Wavelength modulation photoacoustic spectroscopy: Theoretical description and experimental results

A theoretical description of photoacoustic spectroscopy generated by wavelength modulation of a semiconductor laser source is reported for a Lorentzian absorption line. This model describes the first- and second-harmonic photoacoustic signals produced by a current-modulated semiconductor laser. Combined intensity- and wavelength-modulation is considered with arbitrary phase shift. Experimental results obtained when probing a CO2 absorption line with a 2-μm distributed feedback laser are presented and validate the relevance of the reported model. © 2005 Elsevier B.V. All rights reserved

Sub-ppm multi-gas photoacoustic sensor

A photoacoustic multi-gas sensor using tuneable laser diodes in the near-infrared region is reported. An optimized resonant configuration based on an acoustic longitudinal mode is described. Automatic tracking of the acoustic resonance frequency using a piezo-electric transducer and a servo electronics is demonstrated. Water vapour, methane and hydrogen chloride have been measured at sub-ppm level in different buffer gas mixtures. The importance of the system calibration in presence of several diluting gases is discussed. Finally, trace gas measurements have been assessed and detection limits (signal-to-noise ratio = 3) of 80 ppb at 1651.0 nm for CH4, 24 ppb at 1368.6 nm for H 2O and 30 ppb at 1737.9 for HCl have been demonstrated. © 2005 Elsevier B.V. All right reserved

Ethylene spectroscopy using a quasi-room-temperature quantum cascade laser

Spectroscopic measurements on ethylene were performed using a quasi-room-temperature quantum cascade (QC) laser operated in pulsed mode in the 10.3 μm range. Using transmission spectroscopy, a broadening of the ethylene absorption spectrum was observed with increasing laser pulse duration, due to an increase of the laser linewidth. This linewidth was determined from the measured absorption spectra, showing a value of 0.04 cm-1 for a 20 ns pulse duration and an enhancement coefficient of 6.5 × 10-3 cm -1 per ns in the 20-50 ns pulse length range. Photoacoustic (PA) detection of ethylene was also performed using the QC laser and a resonant PA cell, with a detection limit of 60 ppm. © 2002 Elsevier Science B.V. All rights reserved

Ammonia monitoring at trace level using photoacoustic spectroscopy in industrial and environmental applications

An ammonia traces analyser based on photoacoustic spectroscopy is described. The system uses a CO2 laser and a properly designed resonant photoacoustic cell to achieve ammonia detection at sub-parts-per- billion (ppb) level. The instrument features unattended automatic on-line monitoring of ammonia with a detection limit of 0.1 ppb. Interferences from atmospheric CO2 and H2O are efficiently suppressed by a careful selection of the laser wavelength and a compensation of the water vapour signal made with a high-precision hygrometer. The cell design enables continuous measurement at high flow rates (up to 51/min), which guarantees a fast response time of the system for the monitoring of ammonia, a sticky polar molecule that adheres to most surfaces. Various examples of applications of the instrument in the semiconductor industry and for atmospheric pollution monitoring are presented. They demonstrate the excellent performances of the system and its suitability for these applications. © 2004 Elsevier B.V. All rights reserved

All-fiber molecular frequency reference at 2 μm based on a versatile laser modulation sideband locking and a hollow-core fiber gas cell

Sensing of atmospheric trace gases is crucial for climate monitoring and to predict global climate changes. The required global coverage and spatial resolution have driven the studies of space-borne differential absorption lidar (DIAL) instruments to remotely monitor atmospheric gases from a satellite to ground. The performance of such instruments is notably determined by the frequency stability and accuracy of a low-power continuous-wave laser that seeds the pulsed laser transmitter. For a CO2 DIAL, this reference laser needs to be stabilized with an adjustable frequency-detuning from the center of the probed molecular transition and the 2.05-μm spectral range is of high interest from a spectroscopic point-of-view [1].We have developed an all-fiber modulation sideband locking set-up enabling a laser to be locked at a controlled frequency detuning from the center of the CO2 R(30) transition at 2050.97 nm, selected for DIAL applications. The offset frequency can be directly tuned over a span ranging from some hundred MHz up to at least 3 GHz, which is the typical requirement for a space-borne CO2 DIAL. The method is depicted in Fig. 1a. It consists of a distributed feedback (DFB) laser, followed by an intensity electro-optic modulator (EOM) driven by a radio-frequency signal at fEOM provided by an amplified voltage-controlled oscillator (VCO). The EOM generates a pair of sidebands shifted by ±fEOM that are coupled into a reference gas cell. The sidebands are dithered by modulating the VCO at a frequency fm 40 kHz to implement wavelength modulation spectroscopy (WMS). An error signal is produced by demodulating the reference cell transmission signal to servo-lock one of the sidebands at the center of the transition. As a result, the unmodulated laser carrier is detuned from the transition linecenter by the frequency offset fEOM, which can be easily varied, thus making the system versatile

Application of antimonide diode lasers in photoacoustic spectroscopy

First investigations of photoacoustic (PA) spectroscopy (PAS) of methane using an antimonide semiconductor laser are reported. The laser fabrication is made in two steps. The structure is firstly grown by molecular beam epitaxy, then a metallic distributed-feedback (DFB) grating is processed. The laser operates at 2371.6 nm in continuous wave and at room temperature. It demonstrates single-mode emission with typical tuning coefficients of 0.04 nm mA-1 and 0.2 nmK-1. PA detection of methane was performed by coupling this laser into a radial PA cell. A detection limit of 20 ppm has been achieved in a preliminary configuration that was not optimised for the laser characteristics. © 2004 Elsevier B.V. All rights reserved