Dear Taban,
you may use the “Spatial CW Laser” component as a multimode pump. The component can be found in Default/Multimode Library/Transmitters/
You can control the number of modes generated etc.
A very good example (Spatial-Mode-Demultiplexing-Amplification.osd) on this topic can be found in the example library at
C:\Users\xxxxxxxxx\Documents\OptiSystem 15.2 Samples\Multimode systems
Which version of OptiSystem do you use?
Regards,
Ahmad