Lasers with distributed loss have a sublinear output power characteristic
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2015 Optical Society of America. It is a generally accepted fact of laser physics that in a homogeneously broadened gain medium, above threshold the output power of the laser grows linearly with the pump power. The derivation requires only a few simple lines in laser textbooks, and the linear growth is a direct result of the fact that above threshold, the intracavity optical intensity will increase to the point that the gain is saturated to the level of the net lossso-called gain pinning or clamping. Such a derivation, however, assumes that the mirror loss is distributed (the approximation of uniform gain saturation) which is only a good assumption for cavities whose end mirrors have reflectivities close to one. Furthermore, in gain media with a distributed loss there is a maximum achievable intracavity intensity that in turn limits the output power. We show that the approximation of uniform gain saturation leads to output powers that violate this limit. More generally, for lasers with low mirror reflectivities that also have distributed loss, we prove that the output power grows sublinearly with the pump power close to threshold. Furthermore, after threshold the output grows linearly, but with a slope efficiency that can be substantially smaller than predicted by the uniform gain saturation theory, with the largest deviation occurring for traveling-wave lasers and asymmetric FabryPerot lasers. These results are particularly applicable to semiconductor lasers, and specific applications to quantum cascade lasers are discussed.
