Coupled-cavity mode locking: a nonlinear model

Abstract

By distributing the output coupling and the coupling of the external nonlinear cavity along the gain medium of the main cavity, I derive a nonlinear differential equation for a pulse envelope propagating through a coupled-cavity laser. For a phase-mismatch angle of π/2 between the two cavities, a stable hyperbolic-secant short pulse is a solution of the differential equation for negative dispersion (of the soliton into the fiber) as well as for positive dispersion of the nonlinear external medium. The width of the predicted pulse is a function of the coupling factor and the intensity traveling through the optical fiber. The operation of the coupled-cavity laser for this stable regime is shown to be completely similar to passive mode locking by a fast saturable absorber.