Abstract
The quality factor, Q, of plane or radial wave resonators can be increased and controlled using thermoacoustics. When such a resonator is used in photoacoustics, a laser beam passes through it and sound is produced when light absorption occurs. Sound amplitude is proportional to the coefficient for light absorption, to the resonator Q, and to the laser beam power. Traditionally, resonator dimensions have determined both the coupling strength of the laser beam to a particular mode and the quality factor. High coupling strength is desirable, but implies use of a narrow resonator where most of the interior is filled with laser. On the other hand, high Q traditionally implied need for a wide resonator. Thermoacoustics can be used to increase the Q of the narrow resonator to optimize the acoustic signal. Our theory and measurements show that the thermoacoustically enhanced photoacoustic spectrometer can be viewed as an analog narrow-band amplifier for sound where the resonance frequency is f0, the bandwidth is f0/Q, the gain is Q, and the signal-to-noise ratio for discrimination against Gaussian acoustic noise in the resonator is Q1/2. [Work supported by the EPA, ONR.]
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