Optical EngineeringHigh-speed laser speckle photography. Part 1: repetitively Q-switched ruby laser light source
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A system to record laser speckle photographs at framing rates in the range of 105 to 106 frames/s has been developed, based on a repetitively Q-switched ruby laser and rotating mirror high-speed camera. The laser and electro-optic modulator are described. The circuit diagram for an inexpensive high-voltage amplifier, capable of switching 2.5 kV at up to 1 MHz with fall and rise times of 100 and 200 ns, respectively, is given. The resulting optical pulse trains have pulse energy fluctuations at half the driving frequency. We show how these may be suppressed by reducing the time the Q-switch is left open. Both the subharmonic component and its suppression are explained from limiting cases of the laser rate equations. Representative pulse trains over a range of repetition rates are given; pulse energies greater than 20 mJ with pulse energy fluctuations of less than plus or minus one-half of a stop are obtained at rates of up to 500 kHz.