A laser structure for obtaining an optical pulse of a very short duration by using the benefit of a passive mode locking and for improving a repetition rate of an output optical pulse was described. In general, a passively mode-locked laser generates a short optical pulse than a active mode locked laser does, while it has difficulties in increasing the repetition rate of the output optical pulse because the repetition rate of the passively mode-locked laser is determined by the length of the resonator. In accordance with the present invention, a delayed optical path is added to the linear loop of a conventional figure-8 optical fiber laser. The propagation time difference .DELTA.T between the delayed optical path and undelayed short optical path is adjusted to a divisor of the round trip time, T, of the original resonator by properly adjusting the length of the delayed optical path. Thus, the repetition rate of the output optical pulse of the mode locked laser can be increased from 1/T Hz to 1/.DELTA.T Hz. The optical fiber laser which can fully exploit the advantages of the passive mode locking and improve the repetition rate of the output optical pulse in accordance with the present invention can be applied to a wide range of applications including an optical communication and an ultra high speed analysis of optical phenomena.