c. Lasing begins when an excited atom spontaneously emits a photon parallel to
the axis of the crystal (B). (Photons emitted in other directions merely pass out of the
crystal.) The photon stimulates another atom in its path to contribute a second photon,
in step, and in the same direction. This process continues as the photons are reflected
back and forth between the ends of the crystal. (We might think of lone soldiers falling
into step with a column of marching men.) The beam builds up until, when amplification
is great enough (C), it flashes out through the partially silvered mirror at the right--a
narrow, parallel, concentrated, coherent beam of light, ready for many applications.
d. Applications that might be encountered in military use are target acquisition,
range finding, and research. Lasers are also found in many locations in the hospital.
e. Laser equipment available today will operate in one of the following
(1) Continuous wave (CW). A continuous beam of light is produced.
(2) Pulsed laser. A class of laser characterized by operation in a pulsed
mode, i.e., emission occurs in one or more flashes of short duration (pulse length).
(3) Q-switched laser (also known as Q-spoiled). A pulsed laser capable of
extremely high peak powers for very short durations (typical pulse length of 5 to 50
billionths of a second).
f. The output of a laser is measured in either watts or joules, depending on the
operating configuration. Units of power are used primarily to rate CW lasers since the
energy delivered per unit time remains relatively constant (output measured in watts).
However, pulsed lasers have a peak power significantly greater than their average
power and produce effects, which may best be categorized by energy output per pulse.
The output of low power CW lasers is usually expressed in milliwatts (mW = 1/1,000
watts, pulsed lasers in kilowatts (kW = 1,000 watts), and Q-switched pulsed lasers in
megawatts (MW = million watts) or gigawatts (GW = billion watts). Pulsed energy
output is usually expressed in joules. The joule is a unit of energy used in describing a
single pulsed output of a laser. It is equal to one watt-second or 0.239 calories.
a. Laser radiation should not be confused with ionizing radiation (such as x and
gamma rays). The biologic effects of the laser beam are essentially those of visible,
ultraviolet, or infrared energy upon tissues. However, the intensity of the light is of
magnitudes that could previously be approached only by the sun, nuclear weapons,
burning magnesium, or arc lights. This is one of the important properties that make
lasers potentially hazardous.
b. A laser beam striking tissue will be reflected, transmitted, and/or absorbed.
The degree to which each of these reactions occurs depends upon various properties of