Fiber optic fault locators function by shinning a red laser through jacketed fibers to identify breaks, bends, faulty connectors, splices, and other causes of signal loss. Signal loss areas will appear as bright glowing areas as a result of scattering.   Fiber optic fault locators can interface with two types of cables, single mode and multimode.  Single mode is an optical fiber that will allow 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.  Multimode describes a fiber optic cable, which 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 and connector alignment and coupling is less critical than single mode fiber. Distances of transmission and transmission bandwidth are less than with single mode fiber due to dispersion.  Some fiber optic fault locators can be used for both single mode and multimode cables.

Common connector types for fiber optic fault locators include biconic, D4, ESCON, FC, FDDI, LC, loopback, MTP, MT-RJ, MU, SC, SMA, and ST.  Biconic connectors have precision tapered ends for low insertion loss.  D4 connectors are made from a composite zirconia ceramic ferrule for durability. ESCON’s name derives from its application in IBM's ESCON channel interface.  An FC connector is constructed of an all-zirconia ceramic ferrule for durability.  FDDI connectors are designed by ANSI for use in FDDI networks.  LC connectors are precision PC polished zirconia ceramic ferrule. Loopback connectors are used in testing transceiver systems.  MTP connectors are a threaded type fiber optic connector.  The MT-RJ uses a ferrule smaller than the standard MT to hold two fibers.  An MU connectors is a 1.25mm diameter ferrule for compact multiple optical connectors and self-retentive mechanism for backplane applications.  SC connectors are high-precision ceramic ferrules. SMA connectors have a low cost multimode coupling with approval for military applications.  ST connectors are composed of a precise zirconia ceramic ferrule.

Important performance specifications to consider for fiber optic fault locators include wavelength, power output, and distance range.  The wavelength is typically measured over one of three standard wavelengths.  These are 635 nm, 650 nm, and 670 nm.  The power output is the optical power output of the fault locator.  The distance range specifies the maximum length of wire the fault locator is useful for.  Common features of fiber optic fault locators include pulsed and pocket sized.  Pulsed fiber optic fault locators generate a pulsed output.  This enables fiber optic identifiers to differentiate between fibers by tone.  Pocket sized fiber optic fault locators are configured to fit into a pocket.  They are sometimes referred to as pen sized.  An important environmental parameter to consider for fiber optic fault locators is the operating temperature