Brightening of color centers in a glass-fiber neodymium laser

Mechanisms for the brightening of short-lived color centers causing passive modulation of a glass-fiber Nd-laser resonator are examined. It is shown that the brightening of color centers by Nd-laser radiation at 1.06 micron occurs not as the result of absorption but as the result of the extinction of the short-lived color centers. In the absence of short-lived color centers, it is possible to achieve a drastic improvement in the mode-locking pattern of the laser by the use of a dye solution as the passive shutter.