Improvements and applications of non-contacting interferometry

Abstract

The research described in this thesis involves the development and application of a two channel, fiber optic, interferometer. The existing interferometer system consists of an Argon laser, single mode fiber, and an array of Fused Biconical Taper Couplers which act as beamsplitters for optical fiber systems and produce a dual channel interferometer from a single laser beam input. These interferometer signals are recorded with a high speed photo-detection system, also consisting of two channels. The system is well suited to detect the broadband frequencies often used in the ultrasonic investigation of materials. Generation of the ultrasonic signals is usually from a thermal pulse created by a Q-switched ND:YAG laser. In the research reported here, a Graded Index (GRIN) lens was applied to the existing interferometric system and tested. The testing consisted of several comparison experiments between the existing system (FTI) and the modified system (FTI/GRIN). The effects of the increased gap between the detector tip and the test surface made possible by the addition of the GRIN lens was investigated. The interferometer system was applied to hot pressed silicon nitride to investigate the potential use of these measurements of material characteristics with ultrasonic methods. Both ultrasonic velocity and scatter were used to obtain the properties of the ceramic material. An unsuccessful attempt was made to identify and characterize microcracks caused by grinding of a ceramic plate. However, when more severe microcracking was introduced with a Knoop hardness indentor, the FTI/GRIN was able to detect the damage.