Method and apparatus for passive mode locking high power lasers

Abstract

A passively mode-locked laser suitable for use with homogeneously broadened lasing medium or a lasing medium having a high-power output, long relaxation time or a narrow bandwidth. The laser includes a main cavity in which the lasing medium is disposed and an external cavity including a non-linear element. The non-linear element is selected to induce a non-linear phase shift in a coherent beam from the main cavity. In operation, a portion of a laser beam in the main cavity is coupled to the external cavity and a phase-shifted output from the external cavity is reflected back into the main cavity to provide a narrow pulse, mode-locked output from the laser. In one embodiment the non-linear element of the external cavity is an optical fiber with a mirror disposed adjacent one end of the optical fiber to provide dual propagation of a laser pulse through the fiber. In high-power applications the external cavity mirror is disposed proximate but spaced apart from the optical fiber end. The ends of the optical fiber are treated to avoid multiple reflections and Q-switching in lasing media having long relaxation times. In one embodiment the optical fiber ends form surfaces non-perpendicular to the laser beam path. Coupling between the main and external cavity may be provided by a beam splitter disposed either externally or within the main cavity. The beam splitter may also have a reflectance depending upon the polarization orientation of an incident beam. The percentage of a laser beam from the main cavity coupled into the external cavity may then be varied by the addition of a quarter-wave plate to allow changes in the polarization orientation of the beam incident on the beam splitter.