Optical Fiber Definations

8/3/2019 Optical Fiber Definations

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Optical Fiber

A glass or plastic fiber that has the ability to guide light along its axis. The three parts of an

optical fiber are thecore, thecladding, and thecoatingor buffer.

Core

The light-conducting central portion of an optical fiber, composed of material with a higherindex of refraction than the cladding. The portion of the fiber that transmits light.

Cladding

Material that surrounds thecoreof an optical fiber. Its lowerindex of refraction, compared tothat of the core, causes the transmitted light to travel down the core.
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Coating

The material surrounding thecladdingof a fiber. Generally a soft plastic material that protects

the fiber from damage.

Single-mode (SM) Fiber

A small-coreoptical fiberthrough which only onemodewill propagate. The typical diameter is

8-9 microns.

Multimode (MM) Fiber

Anoptical fiberthat has a core large enough to propagate more than one mode of light Thetypical diameter is 62.5 micrometers.
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Index of Refraction

The ratio of the velocity of light in free space to the velocity of light in a fiber material. Always

greater than or equal to one. Also called refractive index.

n = C/V

Where:

C = The speed of light in a vacuum.

V = The speed of the same wavelength in the fiber material.

Attenuation

The decrease in signal strength along a fiber optic waveguide caused by absorption and

scattering. Attenuation is usually expressed in dB/km

Absorption

That portion of optical attenuation in optical fiber resulting from the conversion of optical powerto heat. Caused by impurities in the fiber such as hydroxyl ions.
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Backreflection (BR)

A term applied to any process in thecable plantthat causes light to change directions in a fiberand return to the source. Occurs most often at connector interfaces where a glass-air interface

causes a reflection.

Backscattering

The return of a portion of scattered light to the input end of a fiber; the scattering of light in the

direction opposite to its original propagation.

Bend Radius

The smallest radius an optical fiber or fiber cable can bend before excessive attenuation or

breakage occurs.
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BER (Bit Error Rate)

The fraction of bits transmitted that are received incorrectly. Thebiterror rate of a system can beestimated as follows:

Where N0 = Noise power spectral density (A2/Hz).

IMIN = Minimum effective signal amplitude (Amps).

B = Bandwidth (Hz).

Q(x) = Cumulative distribution function (Gaussian distribution).

Bending Loss

Attenuation caused by high-order modes radiating from the outside of a fiber opticwaveguide

which occur when the fiber is bent around a small radius. See alsomacrobending,microbending.

Macrobending

In a fiber, all macroscopic deviations of the fibersaxisfrom a straight line, that will cause lightto leak out of the fiber, causing signalattenuation.

Microbending

Mechanical stress on afiberthat introduces local discontinuities, which results in light leakingfrom thecoreto thecladdingby a process calledmode coupling.
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Binary

Base two numbers with only two possible values, 0, or 1. Primarily used by communication andcomputer systems.

Bit

The smallest unit of information upon which digital communications are based; also an electrical

or opticalpulsethat carries this information.

Pulse

A current or voltage which changes abruptly from one value to another and back to the originalvalue in a finite length of time. Used to describe one particular variation in a series of wave

motions. The parts of the pulse include therise time,fall time, and pulse width, pulse amplitude.

The period of a pulse refers to the amount of time between pulses.
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Acceptance Angle

The half-angle of the cone (a) within which incident light is totally internally reflected by the

fiber core. It is equal to sin-1(NA).

Critical Angle

In geometric optics, at a refractive boundary, the smallest angle of incidence at whichtotalinternal reflectionoccurs.

Total Internal Reflection

The reflection that occurs when light strikes an interface at an angle of incidence (with respect tothe normal) greater than thecritical angle.

Critical Angle

In geometric optics, at a refractive boundary, the smallest angle of incidence at whichtotalinternal reflectionoccurs.
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Attenuator

1) In electrical systems, a usually passive network for reducing the amplitude of a signal without

appreciably distorting the waveform. 2) In optical systems, a passive device for reducing theamplitude of a signal without appreciably distorting the waveform.

Bandwidth

The range of frequencies within which a fiber opticwaveguideor terminal device can transmitdata or information.

Beamsplitter

An optical device, such as a partially reflecting mirror, that splits a beam of light into two ormore beams. Used in fiber optics for directional couplers.
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Cutoff Wavelength

Insingle-mode fiber, the wavelength below which the fiber ceases to be single-mode.

Center Wavelength

In alaser, the nominal value central operating wavelength. It is the wavelength defined by a peak

mode measurement where the effective optical power resides (see illustration). In an LED, the

average of the two wavelengths measured at the half amplitude points of the power spectrum.

Dispersion
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The temporal spreading of a light signal in an optical waveguide caused by light signals traveling

at different speeds through a fiber either due to modal or chromatic effects.

Chromatic Dispersion

Reduced fiber bandwidth caused by different wavelengths of light traveling at different speeds

down theoptical fiber. Chromatic dispersion occurs because the speed at which an optical pulsetravels depends on its wavelength, a property inherent to all optical fiber. May be caused by

material dispersion,waveguide dispersion, andprofile dispersion.

Profile Dispersion

Dispersionattributed to the variation ofrefractive indexcontrast withwavelength.

Dispersion Penalty
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The result of dispersion in which pulses and edges smear making it difficult for the receiver todistinguish between ones and zeros. This results in a loss of receiver sensitivity compared to ashort fiber and measured in dB. The equations for calculating dispersion penalty are as follows:

Where:

= Laser spectral width (nm)D = Fiber dispersion (ps/nm/km)

= System dispersion (ps/km)f = Bandwidth-distance product of the fiber (Hz km)

L = Fiber length (km)

FF = Fiber bandwidth (Hz)C = A constant equal to 0.5

FR = Receiver data rate (b/s)

dBL = Dispersion penalty (dB)

Cleave

The process of separating anoptical fiberby a controlled fracture of the glass, for the purpose ofobtaining a fiber end, which is flat, smooth, and perpendicular to the fiber axis.

Decibel (dB)
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A unit of measurement indicating relative power on a logarithmic scale. Often expressed inreference to a fixed value, such as dBm or dB.

dB = 10Log10 (P1/P2)

Data Rate

The number ofbitsof information in a transmission system, expressed in bits per second (b/s or

bps), and which may or may not be equal to the signal or baud rate.

DBc

Abbreviation for decibel relative to a carrier level.

dBm

Abbreviation for decibel relative to milliwatt.

dB: Abbreviation for decibel relative to microwatt

Laser

Acronym for light amplification by stimulated emission of radiation. A light source that

produces, through stimulated emission, coherent, near monochromatic light.

Diffraction Grating
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An array of fine, parallel, equally spaced reflecting or transmitting lines that mutually enhancethe effects of diffraction to concentrate the diffracted light in a few directions determined by thespacing of the lines and by the wavelength of the light.

Diameter-mismatch Loss

The loss of power at a joint that occurs when the transmitting fiber has a diameter greater thanthe diameter of the receiving fiber. The loss occurs when coupling light from a source to fiber,

from fiber to fiber, or from fiber to detector.

Evanescent Wave

Light guided in the inner part of an optical fibers cladding rather than in the core, i.e. the portionof the light wave in the core that penetrates into the cladding.

Gap Loss

Loss resulting from the end separation of two axially aligned fibers.
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Graded-index Fiber

Optical fiber in which therefractive indexof the core is in the form of a parabolic curve,decreasing toward thecladding.

Kevlar

A very strong, very light, synthetic compound developed by DuPont which is used to strengthenoptical cables.

Multimode (MM) Fiber

Anoptical fiberthat has a core large enough to propagate more than one mode of light The

typical diameter is 62.5 micrometers.

PC

Abbreviation for physical contact. Refers to an opticalconnectorthat allows thefiberends tophysically touch. Used to minimizebackreflectionandinsertion loss.
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A term applied to any process in thecable plantthat causes light to change directions in a fiberand return to the source. Occurs most often at connector interfaces where a glass-air interface

causes a reflection.

Insertion Loss

The loss of power that results from inserting a component, such as aconnector,coupler(illustrated), orsplice, into a previously continuous path.

Gate

1) A device having one output channel and one or more input channels, such that the outputchannel state is completely determined by the input channel states, except during switching

transients. 2) One of the many types of combinational logic elements having at least two inputs.

Polarization Mode Dispersion (PMD):

Polarization mode dispersion is an inherent property of all optical media. It is caused by thedifference in the propagation velocities of light in the orthogonal principal polarization states of

the transmission medium. The net effect is that if an opticalpulsecontains bothpolarizationcomponents, then the different polarization components will travel at different speeds and arrive

at different times, smearing the received optical signal
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Zero-dispersion Slope

Insingle-modefiber, the rate of change of dispersion with respect to wavelength, at the fiberszero-dispersion wavelength.

Zero-dispersion Wavelength (l0)

In a single-mode optical fiber, the wavelength at which material dispersion and waveguide

dispersion cancel one another. The wavelength of maximum bandwidth in the fiber. Also called

zero-dispersion point.

Zipcord

A two-fiber cable consisting of two single fibercableshaving conjoined jackets. A zipcord cable

can be easily divided by slitting and pulling the conjoinedjacketsapart

Designing Fiber Optic Systems

The first step in any fiber optic system design requires making careful decisions based on operating

parameters that apply for each component of a fiber optic transmission system. The main questions,

given in the table below, involve data rates and bit error rates in digital systems, bandwidth,linearity,

and signal-to-noise ratios in analog systems, and in all systems, transmission distances. These questions

of how far, how good, and how fast define the basic application constraints.

Table 1 - System Design Considerations

System Factor Considerations/Choices

Transmission DistanceSystem Complexity Increases with Transmission

Distance

Types of Optical Fiber Single-modeorMultimode
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DispersionIncorporate SignalRegeneratorsorDispersion

Compensation

FiberNonlinearitiesFiber Characteristics, Wavelengths, and

Transmitter Power

OperatingWavelength 780, 850, 1310, 1550, and 1625 nm Typical

Transmitter Power Typically Expressed indBm

SourceType LEDorLaser

Receiver Sensitivity/Overload Characteristics Typically Expressed in dBm

DetectorType PIN Diode,APD, orIDP

Modulation Code AM,FM,PCM, orDigital

Bit Error Rate(Digital Systems Only) 10-9

, 10-12

Typical

Signal-to-Noise Ratio Specified inDecibels(dB)

Number ofConnectors orSplicesin the SystemSignal Loss Increases with the Number of

Connectors or Splices

Environmental Requirements & Limitations Humidity, Temperature, Exposure for Sunlight

Mechanical RequirementsFlammability, Indoor/Outdoor Application
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Dense Wavelength-division Multiplexing (DWDM)

The transmission of many of closely spaced wavelengths in the 1550 nm region over a single

optical fiber. Wavelength spacings are usually 100 GHz or 200 GHz which corresponds to 0.8nm or 1.6 nm. DWDM bands include theC-Band, theS-Band, and theL-Band.

see full-size

Bandwidth

The range of frequencies within which a fiber opticwaveguideor terminal device can transmit

data or information.

Step-index Fiber

Fiber that has a uniformindex of refraction throughout thecorethat is a step below the index ofrefraction in thecladding.
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Optical Fiber Definations

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