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 Temporal coherence Coherent length l0 = λ2/Δλ = cT0

Spatial Coherence L should be smaller than λD/a

Spatial and Temporal Coherence 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

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.

Spontaneous Emission

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

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

Electron/Photon Interactions

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.

Einstein’s Coefficients

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

Total Absorption Probability

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

Emission and absorption are same

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.

After comparing with Planks Radiation Law And

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.

BASIC PRINCIPLE NEEDED FOR LASER

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

LASER ACTION

Types of Laser According to their sources: Gas Lasers Crystal Lasers   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

2- Level Laser E2 E2 E1 E1 E2 E1 Spontaneous Absorption Emission Stimulated Emission

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

4-Level LASER

PRACTICAL LASERS

He-Ne LASER Construction Helium: Neon= 10:1

Energy Level Diagram of He-Ne . Energy Level Diagram of He-Ne Power output: mW

He: Ne LASER

Nd:YAG LASER

Diode LASER

Emission wavelengths of common lasers

APPLICATION S OF LASER

Not to be Taken Lightly The Weighty Implications of Laser Technology Nuclear fusion Long distance measurement Holography Medical Optical Surgery General Surgery Tattoo removal Entertainment CD Players Applications of Laser Technology DVD Players Video Game Systems Telecommunications Information tech. Holograms Satellites Military Weapons Radar Industry 2017/9/22

Can You See the Light? Military and Space aircraft are equipped with laser guns Airplanes are equipped with laser radar Bad eyesight can be corrected by optical surgery using lasers Cd-Rom discs are read by lasers Dentists use laser drills Tattoo removal is done using lasers Laser tech. is used in printers, copiers, and scanners DVD players read DVD’s using lasers Laser pointers can enhance presentations CD-Audio is read by a laser Bar codes in grocery stores are scanned by lasers Video game systems such as PlayStation 2 utilize lasers 2017/9/22

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