In the paper by Christiansen the novel idea of pumping lasers with the solar radiation trapped in a blackbody cavity is discussed and demonstrated. In principle, the recycling of the radiation trapped in the blackbody cavity might provide much more efficient utilization of the collected solar flux than direct solar pumping. This is illustrated by the relatively high efficiency predicted by Christiansen for a neodymium activated solid state laser. It remains to be demonstrated, however, that this advantage can be achieved in practical blackbody cavities. The loss of thermal energy from the blackbody must be not much larger than, and preferably smaller than, the laser output in order to achieve the predicted efficiencies. Such a demonstration should have a high priority in future research plans. A potential advantage of blackbody pumping is that the collected solar energy is stored in the blackbody during periods of interruption of the solar flux, during which laser operation might continue, perhaps with the infusion of substitute forms of heating of the blackbody during long interruptions.

The solar pumped gas laser discussed by Lee uses an imaging concentrator, which is adequate for pumping gases of molecules containing iodine to above their laser threshold. The utility of such a laser for space-based applications, such as communications and power transfer to space vehicles is being explored. This laser concept has the advantage of readily scaling up to large sizes for high output powers. The anticipated efficiency of this type of laser (about a tenth of a percent or so) is rather low for prospective terrestrial applications in competition with other technologies.

At this stage of their development, it might be said that solar pumped lasers are a possible solution in search of possible problems. While judging the merits of solar pumping of lasers for any application, it is, of course, important to keep in mind the competing technologies that employ either solar or nonsolar energy sources. For example, semiconductor diode laser pumped solid state lasers powered by solar pumped photoelectric cells may provide stiff competition in the future for direct solar pumping in prospective laser applications. At this time too little is known about the performance and cost of solar pumped lasers to make meaningful assessments of their suitability for applications or comparisons with other technologies. It is therefore important to shape research and development programs on the various solar pumping schemes with a view toward clearly establishing the limits to their performance. At the same time, efforts should be made to identify the promising applications of solar pumped lasers and to make credible assessments of their suitability for those applications. If progress is not made in parallel in these two areas, it may be difficult to come up with either the 'solution' or the 'problems' in this period of intense competition for funding.

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