Abstract: The invention relates to a passive mode locked femtosecond laser whose ring resonator comprises the elements: a) a laser-active element 1, b) at least a dielectric mirror 2 having a negative group velocity dispersion GVD such that for a continuous part of the optical spectrum amplifieable by the laser-active element the sum of the group velocity dispersion of the mirror 2 and the positive group velocity dispersion of the laser-active element 1 is negative, i.e. ∑ n ⁢ GVD n < 0 c) two concave mirrors 21 and 22 which are spatial adjacent next to the laser-active element and which are orientated with their concave surfaces towards the laser-active element 1 and d) an optical output coupler 3.