Laser
,Laser
Laser is an acronym for Light Amplification by Stimulated Emission of Ra-
diation and is a device for producing a strong, highly monochromatic, di-
rectional and coherent beam of light of high power density. Its working depends
on the phenomenon of stimulated emission of radiation, the theory of which was
worked out as early as 1917 by Einstein. He was considering the equilibrium
between matter and electromagnetic radiation in a black body chamber at a
constant temperature where energy exchange occurs due to absorption and
spontaneous emission of radiation by atoms. He observed that the absorption and
spontaneous emission alone cannot explain the equi- librium and predicted that
there must be an additional process, now termed as stimulated emission.
In 1954, two Russian physicist N. Basov and A. M. Prokhorov and the Amer- ican
physicist C. H. Townes discovered almost simultaneously and indepen-
dently the phenomenon of Microwave Amplification by Stimulated Emission of
Radiation (MASER). In 1958, Townes and A. L. Schawlow showed that the
principle of MASER can also be extended from the microwave to the field of
visible radiation. Finally, in 1960, LASER was first built by T. H. Maiman using
Ruby as the active medium.
A laser differs from other sources of light in the way that it emits light coherently,
spatially and temporally. Conventional light sources emit light
at different times and in different directions so there is no phase relation
between the emitted photons. On the other hand the photons emitted by lasers
are in phase and move in same direction. Thus laser is a coherent and focused
beam of photons. The conventional light sources does not emit highly directional
and coherent light whereas lasers produce highly direc- tional, monochromatic,
coherent light beam.
, Explanation of laser
A laser not only amplifies or increases the intensity of light but also generates the light.
It emits light through a process called stimulated emission of
radiation which amplifies or increases the intensity of light. In general, when
electron jumps from one higher energy level to another lower energy level, it emits
a photon. The energy of the emitted photon is equal to the energy difference
between the energy levels . The loss of electron energy is attributed to the entire
atom. Therefore, it can be thought that the atom
is moving from a higher energy state to a lower energy state.
Absorption and emission of radiation by matter
Absorption of radiation
An atom has a number of possible quantised energy states characterised by it principal quantum
number n(= 1, 2, 3, ). It remains in the ground state with minimum energy E1 in absence
of external influences. On being subject to some action, say irradiation by photons of right frequency ν, it
transits to a higher energy state E2, absorbing energy hν of the radiation. This process is called absorption
or excitation. The frequency ν is given by,
E2 E1
ν=
h
The absorption of radiation occurs only if the energy of the incident photon (hν) exactly matches the energy
difference of the two energy levels (E2 1).
Figure 1: Absorption or excitation
Spontaneous emission of radiation
Consider now an atom initially in excited state E2. An atom stays in the excited state usually for a
to its initial state E1 of its own, emitting a photon of frequency ν.
The process is
2
,Laser
Laser is an acronym for Light Amplification by Stimulated Emission of Ra-
diation and is a device for producing a strong, highly monochromatic, di-
rectional and coherent beam of light of high power density. Its working depends
on the phenomenon of stimulated emission of radiation, the theory of which was
worked out as early as 1917 by Einstein. He was considering the equilibrium
between matter and electromagnetic radiation in a black body chamber at a
constant temperature where energy exchange occurs due to absorption and
spontaneous emission of radiation by atoms. He observed that the absorption and
spontaneous emission alone cannot explain the equi- librium and predicted that
there must be an additional process, now termed as stimulated emission.
In 1954, two Russian physicist N. Basov and A. M. Prokhorov and the Amer- ican
physicist C. H. Townes discovered almost simultaneously and indepen-
dently the phenomenon of Microwave Amplification by Stimulated Emission of
Radiation (MASER). In 1958, Townes and A. L. Schawlow showed that the
principle of MASER can also be extended from the microwave to the field of
visible radiation. Finally, in 1960, LASER was first built by T. H. Maiman using
Ruby as the active medium.
A laser differs from other sources of light in the way that it emits light coherently,
spatially and temporally. Conventional light sources emit light
at different times and in different directions so there is no phase relation
between the emitted photons. On the other hand the photons emitted by lasers
are in phase and move in same direction. Thus laser is a coherent and focused
beam of photons. The conventional light sources does not emit highly directional
and coherent light whereas lasers produce highly direc- tional, monochromatic,
coherent light beam.
, Explanation of laser
A laser not only amplifies or increases the intensity of light but also generates the light.
It emits light through a process called stimulated emission of
radiation which amplifies or increases the intensity of light. In general, when
electron jumps from one higher energy level to another lower energy level, it emits
a photon. The energy of the emitted photon is equal to the energy difference
between the energy levels . The loss of electron energy is attributed to the entire
atom. Therefore, it can be thought that the atom
is moving from a higher energy state to a lower energy state.
Absorption and emission of radiation by matter
Absorption of radiation
An atom has a number of possible quantised energy states characterised by it principal quantum
number n(= 1, 2, 3, ). It remains in the ground state with minimum energy E1 in absence
of external influences. On being subject to some action, say irradiation by photons of right frequency ν, it
transits to a higher energy state E2, absorbing energy hν of the radiation. This process is called absorption
or excitation. The frequency ν is given by,
E2 E1
ν=
h
The absorption of radiation occurs only if the energy of the incident photon (hν) exactly matches the energy
difference of the two energy levels (E2 1).
Figure 1: Absorption or excitation
Spontaneous emission of radiation
Consider now an atom initially in excited state E2. An atom stays in the excited state usually for a
to its initial state E1 of its own, emitting a photon of frequency ν.
The process is
2
