Contents 1. Introduction
In the fiber to the home (FTTH) era, NTT expects that broadband network provision will require thousands of optical fibers to be employed in optical access networks, An optical fiber line testing system is essential for reducing maintenance costs and improving service reliability in optical access networks. NTT has already developed such a system called AURORA (AUtomatic optical fibeR OpeRAtions support system).[1],[2] Recently, a long wavelength band (L-band) that extends to 1625 nm has started to be used for WDM transmission.[3] As we already use the 1310 and 1550 nm wavelengths for communication, we use the 1650 nm wavelength for maintenance testing in accordance with ITU-T recommendation L41.[4],[5] An attractive way of separating the 1650 nm test light wavelength with a bandwidth of +5 nm from the L-band wavelength of the communication light is to use a chirped fiber Bragg grating (FBG) filter because of its steep optical spectrum.[6] However, it is difficult to measure fiber characteristics with an optical time-domain reflectometer (OTDR), because multi-reflections appear in the OTDR trace when FBG filters are equipped at both ends of an optical fiber line. In this paper, we describe the design of the reflection characteristic required for optical filters located in central offices, and we propose an extended optical fiber line testing system using ultra-low-cost L/U-band crossed optical waveguide couplers, which have a multiarray design fabricated in a planar lightwave circuit (PLC), with a thin dielectric film filter for L-band WDM transmission.
