Dr. Hossam Eldin Sayed Ali
Lecturer of medical Biophysics
The Research Institute of Ophthalmology
What Is LASER
The acronym LASER stands for Light Amplification by
Stimulated Emission of Radiation
The Laser concept was first demonstrated in the microwave
region in 1954 by Charles Townes and co-workers.
They projected a beam of ammonia molecules through a
system of focusing electrodes. When microwave power of
appropriate frequency was passed through the cavity,
amplification occurred and the term microwave
amplification by stimulated emission of radiation
(M.A.S.E.R.) was born.
The term laser was first coined in 1957 by physicist Gordon
A year later, Townes worked with Arthur Schawlow and the
two proposed the laser, receiving a patent in 1960. That
same year, Theodore Maiman, a physicist at Hughes
Research Laboratories, invented the first practical laser.
This laser was a solid state type, using a pink ruby crystal
surrounded by a flash tube enclosed within a polished
aluminum cylindrical cavity cooled by forced air.
The ruby cylinder was polished on both ends to be parallel
to within a third of a wavelength of light. Each end was
coated with evaporated silver. This laser operated in pulsed
mode. Two years later, a continuous ruby laser was made by
replacing the flash lamp with an arc lamp.
components of a laser producing
The fundamental components of a laser producing device
a. Energy source
b. Optical (Resonating) cavity
c. Active lasing medium
d. Cooling system
e. Delivery system
The properties of laser radiation
Laser light has four unique characteristics
that differentiate it from ordinary light:
Electron transition in ordinary light sources is
random in time. The photons emitted from ordinary
light sources have different energies, frequencies,
wavelengths, or colors. Hence, the light waves of
ordinary light sources have many wavelengths.
Therefore, photons emitted by an ordinary light
source are out of phase.
In laser, the electron transition occurs artificially in
specific time. All the photons emitted in laser have
the same energy, frequency, or wavelength. Hence,
the light waves of laser light have single wavelength
or color. Therefore, the wavelengths of the laser light
are in phase in space and time., a large amount of
power can be concentrated in a narrow space.
In conventional light sources photons travel in
random directions. Therefore, these light sources
emit light in all directions.
In laser, all photons will travel in same direction.
The width of a laser beam is extremely narrow.
Hence, a laser beam can travel to long distances
If an ordinary light travels a distance of 2 km, it
spreads to about 2 km in diameter. A laser light
travels a distance of 2 km, it spreads to a diameter
less than 2 cm.
Monochromatic light means a light containing a
single color or wavelength. The photons emitted
from ordinary light sources have different energies,
frequencies, wavelengths, or colors.
In Laser, all the emitted photons have the same
energy, frequency, or wavelength. Hence, the light
waves of laser have single wavelength or color.
Therefore, laser light covers a very narrow range of
frequencies or wavelengths.
The intensity of a wave is the energy per unit time
flowing through a unit normal area. In an ordinary
light source, the light spreads out uniformly in all
directions. If you look at a 100 Watt lamp filament
from a distance of 30 cm, the power entering your
eye is less than 1/1000 of a watt.
In laser, the light spreads in small region of space
and in a small wavelength range. Hence, laser light
has greater intensity when compared to the ordinary
Thus, even a 1 Watt laser would appear many
thousand times more intense than 100 Watt
Types of LASER
There are many types of LASERs available for different
purposes. Depending upon the sources they can be
described as below.
Solid State LASER In this kind of LASERs solid state,
materials are used as active medium. The solid state
materials can be ruby, neodymium-YAG (yttrium
aluminum garnet) etc.
Gas LASER These LASERs contain a mixture of helium
and Neon. This mixture is packed up into a glass tube.
It acts as active medium. We can use Argon or Krypton
or Xenon as the medium. CO2 and Nitrogen LASER can
also be made.
Dye or Liquid LASER In this kind of LASERs organic dyes
like Rhodamine 6G in liquid solution or suspension used as
active medium inside the glass tube.
Excimer LASER Excimer LASERs (the name came from
excited and dimers) use reactive gases like Chlorine and
fluorine mixed with inert gases like Argon or Krypton or
Xenon. These LASERs produce light in the ultraviolet range.
Chemical LASER A chemical laser is a LASER that obtains its
energy from a chemical reaction. Examples of chemical lasers
are the chemical oxygen iodine laser (COIL), all gas-phase
iodine laser (AGIL), and the hydrogen fluoride laser,
deuterium fluoride laser etc
Semiconductor LASER In these lasers, junction diodes are
used. The semiconductor is doped by both the acceptors and
donors. These are known as injection laser diodes. Whenever
the current is passed, light can be seen at the output.