Fiber optic test sources

A fiber optic test source must be chosen that is compatible with the type of fiber being used (single mode or multimode with the appropriate core diameter) and the desired wavelength to perform the test. Most sources are LEDs or lasers of the types that are commonly used as transmitters in existing fiber optic systems, which makes them representative of real applications and improves the usefulness of the test. Some laboratory tests, such as measuring fiber attenuation over a range of fiber lengths, require a variable wavelength source,

Typical wavelengths of the sources are 650 or 665 nm (plastic fiber), 820, 850 and 870 nm (short-wave multimode fiber) and 1300 (long-wave multimode fiber) or 1310 nm and 1550 nm (long wavelength single-mode fiber). LEDs are typically used to test multimode fibers and lasers are used for single-mode fiber, although there is some crossing. High-speed local area networks (LANs) that use multimode fibers can be tested with vertical-cavity and surface emission lasers (VCSEL) such as system sources and short single-mode interconnect cables can be tested with LEDs.

The wavelength of the source can be a crucial point when making accurate loss measurements on long links since the fiber attenuation coefficient is sensitive to the wavelength. Therefore, all test sources must be calibrated for wavelength in case corrections are required for wavelength variations.

Test sources almost always have fixed connectors. Hybrid test jumpers with compatible connectors with the source at one end and the connector being tested at the other should be used as reference cables. This could affect the type of reference adjustment mode used for the loss test.

The factors related to the source that affect the accuracy of the measurement are the stability of the output power and the modal distribution released in the multimode fiber. The stability of the source is mainly a factor of the electronic circuit in the source. Industrial standards have requirements on the modal output of the test sources for multimode fibers that are important for the manufacturers of the test sources. Different standards have required that the mixers, filters and mode separators adjust the modal distribution in the fiber to approximate the actual operating conditions. At present, Most standards require that the sources meet the output requirements and that a mandrel roll mode filter be used in the test. The effects of modal power distribution on multimode measurements are discussed in the chapter on fiber optics.

Fiber optic technician certification

Reference cables

Loss checking requires one or more reference wires, depending on the check performed and the appropriate coupling adapters for the connectors. The reference cables, in general, have a length of 1-2 meters and have fibers and connectors that fit the cables to be tested. The measurement accuracy will depend on the quality of the reference cables since they will be attached to the cable under test. The quality and cleanliness of the connectors in the launch and reception cables are one of the most important factors in the accuracy of loss measurements.(patchcord) or the single-ended method shown below to make sure they are in good condition before you start testing other cables.

The sets of standards have failed to specify the quality of the reference cables in terms of components with restricted tolerance such as fibers and connectors. The standards that require special quality reference test leads now specify cables with low loss connections. The best recommendation to qualify the reference cables is to choose low loss cables, tested with the "single-end" method in accordance with the FOTP-171 cable test standard, as described below.