Helium Neon Laser

Helium-Neon laser consists of a tube 0.5 m long and 5mm in diameter filled with Helium and Neon gas at a pressure of about 1 mm of Hg. Helium and neon are in the ratio 5:1. Two electrodes are inserted to provide potential difference to the gas for electric discharge. The sides of the tube are cut at Brewster’s angle as shown in the figure below.

Construction of H-Ne Laser

Two mirrors, one 100% reflecting and other 99% reflecting are placed across each the tube perpendicular to the axis. These mirrors are at a distance of integral multiple of λ.

Firstly the electrons are accelerated by applying voltage. These electrons collide with the helium atom and excite them to 2s energy level where the energy is -20.61 eV. These atoms then collide with Neon atoms and get excited to 5s level where the energy is – 20.6eV which spontaneously return to 3p level having energy -18.7 eV. The photon which is parallel to the axis of tube suffers multiple reflections producing number of stimulated emission. 1% of these stimulated emissions emerge out the mirror as laser light.

The laser light thus obtained is intense, monochromatic and extremely unidirectional. The wavelength of laser light is 632.8nm. In this way, H-Ne laser is made.

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