The  fiber  brag  grating  reflector( FBG  Reflector)  is  a  low-cost  specific  band  reflector mounted on the optical network unit (ONU) side. It can reflect light pulses ( 1650 +/- 5 nm) from the OTDR on the fiber line terminal (OLT) side nearly 100%, while the working bands of the passive optical network (PON) are transmitted normally. The OTDR can determine whether the  fiber  line  is  normal  by  detecting  the  signal  reflected  by  the  fiber  grating reflector. When the reflected signal is lower than the normal value, the line loss is too large; when the reflection value is 0, the optical fiber line is broken 。Therefore, we can maintain the fiber line in time by the real-time monitoring of the fiber line.