Active interferometers for shape and deformation measurements

It is generally difficult to perform interferometric measurements of surface shape or deformation outside optical benches because fringes are blurred by external perturbations such as mechanical vibration or air turbulence. For overcoming this we have developed active interferometers, in which fringes are stabilized by detecting their movement induced by the perturbations and feeding back the signal to a piezoelectric mirror of the interferometer. We also extend the system to an active phase-shifting speckle interferometer by which the correlation fringes are shifted under feedback control for automatic analysis. Examples of deformation measurement under air turbulence are presented.