Transverse effects in coherently driven nonlinear cavities

We derive reduced field equations describing two cases of optical cavities driven by an external coherent field, taking into account the influence of transverse effects. For the passive-cavity case, we focus on the transistor characteristic and the onset of optical bistability. We prove that, depending on the sign of the detuning, different equations are needed. A global description of the field is possible in terms of a generalized Ginzburg-Landau equation, unless the detuning is finite and positive. That case requires a modal expansion. Furthermore, the good-cavity limit is regular, implying the same differentiation according to the sign of the detuning. For the active-cavity case, the same dependence on the sign of the detuning is found. A global description is obtained only for negative detunings. The good-cavity limit is described by a unique equation for all detunings.