Design and implementation of an optical loop tester

Abstract

Optical Loop Tester (OLT) is an opto‐electronic device which implements a specific functionality of Optical Time Domain Reflectometer (OTDR), that is, to 5 detect the location of a cut in the optical fiber using single ended measurement. A single OTDR can cost millions of dollars and is not manufactured in Pakistan. Thus, OLT is low‐cost and incorporates its advantage of easy‐handling and single‐ended measurement. It is suitable for outdoor, long haul applications and can also be used for research purposes. Light pulses are launched into the fiber under test and the reflected/backscattered light is collected from the same end. At any point in time, the light received by the device is the light scattered from the pulse passing through a region of the fiber. The speed of light in the fiber is known, so the time difference between the outgoing pulse and the incoming light signal is measured. OLT uses this information to calculate the pulse position in the fiber and correlate what it sees in backscattered/reflected light with an actual location in the fiber. Thus it can create a display of the amount of backscattered/reflected light at any point in the fiber. Such a display is commonly called a trace. A trace is a plot of the amplitude of received signal against its distance along the fiber. This plot carriers a lot of information and is used to analyze the fiber characteristics at any point along the fiber. Whenever the pulse of light encounters an anomaly in the fiber characteristics (referred to as an event), there is a change in the amount of backscattered/reflected light. By examining the trace, the nature and location of the event can be predicted.