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Laser. LASER. L ight A mplification by S timulated E mission of R adiation. C.H. Towns, 1954 T.H. Maiman, 1960. Noble prize, 1964. Incandescent vs. Laser Light. Many wavelengths Multidirectional Incoherent. Monochromatic Directional Coherent. Coherence. Coherence.
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LASER • Light • Amplificationby • Stimulated • Emission of • Radiation C.H. Towns, 1954 T.H. Maiman, 1960 Noble prize, 1964
Incandescent vs. Laser Light • Many wavelengths • Multidirectional • Incoherent • Monochromatic • Directional • Coherent
Coherence • Temporal coherence Coherent length l0 = λ2/Δλ = cT0
Spatial Coherence L should be smaller than λD/a
Spatial and Temporal Coherence Spatial and Temporal Coherence: Temporal Coherence; Spatial Incoherence Spatial Coherence; Temporal Incoherence Spatial and Temporal Incoherence Beams can be coherent or only partially coherent (indeed, even incoherent)in both space and time.
Monochromaticity • Degree of non monochromaticity τ = Δν/ν = c/l0ν=1/νT0 • Quality factor = λ/Δλ = l0/λ Ideally coherent: same energy, momentum, polarization τ = Δν/ν = c/l0ν=1/νT0
Absorption and Emission • Excitation potential and critical potential
Stimulated Absorption • Energy is absorbed by an atom, the electrons are excited into vacant energy shells.
Absorption E1 E2
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.
Stimulated Emission atoms in an upper energy level can be triggered or stimulated in phase by an incoming photon of a specific energy.
Stimulated Emission • The stimulated photons have unique properties: • In phase with the incident photon • Same wavelength as the incident photon • Travel in same direction as incident photon
WHY WE NEED META STABLE STATE? ANSWER IS With having the metastable state above the ground level. Atom reaches the meta stable state (after first stimulated emission) can remain there for longer time period. So the number of atom increases in the meta stable state. And when these atoms come back to the original ground level it emits laser beam.
Spontaneous emission A21 :- correspond to spontaneous emission probability per unit time This particular emission can occur without the presence of external field E(v)
Stimulated Absorption B12 :- correspond to stimulated absorption probability per unit time This type of absorption can occur in presence of external field E(v) only
Stimulated Emission B21 :- correspond to stimulated emission probability per unit time This type of emission can occur in presence of external field E(v) only
Total Emission Probability Spontaneous Emission + Stimulated Emission A21 + B21 E(v) Number of atoms that can jump from level E2 to E1 is
Equilibrium condition In case of energy states the number of electron absorbed and emitted should be equal or the rate of change of numbers of atoms in two states should be equal.
The rate of change of atoms in E2 It can be given by differentiation (probability) or
At Equilibrium Then
Maxwell Bolzman Distribution In thermal equilibrium
So the equations become So equation becomes
Planck’s Radiation Law Plank’s gives the formula that how a gas radiate energy.
Einstein’s Coefficients Einstein gives a probability that stimulated emission is same as absorption. Means that if a stimulated absorption can occur then there is same probability that stimulated emission can occur.
Conclusions • Stimulated emission have same probability as stimulated absorption • Ratio between spontaneous and stimulated emission varies with v3 • All we need is to calculate one of the probability to find others.
POPULATION INVERSION • A state of a medium where a higher-lying electronic level has a higher population than a lower-lying level
PUMPING • The method particle of raising a particle from lower energy state to higher energy state is called pumping. • TYPES OF PUMPING : • Optical pumping • Electrical pumping • X-ray pumping • Chemical pumping
LASER COMPONENTS All lasers have 3 essential components: • A lasing or "gain" medium • A source of energy to excite electrons in the gain medium to high energy states, referred to as "pump" energy • An optical path which allows emitted photons to oscillate and interfere constructively as energy is added or "pumped" into the system, ie, a resonator
Types of Laser • According to their sources: • Gas Lasers • Crystal Lasers • Semiconductors Lasers • Liquid Lasers • According to the nature of emission: • Continuous Wave • Pulsed Laser • According to their wavelength: • Visible Region • Infrared Region • Ultraviolet Region • Microwave Region • X-Ray Region • d. According to different levels • 1. 2-level laser • 2. 3-level laser • 3. 4-level laser • e. According to mode of pumping • 1. optical • 2. chemical • 3. electric discharge • 4. electrical
E2 E2 E2 E1 E1 E1 Absorption Spontaneous Emission Stimulated Emission 2- Level Laser
THREE STEP LASER • Stimulated absorption • Spontaneous emission to the meta stable state • Stimulated emission from meta stable state to ground state. E2 E1 E2 – E1 META STABLE STATE E1 – E0 E0
Helium: Neon= 10:1 Construction He-Ne LASER
. Energy Level Diagram of He-Ne Power output: mW