Last updated on Wednesday, May 22nd, 2024
Introduction He-Ne LASER
The Helium-Neon laser was the first continuous laser. It was invented by Javan et. al. in 1961. But how did Javan manage to do this? This shows that it is no coincidence that Javan’s first He-Ne laser oscillated at a Wavelength of 1.5 μm 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 Ridgen. 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 unequaled as far as beam geometry and the purity of the modes are concerned. Laser diodes will have to be improved to a great extent before they pose a serious threat to helium-neon laser
He-Ne laser have many industrial and scientific uses and is 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.
Construction of He-Ne laser
The setup consists of a discharge tube of length 80 cm and a 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 of 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, a high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. He-Ne 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.
He-Ne Energy level diagram
The left side of the representation in the diagram shown in the video tutorial is the lower levels of the helium atoms. The energy scale is interrupted and 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 in which one of the colliding bodies transfers energy to the other atom. A transition from the previous energy state to the next higher takes place. There are two ways; in one an electron collides with the He atom and excites to the He atom. This excited helium atom reaches the ground by colliding with the Ne atom. Both these processes form the basis for the production of a population inversion in the Ne system.
Working of He-Ne laser
A description of the rather complex He-Ne 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.
Transition of Atoms in Energy levels
When the excited Ne atom passes from a metastable state(3s) to a lower level (2p), it emits a photon of wavelength 632 nm. This photon travels through the gas mixture parallel to the axis of the tube, it is reflected back and forth by the mirror ends until it stimulates an excited Ne atom and causes it to emit a photon of 632nm with the stimulating photon. The stimulated transition from the (3s) level to the (2p) level is a laser transition. This process is continued and when a beam of coherent radiation becomes sufficiently strong, a portion of it escapes through a partially silvered end. The Ne atom passes to lower level 1s emitting spontaneous emission. Finally, the Ne atom comes to the ground state through collision with the tube wall and undergoes radiationless transition.
Principle, Construction and Working of He-Ne LASER
Applications of He-Ne laser
The Narrow red beam of the He-Ne laser is used in supermarkets to read bar codes.
The He-Ne Laser is used in Holography to produce 3D images of objects.
He-Ne lasers have many industrial and scientific uses and are often used in laboratory demonstrations of optics.
The inelastic atom-atom collision Resonator cavity:
it is made of a full reflector and partially reflecting plates parallel with high accuracy He-Ne gas laser is the four energy levels in which first He atoms are excited to bring the Ne atoms in the required excited state by the atom-atom inelastic collision pumping process, the population inversion is achieved between the metastable and ground states.
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