Optical time domain reflectometers (OTDR) measure the elapsed time and intensity of light reflected along an optical fiber. They are useful tools for locating problems in an optical network as they can compute the distance to breaks or attenuation. Optical time domain reflectometers function by injecting a series of optical pulses into the fiber under test. They also extract, from the same end of the fiber, light that is scattered or reflected back. The intensity of the return pulses are measured and integrated as a function of time, and are plotted as a function of fiber length. Optical time domain reflectometers may also be used for estimating the fiber's length and overall attenuation, including splice and mated-connector losses. Additionally, they can be used to locate faults, such as breaks.

Types

There are several basic types of optical time domain reflectometers (OTDR): bench top, rack mountable, and handheld. Bench top devices are relatively large and designed to sit on a desk or atop a lab bench. Rack mountable optical time domain reflectometers can be bolted into racks with other electronic equipment. A handheld optical time domain reflectometer is portable and designed for use in the field. 

Selection

Cable type is an important consideration when selecting optical time domain reflectometers (OTDR). A single-mode optical time domain reflectometer is designed for use with optical fiber that allows only one mode to propagate. The fiber has a very small core diameter of approximately 8 µm. It permits signal transmission at extremely high bandwidth and allows very long transmission distances. A multimode optical time domain reflectometer is designed to work with fiber optic cable that supports the propagation of multiple modes. Multimode fiber may have a typical core diameter of 50 to 100 µm with a refractive index that is graded or stepped. It allows the use of inexpensive LED light sources. Connector alignment and coupling are less critical than single-mode fiber. Distances of transmission and transmission bandwidth are also less than with single-mode fiber due to dispersion. A single/multimode optical time domain reflectometer may be used with both single mode and multimode cables.

Uses

Many types of connectors are used with optical time domain reflectometers (OTDR). Biconic connectors have precision-tapered ends for low insertion loss. D4 and FC connectors are durable, zirconia-ceramic ferrules with a keyed body for repeatability. FC connectors are used primarily with single-mode fibers, but are also used in telephone systems, instruments, and high-speed communication links. Designed for use in FDDI networks, FDDI connectors are 2.5 mm ferrules that include a fixed shroud. ESCON connectors have the same measurements, but use an adjustable shroud. LC connectors are high-precision, zirconia-ceramic ferrules that feature an RJ-45 push-pull housing and latching. MT-RJ connectors hold two fibers with a ferrule that is smaller than the one used in MTP connectors, devices that are threaded and well-suited for high-density applications. ST connectors are easy-to-assemble devices that feature a bayonet mounting system. They are used with both single-mode and multi-mode fibers in communications applications. SMA connectors include a low-cost, multi-mode coupling that is suitable for military applications. Loop back connectors are used to test transceiver system