AUTOMATIC STABILIZATION OF A FABRY-PEROT INTERFEROMETER FOR A LASER-ULTRASOUND C-SCAN IMAGING SYSTEM

A confocal Fabry-Perot interferometer for the detection of laser-ultrasound has been employed for C-scan image applications on industrial samples without special surface preparation. Automatic stabilisation of the interferometer has made this possible. A cascade control system has been developed to overcome the complex de-stabilising mechanism inherent in this type of interferometry. It was based on a special version of proportional integral (PI) control, implemented digitally and tailored to the Fabry-Perot characteristics. This was combined with an analogue processing loop to provide a system capable of long term stabilisation throughout C-scan measurements.