Extended optical fiber line testing system using new eight-channel L/U-band crossed optical waveguide coupler for L-band WDM transmission

This paper describes the system design for an extended optical fiber line testing system that uses a new L/U-band crossed optical waveguide coupler and a fiber Bragg grating filter for L-band wavelength-division multiplexing transmission. We describe the reflection characteristic required for optical filters located in central offices in order to suppress the ghost signal caused by multireflection in the optical time-domain reflectometry (OTDR) trace. We design and evaluate an eight-channel crossed optical waveguide coupler with a new thin dielectric film filter that separates a 1650-nm test light from the L-band communication light, and confirm that there was no degradation caused by multireflections in the OTDR trace. We also demonstrate the in-service line monitoring of a 10-Gb/s L-band transmission with no degradation in the transmission quality.

I. Introduction

IN the fiber-to-the-home (FTTH) era, it is expected that broadband network provision will require thousands of optical fibers to be accommodated in a central office for optical access networks [1]–[4]. An optical fiber line testing system is essential for reducing maintenance costs and improving service reliability in optical fiber networks. We have already developed such a system called AURORA (automatic Optical Fiber Operations Support System) [5] [6]– [8]. Recently, a long wavelength band (L-band) that extends to 1625 nm has begun to be used for wavelength-division multiplexing (WDM) transmission [9] [10] [11], and a 10-Gb/s WDM system is being introduced into metropolitan networks [12]. As we already use the 1310- and 1550-nm wavelengths for such communication services as ATM-PON and CATV [13], we use the 1650-nm wavelength for maintenance testing [8], [14] in accordance with ITU-T Recommendation L.41 [15]. With a view to monitoring optical fibers transmitting L-band communication light, an attractive way of separating the 1650-nm test light from the L-band communication light is to use a chirped fiber Bragg grating (FBG) filter because of its steep optical spectrum [8]. However, it is difficult to measure fiber characteristics accurately using an optical time-domain reflectometer (OTDR) because multireflections appear in the OTDR trace when FBG filters are installed at either end of an optical fiber line.