4. Historical facts
The Helium-Neon laser was the first continuous laser.
It was invented by Javan et. al. in 1961. But how did Javen manage to do this?
This shows that it is no coincidence that Javen's first He-Ne laser oscillated at a
Wavelength of 1.5μm, since the amplification at this wavelength is considerably
Higher than the 632 nm line which is reached at what is now commonly known as the red line,
which was made to oscillate only one year later by White and Ridge.
The similarity between the manufacturing techniques of He-Ne lasers and electron valves
helped in the mass production and distribution of He-Ne lasers..
It is now clear that He-Ne lasers will have to increasingly compete with laser diodes in the
future. But He-Ne lasers are still unequalled as far as beam geometry and the purity of the modes
are concerned. Laser diodes will have to be improve to a great extent before they pose a serious
threat to helium-neon laser
5.
6.
7. Introduction
A helium-neon laser, usually called a He-Ne laser, is a type of
small gas laser. He-Ne lasers have many industrial and
scientific uses, and are often used in laboratory
demonstrations of optics.
He-Ne laser is a four-level laser.
Its usual operation wavelength is 632.8 nm, in the red portion
of the visible spectrum.
It operates in Continuous Working (CW) mode.
8.
9. Construction of He-Ne laser
The setup consists of a discharge tube of length 80 cm and bore diameter of 1.5cm.
The gain medium of the laser, as suggested by its name, is a mixture of helium and
neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm
of cavity length ) in a glass envelope.
The energy or pump source of the laser is provided by an electrical discharge of around
1000 volts through an anode and cathode at each end of the glass tube. A current of 5
to 100 mA is typical for CW operation.
The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one
end of the laser tube, and a concave output coupler mirror of approximately 1%
transmission at the other end.
HeNe lasers are normally small, with cavity lengths of around 15 cm up to 0.5 m, and
optical output powers ranging from 1 mW to 100 mW.
13. He-Ne Energy level diagram
The left side of the representation shows the lower levels of the
helium atoms.The energy scale is interrupted and that there is a
larger difference in energy in the recombination process than is
evident in the diagram.
A characteristic of helium is that its first states to be excited, 21S1
and 21S0 are metastable, i.e. optical transitions to the ground state
11S0 are not allowed, because this would violate the selection rules
for optical transitions. As a result of gas discharge, these states are
populated by electron collisions
A collision is called a collision of the second type if one of the
colliding bodies transfers energy to the other so that a transition from
the previous energy state to the next higher or lower takes place.
Apart from the electron collision of the second type there is also the
atomic collision of the second type. In the latter, an excited helium
atom reaches the initial state because its energy has been used in the
excitation of a Ne atom. Both these processes form the basis for the
production of a population inversion in the Ne system.
14.
15. Working of He-Ne laser
A description of the rather complex HeNe excitation process can be given
in terms of the following four steps.
(a)When the power is switched on, An energetic electron collisionally
excites a He atom to the state labeled 21So . A He atom in this excited
state is often written He*(21So), where the asterisk means that the He
atom is in an excited state.
(b) The excited He*(21So) atom collides with an unexcited Ne atom and
the atoms exchange internal energy, with an unexcited He atom and
excited Ne atom, written Ne*(3s2), resulting. This energy exchange
process occurs with high probability only because of the accidental near
equality of the two excitation energies of the two levels in these atoms.
Thus, the purpose of population inversion is fulfilled.
16. Advantages of helium-neon laser
Helium-neon laser emits laser light in the
visible portion of the spectrum.
High stability
Low cost
Operates without damage at higher
temperatures
18. Applications of helium-neon lasers
Helium-neon lasers are used in industries.
Helium-neon lasers are used in scientific
instruments.
Helium-neon lasers are used in the college
laboratories.
