Paraffin oil thermal diffusivity determination using a photothermal deflection setup with a 2.3μm pump: a first step towards methane detection

The photothermal deflection technique, also known as "mirage effect", is a nondestructive method of evaluating thermal properties of solid, liquid or gaseous species. This technique will be used to detect pollutant absorption. As the deflection is stronger in liquids than gases, we will first consider the deflection in paraffin oil. We consider a medium that is heated by a modulated laser diode beam, and we measure the deflection of the probe beam passing through the heated region as a function of the distance between the axes of the beams. After some theoretical considerations and numerical simulations, we present the application of this method to the experimental determination of the thermal diffusivity of a liquid sample in excellent agreement with previously known values. 1. The pump beam: an antimonide laser diode emitting at 2.3μm In this work, we use as a pump beam a tunable GaSb based laser diode with an active zone constituted of GaInAsSb/AlGaAsSb quantum wells. This device has been grown by molecular beam epitaxy at the Institut d'Electronique du Sud (IES). It works in the continuous-wave regime at room temperature, with an emission wavelength of 2.32μm. This kind of device has already been successfully used in tunable laser diode absorption spectroscopy [1].The emission spectrum of the pump laser is shown in figure 1 (inset). Figure 1 shows the absorption peaks of methane. This measurement has been performed using a 12mm-long cell filled with pure methane. The laser beam is directed through this cell and detected with an InGaAs photodiode. Around 2.3μm, there is a large methane absorption band (fig 3), constituted of many absorption lines. Tuning the injected current inside a laser diode increases the active zone temperature. The main effect is a variation of the refractive index thus an increase of the emitted wavelength (tuning effect [2]) of the device. It makes it possible to cross any methane absorption lines. 15th International Conference on Photoacoustic and Photothermal Phenomena (ICPPP15) IOP Publishing Journal of Physics: Conference Series 214 (2010) 012121 doi:10.1088/1742-6596/214/1/012121 c © 2010 IOP Publishing Ltd 1 The laser diode shows a threshold current of 110 mA at 25°C with an emission wavelength around 2.32 μm. Its optical power reaches 1 mW at 100 mA with a current tuning of 0.03 nm.mA. As can be seen figure 2, the 2.3μm range is interesting because CH4 absorption is strong while water absorption is weak [3] in the atmosphere transmission window. 2. Principle of mirage effect detection When a medium like a liquid or a gas is studied, it is excited by nonuniform absorption of a modulated pump-laser source. The medium heats locally, introducing a locally modulated temperature gradient. This temperature gradient induces in turn a refractive index gradient. Then, a probe beam (here an HeNe laser at 0.632 μm) passing through this region will be deflected by an angle related to the thermal gradient at the modulation frequency which is related to thermal properties of the medium. The deflection angle is detected by a quadrant silicon photodiode, linked to a lock-in amplifier locked at the pump-laser modulation frequency. The modulation of the pump beam is made with a chopper. Here the main interest of the technique is that an effect, induced by an infrared device (absorption of the pump laser, at 2.3 μm) can be detected by the deflection of a red laser measured by a cheap silicon detector. 2.3 μm CH4 absorption band Fig. 2: CH4 and H2O lines intensity in the 1-5 μm range. Fig.1: Methane absorption at T=25°C. Inset: laser emission spectrum. 15th International Conference on Photoacoustic and Photothermal Phenomena (ICPPP15) IOP Publishing Journal of Physics: Conference Series 214 (2010) 012121 doi:10.1088/1742-6596/214/1/012121