Argon Ion Laser basics...

1. Term Paper
On
Argon ion laser
Ajay Singh
Physics Department, IIT Delhi

2. An introduction to laser: LASER is an acronym for "light amplification by the stimulated
emission of radiation.” Lasers radiate photons that are 'identical' in phase, direction, and
amplitude which produce a beam that is singularly directional, intense, monochromatic,
polarized, and coherent.
Three main Components of the Laser:
1. Active medium. The active medium consists of a collection of atoms, molecules, or ions
(in solid, liquid, or gaseous form) which is capable of amplifying light waves. Under normal
circumstances, there are always a larger number of atoms in the lower energy state than
in the excited energy state. An electromagnetic wave passing through such a collection of
atoms is attenuated.To have optical amplification, the medium has to be kept in a state of
population inversion, i.e., in a state in which the number of atoms in the upper energy level is
greater than that in the lower energy level—this is achieved by means of the pump.
2. Pumping source. The pump enables us to obtain such a state of population inversion
between a pair of energy levels of the atomic system. When we have a state of population
inversion, the input light beam can get amplified by stimulated emission.
3. Optical resonator. A medium with population inversion is capable of amplification;
however, for it to act as an oscillator, a part of the output energy must be fed back into the
system. Such feedback is brought about by placing the active medium in a resonator; the
resonator could be just a pair of mirrors facing each other.
Types of Lasers:
There are many types of lasers like solid state lasers , gas lasers, chemical lasers,
semiconductor lasers, dye lasers , metal vapour laser , Free electron laser ,gas dynamic lasers
and so on. Gas lasers also have many kind depending upon laser gain medium i.e. Helium-
neon laser, Argon laser, Krypton laser, Xenon ion laser, Nitrogen laser, Carbon dioxide laser,
Carbon monoxide laser, Excimer laserHere I am briefly discussing Argon laser.
A Brief Introduction to Argon Ion Lasers:
Although the argon ion laser was not the first laser invented, it has become one of the most
popular ion lasers in use today. Invented in 1964 by William Bridges at Hughes Aircraft, the
argon ion laser uses, as its name implies, high purity argon gas as the lasing medium. A
multi-line argon ion laser can generate up to 18 discrete laser lines (wavelengths) ranging
from the UV (275.4nm) to visible green (528.7nm) with the majority of the power being
developed at the 488nm and 514.5nm lines. Argon ion lasers are commercially available in a
variety of configurations to accommodate a wide variety of applications. Argon lasers may be
configured to produce a single laser line only or configured to produce multiple laser lines
simultaneously. They may also be fitted with polarizing optics to yield a polarized laser
beam. Additionally, argon ion lasers can be manufactured to produce optical power levels
ranging from a few milliwatts to power levels exceeding 20 watts.
The components of Argon Ion Laser:
The Plasma Tube
The heart of an argon laser is the plasma tube, and the key component of the plasma tube is
the bore. The design of the plasma tube / bore must be such that it can sustain extremely high
temperatures without damage while maintaining an excellent vacuum seal. The material of
choice for the bore of an argon ion laser plasma tube is BeO since it has a low vapor pressure
and can be produced with a high chemical purity. When properly sealed, a plasma tube

3. utilizing a BeO bore will allow the argon gas pressure within the tube to remain at its
approximate 1 torr level. In addition to its afore mentioned valuable properties, BeO is also
an excellent thermal conductor. As such, the large amount of heat, generated by the plasma
discharge within the bore, is readily conducted to the exterior of the BeO bore where it is then
removed by means of forced air cooling (low to medium power argon lasers) or flowing
water in a water jacket (high power argon lasers).
The Resonator Assembly
In order for the plasma tube to produce laser energy, the bore must function as part of an
optically resonant cavity. To accomplish this, mirrors are placed at each end of the bore
facing perpendicular to the length of the bore. As noted in the diagram below, one of these
mirrors is a highly reflective mirror while the other is only partially reflective. Slight
adjustments are then made to the angle of the mirrors until optimal alignment is achieved. In
older argon ion laser designs, both ends of the plasma tube were fitted with Brewster
Windows which allowed the optical energy within the bore to freely exit both ends of the
plasma tube. This type of plasma tube was then installed within a rather bulky resonator
assembly in which the mirrors were rigidly fixed to angle adjustable end plates. Once
achieved, proper alignment was maintained rather well due to the bulky and rigid design of
the resonator assembly.
Sealed Mirror Technology
Today, most argon ion lasers have done away with the bulky resonator assembly. Instead, the
mirrors are permanently bonded, in a vacuum tight manner, to specially designed mounts
directly at each end of the plasma. Thus mounted, these mirrors eliminate the need for
Brewster Windows, and the required optically resonant cavity is thus achieved with
significant reductions in bulk, size, and weight. These "sealed mirror" laser tubes are not only
less bulky and smaller, but they also provide greater long term alignment stability and are less
susceptible to misalignments during transit.
The Power Supply
This power supply is required to supply an initial triggering pulse (6KV to 8KV) to initiate
the plasma discharge, as well as to maintain the plasma discharge. For small to medium size
argon ion lasers, the power supply may be required to deliver up to 12 amps of DC current at
up to 140VDC. For large argon ion lasers, the current may be as much as 45 amps of DC
current at up to 600VDC. Today, state-of-the-art ion laser power supplies operate at very high
efficiencies (>93%), provide Power Factor Correction, and will operate over a wide AC line
voltage range (typically 95VAC to 265VAC).
How does a Argon ion laser work?
a) Atomic structure, radiation, and emission: Radiant emission and absorption take place
within the atomic or molecular structure of materials. Each electron bound to the atom (or
molecule)occupyone of manypossible energylevels,the lowestcalled the ground state. If an atom
isin itsgroundstate,it will staythere until itisexcited by external forces. An electron in an excited
state will radiate a photon to return to a lower energy level because it must conserve energy.
Transition from one energy level to another happens when the atom either absorbs or emits a
photon. This occurs only when the energy of the photon exactly matches the energy difference
between the two different levels.

4. The two types of radioactive decay (decay is a transition to a lower level) are spontaneous
emission and stimulated emission. Spontaneous emission occurs naturally when there is
external perturbation applied to the excited atom or molecule. Stimulated emission is a bit
different. An electron in an excited state, energy E2 can be stimulated to decay to a lower
energy level, energy E1 by the interaction of a photon of energy h =E2-E1. This incoming
photon 'hits' the electron which then decays to E1 producing two final photons of identical
phase, frequency, and direction. A laser takes advantage of absorption, spontaneous and
stimulated emission to create conditions favourable to light amplification or optical gain.
Fig: spontaneous and stimulated emission[Ref. given below]

5. b) Population inversion
The populations of electrons in E2 and E1 are denoted by N2 and N1, respectively. When a
material is in equilibrium, Boltzmann statistics describe the system and predict that nearly all
particles are in the ground state. If enough light of energy h is supplied, the population can
be driven until N2=N1. Under these conditions, the rates of absorption and stimulated
emission are equal. Because every upward transition is matched by one in the opposite
direction, it is impossible (with incoherent excitation) to drive the population beyond
equality, i.e., N2 cannot exceed N1.
[Ref. given below]
However, if three or more energy levels exist, it is possible to create a population inversion
by a cascade process in which for two energy levels N3>N2 and stimulated emission will be
stronger that stimulated absorption at the frequency =(E3-E2)/h.
c) Argon as an excitation medium
The neutral argon atom is pumped to the 4p energy level -the origin of the lasing transition-
by two collisions with electrons. The first ionizes the atom and the second excites it from the
ground state E1 either directly to the 4p level (E3) or to E4, from which it cascades almost
immediately to 4p. The 4p ions eventually decay to 4s (E2), either spontaneously or when
stimulated to do so by a photon of appropriate energy. The wavelength of the photon depends
on the specific energy levels involved, but will be between 400 and 600 nm. The ion decays
spontaneously from 4s to the ground state emitting an ultraviolet photon, about 74 nm.

6. [Ref. given below]
As the above figure shows, there are many competing energy bands. These can be
preferentially selected using a prism in front of one end mirror. This prism selects a specific
wavelength to send back through the cavity to stimulate identical emissions, which stimulates
more, etc. This describes single line operation. Hence the laser can be tuned to different
wavelengths. Removal of the prism allows for broadband operation, that is, several
wavelengths are kept rather than keeping only a particular wavelength. The mirrors reflect a
number of lines within a range of about 70 nm maximum.
[Ref. given below]
Other things affect the laser.
A magnetic field produced by a solenoid envelopes the plasma and enhances population
inversion. It tends to force the free electrons toward the center of the tube, increasing the
probability of a pumping collision.
 Proper pressure inside the cavity must be maintained to optimize the 'gain.' Varying the
number of argon atoms in the volume will change the time between collisions with free
electrons, which varies the average electron energy. The pressure-balanced design of the
plasma tube brings stability to this argon ion laser.

7. [Ref. given below]
Properties of Argon ion laser:
As I have discussed earlier Argon laser has argon ions as gain medium and electrical
discharge as pumping source.Since in case of Argon there are multiple transitions possible ,
so argon laser emits multiple wavelengths correspondingly . An argon laser mainly emits
lines of wavelength 454.6 nm, 488.0 nm, 514.5 nm (351 nm, 363.8, 457.9 nm, 465.8 nm, 476.5 nm,
472.7 nm, 528.7 nm, also frequency doubled to provide 244 nm, 257 nm) between UV to Light .
Applications of Argon ion laser:
Argon ion lasers are used in a wide variety of applications. These applications include, but
are not limited to, Raman Spectroscopy, Microscopy, Flow Cytometry, Holography,
Entertainment, Forensics, Ophthalmic Surgery and sources for optical pumping. Argon ion
lasers are also used extensively in scientific, research and educational applications. Although
advances in laser technology over the years have lead to the development and
commercialization of numerous additional laser light sources, the argon ion laser has been,
and will continue to be, a predictable and reliable laser light source for many applications.
References : http://www.chem.ufl.edu/~itl/4411L_f00/i2_lif/ar_laser.html
http://www.analytical-online.com/Application%20Notes/Lasers/Argon%20Ion%20Basics.pdf
http://www.rp-photonics.com/argon_ion_lasers.html
http://en.wikipedia.org/wiki/Ion_laser#Argon_laser