Past investigators have predicted and, in limited cases, demonstrated the potential utility of using temporally and spatially modulated laser array sources as a means of increasing the signal-to-noise ratio in laser ultrasonic systems without causing the surface damage characteristic of a single high-power laser point source. In an effort to develop a practical and flexible laser array source, a laser system has been designed and implemented which uses a single Nd:YAG pulsed laser and an optical delay system in which the laser pulse passes repetitively through a White cell cavity being sampled by a custom beamsplitter after each pass. Up to ten spatially separated light beams exit from the system each time a single laser pulse is introduced. Time separation between the distinct pulses can be adjusted over a range from 28 to 170 ns, corresponding to a pulse repetition rate from 6 to 36 MHz. Since individual control of the beam paths is possible, one has the flexibility to implement either a single-element, temporally modulated array, or a multielement, ‘‘phased’’ array. From a single point, a multiple spike narrow-band acoustic signal is generated, permitting greater detection sensitivity than can be obtained with broadband detection of a single spike. Alternatively, multielement arrays can be used to increase the acoustic signal amplitude by superposition of the signals generated from each individual element, once again enhancing detection sensitivity.

A configuration for sequential holographic recording based on a new system for obtaining a train of spatially separated light pulses at frequencies ranging from 6 to 35 MHz has been developed. The multipulse system uses a high-energy Q-switched Nd:YAG laser as a light source and incorporates a phase-front-preserving optical delay line and a specially graded beam splitter to produce as many as ten spatially separated light pulses of nearly equal energy. The temporal spacing between successive output pulses may be varied discretely in increments of 28.3 ns from 28.3 to 169.8 ns. The system is currently used for high-speed time-resolved holography of dynamic events with lifetimes between 100 ns and 1.5 micros. Other applications include using the system either as a single point or sequential-array source for laser generation of ultrasound.

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