Mode locking in a fiber laser with a long-period fiber grating

Abstract

We consider modelocking in an optical fiber laser cavity using a passive long period fiber grating. The grating as a passive modelocking element arises from the the nonlinear mode-coupling which occurs between co-propagating core and cladding modes. The underlying concept is as follows: a resonant and linear mode-coupling interaction transfers energy periodically between the core and cladding modes. Nonlinearity, however, can be used to detune the resonant interaction by shifting the propagation constant of each mode via self-phase and cross-phase modulation. Thus the low intensity parts of a pulse which propagate through the grating can be coupled out to the cladding and attenuated while high intensity portions are detuned and transmitted through the grating with minimal loss. This intensity discrimination when acting in combination with chromatic dispersion, self-phase modulation, and a bandwidth limited gain, can lead to stable modelocking operation in a optical fiber laser cavity. Stable pulses are generated for a wide variety of nonlinear coupling strengths between core and cladding modes. Further, dispersive radiation can be completely attenuated while generating the stable pulse trains. Self-starting in the cavity along with stability of the pulse trains under perturbation are considered. In conclusion, the long period fiber grating provides a simple, compact passive component for a modelocked laser source which is robust and efficient.