Laser interferometry for measurement of displacement in hostile environment and in real time

The conventional methods of measuring displacement and strain are severely taxed especially when long-term stability is needed in a situation involving temperatures above 1000 °C. Optical techniques are attractive for measurements in these difficult situations because of their remote, non-contacting and non-destructive capability. The objective of this study was to develop a speckle based laser interferometry system to evaluate whole field out-of-plane deformation of a very small region (<1 mm) of an object at elevated temperatures in real time. The optical system (1) was capable of measuring very small displacements in a small area; (2) was used at high temperatures; and (3) was able to take 250 frames per second. A long distance microscope was used in the system in order to focus onto very small areas on the object and a high-speed camera to take real-time measurements. A CO₂ laser was employed to irradiate a circular thin aluminum oxide plate (object) locally and to generate local deformation as well as a crater. Laser radiation with the power density exceeding the critical power density q_c produces melting and plasma. The plasma appears as a very bright spot and it reduces speckles that give information about surface deformation of the object. As a result, the bright spot limits deformation measurement with the designed laser interferometry system. Some preliminary quantitative data on the form of images and deformation profiles will be presented. The laser interferometry system designed is significant because very small out-of-plane deformations from a small region were measured in real-time with up to 250 frames per second although the object's temperature was locally very high (approximately 2000 °C).