Topic-5

Light Amplification by Stimulated Emission of Radiation.

LASER

CLASSIFICATION OF LASER

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Solid Laser

Liquid Laser

Gas Laser

BASIC CONDITIONS TO PRODUCE LASER

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• There must be a meta stable state in the system

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• The system must achieve population inversion

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• The photons emitted must be confined in the system for a time to allow them further stimulated emission

Absorption of Radiations

Excited state

Ground state

Atoms goes to Excited State

Excited state

Ground state

When the atom is in excited state, it can make

a transition to a lower energy state through the

emission of electromagnetic radiation. This

process occurs in two different ways.

Emission of radiation

1.Spontaneous emission of radiation

2. Stimulated emission of radiation

Spontaneous emission of radiation

An atom in the excited energy state may

jump down to the lower energy state to

emit radiation spontaneously. This process

occurs in the absence of any

incident radiation.

E₂-E₁ = hν

Spontaneous Emission of Radiation

Excited state

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Ground state

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Spontaneous Emission of Radiation

Excited state

Ground state

Stimulated emission of radiation

An incident photon of appropriate

energy triggers an atom in the

excited energy state to

emit radiation. In this process two

photons of same frequency are emitted

Stimulated emission of radiation

Excited state

Ground state

Stimulated emission of radiation

Excited state

Ground state

Stimulated emission of radiation

Excited state

Ground state

Normally at a given temperature the number of

atoms in the ground state are more as compared

to the number of atoms in the excited state.

If the number of atoms in the excited state are

greater than the number of atoms in the

ground state such a state is known as

population inversion.

Population inversion

E₂

E₁

Population inversion

Excited state

Ground state

Einstein’s Coefficients

Consider the energy levels E₁ and E₂ having number of atoms per unit voIume as N₁ and N₂ respectively. When photon of energy hν Incident on the atom in lower energy State then it absorb photon and goes in higher energy state

The rate of absorption r_a is directly proportional to I) N₁ and ii) Energy density u( v)

hence r_a∝ N₁ u( v)

Therefore. r_a = B₁₂ N₁ u( v)

Where B₁₂ is proportionality constant

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Spontaneous Emission

In spontaneous emissions the rate of emissions depends only on number of atoms N₂ in excited state

r_sp ∝ N₂

r_sp = A₂₁ N₂

Here A₂₁ is proportionality constant.

Stimulated emission

The rate of stimulated emissions depends on energy density u( v) of radiations and number of atoms N₂

Hence r_st ∝ u( v) N₂

r_st = B₂₁ u( v) N₂

Where B₂₁ is proportionality constant

All these constants

B₁₂ , B₂₁ and A₂₁ are know as Einstein’s Coefficients

Optical Pumping

Optical pumping is a process in which light is used to raise (or "pump") electrons from a lower energy level in an atom or molecule to a higher energy. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion.

CAVITY RESONATOR

An optical cavity, or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers. some interferometers. Light confined in the cavity reflects multiple times, producing standing waves for certain resonance frequencies.

Cavity Resonator

Cavity Resonator

He-Ne laser

A helium–neon laser or He-Ne laser, is a type of gas laser whose active medium consists of a mixture of helium and neon in a ratio 10:1 at a pressure of about 1 mm of Hg inside of a small electrical discharge tube. The He-Ne laser operates at a wavelength of 632.8 nm, in the red part of the visible spectrum. The two electrodes at two ends are connected to H.T battery.

He-Ne laser

He-Ne laser

Ruby Laser

A ruby laser is a solid-state laser that uses the synthetic ruby crystal asactive medium. Ruby laser is the first successful laser developed by Maiman in 1960. Ruby laser is one of the solid-state lasers that produce visible light. It emits deep red light of wavelength 694.3 nm.

Ruby Laser

Ruby Laser consists of cylindrical rod of Ruby crystal having 10 cm long and 0.8 cm in diameter. One face of rod is fully reflecting and other face is partially reflecting which forms cavity resonator. Xenon flash lamp coiled around the rod provides optical pumping.

Ruby Laser

Ruby Laser

Ruby Laser

Properties of LASER

1-The laser beam is more directional.

2- The beam is highly intense.

3- The beam is monochromatic

4- The beam is highly coherent.

Applications of LASER

• Mechanical Engineering:

For > Drilling , Cutting and Welding

• Electronic Industries:

For > Soldering the Electronic Components

• Medical field :

For > Eye & Brain Surgery, To destroy cancer

Cells and Kidney Stones, to check decay

of teeth

Applications of LASER

• Defence Field:

For > Ranging and guiding weapons to

the target

• Computer Applications :

For > Laser Printers, Audio Systems &

Pointers

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Applications of LASER

• In Optical Fiber Communication System

Laser used to send Telephone Signals.

• Laser are used to measure large distances

such as distance between moon and earth.

• Holography : To take three dimensional

Photography

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