Following the Letokhov model for passive mode locking, which describes the buildup of ultrashort light pulses from noise, the statistical behavior of passive mode locking and the discrimination characteristics of the nonlinear absorber are examined. The probability

of having a second ultrashort pulse of a certain intensity within the resonator round-trip time

is described. This probability is essentially a function of the initial dye absorption coefficient x
0and a pumping parameter t
pthat is a measure for the excitation rate of the optical pump. Experimental results are reported that confirm these theoretical considerations.