Energy transfer efficiency from Cr3+ to Nd3+ in Cr, Nd YAG ceramics laser media in a solar-pumped laser in operation outdoors

Energy transfer efficiencies from Cr3+ to Nd3+: eta(Cr -> Nd) in Cr (0, 0.4, 0.7, and 1.0 at %), Nd (1.0 at%):YAG ceramic laser mediums were assessed for the first time in solar-pumped laser (SPL) in operation under the natural sunlight. In addition to this stimulated emission (StE) conditions, assessment of eta(Cr -> Nd) in solar pumped spontaneous emission (SpE) conditions was performed using the same SPL system but without the optical cavity eliminating the output coupler. In SpE at the incident solar power at the solar concentrator: I-OAP of 1.12 W, eta(Cr -> Nd) were measured to be 0.803 gamma for 0.4 at%, 0.726 gamma for 0.7 at%, 0.766 gamma for 1 at %, where gamma less than or similar to 1. When I-OAP was increased to 3.07 W, eta(Cr -> Nd) in SpE decreased to 0.675 gamma, 0.623 gamma, 0.625 gamma, respectively. In StE, eta(Cr -> Nd) further decreased to 0.448 gamma, 0.213 gamma, 0.166 gamma. These values are lower than those previously reported values measured by the laboratory experiments such as luminescent decay analyses. Both in SpE and StE, the highest values of eta(Cr -> Nd) were given at the Cr content of 0.4 at% at which the output laser power also took the maximum value. The decrease in eta(Cr -> Nd) in the increased I-OAP in SpE can be attributed to the fact that formation of excited Nd3+ ions by the energy transfer from Cr3+ ions became difficult because Nd3+ ions had been already excited directly by the sunlight.