Intrusion-alarmed fiber optic communication link using a planar waveguide bimodal launcher

Abstract

This paper summarizes the results of our study of a point-to-point, intrusion-alarmed, graded-index, multimode optical fiber communication system. In this intrusion-alarmed system, data light is transmitted in the lowest order modes of a graded-index multimode fiber and intrusion alarm monitor light is simultaneously transmitted in high-order modes. An attempted intrusion to extract data light by bending the fiber results in attenuation of the monitor light in high-order modes, thereby sounding an alarm at the receiver. However, the data light in the fundamental mode, or in the lowest order modes, propagates along or near the fiber axis where the refractive index is highest and is exceptionally difficult to extract from the fiber. Even for severe bending of the fiber, light in the lowest order modes remains trapped in a set of lowest order modes. Additional benefits of data light transmitted in the lowest order modes are capabilities for wider bandwidths and lower modal noise. The performance of four fabricated planar waveguide bimodal launchers for simultaneously launching data light into the fundamental mode, or into the lowest order modes, and monitor fight into high-order modes of graded-index multimode fiber is described. The configuration, construction, and performance of a laboratory prototype alarmed system are described.