Abstract

Mode-locking of lasers with coupled cavities takes advantage of the nonlinear effect of self-phase-modulation (SPM) in a nonlinear medium. A pulse from the main cavity is injected to an external cavity, where it propagates through the nonlinear medium and suffers from a phase shift as well as a frequency chirp due to SPM. When this pulse is coupled back into the main cavity, at matched cavity lengths it interferes constructively with a pulse in the main cavity at the center of the pulses and destructively in the wings. As the result of interference the pulses are shortened. In recent years this method known as additive pulse mode-locking (APM) of lasers has established to be a powerful technique for generating ultrashort pulses. This concept, first realized with the soliton laser¹, generally uses an optical fiber as the nonlinear medium, and an external cavity coupled to the main cavity through one of the laser output mirrors². This is known as the linear or Fabry-Perot type configuration. Stable APM operation generally requires an interferometric cavity length control.

© 1998 IEEE