Spectral filtering criteria for U-band test light for in-service line monitoring in optical fiber networks

In the fiber-to-the-home era, thousands of optical fibers will have to be accommodated in the central offices of optical access networks. To reduce maintenance costs and improve the service reliability of optical fiber networks, the authors must develop an optical fiber line testing system with a function for in-service line monitoring that uses a test light with a wavelength different from the communication light wavelength. To monitor an in-service line in an optical network, the effective rejection ratio of the test light must be taken into account. This ratio depends on the spectrum of the test light from the optical time-domain reflectometer and the rejection band of the filter in front of the optical network unit. The dependence of the effective rejection ratio as a function of the sideband suppression ratio (SBSR) and of the ratio of the rejection band to the bandwidth of the sideband noise d/D is clarified. When d/D=0.1 and the target effective rejection ratio of the filter is -40 dB, the SBSR and the filter loss of the termination cable must be -70 and -43 dB, respectively, or the SBSR must be -80 dB. When d/D<0.5 and the target effective rejection ratio of the filter is -40 dB, the SBSR is also required to be -80 dB. In-service line monitoring for a 10-Gb/s transmission using a 1650-nm test light with an SBSR of -80 dB is also demonstrated.

I. Introduction

Broadband optical access services are now commercially available [1]–[3]. The number of fiber-to-the-home (FTTH) subscribers in Japan is increasing rapidly and had exceeded about 4 million by September 2005 [4]. Broadband network provision currently requires thousands of optical fibers to be accommodated in a central office for optical access networks. We have developed an optical fiber line testing system to reduce maintenance costs and improve the service reliability of optical fiber networks [5]. To perform maintenance system testing on in-service fibers without causing any degradation in the transmission quality, we use a test light whose wavelength is different from that of the communication light and install a filter in front of the optical network terminal (ONT) that cuts off only the test light. We also installed the filter that cuts off only the communication light in front of the optical time-domain reflectometer (OTDR) without degradation in the OTDR trace [6], [7].