Fiber Optics Test Equipment - OTDR

What is an OTDR ?

Short for Optical Time Domain Reflectometer, an instrument that analyzes the light loss in an optical fiber cable in optical network trouble shooting. An OTDR injects a short, intense laser pulse into the optical fiber and measures the backscatter and reflection of light as a function of time. The reflected light characteristics are analyzed to determine the location of any fiber optic breaks or splice losses.

The OTDR tester operates by sending an optical pulse of energy, timing and response, a d analyzing the shape of the pulse to determine the type of change in the transmission line that caused the signal reflection.

How does OTDR machine look like ?



This is called mini-OTDR which main functions are to determine fiber length, fiber continuity and approximate fiber damaged location.

There are few brands of mini-OTDR in the market namely by Anritsu, Yokogawa (used to be known as ANDO), NOYES and other few brands either from Europe, USA, Japan or China.

There is another type of lightweight OTDR which is called hand-held OTDR. The handheld OTDR is mainly to be used to test short distance fiber cable length in particular the Fiber-To-The-Home fiber installation. The most common and popular handheld OTDR being used in Malaysia is of NOYES brand.

How Does an OTDR Work ?

Figure 1. Scattering in an optical fiber

In the fiber, light is scattered in all directions, including back toward the source as shown in Figure 1. The OTDR uses this "backscattered light" to make its measurements. It sends out a very high power pulse and measures the light coming back. At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fiber. Think of the OTDR pulse as being a "virtual source" that is testing all the fiber between itself and the OTDR as it moves down the fiber. Since it is possible to calibrate the speed of the pulse as it passes down the fiber, the OTDR can correlate what it sees in backscattered light with an actual location in the fiber. Thus it can create a display of the amount of backscattered light at any point in the fiber, Figure 2.

Figure 2. OTDR Display

There are some calculations involved. Remember the light has to go out and come back, so you have to factor that into the time calculations, cutting the time in half and the loss calculations, since the light sees loss both ways. The power loss is a logarithmic function, so the power is measured in dB.