Abstract

We describe a diode laser powered fiber optic based system for vibrational analysis that incorporates the benefits of electronic speckle pattern interferometry (ESPI) and laser velocimetry. Through phase locking of the common reference arm to the motion of a single point on the target by using the velocimeter output, the combined unit allows automatic heterodyning of the ESPI system and limited compensation for unwanted random out of plane motion of the test object. As the velocimeter point is tracked across the surface, a signal containing the vibrational information but independent of the power level returned from the target and the fringe visibility is obtained by using a dithered servo technique. The frequency shifting is then applied either by using a fiber optic piezoelectric phase modulator in the reference arm or by shifting of the source wavelength through direct modulation of the diode laser injection current. The system was thus able to be successfully demonstrated on a range of surface types, including those showing large spatial variations in reflectivity.

© 1992 Optical Society of America