Area 6: Advanced Concepts and New Emerging Materials for Future PV IMPROVEMENT OF MODE-MATCHING EFFICIENCY OF SOLAR PUMPED LASERS FOR MONOCHROMATIC PHOTOELECTRIC CONVERSION

We have developed a compact solar pumped laser (μSPL) employing an off-axis parabolic mirror with the aperture of 76.2 mm in diameter and a yttrium aluminum garnet (YAG) ceramic rod doped with 1%Nd and 0.1%Cr of φ1×10 mm as a laser medium[1],[2]. The laser oscillation wavelength: 1.06μm, just below the optical absorption edge of Si, is suitable for photoelectric conversion by Si PV cells with minimal thermal loss. Therefore, the laser light can be efficiently utilized for power generation, that is, SPL-PV combined system[3]. In our previous study, a transparent YAG ceramic of 1 mm in diameter and 10 mm long, and homogeneously doped with 0.1% Cr and 1% Nd was used for laser rod. The experimentally obtained values for the total conversion efficiency and the slope efficiency of the μSPL was 1.28% and 2.51%, respectively. These values were roughly reproduced as 0.974% and 2.79% by a ray tracing and a simulated laser oscillation by the commercially available code: ZEMAX and LASCAD. The simulated absorbed incident light energy distribution was broad in the laser rod indicating considerable amount of the incident light was absorbed in the peripheral region in the rod which was quite outside of laser oscillation mode region. To improve this poor mode-matching condition, a core-cladding type longer rod was investigated by the same simulations in this study. Here, the core is supposed to be the same composition as in the previous study, and the cladding is transparent 5% Gd doped YAG ceramic without any Cr and Nd. This cladding has the same refractive index as Cr and Nd doped core and negligible absorption in the wavelength region between 633 nm and 1300 nm in which the main excitation and emission of Nd are included. Figure 1(a) shows the geometry of this core-clad rod and Figure 1(b) shows the absorbed incident light energy distribution by ray tracing (ZEMAX). Typically, as for rods of 0.6 mm across the core while the total diameter is 1 mm, the simulated laser oscillation by LASCAD gave slope efficiency of 5.1% for the rod length of 20 mm and 7.8% for that of 40 mm. The comprehensive survey of the design of the core-cladding rod shows the improvement of the mode matching efficiency from 40% to 80% indicating preferable rods to be produced. This study was partially supported by Advanced Low Carbon Technology Research and Development Program (ALCA), Japan Science and Technology Agency.