This document discusses lasers and their applications. It begins by defining a laser as a device that generates light through stimulated emission. Typical applications of lasers mentioned include using a semiconductor laser to read compact discs by focusing the laser beam onto the disc's reflective layer. Another application is using lasers as light sources for fiber optic communication systems due to lasers' coherent beam properties which reduces data loss during transmission. The document also explains the three mechanisms of light emission: absorption, spontaneous emission, and stimulated emission, with stimulated emission being the basis for laser action.

2. LASER AND ITSPRODUCTIONS

3. Laser

4. 1. Definition of laser
• A laser is a device that generates light by a
process called STIMULATED EMISSION.
• The acronym LASER stands for Light
Amplification by Stimulated Emission of
Radiation
• Semiconducting lasers are multilayer
semiconductor devices that generates a
coherent beam of monochromatic light by laser
action. A coherent beam resulted which all of
the photons are in phase.

5. Typical Application of
Laser
The detection of the binary data stored in the form of pits on the compact disc is
done with the use of a semiconductor laser. The laser is focused to a
diameter of about 0.8 mm at the bottom of the disc, but is further focused to
about 1.7 micrometers as it passes through the clear plastic substrate to strike
the reflective layer. The reflected laser will be detected by a photodiode. Moral
of the story: without optoelectronics there will no CD player!

6. Another Typical Application of
Laser – Fibre Optics
• An example of application is for the light source for fibre
optics communication.
• Light travels down a fibre optics glass at a speed, = c/n,
where n = refractive index.
• Light carries with it information
• Different wavelength travels at different speed.
• This induce dispersion and at the receiving end the light
is observed to be spread. This is associated with data or
information lost.
• The greater the spread of information, the more loss
• However, if we start with a more coherent beam then
loss can be greatly reduced.

7. Fibre Optics Communication

8. 3 Mechanisms of Light Emission
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
Therefore 3 process
of light emission:

9. Absorption
E1
E2

10. Spontaneous Emission

11. Stimulated Emission

12. Background Physics
• In 1917 Einstein predicted that:
 under certain circumstances a photon
incident upon a material can generate a
second photon of
Exactly the same energy (frequency)
Phase
Polarisation
Direction of propagation
In other word, a coherent beam
resulted.

13. Background Physics
• Consider the ‘stimulated emission’ as shown
• Stimulated emission is the basis of the laser
action.
• The two photons that have been produced can
then generate more photons, and the 4
generated can generate 16 etc… etc… which
could result in a cascade of intense
monochromatic radiation.

14. E1
E2
hυ
(a) Absorption
hυ
(b) Spontaneous emission
hυ
(c) Stimulated emission
In
hυ
Out
hυ
E2 E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

15. Stimulated Emission

16. Therefore in a laser….
Three key elements in a laser
•Pumping process prepares amplifying medium in suitable state
•Optical power increases on each pass through amplifying medium
•If gain exceeds loss, device will oscillate, generating a coherentoutput