tsukuba 305-0005, Japan     

Plasma-based nitrogen tail pulse shutter for CO2-TEA lidar dial systems

This paper presents the construction, use and characterisation of a laser-induced sealed plasma shutter to clip off the nitrogen pulse tail of a CO₂-TEA laser-based lidar dial system. Investigation of the optimum gas filling pressure, temporal profile of the clipped pulse, and the laser threshold power intensities to achieve ionisation growth and breakdown in helium, argon, and nitrogen are also presented. Values of these power density thresholds lie between 3×10¹¹ W cm^-2–5×10¹² W cm^-2, 2×10¹¹ W cm^-2–2×10¹² W cm^-2 and 3×10¹³ W cm^-2–2×10¹⁴ W cm^-2 for helium, argon, and nitrogen, respectively. The range resolution attainable with the present clipped pulses is 15 m, which is 30 times better than that readily obtained with the nitrogen-tailed pulses. Field measurements of the lidar returns with the clipped pulse from a co-operative target are presented. Received: 27 December 1999 / Revised version: 11 February 2000 / Published online: 27 April 2000     

The LITE correlative measurements campaign in southern Italy: preliminary results

A,355 and R_A,532); the two data sets appearing to be highly correlated. LITE vs. Potenza LIDAR measurements of R_A,355 and R_A,532 display a correlation coefficient of 0.72 and 0.86, respectively. Stratospheric aerosol dimensional characteristics are determined starting from the measured values of the ?ngstrom coefficient. Received: 24 May 1996/Revised version: 8 October 1996     

Application of lidar in ultraviolet,visible and infrared ranges for early forest fire detection

The efficiencies of direct lidars operating at 355, 532, 1064 and 1540-nm radiation wavelengths for early forest fire detection were compared. For each wavelength, the range for reliable smoke-plume detection was estimated on the basis of a computer simulation plume using a one-dimensional “top-hat” gas dynamic model for the calculation of the backscattering and extinction-coefficient profiles within the plume. The agreement between the predicted signal-to-noise ratio (SNR) and experimental results for 532 and 1064-nm wavelength radiation is good. The decrease of the signal-to-noise ratio with distance is maximum for 355 nm and minimum for 1064 nm. At 1540 nm, the decay of SNR with distance is slightly faster, but the SNR is higher than for other wavelengths, leading to the highest detection efficiency for the same energy of the probing laser pulse. For a burning rate of 2 kg/s and a laser beam divergence of 2.5 mr, the maximum distance for reliable detection varies between 6 and 12 km, depending on the wavelength. Received: 15 May 2002 / Revised version: 13 September 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +351-21/841-8120, E-mail: rui.vilar@ist.utl.pt     

Analysis of lidar measurements using nonparametric kernel regression methods

The lidar technique is an efficient tool for remote monitoring of the distribution of a number of atmospheric species. We study measurements of sulphur dioxide emitted from the Italian volcano Mt. Etna. This study is focused on the treatment of data and on the procedure to evaluate range-resolved concentrations. In order to make an in-depth analysis, the lidar system was prepared to store measurements of individual backscattered laser pulses. Utilizing these repeated measurements a comparison of three different methods to average the returned signals is made. In the evaluation process we use local polynomial regression to estimate the range-resolved concentrations. Here we calculate optimal bandwidths based on the empirical-bias bandwidth selector. We also compare two different variance estimators for the path-integrated curves: local polynomial variance estimation and variance estimation based on Taylor approximations. Results show that the method performs well. An advantage compared to previous methods for evaluation of lidar measurements is that an estimate of the mean squared error of the estimated concentration can be calculated. Received: 9 July 2001 / Revised version: 15 November 2001 / Published online: 17 January 2002     

A LIDAR technique to measure the filament length generated by a high-peak power femtosecond laser pulse in air

We demonstrate a new method for measuring the length of a femtosecond laser pulse induced filament in air using a LIDAR (LIght Detection And Ranging) technique. The LIDAR involves a detector with a fast response time. The back-scattered multiphoton induced fluorescence from nitrogen molecules excited inside the filament is measured, from which the length of the filament can be determined. We find good agreement between the measured filament length and the length estimated from burn patterns on paper. In addition, good qualitative agreement between the experimental measurement and numerical simulations is obtained for the signal features of the filament. We propose that this new method can be used to quantitatively determine filamentation at longer distances. Received: 11 June 2002 / Revised version: 21 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: aiwasaki@phy.ulaval.ca     

Detection of small forest fires by lidar

The possibility of detecting small forest fires with the help of a simple and cheap lidar operating at 0.532-μm wavelength up to distances of about 6.5 km is demonstrated. The values of the signal-to-noise ratio (SNR) achieved in the experiments are consistent with theoretical estimations obtained by computational modeling of the lidar detection process, including simulation of the smoke-plume shape and of the laser beam–plume interaction. This model was used to assess the potential of the lidar technique for fire surveillance in large forest areas. In particular, the upper limiting range for effective detection (SNR>5) of small localized fires in dry- and clear-weather conditions is estimated at 7–15 km depending on operation mode, burning rate, and observation geometry. Received: 29 August 2001 / Published online: 29 November 2001     

Measurement of filament length generated by an intense femtosecond laser pulse using electromagnetic radiation detection

We present a new method to measure the length of a filament induced by the propagation of intense femtosecond laser pulses in air. We used an antenna to detect electromagnetic pulses radiated from multipole moments inside the filament oscillating at the plasma frequency. The results are compared with the values detected from the backscattered fluorescence induced by multiphoton ionization of nitrogen molecules excited inside the filament. The values are found to be in good agreement. Received: 6 November 2002 / Revised version: 27 January 2003 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: shosseini@phy.ulaval.ca     

Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser

Cavity-enhanced absorption spectroscopy is explained in terms of the transmission function of a rapidly swept interferometer, and the integrated transmission is shown to be proportional to the cavity ringdown time. The technique is demonstrated on the b¹Σ_g ⁺-X³Σ_g ^-  (1,0) band in molecular oxygen at 687 nm using a tunable diode laser and a relative-ly high-Q optical cavity (finesse ≈4000). A detection limit of 3×10^-8 cm^-1 s^1/2 is achieved for a 0.8 cm^-1 scanning range. Received: 24 June 2002 / Revised version: 5 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: peverall@physchem.ox.ac.uk     

Applications of Kalman filtering to real-time trace gas concentration measurements

A Kalman filtering technique is applied to the simultaneous detection of NH₃ and CO₂ with a diode-laser-based sensor operating at 1.53 μm. This technique is developed for improving the sensitivity and precision of trace gas concentration levels based on direct overtone laser absorption spectroscopy in the presence of various sensor noise sources. Filter performance is demonstrated to be adaptive to real-time noise and data statistics. Additionally, filter operation is successfully performed with dynamic ranges differing by three orders of magnitude. Details of Kalman filter theory applied to the acquired spectroscopic data are discussed. The effectiveness of this technique is evaluated by performing NH₃ and CO₂ concentration measurements and utilizing it to monitor varying ammonia and carbon dioxide levels in a bioreactor for water reprocessing, located at the NASA–Johnson Space Center. Results indicate a sensitivity enhancement of six times, in terms of improved minimum detectable absorption by the gas sensor. Received: 13 July 2001 / Revised version: 11 October 2001 / Published online: 29 November 2001     

Study of atmospheric water in gaseous and. liquid state by using combined elastic–Raman. depolarization lidar

A combined elastic–Raman lidar system based on a tripled Nd:YAG laser is used for the separate detection of elastic backscatter and Raman signals from atmospheric nitrogen, water vapor and liquid water and for their depolarization measurement. Vertical profiles of water-vapor and liquid-water content measured under clear-sky conditions behave differently: inside the boundary layer the ratio of liquid-water to water-vapor Raman backscatters rises with altitude. The depolarization measurements bring additional information about atmospheric scattering. The observed depolarization ratio of the water-vapor Raman signal is about 14%, while for liquid water this ratio varies in the 30–75% range, which exceeds the depolarization of bulk water and is attributed to the water-aerosol effects. Raman contours of water vapor and liquid water are partially overlapped, and bleed-through of liquid-water Raman backscatter leads to enhancement of depolarization of the water-vapor Raman signal. This parameter may be used as a convenient indicator of liquid-water interference in water-vapor measurements. Received: 12 December 2000 / Revised version: 27 September 2001 / Published online: 7 November 2001     

Franck--Condon factors and r-Centroids for the A 1∑+u-X1∑+g Band System of 107,109Ag2: Comparison of the Observed and Calculated Absorption Band Strengths

Franck-Condon factors and r-centrolds for the, A^1 ∑^＋ u-X^1∑^＋ 9 band system of ^107,109Ag2 are computed using Morse and Rydberg-Klein-Rees potentials for both lower and upper electronic states. The differences between the two sets of results are typically in the third decimal place for transitions involving vibrational levels with ν＇ and ν＂ up to about 15. Somewhat larger deviations appear for higher vibrational levels, but both sets of results follow the same pattern, which is to match well with the relative absorption band strength distribution in our experimental spectrum. The relative absorption band strengths are calculated by assuming that the electronic transition moment has only a weak dependence on the internuclear distance r. Good agreement between our measured and calculated absorption band strength ratios is found, which provides an excellent test of the calculated Franck- Condon factors and relative absorption band strengths. The r-centrold value for the （ν＇ = 0, ν＂ = 0） band is found to be approximately equal to the average value of r＇ and r＇＇e , indicating that the potentials of both states are not significantly aaharmonic around their minimum regions.     

Pulsed cavity ring-down spectroscopy of NO and NO2 in the exhaust of a diesel engine

The application of pulsed cavity ring-down spectroscopy has been demonstrated for the in situ quantitative determination of NO and NO₂ in the exhaust of a diesel engine. NO absorption has been monitored at the transition from the Χ²Π ground state to the A²Σ⁺ state at 226 nm. For NO₂, absorption bands in the spectral region from 438 nm to 450 nm were used. At the selected engine conditions, concentrations of 212±22 ppm and 29±4 ppm have been measured for NO and NO₂, respectively, in good agreement with separate chemical exhaust gas analysis. The method is sensitive enough to meet the European Euro V standard directive on NO_x emissions. This communication discusses the relatively simple setup needed for this type of measurement, the problems encountered, as well as the prospects for single-stroke, simultaneous measurements of both NO and NO₂ at the sub-ppm level. Received: 30 November 2001 / Revised version: 18 February 2002 / Published online: 14 March 2002     

The application of quasi-phase-matched parametric light sources to practical infrared chemical sensing systems

Quasi-phase-matched (QPM) materials allow the generation of spectroscopically useful infrared radiation in an efficient and broadly tunable format. Here, we describe several applications of QPM-based light sources to remote and local chemical sensing. The remote systems are gas imagers that employ a fiber-pumped continuous-wave optical parametric oscillator or a microlaser-pumped, diode-seeded optical parametric amplifier as the illumination source. Technology described for local sensing includes a cavity ring down spectrometer that employs a novel optical parametric generator–amplifier to achieve ≥350 cm^-1 of contiguous tuning and a long-wave infrared light source based on QPM GaAs. In each case the use of QPM materials in conjunction with effective pump sources instills simplicity and ruggedness into the sensing systems. Received: 15 April 2002 / Revised version: 6 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/294-2595, E-mail: tjkulp@sandia.gov RID="**" ID="**"Present address: Corning Inc., Corning, NY 14831, USA RID="*" ID="*"Present address: Corning Inc., Corning, NY 14831, USA RID="**" ID="**"Present address: Blue Leaf Networks, Sunnyvale, CA 94086, USA RID="***" ID="***"Present address: Sandia National Laboratories, Albuquerque, NM 87185, USA     

Sum frequency generation at 309 nm using a violet and a near-IR DFB diode laser for detection of OH

We have used a violet diode laser at 404 nm and a distributed feedback diode laser at 1320 nm to produce 0.8 nW of radiation at 309 nm by sum frequency generation in beta-barium borate. The UV radiation was tuned mode-hop-free over 30 GHz and used to detect OH radicals produced in a microwave discharge. By chopping the UV light at 500 Hz, we observed a concentration of 2×10¹² cm^-3 with a signal to noise ratio of 30:1. Received: 16 November 2001 / Revised version: 23 January 2002 / Published online: 14 March 2002