Peter B. Nagy
Dept. of Welding Eng., Ohio State Univ., Columbus, OH 43210
In spite of its obvious advantages over conventional contacts and immersion techniques, laser interferometry is not feasible in many acoustic measurements since its sensitivity is insufficient at ultrasonic frequencies. Part of the problem is that the maximum signal-to-noise ratio cannot be maintained on ordinary diffusely reflecting surfaces. Although these surfaces reflect a fair amount (5%--50%) of the incident laser light, this energy is randomly distributed among a large number of bright speckles. Unless the detector happens to see one of these bright speckles, the interferometer's noise level will be much higher than the optimum. In most ultrasonic measurements, when the vibrations to be detected are periodically excited by a transmitter, this adverse speckle effect can be almost completely eliminated by a very simple procedure. The conventional interferometer is mechanically modulated to assure the random occurrence of a few very bright speckles and to move the whole speckle pattern around at an appropriate speed. Random but frequent bright flashes detected from the surface of the specimen result. The bright periods are sufficiently long, to trigger the ultrasonic pulser and detect the transmitted signals before the flash subsides and close to maximum signal-to-noise ratio can be maintained everywhere on the surface of a diffuse object. Numerous examples are shown to demonstrate the significant improvement of the detection sensitivity by this novel approach.