Abstract

Imperfections in fiber geometry and constituents can lead to modally dependent loss.^1–5 Measurement of this modal dependence is used to characterize the fiber for use in long-length communication systems and to aid in determining the mechanisms that cause the loss. We have used a powerful new measurement technique, based on the analysis of light back- scattered from a pulse in a fiber waveguide, to observe this phenomenon. The technique is called optical-time-domain reflectometry.^6–9 An optical-time-domain reflectometer (OTDR) uses a directional coupler to launch a pulse of light into the fiber and to observe simultaneously the light backscattered from the pulse. Figure 1 shows that the backscattered signal consists of three distinct segments: (1) an initial pulse due to reflection in the directional coupler barely observable in the figure; (2) a long tail caused by distributed Rayleigh scattering, which occurs as the pulse propagates along the fiber; and (3) pulses caused by discrete reflections, which occur because of imperfections, connectors, and dielectric discontinuities. The Rayleigh-scattered light can be used to extract the local attenuation coefficient. This paper reports the use of an OTDR to measure differential mode attenuation and to observe the approach to steady- state mode distribution and the corresponding loss.

© 1979 Optical Society of America