LASERS
Introduction:
LASER is a device that emits light through a process of optical amplification based on the
Quantum effect called stimulated emission of electromagnetic radiation. LASER is an acronym
for Light Amplification by Stimulated Emission of Radiation.
Distinguish between conventional light and Laser light:
Conventional light Laser light
It emits photons in a narrow, well-
It emits photons in all directions with wide
defined directional beam.
range of wavelengths
It is Incoherent (No fixed phase among the It is highly coherent (constant phase
emitted photons emitted). relationship among the emitted
photons)
The emitted light is monochromatic
The emitted light is polychromatic
Intensity is very high.
Intensity is low
Ex.:- Ruby Laser, He-Ne Laser, CO2 Laser
Ex.:- Electric Bulb, candle, Mercury vapour
etc.,
lamp etc.,
Characteristics of Laser light:
Like ordinary light, laser light is electromagnetic in nature. However, Laser has the following
important characteristics over ordinary light source they are,
1. High Monochromaticity
2. Highly Directional
3. Highly Coherent
4. High Intensity and brightness.
1) Monochromaticity: The light emitted from a
laser is highly monochromatic, that is, it is of
one unique wavelength (color).
The light from a laser typically comes from one atomic transition with a single precise wavelength.
Therefore, the laser light has very small broadening width (∆λ) and is almost the purest
,Monochromatic light.
For Ordinary white light, ∆λ= 300 nm
For monochromatic light, ∆λ= 5-10 nm
For Laser light, ∆λ= 0.2 nm
2. Directionality: Laser light has a high degree of directionality i.e., it can travel over long
distance without much loss of energy. When light travels through the medium it tends to
spread out, this spreading of light is called divergence and the angle at which the light
spreads is called angle of divergence (ϕ). Generally, divergence is more for ordinary light
and less for laser light.
Usually a laser source generates
a beam with divergence less
than 10 -3 radian. This means
that a beam from the laser will
spread to less than 1cm diameter
at a distance of 10m from the
laser.
Coherence: Two sources of
light is said to be coherent if
they exist a constant phase
relationship between them. The
light coming from the laser is
said to be coherent, which
means the group of photons
(Laser beam) are in phase in
space (Spatial) and time
(Temporal). Laser light is much
more coherent than ordinary
light.
Intensity and Brightness: Intensity of a wave is defined as energy per unit time flowing through a unit
normal area. In Laser light, energy is concentrated in small region of space with small wavelengths with
greater intensity. The number of photons emitted from a laser source per unit area per second is of the
order of 1022 to 1034.
Therefore, Laser light is an extraordinary light emitted under stimulated and amplified conditions,
so that the beam is characterized by high intensity, high directionality, high monochromaticity and
high degree of coherence.
, ABSORPTION, SPONTANEOUS AND STIMULATED EMISSION:
The interaction between matter and energy will take place in three ways they are Absorption,
spontaneous emission and stimulated emission.
Let us consider two energy levels having energies E1 and E2
of a given material, their energies being (E1<E2).
Absorption: Let us now assume that the atoms are initially E1
(Ground state). The atoms will remain in the ground state
Unless some external stimulus energy (photons) is applied to it.
We shall assume that, a photon of frequency „ν‟ is incident
on the material.
In this case, there is a finite probability that the atoms will be raised
to E2(Excited state).
The energy difference E2–E1= h ν which is required for the atoms in the ground state to undergo the
transition is obtained from the energy of the incident photon. This is the absorption process.
Spontaneous emission: An atom in excited state, E2 remains only for about 10-8sec. It is then de-excited
to lower energy state E1 by emitting energy in the form of electromagnetic radiation with energy equal to
the difference of two energy levels i.e., E2 – E1 = h.
This process is known as spontaneous (or) radiative emission.
Stimulated emission:
The average life time of an atom in excited state, E2 is nearly 10-8 sec. During this short interval, let a
photon of energy h is incident on the atom as shown in fig. This photon will stimulate the transition from
excited state to ground state. Now atom makes transition to lower energy state emitting an additional
photon of same frequency which is in phase with the incident photon. This process is called stimulated
emission.
Introduction:
LASER is a device that emits light through a process of optical amplification based on the
Quantum effect called stimulated emission of electromagnetic radiation. LASER is an acronym
for Light Amplification by Stimulated Emission of Radiation.
Distinguish between conventional light and Laser light:
Conventional light Laser light
It emits photons in a narrow, well-
It emits photons in all directions with wide
defined directional beam.
range of wavelengths
It is Incoherent (No fixed phase among the It is highly coherent (constant phase
emitted photons emitted). relationship among the emitted
photons)
The emitted light is monochromatic
The emitted light is polychromatic
Intensity is very high.
Intensity is low
Ex.:- Ruby Laser, He-Ne Laser, CO2 Laser
Ex.:- Electric Bulb, candle, Mercury vapour
etc.,
lamp etc.,
Characteristics of Laser light:
Like ordinary light, laser light is electromagnetic in nature. However, Laser has the following
important characteristics over ordinary light source they are,
1. High Monochromaticity
2. Highly Directional
3. Highly Coherent
4. High Intensity and brightness.
1) Monochromaticity: The light emitted from a
laser is highly monochromatic, that is, it is of
one unique wavelength (color).
The light from a laser typically comes from one atomic transition with a single precise wavelength.
Therefore, the laser light has very small broadening width (∆λ) and is almost the purest
,Monochromatic light.
For Ordinary white light, ∆λ= 300 nm
For monochromatic light, ∆λ= 5-10 nm
For Laser light, ∆λ= 0.2 nm
2. Directionality: Laser light has a high degree of directionality i.e., it can travel over long
distance without much loss of energy. When light travels through the medium it tends to
spread out, this spreading of light is called divergence and the angle at which the light
spreads is called angle of divergence (ϕ). Generally, divergence is more for ordinary light
and less for laser light.
Usually a laser source generates
a beam with divergence less
than 10 -3 radian. This means
that a beam from the laser will
spread to less than 1cm diameter
at a distance of 10m from the
laser.
Coherence: Two sources of
light is said to be coherent if
they exist a constant phase
relationship between them. The
light coming from the laser is
said to be coherent, which
means the group of photons
(Laser beam) are in phase in
space (Spatial) and time
(Temporal). Laser light is much
more coherent than ordinary
light.
Intensity and Brightness: Intensity of a wave is defined as energy per unit time flowing through a unit
normal area. In Laser light, energy is concentrated in small region of space with small wavelengths with
greater intensity. The number of photons emitted from a laser source per unit area per second is of the
order of 1022 to 1034.
Therefore, Laser light is an extraordinary light emitted under stimulated and amplified conditions,
so that the beam is characterized by high intensity, high directionality, high monochromaticity and
high degree of coherence.
, ABSORPTION, SPONTANEOUS AND STIMULATED EMISSION:
The interaction between matter and energy will take place in three ways they are Absorption,
spontaneous emission and stimulated emission.
Let us consider two energy levels having energies E1 and E2
of a given material, their energies being (E1<E2).
Absorption: Let us now assume that the atoms are initially E1
(Ground state). The atoms will remain in the ground state
Unless some external stimulus energy (photons) is applied to it.
We shall assume that, a photon of frequency „ν‟ is incident
on the material.
In this case, there is a finite probability that the atoms will be raised
to E2(Excited state).
The energy difference E2–E1= h ν which is required for the atoms in the ground state to undergo the
transition is obtained from the energy of the incident photon. This is the absorption process.
Spontaneous emission: An atom in excited state, E2 remains only for about 10-8sec. It is then de-excited
to lower energy state E1 by emitting energy in the form of electromagnetic radiation with energy equal to
the difference of two energy levels i.e., E2 – E1 = h.
This process is known as spontaneous (or) radiative emission.
Stimulated emission:
The average life time of an atom in excited state, E2 is nearly 10-8 sec. During this short interval, let a
photon of energy h is incident on the atom as shown in fig. This photon will stimulate the transition from
excited state to ground state. Now atom makes transition to lower energy state emitting an additional
photon of same frequency which is in phase with the incident photon. This process is called stimulated
emission.
