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 than the pulse width cannot be seen. Generally longer pulse widths are used to be able to see farther along the cable plant and narrower pulses are used when high resolution is needed, although it limits the distance the OTDR can see. The Dead Zone
Dead zones originate from reflective events (connectors, mechanical splices, etc.) along the link, and they affect the OTDR’s ability to accurately measure attenuation on shorter links and differentiate closely spaced events, such as connectors in patch panels, etc. When the strong optical reflection from such an event reaches the OTDR, its detection circuit becomes saturated for a specific amount of time (converted to distance in the OTDR) until it recovers and can once again measure backscattering accurately. As a result of this saturation, there is a part of the fiber link following the reflective event that can not be “seen” by the OTDR. Analyzing the dead zone is very important to ensure the whole link is measured. Two types of dead zones are usually specified:
1. Event dead zone: This refers to the minimum distance required for consecutive reflective events to be “resolved”, i.e., to be differentiated from each other. If a reflective event is within the event dead zone of the preceding event, it will not be detected and measured correctly. Industry standard values range from 0.8 m to 5 m for this specification.
Figure 5 Common OTDR with 3 m event dead zone
2. Attenuation dead zone: This refers to the minimum distance required, after a reflective event, for the OTDR to measure a reflective or non-reflective event loss. To measure short links and to characterize or find faults in patchcords and leads, the shortest possible attenuation dead zone is best. Industry standard values range from 3 m to 10 m for this specification.
To overcome the problem of dead zones, usually a patchcord of about 100 m is added at the beginning of the system. As a result, all lauch dead zone problems have finished before the fiber (which is to be tested) is reached.
Ghosts When testing short cables with highly reflective connectors, it is likely to encounter “ghosts” like in Figure 6. These are caused by the reflected light from the far end connector reflecting back and forth in the fiber until it is attenuated to the noise level. Ghosts are very confusing, as they seem to be real reflective events like connectors, but will not show any loss. If a reflective event in the trace is found at a point where there is not supposed to be any connection, but the connection from the launch cable to the cable under test is highly reflective, look for ghosts at multiples of the length of the launch cable.
Figure 6 OTDR “Ghosts”
Resolution of the OTDR
Consider that light travels 1 m every 5 ns in the fiber, so a pulsewidth of 100 ns would extend for a distance of 20 m. When the light reaches an event, such as a connector, the light is reflected. The reflection appears to be a 20 m pulse on the OTDR.
However, if two events are separated by a distance of 10 m or less (Figure 7), the two reflections will overlap and join up in returning to the OTDR.
Figure 7 Thus the OTDR will display the two events as one event and the loss at each event is not detected, instead the sum of losses at both events will be shown on the OTDR. Choosing a shorter pulsewidth may give a better resolution but in turn resulting a low energy content (causing shorter detection range).
Besides using a shorter pulse which will provide the required range, a tool that is called a “visual fault locator” can help too. The visual fault locator injects a bright red laser light into the fiber to find faults. If there is a high loss, such as a bad splice, connector or tight bend stressing the fiber, the light lost may be visible to the naked eye. This will resolve event which is close to the OTDR or close to another event that are not resolvable to the OTDR. The limitation of this tool is about 4 km.