High Resolution Time-of-Flight Measurements Employing Laser Ultrasonics with Photo-EMF Detection

Abstract

Laser ultrasonic techniques provide distinct advantages over conventional contact methods for process monitoring and on-line material characterization applications.¹ The most important of these is non-contact measurement capability. Non-contact measurement is particularly desirable in industrial environments where high temperatures or hazardous materials are involved. This latter subject is especially significant concerning the types of materials handled, processed, and inspected at Los Alamos National Laboratory where collateral contamination and generation of secondary low-level waste is a major issue. An important ramification of non-contact laser ultrasonic methods is the ability to probe materials that are in motion such as on a conveyor belt or a part being turned on a lathe. Indeed, this all-optical technology can allow corrective action to be taken during a material removal process before the ideal component geometry is irretrievably altered. Also, optical post-process inspection of fabricated parts are not subject to dimensional measurement errors typical associated with conventional contact probes such as probe wear and probe inclination relative to the surface. In contrast, laser generation of ultrasound offers relative insensitivity of the measurement to the incident optical beam inclination, and has the added benefit of averaging surface roughness effects; contact probes usually ride on the outermost surface profile.