Transcript The Amazing World of Lasers Alexey Belyanin Department

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
Laser is a device which transforms energy from other forms
into (coherent and highly directional) electromagnetic radiation.
•Chemical energy
•Electron beam
•Electric current
•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
Microwave ammonia laser
 = 24 GHz
Ruby laser
Cr+3 ions lightly doped in a corundum crystal matrix
(0.05% by weight Cr2O3 versus Al2O3)
 = 693 nm
Electromagnetic spectrum
Laser radiation
•Monochromaticity
•Directionality
•Coherence
Monochromaticity
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,
W
I ~ 2  0.1mW/cm 2
R
at R = 2 m
He-Ne Laser: W = 1 mW, r = 2 mm, R = r + R /2 = 2.1 mm, I = 8 mW/cm2
Coherence
E   A cos( t   )
i
i
i
Laser radiation is composed of waves at the same wavelength, which start
at the same time and keep their relative phase as they advance.
Interference
Young Interference Experiment
Michelson Interferometer
Nobel Prize in Physics 1907
For a completely coherent wave, defining its phase along particular
surface at specific time, automatically determine its phase
at 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 GHz
Record values ~ kHz
Laser System
1. 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
Amplifier vs. Generator
No (or negative) feedback:
Positive feedback:
Active medium
N1, N2, N3 … – populations of states 1,2,3, …
Population inversion: N2 > N1 or N3 > N2 etc.
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
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
2. Spontaneous emission of a photon
Spontaneous decay rate:
d N2(t)/dt = - g21 N2(t) = - N2(t)/ t2
Solution: N2(t) = N2(0) exp(-g21t) = N2(0) exp(-t/ t2)
Spontaneous photons are emitted randomly and in all directions
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!
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
Three-level laser scheme
For population inversion, more than 50% of all atoms must be in state 2.
Very tough requirement!
Four-level laser scheme
Much lower pumping rate is needed
Helium-Neon laser
Laser Threshold
Sources of losses:
1.
2.
3.
4.
Scattering and absorption losses at the end mirrors.
Output radiation through the output coupler.
Scattering and absorption losses in the active medium, and at the side walls.
Diffraction losses because of the finite size of the laser components.
At threshold the gain should be equal to losses
Gain spectrum can be very broad
Broadening of the gain spectrum
Laser Cavity
Longitudinal modes in
Fabry-Perot cavity
Hole burning in the gain spectrum
Transverse modes
How to make a laser operate in a single basic
transverse mode?
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.
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
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).
Pumping by electric discharge
Argon ion laser
High power, but low efficiency
CO2 Laser
Gas lasers exist in nature!
•Stellar atmospheres
•Planetary atmospheres
•Interstellar medium
Solid state lasers
Nd ions in YAG crystal host
Inertial confinement for nuclear
fusion
Laser Fusion
D + T ==> 4He + n + 17.6 [MeV]
Free electron lasers
Applications
•Industrial applications
•Medical (surgery, diagnostics)
•Military (weapons, blinders, target pointers,…)
•Daily (optical communications, optical storage, memory)
•Research
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