Tomomasa Ohkubo

High-efficiency and economical solar-energy-pumped laser with Fresnel lens and chromium codoped laser medium

The authors achieved 11%–14% slope efficiency of solar-pumped laser by Cr-codoped Nd:yttrium aluminum garnet ceramic and Fresnel lens focusing from natural sunlight. The laser output of 24.4W was achieved with 1.3m2 Fresnel lens. The maximum output for unit area of sunlight was 18.7W∕m2, which is 2.8 times larger than previous results with mirror collector. The utilization of Cr3+ ion enabled efficient absorption and energy transfer to Nd3+ ion of solar spectrum. The fluorescence yield at 1064nm for various pumping wavelengths was measured both for Crcodoped and nondoped laser media, and 1.8 times enhancement of laser output from sunlight is predicted.

Demonstrated fossil-fuel-free energy cycle using magnesium and laser

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.

120 watt continuous wave solar-pumped laser with a liquid light-guide lens and an Nd:YAG rod.

We propose a simple and efficient pumping approach for a high-power solar-pumped laser by using a liquid light-guide lens (LLGL) and a hybrid pumping cavity. A 2×2 m Fresnel lens is used as a primary concentrator to collect natural sunlight; 120 W cw laser power and a 4.3% total slope efficiency are achieved with a 6-mm diameter Nd:YAG rod within a 14-mm diameter LLGL. The corresponded collection efficiency is 30.0 W/m(2), which is 1.5 times larger than the previous record. This result is unexpectedly better than that of Cr:Nd:YAG ceramics. It is because the scattering coefficient of Cr:Nd:YAG ceramics is 0.004cm(1), which is 2 times larger than that of the Nd:YAG crystal, although both have similar saturation gains.

Solar-pumped 80 W laser irradiated by a Fresnel lens

A solar-pumped 100 W class laser that features high efficiency and low cost owing to the use of a Fresnel lens and a chromium codoped neodymium YAG ceramic laser medium was developed. A laser output of about 80 W was achieved with combination of a 4 m(2) Fresnel lens and a pumping cavity as a secondary power concentrator. This output corresponds to 4.3% of conversion efficiency from solar power into laser, and the maximum output from a unit area of Fresnel lens was 20 W/m(2), which is 2.8 times larger than previous results with mirror-type concentrator.

100 W-class solar pumped laser for sustainable magnesium-hydrogen energy cycle

A solar pumped laser system with 7%–9% slope efficiencies has been developed. A Fresnel lens (2×2 m, f=2000 mm) is mounted on a two-axis sun tracker platform and focuses solar radiation toward laser cavity, which embraces Cr:Nd:yttrium aluminum garnet ceramic rod. The maximum emitted laser power is 80 W corresponding to maximum total area performance of 20 W/m2 for the Fresnel lens area. This solar laser system would be used as a section of power plant in a magnesium energy cycle as a cost-efficient solar energy converter. Using direct solar radiation into laser, 4.3% net conversion efficiency has been achieved.

Simulation and Experiments on Laser Propulsion by Water Cannon Target

In previous papers, we reported the successful flight of paper‐airplane about 5 cm. Application of such micro‐airplane to CO2 measurement and tornado observation is proposed. For practical application, repetitive water supply system and levitation system are proposed and examined by experiments. The latter can also be used for launching waste of nuclear reactor and structural materials for space station. Some future applications like stratospheric airplane and microship in human blood vessel are discussed.

Laser-induced Mg production from magnesium oxide using Si-based agents and Si-based agents recycling

We succeeded in laser-induced magnesium (Mg) production from magnesium oxide (MgO) using Si-based agents, silicon (Si) and silicon monoxide (SiO). In these experiments, a cw CO2 laser irradiated a mixture of Mg and Si-based agents. Both experimental studies and theoretical analysis help not only understand the function of reducing agents but also optimize Mg extraction in laser-induced Mg production. The optimal energy efficiencies 12.1 mg/kJ and 4.5 mg/kJ of Mg production were achieved using Si and SiO, respectively. Besides, the possibility of recycling Si and SiO was preliminarily investigated without reducing agents but only with laser-irradiation. As for the Si-based agents recycling, we succeed in removing 36 mol % of oxygen fraction from SiO2, obtaining 0.7 mg/kJ of Si production efficiency as well as 15.6 mg/kJ of SiO one at the same time. In addition, the laser irradiation to MgO-SiO mixture produced 24 mg/kJ of Si with more than 99% purity.

Laser Propulsion Using Metal‐Free Water Cannon Target

We have shown that a metal target covered with water, Water Cannon target (WC), provides higher Cm than a pure metal target. We improved repetitive water supply system and succeeded in driving a miniature car of 70g weight by only 1.4J YAG laser. Then we invented new target, Metal Free Water Cannon target (MFWC) which can be driven into any direction. In addition, we improved it and invented Water Film Cannon (WFC). Cm of 3680[N.s/MJ] was achieved with WFC. By using this system, we propose a piston engine and vertical launch system. We also examined a similar propulsion system with CO2 laser and found some enhancement with water cannon.

Proposal of Liquid Cannon Target Driven by Fiber Laser for Micro‐Thruster in Satellite

We propose a new concept controlling a satellite by a fiber laser loaded in it and demonstrated the acceleration of pendulum with 7kW/2n and 2kHz fiber laser, and measured the Cm of 16Ns/MJ corresponding to the scaling of YAG laser. This laser can be easily bundled to generate much larger power. For more efficient acceleration, we propose “metal‐free water cannon target”, the new concept of propulsion using only water. The momentum coupling coefficient of 2500[Ns/MJ] was achieved with vacuum pump oil instead of water, and we succeeded in controlling the driving direction by the system based on the new concept. This can be used for thrusting a satellite and controlling its posture in combination with fiber lasers.

Laser‐Driven Micro‐Ship and Micro‐Turbine by Water‐Powered Propulsion

In this paper, we report experimental demonstration of propulsion of 100‐weight object with only 668mJ/5ns YAG laser. This is made possible by water overlay structure and the effort to reduce the friction by putting the object on water surface or using a levitating system which we call on “air‐slider”. Furthermore, several water supply systems provided the repetitive propulsion. In addition, we found the laser‐driven micro‐turbine would provide an interesting application area in driving micro‐obstacle.