The quantities (B]2)^, hv, and dx are all positive. Therefore, in order for dlvHv to be positive (i.e., for amplification to occur), the quantity [(gilg^N^ - NJ must be greater than zero. This is a necessary, but not a sufficient condition for obtaining an output laser beam. The sufficient condition is that the quantity in brackets must be greater than a certain positive value in order to compensate for mirror and other losses. This value is a function of the laser's properties. In Fig. 2, the end mirrors are parallel to each other and perpendicular to the long axis of the laser tube. In practice, concave mirrors are usually used to prevent photons traveling in a slightly off-axis direction from “walking out” of the laser tube (3). The energy used to pump the lasant atoms or molecules to the excited state is converted to laser beam energy at less than 100% efficiency. Energy conversion losses appear as waste heat, which must be extracted.
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