Laser Light• “LASER” = Light Amplification by Stimulated Emission of Radiation
What is Laser? Light Amplification by Stimulated Emission of Radiation• A device produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher energy levels• When they return to lower energy levels by stimulated emission, they emit energy.
Properties of Laser• Monochromatic Concentrate in a narrow range of wavelengths (one specific colour).• Coherent All the emitted photons bear a constant phase relationship with each other in both time and phase• Directional A very tight beam which is very strong and concentrated.
Basic concepts for a laser• Absorption• Spontaneous Emission• Stimulated Emission• Population inversion
Absorption E2 E1• Energy is absorbed by an atom, the electrons are excited into higher energy state.
Absorption E2 = E1 + hυ• The probability of this absorption from state 1 to state 2 is proportional to the energy density u(v) of the radiation P = N1 B12u (v) 12 where the proportionality constant B12 is known as the Einstein’s coefficient of absorption of radiation.
Spontaneous Emission• The atom decays from level 2 to level 1 through the emission of a photon with the energy hv. It is a completely random process.
Spontaneous EmissionThe probability of occurrence of this spontaneous emission transition from state 2 to state 1 depends only on the properties of states 2 and 1 and is given by ( P21 ) sp = A21 N 2 where the proportionality constant A21 is known as the Einstein’s coefficient of spontaneous emission of radiation.
Stimulated emission leads to a chain reaction and laser emissionIf a medium has many excited molecules or atoms, one photon can becomemany. Excited mediumThis is the essence of the laser.
Stimulated EmissionThe probability of occurrence of stimulated emission transition fromthe upper level 2 to the lower level 1 is proportional to the energydensity u(v) of the radiation and is given by ( P21 ) st = B21 N 2u (v)where the proportionality constant B21 is known as the Einstein’scoefficient of stimulated emission of radiation.Thus the total probability of emission transition from the upper level2 to the lower level 1 is P21 = ( P21 ) sp + ( P21 ) st P21 = N 2 [ A21 + B21u (ν )]
Relation between Einstein’s CoefficientsLet N1 and N2 be the number of atoms at any instant in the state 1and 2, respectively. The probability of absorption transition foratoms from state 1 to 2 per unit time is P = N1 B12u (v) 12The probability of transition of atoms from state 2 to 1,either byspontaneously or by stimulated emission per unit time is P21 = N 2 [ A21 + B21u (ν )]In thermal equilibrium at temperature t, the emission and absorptionprobabilities are equal and thus P = P21 12
N1 B12u (ν ) = N 2 [ A21 + B21u (ν )] N 2 A21 u (ν ) = N1 B12 − N 2 B21But Einstein proved thermodynamically that probability of(stimulated) absorption is equal to the probability of stimulatedemission, So B12 = B21 N 2 A21 u (ν ) = N1 B21 − N 2 B21 A21 1 u (ν ) = B21 ( N1 / N 2 ) − 1
According to Boltzmann’s law, the distribution of atoms among theenergy states E1 and E2 at the thermal equilibrium at temperature Tis given by N1 e − E1 / kT = − E2 / kT = e ( E2 − E1 ) / kT N2 ewhere k is the Boltzmann constant N1 = e hν / kT N2 A21 1 u (ν ) = (1) B21 e hν / kT − 1
Planck’s radiation formula gives the energy density of radiation u(v)as 8πhν 3 1 u (ν ) = (2) e 3 e hν / kT − 1from equation (1) and (2) A21 8πhν 3 = B21 e3This equation gives the relation between the probabilities ofspontaneous and stimulated emission.
Condition for the laser operationIf N1 > N2• radiation is mostly absorbed• spontaneous radiation dominates.if N2 >> N1 - population inversion• most atoms occupy level E2, weak absorption• stimulated emission prevails• light is amplified Necessary condition: population inversion
Population InversionThis situation in which the number of atoms in the higher stateexceed that in the lower state (N2 > N1) is known as populationinversion. PumpingThe process of moving the atoms from their ground state to anexcited state is called pumping. The objective is to obtain a non-thermal equilibrium. Optical Electrical Pumping Pumping
Optical PumpingThe atoms are excited by bombarding them with photonsexample: Ruby Laser Electrical Pumping The atoms are excited by Electron collision in a discharge tube. example: He-Ne Laser
Lasers that maintain a population inversionindefinitely produce continuous output – termedCW (for continuous wave) lasersLasers that have a short-lived population inversionproduce pulsed output – these are pulsed lasers
Ruby LaserOptical PumpingCoolent requiredHigh Power of 10 kW He-Ne LaserElectronic pumpingCoolent not requiredLow Power of about 0.5 – 5 mW
Applications of LaserLaser beams are very intense so are used forwelding, cutting of materials.Lasers are used for eye surgery, treatment ofdental decay and skin diseases.Lasers are used for barcode scanners in libraryand in super markets.Laser is used in printers (Laser printers).Lasers are used for Nuclear Fusion.Laser are used in CD/DVD PlayerLaser is used in Holography.Laser torch are used to see long distantobjects.
HolographyHolography is the production of three-dimensionalimages of objects.The physics of holography was developed by DennisGabor in 1948. He was awarded the 1971 Nobel Prize.The laser (1960s) met the requirement of coherent lightneeded for making holographic images.
HolographyIn Holography both the amplitude and phasecomponents of light wave are recorded on a lightsensitive medium such as a photographic plate.Holography is a two step process.In First step is the recording of the Hologram where theobject is transformed into a photographic record.Second step is the reconstruction in which the Hologramis transformed into the image.
Principle of HolographyHolography is the interference between two waves, anobject wave which is the light scattered from the objectand the reference wave, which is the light reaching thephotographic plate directly.The film records the intensity of the light as well as thephase difference between the scattered and referencebeams.The phase difference results in the 3-D perspective.
Conventional vs. Holographic photography• Conventional: – 2-d version of a 3-d scene – Photograph lacks depth perception or parallax – Phase relation (i.e. interference) are lost
Conventional vs. Holographic photography• Hologram: – Freezes the intricate wavefront of light that carries all the visual information of the scene – Provides depth perception and parallax – Gives information about amplitude as well as phase of an object. – The hologram is a complex interference pattern of microscopically spaced fringes
Construction of Hologram Mirror Reference BeamIncidentLaser ObjectBeam Object Beam Photographic Plate (Hologram)
Reconstruction of HologramLaserBeam Hologram Virtual Image Real Image
HolographyA hologram is best viewed in coherent light passingthrough the developed film.The interference pattern recorded on the film acts as adiffraction grating.By looking through the hologram, we see virtualimage.
National Geographic• First major publication to put a hologram on its cover• March 1984 issue carried nearly 11 million holograms around the world
Applications of Holography• Design of containers • Improve design of to hold nuclear aircraft wings and materials turbine blades• Credit cards carry • Used in aircraft “heads-up display” monetary value • Art• Supermarket • Archival Recording of scanners fragile museum• Optical Computers artifacts
Holography goes Hollywood• Holodeck from Star Trek Holodeck Clip• Star Wars Chess Game• Body Double in Total Recall• The Wizard in Wizard of Oz