Helium-Neon Laser

A Helium-Neon laser usually called a He-Ne laser, is a type of a small gas laser.
He-Ne lasers have many industrial and scientific uses and are often used in laboratory demonstration of optics.
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 pressures in a glass envelope.
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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.
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A current of 5-100 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.
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The laser process in a HeNe laser starts with collission of electrons from the electrical discharge with the He atoms in the gas.

This excites He from the ground state to the 23 S1 and 21 S0 long-lived metastable excited states. Collision of the excited helium atoms with the ground-state Neon atoms, exciting neon electrons into the 3s2 level.

This is due to a coincidence of energy levels between the helium and neon atoms.

Image courtesy: mi-lasers.com

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