Mid-infrared pulsed laser ultrasonic testing for carbon fiber reinforced plastics.

Laser ultrasonic testing (LUT) can realize contactless and instantaneous non-destructive testing, but its signal-to-noise ratio must be improved in order to measure carbon fiber reinforced plastics (CFRPs). We have developed a mid-infrared (mid-IR) laser source optimal for generating ultrasonic waves in CFRPs by using a wavelength conversion device based on an optical parametric oscillator. This paper reports a comparison of the ultrasonic generation behavior between the mid-IR laser and the Nd:YAG laser. The mid-IR laser generated a significantly larger ultrasonic amplitude in CFRP laminates than a conventional Nd:YAG laser. In addition, our study revealed that the surface epoxy matrix of CFRPs plays an important role in laser ultrasonic generation.

Temperature-controlled picosecond-pulsed high frequency second-harmonic generation by a periodically poled stoichiometric LiTaO(3).

We report experimental results of second-harmonic (SH) generation (SHG) by a quasi-phase-matched periodically poled Mg-doped stoichiometric LiTaO(3) crystal for 1030 nm input radiation of 18 ps pulse duration, within the range of peak input laser intensity I = 0.1-9.5 GW/cm(2) and under repetition rate 10-20 kHz. For I>3 GW/cm(2) SHG efficiency achieves the saturation level of η≈0.35 which can be maintained within a wide range of I = 3-9.5 GW/cm(2). The loss of SHG efficiency observed for I>5 GW/cm(2) can be recovered to the level of η≈0.35 by using temperature-controlled operation. By applying our experimental data we find the value of two-photon absorption (TPA) coefficient for 515 nm radiation, ß≈1.1-2.7 cm/GW, agreeing well with the theoretical estimate ß≈2.6 cm/GW. Our analysis suggests that the inhibition of SHG efficiency, its saturation and stabilization are due to a combined mechanism including: (i) non-steady-state ps effect scaled by ≈ζ(-2)[1-exp(-ζ)](2) as compared with the efficiency for ns pulsed operation (ζ = L/V2τP , L is the crystal length, τP is the pulse duration and V(2) is the group velocity of SH); (ii) dephasing caused by the spectral bandwidth of the input radiation (≈300 GHz); (iii) thermal dephasing caused by TPA of SH; and (iv) strong SH attenuation by TPA of order ≈I(2) (-1)dI2/dz≈-(0.8-8) cm(-1) for I = 1-9.5 GW/cm(2).

Comparison of two-color hologram lifetimes of near-stoichiometric lithium niobate and of tantalate crystals.

Lifetimes of two-color nonvolatile holograms recorded in undoped or in slightly doped near-stoichiometric lithium niobate and tantalate crystals were measured and compared by extrapolation of the high-temperature data. A proton-compensation mechanism dominated the dark decay and yielded similar activation energies, of 1.05 and 1.10 eV, for near-stoichiometric lithium niobate and tantalate crystals, respectively. The lifetime of holograms in lithium tantalate was 1 order of magnitude longer than that in lithium niobate with the same proton concentration, which was consistent with our theoretical estimation. The projected lifetime of two-color holograms in lithium tantalate without observable OH-absorption is longer than 50 years.