Gas traces measurement by photoacoustic spectroscopy using Helmholtz resonator-based sensors

Photoacoustic spectroscopy is a well-established gas traces optical detection technique, which consists in the generation of an acoustic wave in the investigated gas compound excited by a modulated laser beam, and in the detection of this sound wave with a sensitive microphone. The sensitivity of this technique can be greatly enhanced through the use of acoustic resonators. A wide range of resonant configurations has been developed and reported in the literature in the last decades. Among these, Helmholtz resonators are known for their simplicity of implementation, though with a reduced efficiency. The main goal of this work is to demonstrate that Helmholtz resonators can be successfully applied to photoacoustic spectroscopy, delivering sensitivities in the same order of magnitude as other more common configurations, as well as offering a powerful tool to perform differential measurements. Two Helmholtz-based sensors are presented within this thesis. The first sensor has been developed for ammonia sensing, taking benefit from the properties of antimonide-based lasers; the final design has been the result of a carefully optimised compromise between high sensitivity, noise reduction, technical constraints, compact size and straightforward use. After several implemented improvements, an ultimate sub-ppm concentration detection limit has been achieved. The second sensor exploits the intrinsic phase shift existing between the two volumes of a Helmholtz resonator to perform differential measurements. Each of the two volumes of the resonator is filled with a different concentration of the same gas, whereas the exciting laser beam is split in two arms that separately illuminate the volumes, the resulting photoacoustic signal being proportional to the difference between the respective concentrations of the probed gas in the volumes. A particular detection scheme has been implemented to guarantee a linear measurement. The achieved sensitivity is not as high as obtained with the first sensor; nonetheless, the developed sensor offers the possibility to perform measurements that would otherwise require two different sensing devices, resulting in a clear gain in terms of cost and of detection complexity.

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

Mario Mattiello, Luc Thévenaz

A new and compact photoacoustic sensor for trace gas detection in the 2-2.5 μm 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. © 2005 Elsevier B.V. All right reserved.

Elsevier2006

Advanced trends in nonlinear optics applied to distributed optical-fibre sensors

Dario Alasia

The distributed optical-fibre sensors based on the properties of Brillouin scattering is the central object of this thesis. In the past decade, optical fibres have gained a large interest as sensors: attractive solutions based on the non-linear stimulated Brillouin scattering have been proposed in the early 90s and the possibility to achieve long-range fully distributed strain measurements has been extensively demonstrated. The Brillouin interaction is responsible for the coupling between two optical waves and an acoustic wave when a resonance condition is fulfilled. Since the resonance condition is strain and temperaturedependent, by determining the resonance frequency we directly get a measure of temperature or strain. Local information about the acousto-optical resonance condition is typically obtained by using pulsed lightwaves and a classical time-of-flight technique (BOTDA technique). The main goal of this work has been the development of an innovative technique for the generation of optical signals, using a set of locked lasers – instead of the traditional techniques using external modulators. The utilisation of the injection-locking of semiconductor lasers is the key of the entire set-up and represents an entirely new and original approach, since it brings significant improvements in terms of SNR and costs. As long as intense pulses propagate along the fibre, the optical signals can be seriously degraded by several nonlinear interactions occurring inside the fibre; we show that the nonlinear effect exhibiting the lowest threshold power is the modulation instability (MI) process. From the study of the dynamic behaviour of MI we could observe the Fermi-Pasta-Ulam (FPU) recurrence over few periods in very comfortable conditions. One original application of Brillouin sensing has been the dosimetric measurement of ionising radiations in a nuclear environment. The measurement campaign has not only shown that distributed sensors based on Brillouin spectral analysis are radiation tolerant up to very high doses, but has also revealed the first observation – to our knowledge – of the negative compaction of silica in fibres. Distributed fibre sensors based on stimulated Brillouin scattering offer a unique capability for the analysis of optical signals and nonlinear phenomena in optical fibres. We present a generalised theoretical approach to the problem of localised sensing and report on the first distributed measurement – to our knowledge – of the parametric gain in a single-pump fibre-optics parametric amplifier (FOPA).

EPFL2006

Advanced trends in nonlinear optics applied to distributed optical-fibre sensors

Dario Alasia

EPFL2006