Fibrain FBG series are filters integrated into LC and SC adapters that reflect 1625 nm wavelength and transmit other wavelengths (1310 nm, 1490 nm, 1550 nm). They are used to enable monitoring of an active network with an OTDR using the 1625 nm wavelength. OTDR measurements of the whole optical link from the OLT to the ONT are difficult because of the presence of multiport splitters (that from the point of view of the OTDR are a large point attenuation) and because of large spatial resolution required. Application of 1625 nm reflectors is an efficient and cost-effective way to decrease the required dynamic range of the used OTDR. The FBG 1625 nm reflectors are a basic method of implementation of optical layer monitoring on the whole length of a FTTx network from the OLT to the ONT in real time.