Low-frequency harmonic vibration analysis with double-pulsed addition electronic speckle pattern interferometry

We describe the control of a pulsed laser for measuring harmonic vibrations with double-pulsed addition electronic speckle pattern interferometry (ESPI). The basic requirement is to synchronize the laser pulses both to a known position of the object vibration cycle (to obtain a stationary fringe pattern) and to a charge coupled device (CCD) camera (to obtain one double pulse per field.) It is shown theoretically and experimentally that at low object-vibration frequencies (typically <500 Hz) these synchronization requirements can cause instabilities in the laser output. A synchronization scheme to overcome these problems is described and experimental results presented at several low object-vibration frequencies. Addition fringe visibility is improved in real-time with an electronic filter. (C) 1996 Society of Photo-Optical Instrumentation Engineers.