Remote laser generation of narrow-band surface waves through optical fibers

This paper demonstrates the use of a fiberoptic bundle for flexible, compac t, remote, and noncontact laser generation of surface ultrasonic waves in m aterials. The bundle is able to deliver Nd:YAG pulsed light with a 60% deli very efficiency up to an average energy of 55 mJ/pulse for a pulse duration on the order of 10 ns and a pulse repetition rate of 20 Hz without signs o f fiber damage. Details of the bundle construction and surface preparation are given, and pulsed light delivery tests performed with single tapered fi bers are discussed. The high-power light delivery capabilities of the bundle are demonstrated f or the generation of narrow-band surface waves in a Carbon/PEEK composite l aminate by a spatial modulation technique that employs a periodic transmiss ion mask. Single laser pulse ultrasonic tonebursts are clearly detectable u sing a small aperture piezoelectric transducer while ensuring thermoelastic generation conditions. The theory of narrow-band generation of surface aco ustic waves is improved by accounting for the strength nonuniformity of the illumination sources. In addition, the effect of the number of illuminatio n sources on the bandwidth of the generated surface wave is assessed experi mentally, and excellent agreement is shown with the theoretical results pre dicted by the improved model.