6. Laser : A brief introduction
www.optics.rochester.edu/classes/opt100/opt100page.html
Where Does Light Come From?
Æ Accelerated Charges Produce Electromagnetic Radiation
Two-Level Atoms and Howling Dogs
According to Einstein’s Theory of Radiation,
the interaction of electromagnetic radiation with matter can be described in terms of 3 basic processes involving these 2 energy levels:
in terms of 3 basic processes involving these 2 energy levels:
absorption, spontaneous emission, and stimulated emission.
absorption spontaneous emission stimulated emission
absorption spontaneous emission stimulated emission
Blackbody Radiation
Types of Light Sources
1) Thermal Light Source (based on thermal luminescence ) :
Æ Light is produced mainly by spontaneous emission when the atoms relax to the ground state.
2) Electric Discharge Source (based on cathodoluminescence ) :
Æ the excitation is caused by collisions between the atoms and accelerated charged particles (usually electrons).
3) Optically Excited Source (based on photoluminescence ) : Æ the atoms are excited by light itself.y g
1) Thermal Light Source (based on thermal luminescence ) : 2) Electric Discharge Source (based on cathodoluminescence ) : 2) Electric Discharge Source (based on cathodoluminescence ) : 3) Optically Excited Source (based on photoluminescence ) :
4) Chemically Excited Source (based on chemiluminescence ) :
Æ energy to excite the atoms is the product of some chemical reaction.
5) Semiconductor Diode Source (based on electroluminescence ) :
Laser
Light
Amplification by Stimulated
Laser Stimulated
Emission of
Radiation.
How does a laser work ?
Light Amplification by Stimulated Emission of Radiation.
Characteristics of Laser Light
Light Amplification by Stimulated Emission of Radiation.
Monochromaticity :
Directionality :
Spectral brightness :
ÆA little Helium-Neon laser is about 10,000,000,000 brighter than the sun!
Coherence :
The exciting thing about laser light is that it is naturally very coherent!g g g y y coherence lengthA laser is made up of 2 key parts: an optical amplifier and an optical resonator
Light Amplification by Stimulated Emission of Radiation.
A laser is made up of 2 key parts: an optical amplifier and an optical resonator.
1. Laser Amplifier
More atoms must be in the excited state than in the ground state.
Î
“population inversion.”
pumping pumping
Light Amplification by Stimulated Emission of Radiation.
N l ti i i
No population inversion
in 2-level system
2. Laser Resonators
Light Amplification by Stimulated Emission of Radiation.
: Laser mode frequencies
