Monitoring of Drop Optical Fibers in 32-Branched PON using 1.65 μm Pulse-OCDR

Passive optical network (PON) systems based on optical splitters provide low-cost optical access by sharing equipment cost between subscribers. However, it was difficult to monitor a drop fiber section from the central office using reflectometry because of the overlap of reflectometry results derived from multiple drop fiber branches. This difficulty in monitoring has added maintenance costs because in the event of communication faults it is necessary to dispatch service persons for troubleshooting. While it is proposed to integrate the monitoring function in optical network units (ONUs) (1), (2), it is simple and would be cost effective to monitor from the central office using reflectometry. There have been several works on reflectometry for the monitoring drop section. The high-resolution 1.65 μm optical time domain reflectometry (OTDR) (3) can detect reflection from fiber Bragg grating (FBG) filters which are installed at the end of drop fibers, where transmission loss from the OTDR is 31.5 dB. However, since the spatial resolution is as large as 2 m, great care will be required to arrange drop fiber lengths to differ from one another by more than 2 m. Therefore, improvement in spatial resolution has been desired to ease the deployment of drop fibers. With optical frequency domain reflectometry (OFDR), a resolution as high as 0.8 mm is realized (4). However, the spatial range is limited to 2 km by laser linewidth. Although the spatial range of OFDR can be extended to 40 km by using phase noise compensation (5), the spatial resolution degrades to as large as 1 m. Moreover, since those reported OFDR systems use 1.55 μm wavelength band, and further works will be necessary to avoid conflicts with communication channels. We propose pulse-OCDR (Optical Coherence Domain Reflectometry) as a new monitoring method for PON, and present the results of laboratory demonstration, where reflections from the FBG filters installed at the end of drop fibers in a 32-branch PON with a 15 km feeder line are detected with 2.6 cm spatial resolution, and fault in drop fibers is detected as the absence of one FBG reflection. Although the location of a fault is not implemented because the current OCDR does not detect Rayleigh scattering from the drop fibers, fault detection alone is still valuable because it can tell whether the fault is in fiber or in ONU and reduce the dispatches of service persons for troubleshooting. 2. Pulse-OCDR Scheme