Troubleshooting Your Optical Fiber Networks: Introduction to OTDR – Optical Time Domain Reflectometer By Colin Yao At Isnare.com Ezine Articles

How Does an OTDR Work?

In fiber optic networks, OTDR (Optical Time Domain Reflectometer) is an opto-electronic instrument used to characterize an optical fiber. OTDR is both the best known and least understood fiber optic instrument.

OTDR does not measure loss, but instead implies it by looking at the backscatter signature of the fiber. It does not measure cable plant loss that can be correlated to power budgets.

An OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered back and reflected back from points in the fiber where the index of refraction changes. This working principle works like a radar or sonar, sending out a pulse of light from a very powerful laser, that is scattered by the glass in the core of the fiber. The intensity of the return pulses is measured and integrated as a function of time, and is plotted as a function of the fiber length.

An OTDR may be used for estimating the fiber's length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks.

Physical Limitations of OTDR Testing

The OTDR suffers from several serious uncertainties in measurement and physical limitations. The measurement uncertainties come primarily from the variations in backscatter of the fiber. The backscatter coefficient is a function of the material properties of the glass in the core and the diameter of the core.

Variations of the fiber materials or geometry can cause major changes in the backscattered light, making splice or connector measurements uncertain by as much as +/-0.4dB. This has often led to confusion by showing a virtual gain at a connector, where the fibers involved have different backscatter coefficients. Connector or splice loss must be measured from both directions and averaged to remove this source of error.

OTDR Design

The principle optical components in a simple standard OTDR include a laser, a receiver, a coupler and a front-panel connector.

A laser is pigtailed to a connector on the OTDR through a 3dB optical coupler. This coupler is typically a fused bidirectional device but may also be made of discrete optical components.

The laser fires short, intense bursts of light that are directed through the coupler and then out through the front-panel connector and into the fiber under test.

As the pulse travels along the fiber, some of the light is lost via absorption and Rayleigh scattering. The pulse is also attenuated at discrete locations, such as splices, connectors, and bends, where local abrupt changes in the waveguide geometry couples light out the core and into the cladding. When the pulse encounters discontinuities in the index of refraction (such as those found in connectors or the cleaved end of a fiber), part of the pulse's optical energy is reflected back toward the OTDR.

The Applications of Pulse Suppressors

Pulse suppressors, also referred to as OTDR launch boxes, delay lines or "Dummy Fibers" are used to occupy OTDR "dead zones" which enables accurate loss measurements on near end connections of the fiber under test. Suppressors may also be used in an educational setting to simulate networks and during installation and troubleshooting.

With the inclusion of additional loss points, the pulse suppressor becomes a test box or quick verification of your OTDR's calibrated accuracy.

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