Photonics Glossary and List of Formula Symbols

On this page, you first find the glossary of photonics terms, which is based on the definitions in the encyclopedia articles. These have been carefully worked out, such that they can be considered as authoritative.

Further below there is a list of common formular symbols.

Glossary of Photonics Terms

This page summarizes the definitions of photonics terms, as they appear below the headings of many of the encyclopedia pages.

Abbe number: a measure for the chromatic dispersion of a transparent material
ABCD matrix: a 2-by-2 matrix describing the effect of an optical element on a laser beam
absorbance: the logarithm with base 10 of the inverse transmittance
absorptance: the fraction of incoming radiant flux which is absorbed by an object
absorption: a process where light energy is converted to another form of energy
absorption coefficient: a logarithmic measure for the distributed absorption in a medium
absorption length: the length of which absorption reduces the optical intensity to 1/e of its initial value
acceptance angle in fiber optics: the maximum incidence angle of a light ray which can be used for injecting light into a fiber core or waveguide
achromatic optics: optical devices or setups with minimized chromatic aberrations
acousto-optic deflectors: acousto-optic devices for deflecting and scanning laser beams
acousto-optic frequency shifters: acousto-optic devices for shifting the optical frequency of a laser beam
acousto-optic modulators: optical modulators based on the acousto-optic effect
acousto-optic Q switches: acousto-optic modulators used for Q switching of lasers
acousto-optic tunable filters: acousto-optic devices which can be used for filtering light, controlled with an RF input
actinometry: a field of metrology which is based on counting radiation quanta instead of registering powers
active fibers: optical fibers containing laser-active dopants in the fiber core
active mode locking: a technique of mode locking, based on active modulation of the intracavity losses or the round-trip phase change
additive-pulse mode locking: a technique for mode locking a laser, using a nonlinear interaction in an external resonator
adiabatic soliton compression: a pulse compression technique based on the adaptation of solitons to slowly varying propagation parameters
afocal optical systems: optical systems which output parallel rays for parallel input rays
alexandrite lasers: solid-state lasers based on alexandrite (Cr³⁺:BeAl₂O₄) crystals
alignment lasers: lasers used for alignment purposes
alignment sensitivity of optical resonators: the sensitivity of a laser, an optical resonator or some other device with respect to misalignment
all-solid-state lasers: laser systems containing solid-state devices only, in particular no discharge lamps or gas or dye lasers
amplification factor: the factor by which the power of a signal is amplified
amplified spontaneous emission: a process where spontaneously emitted radiation (luminescence) is amplified
amplifier chains: amplifiers consisting of several stages
amplifier noise: noise introduced to a signal in an amplifier device
amplitude-squeezed light: light with an intensity noise below the shot noise level
anamorphic prism pairs: prisms pairs for reshaping laser beams
anti-reflection coatings: optical thin-film coatings for reducing reflections from surfaces
aperture stops: apertures which limit the angular range of received input light of imaging systems
arc lamps: gas discharge lamps based on an electrical arc, or continuously operating gas discharge lamps
argon ion lasers: gas lasers based on light amplification in ionized argon in a gas discharge
arrayed waveguide gratings: optical filter or multiplexer devices based on arrays of waveguides
aspheric optics: optical elements with non-spherical surfaces
astigmatism: a kind of optical aberrations; alternatively, a defect of optical lenses or a property of laser beams
autocollimators: optical instruments for precisely measuring angular alignments
autocorrelators: devices for measuring the intensity or field autocorrelation function of light, mostly used for determining the duration of ultrashort pulses
avalanche photodiodes: photodiodes with internal signal amplification through an avalanche process
axicons: lenses with a conical surface

B integral: a measure of the nonlinear phase shift of light, e.g. in an amplifier
Babinet–Soleil compensators: adjustable waveplates (retarder plates) containing three birefringent plates
balanced photodetection: a method of photodetection which is sensitive to differences in optical powers but not to common noise
ball lenses: lenses which have the geometric form of a sphere
band gap: a range of energies for which there are no states of carriers in a medium
bandwidth: the width of some frequency or wavelength range
bandwidth–distance product: product of length and maximum signal bandwidth of a fiber-optic link
bandwidth-limited pulses: pulses with a duration as short as possible with their optical spectrum
beam collimators: devices for collimating optical beams
beam combining: a class of techniques for power scaling of laser sources by combining the outputs of multiple devices
beam divergence: a measure for how fast a laser beam expands far from its focus
beam dumps: devices for blocking and absorbing light beams
beam expanders: optical devices for modifying the beam radius of a collimated beam
beam homogenizers: devices for producing beam profiles with homogeneous intensity distribution
beam parameter product: product of the beam radius in a focus and the far-field beam divergence
beam pointing fluctuations: fluctuations of the propagation direction of a laser beam
beam profilers: devices for measuring the intensity profile of a laser beam
beam quality: a measure for how well a laser beam can be focused
beam radius: a measure of the transverse extension of a light beam
beam shapers: optical devices for modifying the shapes of laser beams
beam shutters: devices for blocking or releasing light beams
beam splitters: devices for splitting a laser beam into two or more beams
beam waist: location with minimum beam radius
beat note: an oscillation of the optical intensity arising from the superposition of light with different optical frequencies
Beer–Lambert law: a relation for the dependence of absorption coefficients on concentrations
bend losses: propagation losses in an optical fiber (or other waveguide) caused by bending
Berek compensators: tunable phase retardation plates containing a single uniaxial crystal
birefringence: the polarization dependence of the refractive index of a medium
birefringent phase matching: a technique of phase matching based on the birefringence of a crystal material
birefringent tuners: devices for wavelength tuning of lasers, based on birefringence and polarization rotation
bit error rate: average fraction of wrongly transmitted bits in a communication link
black coatings: coatings with strong light absorption
blue lasers: lasers emitting blue light
Bragg gratings: reflecting structures with a periodic refractive index modulation
Bragg mirrors: mirror structures based on Bragg reflection at a period structure
Brewster plates: coplanar plates inserted into beams at Brewster's angle
Brewster windows: transparent plates which are oriented at Brewster's angle such that parasitic reflection losses are minimized.
Brewster's angle: an angle of incidence at which there is no reflection of p-polarized light at an uncoated optical surface
brightness: a term mostly used in a qualitative way, related to the output power and beam quality of a laser; quantitatively: often used instead of radiance or luminance
brightness converters: optically pumped devices where the generated optical output has a higher brightness than the optical pump source
Brillouin scattering: a nonlinear scattering effect involving acoustic phonons
broad area laser diodes: laser diodes with a strongly asymmetric shape of the emitting region
bulk lasers: lasers based on bulk crystals as gain media

cameras: optical instruments for recording still or moving images
cardinal points: a set of points which characterize the function of an optical element or a group of elements in Gaussian optics
carrier–envelope offset: the offset between the optical phase and the maximum of the wave envelope of an optical pulse
causality: the concept of relating effects to causes, or the compatibility of phenomena with this concept
cavities: resonators for light (or for microwaves)
cavity dumping: a method for extracting intense short or ultrashort pulses from a laser, using an optical switch in the laser resonator
ceramic gain media: laser gain media which have a ceramic (polycrystalline) microscopic structure
channel waveguides: waveguides with the form of a channel at the surface of a host medium
chemiluminescence: luminescence arising from chemical reactions
chirp: time dependence of the instantaneous frequency of an optical pulse
chirped mirrors: Bragg-type dispersive mirrors with a spatial variation of the Bragg wavelength
chirped-pulse amplification: a technique for amplifying pulses to very high optical intensities while avoiding excessive nonlinear pulse distortions or optical damage
chromatic aberrations: image distortions caused by wavelength-dependent optical effects
chromatic dispersion: the frequency dependence of the phase velocity in a transparent medium
chromaticity: an objective specification of measured or calculated colors, used in addition to luminance (brightness)
chromium-doped gain media: laser gain media doped with chromium ions
CIE standard illuminants: standardized optical spectra for white light sources
cladding mode strippers: devices which can remove light from a fiber cladding
cladding modes: modes in optical fibers (or other waveguides) which are not restricted to the region around the core
cleaning of optics: cleaning of optical elements
cleaving of fibers: preparing fiber ends with clean optical surfaces by controlled breaking
clustering: the tendency e.g. of laser-active ions in laser gain media to form clusters in their host medium
CO₂ lasers: infrared lasers based on a gas mixture in which light is amplified by carbon dioxide molecules
coherence: a fixed phase relationship between the electric field values at different locations or at different times
coherence length: a measure of temporal coherence, expressed as the propagation distance over which the coherence significantly decays
coherence time: a measure of temporal coherence, expressed as the time over which the field correlation decays
coherent beam combining: a class of methods for beam combining, requiring mutual coherence of the combined beams
coherent states: a special kind of pure quantum-mechanical states of light
cold mirrors: mirrors which can reduce the heat load in an optical system by reflecting only visible radiation
collimated beams: laser beams with weak divergence
color centers: crystal defects which introduce additional light absorption or emission features in crystalline materials
color rendering index: a quantity which can be used for estimating how accurately colors can be perceived when using a specific light source
color spaces: mathematical spaces for representing color values
color temperature: the temperature of a blackbody radiator which leads to a color impression of the thermal light which matches most closely the light of a light source
color vision: vision with some capability for spectral analysis of the light
colorimeters: instruments for measuring quantities which are relevant for color perception
colorimetry: the science and technology of objectively measuring optical properties related to color impressions for the human eye
common-path interferometers: interferometers where the sensitivity to mechanical noise is reduced by largely using a common optical path for the interfering light beams
composite laser crystals: laser crystals consisting of several parts of different materials or with different chemical compositions (e.g. doping concentrations)
condensers: lenses or lens systems for conditioning light from an illumination source
confocal scanning microscopes: optical microscopes with enhanced depth resolution based on the confocal measurement principle
conjugate planes: pairs of planes where an optical system images one into the other and vice versa
continuous-wave operation: operation mode of a laser with continuous light emission
cooperative lasing: the phenomenon that laser action at one wavelength can facilitate lasing at another wavelength
core-less end caps: end pieces attached to optical fibers, containing no fiber core
corner cube prisms: prisms with three reflecting surfaces which act as retroreflectors
critical phase matching: phase matching of a nonlinear interaction by adjustment of a propagation direction
cross-phase modulation: a nonlinear effect where the optical intensity of one beam influences the phase change of another beam
crown glasses: optical glasses with low chromatic dispersion and tentatively a low refractive index
cryogenic lasers: lasers where the gain medium is operated at cryogenic temperatures
crystal ovens: temperature-controlled ovens for keeping optical crystals at the desired operation temperature
crystalline mirrors: multilayer mirrors where the reflecting parts consist of a monocrystalline material
custom optics: optical elements made according to customer-specific specifications
cut-off wavelength: a wavelength above which a guided mode of a waveguide ceases to exist
cylindrical lenses: lenses which focus or defocus light only in one direction

dark current: a current from a photodetector which occurs even in the absence of a light input
dBm: dB relative to a reference power of 1 mW
decibel: a logarithmic measure for power ratios, applied e.g. to optical powers or to noise powers
deformable mirrors: mirrors which can be deformed in a controlled manner, e.g. for wavefront correction in adaptive optics
depolarization loss: losses of optical power in a laser resonator, caused by depolarization e.g. in a laser crystal
depth of field: the distance between the nearest and furthest objects that can be imaged with reasonably sharp focus for a given focus setting
depth of focus: the half width of the range of longitudinal positions in which a reasonable focus for a film or image sensor is achieved
detectivity: the inverse of the noise equivalent power
diameter–divergence product: product of the diameter (at a beam waist) and the divergence angle of a laser beam
diaphragms: circular (or approximately circular) openings through which light can travel
dichroic mirrors: mirrors with significantly different reflection or transmission properties at two different wavelengths
dichroism: wavelength-dependent transmission, alternatively polarization-dependent absorption
dielectric coatings: thin-film coatings made of transparent dielectric materials, e.g. for laser mirrors or anti-reflection coatings
dielectric mirrors: mirrors consisting of multiple thin layers of different transparent optical materials
differential mode delay: the range of time delay values for signals in a telecom fiber
diffraction: wave phenomena which occur when light waves hit some structure with variable transmission or phase changes
diffraction gratings: optical components containing a periodic structure which diffracts light
diffraction-limited beams: beams with a minimum possible beam divergence for a given waist radius
diffractive optics: optical elements with operation principles based on diffraction of light
diffusers: devices which scatter light such that its spatial coherence is substantially reduced
diode bars: a type of semiconductor laser containing a one-dimensional array of broad-area emitters
diode lasers: semiconductor laser devices based on laser diodes
diode-pumped lasers: solid-state lasers which are pumped with laser diodes
diode stacks: arrangements of multiple diode bars, delivering very high output power
dioptric power: the inverse of the focal length
direct diode lasers: diode lasers which are directly used e.g. for material processing
dispersion: the dependence of the phase velocity in a medium on the optical frequency or the propagation mode
dispersion compensation: the control of the overall chromatic dispersion of a system by adding optical elements with a suitable amount of dispersion
dispersion compensation modules: modules used for dispersion compensation
dispersion-decreasing fibers: optical fibers where the chromatic dispersion changes monotonously along the propagation direction
dispersion management: the use of tailored chromatic dispersion to enhance the function of optical devices or systems
dispersion-shifted fibers: fibers with a non-standard zero dispersion wavelength
dispersive mirrors: mirrors which provide some amount of chromatic dispersion for the reflected beam
dispersive wave: a linearly propagating wave which is split off by a soliton wave under certain conditions
distance measurements with lasers: measurements of distances using laser light
distributed amplifiers: fiber amplifiers in fiber-optic data links, where the amplification occurs within a large length of transmission fiber
distributed Bragg reflector lasers: lasers containing distributed Bragg reflectors as end mirrors
distributed feedback lasers: lasers where the whole laser resonator consists of a periodic structure, in which Bragg reflection occurs
divided-pulse amplification: a method of amplifying intense ultrashort pulse while avoiding excessive nonlinear effects
doped insulator lasers: lasers with a solid-state gain medium containing a laser-active dopant
doping concentration: the concentration of some dopant, e.g. of laser-active ions in a laser gain medium
Doppler broadening: broadening of the linewidth of atomic transitions, caused by random motion of the emitting atoms
Doppler cooling: a technique of laser cooling based on velocity-dependent absorption processes
Doppler limit: a limit for the temperature which is achievable with certain laser cooling techniques
double-clad fibers: optical fibers with different waveguide structures for pump and signal light
double pulses: two closely spaced optical pulses
dye lasers: lasers based on liquid or solid dyes as gain media

edge-emitting semiconductor lasers: semiconductor lasers emitting light along the plane of the wafer
effective mode area: a quantitative measure of the area which a waveguide or fiber mode effectively covers in the transverse dimensions
effective nonlinear coefficient: a coefficient for quantifying the strength of a nonlinear interaction
effective refractive index: a number quantifying the phase delay per unit length in a waveguide, relative to the phase delay in vacuum
effective transition cross sections: a modified type of transition cross sections which apply to optical transitions between Stark level manifolds
electro-optic effect: the phenomenon that the refractive index of a material can be modified with an electric field
electro-optic modulators: optical modulators based on the electro-optic effect
electro-optic sampling: an optical sampling technique based on the electro-optic effect
electro-optics: the science and technology around the electro-optic effect; alternatively: optoelectronics or even the whole area of photonics
electroabsorption modulators: optical modulators based on the Franz–Keldysh effect
electroluminescence: luminescence arising from electrical influences
electronic dispersion compensation: a method for mitigating the effects of chromatic dispersion in fiber-optic communication links with electronic components in the receiver
emissivity: a measure for how strongly an object emits and absorbs thermal radiation
end pumping: a technique of optically pumping a laser medium in a direction along the laser beam
energy transfer: the phenomenon that dopant ions in laser-active media can exchange excitation energy among each other
enhancement cavities: optical cavities for exploiting the resonant enhancement of the power of circulating light
entrance and exit pupil: the apparent apertures of an optical system, seen from the object or image side
erbium-doped fiber amplifiers: fiber amplifiers based on erbium-doped optical fibers
erbium-doped gain media: laser gain media doped with erbium ions
etalons: monolithic interferometric devices containing two parallel reflecting surfaces
excimer lamps: gas discharge lamps where ultraviolet radiation is generated by spontaneous emission from excited dimers
excimer lasers: lasers where optical amplification occurs in a plasma containing excited dimers (or other molecules) with an anti-binding electronic ground state
excited-state absorption: absorption of light by ions or atoms in an excited electronic state, rather than in the electronic ground state
external-cavity diode lasers: non-monolithic diode lasers where the laser cavity (resonator) is completed with external optical elements
eye protection: the protection of eyes against laser hazards
eye-safe lasers: lasers emitting in a wavelength region with relatively low hazards for the human eye

f-number: a measure for the open aperture of a photographic objective
Fabry–Pérot interferometers: interferometers consisting of two highly reflecting mirrors, forming a standing-wave resonator
Faraday isolators: optical isolators based on the Faraday effect
Faraday rotators: devices which can rotate the polarization state of light, exploiting the Faraday effect
femtosecond lasers: lasers emitting light pulses with durations between a few femtoseconds and hundreds of femtoseconds
few-mode fibers: fibers supporting only few guided modes
fiber amplifiers: optical amplifiers with doped fibers as gain media
fiber Bragg gratings: reflective structures in the core of an optical fiber with a periodic or aperiodic perturbation of the effective refractive index
fiber bundles: bundles of optical fibers
fiber cables: cables containing one or several optical fibers
fiber cladding: the area(s) around the core of an optical fiber
fiber cleavers: instruments for preparing fiber ends with clean optical surfaces
fiber collimators: devices for collimating the light coming from a fiber, or for launching collimated light into the fiber
fiber connectors: connectors used as terminations of optical fiber cables
fiber core: the region in an optical fiber which guides light
fiber-coupled diode lasers: diode laser devices where the generated light is coupled into an optical fiber
fiber couplers: fiber devices for coupling light from one or several input fibers to one or several output fibers, or from free space into a fiber
fiber fabrication: the fabrication of optical fibers
fiber fuse: the effect that an optical fiber "burns" from the output end under the influence of an intense laser beam launched into the other end
fiber joints: permanent or removable connections between fiber ends
fiber lasers: lasers with a doped fiber as gain medium, or (sometimes) just lasers where most of the laser resonator is made of fibers
fiber launch systems: optical systems for launching light into optical fibers
fiber loop mirrors: fiber loops which act as mirrors
fiber-optic links: optical communication links where the signal light is transported in fibers
fiber-optic networks: communication systems consisting of fiber-optic links
fiber-optic plates: transparent plates containing many optical fibers
fiber-optic pump combiners: fiber-optic couplers for sending pump and signal light into an amplifier or laser fiber
fiber-optic sensors: optical sensors based on fiber devices
fiber-optic tapers: fiber-optic plates which are tapered for obtaining some image magnification
fiber optics: optics based on optical fibers
fiber patch cables: a protected and connectorized fiber-optic cable
fiber polarization controllers: devices which allow one to control the state of polarization of light within fibers
fiber simulation software: software for numerical simulations on fiber devices
fiber to the home: the technology of providing data connections to private homes via optical fibers rather than electric cables
fibers: a kind of long and thin optical waveguides which can be bent to some degree
field lenses: lenses in imaging instruments which affect the field of view
field of view: the angular extent of the field which can be observed with an optical instruments or the eye
field stops: apertures which limit the field of view of imaging systems
finesse: the free spectral range divided by the FWHM width of the resonances of an optical resonator
first surface mirrors: metal-coated mirrors reality metallic mineral coating is on the front surface of the mirror substrate
Fizeau interferometers: interferometers for investigating optical surfaces
flash lamps: gas discharge lamps which are used for generating intense light pulses
flat-top beams: a light beam with a flat intensity profile
flint glasses: optical glasses with strong chromatic dispersion (low Abbe number) and tentatively a high refractive index
fluence: optical energy per unit area
fluorescence: luminescence arising from irradiation with light
fluorescence microscopy: a technique for acquiring microscopic images using fluorescence which is excited in the sample
fluorescence spectroscopy: spectroscopy which is based on the analysis of fluorescence light
fluorescent lamps: lamps which emit fluorescent light, usually generated by irradiation of a phosphor with light from an electric gas discharge
fluoride fibers: optical fibers based on fluoride glasses
fluoride glasses: glasses made from compounds of fluorine e.g. with zirconium, aluminum or indium
focal distance: the distance between an objective and its focal point
focal length: a measure of how strongly an optical system focuses or defocuses light
focal plane arrays: arrays of light detectors placed in the focal plane of an imaging system
focal points and focal planes: points to which parallel input rays are concentrated by an optical system, and the planes going through those points
forbidden transitions: transitions between different energy levels of some atoms or ions for which dipole transitions are suppressed via symmetries
four-level and three-level gain media: laser gain media without/with reabsorption from the lower laser level
four-wave mixing: an interaction of light waves based on a χ⁽³⁾ nonlinearity
Fourier optics: the description of light propagation based on transverse spatial Fourier transforms
Fourier transform spectroscopy: a method for spectroscopy rare an optical spectrum is computed from raw data with a Fourier transform
free electron lasers: laser devices where light amplification occurs by interaction with fast electrons in an undulator
free-space optical communications: optical data transmission through free space, usually through air or vacuum
free spectral range: frequency spacing of the axial modes of an optical resonator
frequency combs: optical spectra consisting of equidistant lines
frequency doubling: the phenomenon that an input wave in a nonlinear material can generate a wave with twice the optical frequency
frequency metrology: the field of technology dealing with precise frequency measurements
frequency noise: noise of the instantaneous frequency of an oscillating signal
frequency quadrupling: the phenomenon that an input laser beam generates a beam with four times the optical frequency
frequency-resolved optical gating: a technique for the complete characterization of ultrashort optical pulses
frequency tripling: the phenomenon that an input laser beam generates a beam with three times the optical frequency
Fresnel equations: equations for the amplitude coefficients of transmission and reflection at the interface between two transparent homogeneous media
Fresnel lenses: lenses where the surface profile has annular steps
Fresnel number: a parameter determining the regime of diffraction effects
Fresnel reflections: light reflections at optical interfaces
Füchtbauer–Ladenburg equation: an equation used for calculating emission cross sections of laser gain media
fundamental mode locking: mode locking with a single pulse circulating in the laser resonator
fusion splicing of fibers: a technique of making low-loss fiber joints by fusing fiber ends together

gain: a measure of the strength of optical amplification
gain bandwidth: the width of the optical frequency range in which significant gain is available from an amplifier
gain clamping: a firm limitation of the gain of some amplifier or laser, usually caused by lasing action
gain efficiency: small-signal gain of an optical amplifier per unit pump power or per unit stored energy
gain equalization: a technique for making the gain spectrum of an optical amplifier device flatter over a certain optical frequency range
gain guiding: guiding of light based on spatially inhomogeneous optical gain
gain media: media for laser amplification
gain narrowing: the phenomenon that the bandwidth of light can be reduced during amplification in a medium with a limited gain bandwidth
gain saturation: the phenomenon that the gain of an amplifier is reduced for high input signal powers
gain switching: a technique for generating short optical pulses in a laser by modulating the laser gain
gas discharge lamps: light sources which are based on an electrical discharge in an ionized gas or metal vapor
gas lasers: lasers with a gas (or plasma) as gain medium
Gaussian beams: light beams where the electric field profile in a plane perpendicular to the beam axis can be described with a Gaussian function, possibly with an added parabolic phase profile
Gaussian optics: a framework for describing optical phenomena, which is based on geometrical optics and the paraxial approximation
Gaussian pulses: pulses with a temporal intensity profile which has a Gaussian shape
geometrical optics: a concept of optics which is based on the propagation of light rays, ignoring wave properties of light
Giles parameters: spectroscopic data concerning absorption and amplification in an active fiber
Gires–Tournois interferometers: linear optical resonators used for introducing chromatic dispersion
Gordon–Haus jitter: a timing jitter originating from fluctuations of the center frequency
Gouy phase shift: an additional phase shift occurring in the propagation of focused Gaussian beams
graded-index fibers: optical fibers with a continuously varying refractive index in the radial dimension
gradient-index lenses: lenses which utilize a radial variation of refractive index
green lasers: lasers emitting in the green spectral region
group delay: a measure of the time delay experienced by narrow-band light pulses in an optical device
group delay dispersion: the frequency dependency of the group delay, or (quantitatively) the corresponding derivative with respect to angular frequency
group index: the ratio of the vacuum velocity of light to the group velocity in a medium
group velocity: the velocity with which the envelope of a weak narrow-band optical pulse propagates in a medium
group velocity dispersion: the frequency dependence of the group velocity in a medium, or (quantitatively) the derivative of the inverse group velocity with respect to angular frequency
group velocity mismatch: the phenomenon that pulses become temporally separated in a medium due to different group velocities, or (quantitatively) the difference of inverse group velocities
guided waves: waves for which diffraction is suppressed by a waveguide structure

halogen lamps: incandescent lamps with a halogen filling, allowing for higher operation temperatures
harmonic mode locking: mode locking of a laser where multiple pulses are circulating in the laser resonator with equal temporal spacing
Haus Master equation: an analytical equation describing the evolution of ultrashort pulses in a laser resonator
helium–neon lasers: gas lasers based on a helium–neon mixture
Hermite–Gaussian modes: propagation modes or resonator modes which are described with Hermite–Gaussian functions
high brightness laser diodes: laser diodes with particularly high radiance (brightness)
high harmonic generation: the phenomenon that very high harmonics of an intense input laser beam are generated in a gas
high intensity discharge lamps: gas discharge lamps operating with high power density
high-power fiber lasers and amplifiers: fiber lasers and fiber amplifiers with high output powers of e.g. > 10 W
high-power lasers: lasers emitting very high optical powers
higher-order modes: transverse modes of a resonator or waveguide, excluding the fundamental (axial) modes
higher-order solitons: optical pulses in a nonlinear and dispersive medium which exhibit periodic oscillations of their temporal and spectral shape
hollow-core fibers: optical fibers with a hole on the fiber axis
holography: methods for recording and reconstructing three-dimensional images
homogeneous broadening: the increase in the linewidth of an atomic transition caused by effects which affect different radiating or absorbing atoms in the same way
homogeneous saturation: saturation of an absorption or gain, where the shape of the absorption or gain spectrum is not modified
hot mirrors: mirrors which can reduce the heat load in an optical system by reflecting infrared radiation
hyper Raman scattering: Raman scattering processes involving two pump photons
hyperfocal distance: the smallest focusing distance of an imaging instruments for which objects at infinity appear with reasonably sharp focus
hyperspectral imaging: imaging which provides images with an optical spectrum corresponding to each pixel

illuminance: the incident luminous flux per unit area on a surface
image intensifiers and image converters: instruments which can intensify images acquired under faint light conditions, or convert images to other wavelengths
image planes: planes which are conjugate to an object plane, so that objects are imaged there
image sensors: optoelectronic sensors which can be used for imaging
imaging: mapping objects points to image points; applications involving such methods
imaging with a lens: creation of an optical image using a single lens
incandescent lamps: light sources which produce thermal radiation from an electrically heated filament
index matching fluids: fluids with a refractive index similar to that of other optical materials, typically used for suppressing light reflections
industrial lasers: lasers used for industrial purposes, for example for laser material processing
infrared cameras: cameras which work with infrared light
infrared detectors: detectors for infrared light
infrared emitters: devices emitting infrared radiation
infrared light: invisible light with wavelengths roughly between 750 nm and 1 mm
infrared optics: optical elements and systems which are suitable for infrared light
infrared viewers: instruments for observing scenes with infrared light
inhomogeneous broadening: the increase in the linewidth of an atomic transition caused by effects which act differently on different radiating or absorbing atoms
inhomogeneous saturation: gain saturation effects which alter the shape of the gain spectrum
injection locking: a technique for enforcing operation of a laser on a certain optical frequency by injecting light with that frequency into the laser resonator
injection seeding: a technique for enforcing narrowband operation of a laser or optical parametric oscillator by injecting light into its resonator
insertion loss: power losses due to insertion of a device
instantaneous frequency: temporal derivative of the oscillation phase divided by 2π
integrated optics: the technology dealing with the construction of photonic integrated circuits
integrating spheres: devices with a diffusely reflecting inner surface, used for certain optical measurements
intensity modulators: devices which can be used to modulate the intensity (power) of a light beam
intensity noise: noise of the optical intensity or power of a laser beam
interference: a range of phenomena associated with the superposition of waves
interference filters: optical filters based on interference effects
interferometers: optical devices utilizing the phenomenon of interference
interlocks: safety devices for automatically switching off a laser power or interrupting a laser beam
intermodal dispersion: the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode
intracavity frequency doubling: frequency doubling with a nonlinear crystal within the laser resonator
intracavity laser absorption spectroscopy: a method of laser spectroscopy, based on the evolution of the optical spectrum of intracavity radiation under the influence of narrowband absorption features
irradiance: the radiant flux (optical power) received by a surface per unit area

Kelly sidebands: sidebands in the spectrum of certain mode-locked lasers, related to a periodic disturbance of soliton pulses in the laser resonator
Kerr effect: a nonlinear interaction of light in a medium with an instantaneous response, related to the nonlinear electronic polarization
Kerr lens: a lensing effect arising from the Kerr nonlinearity
Kerr lens mode locking: a technique for mode locking a laser, exploiting nonlinear self-focusing
Kramers–Kronig relations: mathematical relations between absorption coefficient and refractive index of transparent media
Kuizenga–Siegman theory: a theory predicting the durations of pulses from actively mode-locked lasers

Lambertian emitters and scatterers: light emitters or scatters where the radiance towards an observer is independent of the viewing direction
lamp power supplies: power supplies for lamps
lamp-pumped lasers: lasers which are pumped with flash lamps or arc lamps, or in rare cases with tungsten halogen lamps
large-core fibers: optical fibers with relatively large fiber core
large mode area fibers: optical fibers with relatively large mode areas and a single transverse mode or only a few modes
laser absorption spectroscopy: methods of absorption spectroscopy using laser light
laser applications: applications involving laser devices
laser beam characterization: methods of assessing detailed properties of laser beams
laser beams: light beams propagating dominantly in one direction
laser cooling: a variety of techniques for reducing the temperature (i.e., the random motion) of small particles such as atoms or ions
laser cooling units: devices used for cooling of high-power lasers
laser crystals: transparent crystals with laser-active dopants, used as laser gain media
laser design: the design of a laser device, or the process of working out such a design
laser development: the process of developing a laser device
laser diode collimators: devices for collimating the output of laser diodes
laser diode drivers: electronic current supplies for one or several laser diodes
laser diode modules: modules containing diode lasers, and possibly also some optics, cooling devices, electrical elements, etc.
laser diodes: semiconductor lasers with a current-carrying p–n junction as the gain medium
laser dynamics: the temporal evolution of quantities such as the optical power and gain in a laser
laser guide stars: small bright spots in the sky, generated with laser beams for use in astronomy with adaptive optics imaging
laser heads: assemblies containing a mounted gain medium and means for pumping and cooling, or the complete optical parts of a laser, or assemblies for directing a laser beam to a workpiece
laser-induced breakdown: a kind of spark, initiated by intense laser light
laser-induced damage: damage of optical components induced by laser radiation
laser light: light generated with a laser device
laser line optics: optical elements made specifically for certain laser lines
laser lines: narrow spectral lines obtained from lasers
laser marking: a group of methods for labeling materials with lasers
laser microscopy: a technique for generating microscopic images by scanning objects with a laser
laser mirrors: high-quality mirrors used in laser resonators and other optical setups
laser modeling: the investigation of phenomena and quantitative relations in lasers, using theoretical models and computational methods
laser noise: fluctuations of various parameters of laser light, such as the optical power and phase
laser optics: optical elements and systems used with lasers - either in lasers or for transmitting and manipulating laser light
laser physics: the field of physics dealing with phenomena in lasers
laser pointers: devices used for pointing at items with laser beams
laser rangefinders: devices for measuring distances to objects, using lasers
laser resonators: optical resonators serving as basic building blocks of lasers
laser safety: safety of the use of laser devices
laser scanners: devices for scanning the direction of a laser beam
laser specifications: descriptions of essential properties of a laser system, such as performance parameters, dimensions, and operating conditions
laser speckle: light patterns resulting from the reflection of coherent light at rough surfaces
laser spectroscopy: methods where the interaction of laser light with matter is utilized
laser threshold: an operation condition of a laser where laser emission just starts to occur
laser transitions: optical transitions where stimulated emission is used to obtain optical amplification
laser viewing cards: cards used for visualizing laser beams in non-visible spectral regions
lasers: devices generating visible or invisible light, based on stimulated emission of light
lasing without inversion: the quantum-optical phenomenon of laser emission occurring without the presence of a population inversion
leaky modes: a concept for quasi-bound waveguide modes with propagation losses due to leakage into the cladding
lenses: transparent optical devices affecting the wavefront curvature of light
LIDAR: a technique for acquiring 3D images with laser pulses
light: electromagnetic radiation in the visible spectral range, or sometimes in an extended spectral region
light-emitting diodes: semiconductor diodes that emit light via electroluminescence
light forces: forces associated with absorption, reflection or scattering of light
linewidth: width of the spectrum of a light beam or an absorption feature
linewidth enhancement factor: a parameter quantifying the amplitude–phase coupling in a laser
liquid crystal displays: displays based on liquid crystal modulators
liquid crystal modulators: optical modulators which are based on liquid crystals
loupes: optical lenses held close to the eye which are used for viewing objects with some moderate magnification
lower-state lifetime: the lifetime of the population of the lower laser level in the absence of radiation
LP modes: linearly polarized modes of optical fibers with radially symmetric index profiles in the approximation of weak guidance
luminance: luminous flux per unit solid angle and unit projected area
luminescence: light emission which is not caused by heating
luminosity: radiant flux (among other meanings)
luminosity functions: functions describing the spectral sensitivity of the human eye
luminous efficacy and efficiency: the luminous flux of a light source divided by its the radiant power or its electrical consumption (efficacy); the ratio of reached efficacy to the theoretically possible efficacy (luminous efficiency)
luminous flux: a measure of the perceived power of light, e.g. from a light source
luminous intensity: luminous flux per unit solid angle
Lyot filters: optical filter devices based on birefringence, exhibiting a wavelength-dependent transmission

M² factor: a parameter for quantifying the beam quality of laser beams
magnification: the enlargements of images
magnifying glasses: optical lenses held above objects for viewing them with some moderate magnification
master laser: a single-frequency laser used for injection locking one or several other lasers
master oscillator fiber amplifier: a laser system containing a fiber amplifier for boosting the output power
master oscillator power amplifier: a laser system consisting of a seed laser and a laser amplifier for boosting the output power
McCumber theory: a theory applied to absorption and emission properties of laser gain media, in particular to solid-state media
mechanical fiber splices: permanent or semi-permanent connections of optical fibers which are held in place with mechanical means
medical lasers: laser systems used for medical applications
mercury vapor lamps: metal vapor lamps based on light emission from mercury (Hg) atoms
metal–semiconductor–metal photodetectors: fast photodetector devices based on metal–semiconductor (Schottky) contacts
metal-coated mirrors: mirrors where the reflection is obtained from a metal coating
metal halide lamps: gas discharge lamps containing metal halides
metal vapor lamps: light sources which are based on an electrical discharge in a metal vapor
metastable states: excited states (particularly electronic states in laser gain media) which have a relatively long lifetime due to slow radiative and non-radiative decay
Michelson interferometers: an interferometer with two separate arms and a single beam splitter
microbends of fibers: microscopic bends of optical fibers
microchannel plates: electron amplifiers with many spatial channels
microchip lasers: solid-state lasers with a quasi-monolithic setup
microlens arrays: one- or two-dimensional arrays of microlenses, used e.g. in Shack-Hartmann wavefront sensors
microlenses: optical lenses of particularly small diameter, e.g. below 1 mm
microscope objectives: the optical component of a microscope which is next to the observed object
microscopes: instruments for seeing very small objects
mid-infrared fibers: optical fibers for use with mid-infrared light
mid-infrared laser sources: light sources emitting mid-infrared radiation in the form of a laser-like beam
mirror substrates: substrates on which coatings for laser mirrors can be deposited
mirrors: devices which reflect light
modal bandwidth: the maximum optical bandwidth (limited by intermodal dispersion) which can be used in a telecom fiber
mode cleaner cavities: optical cavities used as filters for improving the beam quality of laser beams
mode cleaners: devices which can improve the beam quality of laser beams
mode competition: the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects
mode coupling: a concept for describing and calculating light propagation in certain situations, e.g. involving nonlinear interactions
mode division multiplexing: a multiplexing technique for data transmission in fibers, where different fiber modes are used for different channels
mode field converters: optical devices which allow for efficient coupling between modes of different sizes
mode hopping: the phenomenon that a laser exhibits sudden jumps of optical frequency, which are associated with transitions between different modes of its resonator
mode-locked diode lasers: diode lasers which are actively, passively or hybrid mode-locked for generating ultrashort pulses
mode-locked fiber lasers: ultrafast fiber lasers which are actively or passively mode-locked for generating ultrashort pulses
mode-locked lasers: lasers which emit ultrashort pulses on the basis of the technique of mode locking
mode locking: a group of techniques for generating ultrashort pulses in lasers
mode locking devices: devices for achieving mode locking of a laser
mode matching: the precise spatial matching of the electric field distributions of laser beams and resonator modes or waveguide modes
mode radius: a measure of the transverse extent of a laser mode or laser beam
modes: self-consistent electric field distributions in waveguides, optical resonators or in free space
modes of laser operation: a mode of operation such as continuous-wave operation, Q-switched or mode-locked operation
modulation depth: a relative modulation amplitude, or (for a saturable absorber) the maximum change in absorption
molecular lasers: gas lasers where the laser-active gas consists of molecules rather than separate atoms or ions
monochromatic light: light with a single optical frequency
monochromators: optical devices which transmit only light in an adjustable small range of optical wavelengths
monolithic solid-state lasers: solid-state lasers where the whole laser resonator consists only of one piece of crystal or glass
multi-core fibers: optical fibers containing more than one fiber core
multi-phonon transitions: transitions between electronic levels of atoms or ions in solid media, involving the emission of multiple phonons
multimode beams: light beams in free space or in transparent media which involve multiple (often many) modes, frequently with highly random characteristics
multimode fibers: fibers supporting more than one guided mode per polarization direction
multipass amplifiers: an amplifier with multiple passes through the gain medium
multipass gas cells: a cell which allows to realize multiple passes of a laser beam through a gas
multiphonon absorption: absorption processes involving multiple phonons
multiphoton absorption: absorption processes where multiple photons are absorbed simultaneously
multispectral imaging: imaging which provides images with multiple spectral channels

nanofibers: optical fibers with transverse dimensions below one micrometer
nanosecond lasers: lasers emitting optical pulses with nanosecond durations
narrow-linewidth lasers: single-frequency lasers with a narrow optical emission spectrum
near field and far field: the optical field either close to a source or object or far from it
neodymium-doped gain media: laser gain media containing laser-active neodymium ions
neutral density filters: optical attenuators with an approximately constant attenuation in a substantial wavelength range
night vision devices: instruments which allow viewing or imaging under conditions of very low light levels
nodal points: points of an optical system for which an incoming ray, directed at a nodal point, leaves the system with the same direction.
noise eaters: devices for reducing the intensity noise of an optical beam by automatically adjusting the degree of power attenuation
noise-equivalent power: the input power to a detector which produces the same signal output power as the internal noise of the device
noise figure: a measure of the excess noise added in an amplifier
noise specifications: specifications for the noise properties of lasers, for example
non-radiative transitions: transitions between energy levels of atoms or ions which are not associated with the emission of light
nonclassical light: light with properties which can be explained only within quantum optics
noncritical phase matching: a technique of phase matching which does not require a critical angular adjustment
nonlinear absorption: absorption where the absorption coefficient depends on the optical intensity
nonlinear crystal materials: crystal materials exhibiting an optical nonlinearity, usually of χ⁽²⁾ type
nonlinear frequency conversion: the conversion of input light to light of other frequencies, using optical nonlinearities
nonlinear index: a parameter for quantifying the Kerr nonlinearity of a medium
nonlinear optics: the part of optics dealing with optical nonlinearities and their applications
nonlinear polarization: the part of the light-induced electric polarization which depends nonlinearly on the electric field of the light
nonlinear polarization rotation: the phenomenon that the change in the polarization direction of light in a fiber can depend on the optical intensity
nonlinear pulse distortion: distortion of the spatial, temporal or spectral characteristics of an optical pulse, resulting from optical nonlinearities
nonlinearities: optical phenomena involving a nonlinear response to a driving light field
nonplanar ring oscillators: a type of monolithic single-frequency lasers, based on a nonplanar ring resonator
numerical aperture: sine of the maximum angle of an incident beam of some optical device, or the sine of the acceptance angle of a waveguide or fiber

object and image space: mathematical spaces for the description of optical elements and systems
objectives: optical elements for collecting light from inspected objects, or for sending light to objects
ocular lenses: lenses or lens systems as used in telescopes and microscopes, for example, close to the observing eye
OEM laser modules: laser modules made for integration into larger devices by another manufacturer
optical aberrations: degradations of optical images caused in imaging instruments
optical activity: the phenomenon that the polarization direction of light is rotated when light passes through certain transparent substances
optical amplifiers: devices for amplifying the power of light beams
optical apertures: structures which limit the propagation of light
optical attenuators: devices which can reduce the optical power e.g. of a light beam
optical breadboards: portable platforms for optical prototyping and for mounting modules on optical tables
optical choppers: devices for periodically interrupting light beams
optical clocks: time measurement devices based on optical frequency standards
optical clockworks: devices which can phase-coherently relate optical frequencies to microwave frequencies
optical coherence tomography: an optical imaging technique where longitudinal resolution is achieved based on interference effects with low-coherence light
optical crystals: crystals for optical applications, usually single crystals, often with polished end faces
optical data transmission: the transmission of information using light beams, e.g. in fibers
optical delay lines: an optical setup which is used to introduce a time delay for a light beam
optical density: a logarithmic measure of the power attenuation, or the refractive index
optical domes: strongly curved optical windows
optical energy meters: instruments for measuring optical pulse energies
optical fabrication: the fabrication of optical components like lenses, prisms and mirrors
optical fiber communications: the technology of transmitting information through optical fibers
optical filters: devices with a wavelength-dependent transmission or reflectance
optical flats: plates with one or two surfaces of very high optical quality and flatness
optical frequency: the oscillation frequency of the electric field of light
optical frequency standards: frequency standards using optical transitions in atoms, ions or molecules
optical glasses: glasses of high quality, made for optical applications
optical heterodyne detection: a sensitive method of photodetection, reaching the standard quantum limit even for weak signal powers
optical intensity: optical power per unit area
optical materials: materials used for optical components
optical metrology: the science and technology of performing measurements with light
optical modulators: devices allowing one to manipulate properties of light beams, such as the optical power or phase
optical molasses: an arrangement of laser beams used for cooling atoms or ions
optical parametric amplifiers: optical amplifiers based on parametric nonlinear interactions
optical parametric chirped-pulse amplification: parametric amplification of chirped ultrashort pulses
optical parametric generators: light sources based on parametric amplifiers with no signal or idler input
optical parametric oscillators: coherent light sources based on parametric amplification within an optical resonator
optical phase: the phase of the electric field of an optical wave, usually considered in the time domain
optical power: the energy per unit time, e.g. transported by a laser beam, or a focusing power
optical power meters: instruments for optical power measurements, based e.g. on heating of an absorber structure or a photodiode
optical power monitors: devices for monitoring optical powers in free-space light beams or in optical fibers
optical profilometers: optical non-contact instruments for precisely measuring surface profiles
optical pumping: electronically exciting a medium with light, or specifically populating certain electronic levels
optical rectification: a nonlinear process where an optical field generates a quasi-DC nonlinear polarization
optical refrigeration: a technique for cooling macroscopic samples via laser-induced fluorescence
optical resonators: arrangements of optical components which allow a beam of light to circulate
optical sampling: a technique of sampling signals using laser light, often in the form of ultrashort pulses
optical sensors: sensors based on optical principles, or sensors for light
optical slits: narrow openings through which light can be transmitted
optical spectrum: the decomposition of the power or energy of light according to different wavelengths or optical frequencies
optical spectrum analyzers: instruments for measuring optical spectra
optical strain sensors: sensors for mechanical strain based on optical technology, e.g. with fiber Bragg gratings
optical tables: platforms for building optical devices with suppression of vibrations
optical temperature sensors: temperature sensors based on optical technology, e.g. with fiber Bragg gratings
optical thickness: a measure either for the absorption or the effective optical path length of a sample
optical time-domain reflectometers: instruments for measuring the spatially resolved reflectivities and losses in optical fibers
optical tweezers: arrangements for capturing and moving particles with laser beams
optical windows: flat transparent plates with optical quality, used for protection against the environment
optics: the science and technology dealing with the properties and the propagation of light
opto-isolators: optoelectronic devices for signal transmission with electrical isolation between two electrical circuits
opto-mechanics: mechanical elements used in optics
optoelectronics: the technology of electronic devices that interact with light
orientation-patterned semiconductors: semiconductors containing a periodic pattern of domain orientation
oscillators: a device producing an oscillation – in contrast to an amplifier, for example
output couplers: partially transparent laser mirrors, used for extracting output beams from laser resonators
output coupling efficiency: a factor influencing the power efficiency of a laser, taking into account intracavity losses

p–i–n photodiodes: photodiodes with a p–i–n semiconductor structure
parabolic mirrors: mirrors with the surface shape of a paraboloid, or at least a parabolic cross section
parabolic pulses: pulses with a parabolic intensity profile
parametric amplification: a process of optical amplification based on a parametric nonlinearity
parametric fluorescence: emission of light by a parametric amplifier without signal or input inputs
parametric nonlinearities: instantaneous optical nonlinearities based on the χ⁽²⁾ or χ⁽³⁾ nonlinear tensor of a transparent medium
parasitic lasing: unwanted laser operation in a laser or amplifier device
paraxial approximation: a frequently used approximation, essentially assuming small angular deviations of the propagation directions from some beam axis
passive fibers: optical fibers without laser-active dopants in the fiber core
passive mode locking: a technique of mode locking, based on a saturable absorber inside the laser resonator
peak power: maximum optical power of a pulse
periodic poling: a technique for achieving quasi-phase matching of nonlinear interactions in a transparent crystal material
phase matching: a group of techniques for achieving efficient nonlinear interactions in a medium
phase-matching bandwidth: the width of an optical frequency range in which some process (e.g. a nonlinear interaction) can be efficient due to more or less precise phase matching
phase modulators: devices for manipulating the phase of a laser beam
phase noise: noise of the optical phase of a beam or of an electric signal
phase shift method for distance measurements: a method for optical distance measurements
phase velocity: the velocity with which phase fronts propagate in a medium
phonons: quantized microscopic vibrations in solid media
phosphate glasses: certain glasses from which certain optical fibers and laser gain media can be made, for example
phosphorescence: a luminescence which lasts for a relatively long time after excitation of a medium
phosphors: substances which exhibit luminescence, used mostly for illumination and displays
photo cameras: optical instruments for taking photographic images
photocathodes: an electrode where electrons can be released based on the photoelectric effect
photochromic materials: optical materials which exhibit increased light absorption when being exposed to light
photoconductive detectors: photodetectors which exhibit a reduction in electrical resistance caused by incoming light
photoconductive sampling: a technique of optical sampling with photoconductive switches
photoconductive switches: electric switches controlled by light via photo-induced conductivity
photocurrent: an electric current which is induced by incident light in a photodetector
photodarkening: the phenomenon that the optical power losses in a medium can grow when the medium is irradiated with light at certain wavelengths
photodetectors: devices used for the detection of light
photodiode arrays: typically linear arrays of photodiodes, provided as compact devices
photodiodes: semiconductor devices with a p–n or p–i–n structure for the detection of light
photoelectric effect: the emission or excitation of electric carriers related to the absorption of light
photoemissive detectors: photodetectors which are based on the external photoelectric effect
photographic objectives: objective lenses used for photography
photoluminescence: emission of light which is caused by the irradiation of a material with other light
photometers: instruments for measuring light properties such as irradiance or illuminance, or optical properties of objects
photometry: the science and technology of measuring properties of light related to its perceived brightness for human eyes
photomultipliers: photodetection devices based on the photoelectric effect and charge multiplication by secondary emission of electrons
photon counting: photodetection at low light levels where single photon absorption events are counted
photonic bandgap fibers: optical fibers where light is guided based on a photonic bandgap effect
photonic crystal fibers: specialty optical fibers with a built-in microstructure, in most cases consisting of small air holes in glass
photonic integrated circuits: integrated circuits with optical functions
photonic metamaterials: nanostructured materials with special optical properties
photonics: the science and technology of light
photons: quanta of light energy
phototransistors: a type of semiconductor-based photodetector with internal current amplification for higher responsivity
phototubes: gas-filled or vacuum tubes which can be used as photodetectors
picosecond lasers: lasers emitting pulses with picosecond durations
pinholes: circular holes, typically with diameters well below one millimeter
planar waveguides: waveguide structures guiding light only in one dimension
plane waves: waves with plane wavefronts
plastic optical fibers: optical fibers made of polymer materials
plastic optics: optical elements made of organic polymer materials
Pockels cell drivers: electronic devices for driving Pockels cells
Pockels cells: electro-optic devices, used for building modulators
Pockels effect: the phenomenon that the refractive index of a medium exhibits a modification which is proportional to the strength of an applied electric field (linear electro-optic effect)
polarimeters: instruments for measuring the rotation angle of optical polarization caused by optically active substances
polarization beam combining: a technique for combining two polarized laser beams
polarization beat length: the period of interference effects in a birefringent medium
polarization-maintaining fibers: specialty optical fibers with strong built-in birefringence, preserving the properly oriented linear polarization of an input beam
polarization mode dispersion: the polarization dependence of the propagation characteristics of light waves in optical fibers
polarization of light: direction of the electric field oscillation of a light beam
polarization waves: patterns of electric (or magnetic) polarization of a medium which are usually caused by light waves
polarizers: optical filters where the light transmission depends strongly on the direction of polarization
polychromatic light: light with multiple optical frequencies
population inversion: a state of a medium where a higher-lying electronic level has a higher population than a lower-lying level
position-sensitive detectors: photodetectors which can detect the position of a light spot in one or two dimensions
power density: intensity or power per unit frequency interval
power over fiber: delivery of power for electronic devices via light in an optical fiber
power scaling of lasers: a procedure for substantially increasing the output power of lasers
power spectral density: optical power or noise power per unit frequency interval
principal dispersion: a crude measure for chromatic dispersion of a transparent optical material
principal points and principal planes: planes from which rays appear to be deflected in an optical system according to paraxial optics, and the intersections of those planes with the optical axis
prism pairs: combinations of two prisms, mostly used for dispersion compensation
prisms: transparent optical devices which refract or reflect light
propagation constant: a mode- and frequency-dependent quantity describing the propagation of light in a medium or waveguide
propagation losses: losses of optical energy during propagation of light
pulse characterization: the measurement of various properties of an optical pulse
pulse compression: linear or nonlinear techniques for reducing the durations of optical pulses
pulse duration: the duration of an optical pulse
pulse energy: energy of an optical pulse
pulse front tilt: the phenomenon that the arrival time of an ultrashort pulse varies across the beam profile
pulse generation: the generation of light pulses (flashes)
pulse pickers: electrically controlled optical switches used for extracting single pulses from a pulse train
pulse propagation modeling: working with physical models describing the propagation of ultrashort pulses e.g. in lasers or optical fibers
pulse repetition rate: the number of pulses emitted per second e.g. by a mode-locked or Q-switched laser
pulse shapers: devices which can be used to modify the temporal or spectral shape of ultrashort light pulses
pulse stretchers: devices for temporally stretching ultrashort light pulses
pulsed laser deposition: a method for fabricating thin films on substrates by evaporating materials with laser pulses
pulsed lasers: lasers emitting light in the form of pulses
pulses: flashes of light
pump–probe measurements: techniques for investigating ultrafast phenomena, where a pump pulse excites a sample and a probe pulse is used for probing the sample after an adjustable delay time
pump absorption: the absorption of pump light, e.g. in a laser or a laser amplifier
pump chambers: a part of a lamp-pumped solid-state laser, containing a laser rod, one or more lamps, reflectors, a cooling arrangement and possibly other parts
pump depletion: reduction in the transmitted pump power in a laser or nonlinear conversion device
pump parameter: the ratio of pump power and threshold pump power of a laser
pyroelectric detectors: light detectors based on the pyroelectric effect

Q factor: a measure of the damping of resonator modes
Q-switched lasers: lasers which emit optical pulses, relying on the method of Q switching
Q-switched mode locking: an operation regime of mode-locked lasers with strong fluctuations of the pulse energy
Q switches: optical switches which are typically used for generating nanosecond pulses in lasers
Q switching: a method for obtaining energetic pulses from lasers by modulating the intracavity losses
Q-switching instabilities: instabilities of passively mode-locked lasers where the pulse energy is unstable
quantum cascade lasers: semiconductor lasers relying on intersubband transitions, normally emitting in the mid-infrared spectral region
quantum cryptography: methods to encrypt information securely, relying on quantum-mechanical phenomena
quantum defect: energy difference between pump and laser photons
quantum dots: microscopic structures confining charge carriers in three dimensions
quantum efficiency: percentage of input photons which contribute to a desired effect
quantum electronics: an area of physics which is the basis of optoelectronics
quantum key distribution: methods for the secure distribution of encryption keys
quantum noise: noise arising from quantum fluctuations
quantum optics: the part of optics which deals with quantum effects
quantum photonics: the science and technology using quantum optics for applications like quantum communications, quantum computing and quantum metrology
quantum wells: thin layers confining carriers in one dimension
quarter-wave mirrors: dielectric mirrors based on a sequence of quarter-wave layers
quasi-continuous-wave operation: an operation mode of lasers (e.g. diode bars) where the pump power is switched on for short time intervals in order to limit thermal effects
quasi-phase matching: a technique of achieving a kind of phase matching, using a periodic structure
quasi-soliton pulses: soliton-like pulses in lasers or fiber-optic links
quenching: the reduction or limitation of an excited-state population, mostly by unwanted effects

Rabi oscillations: oscillations of level populations (or quantum mechanical probability amplitudes) under the influence of an incident light field
radiance: the optical power per unit area and solid angle
radiant energy: energy of some electromagnetic radiation which is emitted, transmitted or absorbed during some interval of time
radiant exitance: radiant flux emitted by a surface per unit area
radiant flux: radiant energy per unit time which is emitted, transmitted, reflected or received by an object
radiant intensity: radiant flux per unit solid angle
radiation-balanced lasers: lasers where the heat generation in the gain medium is compensated by optical refrigeration in the same medium
radiation pressure: a mechanical pressure resulting from incident light or other radiation
radiative lifetime: lifetime of an electronic state in the (hypothetical) situation where only radiative processes depopulate that level
radio and microwave over fiber: the transmission of radio frequency signals through optical fibers
radiometry: the science and technology of measuring properties of electromagnetic radiation, including light
Raman amplifiers: optical amplifiers based on Raman gain
Raman gain: optical gain arising from stimulated Raman scattering
Raman lasers: lasers based on Raman gain rather than on laser gain from stimulated emission
Raman scattering: a nonlinear scattering process involving optical phonons
Raman spectroscopy: optical spectroscopy based on spontaneous or stimulated Raman scattering
rare-earth-doped fibers: optical glass fibers which are doped with rare earth ions
rare-earth-doped gain media: laser gain media which are doped with rare earth ions
rate equation modeling: a method of modeling the dynamics of laser gain media
Rayleigh length: the distance from a beam waist where the mode radius increased by a factor square root of 2
Rayleigh scattering: scattering of light at scattering centers which are much smaller than the wavelength
reciprocity method: a spectroscopic method which is often used for obtaining the scaling of emission spectra of laser gain media
recirculating fiber loops: fiber loops in which light can circulate many times
recoil limit: a lower limit for the temperature which can be reached with some laser cooling techniques
red lasers: lasers emitting red light
reference cavities: optical cavities serving as a kind of frequency reference
reflectance: the ratio of reflected optical power to the incident optical power at some reflecting object
reflectivity: the ratio of reflected optical power to the incident optical power at a surface
refraction: the change of the propagation direction when a wave comes from one medium into another one
refractive index: a measure of the reduction in the velocity of light in a medium
regenerative amplifiers: optical amplifiers with a resonator in which a light pulse can do multiple round trips before being coupled out
relative intensity noise: noise of the optical intensity (or actually power), normalized to its average value
relaxation oscillations: small mutually coupled oscillations of the laser power and laser gain around their steady-state values
resonant frequency doubling: frequency doubling with a nonlinear crystal placed in a resonant enhancement cavity
resonator design: the description of the optical components from which an optical resonator is made, and their exact arrangement
resonator modes: modes of an optical or microwave resonator
responsivity: photocurrent per unit optical power incident on a photodetector
retardance: the difference in optical phase shifts between two polarization directions, e.g. for light transmitted through a sample or reflected at a surface
retroreflectors: optical devices which reflect light in a direction which is parallel to the incident beam
return loss: a measure for how much reflected light is attenuated
RGB sources: source of red, green and blue light, which is usually provided in the form laser beams
ring lasers: lasers with a ring resonator
ring resonators: optical resonators where light can independently circulate in two different directions
rod lasers: solid-state lasers with a rod-shaped gain medium
rod lenses: cylindrical lenses with the geometrical form of a cylinder
ruby lasers: solid-state lasers based on a ruby crystal
rugate filters: dielectric interference filters with a continuous variation of the refractive index

saturable absorbers: light absorbers with a degree of absorption which is reduced at high optical intensities
saturable Bragg reflectors: a special type of saturable semiconductor absorbers
saturation energy: a measure of the incident optical pulse energy required for achieving significant saturation of an absorber or a gain medium
saturation power: a measure of the incident optical power required for achieving significant saturation of an absorber or a gain medium
scanning lenses: lens systems which are designed for use in laser scanners
scattering: processes where light is sent in other directions, usually but not always in random directions
Schawlow–Townes linewidth: linewidth of a single-frequency laser with quantum noise only
scientific lasers: lasers which are designed for scientific applications
scotopic and photopic vision: vision of the human eye at lower and higher light intensity levels, respectively
sech²-shaped pulses: pulses with a temporal intensity profile which has the shape of a sech² function
seed lasers: lasers which are used for generating some seed light into an amplifier or another laser
self-focusing: focusing of a beam in a transparent medium, caused by the beam itself through a nonlinear process in the medium
self-heterodyne linewidth measurement: a technique for laser linewidth measurements, based on a beat note between the beam and a delayed version of itself
self-phase modulation: nonlinear phase modulation of a beam, caused by its own intensity via the Kerr effect
self-starting mode locking: mode locking which is quickly achieved after turning on a laser, without external intervention
self-terminating laser transitions: laser transitions which can not be continuously operated due to the accumulation of population in the lower laser level
Sellmeier formula: an equation for calculating the wavelength-dependent refractive index of a medium
semiconductor lasers: lasers based on semiconductor gain media
semiconductor optical amplifiers: optical amplifiers based on semiconductor gain media
semiconductor saturable absorber mirrors: saturable semiconductor absorber devices acting as nonlinear mirrors
sensitivity: (different meanings, depending on the context)
Shack–Hartmann wavefront sensors: wavefront sensors which are based on a microlens array
shot noise: quantum-limited intensity noise
side pumping: a technique of pumping a solid-state laser in directions which are approximately transverse to its beam direction
signal lamps: lamps for displaying or sending light signals
signal-to-noise ratio: the ratio of signal power to noise power in a detector
silica fibers: optical fibers based on fused silica or related materials
silicon photonics: photonic technology based on silicon chips
single-atom lasers: lasers with only a single atom as the gain medium
single-crystal fibers: long and thin rods made of monocrystalline materials
single-frequency lasers: lasers emitting radiation in a single resonator mode
single-frequency operation: an operation mode of a laser where only a single resonator mode acquires a significant power
single-mode fibers: optical fibers supporting only a single guided mode per polarization direction
single-mode operation: operation of a laser on axial (longitudinal) resonator modes only, or even on a single axial mode
single-polarization fibers: optical fibers which transmit only light with a certain polarization
Sisyphus cooling: a mechanism for laser cooling of atoms or ions
slab lasers: solid-state bulk lasers where the gain medium has the form of a slab
slave laser: a laser which is injection-locked to another laser
slope efficiency: differential power efficiency of a laser
small-signal gain: optical gain for small signal intensities, where no amplifier saturation occurs
sodium vapor lamps: metal vapor lamps based on light emission from sodium (Na) atoms
solar-blind photodetectors: photodetectors which are insensitive to visible light but react to ultraviolet light
solid-state lasers: lasers based on solid-state gain media (usually ion-doped crystals or glasses)
soliton mode locking: a mechanism for laser mode locking based on soliton pulses
soliton period: the distance over which higher-order solitons reproduce their temporal and spectral shape
solitons: pulses with a certain balance of nonlinear and dispersive effects
space division multiplexing: a multiplexing technique for optical data transmission where multiple spatial channels are utilized
spatial hole burning: a distortion of the gain shape in a laser medium (or the loss spectrum in a saturable absorber medium), caused by saturation effects of a standing wave
spatial walk-off: the phenomenon that the intensity distribution of a beam in an anisotropic crystal drifts away from the direction of the wave vector
specialty fibers: optical fibers with special designs or materials
spectral beam combining: a class of methods for beam combining, based on wavelength-sensitive beam combiners
spectral brightness: brightness per unit optical bandwidth
spectral imaging: imaging which also provides spectral information
spectral lamps: low pressure gas discharge lamps emitting light on one or more standard spectral lines
spectral lines: narrow spectral features related to emission or absorption lines of certain substances
spectral phase: the phase of the electric field in the frequency domain
spectral phase interferometry: a class of interferometric methods for pulse characterization, where interference in the frequency domain is exploited
spectral quantities: quantities in radiometry and photometry which describe the distribution e.g. of a radiant flux over different optical frequencies or wavelengths
spectral response of a photodetector: the range of optical wavelengths for which a photodetector is sensitive
spectrograms: a type of diagrams which visualize optical or other signals
spectrographs: instruments for spectrally analyzing light
spectrometers: devices for separating spectral components
spectrophotometers: instruments for measuring the wavelength-dependent transmittance or reflectance of solutions or solid objects
spectroscopy: methods where the interaction of light with matter is utilized, e.g. for detecting substances or measuring concentrations
specular reflection: reflection of light where the angle of reflected light equals the angle of incident light, but on the opposite side of the surface normal
spherical aberrations: optical aberrations resulting from spherical optical surfaces
spiking: the tendency of some lasers to emit spikes of output power, particularly after being switched on
spontaneous emission: a quantum effect, causing the spontaneous decay of excited states of atoms or ions
squeezed states of light: nonclassical states of light with noise below the standard quantum limit in one quadrature component
stability zones of optical resonators: parameter regions of an optical resonator where the resonator is geometrically stable
stabilization of lasers: measures applied to lasers in order to improve their stability in terms of output power, optical frequency, or other quantities
standard quantum limit: a limit for noise levels set by quantum mechanics
standard spectral lines: spectral lines which are often used as wavelength references
standing-wave resonators: optical resonators with two end mirrors
step-index fibers: optical fibers with a step-index refractive index profile
stimulated emission: a quantum effect, where photon emission is triggered by other photons
Stokes shift: the reduction in photon energy in certain processes
streak cameras: devices for measuring the power of an optical pulse versus time
sum and difference frequency generation: nonlinear processes generating beams with the sum or difference of the frequencies of the input beams
supercontinuum generation: a nonlinear process for strong spectral broadening of light
superfluorescence: collective emission of radiation by an ensemble of excited atoms or ions
superluminal transmission: transport of some quantity with a velocity exceeding that of light in vacuum
superluminescence: fluorescence which is enhanced by laser amplification
superluminescent diodes: broadband semiconductor light sources based on superluminescence
superluminescent sources: optical sources based on superluminescence
supermirrors: laser mirrors with a very high reflectivity
superradiance: collective emission of an ensemble of atoms or ions after coherent excitation
surface-emitting semiconductor lasers: semiconductor lasers where the generated light propagates in the direction perpendicular to the wafer surface
synchronization of lasers: the synchronization of light pulses generated with lasers
synchronous pumping: pumping of a laser or an optical parametric oscillator with short light pulses in synchronism with the emitted pulses

tapered amplifiers: optical amplifiers which contain a tapered section, where the beam area is gradually increased
tapered fibers: optical fibers which are over some length stretched out to a very small diameter
tapered laser diodes: laser diodes containing a tapered region for achieving a higher beam quality
telecentric lenses: lenses or objectives having the entrance or exit pupil at infinity
telecom fibers: optical fibers for use in optical fiber communications
telescopes: optical instruments for viewing distant objects, or sometimes more generally afocal systems
temporal walk-off: the loss of temporal overlap of pulses, caused by group velocity mismatch
terahertz detectors: detectors for terahertz radiation
terahertz radiation: electromagnetic radiation with frequencies around 0.1 THz to 10 THz
terahertz sources: sources of electromagnetic radiation with frequencies around 0.1 THz to 10 THz
thermal detectors: light detectors which are based on measuring light-induced temperature rises
thermal imaging: imaging based on thermal radiation
thermal lensing: a lensing effect induced by temperature gradients
thermal radiation: electromagnetic radiation which results from thermal emission
thin-disk lasers: solid-state bulk lasers having a very thin disk of laser-active material as the gain medium
thin-film polarizers: optical polarizers based on a multilayer dielectric coating
third-order dispersion: chromatic dispersion related to a third-order dependence of the phase change on the frequency offset
threshold pump power: the pump power at which the laser threshold is reached
thresholdless lasers: lasers with a threshold power which is virtually zero
time–bandwidth product: product of temporal and spectral width of a pulse
time division multiplexing: a multiplexing technique operating in the time domain
time-of-flight measurements: distance measurements based on measuring the time of flight of a light pulse
time-resolved spectroscopy: spectroscopy where time-dependent signals are measured
timing jitter: fluctuations of the temporal positions of pulses
timing phase: a phase reflecting timing deviations by relating them to the pulse period
titanium–sapphire lasers: lasers based on a Ti:sapphire gain medium
total internal reflection: the phenomenon that light incident on the boundary between two transparent media is fully reflected if the light is coming from the optical more dense medium and the angle of incidence is sufficiently large
transform limit: a limit for the time–bandwidth–product of an optical pulse
transition cross sections: material parameters for quantifying the likelihood or rate of optical transition events
transition-metal-doped gain media: laser gain media which are doped with transition metal ions
transmissivity: the ratio of transmitted optical power to the incident optical power at a surface
transmittance: the ratio of transmitted optical power to the incident optical power at some partial transparent object
triangulation: a method for distance measurements
tunable lasers: lasers the output wavelengths of which can be tuned
tunable optical filters: optical filters with tunable filter parameters, e.g. concerning center wavelength of pass band
tungstate lasers: solid-state lasers based on tungstate crystals as gain media
twisted-mode technique: a technique for achieving single-frequency operation of a laser, based on quarter-wave plates on both sides of the gain medium
two-photon absorption: a nonlinear absorption process where two photons are simultaneously absorbed
Twyman–Green interferometers: an interferometer similar to a Michelson interferometer, but with expanded beams

ultrafast amplifiers: amplifiers for ultrashort (femtosecond or picosecond) optical pulses
ultrafast laser physics: the area of laser physics dealing with ultrashort processes
ultrafast lasers: lasers emitting ultrashort pulses
ultrafast optics: the part of optics dealing with ultrafast phenomena
ultrashort pulses: optical pulses with durations of picoseconds or less
ultraviolet lasers: lasers (or other laser-based light sources) generating ultraviolet light
ultraviolet light: invisible light with wavelengths shorter than ≈ 400 nm
ultraviolet optics: optical elements for use with ultraviolet light
unstable resonators: optical resonators which are dynamically unstable with respect to transverse beam offsets
upconversion: a process where light can be emitted with photon energies higher than those of the light generating the excitation
upconversion lasers: lasers relying on upconversion processes
upper-state lifetime: the lifetime of the population of the upper laser level

V number: a normalized frequency parameter, which determines the number of modes of a step-index fiber
vacuum noise: quantum noise of the vacuum state, or quantum noise introduced into an optical system via a vacuum input
vanadate lasers: lasers based on rare-earth-doped yttrium, gadolinium or lutetium vanadate crystals, usually Nd:YVO₄
variable optical attenuators: optical attenuators with a variable degree of attenuation
variable reflectivity mirrors: mirrors with a spatial variation of the reflectivity (reflectance)
VCSEL arrays: arrays containing many VCSEL laser emitters
velocity-matched photodetectors: a type of photodetector where propagation velocities of light and electrical signals are matched in order to achieve a very high detection bandwidth
velocity of light: the velocity with which light propagates in a medium
vertical cavity surface-emitting lasers: a monolithic kind of semiconductor lasers with beam emission perpendicular to the wafer surface
vertical external-cavity surface-emitting lasers: surface-emitting semiconductor lasers with an external laser resonator
vibronic lasers: lasers based on gain media with a large gain bandwidth, caused by a strong interaction of electronic transitions with phonons
vignetting: a reduction of image brightness in peripheral regions
visible lasers: lasers (or other laser-based light sources) emitting visible light
volume Bragg gratings: Bragg gratings written into some bulk material

wall-plug efficiency: total electrical-to-optical power efficiency of a laser system
wave optics: the description of optical phenomena based on wave models
wave vector: a vector indicating the direction of wave propagation and the phase delay per unit length
wavefronts: surfaces connecting points with the same phase of a wave
waveguide dispersion: chromatic dispersion arising from waveguiding effects
waveguide lasers: lasers with a waveguide structure in the gain medium
waveguides: spatially inhomogeneous transparent structures for guiding light
wavelength: the spatial period of a plane wave
wavelength division multiplexing: a multiplexing technique working in the wavelength domain
wavelength-swept lasers: lasers which allow for period and fast wavelength tuning
wavelength-tunable light sources: light sources where the optical wavelength can be tuned
wavelength tuning: the manipulation of the output wavelength of an optical device such as a laser or an optical parametric oscillator
wavemeters: devices for precise measurements of laser wavelengths
wavenumber: the phase delay per unit length, or that quantity divided by 2π
waveplates: transparent plates with a defined amount of birefringence, used for modifying the polarization of light
wedge prisms: prisms with a small angle between the end surfaces
white light: light which creates a white color impression
white light interferometers: interferometers using broadband light inputs
white light sources: light sources with very broad optical bandwidth
working distance: the distance between an objective and the object

X-ray lasers: lasers emitting in the X-ray spectral region
xenon lamps: lamps based on an electrical discharge in xenon gas

YAG lasers: lasers based on YAG (yttrium aluminum garnet) crystals, usually Nd:YAG or Yb:YAG
yellow and orange lasers: lasers emitting yellow or orange light
YLF lasers: lasers based on YLF (yttrium lithium fluoride) crystals, usually Nd:YLF
ytterbium-doped gain media: laser gain media containing laser-active ytterbium ions

z-scan measurements: a technique for measuring the strength of the Kerr nonlinearity of a material, relying on self-focusing
zero dispersion wavelength: a wavelength where the group delay dispersion of a fiber or a material is zero

List of Formula Symbols

Symbol	Explanation	SI Units
a	core radius of a fiber	m
b	confocal parameter of a laser beam (twice the Rayleigh length)	m
c	velocity of light	m/s
d_eff	effective nonlinear coefficient of a nonlinear crystal material	m/V
d_ij	components of the χ⁽²⁾ nonlinear tensor	m/V
e	elementary electric charge	C
f	focal length (e.g., of a lens)	m
f	noise frequency	Hz
f_rep	pulse repetition rate	Hz
f_ro	relaxation oscillation frequency	Hz
g	gain coefficient	1 or m⁻¹
g(τ)	complex degree of coherence	1
g_B	nonlinear gain coefficient for Brillouin scattering	m/W
g_R	nonlinear gain coefficient for Raman scattering	m/W
g_ss	small-signal gain coefficient	1 or m⁻¹
h	Planck's constant	J s
k	wavenumber	m⁻¹
k_B	Boltzmann constant	J/K
Δk	phase mismatch of a parametric nonlinear process	rad/m
l_rt	round-trip losses of a resonator	1
n	refractive index	1
n_e	extraordinary refractive index	1
n_eff	effective refractive index	1
n_g	group index	1
n_o	ordinary refractive index	1
n₂	nonlinear index	m²/W
q	complex q parameter (→ Gaussian beams)	m
r	radius (e.g., in fiber core or distance from beam axis)	m
r	pump parameter of a laser	1
r_ij	components of the electro-optic tensor	m/V
v_g	group velocity	m/s
v_p	phase velocity	m/s
w	Gaussian beam radius	m
w₀	beam radius at the beam waist	1
z_R	Rayleigh length	m
z_s	soliton period	m
A	amplitude of pulse envelope
A_eff	effective mode area	m²
B	B integral	rad
B	noise bandwidth	Hz
B	brightness	W/(m² rad²)
BPP	beam parameter product	mm mrad
D_λ	dispersion parameter	ps/(nm km)
E	electric field strength	V/m
E_p	pulse energy	J
E_sat	saturation energy	J
F	finesse	1
F	noise figure	1
F_sat	saturation fluence	J/m²
G_X(τ)	autocorrelation function of some variable X
I	optical intensity	W/m²
I_sat	saturation intensity	W/m²
L_coh	coherence length	m
M²	M² factor	1
N_F	Fresnel number	1
N_dop	doping concentration	m⁻³
NA	numerical aperture	1
P	optical power	W
P	electric polarization of a medium	C/m²
P_out	output power	W
P_p	peak power of a pulse	W
P_sat	saturation power of a gain medium or saturable absorber	W
Q	Q factor	1
R	power reflectivity (or reflectance) (of a mirror)	1
R	radius of curvature (e.g., of wavefronts)	m
R	responsivity of a photodiode	A/W
ΔR	modulation depth of a saturable absorber	1
S_φ(f)	power spectral density of a phase value	rad²/Hz
T	temperature	K or °C
T_g	group delay	s
T_oc	output coupler transmission	1
T_rt	round-trip time of an optical resonator	s
V	V number of a step-index fiber	1
α	absorption coefficient	m⁻¹
α	linewidth enhancement factor	1
α	thermal expansion coefficient	K⁻¹
β	propagation constant	rad/m
β₂	group velocity dispersion	s²/m
γ_SPM	self-phase modulation coefficient	rad/(W m)
ε₀	electric permittivity of vacuum	C/Vm
ε	relative electric permittivity	1
η	efficiency	1
η_oc	output coupling efficiency	1
θ	beam divergence half-angle	rad
θ	polar angle	rad
κ	thermal conductivity	W/(m K)
λ	optical wavelength	m
λ_p	pump wavelength	m
λ_s	signal wavelength	m
μ	relative magnetic permeability	1
μ₀	magnetic permeability of vacuum	N/A²
ν	optical frequency	Hz
ν_B	Brillouin frequency shift	Hz
ν_ceo	carrier–envelope offset frequency	Hz
ν(t)	instantaneous frequency	Hz
Δν	optical bandwidth	Hz
ρ	walk-off angle	rad
σ_abs	absorption cross section	m²
σ_em	emission cross section	m²
τ₂	upper-state lifetime	s
τ_coh	coherence time	s
τ_p	pulse duration	s
τ_rad	radiative lifetime	s
φ	azimuth angle	rad
φ	optical phase or phase shift	rad
φ_G	Gouy phase shift	rad
φ_nl	nonlinear phase shift from the Kerr effect	rad
φ_t	timing phase	rad
Δφ_ceo	carrier–envelope offset phase	rad
χ⁽²⁾, χ⁽³⁾	nonlinear tensor	1
ω	angular frequency	rad/s, s⁻¹
Λ	modulation period of a fiber Bragg grating	m

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