We are investigating the output and temperature characteristics of Yb:YAG TRAM (Total-Reflection Active Mirror) laser using zero-phonon line excitation (969-nm pumping) and direct water jet cooling for efficient heat removal. The TRAM configuration has an advantage of cooling the surface of the Yb:YAG disk without the high-reflection coating. We have developed an efficient hydrodynamic cooling system, where the disk is directly cooled by impinging water jet with flow rate of up to 52 liter/min., while the water temperature can be controlled from 7 to 80 degrees Celsius. For the estimation of operating temperatures of the Yb:YAG, we measured fluorescence spectra from Yb:YAG using a spectrometer. We tested several types of TRAM with different layer thicknesses and doping concentrations, which were designed to absorb more than 80% of the pump power in a single bounce at room temperature. A fiber-coupled CW laser diode (FCLD) with 600 W output power at 969 nm was used as a pump source. The dependences of oscillator output power and the laser medium temperature on the cooling water temperature and flow rate were investigated. The direct impinging water jet at high flow rate was demonstrated to be effective for cooling the laser medium. It was also confirmed that the zero-phonon line excitation at 969-nm resulted in lower laser medium temperature and hence higher output power compared to the 940-nm pumping. In addition, we demonstrated kW-class laser oscillation using the cooling system and achieved slope efficiency of 63 %.
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
ceramic.
In the present paper, we overview fabrication methods to produce density-controlled tin and xenon targets for generating
extreme ultraviolet (EUV) light. The target can be classified as a mass-limited target. In the case of tin, EUV was
relatively monochromatic, and its conversion efficiency was higher than bulk tin. Using the nano-template method, the
cellular foam size was controlled by the template size. The density was 0.5 ~1.5 g/cm3. In the case of the 0.5 g/cm3
foam, its morphology was controlled by changing the ethanol content of the precursor tin solution. The morphology
difference was useful to control the angular distribution of EUV radiation. SnO2 nanofiber, which is oriented low-density
material, was fabricated continuously using a electrospinning method. The width and the shape of the fiber were
controlled by optimizing precursor solution. A transparent film with tin and SnO2 elliptic spheres were prepared using
liquid crystalline cellulose derivative. Low density xenon was prepared from liquid xenon using a swirl atomizer to
produce a density of 0.2 g/cm3.
EUV emission from spherical and planer targets were precisely characterized as an experimental database for use in
EUV source generation at high repetition rates. In the single-shot base experiments, conversion efficiency as high as
those for the plasma geometry has been demonstrated. The integrated experiment was made with 10 Hz plasma
generation.
We propose a new scheme for high conversion efficiency from laser energy to 13.5 nm extreme ultra violet emission
within 2 % band width, a double pulse laser irradiation scheme with a tin droplet target. We consider two-color lasers, a
Nd:YAG laser with 1.06 µm in wavelength as a prepulse and a carbon dioxide laser with 10.6 µm in wavelength for a
main pulse. We show the possibility of obtaining a CE of 5 - 7 % using a benchmarked radiation hydro code. We have
experimentally tested the new scheme and observed increase of CE greater than 4 %. We show many additional
advantages of the new scheme, such as reduction of neutral debris, energy reduction of debris ions, and decrease of out
of band emission. We also discuss debris problems, such as ion sputtering using newly developed MD simulations, ion
mitigation by a newly designed magnetic coil using 3-PIC simulations and tin cleaning experiments.
Ablation and amorphization of crystalline Si by ultrashort pulse laser irradiation is reported in this paper. Laser pulse width was changed in the range of 100fs-200ps. We investigated the dependences of ablation rate and fluence for amorphization on laser pulse width. From the dependence of ablation rate, we derived the "effective" light penetration depth, which was much shorter than that calculated from spectroscopic data. At the lower fluence than single shot ablation threshold, femtosecond laser irradiation induced amorphization of crystalline Si. Thickness of the amorphoized layer, which was about 50nm and almost uniform, did not depend on the number of irradiated laser pulses and fluence. The interaction process was investigated by an imaging pump-probe technique.
Laser-produced Sn plasma is an efficient extreme ultraviolet (EUV) light source, however the highest risk in the Sn-based EUV light source is contamination of the first EUV collection mirror caused by debris emitted from the Sn plasma. Minimum mass target is a key term associated with relaxation of the mirror contamination problem. For design of the optimum minimum mass Sn target, opacity effects on the EUV emission from the laser-produced Sn plasma should be considered. Optically thinner plasma produced by shorter laser pulse emits 13.5 nm light more efficiently; 2.0% of conversion efficiency was experimentally attained with drive laser of 2.2 ns in pulse duration, 1.0 × 1011 W/cm2 in intensity, and 1.064 μm in wavelength. Under the optimum laser conditions, the minimum mass required for sufficient EUV emission, which is also affected by the opacity, is equal to the product of the ablation thickness and the required laser spot size. Emission properties of ionized and neutral debris from laser-produced minimum mass Sn plasmas have been measured with particle diagnostics and spectroscopic method. The higher energy ions have higher charge states, and those are emitted from outer region of expanding plasmas. Feasibility of the minimum mass target has been demonstrated to reduce neutral particle generation for the first time. In the proof-of-principle experiments, EUV emission from a punch-out target is found to be comparable to that from a static target, and expansion energy of ion debris was drastically reduced with the use of the punch-out target.
For EUV lithography the generation of clean and efficient light source and the high-power laser technology are key issues. Theoretical understanding with modeling and simulation of laser-produced EUV source based on detailed experimental database gives us the prediction of optimal plasma conditions and their suitable laser conditions for different target materials (tin, xenon and lithium). With keeping etendue limit the optimal plasma size is determined by an appropriate optical depth which can be controlled by the combination of laser wavelength and pulse width. The most promising candidate is tin (Sn) plasma heated by Nd:YAG laser with a pulse width of a few ns. Therefore the generation technology of clean Sn plasma is a current important subject to be resolved for practical use. For this purpose we have examined the feasibility of laser-driven rocket-like injection of extremely mass-limited Sn or SnO2 (punched-out target) with a speed exceeding 100m/s. Such a mass-limited low-density target is most preferable for substantial reduction of ion energy compared with usual bulk target. For high average power EUV generation we are developing a laser system which is CW laser diode pumped Nd:YAG ceramic laser (master oscillator and power amplifier system) operating at 5-10 kHz repetition rate. The design of practical laser for EUV source is being carried out based on the recent performance of >1 kW output power.
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/cm2. 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.
Our institute has been investigating laser-produced tin plasma and EUV emission, and found the highest conversion efficiency of 3% at 13.5 nm in 2% bandwidth. In the present paper, we introduce fabrication methods of density-controlled tin targets to generate relatively monochromatic EUV with keeping similar conversion efficiency.
The first method is the nano-template method, where liquid tin solution was immersed into a polymer film with monodispersed size nanoparticle. The density can be controlled by tin concentration of the solution. The target can be classified into mass-limited target. We have shown a tendency of monochromatic EUV emission around 13.5 nm with decreasing of tin density. The intensity was higher than tin foil with bulk density. The tendency has a merit to mitigate heat effect of the first EUV mirror. The fabrication method has another merit to control not only density but also the poresize of tin oxide to be 100 nm ~ 10 mm. Recent experiments exhibited an EUV character depending on the poresize.
The second is liquid crystalline template method to obtain porous tin oxide. The precursor with tin oxide and cellulose provides mechanically stable and transparent film. The film has wavy sub-microstructure derived from microscopic liquid crystal domain structure. The method is simple and short duration for the hydrolysis reaction to solidify tin compound. This material has a merit of feasibility of fabrication, and was applied for rotation target for 10 Hz and 5 kHz laser repetition.
We have developed an integrated Laser Ablation Fluid Radiation simulation Code (LAFRAC) to estimate the behaviors of highly energetic ions and neutral particles from LPP EUV light sources, and estimated recombination and charge transfer processes between the particles from laser-produced Xe EUV light sources. We clarify that charge transfer effects greatly affect on the number density of neutral particles, especially high energy (more than roughly 500 eV) neutral particles.
It is very effective for mass-limited tin-foil targets to adapt for the EUV source. Tin-foil targets in account of formation, size, and thickness have been developed for debris mitigation. The amount of ions from targets is 40 % decreased tin-foil targets of 1μm or 5μm thickness than tin-bulk targets. The ion velocity is one order of magnitude less than bulk targets. The EUV emission spectra of tin-foil are more narrowing than bulk targets. The targets supply for high repetition rate of 10 kHz is applied for a novel method. It is called "Punch-out" method. The flight of graphite foil that it is a test targets was succeed to observe by using a gated ICCD camera. The target velocity is achieved to be about 120 m/s. This value can be applied for targets supply with high repetition rate of 10 kHz.
Properties of laser-produced tin (Sn) plasmas were experimentally investigated for application to the Extreme Ultra-Violet (EUV) lithography. Optical thickness of the Sn plasmas affects strongly to EUV energy, efficiency, and spectrum. Opacity structure of uniform Sn plasma was measured with a temporally resolved EUV spectrograph coupled with EUV backlighting technique. Dependence of the EUV conversion efficiency and spectra on Sn target thickness were studied, and the experimental results indicate that control of optical thickness of the Sn plasma is essential to obtain high EUV conversion efficiency and narrow spectrum. The optical thickness is able to be controlled by changing initial density of targets: EUV emission from low-density targets has narrow spectrum peaked at 13.5 nm. The narrowing is attributed to reduction of satellite emission and opacity broadening in the plasma. Furthermore, ion debris emitted from the Sn plasma were measured using a charge collector and a Thomson parabola ion analyzer. Measured ablation thickness of the Sn target is between 30 and 50 nm for the laser intensity of 1.0 x 1011 W/cm2 (1.064 μm of wavelength and 10 ns of pulse duration), and the required minimum thickness for sufficient EUV emission is found to be about 30 nm under the same condition. Thus almost all debris emitted from the 30 nm-thick mass-limited Sn targets are ions, which can be screened out by an electro-magnetic shield. It is found that not only the EUV generation but also ion debris are affected by the Sn target thickness.
It is necessary to know the ablation rates of various material surfaces under the irradiation of intense UV laser lights, as one of the ablating beams, when we design the future inertial fusion reacotrs driven by big lasers. So that, we irradiated various materials with a UV laser light. The laser wavelength, the pulse width and the maximum repetition rate were 193 nm, 20 ns, and 50 Hz. Irradiated materials were LiPb, Pb as the wet wall candidates of the inertial fusion reactors, while W, C, Al2O3, SiC and SiC-Si composite material as the dry wall candidates were also irradiated. The changes of teh irradiated surface conditions under different irradiation conditions were observed with three different kinds of optical diagnostic tools. The structures of the produced craters were investigated precisely. We could know how strong the tungsten was against such laser ablations, compared with LiPb or Pb etc..
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 M2=20 at the crystal temperature below 70 K, which was close to the theoretical stokes efficiency of 91.2% (λpump/λlaser=941nm/1030nm). An optical-to-optical efficiency and a laser gain were 74% and 8 cm-1, respectively, at a low pump intensity of 1.3 kW/cm2. 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 M2=1~1.5.
Hydrodynamic instabilities are key issues of the physics of inertial confinement fusion (ICF) targets. Among the instabilities, Rayleigh-Taylor (RT) instability is the most important because it gives the largest growth factor in the ICF targets. Perturbations on the laser irradiated surface grow exponentially, but the growth rate is reduced by ablation flow. The growth rate γ is written as Takabe-Betti formula: γ = [kg/(1+kL)]1/2–βkm/pa, where k is wave number of the perturbation, g is acceleration, L is density scale-length, β is a coefficient, m is mass ablation rate per unit surface, and ρa is density at the ablation front. We experimentally measured all the parameters in the formula for polystyrene (CH) targets. Experiments were done on the HIPER laser facility at Institute of Laser Engineering, Osaka University. We found that the β value in the formula is ~ 1.7, which is in good agreements with the theoretical prediction, whereas the β for certain perturbation wavelengths are larger than the prediction. This disagreement between the experiment and the theory is mainly due to the deformation of the cutoff surface, which is created by non-uniform ablation flow from the ablation surface. We also found that high-Z doped plastic targets have multiablation structure, which can reduce the RT growth rate. When a low-Z target with high-Z dopant is irradiated by laser, radiation due to the high-Z dopant creates secondary ablation front deep inside the target. Since, the secondary ablation front is ablated by x-rays, the mass ablation rate is larger than the laser-irradiated ablation surface, that is, further reduction of the RT growth is expected. We measured the RT growth rate of Br-doped polystyrene targets. The experimental results indicate that of the CHBr targets show significantly small growth rate, which is very good news for the design of the ICF targets.
Extreme ultraviolet (EUV) emission from laser produced tin plasma was investigated for 1064, 532 and 266 nm laser wavelengths. The EUV conversion with tin target tends to be high for shorter laser wavelength and is optimized at 4-5x1010 W/cm2 for 1064 and 532 nm. The EUV emission exhibits laser wavelength dependence in terms of angular distribution and structures of emission spectra. It is found that spectra for 532 nm and 266 nm showed spectral dips at around 13.5 nm and these dips are well replicated in computer simulations. Both the angular distribution together with the spectral dips may suggest existence of opaque plasmas surrounding the EUV emission region.
Extremely ultraviolet (EUV) light at around 13.5 nm of wavelength is the most probable candidate of the light source for lithography for semiconductors of next generation. We have been studying about the EUV light source from laser-produced plasma. Detailed understanding of the EUV plasma is required for developments of modeling with simulation codes. Several parameters should be experimentally measured to develop the important issues in the simulation codes. We focused on density profile, properties of EUV emission, and opacity of the laser-produced plasmas. We present re-cent experimental results on these basic properties of the laser-produced EUV plasmas.
Extreme ultraviolet (EUV) emission from laser produced plasma attracts much attention as a next generation lithography
source. The characterization of EUV emission has been carried out using GEKKO XII laser system. The twelve beams
irradiated tin or tin-oxide coated spherical targets uniformly and dependence of EUV spectra on laser intensity were
obtained with a transmission grating spectrometer and two grazing incidence spectrometers. The EUV Conversion
Efficiency (CE, the ratio of EUV energy at the wavelength of 13.5 nm with 2 % bandwidth to incident laser energy) was
measured using an absolutely calibrated EUV calorimeter. Optimum laser intensities for the highest conversion were
found to be 0.5- 1x1011 W/cm2 with CE of 3 %. The spectroscopic data indicate that shorter wavelength emission
increases at higher laser intensities due to excessive heating beyond optimum temperatures (20- 40 eV). The CE was
almost independent on the initial coating thickness down to 25 nm.
Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of
13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV)
and 1 ~ 2.5 nm (about 0.5 ~ 1.2 keV) are experimentally observed from Sn and/or SnO2 plasmas. These
emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy
balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute
to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity.
To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and
spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme
and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis.
Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of
the corresponding experimental results.
A new research project on extreme ultraviolet (EUV) source development has just been started at the Institute of Laser Engineering, Osaka University. The main task of this project is to find a scientific basis for generating efficient, high-quality, high power EUV plasma source for semiconductor industry. A set of experimental data is to be provided to develop a detailed atomic model included in computer code through experiments using GEKKO-XII high power laser and smaller but high-repetitive lasers. Optimum conditions for efficient EUV generation will be investigated by changing properties of lasers and targets. As the first step of the experiments, spherical solid tin and tin-oxide targets were illuminated uniformly with twelve beams from the GEKKO XII. It has been confirmed that maximum conversion efficiency into 13.5 nm EUV light is achieved at illumination intensity less than 2 x 1011 W/cm2. No significant difference is found between laser wavelengths of one μm and a half μm. Density structure of the laser-irradiated surface of a planar tin target has beem measured experimentally at 1012 W/cm2 to show formation of double ablation structure with density plateau by thermal radiation transport. An opacity experiment has just been initiated.
The present paper summarized recent activity of the target fabrication group at Institute of Laser Engineering (ILE), Osaka University. We focused on (1) organic photovoltaic materials to suppress the damage from laser-shine-through and (2) new emulsion technique to fabricate polyimide capsule. The following topics describes briefly, (3) organic feromagnetic materials for magnetic levitation of the target, (4) ultralow density foams of hydrocarbon whose density is ~2.0 mg/cc with micrometer-sized structures, and (5) new ultrathin (~nm) adhesion technique to provide laser-shock experiment targets.
Laser ablation of Cu, Al, Fe, Zn, Ni, Pb, and Mo by short pulse laser (800nm wavelength, 70fs pulse duration, 0.01-28 J/cm2 fluence range) in air was studied. Three different ablation thresholds were distinguished in all metals. The lowest ablation threshold was of one order of magnitude lower than the one observed previously. In the fluence range of 0.018-0.18 J/cm2 the ablation rate was ≈0.01 nm/pulse. A dependence of the threshold on the pulse duration was demonstrated in the range of 70 fs- 5 ps for cupper. As the laser pulse duration increased, the ablation threshold had the tendency to be higher. A periodic structure was observed at the bottom of the crater in all metals. The spacing d of the patterned structure was determined to be d=300±40 nm for 0.07 J/cm2 and d=600±40 nm for 0.22 J/cm2. The spacing depended on the laser fluence rather than on laser wavelength.
To demonstrate the feasibility of the fast ignitor scheme, the 100 TW Petawatt Module laser PWM was illuminated on an imploded spherical CD shell, which enhanced neutron yield ~106 for the first time. This is due to the fusion reactions between the energetic deuteron beam and cold dense deuterons. Without PWM, we have observed no neutrons above the detection threshold ~105. Proton rear emission and possible gigagauss scale magnetic fields from 60 TW laser illuminated plastic target are observed in a good agreement with a two-dimensional PIC simulation. The PWM is up graded to the world biggest Petawatt Laser (say PW laser). The out put is 1 Petawatt, which is used also for ultra-intense laser matter interactions above the intensity of 1020 W/cm2.
KEYWORDS: Fusion energy, Laser welding, High power lasers, Glasses, Laser development, Pulsed laser operation, Neodymium, Laser applications, Near field optics, Energy efficiency
We are developing high average power diode pumped solid state laser DPSSL for laser fusion power plant, for space propulsion and for various applications in industry. The common features or requirements ofour High Average-power Laser for Nuclear-fusion Application (HALNA) are large pulse energy with relatively low repetition of few tens Hz, good beam quality oforder of diffraction limit and high efficiency more than 10%. We constructed HALNA 10 (10J X 10Hz) and tested the performance to clarify the scalability to higher power system. We have obtained in a preliminary experiment a 8.5J output energy at 0.5Hz with bean quality of2 times diffraction limited far-field pattern. We are also trying to extend the laser technology of high peak power with low repetition to that of high averge power laser with high repetition or CW operation. The progress of power laser technology is now opening new fields of applications in science and industry. The flexibility and variety ofperformance are needed.
The recent progress of high power diode laser is opening new fields of laser and its application. We are developing high average power diode pumped solid state laser DPSSL for laser fusion power plant, for space propulsion and for various applications in industry. The common features or requirements of our High Average-power Laser for Nuclear-fusion Application (HALNA) are large pulse energy with relatively low repetition of few tens Hz, good beam quality of order of diffraction limit and high efficiency more than 10%. We constructed HALNA 10 (10J X 10 Hz) and tested the performance to clarify the scalability to higher power system. We have obtained in a preliminary experiment a 8.5 J output energy at 0.5 Hz with beam quality of 2 times diffraction limited far-field pattern.
As a first step of a driver development for the inertial fusion energy, we are developing a diode-pumped zig-zag Nd:glass slab laser amplifier system which can generate an output of 10 J per pulse at 1053 nm in 10 Hz operation. The water-cooled zig-zag Nd:glass slab is pumped from both sides by 803-nm AlGaAs laser-diode (LD) module; each LD module has an emitting area of 420 mm X 10 mm and two LD modules generated in total 200 kW peak power with 2.5 kW/cm2 peak intensity at 10 Hz repetition rate. We have obtained in a preliminary experiment a 8.5 J output energy at 0.5 Hz with beam quality of 2 times diffraction limited far-field pattern.
We have developed a 'FUNRYU' water cooled heat sink to efficiently cool the LDs and a new assembling system to reliably fabricate LD arrays and evaluated them using a conventional AlGaAs LD bar. We have achieved a maximum CW output power of 115 W limited by the driver circuit and the maximum conversion efficiency of 51% furthermore, we also design the 'FUNRYU' heat sink to be stackable and obtained a power density over 550 W/cm2.
The use of sub-ps laser pulses for material processing results in very precise cutting and drilling with high efficiency. But the energy of femtosecond laser pulse is essentially small. To accomplish high throughput for the industrial applications, it is required to operate the laser system at a high repetition rate and thus at a high average power. Therefore, we decided to develop a LD-pumped femtosecond MHz Yb:YAG MOPA laser system. As a first step, the development of a Yb:YAG oscillator, which employs Kerr-lens mode locking, is in progress.
A 3D analysis code of thermal birefringence in solid-state lasers was developed. Basic equations include thermal conduction, absorption of laser energy, thermal stress and thermal birefringence. Relative phase shift induced by thermal effects are measured and found to be in good agreement with quantitative simulation results. Edge effects of thermal birefringence are quantitatively estimated by 3D simulation. Those are unable to be estimated by 2D analysis.
The gray-tracking of KTiOPO4 nonlinear crystals, which is used for second harmonic generation of solid state lasers, have been investigated. The optical absorptions were measured, and then the susceptibilities of gray-tracking were observed by measuring the reduction of transmittance of the crystal by irradiating of the laser pulses with the second harmonics of Nd:YAG laser. In addition, impurities in the crystals were analyzed. In comparison with the susceptibilities and the absorptions, the susceptibility of the gray-tracking of KTP crystals indicated a dependence on the initial absorption at 532-nm wavelength. Moreover, it is prevented with increasing OH concentration in the crystals. We propose a new approach toward the improvement of gray-tracking for KTP crystals using annealing process under control of the humidity.
A novel laser architecture of laser-diode pumped eight pass 1064-nm Nd:YAG zig-zag slab laser amplifier with thermal birefringence compensation by use of a 90 degree quartz rotator has been developed aiming to achieve a high average power laser with high efficiency and good beam quality. With a 100 W (1 kHz) class module, the basic performance of the novel concept was examined by demonstrating an average power of 68 W with a high energy extraction efficiency of 61% for the laser mode volume at an initial small signal gain of 3.03 on single pass, and an excellent beam quality, undernegligible thermal lensing with the compensation for thermal birefringence.
High peak power laser diode arrays are required for pumping solid-state lasers toward the laser fusion driver. We will present a first successful achievement of a 100 kW peak power AlGaAs diode laser module as the pump source of 10 J X 10 Hz Nd:glass slab laser in order to verify a small scale experiment of the diode-pumped solid-state laser (DPSSL) driver for inertial fusion energy development. The total peak power of 110 kW was obtained at the diode current 120 A, the pulse width 200 microsecond and the duty cycle 0.2% (10 Hz). The peak intensity was more than 2.5 kW/cm2 on the average over the emitting area of 418 mm X 10 mm. The center wavelength of this device was 803 nm at the cooling water temperature 17 degrees Celsius, which designed in order to fit within the peak absorption line of Nd:glass (HAP-4) slab.
The gray-tracking of KTiOPO4 nonlinear crystals, which is used for second harmonic generation of solid state lasers, have been investigated. The susceptibilities of gray-tracking were evaluated by measuring the reduction of transmittance of the crystal with the second harmonics pulse of Nd:YAG laser, and were compared with the optical absorption coefficients and contaminants in each crystal. As result, the susceptibility indicated a dependence on the initial absorption at 532-nm wavelength. Moreover, it was prevented with increasing hydroxyl concentration in the crystals. We propose a new approach toward the improvement of gray-tracking for KTP crystals annealing process under control if the humidity.
The inside defect in carbon steel with the diameter of 100 micrometers was successfully detected by a laser ultrasonics technique. Irradiation of a Q-switched Nd:YAG laser was used for ultrasonic degeneration, and a frequency doubled CW Nd:YAG laser combined with a Fabry-Perot etalon were used for detection of ultrasonic vibration on the specimen surface. The ratio of the inside defect diameter to the ultrasonic wavelength was estimated to be approximately 0.067. We also measured the dependence of the amplitude ratios of the first defect signa and the second longitudinal wave to the first longitudinal wave on the defect diameter.
C60 vapor was irradiated with an intense femtosecond laser pulse. Multiply charged carbon ions, up to C4+, were detected in the time-of-flight spectra. We have measured both energy and angular distributions with respect to the laser polarization direction for ions with different charge number. The kinetic energy of ions are distributed above 1 keV. The angular distribution measurement shows that the highly charged ions of C3+ and C4+ are mostly distributed in the direction parallel to the laser polarization, and the C+ ions are slightly distributed in the direction perpendicular to it. These observations clearly indicate that an anisotropic explosion takes place. The average energy of ions with the different charge number is found to be proportional to the square of the charge number, while the maximum energy is proportional to the charge number. Classical molecular dynamics simulations have been successfully carried out reproducing not only the energy spectra but also the angular distributions of ions. The simulations suggest that the most crucial process for the anisotropic Coulomb explosion of C60 is not the electron impact ionization, but the cascade hopping of electrons.
Multiphoton dissociation BCl3 using free electron laser was demonstrated for the first time. It was found that the dissociant yield in the laser irradiation region where the laser intensity exceeds the dissociation threshold is nearly 1. The maximum selectivity was 1.45 which increased gradually be decreasing the laser frequency toward red side from the vibrational resonance of BCl3. In order to discuss the present result, the dissociation experiments for TEA CO2 laser were also performed.
Optical parametric chirped pulse amplification has apparent advantages over the current schemes for high energy ultrashort pulse amplification. High gain in a single pas amplification, small B-integral, low heat deposition, high contrast ratio and, especially the extremely broad gain bandwidth with large-size crystals available bring people new hope for over multi-PW level at which the existing Nd:glass systems suffered difficulties. In this paper we present simulation and experimental studies for a high gain optical parametric chirped pulse amplification system which may be used as a preamplifier to replace the current complicated regenerative system or multi-pas Ti:sapphire amplifiers. Investigations on the amplification pump and chirped broadband pulse as a seed, respectively. Analysis and discussions are also given.
We have conceptually designed a diode-pumped Nd:glass slab amplifier module for Inertial Fusion Energy (IFE). As a first step of a driver development, we have been developing a diode-pumped zig-zag Nd:glass slab laser amplifier system which can generate an output energy of 10 J per pulse at 1053 nm in 10 Hz operation. The water-cooled zig-zag Nd: glass slab is pumped for both sides by 803-nm AlGaAs laser- diode (LD) module; each LD module has an emitting area of 420 mm X 10 mm and two LD modules generated in total 200kW peak power with 2.5kW/cm2 peak intensity at 10 Hz repetition rate. We have obtained in a preliminary experiment a 8.5 J output energy at 0.5 Hz with a beam quality of 2 times diffraction limited far-field pattern.
Spatial near-resonant effects on laser beam propagation in a multilevel photoionization system have been studied by 2D simulation. It was found that the initial amplitude of nonuniformity grows up during the propagation, which degrades the photoionization rate of the target atom. The spatial effects become large for the longer incident laser pulse. A semi-analytical formula to analyze the growth of nonuniformity in the multilevel photoionization system was derived.
We have proposed and designed a diode-pumped solid-state laser (DPSSL) driver for inertial fusion energy (IFE) which consists of water-cooled zig-zag path Nd:glass slab amplifiers. A driver module has 10 kJ total output energy at 351 nm and operates at 12 Hz with 10.4 percent overall efficiency. The laser driver producing 4 MJ blue output for IFE will consists of 400 modules. To confirm the design, we are developing a small scale DPSSL module of 10J X 10Hz laser output at 1053 nm.
The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.
A novel laser architecture of laser-diode pumped eight pass 1064-nm Nd:YAG zig-zag slab laser amplifier was developed aiming to achieve high energy extraction efficiency and good beam quality. A high energy extraction efficiency of up to 73% for the laser mode volume has been achieved in this amplifier operating at an initial small signal gain of 3.36. The excellent beam quality factor M2 of 1.2 has been obtained. The compensation of thermal birefringence in this amplifier was successfully performed.
Long life, high power and high repetition frequency 2D laser diode (LD) arrays are needed for pumping solid state lasers. The reliability of AlGaAs/GaAs high-power lasers has been studied. Over 1 X 109 operation shots in 4-stack 2D LD arrays with 350 W peak output power and over 1 X 1013 shots in single-stripe laser diodes with 2.3 W peak output power have been obtained. 2D LD arrays of 3.5 kW (emitting area 3.2 cm x 1.0 cm), 2.5 kW (emitting area 1.0 cm x 1.0 cm) and 2.2 kW (emitting area 6.5 cm x 0.12 cm) were demonstrated under quasi continuous wave operation.
The physics of laser-plasma interaction and x-ray generation by laser have been extensively investigated as close relevance laser fusion. Efficient conversion to x-rays and controllability of wavelength, pulse width, and geometrical size have given us the high potentiality of applications to wide scientific and technical fields. An efficient high average power laser with a good beam quality is also under development as the key technology for the applications.
We report the high-sensitivity measurements of 2D thermal birefringence distribution in LD pumped Nd:glass (HAP4) laser under lasing at 1054 nm and non lasing conditions. The thermal birefringence was reduced under the lasing condition compared to the non lasing condition. By inserting a 90 degree(s) quartz rotator between two identically LD pumped Nd:glass rods, almost perfect birefringence compensation was achieved with very little residual birefringence. A considerable reduction of birefringence loss from over 20% before compensation to less than 1% after compensation was observed.
The design and construction of an efficient near-infrared holosteric optical parametric oscillator amplifier based on KTiOPO4 (KTP) crystal are reported. The system is tunable from 840 to 1451 nm and generates up to 0.45 mJ at 920 nm with 27% efficiency in 4 - 6 ns pulses. The 532 nm pump source is a frequency doubled laser-diode pumped Nd:YAG laser producing up to 2 mJ in 8 ns, 50 Hz pulses. We have used the system to study the fluorescence lifetimes of Yb:YAG and Yb:glass as a function of wavelength. The data reveal interesting variations in the reabsorption enhanced Yb-ion lifetime. Some Yb:YAG samples exhibiting a sharp increase in their lifetime near the peak absorption wavelengths. The longest lifetime measured in a few mm thick sample of Yb:YAG was 3.3 ms, which would be highly beneficial for energy storage applications using laser diodes.
We have applied magnetic assist and magnetic pulse compression to copper vapor laser and studied the switching characteristics, especially with the attention to the energy dissipation characteristics. As for magnetic assist, a suitable delay time was studied. And 2-stage MPC was designed using a simulation code and an all solid state driver with SI thyristors was developed.
A computer simulation code to treat atomic excitation and laser beam propagation simultaneously in an atomic laser isotope separation system has been developed. The three- level Bloch-Maxwell equations are solved numerically to analyze the change of pulse shapes, the modification of laser frequencies and the time-varying atomic populations. The near- resonant effects on the propagation of frequency chirped and non-chirped laser pulses have been analyzed. It was found that there are serious differences in pulse shapes, frequency modifications and propagation velocities between laser pulses with and without chirping.
We have developed a new technique for detecting the concentration of Cl in aqueous solution. The concentration of CF is determined by measuring the absorbance of C12 which is formed by
ArF laser irradiation. The absorption maximum of C12 is located at 340 nm. A linear relationship between the absorbance and the concentration on a log-log scale was confirmed. The detection
limit of 1x10-5 M has been achieved. This method is applicable to real-time and in-line measurement, and to other halogen ions.
The C-H bond in the methyl group of toluene, p-xylene, and mesitylene dissociates to the corresponding radicals from the hot molecules with internal energy of 8 eV. The dissociation rate constants were measured by the method of nanosecond laser photolysis. These rate constants can be predicted by a statistical theory on the basis of those measured by 193 nm irradiation. The C-C bond in paracyclophane was found to dissociate by two photons at 193 nm.
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