β factor in a random laser -: art. no. 047601

We develop a definition for the beta factor, the fraction of spontaneous emission that seeds the laser process, for a random laser. With the wavelength-dependence of the gain (and potentially scattering) being the only possible criterion in the competition between gain and loss, our concept of beta is based on the spectral properties of the spontaneous emission and laser light. We find betaapproximate to0.1. We discuss the apparent similarities and differences between the beta for a cavity and a random laser.