The Optical Time Domain Reflectometer (OTDR)
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 ends of fiber reflection is large (Fresnel reflection) which creates spikes to the graph. Length of the fiber can therefore be measured from the width of the graph.
Figure 2 OTDR signal as a function of distance
An OTDR trace is a graphical representation of optical changes or 'events' on a fiber. An event could be a splice, optical connector, a bend, a break, or just normal backscattered light from the fiber itself.
In the OTDR trace faults for instance, are shown as a drop in the power of received signal (Figure 3). Size of a drop depends on an amount of power that is lost due to the component. The lost power represents of course the attenuation of component. Components and faults in fiber are either reflective or nonreflective. Reflective components create a spike to the graph of OTDR the same way as the both ends of fiber do. With nonreflective components there are no spikes because no excess light is reflected back. In most cases reflective attenuation is caused by connectors or other passive components and nonreflective attenuation is usually caused by fusion splice or similar fault in fiber.
Figure 3 Attenuation of different faults
Figure 4 OTDR Trace Information
The slope of the OTDR trace shows the attenuation coefficient of the fiber and is calibrated in dB/km by the OTDR (Figure 4). Whereby,
The height of that peak will indicate the amount of reflection at the event, unless it is so large that it saturates the OTDR receiver. Then the peak will have a flat top and tail on the far end, indicating the receiver was overloaded. Sometimes, the loss of a good fusion splice will be too small to be seen by the OTDR. That's good for the system but can be confusing to the operator. It is very important to know the lengths of all fiber in the network so that the operator is not confused by unusual events. Reflective pulses show the resolution of the OTDR. Two events which are closer...
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