The progress toward developing a technique for the underwater remote detection of gases using Raman lidar is reported. In this work, we describe the development of the marine Raman lidar system for monitoring the CH4 gas of Taketomi submarine hot spring. The transmitter of lidar system was the third harmonic of a standard Q-switched Nd:YAG laser (355 nm). The receiver of this system consisted by a telescope with a diameter of 200 mm, a spectrometer, a CCD camera for measuring Raman spectrum, and a PMT for monitoring Raman lidar signals. In order to evaluate the performance of our system, we demonstrated the Raman spectroscopic measurements from a barge down to the water depth of about 30 m.
Contamination of optics observed in LFEX compression chamber was a critical problem for maintaining high damage
threshold and high optical performance for mirrors and gratings in the vacuum environments. We conducted a study for
understanding this problem, and we found important knowledge on the nature of contamination, namely materials of
contaminants, source of contamination, the invasion mechanism, and removal method of contamination. We also found
the samples for the damage test is easily contaminated in the storage environment. This means the optical coating
accumulate contaminations even in the air. So we tested in-situ damage test in a gas flowing chamber with controlled
contaminants. The degradation was time-dependent phenomena, and proportional to the vapor
contaminants. Several materials were tested, and even in water vapor, the damage threshold was decreased about 10%.
We also found out two methods for removing contaminations from the coatings. According to these studies, our
conclusion is special treatments should be used for knowing the intrinsic damage threshold of the coatings.
We have fabricated a 410 x 468 mm size deformable mirror with 100 Bimorph piezoceramic actuators for the LFEX
laser system at Osaka University. In the case of Bimorph-type deformable mirrors, the mirror surface had to be polished
and coated after bonding the piezoceramic actuators to the rear side of the thin mirror substrate. This provides a good
surface figure, but the coating temperature for the high-reflection mirror was strictly limited because of the thermal
fragility of piezoceramic actuators. The mirror substrate with the actuators was polished, and an ion-assisted multilayer
dielectric coating was produced at 60 degrees Celsius with our 80-inch coating chamber. The flatness of the mirror just
after coating was 7 μm, and reduced by aging to 3.2 μm when the mirror was assembled. The surface figure of the
assembled mirror with 20 piezostack bonded actuators is demonstrated and a laser-induced damage threshold tested with
a witness sample is also reported.
LFEX is the world’s largest high-energy petawatt laser. So far it delivers 3 kJ/1 ps and is planed to finally deliver 10 kJ/10-20 ps. It has been constructed and became partially operational since 2008, and with full beams since 2014. LFEX is synchronized to nsec Gekko-XII laser for variety of experiments with nsec and psec simultaneous laser beams irradiating the targets for fast ignition and other high-energy density physics.
We report total-reflection active-mirror laser experiments by using a cryogenic Yb:YAG composite ceramic. The
composite ceramic has no high reflection coating on the bottom surface, and is cooled with liquid nitrogen directly. We
obtained 273 W output power with optical efficiency of 65% and slope efficiency of 72% against the absorbed pump
power. The laser power and optical efficiency will be improved more when the pump power increases further. To
investigate thermal effects of the laser material in more detail, we have measured the thermal lens focal length and the
temperature of Yb:YAG. We observed thermal lensing effect of f ~ 2000 mm, and the maximum temperature of 150 K
for 400 μm-thick Yb:YAG sample. We have also studied the theoretical analysis of thermal distribution in the composite
The laser-induced damage thresholds in silica glasses at different temperature conditions (123 K - 473 K) by Nd:YAG
laser fundamental (wavelength 1064 nm) and third harmonic (wavelength 355 nm) 4 ns of pulses were measured. In the
results, the damage thresholds increased at low temperature. At 1064 nm, the temperature dependence became strong by
the concentration of impurities. However, at 355 nm, the temperature dependences of almost sample were almost the
same for different concentration of impurities.
We have been developing a high average-power laser system for science and industry applications that can generate an output of 20 J per pulse at 10-Hz operation. Water-cooled Nd:glass zig-zag slab is pumped with 803-nm AlGaAs laser-diode modules. To efficiently extract energy from the laser medium, the laser beam alternately passes through dual zig-zag slab amplifier modules. Twin LD modules equipped on each slab amplifier module pump the laser medium with a peak power density of 2.5 kW/cm<sup>2</sup>. In high power laser system, thermal load in the laser medium causes serious thermal effects. We arranged cladding glasses on the top and bottom of the laser slab to reduce thermal effects.
60 mJ chirped-pulse power was obtained using a diode-pumped regenerative amplifier with a cooled Yb:YLF crystal at 20 Hz repetition rate. After temporal compression, 36 mJ, 795 fs pulses were generated. Our numerical calculation shows that 100 mJ output power at 1 kHz would be obtained by using a sapphire-sandwiched laser material and a 2-pass amplification scheme in regenerative cavity.
Quantum-defect-limited operation in a diode-pumped Yb:YAG oscillator have been demonstrated at low temperature. The highest slope efficiency of 90% was obtained with M<sup>2</sup>=20 at the crystal temperature below 70 K, which was close to the theoretical stokes efficiency of 91.2% (λ<sub>pump</sub>/λ<sub>laser</sub>=941nm/1030nm). An optical-to-optical efficiency and a laser gain were 74% and 8 cm<sup>-1</sup>, respectively, at a low pump intensity of 1.3 kW/cm<sup>2</sup>. After optimizing a spatial mode coupling between a diode pump laser and a TEM00 cavity mode on the crystal, 80% slope efficiency and 70% optical efficiency were still high at M<sup>2</sup>=1~1.5.
We have used a pair of newly constructed electrodes to improve the discharge stability and electrical input power. The electrode shape was designed so that the discharge width became narrower, which lead to the increase of the input power density by 22%. As a result, the maximum output energy increased from 150 to 200 μJ at 147.8 nm. The pulse duration of 250 ns (FWHM) became shorter compared to the previous result (400 ns). This long pulse operation indicated the laser oscillation in an afterglow mode. The laser beam shape was circular with a beam divergence of 2.5 mrad. Because of the long pulse duration, this beam shape reflected on a cavity mode (multi-mode) as a result of the optical feedback. A small signal gain coefficient increased almost linearly with the increase of the discharge voltage. The maximum gain coefficient at 147.8 nm was 3.5%cm<sup>-1 </sup>at 31 kV.
A 24-mJ diode-pumped chirped-pulse regenerative amplifier with a cooled Yb:LiYF<sub>4</sub> crystal has been developed. A 10-mJ pulse energy with sub-ps pulse duration was obtained with a 660 fs pulse duration after compression.
The observed absorption and emission spectra ofYb:YLF at low temperature show that energy extraction efficiency is improved drastically with the reduced saturation fluence of 9 J/cm2 and the expanded spectral width of 38 nm. Yb:glass also shows a similar temperature dependence of the spectral characteristics to that of Yb:YLF.
We have realized a stable self-sustained discharge of high-pressure rare gases (Ar and Kr) using a compact discharge device. The glow discharge was obtained up to 10 atm of pure Kr. The vacuum ultraviolet emission intensity centered at 148 nm abruptly increased when the charging voltage exceeded a certain value. This "threshold" behavior indicates the onset of the stimulated emission at the wavelength. In addition to this threshold behavior, a considerable spectral narrowing was observed when the charging voltage exceeded the threshold value. The deconvoluted spectral width was 0.5 nm (FWFIM), which was much narrower than that of 4 nm (FWHM) at the charging voltage below the threshold. This significant spectral narrowing also strongly indicates the onset of the stimulated emission at 148 nm.
A 1.5 kW high peak power and 140 ns short pulse krypton excimer lamp in VUV spectral region has been developed using a pulsed silent-discharge. In such a high peak power operation, penning ionization was a dominant destructive process as in the same case of a rare gas excimer laser operation.
Efficient VUV excimer lamps with two types of discharge configurations, expanding jet discharge and silent discharge (dielectric-barrier discharge) in a variety of rare gases and their mixtures, are presented. In the jet discharges VUV output power was 9 mW with an efficiency of 10<SUP>-2</SUP>% at 126 nm for argon excimers. Output powers of other excimers were 300 mW with 1.0% efficiency at 146 nm for krypton excimers and 500 mW with 1.6% efficiency at 176 nm for xenon excimers. Simultaneous emissions from hetero-nuclear rare gas excimers (ArKr*, 135 nm) as well as homo-nuclear rare gas excimers (Ar<SUB>2</SUB>* and Kr<SUB>2</SUB>*) were observed by using rare gas mixtures of argon and krypton. Output powers and efficiencies of the silent discharge excimer lamps were 500 mW and 1.6% for argon, 5 W and 13% for krypton, and 5 W and 20% for xenon excimers. In the silent discharge extremely broad band excimer emissions were observed at the center wavelengths of 145 nm for an argon/krypton mixture and of 163 nm for a krypton/xenon mixture. A PMMA plate was photo-chemically etched at the rates of 1 - 2 nm/min by the irradiation of the 172 nm radiation in air and argon gas atmospheres.