stands for ‘light amplification by the stimulated
emission of radiation’. Quantum physics provides
the principle on which LASER emission is produced.
Quantum theory says that sometimes an atom absorbs
energy and goes into an excited state. This state
is short-lived and the atom returns to its resting
state, releasing spontaneously a bundle of energy
or photon which travels in a random direction.
This is called the spontaneous emission of energy.
Examples are light emitted by the sun or a bulb.
The theory further says that photon is the basic
unit that constitutes light.
When millions of atoms in a small container
are simultaneously stimulated, what results is
a laser made up of photons, all of which have
the same wave length and direction.
Laser light, unlike ordinary light, is collimated,
coherent and monochromatic.
Collimated means all laser waves travel in the
same direction with very low divergence over
long distances. Ordinary light waves diverge
widely and become dimmer as they travel further.
Laser beams remain pencil thin and retain their
intensity even after traveling far.
Coherence means all laser waves always travel
together in the same direction. This means all
laser waves have same wave length. Coherence
increases the amplitude or power, increasing
the radiance of a laser beam.
Monochromatic means laser light is of one color
or wave length or composed of a marrow range
of colors or wave lengths. For laser to be coherent
it must be monochromatic.
Lasers come in different wave lengths. They
can be visible and invisible and belong to the
infrared, visible or ultra violet sections of
the electro-magnetic spectrum.
Properties of laser energy
In laser applications, two properties are critical.
They are power and energy density also known
as irradiance and fluence respectively.
Power density or irradiance is energy per area
of application and is expressed as watt/sq.cm.
A small area increases power density and bigger
area lowers it. Tissue cutting needs high irradiance
and coagulation low irradiance.
Energy density or fluence is the power produced
at any time on unit area. It is measured as watts
x time and expressed as joules/sq.cm. Quantity
of joules is a meaningless statistic since 100
joules of energy can be obtained by 100 watts
applied for 1 second or by applying 1 joule for
100 seconds. The first application will produce
marked tissue response while the second would
produce gradual tissue warming.
Laser devices and types
All medical lasers consist of some basic components
like the housing unit or optical resonator which
contain the medium to be excited, a fully and
a partially reflective mirror at both ends of
the housing unit, an excitation source and a
power source like, electricity or another laser.
Laser types are named after the material, whose
atoms are excited to produce the laser. This
material is called the laser medium. Laser mediums
can be solid, liquid or gas.
Solid mediums include certain crystals like,
ruby, doped with rare earth or metal ions like,
ytterbium and titanium. Liquid mediums are certain
dye solutions used in pulse dyed lasers. Gas
mediums include CO2, helium and neon mixture,
argon and metal vapors. For safety reasons the
invisible lasers like, CO2 are linked to visible
lasers like, HeNe laser, to make them visible.
Transmission, reflection, absorption and scattering
Laser like light interacts with tissues in four
ways; transmission, reflection, absorption and
scattering. The laser chosen depends on the outcome
desired. Argon laser is used in eye surgery because
of its high transmissibility. It reaches the
retina without damaging the intervening tissues.
In surgery, absorption and scattering by the
tissues are the more desired outcomes. If tissues
need cutting then lasers which are absorbed are
used. If bleeding has to be controlled, then
lasers which are scattered are used. Scattering
reduces power intensity and allowing coagulation
rather than cutting.
CO2 laser is an efficient cutting tool but ineffective
in fluid containing cavities since it is absorbed
by water. It is also ineffective in controlling
bleeding since most of the energy is absorbed.
Mode of delivery and applications
Laser is delivered in the continuous, and single
or repeat pulse mode. Continuous mode delivers
laser, without interruptions, at selected wattage
for as long as wanted. In single pulse, laser
at a selected wattage and duration is delivered
once. In repeat pulse, the single delivery is
repeated. The pulse mode is generally preferred
in precision surgery because it causes less thermal
The highly collimated laser beam, which can
be focused to a microscopic dot of extremely
high energy density, finds applications in many
fields including surgery, telecommunication,
the welding and cutting industry, laser printing,
CDs, optical discs, and laser heating.