LASERLASER

LASERLASER stands for stands for LIGHT APLIFICATION LIGHT APLIFICATION by STIMULATED EMISSION of by STIMULATED EMISSION of RADITIONSRADITIONS

First laser was constructed by MaimanFirst laser was constructed by Maiman Laser action has been obtained with Laser action has been obtained with

atoms, ions & molecules in gases, liquids, atoms, ions & molecules in gases, liquids, solids, semiconductors at wavelength( solids, semiconductors at wavelength( λλ ) ) ranging from ultraviolet to radio frequency ranging from ultraviolet to radio frequency regions and with power ranging from a few regions and with power ranging from a few milli watts to mega watts.milli watts to mega watts.

Basic principle involved in laser

Stimulated absorption

Spontaneous emission Stimulated

emission

Stimulated AbsorptionStimulated Absorption EE11 & & EE00 are two energy are two energy

levels such that levels such that EE11 is is excited energy level & excited energy level & EE00 is lower energy level.is lower energy level.

If hIf hƒ =Eƒ =E11- E- E00 ,where ƒ is ,where ƒ is frequency of incident frequency of incident radiation, then radiation radiation, then radiation photon is absorbed & photon is absorbed & electron jumps from Eelectron jumps from E00 to to

EE1.1.

Spontaneous EmissionSpontaneous Emission The excited atom does The excited atom does

not remain in higher not remain in higher energy state for long.It energy state for long.It gives out a photon of gives out a photon of energy henergy hƒ= Eƒ= E11- E- E00 & & returns to ground returns to ground state. As this process state. As this process does not require does not require stimulus from stimulus from outside,so it is termed outside,so it is termed as spontaneous as spontaneous emission.emission.

Stimulated EmissionStimulated Emission The excited atom in energy The excited atom in energy

state Estate E11 may not come to may not come to ground state at given instant. ground state at given instant. So it requires external So it requires external stimulation. When a photon stimulation. When a photon of energy hof energy hƒ= Eƒ= E11- E- E00 strikes strikes the atom in excited state , it the atom in excited state , it comes to ground state. The comes to ground state. The photon emitted goes parallel photon emitted goes parallel to incident photon & both of to incident photon & both of these photons are in phase. these photons are in phase. For laser to have Stimulated For laser to have Stimulated Emission, the following two Emission, the following two conditions must be satisfiedconditions must be satisfied

1)The higher energy state 1)The higher energy state should have longer life timeshould have longer life time

2) The number of atoms in 2) The number of atoms in higher energy state Ehigher energy state E11 must must be more than that in Ebe more than that in E00

Population InversionPopulation Inversion The situation in which The situation in which

number of atoms in higher number of atoms in higher energy state is greater energy state is greater than in lower energy state is than in lower energy state is called population inversion. called population inversion. It can be obtained even at It can be obtained even at room temperature by laser room temperature by laser pumping.various pumping pumping.various pumping methods are:optical methods are:optical pumping,chemical pumping, pumping,chemical pumping, electrical pumping. when electrical pumping. when population inversion is population inversion is obtained, emitted photons obtained, emitted photons have same frequency and have same frequency and phase. They travel in same phase. They travel in same direction. Thus the number direction. Thus the number of photons goes on of photons goes on multiplying by stimulated multiplying by stimulated emission. Hence we get a emission. Hence we get a highly intense, highly intense, monochromatic, coherence monochromatic, coherence and unidirectional beam.and unidirectional beam.

Active mediumFully

Reflecting mirror

Partially reflecting mirror

RESONANCE CAVITY

A medium with population inversion is capable of amplification but in order to get a continous supply of light energy it is required to couple a part of output back into the medium. It can be made by placing an active medium between two mirrors such that one is fully reflecting and other is partially reflecting.

Active medium: A medium which on excitation produces population inversion and promotes stimulated emission

Characteristics of LaserCharacteristics of Laser

1)Directionality1)Directionality 2)Intensity2)Intensity 3)Monochromaticity3)Monochromaticity 4)Coherence4)Coherence

1)Directionality1)Directionality The conventional source The conventional source

emits radiation in all emits radiation in all direction. But Laser direction. But Laser emits light in particular emits light in particular direction. Directionality direction. Directionality of laser can be of laser can be expressed in terms of expressed in terms of full angle beam full angle beam divergencedivergence,, which is which is twice the angle that twice the angle that outer edge of beam outer edge of beam makes with the axis of makes with the axis of beam.beam.

The angular spread The angular spread ΔθΔθ ==λλ/d where d is /d where d is diameter of aperture.diameter of aperture.

2) Intensity2) Intensity

Laser gives out light in form of narrow beam with Laser gives out light in form of narrow beam with energy concentrated in a small region.energy concentrated in a small region.

Even 1 watt laser appears thousands times more Even 1 watt laser appears thousands times more intense than 100 watt ordinary lamp.intense than 100 watt ordinary lamp.

The power output from a small gas laser is 10The power output from a small gas laser is 10-3-3 watt whereas that from a pulsed solid state laser watt whereas that from a pulsed solid state laser may be as high as 10may be as high as 1099 watt. watt.

3) Monochromaticity3) Monochromaticity

Laser emits light which is more monochromatic than any other monochromatic conventional source.

Light emitted from a monochromatic conventional source is not perfectly sharp.It spreads whereas laser light spreads very little as shown in figure.

4) Coherence4) Coherence

It is measure of degree of phase It is measure of degree of phase correlation that exists in radiation field of correlation that exists in radiation field of light source at different locations and light source at different locations and different times.different times.

Cohernce is of two typesCohernce is of two types

a) Spatial Coherencea) Spatial Coherence

b) Temporal Coherenceb) Temporal Coherence

Temporal CoherenceTemporal Coherence

If the phase difference between any If the phase difference between any two points at an instant of time lying two points at an instant of time lying in direction of propagation of the in direction of propagation of the electromagnetic wave is independent electromagnetic wave is independent of time, the coherence is temporal of time, the coherence is temporal coherence.coherence.

Spatial CoherenceSpatial Coherence

If the phase difference between any If the phase difference between any two points at an instant of time lying two points at an instant of time lying in the plane perpendicular to in the plane perpendicular to direction of propagation is direction of propagation is independent of time, the coherence independent of time, the coherence is spatial coherence.is spatial coherence.