The Probability of Occurrence of Absorption from state 1 to state 2 is proportional to the energy density u(v).. of the radiation
> P12 = B12 u(v)
Where as , the proportionality constant B12 is known as the Einstein’s Co-efficient of absorption of radiation .
The probability of of occurrence of this spontaneous emission transition from State 2 to state 1 only on
 P’12 = A21…,
In a system of atoms in thermal equilibrium the number of atoms in ground state is generally much greater than in a Higher Energy state.This is known as Normal Inversion population of atoms among the available energy states is greater than that of Lower energy state is known as Populatio Inversion.
i) Metastable state
ii) optical pumping iii) population inversion
Metastable state: The excited state, which has a long life time, is known as metastable state.
Optical pumping: This process is required to achieve population inversion and used in Ruby laser.
Pumping process is defined as: “The process which excites the atoms from ground state to excited state to achieve population inversion”.
Population Inversion:
Generally, number of atoms in the ground state is greater than the number of atoms in higher energy states.
But in order to produce a laser beam, the minimum requirement is stimulated emission.
Stimulated emission takes place only if the number of atoms in the higher energy level is greater than the number of atoms in the lower energy level.
Simply population inversion is nothing but number of atoms in higher energy level is greater than the number of atom in lower energy level.
2) Define spontaneous and stimulated emission of radiation?
Spontaneous Emission: When an atom in the excited state emits a photon of energy ‘hv’ coming down to ground state by itself without any external agency, such an emission is called spontaneous emission. Atom* atom + hv.
Photons released in spontaneous emission are not coherent. Hence spontaneous emission is not useful for producing lasers.
Stimulated Emission: When an atom in the excited state, emits two photons of same energy ‘hv’ while coming to down to ground state with the influence of an external agency, such an emission is called stimulated emission. Atom* atom + 2hv.
In the two photons one photon induces the stimulated emission and the second one is released by the transition of atom from higher energy level to lower energy level.
Both the photons are strictly coherent. Hence stimulated emission is responsible for laser production.
3) Explain the basic principle of optical fiber?
Optical fibers are the waveguides through which electromagnetic waves of optical frequency range can be guided through them to travel long distances.
An optical fiber works on the principle of total internal reflection (TIR).
Total Internal Reflection: when a ray of light travels from a denser medium into a rarer medium and if the angle of incidence is greater than the critical angle then the light gets totally reflected into the denser medium
4) Explain i) Numerical Aperture ii) Acceptance angle
i) Numerical Aperture:
Numerical aperture of a fiber is a measure of its light gathering power.
“The Numerical Aperture (NA) is defined as the sine of the maximum acceptance angle”
The light gathering ability of optical fiber depends on two factors i.e.,
 NA
NA is defined as sine of the acceptance angle A i.e NA = 12-n22
The efficiency of optical fiber is expressed in terms of NA; it is called as figure of merit of optical fiber.
ii) Acceptance Angle:
All right rays falling on optical fiber are not transmitted through the fiber. i -cladding interface are transmitted through the fiber by undergoing TIR. For which angle of incidence, the refraction angle is greater than 900 will be propagated through TIR.
There by Acceptance Angle is defined as: The maximum angle of incidence to the axis of optical fiber at which the light ray may enter the fiber so that it can be propagated through TIR.
5. What are the main sections of optical fiber? Describe the step index optical fiber? o-axial regions. -guiding region known as “Core”. It is surrounded by a middle co-axial regional known as “cladding”. The outer most regions which completely covers the core & cladding regions is called “sheath or buffer jacket”. strength to the fiber.