What are the design principles for our miniaturized cells?
The cell performance
is specified usually in terms of the signal-to-noise ratio. In order to obtain
a high magnitude for the ratio the cell design must satisfy some requirements.
First of all, the design has to provide detection of the highest possible useful
signal (that is, a photoacoustic response from the gas inside the cell)
at a selected acoustic resonance. The cell must be reliably isolated from external acoustic noise
and parasite electric pickups. And, finally, the background effect of cell windows
on the measurements should be reduced to a minimum.
According to our approach, an efficient way to miniaturize the resonant photoacoustic
cell can be implemented with the help of a properly optimized design of the cell.
The cell design (the geometry shape for the cell cavity, the microphone location
and arrangement of inlet/outlet gas holes) is fitted with the help of numerical simulation
in order to enhance the cell performance for an individual acoustic eigen-mode. The design
is fitted in such a way as to increase the useful response from the detected chemicals,
to minimize the possible parasite signals (first of all, the background signals arising
due to absorption or reflection of laser beam by the cell windows) and to provide
a reliable acoustic isolation of the cell from the environment. In a sense, we ‘manipulate’
by the node-antinode structure of spatial mode distribution in order to enhance the cell performance.
The design optimization in action.
1. Inclined cells. We made an experimental examination of a prototype cell (a cell of inclined geometry with ZnSe windows, the internal volume ~ 500 mm3 and clear aperture d = 8 mm). The cell design is optimized to the best performance for an acoustic resonance (the acoustic mode n2 of the cell cavity) near 14.2 kHz.Figure 1 represents results of experimental examination for the prototype: the amplitude |Pω| and root-mean-square deviation σω (that is, a measure of experimental error) of detected photoacoustic signals as functions of modulation frequency ω.
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Conclusion: Analysis of the examination testifies that the design optimization provides:
- a higher Q-factor
- low window background
- reliable acoustic isolation from the environment
Obviously, the gas-detection sensitivity for the cell is maximal when the modulation frequency ω is close to ω2 ~ 14.2 kHz. The cell performance is specified in terms of the normalized noise equivalent absorption (NNEA). We accept this quantity as the minimal detectable absorption αmin, which can be obtained if the signal-to-noise ratio is equal to 1 provided that the laser power is 1 W and the averaging time is 1 sec. According to our measurements, the minimal detectable absorption αmin is ~ 1.2 10-8 cm-1 W Hz-1/2.