A laser displacement sensor using the self-mixing effect for modal analysis and defect detection

A low-cost, simple, noncontact displacement sensor using the self-mixing effect inside a cw single-mode laser diode has been developed for modal analysis and defect detection. The resonance frequencies of thin clamped metallic plates have been determined with a resolution of 0.2 Hz. With such targets, classic spectrum analyzers generate a measurement error up to 10% because of the weight of the accelerometer. The frequency-shift of the fundamental resonance frequency between a standard plate and the same one with a known defect can also be measured, the modal vector associated to this frequency being distorted at the place of the defect