Non Contact Generation and Detection of Ultrasound with Lasers

A broad overview of laser-ultrasonics, which is a technique for non-contact generation and detection of ultrasound is presented. The principles of generation and detection are outlined, stressing a few key characteristics of laser-ultrasonics: the material is actually the emitting transducer and transduction is made by light, thus allowing generation and detection at a distance. These features carry both advantages and limitations that are explained. Another feature, which has been an impediment, is actually the complexity of the “laser-ultrasonic transducer”, but in spite of this complexity, it can be made very reliable for use in severe industrial environments. It also can be very cost effective for a number of applications. Applications that have been transitioned to the industry are presented as well as many other potential industrial applications. Introduction: A main constrain of piezoelectric-based ultrasonics is the need of contact or fluid medium for ultrasound coupling to the tested part. Others are the limited bandwidth of piezoelectric transducers and the sensitivity to the phase of the ultrasonic wavefront across the transducer element, which makes the inspection of parts with acute curvature difficult. As recognized by researchers for many years, these limitations could be eliminated by using lasers and light for generating and detecting ultrasound, so called laser-ultrasound, laser-based ultrasound or laser-ultrasonics. Progress to take this concept out of the laboratory into industry has been quite slow, but during the last few years, we have seen the start of real industrial use, while observing at the same time increasing research activities. In laser-ultrasonics transduction is made by light, thus eliminating any contact and the material is actually the emitting transducer, which has both advantages and limitations. Another feature, which has been actually an impediment, is the complexity of the “laser-ultrasonic transducer”, which is sketched in Fig. 1. As shown in Fig. 1, such a transducer has typically three basic elements, a generation laser, a detection laser and an interferometer, followed by data acquisition and processing electronic hardware. In spite of this complexity, such a transducer can be made very reliable for use in severe industrial environments and it also can be made very cost effective for a number of applications. Two applications that have been transitioned from the laboratory to the plant floor are presented below. An extensive collection of references for the topics presented in this paper can be found in [1]. Basics of generation and detection developed before 1990 can also be found in [2].