Hello Runtao,
If I understood you correctly, you are using Brillouin Optical Time Domain Reflectometry (BOTDR) in your project. However, I am not exactly sure how you are planning to realize your sensor in OptiSystem. Do you mean that you want to test it in the design of some scheme? Otherwise, there were some topics in Optiwave forums about sensors realization. Please, take a look at the following links below: https://optiwave.com/forums/topic/evanescent-wave-optical-fiber-sensor/