Helium-Neon Laser TIT GROUP Of INSTITUTIONS, BHOPAL [M.P.] By Dr. Namrata Yaduvanshi Engg. Physics

Outlines Historical facts Introduction Construction of He-Ne Laser He-Ne Energy Level Working of He-Ne Laser Advantages and Disadvantages of Laser Applications of He-ne Laser

Historical facts 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 1.5μm, 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 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 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

Introduction A helium-neon laser, usually called a He-Ne laser, is a type of small gas laser. HeNe 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.

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.

Fig.1. Laser Daigram

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.

Fig.2. Excitation and Laser process for the visible Laser emission

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. Conti…..

Conti….. When the excited Ne atom passes from metastable state(3s) to lower level(2p), it emits photon of wavelength 632 nm. This photon travels through the gas mixture parallel to the axis of 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 (3s) level to (2p) level is laser transition. This process is continued and when a beam of coherent radiation becomes sufficiently strong, a portion of it escape through partially silvered end. The Ne atom passes to lower level 1s emitting spontaneous emission. and finally the Ne atom comes to ground state through collision with tube wall and undergoes radiationless transition.

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

Disadvantages of helium-neon laser Low efficiency Low gain Helium-neon lasers are limited to low power tasks

Applications of He-Ne laser The Narrow red beam of He-Ne laser is used in supermarkets to read bar codes. The He- Ne Laser is used in Holography in producing the 3D images of objects. He-Ne lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics.

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