Study of intracavity absorption and laser behavior for C2H4, C2H6, CH3F, CH3Cl, and CH4/N2 by photoacoustic resonance spectroscopy

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Abstract
The first radial mode of a cylindrical resonator was excited in extracavity and intracavity configurations employing a modulated 3.39 μm HeNe laser. The resonance curves were measured as a function of pressure by a computer-controlled method and fitted to Lorentzian profiles. Measurements were performed for C2H4, CH3Cl, CH3F, C2H6, and CH4, where the optical absorption coefficient determined by transmission experiments increases from C2 H4 to CH4 . From the signal strengths and the Q factors of the resonances the so-called photoacoustic signal (PAS) was determined and compared with theory. Good agreement between theory and experiment was observed for all extracavity measurements and in the case of intracavity experiments with low optical losses in the laser resonator. In order to describe the large discrepancies between the experimentally determined PAS and the existing theory for high losses, improved models were developed, which are based on an inhomogeneous distribution of laser power in the optical cavity. Deviations from these models were found near threshold. The PAS observed above threshold is attributed to amplified spontaneous emission.