Title: Development of compact solar-pumped lasers

Abstract

We have developed a compact solar-pumped laser (μSPL) employing an off-axis parabolic mirror with an aperture of 76.2mm diameter and an Cr doped Nd (0.1 at %): YAG transparent ceramic rod of φ1 mm x 10 mm as a laser medium (LM). Here, the stimulated emission takes place by the transition of Nd3+ in YAG from its excited state to lower energy states. Cr is doped to absorb the sunlight in broad wavelength ranges and to transfer the absorbed solar energy to Nd3+ because Nd3+ absorbs the sunlight only in the limited narrow wavelength ranges. The laser oscillation wavelength of 1.06 μm, just below the optical absorption edge of Si solar cells, is suitable for photoelectric conversion with minimal thermal loss after optical wireless (laser) power transmission to distant places. The small LM and solar concentrator realize more stable oscillation by rapid natural/air convection cooling and increased mechanical stability during wind exposure in contrast to the conventional large SPLs employing typically a 2 m size solar concentrator. Outdoor operation tracking the sun yielded continuous oscillation exceeding 6.5 h, improving upon the previously reported 11 min. This showed applicability of SPLs to whole day operation and terrestrial solar energy utilization. Here, the SPL output increased more than eightfold between LMs with Cr contents of 0.0 and 0.4 at%, and then decreased at 0.7 at% and further at 1.0 at%, where the round-trip loss due to scattering by Cr dopants became significant. Energy transfer efficiencies from Cr3+ to Nd3+ were assessed to be lower than 50%. This can be attributed to the fact that the energy transfer from excited Cr3+ to unexcited Nd3+ became difficult because Nd3+ had been already excited directly by the sunlight. Although Cr doped Nd: YAGs are renowned LMs, development of more efficient LMs for SPL sis eagerly awaited.