Automatic heterodyning of fiber-optic speckle pattern interferometry

The operational advantages of the complementary techniques of electronic speckle pattern interferometry (ESPI) and laser Doppler velocimetry (LDV) are combined for out-of-plane vibrational analysis. A cw laser was used in a fiberoptic system to generate time-averaged ESPI fringes. By mutual phase-locking of the ESPI and LDV signals, automatic heterodyning of the ESPI signal was achieved, together with compensation for unwanted nondeterministic out-of-plane whole body motion of the object under study. Automatic heterodyning extends the operating range of time-averaged ESPI systems to higher vibrational amplitudes, and reveals phase relationships in the object vibration. A practical demonstration of this new technique is described.