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PRINCIPLES ANDAPPLICATIONS OF
LASER
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LASERS ARE EVERYWHERE…
5 mW diode laserFew mm diameter
Terawatt NOVA laserLawrence Livermore Labs
Futball field size
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1. What is laser?2. Brief laser history3. Principles of laser4. Properties of laser light5. Types of laser6. Biomedical applications of laser
Laser
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E1
E2h
h
MASER: Microwave Amplification by Stimulated Emission of Radiation
LASER:Light Amplification by Stimulated Emission of Radiation
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LASER HISTORY IN A NUTSHELL
1917 - Albert Einstein: theoretical prediction of stimulated emission
1946 - G. Meyer-Schwickerather: first eye surgery with light
1950 - Arthur Schawlow and Charles Townes: emitted photons may be in the visible range
1954 - N.G. Basow, A.M. Prochorow, and C. Townes: ammonia maser
1960 - Theodore Maiman: first laser (ruby laser)
1964 - Basow, Prochorow, Townes (Nobel prize): quantum electronics
1970 - Arthur Ashkin: laser tweezers
1971 - Dénes Gábor (Nobel prize): holography
1997 - S. Chu, W.D. Phillips and C. Cohen-Tanoudji (Nobel prize):atom cooling with laser
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Laser principles I. Stimulated emission
Elementary radiative processes:
1. Absorption 2. Spontaneous emission 3. Stimulated emission
A21
Explanation: two-state atomic or molecular systemE1, E2 : energy levels, E2>E1
() : spectral power density of external fieldN1, N2 : number of atoms, molecules on the given energy levelB12, A21, B21: transition probabilities between energy levels (Einstein coefficients), B12 = B21
•Frequency of transition:n12=N1B12()
E= E2-E1=h energy quantumis absorbed.
E1
E2
N1
N2
B12
() B21()
•Frequency of transition:n21=N2A21
•E2-E1 photons radiate independently in all directions.
•Frequency of transition:n21=N2B21()
•Upon external stimulation.•Field energy increases.•Phase, direction andfrequency of emitted andexternal photons are identical.
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Laser principles II. Population inversion
•Amplification dependson the relative populationof energy levels
ActivemediumF F+dF
dz
A
dF=FA(N2-N1)dz
E1
E2
E1
E2
Thermal equilibrium Population inversion
•Population inversion onlyin multiple-state systems!•Pumping: electric,optical, chemical energy
E0
E2
E1
Pumping
Fast relaxation
Laser transition
Metastable state
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Laser principles III. Optical resonance
Active medium
PumpingEnd mirrorPartially
transparent mirror
Laser beam
d=n/2
Resonator:• two, parallel planar (or concave) mirrors• Couples part of the optical power back in the active medium• Positive feedback -> self-excitation -> resonance
• Optical shutter in the resonator: Q-switch, pulsed mode
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Properties of laser I.
1. Small divergenceParallel, collimated beam
2. High powerIn continuous (CW) mode: tens, hundreds of watts (e.g., CO2)Q-switched mode: instantaneous power is enormous (GW)Large spatial power density due to small divergence
3. Small spectral width“Monochrome”Large spectral power density
4. Polarized
5. Possibility of very short pulses ps, fs
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Properties of laser II.
6. Coherencephase equivalence, ability for interferenceTemporal coherence (phase equivalence of photons emitted at different times)Spatial coherence (phase equivalence across beam diameter)
Application: holography
OBJECTOBJECT
Photo plate
Reference beam
Laser beam
Beamsplitter
Rays reflected from object
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Types of laser
Based on active m edium:1. Solid state lasersCrystals or glasses doped with metal ions; Ruby, Nd-YAG, Ti-zaphireRed - infrared spectral range; CW, Q-switched modes, high power
2. Gas lasersBest known: He-Ne laser (10 He/Ne). Small energy, wide useCO2 laser: CO2-N2-He mixture; ~10 µm; enormous power (100 W)
3. Dye lasersDilute solution of organic dyes (e.g., rhodamine, coumarine); pumped with another laserLarge power (in Q-switched mode); Tunable
4. Semiconductor lasersAt the junction of p- and n-type, doped semiconductors.No need for resonator mirrors (internal reflection)Red, IR spectral range. Large CW power (up to 100 W)Beam characteristics not ideal. Wide use due to small size.
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Biomedical applications of laser I.
Principles:1. Interaction of light with biological matter
2. Properties of laser beamFocusing, wavelength, power
3. Properties of biological tissueTransmittivity, absorbance, light-induced
reactions
Absorption
Reemission
Scatter
Transmission
ReflectionIncident beam
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Biomedical applications of laser II. Surgical disciplines: “laser scalpel”, coagulation, bloodless operation.Removal of tumors, tattoo. CO2 and Nd:YAG lasers.
Dentistry: caries absorbs light preferentially (drilling).
Photodynamic tumor therapy: laser activation of photosensitive chemicals (e.g., hematoporphyrin derivatives) preferentially taken up by tumorous tissue.
Ophthalmology: Retina displacement, photocoagulation of fundus, glaucoma, photorefractive keratectomy (PRK).
Visible laser UV laser
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Biomedical applications of laser III.Optical tweezers
F F
Microscope objective
Scatteringforce
(light pressure)
Gradientforce
Laser
Refractilemicrobead
EQUILIBRIUM
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Tying a knot on an actin filament with laser tweezers
Arai et al. Nature 399, 446, 1999.
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KEY WORDS
What is needed for laser operation?•Stimulated emission•Population inversion•Pumping•Optical resonance
What are the main properties of laser light?•Monochromatic•Coherent•Large optical power