Miniaturized resonant photoacoustic cells
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We support a traditional approach to photoacoustic detection of gases:
through a small hole in the cell shell, a condenser microphone registers
the photoacoustic response to the laser beam modulated with the frequency
of an acoustic resonance of the cells. This approach implies a possibility
to adapt the photoacoustic cell to the application needs. For instance,
the detection of slow-relaxing chemicals can be realized by an efficient way
if the cell design enables the resonant operation with the probing beam modulated
at low frequencies. An optimal gas detection can be realized also when the clear
aperture of the cell is close to the diameter of laser beam. And, certainly,
the best gas-detection performance is reached if
the cell design is optimized
with respect to an acoustic resonance.
Our photoacoustic cells are equipped by Brewster windows. Therefore, the parasite reflection of laser beam from the cell windows is canceled essentially if a plane-polarized and collimated laser beam is applied. The cell can be inserted inside the laser cavity for implementation of high-sensitivity laser-intracavity gas-detection applications. Since the cell windows have no coating the cells each can be applied for multi-component analysis of gases detected in wide wavelengths range.
To the moment we are experienced in manufacturing and application of miniaturized resonant photoacoustic cells of two types. They are inclined and folded cells. The cells of inclined geometry were the first ones where our optimization principle was applied and examined. The acoustical properties and application features are studied in detail for these cells (see our recent publications). From our viewpoint, the most efficient gas-analyzing application of the inclined cell is associated with the laser beam of the diameter d greater than 2 mm (for instance, inside the cavity of a high-power gas laser). The laser beam of smaller diameter (d smaller than 1 mm), generated usually by miniature lasers (such, for instance, as near-infrared laser diodes or mid-infrared quantum cascade lasers) can be efficiently applied with our so-called folded cells. At the moment, the aspects associated with the folded cells are under our detailed investigation. Nevertheless, our preliminary analysis shows that these cells are distinguished by small sizes and the capability to enhancing the the gas-detection sensitivity. Some cells with optimized design are represented in photo.
The internal volume for the cells is pointed by arrows. A traditionally sized photoacoustic cell is applied as the background. Despite the reduced sizes, the shown cells are not worse in the performance (that is, in the sensitivity) in comparison to non-miniaturized ones. The minimal detectable absorption estimated from our experiment for the cells is better than ~ 10-8 cm-1 W Hz-1/2.