Optical transduction methods for the photoacoustic and photothermal detection of trace gas

This thesis focuses on the study of Fabry - Perot cavities, used as optical transduction alternatives for the resonant detection of gas traces, by the photoacoustic and photothermal method. The principle of measurement is based on the shift of the optical cavity resonance, induced by acoustic or thermal perturbations. Two approaches have been studied, then the transduction systems were fabricated and characterized, and finally implemented in sensors for trace gas detection.The first approach is based on a Fabry - Perot cavity of low finesse assembled at a fiber tip and interrogated by a low - cost laser diode emitting at 1.55 µm. The second mirror of the cavity is a mechanically resonant cantilever beam, suitable for the measurement of weak acoustic signals. In a first step, a reduced element model was set up to predict the coupled behavior of the mechanical structure, the surrounding fluid, and its interaction with the excitation light source in the sensor. The model has been successfully compared with resolutions performed by the finite element methods and experimental characterization results. The limit of detection at the resonance frequency of the system, is of the order of 2 μPa/√Hz. Finally, the transduction system was integrated in a photoacoustic sensor and calibrated on a NO absorption line, leading to a detection limit lower than 10 ppb.The second approach, more innovative, consists of a phase shifted Fabry - Perot cavity, fabricated on an optical fiber and of very high quality factor, stabilized by a Pound-Drever-Hall optical locking technique. The implemented optical locking permits to be extremely sensitive to optical resonance shifts, which depends here mainly on thermal perturbations. After an implementation of the reduced element model, adapted to this new problem, the optical guide is mounted in a photothermal sensor and tested for the detection of atmospheric CO2. The limit of detection, estimated by extrapolation, is here in the order of one ppm.