An optical time-domain reflectometer (otdr) is an optoelectronic instrument used to measure the time and intensity of the light reflected on an optical fiber. It can verify splice loss, measure length and find faults. As a result, you can use the OTDR to pinpoint breaks in the cable, splices, and connectors, as well as to measure light loss in the system. OTDRs are invaluable test instruments that can illuminate problems in your optical fiber before they bring your system to its knees.
How Does OTDR Work?
An OTDR uses a light backscattering technique to analyze fibers. In essence, it takes a snapshot of the fiber’s optical characteristics by sending a high-powered pulse into one end of the fiber and measuring the light scattered back toward the instrument.
When high power light pulses are sent in to an Optical Fiber Cable (Core), a small portion of this light is reflected back to the source due to small defects in the glass that fibers are made of. This is called backscatter.
Reflections can also be caused inside an optical fiber cable (core) due to sharp bends, cracks, splices, cable terminations, cable cuts, etc. So, an OTDR Tester sends a series of short but high intensity light pulses in to the core of an optical fiber cable and records the power of the reflected light (at various points) as each pulse travels across the fiber cable. It uses specialized pulse laser diodes and high gain light detectors for this purpose.
When do you need an OTDR?
You can use an OTDR to locate a break or similar problem in a cable run, or to take a snapshot of fibers before turning an installation over to a customer. This snapshot, which is a paper copy of the ODTR trace, gives you a permanent record of the state of that fiber at any point in time. This can help installers when fibers have been damaged or altered after installation, proving where responsibility for the damage lies. In fact, some customers will demand OTDR testing as a condition for system acceptance.
OTDR can be used to predict the distance (in meters from the source) where the optical fiber cable has been disconnected. OTDRs are most effective when testing long cables (more than aproximatley 250 meters or 800 feet) or cable plants with splices. The data that the OTDR produces are typically used to create a picture called a “trace” or “signature” that has valuable information for the trained user and can be stored for later reference or to check against a blueprint when network trouble arises.
The OTDR is not perfect, of course. OTDRs should not be used for measuring insertion loss in the fiber optic cable – that task is better left to a fiber optic test equipment and optical power meter. OTDRs simply show you where the cables are terminated and confirm the quality of the fibers, connections and splices. Of course, OTDR traces are also used for troubleshooting, since they can show where breaks are in fiber when traces are compared to installation documentation. Although OTDRs are not especially accurate for loss testing, they can be used to conduct loss testing on long, outdoor runs of singlemode fiber where access to both ends of the cable is not practical.
