He-Ne LASER

TOPICS TO BE DISCUSSED

HISTORY CHARACTERSTICS CONSTRUCTION PRINCIPLE AND WORKING LIMITATIONS APPLICATIONS

HISTORY

The He-Ne laser was the first laser to be operated successfully and was fabricated by Ali Javan and his coworkers in Bell Telephone laboratories in U.S.A in 1961.

CHARACTERSTICS It is a four level laser.Therefore population

inversion can be easily obtained.

It works in continuous mode(CW mode)

The power output of He-Ne laser lies in the range 1-50mW with input electrical power of 10 watt.

The laser is capable of supplying continuous laser beam without need of elaborate cooling arrangement.

The light from He-Ne Laser is more directional and much more monochromatic as compared to that from solid state Laser.

The He-Ne laser employs electrical pumping.

CONSTRUCTION The He-Ne laser consists of

a long and narrow discharge tube of length=25 cm to 100 cm and diameter=1cm.It is filled with He and Ne gases at a pressure of about 1mm of Hg(for He)and 0.1 mm of Hg(for Ne).

The gas mixture of He &Ne forms the lasing medium and is enclosed between a set of mirrors forming a resonant cavity.

A stationary glow discharge is excited in the gas by applying a few kilovolt d.c supply.

PRINCIPLE &WORKING IN this laser , the neutral

neon atoms form the active laser material while the helium acts as a buffer gas and is used to selectively populate the levels of neon .

When the discharge is passed through gas mixture, electrons near the cathode are accelerated towards the anode. Depending upon the applied voltage and pressure of gas the accelerated electrons acquire enough energy to excite the helium atoms to 21S and 23 S states.

The 2 1S and 23S levels of helium have relatively long life (i.e. they are metastable levels). Since the only lower lying state is a singlet ground state (11S), no transition from states 23S to 11S is optically allowed. These metastable states of helium nearly coincide in energy with the 3S & 2S states of Ne. The closeness of levels indicates that collisions of second kind are effective in resonant energy transfer from He atoms to Ne atoms. The probability of this energy transfer is very high since The 21S and 23S levels of helium are metastable

The excitation process can be represented as

Helium+electron=Helium*

Helium*+Neon = Helium+Neon*

These excitation processes show the pumping mechanism of He & Ne laser .The 3S and 2S bands of Ne play the role of the upper laser levels while 3P& 2P are the corresponding lower levels.

The main transitions are 3S to 3P (3.39 um), 2S to 2P (1.15 um) and 3S to 2P(6328 Å).

The lower lasing levels 3P & 2P are relaxed to 1S levels by spontaneous emissions .However ,1s level is metastable level and affects the efficient working of the laser. This problem is minimized by reducing the diameter of the discharge tube. Since 1S is metastable level,the Ne atoms in 1S state collide with the walls of the discharge tube and get de-excited to the ground state of Ne. This cycle continues and He-Ne laser gives out light continuously (CW-mode).

LIMITATIONS The mirrors used in the internal

cavity lasers get eroded by gas discharged and have to be replaced . The other configuration as shown in fig.using external mirror cavity provides greater flexibility. However external mirror cause a loss of light energy due to reflection at the ends of the discharge tube. This can be avoided by using

1. Brewster windows at the two ends of discharge tube.When such windows are used the output laser beam is polarized in the plane of the figure.

2. Spherical mirrors in the co nfocal arrangement (i.e.cavity length is equal to radius of curvature of each mirror).In this configuration, diffraction losses are minimum and alignment is much less critical .

APPLICATIONS:-

1. They are often used in integrated bar code readers.

2. He-Ne laser are used in laser surgery to position the powerful infrared cutting beam.

3. Surveyors take advantage of good quality of He-Ne laser beam to take measurement over long distance.

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