Lasing in a gain slab as a consequence of the causality principle

Transmission of a semi-infinite light pulse through a slab of an gain medium is considered. It is demonstrated that, in the presence of lasing, the Fresnel formulas yield a finite field distribution violating the causality principle: lasing in the slab begins starts the moment at which the pulse front reaches the slab. A modification of the Fresnel approach providing for causality through adding time-diverging terms for description of the linear stage of lasing is proposed. The correctness of the proposed approach is proved by the comparison with the numerical solution of the system of Maxwell-Bloch equations. It is shown that fulfillment of lasing conditions does not contradict the causality principle.