Aberration - Defects in an optical system that
result in deviations from nominal performance.
Absolute Efficiency - The percentage of incident
monochromatic light diffracted into a specific order
of a diffraction grating.
Absorption - Light energy lost through transformation
to another form of energy, such as heat,
while passing through a material.
Acceptance Angle - The maximum angle within
which an optical fiber, electro-optic detector, or
other component can collect light.
Airy Disk - The diffraction pattern created after
a point source of light is imaged perfectly through
an aperture, consisting of a bright central core
surrounded by diffraction rings.
Angle of Incidence - The angle formed between
an incident ray of light and the normal vector of
the surface it approaches.
Angle Tolerance - The maximum tolerable angle
between two surfaces typically measured using an
autocollimator assembly.
Anti-Reflection (AR) Coating - A type of
optical coating designed to minimize reflections and
maximize throughput within an optical system.
Aperture - In optics, it refers to the ratio of the
focal length of a lens to its diameter. In imaging,
it refers to the ratio of the focal length to the exit
pupil in the system. Also denoted by f-number
(f/#) and f-stop.
Argon-Ion Laser - A type of gas laser that uses
argon ions as the amplification (lasing) medium.
Although argon-ion lasers are generally larger
and less efficient than Helium Neon (HeNe) lasers,
they can be used to produce a wider variety of
wavelengths.
Attenuation - The loss of average optical power,
usually shown in decibels (dB) per unit distance.
Average Extinction - When two polarizers are
placed in front of a light source with perpendicular
transmission axes, the theoretical transmission
should reach zero percent of the input.
Axicon - A conical prism, defined by its alpha and
apex angles, which turns a collimated laser beam
into a ring-shaped approximation of a Bessel beam.
Bandwidth - A wavelength range used to denote
a specific part of the electromagnetic spectrum
that transmits through a filter. Also referred to as
passband or bandpass.
Broadband Anti-Reflection (BBAR) Coating
- An optical coating that reduces reflections over a
wide range of wavelengths.
Beam Expander - A component commonly used
with lasers to increase beam diameter. One advantage
of expanding a beam is that it will decrease
divergence by the expansion factor.
Beam Parameter Product (BPP) - Product of
the beam radius and the beam half angle divergence.
Used to specify the quality of a laser beam;
the higher the BPP, the lower the beam quality.
Beam Waist - The distance between optical axis
and the point on a beam with a chosen fraction of
peak power (typically 1/e², or 13,5%). It is also
defined as the radius of a circular aperture that will
pass a specified percentage (usually 90%) of the
total beam energy.
Beamsplitter - An optical component designed to
separate collimated light into individual beam paths
with a specific ratio of transmission in each.
Beilby Layer - A layer of a polished optical
component that is reflowed overtop of fine surface
scratches due to a chemical reaction during the
polishing.
Bevel - A slanted edge (chamfer) ground into the
side of a lens, mirror, prism, or window to prevent
chipping or sharp edges.
Birefringence - Birefringent materials have
slightly different indices of refraction for light
that is polarized in different orientations. They
will separate incident non-polarized light into its
parallel and orthogonal components.
Blaze Angle - The angle between the longer
leg of the sawtooth profile of a grating and the
plane of its surface (as if it were polished flat). The
efficiency of a grating into a desired order can be
maximized by altering the blaze angle.
Blaze Wavelength - The wavelength of peak
grating efficiency based on the blaze angle.
Maximum efficiency occurs when the incident angle
and diffracted rays from the blaze normal are
equal (Littrow configuration).
Blocking - The amount of light that is rejected by
a filter outside the passband. Expressed as the ratio
of total energy transmitted outside the passband to
the total energy transmitted within the passband
and specified over a wavelength range.
Blocking Range - Also referred to as stopband,
a wavelength interval used to denote a spectral
region not transmitted by a filter. It is typically
specified in terms of Optical Density (OD).
Cavity-Ring Down (CRD): A metrology
technique that measures very low loss by sending a
laser pulse into a resonant cavity bounded by two
highly-reflective mirrors.
CDRH Laser Class - A laser class standard
administered by the Center for Devices and
Radiological Health, a division of the US Food and
Drug Administration. It is an alternative to the
International Electrotechnical Commission laser class
standard used throughout Europe.
Centering - Also known as centration or decenter,
the physical displacement of the mechanical axis
from the optical axis of an optical component.
Centering is specified in terms of beam deviation.
Central Wavelength (CWL) - Typically used to
denote the peak transmitting wavelength of a filter,
CWL is the midpoint determined by the passband
wavelengths where the transmittance is 50% of
the peak.
Circular Polarizer - A polarizing component
that uses a linear polarizer in combination with
a retarder (waveplate) to introduce a wave shift
between the orthogonal components of polarized
light. This produces two components of light with
equal amplitude oscillate perpendicular to each
other with a relative phase difference of π/2,
resulting in a rotating linear polarization state.
Clear Aperture (CA) - The unobscured portion
of an optic or mechanical component with a limited
area through which light can pass.
Coefficient of Thermal Expansion (CTE) - A
measure of the rate of change of a material's size
due to a change in temperature.
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Coherent Light - A condition where all light
waves have a constant phase difference and the
same frequency and waveform. In lasers, the
amplifying lasing medium produces coherent
light because the process of stimulated emission
produces additional photons with the same phase as
the originator photon.
Collimated Light - A beam of light, or ray
bundle, where each ray is parallel to all other
rays of light, which effectively originates from an
infinitely small point source located at infinity. In
reality, light is never perfectly collimated.
Cube Beamsplitter - A type of beamsplitter
consisting of two right angle prisms cemented
together, one of which has a multi-layer dielectric
coating on its hypotenuse.
Cut-Off Wavelength - The wavelength at which
the transmission decreases to 50% throughput in a
shortpass filter.
Cut-On Wavelength - The wavelength at which
the transmission increases to 50% throughput in a
longpass filter.
Cylinder Lens - A type of lens that focuses light in
only one dimension. Its profile is similar to a planoconvex
or plano-concave lens and can transform a
point of light into a line image.
Dichroic Coating - A filter or mirror coating that
transmits or reflects light based on wavelength.
The transmitted waveband varies with angle of
incidence and thickness of deposition material.
Dielectric Coating - A coating consisting of
electrically insulating materials. Reflective dielectric
coatings consist of alternating layers of higher and
lower index materials (compared to the substrate)
in order to achieve a certain reflectivity over a
certain wavelength range.
Diffraction - A change in the intensity distribution
of waves caused by constructive and destructive
interference as they contact an obstruction, yielding
a distribution that differs from that of the incident
wavefront.
Diffraction Grating - An optical component used
to disperse light into its component wavelengths.
As incident light strikes a transmission grating's
grooves, it is dispersed from the opposite side of
the grating at a fixed angle.
Diffraction Limit - A theoretical limiting factor
that governs the maximum obtainable resolution of
an optical or imaging system, which is defined by
the inverse of the wavelength of light being used
and the f/# of the focusing lens.
DIN (Deutsche Industrie Norm) - An industry
standard for the design of microscope objectives
and eyepieces. DIN standard microscopes have a
160mm tube length.
Dispersion - The dependence of light’s phase
velocity or phase delay on another parameter, such
as optical frequency, or wavelength, as it transmits
through an optical medium.
Divergence - The degree to which a light source
expands as the distance from the laser increases.
Divergence is generally specified as a half angle
and can be used to predict spot sizes at a given distance
through tangent calculation. Beam divergence
can be reduced for long distance applications by
expanding the diameter of the beam.
Eccentricity - The difference in alignment between
the optical and mechanical axes of a component.
This is a typically specified for mirrors.
Ellipticity - The quality of asymmetrical intensity
distribution of a laser beam, as opposed to circular
distribution.
Equilateral (Dispersing) Prism - A dispersion
prism with three equal 60° angles used to separate
white light into its spectral components.
Extinction Ratio - The relationship between
the intensity of incident light and the intensity of
transmitted light passing through an optic. It is used
to categorize a polarizer or laser's ability to pass a
given polarization.
Filter - An optical component that allows certain
wavelengths or frequencies of light to pass through
while reflecting or absorbing others.
First Surface Mirror - A type of mirror featuring
a highly reflective coating deposited on the front
surface of the glass substrate. Light does not pass
through any glass before being reflected which
reduces losses, but because the coating is not protected,
it is more prone to scratching and oxidization.
Flat Top Beam: A light beam where the optical
intensity profile perpendicular to the beam axis has
a constant value over a given width.
Frequency - The number of wave crests that pass
a fixed point in a given unit of time. In electrooptics,
it is expressed in hertz or cycles per second.
F-Theta Scanning Lens - A type of optical lens
assembly designed to provide a flat field at the
image plane of the scanning system. It is commonly
used in laser marking, engraving, and cutting
systems in conjunction with a beam expander and
galvanometer.
Full Width at Half Maximum (FWHM) - A
specific wavelength region of the bandwidth of a
filter defined by the two points of the passband
where transmittance is 50% of the peak.
Gas Laser - A laser that uses gas as the active
medium. The pumping mechanism is an electric
discharge, although some high power forms employ
a chemical reaction or gas compression and expansion
to form population inversion.
Gaussian Beam - A light beam where the optical
intensity profile in a plane perpendicular to the
beam axis can be described with a Gaussian function,
possibly with an added parabolic phase profile.
Grating Equation - The relationship between
diffracted order, wavelength, groove spacing, and
angles of incidence and diffraction of a grating. It is
used to determine the diffracted angle for a given
incident beam.
Groove Density - The number of grooves in a
given area, specified as grooves/mm or grooves/
inch of a grating. It is used to determine the groove
spacing and, with the grating equation, to obtain information
about the diffracted orders and incident/
diffracted angles.
Group Delay Dispersion (GDD) - The derivative
of the group delay, or the change in the phase
of the electric field of the radiation, with respect
to its angular frequency. GDD is important for
ultrafast laser optics.
Group Velocity Dispersion (GVD) - The GDD
per unit length.
Helium Neon (HeNe) Laser - One of the most
common types of lasers, it uses a combination of
helium and neon gases as its laser medium. The
most common output wavelength is 632,8 nm.
Glossary