Fiber Optic Components

Introduction to Fiber Optic Data Links

As a communications subsystem, a fiber optic data link connects inputs and outputs (I/O) from electronic subsystems and transmits these signals over fiber optic cables. More specifically, this data link converts an electrical input signal into an optical signal, then sends the optical signal over an optical fiber, and finally converts the optical signal back to an electrical signal, actually operating an alternative to copper cabling or a wireless subsystem. There are three parts included in a fiber optic data link: transmitter, optical fiber, and receiver. And most systems use a transceiver which includes both transmitter and receiver in a single module. This article is intended to introduce some basic information about fiber optic data link, such as its parts, signals and protocols, as well as performance.

Fiber Optic Data Link Parts

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. The typical data link transmits over two fibers for full duplex links, one fiber in each direction. Some links may allow transmission at several wavelengths of light simultaneously over a single fiber in each direction, called wavelength-division multiplexing (WDM).

signal transmission over two/one optical fiber

The transmitter consists of an interface circuit and a source drive circuit. The transmitter’s drive circuit converts the electrical signals to an optical signal by varying the current flow through the light source. The two types of optical sources are light-emitting diodes (LEDs) and laser diodes.

The receiver converts the optical signal exiting the fiber back into an electrical signal. It consists of two parts: the optical detector and the signal-conditioning circuits. An optical detector detects the optical signal. The signal-conditioning circuit conditions the detector output so that the receiver output matches the original input to the transmitter. The receiver should amplify and process the optical signal without introducing noise or signal distortion. Noise is any disturbance that obscures or reduces the quality of the signal. Noise effects and limitations of the signal-conditioning circuits cause the distortion of the receiver’s electrical output signal.

Transceivers are designed to work over one specific type of fiber decided by the distance and bandwidth of the communications. For instance, Cisco MGBLX1, a 1000BASE-LX SFP, is dedicated to work over single-mode fiber (SMF) for maximum 10km link length at 1310nm wavelength. HP J4858C, a 1000BASE-SX SFP is designed to operate through multi-mode fiber (MMF) for maximum 550m link length at 850nm wavelength. SMF is used for significantly longer links while MMF for shorter links.

Signals & Protocols

Fiber optic data links 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 links may be protocol transparent, but may also include data encoding to provide more robust data communications. They may be specified by the application or standardized network which they are intended to support, or by the types and bandwidth of signals that they are designed to transmit.

Analog signals are continuously variable signals where the information in the signal is contained in the amplitude of the signal over time. They are the natural form of most data, but are subject to degradation by noise in the transmission system. As an analog signal is attenuated in a cable, the signal to noise ratio becomes worse so the quality of the signal degrades. In contrast, digital signals are sampled at regular time intervals and information is a digital number. They can be transmitted long distances without degradation as the signal is less sensitive to noise.

analog vs. digital

Power Budget

The power budget is the difference between the output power of the transmitter and the input power requirements of the receiver. The receiver has an operating range determined by the signal-to-noise ratio (S/N) in the receiver. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N.

power budget

The power of the receiver is determined by the output power of the transmitter, and diminished by the loss in the cable plant primarily, but other factors may also affect power budget performance. When the power budget is inadequate for the loss in the cable plant, higher power transmitters or more sensitive receivers or higher bandwidth cable plant are required.

Conclusion

In typical applications, a fiber optic data link serves as a communications medium attached to electronics on either end that provide the other services necessary for communications over the link. Its each part is responsible for the successful transfer of the data signal. As a professional fiber optic product supplier, Fiberstore offers various kinds of fiber optic transceivers e.g. MGBLX1), and optical fibers (e.g. LC fiber cable). You can visit Fiberstore for more information about fiber optic data link products.

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