Therefore, solar-pumped lasers could be extremely useful in off-grid locations, such as spacecraft applications (including military), on-board aircraft laser systems, and space communication. Until now, however, for most solar-pumped laser systems, power-conversion efficiency is very low, and all efforts in this field of research are directed toward increasing efficiency as much as possible. For example, instead of large mirrors, a small Fresnel lens is used in the pump module to increase the portion of incident solar light. Another strategy to get better efficiency is to find an appropriate active medium with broad absorption spectrum and high absorption and emission cross sections.
In this paper, Suzuki et al. investigate ZBLAN glass as a possible candidate for a solar-pumped laser medium and found it extremely promising. Indeed, this medium is suitable for manufacturing laser-quality optical fibers, owing to its very low passive losses. Additionally, the doping level of rare-earth ions for this host can be as high as 10 mol%, which translates into larger absorption of pump light, and therefore greater efficiency. The disordered structure of glass implies a broader absorption spectrum of rare-earth ion dopants in comparison with crystaline media such as garnets or oxides, which also helps in utilizing a larger portion of incident solar light. Finally, the low phonon ZBLAN fluoride structure helps to avoid unwanted nonradiative energy losses.
The summary of the investigation by Suzuki et al. is the following: The product of the stimulated emission cross section and the radiative lifetime for laser transition at 1050 nm, which is inversely proportional to the laser threshold, is the largest among those of all Nd3+ doped glass media, whereas the integrated absorption strength in the 450–900-nm region of ZBLAN: Nd3+ is the largest among those of all Nd3+ doped fluoride glasses. Let us wish the authors success in their continuing efforts to obtain increasingly efficient solar-pumped laser operation for this medium.
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