Passive mode locking of inhomogeneously broadened lasers is studied in three typical regimes: pure self-amplitude modulation (SAM) mode locking, soliton mode locking, and mode locking in the presence of self-phase modulation (SPM) and positive group-delay dispersion (GDD). When the mode-locking strength becomes weak and/or the GDD is large, the lockable spectral width reduces and mode locking becomes unstable, which is a common and unique feature for broadband inhomogeneously broadened lasers. Mode locking by the pure SAM is essentially a phase-locking process. The soliton mode locking best resists the impact of insufficient gain filtering, while mode locking in the presence of strong SPM and positive GDD is the weakest, and a certain amount of gain filtering (narrowing) is necessary for stable mode locking. The scaling relations of mode-locking characteristics with the gain linewidth, GDD, and nonlinearities, and the influence of the degree of inhomogeneity of the gain medium are examined. A linearized saturable absorber may not be able to mode lock a strongly inhomogeneously broadened laser, and a minimum unsaturated absorber loss is required for stable mode locking in most situations.
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