Optical resonant ultrasound spectroscopy for fluid properties measurement

The properties of fluids are studied using unusually small containment sphe rical resonators. Proper identification of resonant fluid signatures allows determination of pressure and density of the internal gas with great accur acy using an appropriate equation of state (EOS). Low noise and high sensit ivity detection of vibration are critical parameters to characterizing the contained gas when its pressure approaches 1 atm or less. The benefits of u sing spherical resonators to determine fluid properties are discussed, and some example calculations of sound speed are presented. In addition to meas uring fluids, a comparative experimental approach is taken to explore and, eventually, to optimize vibration detection. In the experiments, two detect ion methods, a contact piezoelectric transducer (PZT) device and a non-cont act optical device, are compared simultaneously and quantitatively. This is done in a unique manner without change in vibration coupling to the sample between tests. A commercially available resonant ultrasound spectroscopy s ystem is used as the contact system, while another commercial device (used as the non-contact vibration detector) combined with the same excitation so urce (used in the contact system) comprises the other system. The non-conta ct detector is an optical interferometric receiver that provides adaptation to optically rough surfaces and high sensitivity to acoustic displacements through optical interference in photorefractive GaAs. Both vibration detec tion systems are compared with particular emphasis on displacement sensitiv ity, frequency response, and noise level. Furthermore, the results from com paring detection modalities are presented, and their effects on fluid prope rties measurement are discussed.