Optical time domain reflectometer for backscattered light

Abstract

It is possible to detect the point of breakdown and to obtain the loss of optical fibers by measuring the backscattered light caused by Rayleigh scattering. This paper describes design and test results for a small, light‐weight, high‐performance optical time‐domain reflectometer. To obtain masking functions for excessive pulses such as those obtained by Fresnel reflection, we use an image dissector as the detector and obtain a 50‐dB masking attenuation. A digital averager is used to improve the S/N ratio. The time required for 1000 averagings is only 40 seconds. To prevent drift, an AC coupled amplifier and a zero‐level sampling system are used and sufficient temperature stability is realized. The experimental reflectometer is able to detect the breakdown point as far away as 8 km with backscattered light and at 14 km with perfect end Fresnel reflection. The error in the loss measurement is less than 0.05 dB for the distances up to 4 km. From these results, the tested reflectometer is found usable in practice.