Laser Fundamentals

introduction
the word "laser" is an acronym for light amplification by stimulated emission of
radiation. lasers are finding ever increasing military applications principally for target
acquisition, fire control, and training. these lasers are termed rangefinders, target
designators, and direct-fire simulators. lasers are also being used in communications,
laser radars (lidar), landing systems, laser pointers, guidance systems, scanners, metal
working, photography, holography, and medicine.
in this document the word laser will be limited to electromagnetic radiation emitting
devices using light amplification by stimulated emission of radiation at wavelengths from
180 nanometers to 1 millimeter. the electromagnetic spectrum includes energy ranging
from gamma rays to electricity. figure 1 illustrates the total electromagnetic spectrum
and wavelengths of the various regions.
figure 1. electromagnetic spectrum
the primary wavelengths of laser radiation for current military and commercial
applications include the ultraviolet, visible, and infrared regions of the spectrum.
ultraviolet radiation for lasers consists of wavelengths between 180 and 400 nm. the
visible region consists of radiation with wavelengths between 400 and 700 nm. this is
the portion we call visible light. the infrared region of the spectrum consists of radiation
with wavelengths between 700 nm and 1 mm. laser radiation absorbed by the skin
penetrates only a few layers. in the eye, visible and near infrared radiation passes through
the cornea, and is focused on and absorbed by the retina. it is the wavelength of the light
that determines the visible sensation of color: violet at 400 nm, red at 700 nm, and the
other colors of the visible spectrum in between. when radiation is absorbed, the effect on
the absorbing biological tissue is either photochemical, thermal, or mechanical: in the
ultraviolet region, the action is primarily photochemical in the infrared region, the action
is primarily thermal and in the visible region, both effects are present. when the intensity
of the radiation is sufficiently high, damage to the absorbing tissue will result.
laser theory and operation
a basic understanding of how a laser operates helps in understanding the hazards when
using a laser device. figure 2 shows that electromagnetic radiation is emitted whenever a
charged particle such as an electron gives up energy. this happens every time an electron
dropings from a higher energy state, q1, to a lower energy state, q0, in an atom or ion as
occurs in a fluorescent light. this also happens from changes in the vibrational or
rotational state of molecules.
the color of light is determined by its frequency or wavelength. the shorter wavelengths
are the ultraviolet and the longer wavelengths are the infrared. the smallest particle of
light energy is described by quantum mechanics as a photon. the energy, e, of a photon
is determined by its frequency, n, and planck s constant, h.
e = h × v (1)
the velocity of light in a vacuum, c, is 300 million meters per second. the wavelength,
l, of light is related to n from the following equation:
n
l
c
= (2)
the difference in energy levels across which an excited electron dropings determines the
wavelength of the emitted light.
figure 2. emission of radiation from an atom by transition of
an electron from a higher energy state to a lower energy state
components of a laser
as shown in figure 3, the three basic components of a laser are:
· lasing material (crystal, gas, semiconductor, dye, etc...)
· pump source (adds energy to the lasing material , e.g. flash lamp, electrical
current to cause electron collisions, radiation from a laser, etc.)
· optical cavity consisting of reflectors to act as the feedback mechanism for light
amplification
figure 3. solid state laser diagram
electrons in the atoms of the lasing material normally reside in a steady-state lower
energy level. when light energy from the flashlamp is added to the atoms of the lasing
material, the majority of the electrons are excited to a higher energy level – a
phenomenon known as population inversion. this is an unstable condition for these
electrons. they will stay in this state for a short time and then decay back to their original
energy state. this decay occurs in two ways: spontaneous decay – the electrons simply
fall to their ground state while emitting randomly directed photons and stimulated decay
– the photons from spontaneous decaying electrons strike other excited electrons which
causes them to fall to their ground state. this stimulated transition will release energy in
the form of photons of light that travel in phase at the same wavelength and in the same
direction as the incident photon. if the direction is parallel to the optical axis, the emitted
photons travel back and forth in the optical cavity through the lasing material between the
totally reflecting mirror and the partially reflecting mirror. the light energy is amplified
in this manner until sufficient energy is built up for a burst of laser light to be transmitted
through the partially reflecting mirror.
as shown in figure 4, a lasing medium must have at least one excited (metastable) state
where electrons can be trapped long enough (microseconds to milliseconds) for a
population inversion to occur. although laser action is possible with only two energy
levels, most lasers have four or more levels.