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LASER. German-born American physicist Birth: March 14, 1879 Death: April 18, 1955 Place of Birth: Ulm, Germany Known for: Proposing the theory of relativity, a physical theory of gravity, space, and time, Explaining the photoelectric effect and Brownian motion.

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albert einstein
German-born American physicist

Birth: March 14, 1879

Death: April 18, 1955

Place of Birth: Ulm, Germany

Known for: Proposing the theory of relativity, a physical theory of gravity, space, and time, Explaining the photoelectric effect and Brownian motion.

Albert Einstein first proposed stimulated emission, the underlying process for laser action, in 1917. Translating the idea of stimulated emission into a working model, however, required more than four decades.

Albert Einstein
charles townes
Charles Townes
  • American physicist Charles Townes won the 1964 Nobel Prize in physics. He made fundamental contributions in quantum theory and significantly improved radar technology.
  • In December 1953 Townes and his students constructed a device producing microwaves in a beam.
  • They dubbed the process “microwave amplification by stimulated emission of radiation,” which led to the more commonly used term maser.
  • The maser quickly found many applications for its ability to send strong microwaves in any direction.
  • In 1958 Townes developed the concepts for the visible-light maser, or laser (derived from “light amplification by stimulated emission of radiation”), which delivers infrared or visible light instead of microwaves.
theodore maiman
Theodore Maiman
  • Theodore Maiman, born in 1927, American physicist. Theodore Harold Maiman was born in Los Angeles and educated at the University of Colorado and Stanford University.
  • He was the first to successfully produce a pulse of coherent light from a laser, accomplishing this in May 1960, using ruby as the laser medium.
  • The first continuously operating laser was achieved a few months later.
  • Due to his work on the laser, he was twice nominated for a Nobel Prize and was given membership in both the National Academies of Science and Engineering


The laser perhaps is the most important optical device past fifty years.

The laser is essentially an optical amplifier. The word Laser is acronym that stands for Light Amplification by Stimulated Emission of Radiation.

  • Importance( Introduction & Applications,)
  • Induced absorption
  • Spontaneous and stimulated emission
  • Einstein’s coefficient.
  • Population inversion
  • Meta-stable state
  • Requisites of laser system
  • Ruby construction and working
  • He-Ne laser construction and working
Medical field

a. Eye surgery

b. Cosmetic / plastic surgery

c. Brain tumor

d. Endoscopic

e. Dental treatment and extraction

Defense field

a. Death ray

b. Defensive applications

c. Strategic defense initiative

d. Laser sight

e. Illuminator

f. Range finder( Measure of distance)

g. Target designator ( Measurement of mobile object with high accuracy)

Scientific and research field

a. Spectroscopy b. Lunar laser ranging

c. Photochemistry


Excimerlaser used for eye surgery.


Law Enforcement

Target Designators





L ight

A mplification by

S timulated

E mission of

R adiation



1. Monochromaticity

The light emitted by a laser is almost pure in color, almost of a single wavelength or frequency.


3. Directionality

The astonishing degree of directionality of a laser light is due to the geometrical design of the laser cavity and to the monochromaticity and coherent nature of light generated in the cavity.



The intensity of laser light is highly intense. For example intensity of light from a 1mW Helium- Neon laser is hundreds of times more intense than the light starting from an equal area on the surface of sun.


Focusing light to a tiny , diffraction limited spot is a challenge. Due to the incoherence and non point source ,it is difficult to focus the ordinary light to tiny spot. But as laser emits intense, coherent light that appears to come from distant point source, it can be focused to a diffraction limited spot.


Population of atoms in various energy levels

Normal Population & Population Inversion



Hence at thermal equilibrium, the population of higher energy state is always lesser than any of its lower states.



Stimulated/induced absorption

Spontaneous emission

Stimulated emission


Stimulated absorption: When an electromagnetic radiation of frequency  is incident on a sample of atoms, the electrons in the lower energy state (E1) absorb the energy from the incident radiation & rise to the higher energy state (E2). This process is called stimulated absorption. [Figure (a) above].


Spontaneous emission:The atoms excited to higher energy state are unstable there. Their life time  in these states is of the order of 108s. The electrons in these states spontaneously make transition to lower energy states emitting a photon whose energy h is the difference between the two energy states E2 and E1 i.e, E2 – E1.

This type of transition of an electron from a higher to a lower energy state without any outside stimulus is called spontaneous emission. The photons so emitted are in random phases and random directions.[Figure (b) previous slide].


Stimulated emission: When a photon of energy h = E2E1 is incident on an atom which is already in an excited state E2, the atom being disturbed or stimulated by the incident photon, makes a transition to a lower energy state E1 emitting a photon. The emitted photon has the same frequency, phase & direction as the incident photon. This type of emission is called stimulated emission. The net effect is two identical photons in the place of one thereby increasing the intensity of the incident beam. It is this process of stimulated emission that makes possible the amplification of light in lasers. [Figure (c) previous slide].

spontaneous and stimulated emission
Emission take place without external agency

Independent on incident light intensity

Transition take place b/n two states

Ordinary light radiation is emitted

Emission take place with external agency namely photon of right frequency

Dependent on incident light intensity

Transition take place b/n three states

Laser radiation is emitted

Spontaneous and Stimulated emission

Einstein A and B coefficients

Relation between radiation and matter

It was demonstrated by Einstein in 1917 that the rates of the three processes – stimulated absorption, spontaneous emission, and stimulated emission are related mathematically. This is the first step toward understanding lasing action in atomic system.

Consider two energy levels E1 and E2 with populations N1 and N2 in an atomic system in thermal equilibrium. E2 is larger than E1 and normally N2 is smaller than N1. There will be a transition between these two energy levels.


Radiation of a proper frequency (21) will cause transitions to occur between energy levels E1 and E2. In such a system, the rate of upward transitions must be equal to the rate of downward transitions.

The populations of two levels (at thermal equilibrium) are related by Boltzmann statistics, assuming non-degeneracy, as


Now consider such an atomic system in presence of radiation. Three possible processes then are,

  • Stimulated absorption or induced absorption – occur in presence of external radiation
  • Spontaneous emission – occur even in absence of radiation
  • Stimulated emission or induced emission – occur in presence of stimulating radiation

a)Stimulated absorption or induced absorption – occur in presence of external radiation.

The rate of stimulated absorption

B12 is a constant, characteristic of the atom called Einstein’s coefficient of stimulated absorption.


b) Spontaneous emission – can occur even in absence of radiation

The rate of spontaneous emission

A21 is a constant, characteristic of the atom called Einstein’s coefficient of spontaneous emission.


c) Stimulated emission or induced emission – occur in presence of stimulating radiation

The rate of stimulated emission

B21 is a constant, characteristic of the atom called Einstein’s coefficient of stimulated emission and .


At thermal equilibrium, the number of upward transitions must be equal to the number of downward transitions. Thus,

Dividing both numerator & denominator of above equation by N2,


Using Eq. (8) in Eq. (7), we have,

Dividing both numerator and denominator of R.H.S. by B21, we get


Now, the black-body radiation law developed by Planck gives an alternate expression for the energy density per unit frequency range in a photon cloud given by,


Comparing Eqs. (10) and (11), with h21 = h12 = h

That is, or a simple electronic system with no degeneracy.


Thus for a simple electronic system with no degeneracy, the probabilities of stimulated absorption and stimulated emission are the same [Equation (13), B12 = B21] and the ratio of A and B coefficients is given by equation (12)


We have

At thermal equilibrium, the ratio of the spontaneous to stimulated emission is given by, [using Eqs. (11) and (12)]


Special cases – Case 1

We have the ratio of the spontaneous to stimulated emission as

  • Case – 1: when h << kT,

That is, whenh << kT, the number of stimulated emission far exceed the number of spontaneous emission. LASING ACTION CAN OCCUR


Special cases – Case 2

  • Case – 2: When h >>kT

That is, whenh >> kT, the number of spontaneous emission far exceed the number of stimulated emission. LASING ACTION CAN NOT OCCUR.

This is the reason why at visible and ultraviolet frequencies lasing action is difficult as compared to microwave frequencies.



  • Population Inversion
  • Metastable states

 Depends on population N2

 Depends on population N1

If N2> N1 photons will be added to the field

 amplification

If N1> N2 photons will be added to the field

 attenuation

Thus for Laser to operate it is necessary to have

N2> N1 ,,,that is, population inversion.


b) Metastable states

  • Life time of the atom for a normal energy state is  108s.
  • Life time of the atom in metastable state is  103s.
  • Spontaneous transition from metastable state to lower state is forbidden.
  • Pupulation inversion N2 > N1 is possible

Essential components of a Laser

  • The laser device is an optical oscillator that emits an intense, highly collimated beam of coherent radiation . The device consists of,
  • External energy source or pump
  • An amplifying medium
  • Optical cavity or resonator

External energy source or pump

The pump is an external source of energy that produces population inversion in the laser medium. Pumps can be,

a) Electrical (Gas laser – He-Ne laser)

b) Optical (Solid state laser,– ruby, Nd:YAG lasers)

c) Chemical

d) thermal


2. An amplifying medium

  • The most important requirement of the amplying medium is its ability to support a population inversion between two energy levels of the laser atoms.
  • The laser medium determines the wavelength of the laser radiation.
  • The laser medium may be
  • A collection of atoms or molecules in a gas (He-Ne, Co2 laser)
  • Atoms or molecules or ions in solids (Ruby, Nd:YAG lasers)
  • Atoms or molecules or ions in liquids etc.
  • In some lasers, amplyfying medium consists of two parts: the laser host medium and laser atoms (dopants). Example: In ruby (Al2O3 doped with chromium atoms) laser, Al2O3 is the host and chromium is the dopant.

3. Optical cavity or resonator

It is an optical feed back device that directs photons back and forth through the laser medium.

Most basic form of a resonator – a pair of carefully aligned plane or curved mirrors.

One of the mirrors is chosen with a reflectivity of close to 100%, the other is selected with reflectivity some what less than 100 (99%) in order to allow part of the internally reflecting beam to escape and become the useful laser output beam.



Pumping radiation

λb = 400 nm

λg = 660 nm

RUBY, precious stone that occurs as a red, transparent variety of the mineral corundum. The color varies in different specimens from rose red through so-called ruby red and carmine to a deep purplish red, called pigeon blood.

Clear stones of the deeper shades are the most highly prized. When cut into a cabochon (a nonconvex) form, some specimens of ruby exhibit asterism; that is, a six-rayed star can be seen in the interior of the stone.

Such rubies, called star rubies, are also highly prized. Many stones that are not rubies are nevertheless called rubies. The balas, or balas ruby, for example, is a type of spinel; the Bohemian ruby is rose quartz; the Siberian ruby is red or pink tourmaline; American ruby, Cape ruby, Montana ruby, and Rocky Mountain ruby are varieties of garnet.

Corundum, mineral consisting of aluminum oxide, Al2O3.


1. The laser in its non-lasing state

2. The flash tube fires and injects light into the ruby rod. The light excites atoms in the ruby.


3. Some of these atoms emit photons

4. Some of these photons run in a direction parallel to the ruby\'s axis, so they bounce back and forth off the mirrors. As they pass through the crystal, they stimulate emission in other atoms.

5. Monochromatic, single-phase, columnated light leaves

the ruby through the half-silvered mirror -- laser light!


Helium – Neon Laser

It is an example of gas Laser. And uses electric discharge to produce population inversion.


W is the Brewster window Bis the Brewster angle

M1 is a 100% reflecting mirror M2 is 99% reflecting mirror


Neon atoms

Return to ground

State by collision with wall.


1. A three level laser emits light of wavelength 550 nm. (a) What is the ratio of population of the upper level (E2) to that of the lower level (E1) in laser transition, at 300 K? (b) At what temperature the ratio of the population of E2 to that of E1 becomes half? (c) At what negative temperature the population of the upper level exceeds that of the lower by 10%?


2. A hypothetical atom has two energy levels with a transition wavelength of 582 nm. In such a sample at 300 K, 4 x 1020 atoms are there in the lower state. (a) How many occupy the upper state under conditions of thermal equilibrium? (b) Suppose, instead, that 3.0 x 1020 atoms are pumped into upper state, with 1.0 x 1020 remaining in the lower state. How much energy could be released in a single laser pulse?


3. Compare the relative probabilities of spontaneous and stimulated emission in an equilibrium system at room temperature (T=300K) for transitions that occur in (a) the visible (h = 2eV) (b) the microwave regions (h = 104eV) of the spectrum.


4. The transition to the ground state from two closely spaced upper and lower states in a ruby laser results in the emission of photons of wavelengths 692.8nm and 694.3nm respectively at 300 K. Calculate the energy values of the two levels and also the ratio their populations.


5. A ruby laser delivers a 20.0-ns pulse of 0.1MW average power/pulse. If the number of photons emitted per second is 6.98×1015. Calculate wavelength of the photons.


6. A He-Ne laser emits light of wavelength of 632.8 nm and has an output power of 2.3 mW. How many photons are emitted each minute by this laser when operating?