Passive mode-locking of lasers is based on the insertion of a saturable absorber (SA) into the laser cavity. The SA encourages oscillation in short pulses, where the light intensity is large and the loss in the absorber is minimized. We could consider, however, another way for obtaining energy compression by using the transverse dimension of the oscillating beam in the cavity rather than the longitudinal one. In this case, transverse mode-locking would give a reduced cross section with high power density, which would result in continuous oscillation with low loss in the SA. This scenario is, however, problematic and was not considered in the past because it required the oscillator to support a very large number of transverse modes with low losses.

© 1991 Optical Society of America

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