Einstein coefficient for spontaneous emission, while T is the rate of stimulated emission. The rate equations for the eight species present are The quantity G in Eq. 9 is the geometrical factor 2rf/L2, where rb is the radius of the laser beam and L the length of the laser; for isotropic spontaneous emission only this fraction of photons is confined within the laser cavity. The last term represents the loss through the end mirrors of reflectivites and r2, and Tc *s the lifetime of an average stimulated emission photon travelling parallel to the axis;
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