Dear Xiaoyue,
Please check the example (Comb generation_Optoelectronic scheme.osd) on comb lasers generation in the Example Library at the location below.
I don’t think you will find the example in OptiSystem version 7, which is a hacked version.
C:\Users\USER NAME\Documents\OptiSystem 21.0 Samples\Component sample files\Transmitters Library\Comb\.
Regards,
Ahmad