Hello Jiaqing,
What do you mean LP11a and LP11b?
The multimode generator component is able to generate as many modes as you want. Please refer to the example (Multimode signal generator.osd) in the example library located at the following link for more information how to set the component. You may check the datasheet of the component.
C:\Users\USER NAME\Documents\OptiSystem 17.0 Samples\Multimode systems
Regards,
Ahmad