Hello Jaffar,
In OptiSystem, you can get the Brillouin backscattered signal in bidirectional fiber as a parametrized signal. Also, you can insert the tunable laser source on the right hand side. So in the left hand side you would have a parametrized Brillouin backscattered signal and the transmitted signal from the tunable laser (parametrized or sampled as you define the tunable laser source).
Hope that clears and helps.
thanks
Mohamed Abouseif