INTRODUCTION TO LASER

Dr. N. Venkatanatha

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LASER (Light Amplification by the Stimulated Emission of Radiation)

A device that creates a uniform and coherent light.

Laser output can be continuous or pulsed and is used in many applications.

LASER

Gas lasers are used to cut steel and perform delicate eye surgery.

solid state lasers create the ultra-high-speed, minuscule pulses traveling in optical fibers traversing the backbones of all major communications networks.

APPLICATIONS

A laser is an optical oscillator, which is made out of a solid, liquid or gas with mirrors at both ends.

To make the laser work, the material is excited or "pumped," with light or electricity. The pumping excites the electrons in the atoms, causing them to jump to higher orbits, creating a "population inversion."

A few of the electrons drop back to lower energy levels spontaneously, releasing a photon (quantum of light).

WORKING

The photons stimulate other excited electrons to emit more photons with the same energy and thus the same wavelength as the original.

The mirrors at both ends keep reflecting the light back and forth creating a chain reaction and causing the laser to emit laser light continuously.

In simple laser cavities, one mirror has a small transparent area that lets the laser beam out.

In semiconductor lasers, both mirrors often transmit a beam, the second one being used for monitoring purposes.

Properties of LASER

Directionality

Monochromacity

Coherence

High Intensity

PROPERTIES OF LASER

The directionality of the laser beam is usually expressed in terms of full angle of beam divergence,

it is twice the angle made by the outer edge of the beam with the axis of the beam.

At the outer edge the strength of the beam drops to 1/e times compared to its strength at the centre.

DIRECTIONALITY

ФФ1

Ф1

The angular spread (Ф) = λ/da

Where, da- Diameter of the aperture

λ- Wavelength of the beam Rayleigh Range: The range for which the laser light propagates as a parallel beam is called Rayleigh range, which is equal to da

2/λ.

For a typical laser the beam divergence is less than 0.01 milliradian (it spreads 0.01 mm/m), but the ordinary light the spread is 1m for every 1m of travel.

Since the angular spread depends upon distance from the source, the angular spread (Ф) = a2-a1 / [2(d2-d1)]

Where, a1 & a2 – diameters of laser radiation

at distances d1 & d2 from the laser source.

The laser emits light into a narrow beam; the concentration of energy is both spatially and spectrally. Therefore the intensity of the laser beam is very high.

For example, from a 100 W bulb emitting ordinary light, one can perceive only 1/1000 watt of light if he/she stands 1foot away from it.

HIGH INTENSITY

But laser can damage eye of a person, if allowed observe at a same distance.

The number of photons emitted by laser source is 1012

to 1028 times more than that of ordinary light.

monochromaticity is the function of wavelength spread of radiation.

It is, Δλ = (-c/v2) Δυ

MONOCHROMACITY

The light from a laser source is highly monochromatic compared to light from conventional incoherent monochromatic source.

The value of Δλ is in the order of 300 nm for white light, 0.01 nm for gas discharge lamp, and for laser it is 0.0001 nm.

A typical laser emits light in a narrow, low-divergence beam and with a well-defined wavelength.

This is in contrast to a light source such as the incandescent bulb, which emits into a large solid angle and over a wide spectrum of wavelength.

Laser beams can be focused to very small area of 0.7 µm thickness.

Spatial Coherence & Temporal Coherence

COHERENCE

For any electro-magnetic wave, if at times t1 and t2, the phase difference between two points in space remains the same, then it is called as spatial coherence.

If at a point P, the electro-magnetic wave at t and t+dt has same phase difference, if dt is the same, temporal coherence exists.

Types of Coherence

Laser material (crystal, gas, semiconductor, dye, etc...)

Pump source (adds energy to the lasing material, e.g. flash lamp, electrical current to cause electron collisions, radiation from a laser, etc.)

Optical cavity consisting of reflectors to act as the feedback mechanism for light amplification.

COMPONENTS OF A LASER

LASER SETUP

RUBY LASER

Theory of interaction of radiation with matter. (Emission and absorption of electromagnetic radiation by matter is in the form of photon).

Consider an atom that has only two energy levels. E1 and E2. When it is exposed to radiation having a stream of photons, each with energy hν, three distinct processes are possible.

Absorption, Spontaneous emission and Stimulated emission.

PRINCIPLE OF LASER

ABSORPTION

R12 α ρν

α N1

R12 = B12 ρν N1

Where,N1 - Number of atoms per unit volume andB12 – Probability of absorption per unit time.

Since higher energy state is unstable state the atoms will come back to the lower energy state with the emission of a photon. It may takes place in two ways.

Spontaneous emission and Stimulated emission

EMISSIONS

The atom or molecule in the higher energy state E2 is coming back to the ground state by emitting excess energy spontaneously.

This process is independent of the external radiation.

The rate of spontaneous emission is directly proportional to the population of the energy level E2.

Spontaneous Emission

SPONTANEOUS EMISSION

R21 (SP) α N2

R21 (SP) = A21 N2

Where, A21 – Probability per unit time that the atoms will spontaneously fall to the ground state. N2 – Number of atoms per unit volume in E2.

A photon of energy E, equal to the difference between the two levels E2 and E1, stimulate the atom to transit to ground state from the higher energy state by emitting second photon.

The rate of stimulated emission R21 (ST) is,

R21 (ST) = B21 ρν N2

Where, B21 – Probability per unit time that the atoms

undergo from higher energy state to lower energy state by stimulated emission.

STIMULATED EMISSION

In a system containing a very large number of atoms, the dominant process will depend on the virtual number of atoms in the excited and ground state.

If there are more number of atoms in the ground state (N1>N2), then there will be more absorption than stimulated emission.

If large number of atoms are present in the excited state (N1<N2), then stimulated emission will be more.

POPULATION INVERSION

Under the conditions of thermal equilibrium, N2/N1 = e-(E2- E1) /kT = e-hν /kT Where, k – Boltzmann’s constant & T – Absolute temperature. For Laser action, Stimulated emission

should be a dominate process. So it is necessary to increase the population of excited state and it is greater than that of ground state. This is known as Population inversion (P.I.).