Datalinks
A fiber optic datalink is a communications subsystem that connects inputs and outputs (I/O) from electronic subsystems and transmits those signals over optical fiber. In this function, a fiber optic datalink operates as an alternative to copper cabling or a wireless subsystem. In typical applications, a fiber optic datalink acts as a communications medium attached to electronics on either end that provide the other services necessary for communications over the link. In the OSI (Open Systems Interconnection) Network Model, the datalink is basically the first layer, called the Physical Layer or PHY.

Signals and Protocols
Fiber optic datalinks may transmit signals that are either analog or digital and of many different, usually standardized, protocols, depending on the communications system(s) it supports. Datalinks may be protocol transparent but may also include data encoding to provide more robust data communications. Datalinks may be specified by the application or standardized network (e.g. Ethernet) they are intended to support or by the types and bandwidth of signals they are designed to transmit.

A fiber optic datalink consists of fiber optic transceivers or individual transmitters and receivers at either end that transmit over optical fibers. The typical datalink transmits over two fibers for full duplex links, one fiber in each direction. The fibers may be of any type, multimode (graded index or step index) or singlemode.

Some links may use couplers and wavelength-division multiplexing to transmit bi-directionally over a single fiber as in FTTH PONs passive optical networks or OLANs, optical LANs. Some links may allow transmission at several wavelengths of light simultaneously over a single fiber in each direction, called wavelength-division multiplexing.

Extremely long cable plant lengths may require regeneration using repeaters, essentially datalinks in series. Optical fiber amplifiers may be used as repeaters in long singlemode systems. Singlemode systems using fiber amplifiers and wavelength-division multiplexing may require concern for nonlinear effects from high optical power involved.

Fiber Optic Transceiver for Datalinks
A fiber optic transceiver used on each end of a link includes a transmitter and receiver that convert electrical signals to optical signals and vice versa for transmission over optical fiber. Appropriate interfaces are included in the datalink to mate with the electrical and optical signals it connects with. These are typically standardized electrical and fiber optic connectors.

The transmitter consists of an electrical input and signal conditioning circuitry to drive an optical source, a light-emitting diode or laser that provides the electrical to optical conversion and alignment hardware for coupling of the optical signal into an optical fiber for transmission.

The receiver consists of a detector that connects to the optical fiber to accept an optical signal, convert the optical signal that has been transmitted through the optical fiber to an electrical signal and conditioning circuitry that creates an electrical output compatible with the communications system.

Transceivers are dedicated to one type of fiber determined by the distance and bandwidth of the communications being transmitted. Multimode fiber may be used for shorter and/or slower datalinks while singlemode fiber is used for longer links. The source in the transceiver will also depend on the application.

LEDs are used for slower (<~100 Mb/s) multimode links and VCSELs are used for faster multimode links. Some standard networks have options for using singlemode 1310nm lasers on multimode fiber. There are currently four types of multimode fiber used for datalinks, designated OM1-4. OM-1 is a fiber with larger core (62.5 microns) used primarily in legacy systems with LEDs at 850 or 1300 nm wavelengths. Faster multimode links use OM-2, OM-3 or OM-4 fiber with a 50 micron core, generally the faster fibers designated OM-3 or OM-4 which are optimized for 850 nm VCSELs.

Singlemode systems use lasers at 1310 or ~1550nm. 1310nm lasers are used for shorter links. The longer wavelengths around 1550 nm are used for long links and those using wavelength-division multiplexing. There are several specialized singlemode fibers which are optimized for special applications.

Performance
Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter. The discussion of performance on datalinks applies directly to transceivers which supply the optical to electrical conversion.

Every manufacturer of transceivers specifies their product for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Those specifications will end up being the datalink specifications on the final product used in the field.