Automatic heterodyning in fiber optic speckle pattern interferometry using laser velocimetry

We combine the operational advantages ofthe complementary techniques of electronic speckle pattern interferometry (ESPI) and laser Doppler velocimetry (LDV) for out-of-plane vibrational analysis. A cw laser was used in a fiber optic 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 (i.e., without operator intervention) 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 the technique is described.