MULTI-gas and multi-source Photo Acoustic Spectroscopy Measurement Platform – MULTIPAS-2

The MULTIPAS project proposes to develop a gas sensor dedicated to environmental measurements. It focuses on the detection of several species of environmental interest linked to problems of ambient air pollution, indoor or outdoor. Mention may thus be made of CO, CO2, CH4, NO, but also NH3, NO2 or SO2.

The developed sensor uses infrared emitting lasers whose light can be absorbed by a target gas. It is made up of two parts: the laser itself and the detection system. The detection is a photo-acoustic detection made with a very inexpensive commercial quartz tuning fork. Depending on the target species, the lasers used will be semiconductor lasers emitting in a wavelength range of 2 to 15 µm. By tuning the laser in wavelength, one or more gas absorption lines are scanned. The light is absorbed and causes local heating, which in turn gives rise to a sound wave. It is this sound wave that is detected by the tuning fork. Taking into account the specificities of lasers, the technique is selective and sensitive. It makes it possible to measure gas concentrations much lower than ppmv (part per million by volume).


Infrared photoacoustic spectroscopy is based on light sources whose characteristics are very specific to their use. It is necessary to have sources of very good spectral quality, single-frequency, powerful and tunable. The MULTIPAS project therefore relies on the MIRSENSE company and the IES laboratory to study and produce quantum cascade laser sources making it possible to reach spectral ranges of environmental interest. While quantum cascade lasers (QCL) have been known since the end of the 1990s, only a few material families are experiencing real technological maturity. Within the framework of MULTIPAS, two material routes are considered for QCLs: InAs/AlSb/InAs for wavelengths higher than 11 µm and GaInAs/AlInAs/InP below, up to 5 µm.

The work in this field will focus on the realization of good quality sources, preferably operating in continuous wave regime. These sources can then be coupled to multiplexers to benefit from the flexibility that would be offered by a chip containing several potential emission wavelengths. Efforts are therefore also focused on this possibility, offering prospects for SiGe coupling.

QEPAS (Quartz Enhanced Photoacoustic Spectroscopy) detection comes in several configurations, all of which offer certain advantages. The IES and the LPCA decided to work on different configurations to compare the performances and explore their possibilities. By exploring these possibilities, the MULTIPAS project made it possible to propose various studies on optical cavities, which have been the subject of publications. In parallel with these optical and opto-acoustic studies, a development work was carried out in collaboration with the company VALOTEC, to perfect an electronic card for controlling sources, modulation and detection, and data processing.

From a scientific point of view, several publications confirm the scientific production from the MULTIPAS project.
We have thus learned to better understand the QEPAS environment and the design of acoustic cavities coupled to QTF in order to improve the detection performance. We have developed a technique for real-time measurement of the physical parameters of the QTF, knowledge of which enables the measurements and processing of QEPAS measurements to be optimized.
The use of QCL sources has produced results with very good detection limits, on several gases and in environmental and other applications. A compact sensor has been proposed, employing a lower wavelength commercial laser and achieving detection limits for three gas species compatible with the requirements set out by ENVEA.

The project enabled ENVEA to discover QEPAS detection. ENVEA is interested in this technique and may extend its collaboration with IES on this type of sensor.
MIRSENSE has joined forces with IES to create a branch in Montpellier dedicated to the manufacture of InAs/AlSb QCL through technology transfer. The links between these two partners are therefore very important and this synergy will promote joint work in the future.
The LPCA worked in close interaction with IES on the development of its iQEPAS device and the joint characterization of laser sources. The collaboration between the two laboratories will continue in order to complete the work delayed by the sanitary crisis.

Scientific production is at a very good level with several publications peer-review journals. Unfortunately, the health context has not allowed us to communicate in congresses as much as we would have liked during the last two years of this project.
The results are as follows:
- 6 articles in international journals
- 8 international conferences with proceedings (oral)
- 3 international conferences without proceedings (oral)
- 8 posters in international conferences
- 2 workshops
- 1 invited laboratory seminar

The MULTIPAS-2 project aims to study and develop a multi-gas/multi-laser sensor, based on tunable lasers photoacoustic spectroscopy, dedicated to ambient air sensing and quantification, as defined by the project end-user, Environnement SA. Even if sensors with high sensitivity already exist, they are limited with some specifications such as: single-gas sensors, very cumbersome, complex to use and maintenance needed, complex calibration…. The proposed sensor will provide equivalent or much better performances in terms of sensitivity and selectivity, but for a much smaller footprint and addressing several gas species simultaneously, which is a major technological breakthrough.
The high performances required for optical sensors with low cost can be achieved by using resonant quartz photoacoustic detection technique (QEPAS, Quartz Enhanced Photoacoustic Spectroscopy). MULTIPAS-2 proposes technology progresses both in the development of laser sources and in QEPAS spectrophone, paying significant efforts on compacification of optical sensing platform and electronic boards. 5 partners are involved, 2 are academic and 3 are industrial partners: the IES, the LPCA, MIRSENSE, VALOTEC and ENVIRONMENT SA.
Semiconductor laser sources (emitting between 2 and 10 µm, single-frequency, high power) will be provided by the IES and MIRSENSE, and implanted by MIRSENSE on a SiGe platform to integrate several sources addressing different gases at different wavelengths on a single chip. This step is the first technological breakthrough that will be achieving for the first time an hybrid multi-source component (SiGe/GaAs/GaSb). Individual sources may initially be supplied to laboratories working on the development of the QEPAS measurement technique. QEPAS offers the advantage of being a simple, compact, yet highly sensitive and selective. IES and LPCA will work on the development and optimization of the QEPAS setups, first with individual sources provided by the IES and MIRSENSE and then with the SiGe integrated source. The compacification of the whole system without losing the sensitivity combined with a simultaneous measurement of various gaseous species is another technological breakthrough. VALOTEC will be responsible of the electronic maps implementation for various needed functions for the laser sources operation and the QEPAS signal processing. Environnement SA will qualify the developed sensor by comparative measurements of single-gas sensors.
Thanks to the compact size of the device combined with its multi-species specificity, the sensor is not only suitable for significant industrial and societal applications (measurement of gas production sites, impact of air quality in urban areas ...), but also for the general public (multi-gas control in individual houses, control of indoor air in vehicles ...). In the future, mass production could swing against the currently still-high manufacturing cost of semiconductor lasers, with a sensor whose production cost may not exceed 1.000 euros, knowing that at the moment, single-gas analyzers prices present in the market are sold between 8000 € and 12000 €. This project proposes a close collaboration between academic and industrial partners, with a carefully adjusted budget.