| Popis: |
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. |
