Soliton stability conditions in actively mode-locked inhomogeneously broadened lasers

We examine the conditions for soliton stability in actively mode-locked inhomogeneously broadened lasers. Numerical simulation shows that in the small negative group-velocity dispersion (GVD) region, excessive gain filtering prevents the generation of a single stable soliton pulse and that in the large negative GVD region, the free-running spectral components destabilize the soliton pulse as a result of insufficient gain filtering. As the laser gain medium becomes more inhomogeneously broadened or the unsaturated gain linewidth becomes broader, the gain filtering becomes weaker, and both the lower and the upper stability boundaries shift to small negative GVD. A larger self-phase modulation, however, changes both boundaries toward more-negative GVD and yields a broader stable GVD range. Experimental results for actively mode-locked Nd:silicate glass and Nd:phosphate glass lasers confirm the soliton instability boundary and the influence of the gain medium's degree of inhomogeneity. (C) 2003 Optical Society of America.