Module-1
LASER AND OPTICAL FIBERS
Introduction
Laser is the acronym for Light Amplification by Stimulated Emission of Radiation. It is a device for
producing very intense, almost unidirectional, monochromatic and coherent visible light beams. The
laser operation is based on the principle of stimulated emission. The lasers find applications in many
fields. They brought amazing changes in many areas and caused spectacular developments in the
field of communications.
Characteristics of Lasers
Coherence: The light from a laser is said to be coherent, which means the wavelengths of the laser light
are in phase in space and time. All the photons emitted in laser have the same energy, frequency, or
wavelength. Hence, the light waves of laser light have single wavelength (color). Therefore, the
wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated
emission is used to produce light.
Directionality: In a laser, all photons will travel in same direction. Therefore, laser emits light only in
one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow.
Hence, a laser beam can travel to long distances without spreading.
Monochromatic: The word monochromatic means a light beam which has a single wavelength. Lasers
are monochromatic whereas ordinary light sources contain or range of wavelengths energies or colors.
High intensity:. The intensity depends on the amount of flow of energy that is through a unit area per
unit time. The ordinary source light spreads out in all directions while laser light is very particular
and travels in a particular direction. Because of this, it has a very high intensity.
Basic Principles:
1. Induced absorption:
It is a process in which an atom in the ground state undergoes transition to the higher energy state by
absorbing incident photon. It is represented as
Atom + Photon → Atom*
Let E1 and E2 be the energy levels in an atom and N 1 and N2 be the number density in these levels
respectively. Let Uγ be the energy density of the radiation incident.
1
, Rate of absorption is proportional to the number of atoms in lower state and also on the energy
density Uγ.
Rate of absorption = B12 N1 Uγ
Here B12 is a constant known as Einstein’s coefficient of Induced absorption.
2. Spontaneous emission:
It is a process in which, atoms at the higher level undergoes transition to the ground state without any
external aid. The photons emitted in spontaneous emission may not have same direction and
phase similarities. It is incoherent.
Ex: Glowing electric bulbs, Candle flame etc.
The process is represented as
Atom* → Atom + Photon
The rate of spontaneous emission representing the number of such deexcitations is proportional to
number of atoms in the excited state.
Rate of spontaneous emission = A21 N2
Here A21 is a constant known as Einstein’s coefficient of spontaneous emission.
E2 𝐸2 −𝐸1
𝛾= =∆𝐸
ℎ
𝛾
E1
2
, 3. Stimulated emission:
It is a process in which an atom in the excited state undergoes transition to the ground state by the
influence of passing photon. During this process a stimulated photon emitted along with the incident
photon and these photons are found to be coherent. The emitted two photons have same phase,
frequency, direction and polarization with the incident photon. This kind of action is responsible for
lasing action. This process represented as follows
Atom* + Photon → Atom + (Photon + Photon)
E2
𝐸2 − 𝐸1
𝛾 𝛾 𝛾=
ℎ
𝛾
E1
The number of stimulated emissions is proportional to the number of atoms in higher state N2 and
also on the energy density Uγ.
Rate of stimulated emission = B21 N2 Uγ
Here B21 is the constant known as Einstein’s coefficient of stimulated emission.
3
, Expression for energy density:
Consider a system of atoms in thermal equilibrium with radiation of energy density U ν & frequency
‘v’. Let N1 & N2 be the population of the energy states E1 & E2 respectively, where (E2 >E1 )Rate of
absorption is proportional to the number of atoms in lower state and also on the energy density U γ.
Rate of absorption = B12 N1 Uγ
Here B12 is a constant known as Einstein’s coefficient of Induced absorption.
Rate of spontaneous emission = A21 N2
Here A21 is a constant known as Einstein’s coefficient of spontaneous emission.
Rate of stimulated emission = B21 N2 Uγ
Here B21 is the constant known as Einstein’s coefficient of stimulated emission.
At thermal equilibrium,
Rate of absorption = Rate of spontaneous emission + Rate of stimulated emission
B12 N1 Uγ = A21 N2 + B21 N2 Uγ
𝐴21 𝑁2
𝑈𝛾 = 𝐵
12 𝑁1 −𝐵21 𝑁2
Rearranging this, we get
4
LASER AND OPTICAL FIBERS
Introduction
Laser is the acronym for Light Amplification by Stimulated Emission of Radiation. It is a device for
producing very intense, almost unidirectional, monochromatic and coherent visible light beams. The
laser operation is based on the principle of stimulated emission. The lasers find applications in many
fields. They brought amazing changes in many areas and caused spectacular developments in the
field of communications.
Characteristics of Lasers
Coherence: The light from a laser is said to be coherent, which means the wavelengths of the laser light
are in phase in space and time. All the photons emitted in laser have the same energy, frequency, or
wavelength. Hence, the light waves of laser light have single wavelength (color). Therefore, the
wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated
emission is used to produce light.
Directionality: In a laser, all photons will travel in same direction. Therefore, laser emits light only in
one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow.
Hence, a laser beam can travel to long distances without spreading.
Monochromatic: The word monochromatic means a light beam which has a single wavelength. Lasers
are monochromatic whereas ordinary light sources contain or range of wavelengths energies or colors.
High intensity:. The intensity depends on the amount of flow of energy that is through a unit area per
unit time. The ordinary source light spreads out in all directions while laser light is very particular
and travels in a particular direction. Because of this, it has a very high intensity.
Basic Principles:
1. Induced absorption:
It is a process in which an atom in the ground state undergoes transition to the higher energy state by
absorbing incident photon. It is represented as
Atom + Photon → Atom*
Let E1 and E2 be the energy levels in an atom and N 1 and N2 be the number density in these levels
respectively. Let Uγ be the energy density of the radiation incident.
1
, Rate of absorption is proportional to the number of atoms in lower state and also on the energy
density Uγ.
Rate of absorption = B12 N1 Uγ
Here B12 is a constant known as Einstein’s coefficient of Induced absorption.
2. Spontaneous emission:
It is a process in which, atoms at the higher level undergoes transition to the ground state without any
external aid. The photons emitted in spontaneous emission may not have same direction and
phase similarities. It is incoherent.
Ex: Glowing electric bulbs, Candle flame etc.
The process is represented as
Atom* → Atom + Photon
The rate of spontaneous emission representing the number of such deexcitations is proportional to
number of atoms in the excited state.
Rate of spontaneous emission = A21 N2
Here A21 is a constant known as Einstein’s coefficient of spontaneous emission.
E2 𝐸2 −𝐸1
𝛾= =∆𝐸
ℎ
𝛾
E1
2
, 3. Stimulated emission:
It is a process in which an atom in the excited state undergoes transition to the ground state by the
influence of passing photon. During this process a stimulated photon emitted along with the incident
photon and these photons are found to be coherent. The emitted two photons have same phase,
frequency, direction and polarization with the incident photon. This kind of action is responsible for
lasing action. This process represented as follows
Atom* + Photon → Atom + (Photon + Photon)
E2
𝐸2 − 𝐸1
𝛾 𝛾 𝛾=
ℎ
𝛾
E1
The number of stimulated emissions is proportional to the number of atoms in higher state N2 and
also on the energy density Uγ.
Rate of stimulated emission = B21 N2 Uγ
Here B21 is the constant known as Einstein’s coefficient of stimulated emission.
3
, Expression for energy density:
Consider a system of atoms in thermal equilibrium with radiation of energy density U ν & frequency
‘v’. Let N1 & N2 be the population of the energy states E1 & E2 respectively, where (E2 >E1 )Rate of
absorption is proportional to the number of atoms in lower state and also on the energy density U γ.
Rate of absorption = B12 N1 Uγ
Here B12 is a constant known as Einstein’s coefficient of Induced absorption.
Rate of spontaneous emission = A21 N2
Here A21 is a constant known as Einstein’s coefficient of spontaneous emission.
Rate of stimulated emission = B21 N2 Uγ
Here B21 is the constant known as Einstein’s coefficient of stimulated emission.
At thermal equilibrium,
Rate of absorption = Rate of spontaneous emission + Rate of stimulated emission
B12 N1 Uγ = A21 N2 + B21 N2 Uγ
𝐴21 𝑁2
𝑈𝛾 = 𝐵
12 𝑁1 −𝐵21 𝑁2
Rearranging this, we get
4
