An automatic stabilization system for a confocal Fabry-Perot interferometer used in the detection of laser-generated ultrasound

A laser confocal Fabry-Perot interferometer (CFPI) is described for the sensing of ultrasound in opaque solids having optically rough surfaces. Stabilization was required to eliminate the optical frequency drift arising from various sources in both the CFPI and the argon-ion laser. Measurements showed that conventional proportional control was inadequate when using feedback to the piezoelectrically-controlled interferometer mirror. Instead, a cascade control system was developed with a digital proportional-integral controller as the primary controller and an analogue proportional controller as the secondary controller. Two control loops were nested, with the secondary loop used to stabilize fast fluctuations and the primary loop used to eliminate the drift. When the interferometer mirror reaches the limit of its dynamic range, the intelligent digital controller was able to reset automatically the interferometer at a new operating point within its proper dynamic range. In this way, it is shown that the interferometer sensor can be used in a fully automatic way for on-line monitoring applications.