2. Introduction
LASER or laser (Light Amplification by
Stimulated Emission of Radiation) is a
mechanism for emitting electromagnetic
radiation, often visible light, via the process of
stimulated emission.
Electromagnetism
3. Introduction
The main use of laser as a light amplifier:
is as an optical oscillator or transducer for
converting energy into highly collimated beam of
optical radiation ( light).
4. Introduction
Characteristics of Lasers: Laser light has three
unique characteristics, that make it different than
"ordinary" light. It is:
Monochromatic means that it consists of one single
color or wavelength. Even through some lasers can
generate more than one wavelength, the light is
extremely pure and consists of a very narrow spectral
range.
Directional means that the beam is well collimated
(very parallel) and travels over long distances with
very little spread.
Coherent means that all the individual waves of light
are moving precisely together through time and
5. Laser Physics
We need to begin with the idea of atomic energy
levels.
In chemistry you will probably remember doing
'Shell' diagrams of atoms.
The electrons placed on circles around the
atom, two in the first layer, eight in the
second, etc. thus for Oxygen with eight electrons
we drew:
6. Laser Physics
Now, we shall consider an idealized atom with
two electron energy levels and one electron. The
electron may be in either of the two energy levels,
thus:
7. Laser Physics
We may make the analogy of a person who is
only allowed to stand on a chair or on the
ground, two defined levels.
If the electron is in the higher level it may fall
down into the lower level.
In its doing this it must give up an amount of
energy equal to the energy difference between
the two levels.
This is the law of conservation of energy being
applied.
This energy is given up in the form of light.
8. Laser Physics
Light is also quantized. It may be represented as
groups of photons. Each photon carries one
quantum of light energy. The amount of energy in
a quantum depends upon the wavelength (
colaour) of the light.
9. Laser Physics
So we see that a short wavelength such as blue light
at 470 nm has a high energy, and red light at 670 nm
has a low energy per photon.
The important point is that the wavelength of light is
linked to the energy of a photon in a defined way.
Thus our electron in the idealised atom which has
given out a photon of defined energy emits light of a
defined wavelength or 'colour'.
This is seen in street lights. They contain Sodium
atoms which take electrical energy to move their
electrons into higher levels, these electrons then fall
back down to the initial, lower, state giving light at 589
nm the characteristic orange of street lights.
10. Laser Physics
This process is known as SPONTANEOUS
EMISSION
The atom emits light spontaneously, without
external influences.
If however the atom is not isolated, other effects
may occur. Photons of the same energy as the
energy of the upper level may use their energy to
move an electron from the lower level to the
upper one.
This is known as ABSORBTION, as the photon is
destroyed in the process.
11. Laser Physics
If a photon of the correct energy passes an atom
with its electron in the upper level, then it may
cause the electron to fall to the lower level.
This STIMULATED EMISSION is very different
from spontaneous emission.
In the spontaneous process the photon may
travel in any direction and be emitted at any
time, Stimulated emission, however, causes the
emitted photon to travel in the identical direction
to the passing photon and at the same time.
13. Laser Physics
Now we finally get to lasers. L.A.S.E.R. is an
acronym for Light Amplification by the Stimulated
Emission of Radiation. Which is why I have had
to explain all about Stimulated emission.
The three processes above all happen if we have
a group of N atoms some of which (N2) have their
electrons in the upper level and some (N1) with
their electrons in the lower level. In a laser we
want stimulated emission to be the biggest effect
( as indicated by the acronym ). Let us then look
at the rate at which each process occurs:
14. Laser Physics
Spontaneous emission.
This is only dependent on the electron being in the upper
level. A certain proportion ( call it a ) of the upper level
electrons will emit in a given time, so:
Spontaneous rate = a N2
Absorbtion.
This depends upon the electron being in the lower level and
there being a photon present. Using the number of
photons as n and the proportion of possible interactions
which occur as b :
Absorbtion rate = b N1 n
Stimulated emission.
This is the same as for absorbtion, but the electron must
start in the upper level. :
Stimulated emission rate = b N2 n
15. Laser Physics
We require that the last of these expressions is
the largest.
Now a and b are constants which depend upon
the particular atom used and are thus not under
our control. So for stimulated emission to be
greater than spontaneous emission, we require n
to be large - we need many photons in the laser.
For stimulated emission to be greater than
absorbtion we require N2 to be greater than N1 -
more atoms have their electrons in the upper
level than have their electrons in the lower level.
16. Laser Physics
This is known as an Inversion as it is normal for
electrons to be in their lowest energy level. This
may be readily seen from the fact that in the
absence of external influences, ie. no photons
n=0, the only process which can occur is
spontaneous emission which will allow any
electron which began in the upper level to fall to
the lower level, but not vice-versa.
17. Laser Physics
The fundamental difficulty in producing a laser is
creating this necessary inversion in the
populations of the two levels.
It must be stressed here that we are talking
about many atoms each with a single electron
which may be in one of two levels.
We have not got many electrons in one atom
which would be restricted in their movements
between levels by the rules governing how many
electrons may occupy a single level of a particular
atom.
18. Laser Physics
Now assuming that we have an inversion, N2
greater than N1 , then we can get the SE part of
laSEr.
Now how do we amplify light using this? consider
a single photon entering a region with the atoms
in.
This photon will pass by an atom with its electron
in the upper level and cause it to emit a second
photon travelling in the same direction, by the
process of stimulated emission.
There are now two photons, each of which can
cause stimulated emission in two more atoms to
give four photons , and so on.
19. Laser Physics
Thus we have amplification, which is also known
as gain. The region containing the atoms is
known as the gain medium. The final stage in a
laser is to get this first photon to amplify. This is
done by placing the gain medium between two
mirrors. This forms what is known as a laser
20. Laser Physics
Initially there is no light in the cavity. The only
possible process for the atoms to undergo is
therefore spontaneous emission, and this duly
occurs.
As stated earlier, this may travel in any direction
out of the gain medium, and most is lost from the
cavity.
However out of the millions of photons emitted by
the millions of atoms in any real medium, there is
bound to be at least one which travels directly to
one of the mirrors and is reflected back to the
gain medium.
21. Laser Physics
This is now our first photon. As it passes through the gain
medium, it causes stimulated emission as described above and
by the end of the gain medium there are, say, ten photons.
Now the important part is that these are all travelling int the
SAME DIRECTION as the first photon, so will be reflected back
to the gain region by the other mirror.
These ten photons now each cause stimulated emission, and
when they get out of the medium to the first mirror again there
are one hundred which are reflected back to the gain medium
again and are amplified to 1000 etc...
22. Laser Physics
Thus we very rapidly get very many photons
travelling back and forward in the cavity.
Obviously in this idealised case where no
photons are lost from the steadily amplified
beam, the photon number just goes on
increasing.
In any real laser some photons are lost, for many
various reasons.
One of these is quite deliberate.
One of the mirrors is made to reflect only part of
the light, and to allow the rest through.
23. Laser Physics
This is then the output beam of the laser and the
'leaky' mirror is referred to as the output coupler.
A steady state may then be reached where the gain
exactly replaces the photons lost from the cavity by
the output coupler.
There is then a constant number of photons in the
cavity at any time. For example a laser with a gain of
1.12 ( much more realistic than the gain of ten used
earlier as the illustration ) and an output coupler which
reflects just 80% of the light we have:
24. Laser Physics
The output beam thus has photons which are
travelling in a fixed direction and also have a fixed
wavelength (colour) defined by the energy levels
of the electrons in the atoms of the gain medium.
