UNISEL: Unipolar semiconductor lasers

• Research unit: EU RFP
• Project number: 97.0453-2

Key words (English)

Gas sensing; photoacoustic effect; Helmoltz resonator

Alternative project number (English)

EU project number: BRPR-CT-0557

Research programs (English)

EU-programme: 4. Frame Research Programme - 2.1 Industrial and materials technologies

Short description (English)

See abstract

Partners and International Organizations (English)

Thomson CSF, Tech. Universität Wien, Uni Paris Sud, Uni Neuchâtel, Mütek GmbH

Abstract (English)

A photoacoustic gas sensor exploiting a quantum cascade laser (QCL) as radiation source is demonstrated. Our approach was that of building a system suitable for gas sensing applications in industrial environments, therefore sensitivity yet robustness and low-cost were the main guidelines driving the development. To this end we decided to exploit the potentialities offered by photoacoustic spectroscopy which has emerged as a valid alternative to more conventional spectroscopic techniques like transmission or reflectance spectroscopy.
We designed a photoacoustic cell composed by two cascaded Helmholtz resonators. The first one presents a resonance at a relatively low frequency f0 = 400 Hz while the inner chamber, where the detection microphone is placed at one end, resonates at f1 = 1100 Hz. The first resonator behaves as a low-pass filter for the acoustic noise coming from the external environment, providing an effective way of reducing the noise level at the opening of the inner chamber. The QCL beam is focused on the opening of the first resonator and amplitude modulated with a frequency equal to the resonance frequency of the inner chamber. Gas molecules are therefore directly excited inside the first cell producing a pressure wave of frequency f1. In this way the acoustic filtering is bypassed by the photoacoustically generated sound wave. The latter finally excites the inner resonator and is detected by the microphone. The original design of the present photoacoustic cell fully exploits two important characteristics of the QCL, i.e. reduced spotsize and possibility of electrical amplitude modulation.
A detection limit of 50 ppm is found for our apparatus using a Peltier-cooled Fabry-Pérot InGaAs-based QCL emitting at 5.7 microns and acetone vapors as absorbing gas. This value largely underestimates the possibilities of the present detector, which we believe can be greatly improved (at least one order of magnitude) after a suitable optimization. To this extent studies are presently under way.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 97.0453-2