Sensitivity enhancement in laser ultrasonics using a versatile laser array system

Abstract

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 Model of Temporal Intensity Modulation for Laser Generated Ultrasound

“…But it would be desirable in some NDE applications to narrow the signal bandwidth to improve the signal to noise ration (SNR), and also to have direct control over the center frequency of the generated ultrasound. In principle, this may be achieved by temporal [3,4] or spatial modulation [5,6] of the Iaser pulse, or both [7]. The purpose of this work was to develop a numerical model of a single, tem porally modulated Iaser source of ultrasound in the thermoelastic regime, for isotropic metals.…”

Narrow-Band Hybrid Pulsed Laser/EMAT System for Non-Contact Ultrasonic Inspection Using Angled Shear Waves

“…pulse train required to create the narrow-band ultrasound can be accomplished in a number of ways. In the past researchers have used mode locked lasers [18] and single pulsed laser/optical delay (White cell) combination [19] to generate the necessary pulse trains. However since the EMA T is typically limited to less than 5 MHz operation frequency (because of stand-off issues) the mode locked and White cell laser methods become difficult to implement since long optical delays are needed.…”

A frequency domain laser based ultrasonic system for time resolved measurement of broadband acoustic transients

“…Techniques for generating acoustic waves with controllable, periodic sequences of laser pulses have been demonstrated. [9][10][11][12][13][14][15] An alternate approach to temporal modulation has been developed that uses an amplitude modulated continuous wave a͒ Electronic mail: twmurray@bu.edu laser source. [16][17][18] In this approach, the laser is modulated with a pseudorandom sequence and the resulting acoustic signals, in the low megahertz frequency range, are measured with an optical interferometer or piezoelectric transducer.…”