Seven-Rod Pumping Approach for the Most Efficient Production of TEM₀₀ Mode Solar Laser Power by a Fresnel Lens

A seven-rod/seven-TEM₀₀ mode beam Fresnel lens solar laser pumping approach is here proposed. The Fresnel lens with 4.0 m² collection area was used as the primary solar concentrator to pump seven 2.5 mm diameter, 15 mm length Nd:YAG rods within a conical pump cavity through a secondary fused silica aspheric concentrator. Within the pump cavity, solar pump rays not completely absorbed by one of the seven rods were furtherly absorbed by other rods, ensuring hence a high absorption efficiency and avoiding the serious thermal lensing and thermal stress issues associated with classical large rod solar lasers. Seven individual plane-concave large-mode resonators were adopted to enable a good overlap between solar pump mode and TEM₀₀ laser oscillating mode. By using both ZEMAX^® and LASCAD^® software, the maximum total TEM₀₀ mode solar laser power of 54.65 W was numerically calculated by optimizing the radius parameter of the Fresnel lens, the diameter of the laser rod, and the radius of curvature of the laser resonator output mirror. TEM₀₀ mode solar laser collection efficiency of 13.66 W/m² and solar power-to-TEM₀₀ mode laser power conversion efficiency of 1.44% were calculated, representing substantial enhancements of 4.66 times and 4.38 times, respectively, as compared with previous experimental records of the TEM₀₀ mode solar laser pumped through a Fresnel lens with 0.785 m² collection area. The feasibility of TEM₀₀ mode solar laser power delivery by hollow-core photonic crystal fibers was finally studied.