The first laser was constructed by T. H. Maiman
in 1960. Up to this date, there are practically hundreds of
different types of lasers and the applications of lasers are
increasing everyday. Here we briefly describe the basic principles
of lasers, how to construct a laser, some applications. Some
exercises are also given to further clarify some points.
LASER - acronym (short form) for Light
Amplified Stimulated Emission
of Radiation
We shall explain each word in this name.
(Laser Pointer - This photo was provided
by Physics World)
(fig. 1)
•Electron excited to an upper energy level will, typically
after sec, jump to a lower energy level and a photon will
be emitted. (No external perturbation.) Also represented by:
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Let
v = photon frequency
h = Planck's constant  J.
sec.
where  is
energy of electron in level 2, and 
is energy of electron in level 1. |
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Example:
 ,
 .
 .
Then wavelength
= 621.1 nm (red light)
nm (nanometer =  m)
is a commonly used unit in optical sciences.
Remark: similar emissions also occur in
(i) molecules and (ii) ions in solids.
A photon is emitted from an excited atom (molecule, ion)
when it is "disturbed" (stimulated) by other photons.
(Fig. 3)
• Requirement: both photons have the same wavelength
• Characteristics: The two photons vibrating in parallel
planes, travel along the same direction, and vibrating in
phase.
• Stimulated emission occurs "immediately".
(Fig. 4)
An electron excited to level A will stay much
longer (say  sec)
compared to a "normal" energy level where an electron
typically stays for 
sec (provided there is no external perturbation, stimulated
emission can occur immediately).
Consider some atoms (or molecules) in space as shown in the
diagram below.
(Fig. 5)
When there are more atoms with an electron in
level A, we say that there is population inversion.
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