Mid IR pulsed light source for laser ultrasonic testing of carbon-fiber-reinforced plastic

Abstract

A quasi-phase-matched (QPM) optical parametric oscillator (OPO) was developed using a periodically poled Mg-doped stoichiometric LiTaO3 crystal to generate mid-IR light for excitation of laser ultrasound in carbon fiber reinforced plastic (CFRP). The ultrasound generation efficiency was measured at the three different wavelengths that emanate from the OPO: 1.064 μm, 1.59/1.57 μm, and 3.23/3.30 μm. The measurements indicate that mid-IR 3.2–3.3 μm light generates the most efficient ultrasonic waves in CFRP with the least laser damage. We used mid-IR light in conjunction with a laser interferometer to demonstrate the detection of flaws/defects in CFRP such as the existence of air gaps that mimic delamination and voids in CFRP, and the inhomogeneous adhesion of CFRP material to a metal plate was also clearly detected.

Mid-IR laser ultrasonic testing for fiber reinforced plastics

“…Then, we have developed the LUT system composed of the Nd:YAG laser, the wavelength conversion device, and the laser interferometer for detecting ultrasonic waves. It was confirmed that the system can generate and receive ultrasonic waves in CFRPs, and successfully detected un-bonded area in transmission C-scan mode [8].…”

“…The technique of the wavelength conversion device using optical parametric oscillation (OPO) was previously described in detail [8]. A specially developed crystal, a periodic polled stoichiometric lithium tantalate crystal (PPSLT) was placed between two plano-plano resonator mirrors in the device.…”

“…The Nd:YAG laser pulse as a pump light was input into the resonator and then two different wavelength pulsed light, idler and signal waves, were output from the opposite side of the resonator. The respective wavelengths of the idler and signal waves are decided by the wavelength of the pump laser, the grating pitch of the crystal, and the refractive indices of pump, idler, and signal wave depending QPM(quasi phase matching) grating pitch [8] crystal were 35 mm and 4 mm × 4 mm, respectively. The output idler wave was separated from the pump and the signal waves by filters.…”

“…In order to emit the mid-IR laser pulse, we came up with the application of an optical parametric oscillator and developed an efficient wavelength conversion device by pumping a compact Nd:YAG solid-state laser [8]. Since the wavelength conversion device and the solid-state laser are compact and robust, our mid-IR pulsed laser is most suitable for ultrasonic generation in polymeric materials.…”

Non-Contact Ultrasonic Inspection of Impact Damage in Composite Laminates by Visualization of Lamb wave Propagation

“…The averaging time should be selected according to the damage (type, size), objective materials, inspection time, inspection resolution, and so on. Moreover, the signal-to-noise ratio could be drastically improved and inspected quickly using the mid-infrared laser developed by Hatano et al [24], as it can generate significantly larger ultrasonic amplitude in CFRP laminates than the conventional Nd: YAG laser [25]. therefore used a time-consuming signal averaging process to improve the signal-to-noise ratio.…”