An analytic model that takes into account dispersion, chirp, self-phas
e modulation, bandwidth limitation, frequency walkoff, and cavity leng
th detuning is applied to lasers passively mode locked by two saturabl
e absorbers-one fast and one slow. This general model predicts that th
e balance between slow gain and slow absorber affects pulse timing shi
fts and frequency changes. Such shifts limit the range of permitted in
tracavity dispersion.