Biomedical subjects

Luc Thévenaz

Publications and source records attributed to Luc Thévenaz.

11 recordsLinked to original sources

PubMed2006

Time biasing due to the slow-light effect in distributed fiber-optic Brillouin sensors.

The influence of the slow-light effect on the performance of distributed Brillouin sensors is studied. We show that, while in most situations it can be neglected, it may greatly affect the results obtained for certain experimental configurations. More specifically, for one of the experimental arrangements described in the literature (a strong continuous-wave pump and a weak pulsed probe) we show that this effect induces a large time biasing of the traces that depends not only on the fiber length but also on the frequency separation between pump and probe. This biasing reduces the available resolution in this experimental arrangement.

Journal Article↗

PubMed2006

Novel Helmholtz-based photoacoustic sensor for trace gas detection at ppm level using GaInAsSb/GaAlAsSb DFB lasers.

A new and compact photoacoustic sensor for trace gas detection in the 2-2.5 microm atmospheric window is reported. Both the development of antimonide-based DFB lasers with singlemode emission in this spectral range and a novel design of photoacoustic cell adapted to the characteristics of these lasers are discussed. The laser fabrication was made in two steps. The structure was firstly grown by molecular beam epitaxy then a metallic DFB grating was processed. The photoacoustic cell is based on a Helmholtz resonator that was designed in order to fully benefit from the highly divergent emission of the antimonide laser. An optimized modulation scheme based on wavelength modulation of the laser source combined with second harmonic detection has been implemented for efficient suppression of wall noise. Using a 2211 nm laser, sub-ppm detection limit has been demonstrated for ammonia.

Acoustics↗

PubMed2006

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 CH(4), 24 ppb at 1368.6 nm for H(2)O and 30 ppb at 1737.9 for HCl have been demonstrated.

Chemistry Techniques, Analytical↗

PubMed2005

Long optically controlled delays in optical fibers.

Optically controlled delay lines in optical fibers are demonstrated by use of the group-velocity control of signal pulses based on stimulated Brillouin scattering. We achieve continuous time delay within the range of 150 ns, much larger than the width of the 40 ns signal pulse, using cascaded fiber segments joined by unidirectional optical attenuators. In the meantime, we also observe a large amount of pulse broadening, which agrees well with a theoretical prediction based on linear theory.

Journal Article↗

PubMed2004

Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation.

An experimental method is presented for characterization of the combined intensity and frequency modulation produced when the injection current of a laser diode is modulated. The reported technique is based on the analysis of the harmonic signals produced when a modulated laser is used to probe a gas absorption line by the so-called wavelength-modulation spectroscopy method. Based on a theoretical model of this technique, we present two methods that facilitate the determination of (i) the deviation in laser frequency and (ii) the phase shift between intensity and frequency modulation. These methods are illustrated experimentally by measurement of the modulation parameters of a 2-microm distributed-feedback laser by use of a CO2 absorption line. The experimental results have been compared with those obtained with another traditional method and have shown full agreement in the frequency range (400 Hz-30 kHz) considered.

Journal Article↗

PubMed2004

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 CO(2) 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 CO(2) and H(2)O 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 5 l/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.

Air↗

PubMed2004

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 nm K(-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.

Antimony↗

PubMed2004

Multi-gas sensing based on photoacoustic spectroscopy using tunable laser diodes.

Multi-hydrogenated compounds detection based on photoacoustic (PA) spectroscopy is reported. Three near-infrared semiconductor lasers are used with a resonant PA cell operated in its first longitudinal mode to monitor methane, water vapour and hydrogen chloride in the parts per million range. The design of our cell results from simulations performed in order to optimise its performances. Influence of the buffer gas on the PA signal has also been analysed, both theoretically and experimentally. A reduction of the PA signal of almost one order of magnitude has been observed between N(2) and He, which demonstrates the importance of the buffer gas in PA spectroscopy. Finally, detection limits of 0.5 ppm of CH(4) and 3 ppm of HCl has been achieved experimentally in nitrogen and an H(2)O sensitivity of 0.2 ppm has been estimated.

Chemistry Techniques, Analytical↗

PubMed2003

Wavelength modulation spectroscopy: combined frequency and intensity laser modulation.

A theoretical model of wavelength modulation spectroscopy that uses a laser diode on a Lorentzian absorption line is presented. This theory describes the general case of a current-modulated semiconductor laser, for which a combined intensity and frequency modulation with an arbitrary phase shift occurs. On the basis of this model, the effect of several modulation parameters on the detected signals is evaluated. Experimental signals measured on an absorption line of CO2 by use of a 2-microm distributed-feedback laser are also presented and validate this analysis. These experimental results agree with the calculated signals, confirming the relevance of the model.

Journal Article↗

PubMed2003

Distributed measurement of chromatic dispersion by four-wave mixing and Brillouin optical-time-domain analysis.

A new nondestructive method for measuring the spatial distribution of chromatic dispersion along an optical fiber is presented. It is based on using Brillouin optical time-domain analysis to probe the power distribution of the four-wave mixing generated by two continuous-wave lasers. The results obtained prove that this new method is capable of providing better performance than comparable techniques. Furthermore, sensing the variations of Brillouin gain maximum produces additional information about the fiber, such as presence of strain and concentration of GeO2.

Journal Article↗

PubMed2002

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 microm 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 x 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.

Ethylenes↗