Glossary
of Laser Terms
Absorption of radiation
Receiving electromagnetic radiation by
interaction with the material, and transforming it to different form, which is
usually heat. The absorption process is dependent on the wavelength of the
electromagnetic radiation and on the absorbing material.
Active Medium
Collection of atoms or molecules which can be
stimulated to a population inversion, and emit electromagnetic radiation in a
stimulated emission. Amplification The process in which the electromagnetic radiation
inside the active medium within the laser optical cavity increase by the
process of stimulated emission.
Amplitude
The maximum value of a wave, measured from its
equilibrium.
Aperture
A small opening through which the
electromagnetic radiation pass.
Attenuation
The decrease in radiation energy (power) as a
beam passes through an absorbing or scattering medium.
Beam Diameter
Defined as the diameter of a circular beam at a
certain point where the intensity drop to a fraction of its maximum value. The
common definitions are 1/e (0.368) and 1/e2 (0.135) of the maximum value.
Beam
Divergence
Angle of beam spread, measured in
(milli)radians. Can be approximated for small angle by the ratio of the beam
diameter to the distance from the laser aperture.
Black
Body Radiation
Any object surface can radiate heat to and
receive heat from outside, if an object can absorb all the incident radiation,
regardless of the frequencies and directions, this object is called Black Body.
A ball cavity with a small hole can be regarded as a black body, since any
radiation entering the ball cavity can only reflect inside it, thus totally
absorbed.
Brightness
The brightness of a light source is defined as
the power emitted per unit surface area per unit solid angle.
Coherence
Coherence can be divided into spatial and
temporal coherence. For any electromagnetic wave, if at time t=0 and t0 the
phase difference between two points in space remains the same, we say the
electromagnetic wave has spatial coherence; If at a point P, the electromagnetic
wave at t and t+dt has same phase difference if dt is the same, temporal
coherence exists.
Hologram
An interference phenomena captured on a plate
(or film). It can contain enormous amount of information and a 3 dimensional
image can be constructed from it.
Laser
Laser is the acronym of Light Amplification by
Stimulated Emission of Radiation. Laser is light of special properties, light
is electromagnetic (EM) wave in visible range. Lasers, broadly speaking, are
devices that generate or amplify light, just as transistors generate and
amplify electronic signals at audio, radio or microwave frequencies. Here light
must be understand broadly, since lasers have covered radiation at wavelengths
ranging from infrared range to ultraviolet and even soft x-ray range.
Laser Mode
Laser mode is the possible standing electromagnetic
waves in laser cavity.
Longitudinal
(Axial) Modes
Axial standing electromagnetic waves within the
laser cavity.
Laser
Resonator or Laser Cavity
The optical mirrors, active medium and pumping system form the laser resonator, which is also called Laser Cavity. Laser cavities
can be divided into Stable Cavities and Unstable Cavities according to whether
they make the oscillating beam converge into the cavity or spread out from the
cavity.
Linewidth
The linewidth of laser is the width of laser
beam frequency. Laser linewidth is much narrower than normal light.
M2 of the beam
M2 is a beam quality index that measures the difference between an
actual beam and the Gaussian beam.
Mode Locking
A method to create very short laser pulses. It
makes the phase difference of many modes (frequencies) in the laser cavity
fixed, or locked, thus very narrow pulses (in time) are created.
Polarized
Light
If the light has a dominant direction of the E
vector, we say the light is polarized. Natural light is not polarized, while
laser beam is polarized. Polarization can be created and adjusted by polarizer.
Pumping
The process to raise atoms from lower level to
upper level is called pumping.
Q-Switching
A method to create laser pulses. It modulates
the Q (Quality) of laser cavity to build population inversion first, then
release the accumulated energy suddenly, in this way high energy pulses can be
created.
Solid
State Laser
A laser in which the active medium is in solid
state (usually not including semiconductor lasers).
Semiconductor Laser
Lasers which use semiconductor as active medium.
The majority of semiconductor materials are based on a combination of elements
in the third group of the Periodic Table (such as Al, Ga, In) and the fifth
group (such as N, P, As, Sb) hence referred to as the III-V compounds.
TEM Mode
Transverse Electromagnetic Mode (TEM) of laser
beam is called TEM mode. Three indexes are used to indicate the TEM modes. TEM plq,
p is the number of radial zero fields, l is the number of angular zero fields,
q is the number of longitudinal fields.
YAG
yttrium/aluminum garnet