Use of Optical Time-Domain Reflectometer in Fiber Optics

Fiber optic network deployment staff and optical fiber and cable production/testing personnel are end users of Optical Time-Domain Reflectomtery in the fiber optic industry. The equipment that make use of the time domain reflection of the light pulses is known as OTDR (Optical Time-Domain Reflectometer. Those who work in fiber optic industry uses optical time-domain reflectometry to characterize optical fiber and optical connection properties in the field.

In optical time-domain reflectometry, the equipment called OTDR transmits an optical pulse through an optical fiber. The transmitted signal travels along the length of the fiber and when it meets the end of the fiber, it returns back to the OTDR. The OTDR’s receiver unit receives the returned signal and its power level. Note that a fraction of light is reflected back due to Rayleigh scattering and Fresnel reflection. By comparing the amount of light scattered back at different times, the OTDR can determine fiber and connection losses.

Optical Time Domain Reflectometer

In an outside plant fiber optic network, where several fibers are connected to form an installed cable plant, the OTDR can characterize optical fiber and optical connection properties along the entire length of the cable plant. A fiber optic cable plant consists of optical fiber cables, connectors, splices, mounting panels, jumper cables, and other passive components. A cable plant does not include active components such as optical transmitters or receivers.

OTDR’s display unit, which is mounted at the front side displays the back-scattered and reflected optical signal as a function of length. The OTDR plots half the power in decibels (dB) versus half the distance. You may wonder why half the power and half the distance. Signal travels to the end of the fiber and then returns back to the origin, thus traveling two times the fiber length. Thus, plotting half the power in dB and half the distance corrects for round trip effects makes sense.

Another interesting feature that can be observed in the OTDR is a reflection peak at the end of the fiber. This reflection known as Fresnel reflection happens when the signal traveling inside the glass medium meets the air at the end of the fiber. Fresnel reflection is caused by the change in refractive indices of two medium (glass and air). By analyzing the OTDR plot, or trace, end users can measure fiber attenuation and transmission loss between any two points along the cable plant. End users can also measure insertion loss and reflectance of any optical connection. In addition, end users use the OTDR trace to locate fiber breaks or faults.

OTDR traces can have several common characteristics. An OTDR trace begins with an initial input pulse. This pulse is a result of Fresnel reflection occurring at the connection to the OTDR. Following this pulse, the OTDR trace is a gradual down-sloping curve interrupted by abrupt shifts. Periods of gradual decline in the OTDR trace result from Rayleigh scattering as light travels along each fiber section of the cable plant.

Periods of gradual decline are interrupted by abrupt shifts called point defects. A point defect is a temporary or permanent local deviation of the OTDR signal in the upward or downward direction. Point defects are caused by connectors, splices, or breaks along the fiber length. Point defects, or faults, can be reflective or non-reflective. An output pulse at the end of the OTDR trace indicates the end of the fiber cable plant. This output pulse results from Fresnel reflection occurring at the output fiber-end face.

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