Matrix laser vibrometer for transient modal imaging and rapid nondestructive testing

This article describes the development and application of a 16x16 array (matrix) laser vibrometer based on a parallel architecture which supports fast 2D measurement of arbitrary (steady state, non-steady state, transient) solid body vibrations without beam scanning. The small size and low weight of the measurement probe, which is linked to a remote detector/processor unit via a flexible armoured fibre-optic umbilical, enables deployment in areas with restricted access. Incorporating aspects of high-speed electronic speckle pattern interferometry (ESPI) and laser Doppler vibrometry (LDV), the design is based on a hybrid fiber-optic/bulk optic interferometer which operates at a wavelength of 1550 nm. Test data illustrate high-speed capture of transient vibrations, showing the full 2D temporal evolution of surface deformation, including multiple resonant modes, of a center-pinned metal plate excited by a 1-50 kHz frequency chirp of 109 ms duration. We discuss preliminary data showing detection of sub-surface defects in composite materials, based on non-contact (frequency chirped) acoustic resonance of the locally damaged structure. For large area NDT the probe can be mounted on a lightweight XY gantry for automated multi-frame measurements.