The theoretical efficiency of various pumping power systems that have been adopted in optical masers is studied. A thermodynamical method is used to calculate the power transferred from the flashtube to the active rod. In this way a general expression is obtained which allows one to calculate the total power and the power per unit area required from the flashtube, for a given power absorption by the active rod. When the active rod, with an index of refraction equal to n, is covered by a cylindrical sheath with the same index of refraction, then: (i) the power per unit area required from the flashtube for a given power absorption of the rod is at best n times lower than in the case of a simple rod; (ii) the total power required from the flashtube is, however, the same in both cases. The rod covered by a sheath is not the most efficient system. The theory, in fact, shows that the most efficient case occurs when the pumping power is allowed to enter the active rod from all possible directions. In this case the power per unit area required from the flashtube is n2 times lower than in the case of a simple rod.
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