Fiber Optics

Fiber optics uses light to send information (data). More formally, fiber-optics is the branch of optical technology concerned with the transmission of radiant power (light energy) through fibers.

FIBER OPTIC DATA LINKS

A fiber-optic data link sends input data through fiber-optic components and provides this data as output information. It has the following three basic functions:

1. To convert an electrical input signal to an optical signal

2. To send the optical signal over an optical fiber

3. To convert the optical signal back to an electrical signal

A fiber-optic data link consists of three parts - transmitter, optical fiber, and receiver. The picture below is an illustration of a fiber-optic data-link connection. The transmitter, optical fiber, and receiver perform the basic functions of the fiber-optic data link. Each part of the data link is responsible for the successful transfer of the data signal.

A fiber-optic data link needs a transmitter that can effectively convert an electrical input signal to an optical signal and launch the data-containing light down the optical fiber. A fiber optic data link also needs a receiver that can effectively transform this optical signal back into its original form. This means that the electrical signal provided as data output should exactly match the electrical signal provided as data input.

Parts of a fiber-optic data link.

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The transmitter converts the input signal to an optical signal suitable for transmission. The transmitter consists of two parts, an interface circuit and a source drive circuit. The transmitter's drive circuit converts the electrical signals to an optical signal. It does this by varying the current flow through the light source. The two types of optical sources are light-emitting diodes (LEDs) and laser diodes.

The optical source launches the optical signal into the fiber. The optical signal will become progressively weakened and distorted because of scattering, absorption, and dispersion mechanisms in the fiber waveguides. Chapter 2 discusses the fiber mechanisms of scattering, absorption, and dispersion. Chapter 3 discusses the types of optical fibers and cables.

The receiver converts the optical signal exiting the fiber back into an electrical signal. The receiver 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.

An optical detector can be either a semiconductor positive-intrinsic-negative (PIN) diode or an avalanche photodiode (APD).

A PIN diode changes its electrical conductivity according to the intensity and wavelength of light. The PIN diode consists of an intrinsic region between p-type and n-type semiconductor material.

A fiber-optic data link also includes passive components other than an optical fiber. The picture above does not show the optical connections used to complete the construction of the fiber-optic data link. Passive components used to make fiber connections affect the performance of the data link. These components can also prevent the link from operating. Fiber optic components used to make the optical connections include optical splices, connectors, and couplers.

Proof of link performance is an integral part of the design, fabrication, and installation of any fiber optic system. Various measurement techniques are used to test individual parts of a data link. Each data link part is tested to be sure the link is operating properly. Chapter 5 discusses the laboratory and field measurements used to measure link performance.

Fiber-Optic History

Fiber-Optics Systems

Fiber-Optic Light Transmission

Properties of Light

Refraction of Light

Light-Reflection in Fiber Optics

Light diffusion, absorption, and transmission

Basic Optical Material Properties

Optical Fiber Structure

Light Propagation

Mode Theory

Modes

Optical-Fiber Types

Optical-Fiber Properties

Absorption

Scattering

Dispersion

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