A non-contact method of determining material properties and structural integrity through the analysis of laser generated ultrasound

This paper describes a technique to evaluate the mechanical properties of a structure without involving any physical contact with that structure. The basic principle is to monitor the ultrasonic transfer function of the structure. However by utilising Lamb wave dispersion characteristics whose shape depends on a multitude of parameters, this approach is capable of extracting far more structural data than straightforward compressional mode propagation measurements. In this paper we shall describe initially how optical techniques are used to both launch and receive Lamb wave signals. Typically a very short harmonically rich pulse of laser light is used to launch a wide spectrum of ultrasonic frequencies. From this impulse response the dispersion curves can be extracted. In turn these dispersion curves can be inverted to produce values for important parameters such as Young's modulus, material thickness and Poisson's ratio. We demonstrate that this inversion technique is capable of producing values for mechanical parameters with a reproducibility of a few percent. Consequently any deviation in these values becomes immediately obvious. Such deviations can be indicator of structural damage or deterioration. Examples demonstrating this discrimination capability are also included in the paper

A gravitational wave observatory operating beyond the quantum shot-noise limit

Around the globe several observatories are seeking the first direct detection of gravitational waves (GWs). These waves are predicted by Einstein's general theory of relativity and are generated, for example, by black-hole binary systems. Present GW detectors are Michelson-type kilometre-scale laser interferometers measuring the distance changes between mirrors suspended in vacuum. The sensitivity of these detectors at frequencies above several hundred hertz is limited by the vacuum (zero-point) fluctuations of the electromagnetic field. A quantum technology--the injection of squeezed light--offers a solution to this problem. Here we demonstrate the squeezed-light enhancement of GEO600, which will be the GW observatory operated by the LIGO Scientific Collaboration in its search for GWs for the next 3-4 years. GEO600 now operates with its best ever sensitivity, which proves the usefulness of quantum entanglement and the qualification of squeezed light as a key technology for future GW astronomy