Gain saturation in fiber distributed feedback lasers

Systematic numerical studies of the nonlinear operation of a fiber distributed feedback (DFB) Nd:YAG laser are presented. An approximate formula relating small-signal gain to output power and the parameters of the structure for this kind of laser is derived. Characteristics revealing saturation effects for weak and strong coupling regions are presented. The fiber DFB structure provides the discrimination between the different kinds of transverse modes. An interesting behavior of the laser characteristics is revealed. For large coupling strength the longitudinal mode spectrum is inversed to the one observed at the threshold or to the one above the threshold for weak coupling. This mode inversion strongly depends on the output power level and the normalized loss coefficient. Similar phenomena are observed for the transverse mode.<
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