Solar-pumped laser is a device that directly converts sunlight into laser. It has the advantages of reliable system structure, few energy conversion links, and efficient energy transmission from sunlight to laser. Improving the sunlight convergence system, designing an effective pump cavity and finding suitable laser materials are useful to improving the collection efficiency and conversion efficiency of the solar-pumped laser system for sunlight. Sunlight is a wide-spectrum light source, while the bandwidth of the absorption band of the laser material is limited, and the laser materials with different doping ions and ion concentrations have different absorption capacities for sunlight, so the utilization and conversion of sunlight by solar-pumped lasers are limited. Studying the matching of different laser materials with the solar spectrum and the influence of the length of the laser material on the spectral matching is helpful to analyzing the potential of different materials in the solar-pumped laser, and providing a reference for the parameter design of the length of the laser material. In this paper, Nd∶YAG, Cr∶Nd∶YAG and Ce∶Nd∶YAG laser materials widely used in solar-pumped lasers are taken as the research objects. Based on the analysis of their energy level structures, the spectral matching of three laser materials under solar pumping is analyzed by establishing a refined solar spectral model, and their application potential in solar-pumped lasers is discussed.
Based on the theory of solar-pumped laser materials, the energy level structures of Nd∶YAG, Cr∶Nd∶YAG and Ce∶Nd∶YAG are analyzed, and the absorption bands of the three laser materials are determined. Then, the solar spectral data models are established by using the space-application-oriented solar spectral irradiance standard AM0 and the ground-application-oriented solar spectral irradiance standard AM1.5 provided by ASTM International. The overlapping efficiency between the solar radiation spectrum and the laser material absorption spectrum is calculated under the two models. On this basis, the absorption coefficient of the laser material is introduced into the overlapping efficiency. The variation of solar spectral matching efficiency of laser materials with absorption length after absorption coefficient correction is analyzed. Finally, the advantages of Nd∶YAG, Cr∶Nd∶YAG and Ce∶Nd∶YAG under solar pumping are compared by analyzing the absorption length required by the three materials when approaching the solar spectrum matching limit.
Under the AM1.5 ground solar spectrum model, the overlapping efficiencies between the solar radiation spectrum and the laser material absorption spectrum for Nd∶YAG, Cr∶Nd∶YAG, and Ce∶Nd∶YAG are 17.34%, 35.55%, and 33.41%, respectively. Under the AM0 space solar spectrum model, the overlapping efficiencies are 15.4%, 33.13%, and 32.92%, respectively (Table 3). The overlapping efficiency under the AM1.5 solar spectrum model is slightly higher that under the AM0 solar spectrum model, indicating that the useful irradiation amount absorbed by laser materials in the AM1.5 model accounts for a larger proportion of the total solar spectrum irradiation. After the absorption efficiency is introduced, the spectral matching efficiency changes exponentially with the absorption length for the three laser materials. The solar spectral matching efficiencies of Cr∶Nd∶YAG and Ce∶Nd∶YAG are significantly higher than that of Nd∶YAG, about doubled. When approaching the solar spectral matching limit, the absorption lengths required for Nd∶YAG, Cr∶Nd∶YAG, and Ce∶Nd∶YAG under the AM1.5 model are 6.5 cm, 4.4 cm, and 3.7 cm, respectively (Fig. 4). Under the AM0 model, the corresponding required absorption lengths are 7.3 cm, 4.3 cm, and 3.8 cm, respectively (Fig. 5). Considering the solar spectral matching efficiency of the laser material and the scattering loss of the pump light inside the material, the actual length of the laser material should not exceed the absorption length studied. This analysis provides a maximum value for the design of the length of the laser material.
In this paper, three kinds of laser materials, Nd∶YAG, Cr∶Nd∶YAG and Ce∶Nd∶YAG, are taken as the research objects. Two solar spectral models oriented to space application and ground application are adopted to calculate the overlapping efficiency between solar radiation spectrum and material absorption spectrum. The absorption coefficient of laser material is then introduced into the overlapping efficiency, and the relationship between the solar spectral matching efficiency of laser material and the absorption length is analyzed after the absorption coefficient is corrected. When approaching the spectral matching efficiency limit, the absorption lengths of Nd∶YAG, Cr∶Nd∶YAG and Ce∶Nd∶YAG in the ground solar spectrum model are 6.5 cm, 4.4 cm and 3.7 cm, respectively, and the absorption lengths in the space solar spectrum model are 7.3 cm, 4.3 cm and 3.8 cm, respectively. The absorption length studied provides a maximum value for the length parameter design of laser materials. At the same time, the calculated length of Ce∶Nd∶YAG crystal is the shortest, and it is expected that the scattering loss introduced in this material will be the smallest. Therefore, using Ce∶Nd∶YAG crystal as the laser material is expected to further improve the output performance of solar-pumped lasers.
