Dear Samuel,
you need to make sure that you set the Iterations parameter in the Signals tab of the Layout Parameters window to 25 or higher number to allow light to circulate inside the cavity to produce the laser. Please check the examples in OptiSystem Example library located at the following link for more information.
C:\Users\USER NAME\Documents\OptiSystem 19.0 Samples\Optical amplifiers\Fiber lasers and amplifiers
regards,
Ahmad