Origin of Short-Pulse Emission by Passively Switched Lasers

Abstract

The pulsed emission of passively switched lasers is analyzed in terms of traveling waves. Partial differential equations governing waves proceeding in opposite directions inside the laser cavity are derived by using a rate‐equation approximation. The essential equivalence between the resulting model and theories based on normal mode expansions of the laser field is demonstrated. Numerical solution of the equations shows emission as a periodic train of short pulses for sufficiently short lifetimes of the upper absorber state. Single pulses in a round‐trip time 2L/c occur if the saturable absorber is next to one mirror, whereas multiple pulses per round trip occur for absorber positions intermediate between the mirrors. The pulse patterns can be interpreted simply in terms of the nonlinear absorption properties of the saturable absorber.