Extended Optical Fiber Line Testing System with L/U-Band Optical Coupler Employing 4-Port Circulators and Chirped Fiber Bragg Grating Filters for L-Band WDM Transmission

摘要

An optical fiber line testing system is essen-tial for reducing maintenance costs and improving service reliability in optical access networks. NTT has already developed such a system called AURORA (Automatic optical fibeR opeR-Ations support system). 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 L.41. Recently, a long wavelength band (L-band) that extends to 1625 nm has begun to be used for WDM transmission. With a view to monitoring optical fiber cables transmitting L-band communication light, an attractive way of separating the U-band wavelength of the test lights 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. 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 installed at both ends of an optical fiber line. In this paper, we consider this problem and show that the reflection loss at the port of a test access module (TAM) must be more than 36.6 dB. We also describe the system design for an extended optical fiber line testing system using an L/U-band optical coupler, which has two chirped FBGs between two 4-port circulators for L-band WDM transmission. In this system, the reflection loss at a TAM port was 38.1dB, and we confirmed that there was no degradation in the OTDR trace caused by multi-reflections at the optical filters.