LASER DEFINITIONS (from LaserNet)
Definitions useful to astrophysical lasers were retained, certain definitions
were changed.
Absorb
To transform radiant energy into a different form, with a resultant rise
in temperature
Absorption
Transformation of radiant energy to a different form of energy by the
interaction of matter, depending on temperature and wavelength.
Absorption Coefficient
Factor describing light's ability to be absorbed per unit of path length.
Active Medium
Collection of atoms or molecules capable of undergoing stimulated emission
at a given wavelength.
Amplification
The growth of the radiation field in the laser resonator cavity. As the
light wave bounces back and forth between the cavity mirrors, it is amplified
by stimulated emission on each pass through the active medium.
Amplitude
The maximum value of the electromagnetic wave, measured from the mean to the
extreme; simply stated: the height of the wave.
Angstrom Unit
A unit of measure of wavelength equal to 10-10 meter, 0.1 nanometer, or 10-4
micrometer, no longer widely used nor recognized in the SI system of units.
Anode
An electrical element in laser excitation which attracts electrons from a
cathode.
Attenuation
The decrease in energy (or power) as a beam passes through an absorbing or
scattering medium.
Cathode
A negatively charged electrical element providing electrons for an
electrical discharge.
Coherence
A term describing light as waves which are in phase in both time and space.
Monochromaticity and low divergence are two properties of coherent light.
Continuous Wave (CW)
Constant, steady-state delivery of laser power. CW is the
abbreviation for continuous wave; the continuous-emission mode of a laser as
opposed to pulsed operation.
Electric Vector
The electric field associated with a light wave which has both direction and
amplitude.
Electromagnetic Radiation
The propagation of varying electric and magnetic fields through space at the
velocity of light.
Electromagnetic Spectrum
The range of frequencies and wavelengths emitted by atomic systems. The
total spectrum includes radiowaves as well as short cosmic rays. Wavelengths
cover a range from 1 hz to perhaps as high as 1020 hz.
Electromagnetic Wave
A disturbance which propagates outward from an electric charge that
oscillates or is accelerated. Includes radio waves; X-rays; gamma rays; and
infrared, ultraviolet, and visible light.
Electron
Negatively charged particle of an atom.
Emission
Act of giving off radiant energy by an atom or molecule.
Emissivity
The ratio of the radiant energy emitted by a any source to that emitted by a
blackbody at the same temperature.
Emittance
The rate at which emission occurs.
Energy
The product of power (watts) and duration (seconds). One watt second = one
Joule.
Energy (Q)
The capacity for doing work. Energy is commonly used to express the out put
from pulsed lasers and it is generally measured in Joules (J). The product of
power (watts) and duration (seconds). One watt second = one Joule.
Energy Source
High voltage electricity, radiowaves, flashes of light, or another laser
used to excite the laser medium. Also indirect pumping schemes such as
rapid adiabatic expansion, or rapid cooling by contact with gas or dust.
Excimer
"Excited Dimer". A gas mixture used as the active medium in a family
of lasers emitting ultraviolet light.
Excitation
Energizing a material into a state of population inversion.
Excited State
Atom with an electron in a higher energy level than it normally occupies.
Femtoseconds
10-15 seconds.
Flashlamp
A tube typically filled with Krypton or Xenon. Produces a high intensity
white light in short duration pulses.
Fluorescence
The emission of light of a particular wavelength resulting from absorption
of energy typically from light of shorter wavelengths.
Flux
The radiant, or luminous, power of a light beam; the time rate of the flow
of radiant energy across a given surface.
Frequency
The number of light waves passing a fixed point in a given unit of time, or
the number of complete vibrations in that period of time.
Gain
Another term for amplification.
Gas Discharge Laser
A laser containing a gaseous lasing medium in a glass tube in which a
constant flow of gas replenishes the molecules depleted by the electricity or
chemicals used for excitation.
Gas Laser
A type of laser in which the laser action takes place in a gas medium.
Gaussian Curve Normal
Statistical curve showing a peak with even distribution on either side. May
either be a sharp peak with steep sides, or a blunt peak with shallower sides.
Used to show power distribution in a beam. The concept is important in
controlling the geometry of the laser impact.
Ground State
Lowest energy level of an atom.
Half-Power Point
The value on either the leading or trailing edge of a laser pulse at which
the power is one-half of its maximum value.
Hertz (Hz)
Unit of frequency in the International System of Units (SI), abbreviated Hz;
replaces cps for cycles per second.
Infrared Radiation (IR)
Invisible Electromagnetic radiation with wavelengths which lie within the
range of 0.70 to 1000 micrometers. This region is often broken up into IR-A,
IR-B and IR-C.
Integrated Radiance
Product of the exposure duration times the radiance. Also known as pulsed
radiance.
Intensity
The magnitude of radiant energy.
Ionizing Radiation
Radiation commonly associated with X-Ray or other high energy
electromagnetic radiation which will cause DNA damage with no direct, immediate
thermal effect. Contrasts with non-ionizing radiation of lasers.
Irradiance (E)
Radiant flux (radiant power) per unit area incident upon a given surface.
Units: Watts per square centimeter. (Sometimes referred to as power density,
although not exactly correct).
Irradiation
Exposure to radiant energy, such as heat, X-rays, or light.
Joule (J)
A unit of energy (1 watt-second) used to describe the rate of energy
delivery. It is equal to one watt-second or 0.239 calorie.
Joule/cm2
A unit of radiant exposure used in measuring the amount of energy incident
upon a unit area.
Laser
An acronym for light amplification by stimulated emission of radiation. A
laser is a cavity, with mirrors at the ends, filled with material such as
crystal, glass, liquid, gas or dye. A device which produces an intense beam
of light with the unique properties of coherency, collimation and
monochromaticity.
Laser Oscillation
The buildup of the coherent wave between laser cavity end mirrors producing
standing waves.
Light
The range of electromagnetic radiation frequencies detected by the eye, or
the wavelength range from about 400 to 760 nanometers. The term is sometimes
used loosely to include radiation beyond visible limits.
Longitudinal or Axial Mode
Determines the wavelength bandwidth produced by a given laser system
controlled by the distance between the two mirrors of the laser cavity.
Individual longitudinal modes are produced by standing waves within a laser
cavity.
Metastable State
The state of an atom, just below a higher excited state, which an electron
occupies momentarily before destabilizing and emitting light. The upper of the
two lasing levels.
Micrometer
A unit of length in the International System of Units (SI) equal to
one-millionth of a meter. Often referred to as a "micron".
Micron
An abbreviated expression for micrometer which is the unit of length equal
to 1 millionth of a meter. See micrometer.
Mode
A term used to describe how the power of a laser beam is geometrically
distributed across the cross-section of the beam. Also used to describe the
operating mode of a laser such as continuous or pulsed.
Monochromatic Light
Theoretically, light consisting of just one wavelength. No light is
absolutely single frequency since it will have some bandwidth. Lasers provide
the narrowest of bandwidths that can be achieved.
Multi-mode
Laser emission at several closely-spaced frequencies.
Nanometer (nm)
A unit of length in the International System of Units (SI) equal to
one-billionth of a meter. Abbreviated nm- a measure of length. One nm equals
10-9 meter, and is the usual measure of light wavelengths. Visible light
ranges from about 400 nm in the purple
to about 760 nm in the deep red.
Nanosecond
One billionth (10-9) of a second. Longer than a picosecond or femtosecond,
but shorter than a microsecond. Associated with Q-switched lasers.
Optical Cavity
(Resonator) Space between the laser mirrors where lasing action occurs.
Optical Pumping
The excitation of the lasing medium by the application of light rather than
electrical discharge.
Optical Radiation
Ultraviolet, visible and infrared radiation (0.35-1.4 æm) that falls in the
region of transmittance of the human eye.
Optically Pumped Lasers
A type of laser that derives energy from another light source such as a
xenon or krypton flashlamp or other laser source.
Output Coupler
Partially reflective mirror in laser cavity which allows emission of laser
light.
Phase
Waves are in phase with each other when all the troughs and peaks coincide
and are "locked" together. The result is a reinforced wave in increased
amplitude (brightness).
Photon
In quantum theory, the elemental unit of light, having both wave and
particle behavior. It has motion, but no mass or charge. The photon energy (E)
is proportional to the EM wave frequency (v) by the relationship: E=hv; where h
is Planck's constant (6.63 x10-34 Joule-sec).
Picosecond
A period of time equal to 10-12 seconds.
Point Source
Ideally, a source with infinitesimal dimensions. Practically, a source of
radiation whose dimensions are small compared with the viewing distance.
Polarization
Restriction of the vibrations of the electromagnetic field to a single
plane, rather that the innumerable planes rotating about the vector axis.
Various forms of polarization include random, linear, vertical, horizontal,
elliptical and circular.
Population Inversion
A state in which a substance has been energized, or excited, so that more
atoms or molecules are in a higher excited state than in a lower resting state.
This is necessary prerequisite for laser action.
Power
The rate of energy delivery expressed in watts (joules per second). Thus:
1 Watt = 1 Joule x 1 Sec.
Pulse
A discontinuous burst of laser, light or energy, as opposed to a continuous
beam. A true pulse achieves higher peak powers than that attainable in a CW
output.
Pumping
Addition of energy (thermal, electrical, or optical) into the atomic
population of the laser medium, necessary to produce a state of population
inversion.
Radiance
Brightness; the radiant power per unit solid angle and per unit area of a
radiating surface.
Radiant Energy (Q)
Energy in the form of electromagnetic waves usually expressed in units of
Joules (watt-seconds).
Radiant Exposure (H)
The total energy per unit area incident upon a given surface. It is used to
express exposure to pulsed laser radiation in units of J/cm2.
Radiant Flux
Radiant Power - The time rate of flow of radiant energy. Units-watts. (One
[1] watt = 1 Joule-per-second). The rate of emission of transmission of radiant
energy.
Radiant Intensity
The radiant power expressed per unit solid angle about the direction of the
light.
Radiant Power
See radiant flux.
Radiation
In the context of optics, electromagnetic energy is released; the process of
releasing electromagnetic energy.
Radiometry
A branch of science which deals with the measurement of radiation.
Resonator
The mirrors (or reflectors) making up the laser cavity including the laser
rod or tube. The mirrors reflect light back and forth to build up amplification.
Solid Angle
The ratio of the area on the surface of a sphere to the square of the radius
of that sphere. It is expressed in steradians (sr).
Spectral Response
The response of a device or material to monochromatic light as a function of
wavelength.
Spontaneous Emission
Decay of an excited atom to a ground or resting state by the random emission
of one photon. The decay is determined by the lifetime of the excited state.
Steradian (sr)
The unit of measure for a solid angle.
Stimulated Emission
When an atom, ion or molecule capable of lasing is excited to a higher
energy level by an electric charge or other means, it will spontaneously emit a
photon as it decays to the normal ground state. If that photon passes near
another atom of the same frequency, the second atom will be stimulated to emit a
photon.
TEM
Abbreviation for: Transverse ElectroMagnetic modes. Used to designate the
cross-sectional shape of the beam.
TEMoo
The lowest order mode possible with a bell-shaped (Gaussian) distribution of
light across the laser beam.
Thermal Relaxation Time
The time to dissipate the heat absorbed during a laser pulse.
Threshold
The input level at which lasing begins during excitation of the laser
medium.
Transmission
Passage of electromagnetic radiation through a medium.
Transmittance
The ratio of transmitted radiant energy to incident radiant energy, or the
fraction of light that passes through a medium.
Transverse Electromagnetic Mode
The radial distribution of intensity across a beam as it exits the optical
cavity. (See TEM.)
Ultraviolet (UV) Radiation
Electromagnetic radiation with wavelengths between soft X-rays and visible
violet light, often broken down into UV-A (315-400 nm), UV-B (280-315 nm), and
UV-C (100-280 nm).
Visible Radiation (light)
Electromagnetic radiation which can be detected by the human eye. It is
commonly used to describe wavelengths which lie in the range between 400 nm and
700-780 nm.
Watt
A unit of power (equivalent to one Joule per second) used to express laser
power.
Watt/cm2
A unit of irradiance used in measuring the amount of power per area of
absorbing surface, or per area of CW laser beam.
Wavelength
The length of the light wave, usually measured from crest to crest, which
determines its color. Common units of measurement are the micrometer (micron),
the nanometer, and (earlier) the Angstrom unit.
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