Signals and Protocols in Fiber Optic Data Links

A fiber optic data link 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 data link operates as an alternative to copper cabling or a wireless subsystem. In typical applications, a fiber optic data link 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 data link is basically the first layer, called the Physical Layer or PHY.

Fiber optic data link may transmit signals that are either analog or digital and of many different, usually standardized, protocols, depending on the communications system(s) it supports. Data link may be protocol transparent but may also include data encoding to provide more robust data communications. Data link 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.

Figure 1. Components Of A Fiber Optic Data link

A fiber optic data link consists of fiber optic transceivers or individual transmitters and receivers at either end that transmit over optical fibers. The typical data link 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 single mode.

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.

Figure 2. Components Of A Fiber Optic Data link Using A Single Fiber

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

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