Active stabilization and real-time analysis of interference fringes

An active two-beam interferometer has been developed which locks fringes in the presence of external perturbations such as vibration and air flow and enables measurement on an unstabilized table. Movement of carriered fringes caused by the perturbations are detected by a spatial filtering detector whose output signal is fed back to injection current of a laser diode to compensate for path variations by the resulting wavelength shift. The video image of the fringes is supplied to a real time fringe analyzer which delivers the phase distribution at the video rate. The analyzer is based on electronic moire principle for simultaneous generation of three phase shifted fringe patterns. The results from a spherical mirror of 130 mm- diameter have shown good coincidence in spherical errors with that obtained from measurement on an optical bench. Only its repeatability proved to be worse by 1.7 times.