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Laser-induced damage thresholds (LIDTs) of fluoride thin films at 193-nm were estimated by irradiation of 2 laser pulses with different separate time of 0.01 to 1.0 seconds. As the separate time was 0.01 second, the LIDT was reduced to about half of 1 pulse LIDT. The transmittance at 100Hz repetition pulses also decreased remarkably. As a result, it would be concluded that the mechanism of the decrease in LIDT was due to the formation of defects and free-electrons.

We report highest power conductive-cooled Yb:YAG active-mirror amplifier (CcAMA) delivering 1 kW average power in 10 J, 100 Hz.

This work investigates the energy-transfer processes from Cr3+ to Nd3+ in yttrium aluminum garnet (YAG) ceramics by comparing the fluorescence decay of Cr3+ for Nd/Cr:YAG with that of Cr:YAG from 293 to 473 K. The experimentally determined fluorescence decay of Nd3+ excited through Cr3+ is consistent with the theoretical predictions when energy-transfer processes are taken into account by including in the rate equations the dipole-dipole interaction and energy transfer via excitation-energy migration. Furthermore, the results presented herein clarify that the efficiency of energy transfer from Cr3+ to Nd3+ increases slightly with increasing temperature.

UV light sources have many applications in science and technology. Generation of ultraviolet light from near-infrared lasers requires twice frequency conversion processes, such as the third- or fourth-harmonic generations (THG or FHG). In recent years, however, due to the increase in power of InGaN laser diode (InGaN-LD) and the emergence of primary visible solid-state lasers, it has become possible to generate ultraviolet laser light by once second-harmonic generation (SHG) [1] . In order to develop a more compact and simpler UV laser light source, we have been developing a single-mode primary visible fiber laser. Firstly, we successfully drew a single-mode double-clad structured Pr-doped waterproof fluoride glass fiber (Pr:SM-DC-WPFGF) and demonstrated laser oscillation with the power of 251.7 mW in 2016 [2] , then output power was improved to 2-W in 2019 [3] . We think that this kind of visible light source can be applied for a generation of UV laser.

In this study, the energy transfer mechanism in Cr,Nd,Yb:YAG (yttrium aluminum garnet) materials was investigated. Based on the emission/excitation spectra and the temporal evolutions of the donor and acceptor ions in Cr,Nd:YAG, Nd,Yb:YAG, and Cr,Nd,Yb:YAG materials, the role of the cascaded Cr3+→Nd3+→Yb3+ energy transfer was elucidated in terms of Yb fluorescence enhancement. The quantum yield balance among Cr, Nd, and Yb fluorescences was determined by solving rate equations, which utilized parameters such as the lifetimes of Cr3+, Nd3+, and Yb3+ ions and the energy transfer parameters related to these materials. The quantum yield for Yb fluorescence was determined to be 35–81% at a Yb concentration of 1–7 at%.

A stable operation of 1.1 J/100 Hz 10 ns laser pulses were achieved from a single cryogenically cooled Yb:YAG rod amplifier with ink-cladding. The efficiency and gain coefficient were 44% and 383, respectively.

We evaluated laser-induced damage thresholds (LIDTs) on silica glasses by two kinds of ultra-short soft X-ray laser pulses (13.5 nm, 70 fs, and 13.9 nm, 7 ps). The comparison of our experimental results and the reported values observed by nanosecond soft X-ray pulse revealed a pulse width dependence of LIDTs on silica glasses in the soft X-ray region. The relationship between the pulse width and LIDTs provided valuable discussions of the laser-induced damage mechanism.

High average power lasers with a high pulse energy are of considerable interest in various fields such as high-energy-density physics. Light-absorbing edge cladding is effective in suppressing parasitic oscillations in high-pulse-energy disk lasers; however, the large amount of heat generated from the cladding can affect the laser medium. We develop an improved conduction-cooled active-mirror laser with a double-sided cooled-edge cladding. A stable laser output with a pulse energy of 10 J at a repetition rate of 100 Hz was achieved using six liquid-nitrogen-cooled active-mirrors in the main amplifier. This study shows that aggressive cooling of the edge cladding is highly effective in decreasing the temperature rise and controlling the temperature distribution in the laser medium.

The concentration dependence of energy transfer from Cr3+ to Nd3+ at the 4 T 1 level excitation in Nd/Cr:YAG was investigated by the fluorescence decay curves of Cr3+ and Nd3+ for Nd/Cr:YAG and Cr:YAG ceramic powders in the Cr3+ concentration range of 0.1–6.0 mol%. The energy transfer process between Cr3+ and Nd3+ at the 4 T 1 level excitation is tried to explain using a rate equation that assumes energy transfer from the 2 E–4 T 2 level to Nd3+ on the basis of dipole–dipole interactions, the same as the 4 T 2 level excitation. In conclusion, the energy excited to the 4 T 1 level will relax non-radiatively to the 2 E–4 T 2 level and then transfer to Nd3+. It is presumed there will be no direct transfer from the 4 T 1 level to Nd3+. Our rate equations will be useful when simultaneously exciting the 4 T 1 and 4 T 2 levels of Cr3+ in Nd/Cr:YAG using broadband pumping sources.

Development of high-energy and high-repetition rate laser using Yb:YAG is a subject of growing attention to use many scientific and industrial applications. Especially, The Laser Wake Field Acceleration (LWFA) is receiving a lot of attention in the world [1–3]. Laser driven plasma accelerator (LPA) has potential to accelerate electron bunches to high energies in mm-size acceleration length, and still provides high quality bunch characteristics like: high bunch charge, femtosecond duration bunch length, and high pointing stability [4]. However, the accelerated electron charge is no many compare to radio frequency accretion. Therefore, the high repetition rate and high peak power laser is required. We have developed 1 J, 10 Hz cryogenically-cooled active mirror amplifier using Total-reflection active-mirror (TRAM) architecture [5]. At present, we are developing the 100 J, 100 Hz cryogenically-cooled active-mirror amplifier using Yb:YAG. The front end for the laser system is consists of CW distributed feedback laser diode, EO pulse slicer, which generates laser pulse of several ns duration at a pulse energy of few nJ. This pulse amplified up to around 200 mJ in the regenerative amplifier and multi-pass amplifiers using by cryogenically-cooled Yb:YAG. This pulse is amplified energy of 100J through two stage cryogenically-cooled active-mirror amplifier. We are developing the conductive cooled active mirror using the liquid nitrogen circulation. It is possible to 10kW (100J, 100Hz) operation. In order to conductive cooled active mirror, it is necessary to develop a cooling method for suppresses wave front changes in cooling. Therefore, we are developing Key technologies about cooling structure, bonding of amplifier medium and heat sink, wave front compensation. Also, we made the wave front measurement system of cryogenically-cooled active mirror. Figure. 1 (a) show the layout of wave front measurement system for cryogenically-cooled active mirror. The reference laser is fibre coupled CW distributed feedback laser diode at 1030 nm of wavelength. The reference laser is expanded to 50 m diameter by beam expander. The Yb:YAG coated AR and HR coat for 1030 nm. The Yb:YAG bonding at Heat-sink and cooling by cryocooler (Stirling cooler). The refracted laser is reduced to 5 mm diameter by beam reducer and input to the wave front sensor (SID4, PHASICS). Figure.1(b) show the measurement result of wave front shift. The wave front shift is change amount (P-V change) from the reference at room temperature (300K). The wave front shift increases with cooling, it is small value. Also, we measured the Zernike coefficient. It is dominant in low order and easy to compensation.

Visible lasers have many applications, such as, optical storage, display technologies, medicine, and flow cytometry. Especially primary visible solid-state laser (PVSSL) is a very attractive fundamental laser source of UV laser by second harmonic generation (SHG), because such UV laser source is expected to be higher conversion efficiency than typical UV laser system with third or fourth harmonic generation technique with twice nonlinear conversion process. Recently, fundamental visible laser researches have been increasing and the power also becomes enough large for applications. We have been developing visible fiber lasers based on Pr-doped waterproof fluoride glass fiber (WPFGF) [1], and the power progressed remarkably and reached over 1.0 W in red, green and blue colors [2]. Since then we developed a double-clad (DC) structured fiber [3], the fundamental visible fiber laser becomes a promising item to build a new UV laser system. In this paper, we report a 2 W single-mode laser oscillation in Pr:DC-WPFGF.

A grazing incidence, shallow liquid metal mirror could be used as the final optic for the heating laser in a fast-ignition fusion power plant. The relaxation of vibrations on the surface of liquid ...

Energy transfer from Cr3+ to Nd3+ in Cr3+ and Nd3+ co-doped Y3Al5O12 (Nd/Cr:YAG) was investigated by measuring the decay curves of Nd3+ and Cr3+ fluorescence for Cr:YAG and Nd/Cr:YAG ceramic powder samples that were produced by the sol–gel method in a Cr3+ concentration range of 0.1–6.0 mol%. The energy transfer processes in Nd/Cr:YAG could be explained based on dipole–dipole interactions even when the concentration quenching of Cr3+ was included, and the experimental fluorescence decay characteristics of Nd3+ excited through Cr3+ were consistent with the theoretical predictions based on dipole–dipole interactions. For a high-quality Nd/Cr:YAG ceramic with negligible concentration quenching, the energy transfer efficiency from Cr3+ to Nd3+ increased with an increase in Cr3+ concentration.

We report, to the best of our knowledge, the highest power conductive-cooled active-mirror amplifier (CcAMA) using Yb:YAG with a pulse energy of 10 J. By using four liquid-nitrogen circulating cooled laser heads, we achieved a repetition rate, pulse energy, and average power of 33.3 Hz, 9.3 J, and 310 W, respectively. The problem of wavefront distortion, which is difficult to solve with a large-aperture active-mirror laser, is suppressed by using reinforcing materials with the same thermal expansion coefficient. We have confirmed that the wavefront distortion is small ( 0.15 λ P-V per head) at 100 Hz operation, which paves the way for 100 Hz operation with the CcAMA concept.

Laser-induced damage thresholds (LIDTs) of silica glasses obtained by the femtosecond soft x-ray free-electron laser (SXFEL, 13.5 nm, 70 fs) and the picosecond soft x-ray laser (SXRL, 13.9 nm, 7 ps) are evaluated. The volume of the hydroxyl group in the silica glasses influenced its LIDTs. The LIDTs obtained in this research by the femtosecond SXFEL and the picosecond SXRL were nearly identical, but were different from that by the nanosecond soft x-ray pulse. The photoionization processes of silica glass in context of the laser-induced damage mechanism (LIDM) are also discussed. In the ultra-short soft x-ray pulse irradiation regime, the LIDM can be speculated to include the spallation process with a scission of bondings.

A Pr-doped double-clad structured waterproof fluoride glass fibre (Pr:DC-WPFGF) was successfully drawn, and the laser oscillation was demonstrated. The maximum output power was measured as 251.7 mW at 639.0 nm. The Pr:DC-WPFGF was designed to be single-mode fibre, with 0.08 of numerical aperture and 5.2 μm of core diameter. The beam-quality factor, M 2 , was measured and evaluated to be 1.0.

The energy transfer from Cr3+ to Nd3+ for Nd/Cr:YAG (Nd: 1.0%, Cr: 2.0%) materials was investigated by measuring the temperature dependences of fluorescence characteristics. The fluorescence intensity of Nd3+ increased with temperature owing to enhancement of the absorption coefficient of Cr3+. The energy transfer efficiency was constant from 77 to 450 K. The energy transfer time decreased with increasing temperature.

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.

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 pressure of 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.

We evaluated the laser-induced damage resistance of dielectric coatings at temperature from 123 K to 473 K with 1064-nm wavelength at 4-ns pulse duration. In the results, laser-induced damage resistances were improved at low temperature for dielectric coatings (single layer of SiO2 and MgF2). These results are important for understanding the laser-induced damage mechanisms.

The mechanical properties of lanthanum fluoride (LaF3) films are investigated. The films are deposited by using a resistive heating (RH) boat evaporation method and the advanced ion plating (AIP) method. A severe abrasion resistance test and a laser induced damage threshold (LIDT) test are done on the films. Abrasion resistance is increased by increasing the deposition temperatures for RH films. On the other hand, for AIP films, abrasion resistance is increased by adding direct current (DC) bias voltage. It is confirmed that the AIP films have a superior LIDT characteristics than those of RH films. With an additional DC bias voltage superimposed to a self bias voltage, the abrasion resistance and LIDT value is further improved. Based on the proposed mechanism of LIDT process, the microstructures of LaF3 films are discussed.

We demonstrated laser oscillation in an only 4-mm Nd-doped silica fiber, which is 10 times shorter than the previously reported one. This fiber-core glass, which was fabricated by the zeolite method, contained 1.25 wt% of Nd 2 O 3 . The laser output power was observed to be 3.1 mW at 1060 nm, and the laser threshold power and slope efficiency were 7.7 mW and 8.9%, respectively. Nd-doped silica fiber can produce high-repetition and extremely compact short-pulse laser devices and will also be applied to optical integrated circuits with environmental toughness in practical use.

We examined luminescence properties of highly chromium co-doped Nd:YAG powders fabricated by the sol–gel method. Though the maximum luminescence intensity of Cr:YAG was at 3.0 mol% of Cr3+ concentration, we found that the enhancement effect on Nd:YAG by co-doping Cr3+ was continued until 6.0 mol% of Cr3+ concentration. This phenomenon is well-explained by a simple energy transfer model derived from the rate equations between Cr3+ and Nd3+. The enhancement of sensitization for Nd3+ activator by high co-doped Cr3+ is very effective for making a miniaturized or thin film-type solar laser.

We propose a convenient method for pre-evaluating the spectroscopic properties of luminescent impurity doped YAG by sol–gel synthetic powder. Three synthetic methods, the sol–gel, normal-strike, and reverse-strike methods, are examined for their ability as a predictor and the sol–gel method proved the most effective of the three in experiments on crystallinity, particle size, and spectroscopic properties. The sol–gel Nd:YAG powder agrees well with that of a single crystal or ceramic YAG regarding the luminescent spectrum, the intensity, and the lifetime, even though the powder sintered at a low temperature of 1100 °C in comparison with the YAG crystal melting point of 1970 °C. These facts suggest that the sol–gel synthetic powder is the most effective and the easiest technique for evaluating spectroscopic properties of bulk YAG materials before fabricating the final materials.

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.

The laser damage on a total internal reflection (TIR) surface as well as on entrance and exit faces was evaluated to design a zig-zag slab laser. Incorporating a phase shift by the Goos–Hanchen effect on a TIR surface of a laser slab in contact with water, formulas for a laser-induced damage threshold (LIDT) were modified. We determined a method for the designing the geometry of a zig-zag slab laser based on LIDT. A thin film for anti-reflection (AR) with a high LIDT must be formed on the entrance and exit faces of a laser slab glass. A sol–gel thin film was adopted for the AR coating of a high-power laser due to its high LIDT. We devised an effective dip-coating method to form a sol–gel thin film uniformly on a small area. The LIDT of the sol–gel thin film was measured and evaluated with a single mode Q-switch Nd:YAG laser by a one-on-one measurement technique.

A new type of solar pumped laser system that has features of high-efficiency and low cost due to the use of a Fresnel lens and a Cr codoped Nd:YAG ceramic laser medium is reported in this paper. Slope efficiencies of 7 to 9% have been achieved. The laser output of about 80 W was achieved with the combination of a 4-m2 Fresnel lens and solar light chamber as a secondary power concentrator. The maximum output from a unit area of the Fresnel lens was 20 W/m2, which is 2.8 times larger than the previous results with a mirror type concentrator.

We examined the laser induced damage threshold (LIDT) of dual wavelength mirrors under the simultaneous irradiation of two wavelengths. The effects of interferences between the two waves were discussed.

We demonstrated a visible laser oscillation in a Pr-doped double-clad structured waterproof fluoro-aluminate glass fiber. The maximum output power and the slope efficiency were measured to be 251.7 mW at 639.0 nm and 43.4%, respectively.

Cr3+ fluorescences excited in Cr3+ 4T 1 level for Cr:YAG and Nd/Cr:YAG powders were measured. The energy transfer efficiency from Cr3+ to Nd3+ was increased from 68% to 78% with increasing concentration of Cr3+.

In this paper, we describe the development of a high-average-power solid-state laser system and the derivation of equations for the amplification of a laser beam. This laser system is capable of generating an output energy of 10 J per pulse at a wavelength of 1,053 nm in a 10 Hz operation for scientific and industrial applications. The main amplifier of our system is a laser-diode-pumped solid-state amplifier. A water-cooled Nd:glass slab is pumped with two 803 nm AlGaAs laser-diode modules. The laser beam propagates through zig-zag optical paths four times and is amplified. To estimate laser output energy, we have derived and evaluated equations for the amplification of the laser beam, and designed and constructed a laser system based on the calculated results. Experimental results reveal an output energy of 10.6 J at 1 Hz, which closely fits the results calculated using the derived equations.

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.