@PROFDOCDIGITALLIBRARIES
Experiment: Properties of a LASER Beam
Objective:
1. To study the shape of the LASER beam cross-section and evaluation of the
spot size of the beam.
2. To find the divergence angle of the LASER beam.
3. To study the polarizing nature of the LASER beam.
Introduction:
The LASER action is preceded by three processes, namely absorption,
spontaneous emission and stimulated emission; absorption of energy to
populate upper levels, spontaneous emission to produce the initial photons for
stimulation and finally, stimulated emission for generation of coherence output
or LASER.
PR
Generally, the electronic, atomic, molecular or ionic systems the upper energy
levels are less populated than the lower energy levels under equilibrium
conditions. The pumping mechanism excites atoms to a higher energy level by
absorption. The atom stays at the higher level for a certain duration and decays
O
to the stable ground (lower) level. But it is not necessary that the atom is always
de-excited to ground state. It can go to an intermediate state, called metastable
state where it stays for a much longer period than upper level and comes down
FD
to lower level or to the ground state. Since the lifetime of the atoms in the
metastable state is large, it is possible to have a much larger number of atoms in
the metastable level in comparison to the lower level so that the population of
the metastable state and the lower or ground state is reversed. i.e., there are
more atoms in the upper metastable level than the lower level. This condition is
O
referred to as population inversion.
C
Once this is achieved, LASER action is initiated in the following fashion. The
atoms in the upper metastable state come down to the ground state emitting a
photon. This photon can stimulate an atom in the metastable state to release its
photon in phase with it. The photon thus released is called a stimulated photon.
It moves in the same direction as the initiating photon, has the same wavelength
and polarization and is in phase with it, thus producing amplification. Since
there are a large number of initiating photons, it forms an initiating
electromagnetic radiation field. An avalanche-stimulated photon is generated, as
the photons traveling along the length of the active medium stimulates a
number of excited atoms in the metastable state to release their photons. This is
1
, @PROFDOCDIGITALLIBRARIES
referred to as the stimulated emission. These photons are fully reflected by the
rear reflector (100 reflective) and consequently the intensity of stimulated
photons increases as they traverse through the active medium, thus increasing
the intensity of the radiation field of stimulated emission. At the output of a
transmissive mirror, a part of these photons is reflected and the rest is
transmitted as the LASER output.
This action is repeated and the reflected photons after striking the rear mirror,
reach the output transmissive mirror in the return path. The intensity of the
LASER output increases as the pumping continues. When the input pumping
energy reduces, the available initiating and subsequently the stimulated photons
decrease considerably and the gain of the system is not able to overcome the
losses, thus laser output ceases.
Since the stimulation process was started by the initiating photons, the emitted
PR
photons can combine coherently, as all of them are in phase with each other.
Though the LASER action continues as long as the energy is given to the active
medium, it may be stated that pulsed LASER is obtained if the population
inversion is available in a transient fashion and continuous wave (CW) LASER is
possible if the population inversion is maintained in a steady-state basis. If the
O
input energy is given by the flash lamp, the output will be a pulse output and the
LASER is called a pulsed laser. If equilibrium can be achieved between the
number of the photons emitted and the number of atoms in the metastable level
FD
by pumping with a continuous arc lamp instead of a flash lamp, then it is
possible to achieve a continuous laser output, which is called continuous wave
LASER.
The optical property of light that most distinguishes the LASER from other light
O
is coherence. Coherence is a measure of the degree of phase correlation that
exists in the radiation field of a light source at different locations/times.
There are two types of coherence:
C
i. Temporal coherence
ii. Spatial coherence
• Temporal coherence:
The coherence time 𝜏c of a LASER is a measure of the average time
interval over which the laser wave remains in phase as they travel in
space.
𝜏c can be defined as
𝜏c = 1
∆𝑣
2𝜋
= 2𝜋∆𝑣
2𝜋
= ∆𝑤
𝜏c = 2𝜋∆𝜏re
2
Experiment: Properties of a LASER Beam
Objective:
1. To study the shape of the LASER beam cross-section and evaluation of the
spot size of the beam.
2. To find the divergence angle of the LASER beam.
3. To study the polarizing nature of the LASER beam.
Introduction:
The LASER action is preceded by three processes, namely absorption,
spontaneous emission and stimulated emission; absorption of energy to
populate upper levels, spontaneous emission to produce the initial photons for
stimulation and finally, stimulated emission for generation of coherence output
or LASER.
PR
Generally, the electronic, atomic, molecular or ionic systems the upper energy
levels are less populated than the lower energy levels under equilibrium
conditions. The pumping mechanism excites atoms to a higher energy level by
absorption. The atom stays at the higher level for a certain duration and decays
O
to the stable ground (lower) level. But it is not necessary that the atom is always
de-excited to ground state. It can go to an intermediate state, called metastable
state where it stays for a much longer period than upper level and comes down
FD
to lower level or to the ground state. Since the lifetime of the atoms in the
metastable state is large, it is possible to have a much larger number of atoms in
the metastable level in comparison to the lower level so that the population of
the metastable state and the lower or ground state is reversed. i.e., there are
more atoms in the upper metastable level than the lower level. This condition is
O
referred to as population inversion.
C
Once this is achieved, LASER action is initiated in the following fashion. The
atoms in the upper metastable state come down to the ground state emitting a
photon. This photon can stimulate an atom in the metastable state to release its
photon in phase with it. The photon thus released is called a stimulated photon.
It moves in the same direction as the initiating photon, has the same wavelength
and polarization and is in phase with it, thus producing amplification. Since
there are a large number of initiating photons, it forms an initiating
electromagnetic radiation field. An avalanche-stimulated photon is generated, as
the photons traveling along the length of the active medium stimulates a
number of excited atoms in the metastable state to release their photons. This is
1
, @PROFDOCDIGITALLIBRARIES
referred to as the stimulated emission. These photons are fully reflected by the
rear reflector (100 reflective) and consequently the intensity of stimulated
photons increases as they traverse through the active medium, thus increasing
the intensity of the radiation field of stimulated emission. At the output of a
transmissive mirror, a part of these photons is reflected and the rest is
transmitted as the LASER output.
This action is repeated and the reflected photons after striking the rear mirror,
reach the output transmissive mirror in the return path. The intensity of the
LASER output increases as the pumping continues. When the input pumping
energy reduces, the available initiating and subsequently the stimulated photons
decrease considerably and the gain of the system is not able to overcome the
losses, thus laser output ceases.
Since the stimulation process was started by the initiating photons, the emitted
PR
photons can combine coherently, as all of them are in phase with each other.
Though the LASER action continues as long as the energy is given to the active
medium, it may be stated that pulsed LASER is obtained if the population
inversion is available in a transient fashion and continuous wave (CW) LASER is
possible if the population inversion is maintained in a steady-state basis. If the
O
input energy is given by the flash lamp, the output will be a pulse output and the
LASER is called a pulsed laser. If equilibrium can be achieved between the
number of the photons emitted and the number of atoms in the metastable level
FD
by pumping with a continuous arc lamp instead of a flash lamp, then it is
possible to achieve a continuous laser output, which is called continuous wave
LASER.
The optical property of light that most distinguishes the LASER from other light
O
is coherence. Coherence is a measure of the degree of phase correlation that
exists in the radiation field of a light source at different locations/times.
There are two types of coherence:
C
i. Temporal coherence
ii. Spatial coherence
• Temporal coherence:
The coherence time 𝜏c of a LASER is a measure of the average time
interval over which the laser wave remains in phase as they travel in
space.
𝜏c can be defined as
𝜏c = 1
∆𝑣
2𝜋
= 2𝜋∆𝑣
2𝜋
= ∆𝑤
𝜏c = 2𝜋∆𝜏re
2
