We have developed a compact solar-pumped laser (µSPL) employing an off-axis parabolic mirror with an aperture of 76.2 mm diameter and an yttrium aluminum garnet (YAG) ceramic rod of φ1 mm × 10 mm doped with 1% Nd and 0.1% Cr as a laser medium. The laser oscillation wavelength of 1.06 µm, just below the optical absorption edge of Si cells, is suitable for photoelectric conversion with minimal thermal loss. The concept of laser beam power feeding to an electric vehicle equipped with a photovoltaic panel on the roof was proposed by Ueda in 2010, in which the electricity generated by solar panels over the road is utilized to drive a semiconductor laser located on each traffic signal along the road. By substituting this solar-electricity-driven semiconductor laser with a solar-pumped laser, the energy loss of over 50% in converting the solar electricity to a laser beam can be eliminated. The overall feasibility of this system in an urban area such as Tokyo was investigated.

We have designed silicon (Si) photovoltaic (PV) cells coupled with solar-pumped fiber lasers (SPFLs), by considering the unique illumination conditions for PV cells: (1) monochromatic (1064 nm) and normal incidence, (2) large intensity, and (3) small area. We have revealed that a multilayered bandpass filter formed on the front surface of a PV cell equipped with a diffuse reflector on the back surface sufficiently traps the illumination. This, in turn, allows us to use a thin Si wafer to eliminate the detrimental effect of the series resistance in the cell under intense illumination, along with a small lateral size that is suitable for direct connection with an SPFL. Simulated conversion efficiency of the newly designed 20 μm-thick Si PV cell with the 14-layerd bandpass filter and Lambertian back reflector is 61% under normally incident 1064 nm illumination at 1 kW/cm², which is twice as high as that of a conventional surface-textured Si solar cell of 75 μm in thickness.

We report energy transfer efficiency from Cr³⁺ to Nd³⁺ in Nd (1.0 at.%)/Cr (0.4 at.%) co-doped Y₃Al₅O₁₂ (YAG) transparent ceramics in the laser oscillation states. The laser oscillation has performed using two pumping lasers operating at 808 nm and 561 nm; the former pumps Nd³⁺ directly to create the 1064 nm laser oscillation, whereas the latter assists the performance via Cr³⁺ absorption and sequential energy transfer to Nd³⁺. From the laser output power properties and laser mode analysis, the energy transfer efficiency was determined to be around 65%, which is close to that obtained from the spontaneous Nd³⁺ emission.

We have proposed a new configuration of solar-pumped lasers employing transparent ceramic rods. The laser rod has a composite structure consisting of a Nd/Cr:YAG gain domain surrounded by Gd:YAG with the same refractive index as that of Nd/Cr:YAG. The lasing mode is well controlled by the output coupler, and the parasitic oscillation is suppressed, owing to the refractive index matching. A high laser slope efficiency and a low laser oscillation threshold were achieved owing to the suppressed absorption outside the lasing mode, which was previously a serious issue for the end-pumping configuration using a high-NA focusing optics. The laser oscillation threshold of 136 mW and the slope efficiency of 25.3% were derived. Thus, we have resolved the issue of useless absorption associated with the high-NA end-pumping, and achieved significant improvements compared with the conventional structure of uniform Nd/Cr:YAG.

A monochromatic laser light with a photon energy just above the band edge of photovoltaic cells can be converted into electricity with minimal thermal loss. To attain efficient conversion of sunlight to laser light, a coordinated solar tracking system for an array of originally designed compact solar-pumped lasers of 50 mm aperture diameter is being constructed. As for the feasibility of this system, a prototype with a holding capacity of 25 compact solar-pumped lasers has been fabricated. The primary requisite of this system is that the angular accuracy of tracking should be below 1 mrad for all 25 compact solar-pumped lasers to sustain their continuous lasing. To realize this, imperative challenges have been elucidated including thermal expansion under sunlight. A prototype fabricated with its main frame made of Super Invar alloy was found to fulfill the requisite by measurement using a three-dimensional coordinate measuring machine.