Fiber Optics Glossary of Terms:
absorption: Together with scattering, absorption
forms the principal cause of the attenuation of an
optical waveguide. It results from unwanted impurities
in the waveguide material and has an effect only at
angle of incidence: The angle between an incident
ray and the normal to a reflecting surface.
attenuation: The reduction of average optical power
in an optical waveguide, expressed in decibels. The main
causes are scattering and absorption, as well as optical
losses in connectors and splices. Attenuation α or loss is
α(dB) = -10 log10 (Po/Pi)
where Pi is the optical power measured at the input and Po
the optical power measured at the output. Since Po is less
than Pi, a negative sign is placed before the 10 to yield a
positive number for α.
attenuator: An optical element that reduces intensity
of a optical signal passing through it (i.e., attenuates
it). Example: Manufactures make attenuators built into
connectors which incorporate a sleeve consisting of a
carbon‑coated mylar filter. They come in steps of 6 dB,
12 dB,16 dB, and 22 dB values.
avalanche photodiode (APD): A photodiode designed
to take advantage of avalanche multiplication of photocurrent.
As the reverse‑bias voltage approaches the break‑down
voltage, hole‑electron pairs created by absorbed photons
acquire sufficient energy to create additional hole‑electron
pairs when they collide with ions; thus a multiplication or signal
gain is achieved.
axial ray: A light ray that travels along the axis of an
backscattering: A small fraction of light that is deflected
out of the original direction of propagation by scattering
suffers a reversal of direction. In other words, it propagates
in the optical waveguide towards the transmitter.
bandwidth: The lowest frequency at which the magnitude of
the waveguide transfer function decreases to 3 dB (optical
power) below its zero frequency value. The bandwidth will
be a function of length of the waveguide, but may not be
directly proportional to the length.
beamsplitter: A device used to divide or split an optical
beam into two or more separate beams.
beamwidth: The distance between two diametrically opposed
points at which the irradiance is a specified fraction of the
beam’s peak irradiance; most often applied to beams that
are circular in cross‑section.
ber (Bit Error Rate): In digital applications, the ratio of
bits received in error to bits sent. BERs of one errored bit
per billion (1×10-9) sent are typical.
buffer: Material used to protect optical fiber from physical
damage, providing mechanical isolation and/or protection.
Fabrication techniques include tight or loose tube buffering,
as well as multiple buffer layers.
Burrus LED: A surface‑emitting LED with a hole etched to
accommodate a light‑collecting fiber. Named after its inventor,
Charles A. Burrus of Bell Labs.
chromatic dispersion: Spreading of a light pulse caused
by the difference in refractive indices at different wavelengths.
cladding: The dielectric material surrounding the core of
an optical fiber.
coherent: Light source (laser) in which the amplitude of
all waves is exactly equivalent, and rise and fall together.
core: The central region of an optical fiber through which
light is transmitted.
coupler: An optical component used to split or combine
optical signals. Also known as a “Splitter”,”T‑coupler”,
“2×2” or “1×2” coupler.
coupling loss: The power loss suffered when coupling light
from one optical device to another.
critical angle: The smallest angle from the fiber axis at
which a ray may be totally reflected at the core/cladding
cutoff wavelength: The shortest wavelength at which only the
fundamental mode of an optical waveguide is capable of
dark current: The external current that, under specified
biasing conditions, flows in a photodetector when there is
no incident radiation.
data rate: The maximum number of bits of information which
can be transmitted per second, as in a data transmission
link. Typically expressed as megabits per second (Mb/s).
decibel (dB): The standard unit of level used to express
gain or loss of optical or electrical power.
detector: A transducer that provides an electrical output
signal in response to an incident optical signal. The current
is dependent on the amount of light received and the type
dispersion: Spread of the signal delay in an optical wave‑
guide. It consists of various components: modal dispersion,
material dispersion, and waveguide dispersion. As a result
of its dispersion, an optical waveguide acts as a low‑pass
filter for the transmitted signals.
ferrule: A component of a fiber optic connection/connector
that holds a fiber in place and aids in its alignment.
fiber optics: A branch of optical technology concerned with the transmission of light through micro fibers made of transparent materials such as glass, fused silica, or plastic, to carry (data) information.
fiber optic link: A fiber optic cable with connectors
attached to a transmitter (source) and receiver (detector).
fresnel reflection: The reflection of a portion of the light
incident on a planar surface between two homogeneous
media having different refractive indices. Fresnel reflection
occurs at the air/glass interfaces at entrance and exit ends
of an optical fiber.
fundamental mode: The lowest order mode of a waveguide.
fiber data distributed interface (FDDI):‑ An emerging
standard developed by AT&T, Hewlett‑Packard Co, and Siemens
Corp.,using a 100 Mbps token‑ring network that employs dual
optical fibers. Critical portions of the specification have not
yet been defined.
graded index fiber: An optical fiber with a variable
refractive index that is a function of the radial distance
from the fiber axis.
incoherent: Light source (LED) emits incoherent light as
opposed to the laser which emits coherent light. (See
index matching material: A material, often a liquid or
cement whose refractive index is nearly equal to the core
index, used to reduce Fresnel reflections from a fiber end
index of refraction (IOR): Refractive index.
injection laser diode (ILD): Laser diode.
insertion loss: The attenuation caused by the insertion of
an optical component; in other words, a connector or coupler
in an optical transmission system.
intermodal distortion: Multimode distortion.
integrated optical components (IOCs): Optical devices
(singly or in combinations) that use light transmission
in waveguides. The waveguides structure and confine the
propagating light to a region with one or two very small
dimensions of the order of the wavelength of light. A
common material used in the fabrication process of an IOC
is Lithium Niobate (LiNbO ).
irradiance: Power density at a surface through which radiation
passes at the radiating surface of a light source or at the cross
section of an optical waveguide. The normal unit is Watts per
centimeters squared, or W/cm.
laser diode (LD): Semiconductor diode which emits coherent
light above a threshold current.
launch angle: Angle between the propagation direction of the
incident light and the optical axis of an optical waveguide.
launching fiber: A fiber used in conjunction with a source
to excite the modes of another fiber in a particular way.
Launching fibers are most often used in test systems to
improve the precision of measurements.
light emitting diode (LED): A semiconductor device which
emits incoherent light from a p‑n junction when biased with
an electrical current in the forward direction. Light may exit
from the junction strip edge or from its surface, depending
on the device’s structure.
light: In the laser and optical communication fields, the
portion of the electromagnetic spectrum that can be handled
by the basic optical techniques used for the visible spectrum
extending from the near ultraviolet region of approximately
0.3 micron, through the visible region and into the mid-infrared
region of about 30 microns.
lightwaves: Electromagnetic waves in the region of optical
frequencies. The term “light” was originally restricted to
radiation visible to the human eye, with wavelengths between
400 and 700 nanometers (nm). However, it has become
customary to refer to radiation in the spectral regions adjacent
to visible light (in the near infrared from 700 to about
2000 nm) as “light” to emphasize the physical and technical
characteristics they have in common with visible light.
macrobending: Macroscopic axial deviations of a fiber from
a straight line, in contrast to microbending.
microbending: Curvatures of the fiber which involve axial
displacements of a few micrometers and spatial wavelengths
of a few millimeters. Microbends cause loss of light and
consequently increase the attenuation of the fiber.
micron: Micrometer (μm). One millionth of a meter (1×10-6 m).
modal dispersion: Pulse spreading due to multiple light rays
traveling different distances and speeds through an optical fiber.
modal noise: Disturbance in multimode fibers fed by laser
diodes. It occurs when the fibers contain elements with mode
dependent attenuation, such as imperfect splices, and is more
severe the better the coherence of the laser light.
mode: A ray or beam of light.
modes: Discrete optical waves that can propagate in optical
waveguides. They are eigenvalue solutions to the differential
equations which characterize the waveguide. In a single mode
fiber, only one mode, the fundamental mode, can propagate.
There are several hundred modes in a multimode fiber which
differ in field pattern and propagation velocity.
The upper limit to the number of modes is determined by the
core diameter and the numerical aperture of the waveguide.
modified chemical vapor deposition (MCVD) technique:
A process in which deposits are produced by heterogeneous gas/
solid and gas/liquid chemical reactions at the surface of a
substrate. The MCVD method is often used in fabricating optical
waveguide preforms by causing gaseous material to react
and deposit glass oxides. Typical starting chemicals include
volatile compounds of silicon, germanium, phosphorus, and
boron, which form corresponding oxides after heating with
oxygen or other gases. Depending on its type, the preform may
be processed further in preparation for pulling into an optical fiber.
monochromatic: Consisting of a single wavelength. In
practice, radiation is never perfectly monochromatic but,
at best, displays a narrow band of wavelengths.
multimode distortion: The signal distortion in an optical
waveguide resulting from the superposition of modes with
multimode fiber: Optical waveguide whose core diameter is
large compared with the optical wavelength and in which,
consequently, a large number of modes are capable of
nanometer (nm): One billionth of a meter (1×10-9 m).
noise equivalent power (NEP): The RMS value of optical
power which is required to produce an RMS signal‑to‑noise
ratio of 1; and indication of noise level which defines the
minimum detectable signal level.
numerical aperture: A measure of the range of angles of
incident light transmitted through a fiber. Depends on the
differences in index of refraction between the core and the
optical time domain reflectometer (OTDR): A method for
characterizing a fiber wherein an optical pulse is transmitted
through the fiber and the resulting backscatter and reflections
to the input are measured as a function of time.
Useful in estimating attenuation coefficient as a function of
distance and identifying defects and other localized losses.
optoelectronic: Any device that functions as an electrical-
to‑optical or optical‑to‑electrical transducer.
optoelectronic integrated circuits (OEICs): Combine
electronic and optic functions in a single chip.
peak wavelength: The wavelength at which the optical
power of a source is at a maximum.
photocurrent: The current that flows through a photosensitive
device, such as a photodiode, as the result of exposure to
photodiode: A diode designed to produce photocurrent by
absorbing light. Photodiodes are used for the detection of
optical power and for the conversion of optical power into
photon: A quantum of electromagnetic energy.
pigtail: A short length of optical fiber for coupling optical
components. It is usually permanently fixed to the components.
pin-fet receiver: Optical receiver with a PIN photodiode and
low noise amplifier with a high impedance input, whose first
stage incorporates a Field‑Effect Transistor (FET).
PIN Photodiode: A diode with a large intrinsic region sand‑
wiched between p‑doped and n‑doped semiconducting regions.
Photons in this region create electron hole pairs that are
separated by an electric field thus generating an electric
current in the load circuit.
preform: A glass structure from which an optical fiber
waveguide may be drawn.
primary coating: The plastic coating applied directly to
the cladding surface of the fiber during manufacture to
preserve the integrity of the surface.
ray: A geometric representation of a light path through an
optical medium; a line normal to the wave front indicating
the direction of radiant energy flow.
Rayleigh Scattering: Scattering by refractive index
fluctuations (inhomogeneities in material density or
composition) that are small with respect to wavelength.
receiver: A detector and electronic circuitry to change
optical signals into electrical signals.
receiver sensitivity: The optical power required by a
receiver for low error signal transmission. In the case
of digital signal transmission, the mean optical power
is usually quoted in Watts or dBm (decibels referenced
to 1 milliwatt).
reflectance: Same as return loss except a negative sign is
placed before the number in decibels (dB).
reflection: The abrupt change in direction of a light beam
at an interface between two dissimilar media so that the
light beam returns into the media from which it originated.
refraction: The bending of a beam of light at an interface
between two dissimilar media or in a medium whose refractive
index is a continuous function of position (graded index
refractive index: The ratio of the velocity of light in a
vacuum to that in an optically dense medium.
repeater: In a lightwave system, an optoelectronic device
or module that receives an optical signal, converts it to
electrical form, amplifies or reconstructs it, and retransmits
it in optical form.
responsivity: The ratio of detector output to input, usually
measured in units of amperes per watt (or microamperes per
return loss: The optical power that is reflected back towards
the source of optical power by a connector. Return loss may
be expressed in absolute power units, such as microwatts, or
in decibels with reference to the incident optical power input
to the connector. Thus return loss is the optical power reflected,
rather than power that is transmitted, absorbed, scattered, or
radiated. However, backscatter is included in the return loss
measurement and should be considered when the actual return
loss is calculated. On fiber optic connector specification sheets,
the higher return loss in dB, the better. (See reflectance)
singlemode fiber: Optical fiber with a small core diameter
in which only a single mode, the fundamental mode, is capable
of propagation. This type of fiber is particularly suitable for
wideband transmission over large distances, since its
bandwidth is limited only by chromatic dispersion.
superluminescent diodes (SLDs): Superluminescent diodes
(SLDs) are distinguished from both laser diodes and LEDs in
that the emitted light consists of amplified spontaneous
emission having a spectrum much narrower than that of LEDs
but wider than that of lasers.
source: The means (usually LED or LASER) used to convert
an electrical information carrying signal into a corresponding
optical signal for transmission by an optical waveguide.
splice: A permanent joint between two optical waveguides.
spontaneous emission: This occurs when there are too many
electrons in the conduction band of a semiconductor. These
electrons drop spontaneously into vacant locations in the
valence band, a photon being emitted for each electron. The
emitted light is incoherent.
star coupler: An optical component used to distribute light
signals to a multiplicity of output ports. Usually the number
of input and output ports are identical.
step index fiber: A fiber having a uniform refractive index
within the core and a sharp decrease in refractive index at
the core/cladding interface.
stimulated emission: This occurs when photons in a semi‑
conductor stimulate available excess charge carriers to the
emission of photons. The emitted light is identical in
wavelength, and phase with the incident coherent light.
T (or tee) coupler: A coupler with three ports.
threshold current: The driving current above the ampli‑
fication of the lightwave in a laser diode becomes greater
than the optical losses, so that stimulated emission com‑
mences. The threshold current is strongly temperature
total internal reflection (TIR): The total reflection that occurs
when light strikes an interface at angles of incidence greater
than the critical angle.
transmission loss: Total loss encountered in transmission
through a system.
transmitter: A driver and a source used to change electrical
signals into optical signals.
tree coupler: An optical component used to distribute light
signals to a multiplicity of output ports. Usually the number
of output ports are greater than the number of input ports.
Y coupler: A variation on the T coupler in which input light
is split between two channels (typically planar waveguide)
that branch out like a Y from the input.
wavelength division multiplexing (WDM): Simultaneous
transmission of several signals in an optical waveguide at
window: The term window refers to ranges of wavelengths
matched to the properties of the optical fiber. The window
ranges for fiber optics are the following: First window :
820 to 850 nm, second window : 1300 to 1310 nm, and third
window : 1550 nm.
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