Jan C. Petersen

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A Versatile Integrating Sphere based Photoacoustic Sensor for Trace Gas Analysis

Compact, low cost, highly sensitive and automated trace gas detection systems are important for a number of applications, including environmental and industrial monitoring. Photoacoustic spectroscopy (PAS) has turned out to be a very promising technique, empowered by the simplicity of the technique and low cost detection components, however still allowing measurements of molar mixing ratios at levels below 10-9.

The concentration of trace gases is measured by quantifying the optical absorption at specific wavelengths depending on which molecular species is of interest. In transmission spectroscopy the Beer-Lamberts law determines the dependence between optical attenuation and interaction length, thus quantifying the molecular concentration. In PAS a pressure wave is created by heating and subsequently cooling of the absorbing molecules by intensity modulation of light.

The photoacoustic (PA) signal strength is proportional to the concentration, thus the concentrations can be quantified. Using an optical integrating sphere as the measuring cell the light intensity experienced by the gas is enhanced, due to multiple reflections inside the sphere, and the optical alignment is simplified. In our experiment the optical power is enhanced by a factor of 20 for a 2 ┬Ám laser used to probe specific rovibration lines of CO2. Driving the light modulation at a frequency, which is acoustically resonant to the absorption cell, enhances the PA signal. However, due to the uniform distribution of the light field inside the integrating sphere, acoustic resonances cannot be exploited directly. By attaching an organ pipe onto the integrating sphere, a new acoustic resonance condition is established and resonances can be exploited.

The figure shows a simulation of the acoustic field pressure and frequency response, with low acoustic amplitude (blue) in the sphere and a strong (red) standing wave at the end of the organ pipe. The plot on the right, shows measurements of the acoustic spectrum experienced by a microphone mounted at the end of the organ pipe and inside the sphere. It is observed that the background absorption signals are highly attenuated due to the thermal conduction and diffusion effects in the cell walls. These features makes the sensor suitable for various practical sensor applications in the ultraviolet to the mid infrared wavelength region. The total enhancement factor of the integrating sphere based sensor is approximately 1400 compared to a single-pass non-resonant PA sensor making it useful for many practical applications, where sensitivity, low cost and compactness are needed.

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