Optical Time Domain Reflectometer (OTDR), as an effective means to accurately locate the fault of the optical distribution network (ODN) and monitor the performance of the ODN, has been widely used in the backbone network of the operators, but it is represented by the passive optical network (PON). In the access network of China, it is facing huge challenges. Factors such as the star structure of the PON ODN network, the high-loss optical splitter, and the complex ODN deployment environment all increase the difficulty of OTDR's measurement and diagnosis of fiber faults. Especially when a high-loss optical splitter is introduced in the PON ODN network, the intensity of the OTDR detection optical signal is reduced, making it difficult for the current OTDR technology to identify attenuation events (such as bending and other attenuation events) on the ODN branch fiber. Moreover, when the end of the home fiber is connected to an optical network unit (ONU), the weak OTDR detection signal cannot form an effective reflection peak to distinguish and locate different ONU branches. In order to improve the OTDR's control and measurement sensitivity to the end-to-end performance of the optical link, the industry proposes to install a low-cost wavelength selective reflector on the ONU side to achieve accurate detection of the end-to-end attenuation of the optical link.
Fiber grating reflector (FBG reflector) is just such a wavelength selective reflector. The principle of fiber grating is to use the photosensitivity of the fiber material to write the coherent field pattern of incident light into the core by ultraviolet light exposure, and produce periodic changes in the refractive index along the axis of the core in the core, thereby forming permanent The role of the spatial phase grating is essentially to form a narrow-band (transmission or reflection) filter or mirror in the core. When a beam of broad spectrum passes through the fiber grating, only the wavelength that meets the Bragg condition of the fiber grating will be reflected, and the remaining wavelengths will continue to be transmitted through the fiber grating.
The special pigtails written into the fiber grating or the fiber grating reflector embedded in the fiber grating in the adapter are installed on the ONU side, due to the reflection center wavelength of the fiber grating and the optical line terminal (OLT) The test light pulses sent by the OTDR on the side are consistent, so that the test light pulses are strongly reflected with a reflectivity close to 100%, while the normal PO system working wavelength does not meet the fiber grating Bragg condition and passes through the reflector with a small attenuation. OTDR can accurately calculate the attenuation of the optical link from the OLT to the ONU by detecting the intensity of the optical signal reflected by the fiber grating reflector, and can accurately measure the return loss by comparing the failure link with the return loss of the health file The value of the fiber loss introduced by the attenuation fault on the fiber link. Secondly, in the second-level optical splitting scene, it is also possible to accurately locate whether the attenuation fault occurs in the distribution fiber section or the home fiber section. At the same time, after installing the fiber grating reflector, each ONU branch can be effectively distinguished by reflection events on the OTDR curve to help locate which ONU branch has an attenuation fault. So as to realize the effective detection of the branch fiber attenuation event.