Industrial Laser-ultrasonic Receivers: Recent Progress in Multi-channel Random-quadrature Interferometers

Laser-based ultrasonics (LBU) is now becoming a more mature technology, making the transition from research laboratory equipment to industrial on-line measurement system possible. The non-contact and remote capability of LBU is an undeniable advantage for high-speed inspection. A major challenge for LBU receivers is to overcome the random phase and intensity distributions (speckles) of laser light reflected off optically rough surfaces. In this context, we proposed and demonstrated in 2005 a new type of LBU receiver based on Multi-Channel Random-Quadrature (MCRQ) detection. We showed that we can transform a high-sensitivity but delicate laboratory interferometer into a rugged system capable of enduring tough industrial environment. This MCRQ architecture combines a classic interferometric design (Michelson interferometer) with an innovative multi-speckle processing technique that takes advantage of the random distribution of speckles for quadrature detection. Robust behavior is achieved. The interferometer does not require critical alignment and rejection of unwanted environmental perturbations is set electronically. Its ability to work with speckled light allows an easy integration of multi-mode fibers for remote detection. We will review the recent developments in order to increase the MCRQ interferometer performances and examples of measurements will be shown and discussed.