Introduction to LASER

Photons of

• Spontaneous emission are coherent

• Stimulated emission are not coherent

• Stimulated emission are in phase with stimulating photons

• Stimulated emission have no phase relationship

Spontaneous emission

• is proportional to the number of atoms per unit volume of excited state

• is proportional to the number of atoms per unit volume of ground state

• takes place when pumping is done

• results in photons of same phase

The rate of stimulated radiation

• does not depend on the number of atoms in excited state

• depends on number of atoms in ground state

• is responsible for production of population inversion

• depends on intensity of external radiation

The working of Laser is based on the principle of

• induced absorption

• spontaneous emission

• ionization

• stimulated emission

The lifetime of an atom in a metastable state is of the order of

• 10 ^-10 sec

• 10 ^-8 sec

• 10 ^-3 sec

• None of these

Rate of induced absorption depends on

• number of atoms in the lower energy state

• the energy density of incident radiation

• number of atoms in the higher energy state

• both number of atoms in the lower energy state and the energy density of incident radiation

LASER is a short form of

• Light Amplification by Spontaneous Emission Radiation

• Light Amplification by Stimulated Emission of Radiation

• Light Absorption by Stimulated Emission of Radiation

• Light Absorption by Spontaneous Emission of Radiation

Example of solid state laser is

• He-Ne Laser

• CO₂ Laser

• Nd:YAG Laser

• Dye Laser

What requirement needs to be met before a medium can begin to ‘laser’?

• 100% slope efficiency

• threshold voltage

• Population inversion

• Equal density of states

Usually, population inversion in gas laser is achieved by

• electric discharge

• heating

• optical pumping

• direct conversion