An optical transmission system comprising, at the most basic level, a light source, an optical fiber, and a light detector. The light source can be in the form of a light-emitting diode (LED), vertical cavity surface-emitting laser (VCSEL), or laser diode.The optical fiber generally is one of many types of glass optical fiber (GOF), although plastic optical fiber (POF) is sometimes used. The light detector can be a positive-intrinsic-negative (PIN) diode or avalanche photodiode (APD). As is true of any transmission system, attenuation can be an issue over a long haul, so some form of amplification can be introduced. As FOTS systems generally are digital in nature, the typical approach is to apply a regenerative repeater, which detects the weak incoming signal, which it amplifies electrically, reshapes, retimes, and retransmits as an improved outgoing signal. Raman amplifiers increasingly are used, often in conjunction with repeaters. Long haul systems also typically conform to a set of ITU-T standards, with Synchronous Optical Network (SONET) being the North American version and Synchronous Digital Hierarchy (SDH) being preferred elsewhere.The optical signal is in the infrared (IR) range, within one of a number of windows, or wavelength bands, specified by the ITU-T. A single wavelength may be involved, or multiple wavelengths may coexist in a single fiber through a process known as wavelength division multiplexing (WDM), of which there are several levels, coarse (CWDM) and dense (DWDM). As a conducted system, FOTS systems are unparalleled in terms of bandwidth, error performance, signal attenuation, and security. See also APD, bandwidth, diode, diode laser, DWDM, GOF, IR, laser, LED, PIN, POF, repeater, SDH, SONET, VCSEL, wavelength, WDM, and window.
(Fiber Optic Transmission System) Referred to the first optical fiber systems used by the telephone companies. Deployment started in the late 1960s and developed slowly. There was no vendor interoperability in these early systems. See optical fiber.