The Optical Time Domain Reflectometer (Otdr)

Topics: Optical fiber, Light / Pages: 6 (1367 words) / Published: Jun 23rd, 2013
The Optical Time Domain Reflectometer (OTDR)

Introduction

In fiber optic networks, OTDR (Optical Time Domain Reflectometer) is an opto-electronic instrument used to characterize an optical fiber. Unlike power meters OTDR does not measure loss, but instead implies it by looking at the backscatter signature of the fiber. Generally, OTDR are used to determine the loss of any part of a system, the length of the fiber and the distance between any points of interest.

Most of the light which is sent to the fiber can be detected at the other end, but a part of it is always absorbed or scattered. Absorption and scattering are caused by imperfections of fiber, small grains of dirt, for instance. Scattering means that light is not absorbed but it is just sent in different angle after it hits small particles in optical fiber (Figure 1). Some of the light is scattered to the direction it came from. This is called backscattering. Backscattering forms the basis to the use of the optical time domain reflectometry.

Figure 1 Rayleigh –scattering in optical fiber

Optical time domain reflectometry is based on scattering and reflections. OTDR sends an optical pulse to the fiber and measures the received backscattering. The signal which is received consists naturally only of scattering and reflections of pulse which was sent. By interpreting signal as a function of time OTDR can draw an attenuation of a fiber as a function of distance.

Theory of the OTDR

Optical time domain reflectometry measures backscattering as a function of time and graph is then drawn as a function of distance (Figure 2). The graph represents the power of signal which the detector of the OTDR receives. The graph of fiber probed by OTDR consists of two spikes with gradually decreasing line between them. The line between spikes is decreasing because the received signal is decreased as a function of distance in accordance with attenuation coefficient of fiber. At the both

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