This includes basic laser principles which are required to understand working of any laser system. It consists of the terms like excitation, de-excitation, population inversion, optical resonator, pumping source, active system and basic laser action.

2. Contents:
• Basic laser principles
• Laser Types: Semiconductor Laser diode and Nd-YAG laser
• Applications of Lasers: Laser printer, drilling and cutting

3. LASER:
• LASER is a device which produces highly intense light through the
process of Stimulated Emission
• Properties:
• Highly monochromatic
• Highly coherent (Temporal as well as Spatial)
• High intensity (more bright)
• High unidirectionality (low beam divergence)
Light Amplification by Stimulated Emission of Radiation

4. Basic Principles of LASER :
Stimulated Absorption (Excitation)
It is the process of transition of an electron from normal state
(ground state) to excited state due to absorption of energy
Before During After
hυ
. .
.
E1
E2

5. Basic Principles of LASER :
Spontaneous Emission
• In this process electron undergo transition from higher state to lower state
spontaneously by emitting a photon of energy hυ
• This is natural and instantaneous process, independent of outside circumstances
• Photons emitted by spontaneous emission process are random in direction and hence
they have random phase. Thus, the radiation is incoherent and has a broad spectrum

6. Basic Principles of LASER :
Stimulated Emission
• The forced transition of electron from higher state to lower state is referred to be as stimulated
emission
• In this process, an atom in excited state is triggered or stimulated by incident energy of suitable
value
• A photon (having same energy to that photon which would be emitted due to de-excitation)
applied to excited system stimulates de-excitation process.
• During this process a photon is emitted having energy hυ= (E2-E1) along with one more photon of
same energy

7. Features of Stimulated Emission
• Photons emitted are identical with each other
• Photons have same energy, frequency, wavelength, direction
• Thus, radiation produced by stimulated emission results into a
monochromatic, coherent and unidirectional beam of light.
• Also, a stimulated emission initiated by a photon produces two same
photons, these two photons de-excites two more electrons and give
out four photons.
• Stimulated emission caused by these four photons gives away eight
photons and so on.
• This way number of photons increases which in turn increases the
intensity of light

8. Basic Principles of LASER :
• Metastable state: An excited state having lifetime more than 10-8sec
• Pumping Source: It is a type of energy required to transfer electrons from
lower to excited state. e.g. electrical or optical depending upon type of
laser.
• Population Inversion: An artificial situation created in which we get N2>N1
• Optical Resonator: It is a pair of reflecting surfaces, one completely
reflecting and other partially reflecting

9. Basic Principles of LASER :
• Active system or Active medium:
collection of atoms, ions or molecules
e.g. He-Ne laser: mixture of Helium and Neon gases
Ruby Laser: Ruby crystal in the form of rod
• In general, active system may be gas, liquid or solid and its
physical state decides the type of laser

10. Basic Principles of LASER :
Basic Laser action
Due to optical resonator, these photons are reflected back and forth millions of times. As a result of this,
amplification takes place (i.e. intensity increases)
Finally, through partially reflecting surface beam emerges out in the form om of laser beam