How to evaluate the repetition rate of a ultrashort pulse laser?

In optisystem, here is a program in the example library named ultrashort fiber laser. As the optisystem tutorials says, this fiber laser is based on the passive mode locking.
Theoretically speaking, in a passively mode-locked fiber laser, the generated pulses are emitted after cycling once in the cavity. Observed in time, the laser will generate a pulse sequence with a stable repetition frequency, whose repetition frequency is inversely proportional to the cavity length.
But in the ultrashort fiber laser program, no matter how long I extend the time window, even if it has a length of 1s, I can only see a single pulse on the visualizer. Why is that？
Thanks

• This topic was modified 3 years, 9 months ago by Shen Kaiyuan. Reason: forgot to add the figure