Learning About Lasers
The Unique Nature of Laser Radiation
A. Coherent vs Non-Coherent Radiation Sources
The laser is unique in that it creates a radiation beam that
is coherent (in-phase).
In a coherent light source, the amplitude of the radiated
waves is added (constructive interference) and results in
a radiation beam of great intensity. Non-coherent radiation
sources (like a light bulb) produce radiation that is out
of phase. This results in the reduction of the amplitude by
cancellation of overlapping wave forms (destructive interference).
The intensity of coherent radiation sources normally exceed
the intensity of non-coherent sources by orders of magnitude.
B. Monochromatic Radiation Sources
Many sources produce a broad range of radiation wavelengths.
Lasers will normally produce only one or two wavelengths.
The single wavelength is called monochromatic radiation and,
depending on the type of laser, the radiation produced can
fall anywhere in the electromagnetic spectrum between 10 nm
(extreme ultraviolet) and 1 mm (far infrared). Monochromatic
radiation does not scatter much (as does polychromatic radiation)
when interacting with lenses or mirrors (chromatic aberration).
This reduction in scattering can result in very intense specular
or diffuse reflections.
C. Irradiance (Power Density) and Continuous Wave (CW) Lasers
An important factor in determining the hazard of continuous
wave lasers is the irradiance (power density) of the laser
beam. Irradiance is normally expressed in W/cm2 and is a function
of the beam power divided by the beam area. Beam area is dependent
on: the beam size at the aperture, the divergence (spreading)
of the beam and the distance from the aperture. Focusing or
defocusing the laser will dramatically affect the irradiance.
Obviously, the greater the irradiance, the greater the potential
hazard.
D. Radiant Exposure (Energy Density) and Pulsed Lasers
Not all lasers are operated in a continuous wave mode. Many
operate in a pulsed mode with a pulse duration and a pulse
repetition frequency. These lasers cannot be characterized
by their irradiance and we instead refer to their radiant
exposure (energy density) that is expressed in J/cm2. Radiant
exposure is a function of power density, pulse duration and
pulse frequency. Again, the greater the radiant exposure,
the greater the hazard. The averaged power (pulses/sec x J/pulse
= J/sec or Watts) of a pulsed laser will usually be less than
a CW laser, however the peak power in the pulse may be very
large if the pulse duration is very short.
Understanding the Laser
A. Basic Operation of the Laser
The basic operating concept of the laser is very simple.
Electrons in the atoms of the lasing medium are moved from
a ground state into a higher energy state by absorbing energy
from an energetic excitation source. For the laser to work,
more electrons must be an excited state than in a ground state
(population inversion). When these electrons descend to their
ground state, photons of a specific (monochromatic) wavelength
are emitted in a process called "spontaneous emission."
These photons are allowed to oscillate inside a mirrored resonator.
This increases the laser radiation intensity through stimulating
the emission of additional photons with the same wavelength
and phase. Finally, the photons are allowed to escape via
an output coupler (semi-mirrored mirror) as an intense laser
beam. See Diagram 1
B. Types of Lasing Media
Lasing media can be solids, liquids, or gases. The type of
medium dictates the wavelength of the laser beam. Some media
can be manipulated to allow for tuning of the wavelength.
Solid state media (polished crystal rods), gases or vapors
(sealed in a glass tube), liquid dyes, and semiconductors
(laser diodes) are all common lasing media. Halogen gases
mixed with noble gases can combine in an excited state to
create pseudo molecules called "excited dimers"
or excimers. Excimer lasers emit laser radiation in the ultraviolet
region of the spectrum. It is also possible to use an accelerated
beam of free electrons as a lasing media. Free electron lasers
(FEL) use a "wiggler" magnet to propagate photons
from the electron beam. See Appendix C for a listing of laser
types (media) typically found at UCB.
C. Types of Excitation Sources
Flashlamps, plasma discharge tubes, high voltage current
and radio frequency devices are all energy sources used to
excite the lasing media. Some laser beams are used to "pump"
(excite) other lasers (liquid dyes, Ti-Sapphire, etc.). It
is important to remember that the excitation device itself
can present a serious non-beam hazard (radiation, electrical,
etc.).
All of the above information is from Dewey Sprague, Laser
Safety Officer, at the University of California, Berkeley.
He is a highly respected authority in laser safety and we
are grateful for the information that he provides to laser
users around the country and worldwide.
