Abstract

Phase measuring holographic interferometry is a powerful technique for quantitatively determining surface deformations. The phase measuring method requires that the average optical phase between the interfering object and reference waves be shifted with a high degree of accuracy. Improved accuracy in deformation measurements has been obtained by directly measuring the average phase between the two beams and then employing that information in a real-time feedback loop to control the motion of a piezoelectric translator, which serves as the phase stepping device. The average phase is determined by dithering the piezoelectric translator and measuring a portion of the resultant interference distribution with a photodiode. The signal from the photodiode is demodulated with a lock-in amplifier and used as an error signal for a digital controller to obtain phase steps of exactly 90°. The digital controller can also be useful in compensating for low-frequency phase fluctuations in the optical system caused by vibrations. The end result is that the need for PZT calibration is eliminated, the phase steps are of higher accuracy, and errors in the deformation calculation due to vibration-induced phase fluctuations are reduced.

© 1988 Optical Society of America