| How Does an OTDR Work? | | | | where the fibers involved have different backscatter |
| In fiber optic networks, OTDR (Optical Time Domain | | | | coefficients. Connector or splice loss must be |
| Reflectometer) is an opto-electronic instrument used | | | | measured from both directions and averaged to |
| to characterize an optical fiber. OTDR is both the | | | | remove this source of error. |
| best known and least understood fiber optic | | | | OTDR Design |
| instrument. | | | | The principle optical components in a simple standard |
| OTDR does not measure loss, but instead implies it | | | | OTDR include a laser, a receiver, a coupler and a |
| by looking at the backscatter signature of the fiber. | | | | front-panel connector. |
| It does not measure cable plant loss that can be | | | | A laser is pigtailed to a connector on the OTDR |
| correlated to power budgets. | | | | through a 3dB optical coupler. This coupler is typically |
| An OTDR injects a series of optical pulses into the | | | | a fused bidirectional device but may also be made of |
| fiber under test. It also extracts, from the same end | | | | discrete optical components. |
| of the fiber, light that is scattered back and reflected | | | | The laser fires short, intense bursts of light that are |
| back from points in the fiber where the index of | | | | directed through the coupler and then out through |
| refraction changes. This working principle works like a | | | | the front-panel connector and into the fiber under |
| radar or sonar, sending out a pulse of light from a | | | | test. |
| very powerful laser, that is scattered by the glass in | | | | As the pulse travels along the fiber, some of the light |
| the core of the fiber. The intensity of the return | | | | is lost via absorption and Rayleigh scattering. The |
| pulses is measured and integrated as a function of | | | | pulse is also attenuated at discrete locations, such as |
| time, and is plotted as a function of the fiber length. | | | | splices, connectors, and bends, where local abrupt |
| An OTDR may be used for estimating the fiber's | | | | changes in the waveguide geometry couples light out |
| length and overall attenuation, including splice and | | | | the core and into the cladding. When the pulse |
| mated-connector losses. It may also be used to | | | | encounters discontinuities in the index of refraction |
| locate faults, such as breaks. | | | | (such as those found in connectors or the cleaved |
| Physical Limitations of OTDR Testing | | | | end of a fiber), part of the pulse's optical energy is |
| The OTDR suffers from several serious uncertainties | | | | reflected back toward the OTDR. |
| in measurement and physical limitations. The | | | | The Applications of Pulse Suppressors |
| measurement uncertainties come primarily from the | | | | Pulse suppressors, also referred to as OTDR launch |
| variations in backscatter of the fiber. The | | | | boxes, delay lines or "Dummy Fibers" are used to |
| backscatter coefficient is a function of the material | | | | occupy OTDR "dead zones" which enables accurate |
| properties of the glass in the core and the diameter | | | | loss measurements on near end connections of the |
| of the core. | | | | fiber under test. Suppressors may also be used in an |
| Variations of the fiber materials or geometry can | | | | educational setting to simulate networks and during |
| cause major changes in the backscattered light, | | | | installation and troubleshooting. |
| making splice or connector measurements uncertain | | | | With the inclusion of additional loss points, the pulse |
| by as much as +/-0.4dB. This has often led to | | | | suppressor becomes a test box or quick verification |
| confusion by showing a virtual gain at a connector, | | | | of your OTDR's calibrated accuracy. |