NewTrans1.txt[9/15/2024 8:15:35 PM] 2.1 The Media Gaseous In practice, the energy levels of these are very narrow, typically XEIE << 10'3 in the temperature and pressure conditions in which these media can maintain laser oscillation. We then note that the overlap between the solar spectrum and that of gas absorption is very limited, the energy efficiency cannot, at best, exceed a few . The situation is even more unfavorable if, as is common, the frequency associated with the pumping transition is far from the maximum solar radiation. In the case of CO2 and N2 (CO2 lasers), whose absorption lines are located around 4.2 /zm (Fig. 2), the overlap Rv between the solar spectrum and absorption lines does not exceed a few 10-4 (Fig. 3), to rise to 10-3 in the case of CO (lines around 2.3 /zm). Yet these evaluations only constitute an upper limit of the pumping efficiency, because the En —> Et transitions in these gases are relatively weak, and correspond to a weak absorption. The situation therefore appears very unfavorable for direct use of solar pumping by a gas laser. Higher efficiencies would require amplifying gases with absorption lines located in the visible or near UV — for example, alkali metal vapors. Colger et al. (3) have shown that cesium would theoretically allow an efficiency of up to 3.7%, in conditions allowing the realization of a 7 kW laser with a collector surface of 200 m2. Another solution would consist in using an intermediate gas, absorbing the solar radiation, then transferring its energy to the amplifying medium. A Br2-CO2 mixture would thus theoretically allow an efficiency of 1% (4). However, no experimental studies have yet confirmed these assessments. 2.2 Condensed Media In these, the E„ - E2 transitions, notably broadened by the mutual interactions between neighboring atoms, can come to cover an extended portion of the solar spectrum. Figs. 4 and 5 thus show the levels of ruby and neodymium, while the bands shown in Fig. 3 illustrate the corresponding overlap. We thus reach notable values of Rp (several percent), all the more effective as the absorption of the Eo - E2 transitions is strong. These theoretical assessments are fairly widely confirmed by experience, since several experimental realizations have made it possible to approach efficiencies of 2%, with a power greater than 1 W (5). Unfortunately, we can fear that these values could easily be exceeded. In fact, within a solid, the evacuation of heat (1 - R,) (E2 - Eo) released by each transition of pumping can only be carried out in the presence of a temperature gradient 8T/8t determined by the thermal conductivity. As soon as the dimensions of the medium exceed a certain value/), the center reaches a temperature incompatible with the mechanical integrity of the laser. In practice, the limit D is very low: less than a millimeter in poor conductors (glass), a few millimeters in crystals, whose conductivity is generally higher (YAG, A12O3). These reduced dimensions limit the emitted power to a few watts - a few tens at most. The only solution allowing, in a condensed medium, the emission of significant powers, would be to have a liquid amplifier, the cooling could then be ensured by circulation. However, no solvent has yet been found to constitute a liquid medium with chromium. As for neodymium, such a solvent exists, but it is so corrosive and toxic that very little work has been devoted to it. Among the liquids, we could also think of organic dyes (in particular rhodamines), which have already made it possible to produce continuous lasers of several watts, with efficiencies reaching 30%.
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