Laser Ultrasonic Technology And Its Application In High-pressure Sound Velocity Measurements

Laser ultrasonics, especially high frequency laser ultrasonics is a very active forward project in the area of non-destructive testing and its applications in the high pressure physics is in the ascendant too.In this dissertation,the physical mechanisms of ultrasonic generation by laser pulse, the properties of laser ultrasonics and laser ultrasonics's application in the high pressure physics were briefly introduced.On the basis of discussing the principles of detecting ultrasonic signals with displacement interferometer and velocity interferometer, an all optical fiber displacement and velocity mixing interferometer was developed successfully. The interferometer with high bandwidth and high time resolution is ideally suited to the detection of high frequency laser ultrasonic signals with the order of GHz frequency.A cat's eye retroreflector and a microscope lens with large numerical aperture were applied to increase the intensity of signal light input into the interferometer. With the space filtering effect of single mode optical fiber, the speckle effect at the diffused surface of sample was effectively restrained. This two measures enormously improved the ratio of signal to noise and make the mixing interferometer become an effective tool detecting the laser ultrasonic signals in the diamond anvil cell.A noncontact optical method for measuring the thickness of sample in the DAC was presented. The method used the least diameter of focus of a beam of monochromatic light as the indicator of measuring and precise positioning stages as the distance calibrator. It can measure the thickness of sample in the DAC with 7p.m resolution. On the thickness of sample in the DAC measuring method basis, a method which can measure the sound velocity of anisotropic materials under high pressure in the DAC with laser ultrasonics technique was developed.Another method can measure the thickness and sound velocity of sample under high pressure in the DAC with laser ultrasonics technique was developed successfully. In the method pump laser and probe laser irradiated on the two sides of sample respectively, furthermore the distance between the pump laser and probe laser can be changed at will. The method has advantages that ultrasonic wave spectrum is simple, the intensity of ultrasonic signal is strong and thus ultrasonic signal is easy to be detected and analyzed.A prototype laser-based ultrasound system was designed and set upt which can be used to measure the sound velocity of samples under high pressure in the diamond anvil cell. The longitudinal sound wave velocity of copper foils with different thicknesses at normal pressure were measured with this system. Meanwhile,the experimental researches for measuring the longitudinal sound wave velocity and thickness of copper foils at the pressure of 1.5GPa and 2.6GPa in the DAC with the above methods respectively were carried out. The results showed that it is practical and effective to measure longitudinal sound wave velocity of sample in the DAC with this system and the above methods.