OTDR MODE
OVERVIEW
TEST PROCEDURES
All personnel testing optical fibers should be adequately trained in the field of fiber optics before using any fiber optic test equipment.
If the user is not completely familiar with testing fiber optics, they should seek competent training. Such training can be acquired from
a variety of sources, such as local hands-on training classes or online courses.
Valuable information about fiber optic testing can also be gathered from reading printed literature carefully or by thoroughly reading
supplied operations manuals.
OTDR basics
The above is especially true for Optical Time Domain Reflectometers (OTDRs). OTDRs are complicated technical devices, requiring
a great amount of technical skill, knowledge, and expertise to operate. Proper setup and trace interpretation are paramount to a
successful OTDR test, and the consequences of being inadequately trained could result in a significant amount of network down-time
and repair costs.
In addition, OTDRs are delicate scientific instruments, and should be treated as such. Great care should be taken to ensure that all
optical ports are kept clean and free from debris. The reasons for this are two-fold: 1) a clean OTDR produces accurate and precise
results; and 2) if debris is allowed to build up in the OTDR test port, over time connector insertions will grind the debris into the OTDR
port endface, resulting in scratches or “pits” that require extensive and costly repairs.
The main purpose of an OTDR is to locate faults in an optical fiber that exhibits unacceptable amounts of optical loss, or is no longer
functioning properly. These faults are commonly called “events” and include anomalies such as breaks, shatters, connector
endfaces, patch panels, splices, macro-bends, and micro-bends. By knowing the precise distance to an event, the technician can
determine the nature of the event and quickly restore the network to its former working state.
There are two types of events detectable by an OTDR: Fresnel (reflective) and backscatter (non-reflective).
Fresnel events are caused by “glass-to-air” boundaries in the optical fiber, which causes a high amount of light to be reflected directly
back toward the OTDR. Common Fresnel events include breaks, shatters, connector endfaces, patch panels, or even the end of the
fiber. End-of-fiber detection can also be used to measure the end-to-end length of the fiber.
Backscatter events are caused by the intrinsic properties of the optical fiber. The make-up of the optical fiber scatters the light in all
directions, including a small amount that gets scattered back towards the OTDR. Common backscatter events are splices (either
fusion or mechanical), macro-bends, and micro-bends. Backscatter can also be used to measure the attenuation (loss) on a certain
section of an optical fiber.
