damian,thank you for your answer.I was learning the double rayleigh backscattering in my unidirectional system .the signal travels forward and it will be backscattered for the first time and twice time ,which will interface with the forward traveling signal.so i want to research how the double rayleigh back scattered signal affect the forward signal and i wonder if the double backscattered light will be added to the forward traveling signal and how we can measure the double rayleigh backscattering power in optisystem?