A simplified flash-lamp pumped high-average-power Nd:YAG Q-switched laser system based on a master oscillator power amplifier platform was developed toward outside laser remote sensing. The performance of the laser system was demonstrated, obtaining 4.7 J output pulse energy with a 50 Hz operating frequency on the optical breadboard of 1.8 m x 0.7 m size. The pulse energy from master oscillator was approximately 250 mJ with 14 ns pulse duration that was amplified by first Nd:YAG rod crystals with double pass amplification. Then, output laser pulse from first YAG rod was amplified by second and third Nd:YAG rod crystals. The beam pattern was image relayed using lens pair between all Nd:YAG rods to maintain the good beam spatial profile in rod amplifiers to avoid the optical damages induced by non-uniform beam profile. The focal lengths of thermal lens effect in each Nd:YAG rod crystal was about 2 m that were compensated by an adjustment of lens pairs. The amplified pulse laser was focused using focusing lens pair on the concrete surface to generate panel vibrations by laser ablation and/or thermal stress, acting thus as a hammer. The focal length of lens pair was approximately 7 m that is assumed the typical a tunnel roof in Japan. The energy transfer efficiency from final amplifier to concrete surface was approximately 87%, its main reason of reduction of efficiency was beam quality of master oscillator. That efficiency was 89% with only oscillator beam.
High-speed laser remote sensing of defects inside a concrete specimen was demonstrated. In the proposed measurement setup, high-power laser pulses irradiated a concrete surface to generate vibration that can be detected by an optical interferometer, which was constructed using photorefractive crystal. The laser-based remote sensing system achieved inspection speeds of 25 Hz. The predominant frequency of a mock-up defect that was embedded in a concrete specimen was measured. The inspection result was identical to that obtained using a conventional hammering method.
We developed high-resistant anti-reflection (AR) coating by using Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> multilayer for Yb:YAG thin disk
amplifier. The AR coating was designed both for 940 nm of pump laser at an incident angle of 30 degrees and for 1030
nm of seed laser at 5 degrees. The Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> multilayer was deposited by using the electron beam evaporation
technique on a fused silica substrate and then the laser induced damage threshold was evaluated. The sample was
irradiated by 1030 nm laser with 520 ps duration delivered from the Yb:YAG thin-disk regenerative amplifier. The
measured damage threshold of the Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> AR coating was 75 J/cm<sup>2</sup>.
Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs
during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through
laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and
an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection
and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on
collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the
forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The
source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.
In this paper, we analyze the steady state and the temporal response of cross-polarized four-wave mixing and show that it is possible to stabilize the gain of optical bus line with the PRCM (Photorefractive Connection Module). We analyze the temporal response of the signal intensity and find the optimum setting for the signal and control beam intensities. We experiment on the two stage optical bus system and evaluate the stability of the bus line.