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We adopted a single-mode, single-wavelength volume holographic grating (VHG) wavelength-stabilized wavelength laser diode (LD) as a pumping LD for an end-pumped microchip Nd:YAG and Nd:YVO4 lasers we developed during CW and pulse operations. Higher optical-optical and slope efficiencies during CW operation have been obtained than when using a VHG LD experimentally. Output laser power is insensitive to the temperature of the LD when using a wavelength-stabilized LD and can remain stable and almost constant until the temperature of LD increases up to 40°C. The improved optical-optical conversion efficiency of 58% for the Nd:YVO4 laser has been obtained and calculated the output laser power during CW operation and compared it with the experimental results. We found that the output laser power of the Nd:YVO4 laser using the VHG wavelength-stabilized LD was more than twice as high as that using an LD without VHG. When the ambient temperature increases, the difference in output laser power should be large. In the future, a low-cost end-pumped microchip laser that does not require a temperature control should be developed.

A mode-locked and Q-switched short pulse laser using the Nd3+/Cr3+:YAG ceramic has been constructed with a SESAM and Cr4+:YAG crystal optical switch based on excite state absorption (ESA). Laser oscillations of the pulse laser were observed experimentally. The Nd/Cr:YAG ceramic laser has a high conversion efficiency from white light (such as lamp light or solar light) to the laser. The Nd/Cr:YAG ceramic has a higher laser gain than the Nd:YAG laser for the same pumping power. The laser oscillation can be obtained very easily. A single-mode-locked laser pulse with fast modulation on the order of 100 ps was obtained in some pump power regimes when using the Cr4+:YAG crystal. The obtained pulse duration of the short pulse was a few hundred ps. A maximum peak power of 60 kW was obtained when using a SESAM. The same level of peak power (60 kW) was also obtained when using the Cr4+:YAG crystal.

An output peak power of over 1 kW for a Nd/Cr3+:YAG ceramic multi-amplifier system with white-light pumping was obtained. A four-stage active-mirror amplifier system was adapted for obtaining kW laser peak power. Output spectrum of pumping light used was close to that of solar light. Enhancement of saturated output laser power at high temperatures was observed owing to the phonon-assisted cross-relaxation effect. An output laser peak power of 1.6 kW were achieved at a ceramic disk temperature of 100 °C. The evaluated optical-optical conversion efficiency for volume of ceramics through which laser passed was 63% for the maximum output laser peak power of 1.6 kW. The laser system will be applicable to solar-pumped lasers with high optical-optical conversion efficiency.

Reduction in the skin effect for the sintered Si nanopolycrystalline body as an electricity conductor at a high frequency due to its nanostructure was studied. Singular disappearance of electrical resistances near a local high magnetic harmonic frequency of a few MHz was observed. This phenomenon has not been observed for conventional ferromagnetic metals. The measured electrical resistances changed to almost 0 mΩ at room temperature. At the same time, negative resistance of the sintered Si nanopolycrystalline body was observed. It will be applicable to electronic transmittance lines or semiconductors. Numerical calculation was also performed on the electrical resistance with frequency dependency while considering the electric field and magnetic field in the sintered Si nanopolycrystalline body. The experimental and calculated results are compared. The calculation could explain the variation of the relative permittivity of the Si nanopolycrystalline body and the phenomenon for the theoretical disappearance of the resistivity at the MHz frequency. Reduced Si nanoparticles from SiO2 powder were synthesized by laser ablation in liquid. A Si nanopolycrystalline body made of the reduced Si nanoparticles was fabricated. It was found by measuring the magnetization property of the body of the sintered Si nanopolycrystalline body which is ferromagnetic. Dangling bonds (unpaired electrons) have long been known to occur due to defects in Si crystals. Perfect Si without defective crystals has no dangling bonds. However, Si nanoparticles have many dangling bonds. High-density dangling bonds cause the sintered Si nanopolycrystalline body to have ferromagnetism. In this study, the density of the unpaired electrons in the sintered Si nanopolycrystalline body was observed using ESR. It has been clarified that the Si nanopowder and the sintered Si nanopolycrystalline body have numerous dangling bonds. Both densities of the dangling bonds were evaluated.

Pulse power sources can produce high temperature or high-density extreme conditions within a short time. They have been introduced to various fields such as those in laser, fusion research, the production of plasma, shockwaves in water, water treatment, and exhaust gas treatments. Various high voltage pulse sources using insulated-gate bipolar transistor (IGBT) semiconductors with high voltage resistivity have been used. Also, Marx bank circuit is well known as an instrument that produces voltage pulses with low repetitive rates. These instruments have several advantages. However, their problems are a complex structure, high cost, and excessive weight. A simple method of producing high voltage pulses with short rising times based on electrostatic induction in external capacitor used for pulse power applications is proposed. The circuit has a simple structure and contains a minimum number of parts, which makes the instrument small and light weight. In fact, the generation of sawtoothed high voltage pulses with short rising times and low repetitive rates of a few 100 Hz was successfully conducted in experiments. Theoretical analysis was simultaneously undertaken. The numerically calculated results for generating high voltage pulses were goodly consistent with the experimental ones. Moreover, it has been confirmed that amplification of the output voltage by electro-hydrodynamics (EHD) electricity generation using a jet flame resulted in higher voltage pulses, lower electricity consumption, and high repetition rates.

We investigated the optimized sensitizing effect of Ce/Cr/Nd:YAG material for solar-pumped solid-state lasers using the sol–gel synthetic YAG powder method. After a parameter mapping survey on Ce 3+ and Cr 3+ additives in Nd:YAG for the sensitizing effect on sunlight, we found that the most efficient composition of Ce/Cr/Nd:YAG is where Ce 3+ and Cr 3+ were 0.9 and 8.0 mol% respectively, when the Nd 3+ concentration was 1.0 mol% and the sensitizing effect of the optimized Ce/Cr/Nd:YAG was increased four times more than the Cr/Nd:YAG one. This result indicates that Ce 3+ can work as an effective sensitizer to Nd 3+ and optimized Ce/Cr/Nd:YAG ceramics can be useful to construct a space solar power system using a laser in space.

Spectral broadening of the fluorescence of a Nd/Cr:YAG ceramic at the 1064 nm lasing wavelength was observed, and the amplification properties at a high temperature were investigated by considering cross-relaxation. These ceramics are promising for use as a solid-laser material pumped with solar or lamp light. It has been found that whenever the temperature of a laser medium is high and a spectral shift occurs, a high small-signal gain remains owing to the broad spectral band and the cross-relaxation. This optical property is remarkably different from that of a Nd:YAG laser. For a conventional Nd:YAG laser, the bandwidth at 1064 nm is 0.45 nm, and a reduction in small-signal gain occurs at a temperature of 373 K because the spectral peak shift is 0.005 nm/K. However, for the Nd/Cr:YAG ceramic, the bandwidths are 1.2 and 1.9 nm in the case of 0.1% Cr ion doping and 3% Cr ion doping, respectively, owing to the existence of excited Cr ions and the shortening of the effective Nd ion coherence time. It is prospected that the laser medium can be used at a high temperature of 600 K.

Laser pulses with high repetitive rate generated from a Q-switch microchip Nd:YAG oscillator were amplified by Active mirror composed of Nd/Cr:YAG ceramic pumped by CW arc-lamp. The laser pulses were amplified, and saturation of averaged output laser power was observed. Repetitive ratio of injected laser pulses was changed from 20 to 100 kHz. Averaged output laser power and gain were measured, and gain coefficient and power-extraction efficiency were evaluated. Output laser power was calculated and compared to experimentally measured one, and the calculated results are consistent with the experimental ones. For amplification of laser pulses with high repetitive ratio, this laser material can realize very high laser gain with low pumping power density and high optical–optical conversion efficiency under CW-lamp-light pumping.

Laser pulses generated in a laser oscillator were amplified by Nd/Cr:YAG ceramic amplifiers. The saturation of output-laser energy was experimentally observed. We found that the stimulated emission cross-section of Nd/Cr:YAG ceramics was effectively increased and the saturation fluence was effectively reduced to 0.1 J/cm 2 due to the energy transfer between excited Cr and Nd ions. The output-laser energy we calculated was compared with the experimental ones and the extractable stored energy in the Nd/Cr:YAG ceramics was evaluated. This laser material is suitable for high-repetition-rate operations because of the high laser gain with low pumping intensity and low effective saturation fluence.

The concept of an Active mirror with transparent Nd/Cr:YAG ceramics is proposed. A new ceramic material pumped by an arc-metal-halide lamp has been experimentally implemented. CW-laser light generated in a 1064 nm Nd:YAG micro-tip oscillator was amplified by an Active mirror made of Nd/Cr:YAG ceramic and pumped by CW arc-lamp light. The saturation power density of the Nd/Cr:YAG ceramic was lower than that of Nd:YAG because the stimulated-cross-section and lifetime were effectively enhanced by the sensitization of Cr ions. The maximum output laser power while keeping spatial beam quality was up to 3 W with an input lamp power of 9 W; the resulting optical–optical conversion efficiency of 33% was close to the theoretical prediction of nearly 50%.

Remarkable improvements in the lifetime of the Nd upper level and in the effective stimulated emission cross-section of Nd/Cr:YAG ceramics have been theoretically and experimentally studied. Until recently, it had been thought that the long energy transition time from Cr ions to Nd ions of Nd/Cr:YAG adversely affects laser action, degrading optical–optical conversion efficiency under CW and flash lamp pumping. However, current research showed that high-efficiency energy transition has a positive effect on laser action. The effective lifetime is increased from 0.23 to 1.1 ms and the emission cross-section is effectively increased to three times for that of the conventional Nd:YAG. A small signal gain is significantly improved, and the saturation power density is reduced to 1/10 that of the Nd:YAG for the same pumping power density. A CW laser light generated in a laser diode (LD)-pumped 1064 nm Nd:YAG laser oscillator was amplified, and the measured output power was saturated. The output laser power calculated using theoretical saturation power density was consistent with the experimental results.

We observed laser oscillations of rod-type Nd/Cr:YAG ceramics experimentally pumped using an arc-metal-halide lamp having a similar spectrum to solar light. An optical–optical conversion efficiency of 43% was obtained by chromium-ion codoping of Nd:YAG with an efficiency of 26%.

We observed the lasing of highly sensitized Nd/Cr:YAG ceramic disks that use artificial solar radiation pumping. The disk material can be used for high power multi-stage amplifiers pumped by lamplight or sunlight because of its scalability and ability to handle high power densities. A maximum output power of 86 mW was experimentally obtained, and this value correlated well with the calculated results. A small signal gain of 1.9 was obtained, and the estimated stored energy was 0.6 J/cm 3 , which is three or four times higher than that from a Nd:YAG medium.

Marked increase in output laser power of a high-power Nd/Cr:YAG ceramic amplifier based on the cross-relaxation effect at high temperature under quasi-solar light pumping was observed. The solar-pumped laser is applicable to renewable energy production.

The multi-pass laser amplifier has been often used as the final power amplification stage in the high peakpower laser systems. The Amplified Spontaneous Emission (ASE) at such a strongly pumped laser generally results in a parasitic oscillation that limits a high gain operation of the amplifier. The three dimensional propagation of the finite-band-width ASE from laser glasses has been calculated for a 30-m long Cassegranian 3-pass disk amplifiers of 35-cm clear aperture. The computational code includes the temporal behavier of flashlamp pumping, the gain reduction in a laser disk due to ASE generation, the long-path back-and-force propagation of ASE between Cassegranian mirrors, and the reduction effect on ASE of spatial filter installed after the amplifier. The ASE energy and its profile have been compared for the on-axis and off-axis arrangement of Cassegranian optics, and also z-pass geometries for three pass configuration. The off-axis alignment of the mirror pairs is very preferable to reduce ASE to one hundredth level than that appeared in the on-axis geometry.

Thermal dependence on the small signal gain of Nd/Cr:YAG (yttrium aluminum garnet) ceramics was observed experimentally. Usually, Nd:YAG crystal and ceramics have remarkable gain reduction when optical pumping is performed and the temperature of the laser media is upped to 373 K. A CW laser light generated in a 1064 nm Nd:YAG laser oscillator was amplified by Nd/Cr:YAG ceramic amplifier, and the output power was measured at non-saturation level. Laser small signal gain of the ceramic disk was kept to 470 K. This property was remarkably different from one of Nd:YAG crystals or ceramics. The peak shift of the fluorescence was observed experimentally when the temperature is high.

We calculated ASE propagation in a Cassegrain-type amplifier. The code used in the calculations is applicable to a multipass amplification system. Output fluence of the ASE at the exit of the system can be decreased by optimization of design parameters. Ratio of diameter of main mirror hole to sub mirror diameter is the most sensitive parameter, and should be larger than 1.2. It is suitable to set gain medium (laser glasses) as near to the main mirror as possible. We found optimal values for a new amplifier chain in GXII laser facility which can deliver ultrashort and high power pulses for the basic research on fast ignition (Fujita et al., CLEO '97, CFK 3, Baltimore MD, 1997; [1]).

The upgrade of the actual large laser facilities (GEKKO XII in our case) implies an increase of filling-factor, but avoiding the generation of spatial high frequencies. To reach that purpose we intend to construct the beams using serrated apertures with an optimised shape. The performances of our system led after simulation to a 70% filling factor with less than 2% fluctuation on the laser pattern intensity map. The optimisation was also checked from a financial point of view. The usual serrated apertures are already giving good results, so we attached ourselves to seek for financial efficiency.

Efficient lasers as energy transfer devices from broadband solar light to coherent laser light have been developed recently. Research on efficient amplification of Nd3+/Cr3+:YAG ceramics based on cross-relaxation pumped by white light was performed.

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.

We report on a multiple total-reflection active-mirror laser with cryognic Yb:YAG ceramics using zigzag optical path. A 214W output power with 62% slope efficiency has been demonstrated.

With the rapid development of high transparent ceramics, it is possible to achieve high laser output powers. Currently, high average power lasers with high efficiencies and good beam qualities are desired for many applications. To develop such an ideal optical source, it is necessary to avoid thermal effects. Giesen et al. [1] proposed a thin disk laser concept. This concept used a face cooled thin, disk-shaped active medium. This face cooling minimized the temperature gradient and the phase distortions transversal to the direction of the beam propagation, and allowed realization of a laser with high output power (≫ 5.3 kW), high efficiency (≫ 65%), and good beam quality (M2 ≪ 24), simultaneously from a single Yb:YAG disk. The laser was pumped up to about 4 kW/cm2. Alternatively, Tsunekane et al. [2] reported a diode edge pumped microchip laser, and 414 W CW output powers were obtained at incident pump powers of 946 W from a 200-µm thick microchip laser. In their work, the pump density was about 8.8 kW/cm2 and the slope and optical-to-optical efficiency were 52% and 44%, respectively. However, the temperature rise at the high-reflection coating was considerable due to its low thermal conductivity. In fact, the temperature of the opposite surface which was non-cooled was measured to be 228 µC at the input of 946 W. Without the high-reflection coating and the heat sink, the temperature rise can be suppressed, leading to higher operation.

We report on the efficient operation of directly liquid-nitrogen-cooled Yb:YAG active-mirror laser. A CW output power of 273 W with slope efficiency of 72% was demonstrated. High heat removal ability was ensured at 743 W/cm2.

We report on the efficient high-power laser operation by using a cryogenic Yb:YAG composite ceramics with a total-reflection active-mirror arrangement. A 273W CW output power was demonstrated at 65% optical efficiency and 72% slope efficiency.

Electro hydro dynamic (EHD) and electrostatic propulsion devices were developed in the 1920s by Thomas Townsend Brown. One such device, called a “lifter”, has no moving parts and, in the air, operates on electrical energy. It is a fashionable device and has a very simple structure, basically consisting of a narrow wire electrode and a large, flat one. However, it has a low ratio of propulsion force to unit electrical input power. According to theory, the propulsion force it generates depends on the interaction between the ion density of the ionized air and the charges on the surface of the large electrode. EHD and electrostatic propulsion models using the polarization effect are proposed to improve the ratio of the propulsion force to unit electrical input power. The propulsion device generates propulsion force through the use of an asymmetrical metal structure with charges generated by the polarization effect. The propulsion force the new devices generated for the same electric energy was 5.7 times higher than that of a basic type lifter owing to additional propulsion force being generated by the maximum polarization effect in the experiments. It was found that combining other effects with this polarization effect results in the ratio of generated propulsion force to electric power being close to 100N/kW when the electric power is high. This value is as high as that of a helicopter. We also performed numerical analysis was also performed for capacitances and charges for various kinds of EHD and electrostatic propulsion devices. An optimized system was developed and is discussed in this paper.

We observed laser oscillations of rod-type Nd/Cr:YAG ceramics experimentally pumped using an arc-metal-halide lamp having a similar spectrum to solar light. An high optical-optical conversion efficiency of 43% was obtained by Cr ions co-doping.

Propagation and amplification of partial coherent light (PCL) is simulated with a 3-D propagation code and fast Fourier transformation. The PCL is useful to realize highly uniform irradiation on a target in internal confinement fusion. On the other hand, the PCL has speckle structure in space and time owing to partial coherence. The intensity modulation due to speckles is enhanced by self-focusing in laser glasses due to nonlinear refractive index. Calculations on the laser propagation for the PCL show that the speckles of cm scale self-focus to 10 mm scale. As a result, average fluence of the PCL is limited at lower level comparing with that of the coherent laser. Experimental results are also coincident with the simulation results.

The upgrade of the actual large laser facilities (GEKKO XII in our case) implies an increase of filling factor avoiding generation of high spatial-frequencies. To reach that purpose we intend to construct the beam using serrated aperture with optimized shape. The performances of our system lead after simulation to 70% filling factor with less than 2% fluctuation on laser pattern intensity map. As only a very few articles were written on that subject although it is not very new, this paper includes a theoretical approach of the serration effects. It presents mainly results from simulation and some experimental data.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on a novel amplifier configuration concept for a 10 kW laser system using a zig-zag optical path based on a cryogenic Yb:YAG Total-Reflection Active-Mirror (TRAM) laser. The laser material is a compact composite ceramic, in which three Yb:YAG TRAMs are combined in series to increase the output power. Output powers of up to 214 W with a slope efficiency of 63% have been demonstrated for CW operation, even at a quite low pump intensity of less than 170 W/cm2. Further scaling could achieve output powers of more than 10 kW.

We constructed a theory to explain the mechanism of laser generation with a high optical–optical conversion efficiency for Nd3+- and Cr3+-doped yttrium aluminum ceramics when sunlight or lamplight sources are used for pumping. As a result, a unique mechanism of laser action was found where the solar or lamp-light power could be converted to laser power with a high efficiency close to 80%, which has not previously been observed. The high conversion efficiency was not only considered to be based on one-to-one photon conversion but on two-photon excitation by a single photon with phonon assistance. Thus, the mechanism of lasing action should include a process where thermal energy is converted to photon energy. The theoretical results we obtained were consistent with those of the experiments.

A passively Q-switched Nd3+-and Cr3+-doped yttrium aluminum garnet (YAG) ceramic laser using Cr4+:YAG crystal as a Q-switch element is reported for the first time. Unique temporal waveforms of the output laser pulses were observed. A temporal waveform of a Q-switched laser pulse was compared with one in a free-running pulse. Laser pulses of 130–300 ns duration were generated. A maximum output laser energy of 1.1 J and an average peak power of 0.1 MW were obtained for multiple Q-switched pulses by a single shot, and a maximum output laser energy of 80 mJ was obtained for a single pulse. The evaluated maximum peak power was 0.6 MW as determined from measured pulse energy and duration. The effective saturation fluence and maximum stored energy were lower than those of Nd:YAG. However, a high peak power can be obtained using an output coupler with a low reflectivity owing to the high gain as in the case of Nd:YAG. The threshold of the Nd/Cr:YAG ceramic laser oscillator is reduced to less than 10% of that of a Nd:YAG oscillator. It was determined by calculation that the output laser energy of the Nd/Cr:YAG ceramic laser was close to 4 times that of Nd:YAG.

The authors propose an energy cycle based on a renewable fuel. Magnesium is chosen as an energy carrier and is combusted with water to retrieve energy using many power devices. MgO, the combustion residue, is reduced back to Mg by laser radiation generated from solar and other renewable energy sources. They have achieved an energy recovery efficiency of 42.5% for converting MgO to magnesium, using a laser. Combined with a demonstrated 38% efficiency for converting an artificial sunlight source (metal halide lamp) into laser output energy indicates that the proposed energy cycle is already in a feasible range for practical use.

An efficient high-power laser operation has been demonstrated by using a cryogenic Yb:YAG composite ceramic with a total-reflection active-mirror arrangement. The composite ceramic, which had no high-reflection coating and was cooled with liquid nitrogen directly, showed four-level operation even at 67 kW/cm(3) of high pump density. A 273 W cw output power was obtained with 65% optical efficiency and 72% slope efficiency.

We report a new compact high power laser system using zig-zag optical path based on the cryogenic Yb:YAG total-reflection active-mirror laser for ten-kilowatts. The thermal analyses of the laser have also been discussed.