the amazing world of lasers alexey belyanin department of physics, tamu
DESCRIPTION
The Amazing World of Lasers Alexey Belyanin Department of Physics, TAMU. Laser Definition and History Laser Radiation Laser System Active Medium and Pump Laser Cavity Laser Types and Applications. LASER = Light Amplification by Stimulated Emission of Radiation. - PowerPoint PPT PresentationTRANSCRIPT
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The Amazing World of Lasers
Alexey BelyaninDepartment of Physics, TAMU
• Laser Definition and History• Laser Radiation• Laser System
– Active Medium and Pump– Laser Cavity
• Laser Types and Applications
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LASER = Light Amplification by Stimulated Emission of Radiation
Laser is a device which transforms energy from other forms into (coherent and highly directional) electromagnetic radiation.
•1917 – A. Einstein postulates photons and stimulated emission•1954 – First microwave laser (MASER), Townes, Shawlow, Prokhorov•1960 – First optical laser (Maiman)•1964 – Nobel Prize in Physics: Townes, Prokhorov, Basov
•Chemical energy•Electron beam•Electric current•Electromagnetic radiation
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Microwave ammonia laser
= 24 GHz
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Ruby laser
Cr+3 ions lightly doped in a corundum crystal matrix (0.05% by weight Cr2O3 versus Al2O3) = 693 nm
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Electromagnetic spectrum
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Laser radiation
•Monochromaticity•Directionality•Coherence
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Monochromaticity
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Directionality
Radiation comes out of the laser in a certain direction, and spreads at a defined divergence angle ()
This angular spreading of a laser beam is very small compared to other sources of electromagnetic radiation, and described by a small divergence angle (of the order of milli-radians)
Lamp: W = 100 W, 22
mW/cm1.0~ R
WI
at R = 2 m
He-Ne Laser: W = 1 mW, r = 2 mm, R = r + R /2 = 2.1 mm, I = 8 mW/cm2
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Coherence )cos( iii tAE
Laser radiation is composed of waves at the same wavelength, which startat the same time and keep their relative phase as they advance.
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InterferenceYoung Interference Experiment
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Michelson Interferometer
Nobel Prize in Physics 1907
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For a completely coherent wave, defining its phase along particularsurface at specific time, automatically determine its phaseat all points in space at all time.
•Temporal Coherence is related to monochromaticity. •Spatial Coherence is related to directionality and uniphase wavefronts.
Coherence time tc ~ 1/, where is linewidth of laser radiation
Coherence Length (Lc) is the maximum path difference
which still shows interference: Lc = ctc = c/
Typical laser linewidths: from MHz to many GHzRecord values ~ kHz
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Laser System1. Active (gain) medium that can amplify light that passes
through it 2. Energy pump source to create a population inversion in
the gain medium 3. Two mirrors that form a resonator cavity
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Amplifier vs. Generator
No (or negative) feedback:
Positive feedback:
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Active medium
N1, N2, N3 … – populations of states 1,2,3, …Population inversion: N2 > N1 or N3 > N2 etc.
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Thermodynamic equilibrium
N2/N1 = = exp(-(E2-E1)/kT)
In optics E2 – E1 ~ 1 eV while at room temperature kT = 0.025 eV.Therefore, N2/N1 ~ 10-18
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Three one-photon interactions between radiation and matter
1. Photon Absorption
Absorption rate:
d N2(t)/dt = K n(t) N1(t)
n(t) - number of incoming photons per unit volume
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2. Spontaneous emission of a photon
d N2(t)/dt = - g21 N2(t) = - N2(t)/ t2
Solution: N2(t) = N2(0) exp(-g21t) = N2(0) exp(-t/ 2)
Spontaneous decay rate:
Spontaneous photons are emitted randomly and in all directions
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3. Stimulated emission of a photon
d N2(t)/dt = - K n(t) N2(t)
Proportionality constant (K) for stimulated emission and (stimulated) absorption are identical.
•Stimulated photons have the same frequency and direction. •Stimulated emission is a result of resonance response of the atom to a forcing signal!
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Rate Equations
dN2(t)/dttot = dN2(t)/dtabsorp+ dN2(t)/dtStimul+ dN2(t)/dtSpontan
= +Kn(t)[N1(t)-N2(t)]-g21N2(t) = - dN1(t)/dttot
dn(t)/dt = -K [N1(t)-N2(t)] n(t)
n(t) = n(0) exp[-K(N1-N2)t]; N2 > N1 is needed for amplification
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Three-level laser scheme
For population inversion, more than 50% of all atoms must be in state 2.Very tough requirement!
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Four-level laser scheme
Much lower pumping rate is needed
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Helium-Neon laser
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Laser Threshold
1. Scattering and absorption losses at the end mirrors. 2. Output radiation through the output coupler. 3. Scattering and absorption losses in the active medium, and at the side walls. 4. Diffraction losses because of the finite size of the laser components.
At threshold the gain should be equal to losses
Sources of losses:
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Gain spectrum can be very broad
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Broadening of the gain spectrum
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Laser Cavity
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Longitudinal modes in Fabry-Perot cavity
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Hole burning in the gain spectrum
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Transverse modes
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How to make a laser operate in a single basic transverse mode?
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Laser Types
Lasers can be divided into groups according to different criteria:
1. The state of matter of the active medium: solid, liquid, gas, or plasma. 2. The spectral range of the laser wavelength: visible, Infra-Red (IR), etc. 3. The excitation (pumping) method of the active medium: Optical
pumping, electric pumping, etc. 4. The characteristics of the radiation emitted from the laser. 5. The number of energy levels which participate in the lasing process.
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Classification by active medium
• Gas lasers (atoms, ions, molecules)• Solid-state lasers• Semiconductor lasers
– Diode lasers– Unipolar (quantum cascade) lasers
• Dye lasers (liquid)• X-ray lasers• Free electron lasers
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Gas Lasers
The laser active medium is a gas at a low pressure (A few milli-torr).
The main reasons for using low pressure are: •To enable an electric discharge in a long path, while the electrodes
are at both ends of a long tube. •To obtain narrow spectral width not expanded by collisions between
atoms.
The first gas laser was operated by T. H. Maiman in 1961, one year after the first laser (Ruby) was demonstrated.
The first gas laser was a Helium-Neon laser, operating at a wavelength of 1152.27 [nm] (Near Infra-Red).
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Pumping by electric discharge
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Argon ion laser
High power, but low efficiency
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CO2 Laser
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Gas lasers exist in nature!
•Stellar atmospheres•Planetary atmospheres•Interstellar medium
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Solid state lasers
Nd ions in YAG crystal host
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Inertial confinement for nuclear
fusion
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Laser Fusion
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D + T ==> 4He + n + 17.6 [MeV]
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Free electron lasers
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Applications•Industrial applications•Medical (surgery, diagnostics)•Military (weapons, blinders, target pointers,…)•Daily (optical communications, optical storage, memory)•Research
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