The absorption spectrum of high-frequency sound (frequency f much larger than 1 MHz) in gases has traditionally been very difficult to measure because of the lack of a controllable high-frequency source with good acoustic coupling to the gas and of a sensitive detection method with fast response. Thus classical methods1 using ultrasonic transducers are usually limited to frequencies of < 1 MHz. The propagation of high-frequency waves (e.g., with f approaching a molecular relaxation frequency or wavelength approaching a collisional mean free path) remains an open question for many gases. The present experiment is a first attempt to measure ultrasonic absorption spectra of gases and mixtures near normal temperature and pressure conditions at frequencies up to tens of megahertz. Our technique is new in two respects: (1) it is all optical and hence is noncontact and applicable for corrosive or inaccessible samples. (2) It is a frequency-multiplexed technique relying on pulse-broadening measurement and fast Fourier transform (FFT) analysis and hence is fast and applicable for real-time measurements under rapidly changing conditions.

© 1984 Optical Society of America

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