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He Ne LASER

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OBJECTIVE: To produce a monochromatic, coherent beam of light

PRINCIPLE:

1. STIMULATED EMISSION: photons of the same phase, frequency and polarization is emitted by excited atoms when a photon is passed through them due to electromagnetic resonance

2.POPULATION INVERSION: a condition when no. of excited atom > ground state atoms


APPARATUS:


1. a vacuum tube with He Ne mixture (9:1) at 1 mm Hg

2. a high voltage supply (20 kV)

3. a completely reflective mirror and a partially reflective mirror


WORKING MECHANISM

1.the electric discharge excites He atoms to 2s (20.61 eV) energy level. it is a metastable state ie lifespan of this excited state is larger. 

2. the abundant He atoms by collision excite the Ne atom to 5s (20.66eV) energy level. thus, He helps to obtain population inversion of Ne.


3. the Ne atom drops to ground level through the following sequence of emissions;

i. laser transition; emits a photon of 6.32x 10^-7m wavelength

ii. spontaneous emission; emits non-coherent photons

iii. radiationless emission

4. the emitted photons are reflected by mirrors, increasing the intensity through optical pumping

5. finally coherent intense red light is emitted by a partially reflective mirror.