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OTDR stands for Optical Time Domain Reflectometer. Its name means that an OTDR checks the reflected light from the fiber under test along time, shown as the yellow trace line in this picture.
The fiber connected to this OTDR is called a launch cable. The launch cable is then connected to the fiber under test.
An OTDR provides a view of the fiber link by reading the level of light that is reflected back from the fiber under test.
There are actually two types of light being reflected back.
The first type is a constant low level reflection created by the fiber called "Rayleigh backscattering".
Rayleigh backscattering comes from the natural reflection and absorption of impurities inside optical fiber. When hit, some particles redirect the light in different directions, creating both signal attenuation and backscattering.
The second type of reflection is called "Fresnel reflection". When the light hits an abrupt change in refractive index, for example, from glass to air at a fiber connector interface, a higher amount of light is reflected back, which can be thousands of times bigger than the Rayleigh backscattering.
Fresnel reflection can be seen as spikes in an OTDR trace. Examples of such reflections are connectors, mechanical splices, bulkheads, fiber breaks or opened connectors.
This illustration shows the construction of an OTDR.
The pulse generator generates a short electrical pulse which drives the laser to generate a corresponding light pulse. The light pulse is coupled into the fiber under test.
When some light is reflected back from the fiber, it is coupled into a photodiode by the directional coupler. This photodiode converts the light signal into electrical signal, which is then amplified and recorded. This process produces the trace as shown here.
The signal sent is a short pulse that carries a certain amount of energy. A clock then precisely calculates the time of flight of the pulse, and time is converted into distance based on the speed of light in this fiber.
When the pulse has entirely returned to the detector, another pulse is sent-until the acquisition time is complete.
So many acquisitions will be performed and averaged in a second to provide a clear picture of the link's components.
There are three types of OTDRs on the market now.
Laboratory OTDRs are typically used in test labs. They have an extremely long range with many options.
Mini OTDRs are portable and designed for field testing. They have built in screens and provide data storage as traces are collected in the field.
PC-based OTDRs connect with a personal computer and operate with windows based software. They allow saving traces on a disk and then transfer data between computers.
Generally OTDRs are used for testing with a launch cable and may use a receive cable.
The launch cable allows the OTDR to settle down after the test pulse is sent into the fiber and provides a reference connector for the first connector on the cable under test to determine its loss.
A receive cable may be used on the far end to allow measurements of the connector on the end of the cable under test also.
Fresnel reflections lead to an important OTDR specification known as "dead zones". Dead zones are expressed in distance such as meters.
A dead zone is defined as the length of time during which the detector is temporary blinded by a high amount of reflected light, until it recovers and can read light again.
Think of when you drive a car at night and you cross another car in the opposite direction; your eyes are blinded for a short period of time.
In the OTDR world, time is converted into distance; therefore, more reflection causes the detector to take more time to recover, resulting in a longer dead zone.
The most common place you see this as a problem is caused by the connector on the OTDR itself. The reflection causes an overload which can take the equivalent of 50 meters to one kilometer to fully recover, depending on the OTDR design, wavelength and magnitude of the reflection.
For this reason, most OTDR manuals suggest using a launch cable, which gives the OTDR time to recover before you start looking at the actual fiber under test.
The slope of the fiber trace shows the attenuation coefficient of the fiber and is calibrated in dB/km by the OTDR.
Connectors and splices are called "events". Both should show a loss, but connectors and mechanical splices will also show a reflective peak. 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 peak will have a flat top and tail on the far end, indicating the receiver was overloaded.
Reflective pulses can show you the resolution of the OTDR. You cannot see two events closer than is allowed by the pulse width.
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