The Amazing World of LasersAlexey BelyaninDepartment of Physics, TAMU: The Amazing World of Lasers Alexey Belyanin Department of Physics, TAMU Laser Definition and History
Active Medium and Pump
Laser Types and Applications LASER = Light Amplification by Stimulated Emission of Radiation: 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
… Microwave ammonia laser: Microwave ammonia laser = 24 GHz Ruby laser: Ruby laser Cr+3 ions lightly doped in a corundum crystal matrix
(0.05% by weight Cr2O3 versus Al2O3)
= 693 nm
Electromagnetic spectrum: Electromagnetic spectrum Laser radiation: Laser radiation Monochromaticity
Coherence Monochromaticity: Monochromaticity Directionality: 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, 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: Coherence 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: Interference Young Interference Experiment Michelson Interferometer: Michelson Interferometer Nobel Prize in Physics 1907 Slide13: 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: Laser System
Active (gain) medium that can amplify light that passes through it
Energy pump source to create a population inversion in the gain medium
Two mirrors that form a resonator cavity
Amplifier vs. Generator: Amplifier vs. Generator No (or negative) feedback: Positive feedback: Active medium: Active medium N1, N2, N3 … – populations of states 1,2,3, …
Population inversion: N2 > N1 or N3 > N2 etc. Thermodynamic equilibrium: 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: Three one-photon interactions between radiation and matter Photon Absorption Absorption rate: d N2(t)/dt = K n(t) N1(t)
n(t) - number of incoming photons per unit volume Slide19: 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/ t2) Spontaneous decay rate: Spontaneous photons are emitted randomly and in all directions Slide20: 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: 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: 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: Four-level laser scheme Much lower pumping rate is needed Helium-Neon laser: Helium-Neon laser Laser Threshold: Laser Threshold 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 Sources of losses: Gain spectrum can be very broad: Gain spectrum can be very broad Broadening of the gain spectrum: Broadening of the gain spectrum Laser Cavity: Laser Cavity Longitudinal modes in Fabry-Perot cavity: Longitudinal modes in Fabry-Perot cavity Hole burning in the gain spectrum: Hole burning in the gain spectrum Transverse modes: Transverse modes Slide34: How to make a laser operate in a single basic
Laser Types: Laser Types Lasers can be divided into groups according to different criteria:
The state of matter of the active medium: solid, liquid, gas, or plasma.
The spectral range of the laser wavelength: visible, Infra-Red (IR), etc.
The excitation (pumping) method of the active medium: Optical pumping, electric pumping, etc.
The characteristics of the radiation emitted from the laser.
The number of energy levels which participate in the lasing process.
Classification by active medium: Classification by active medium Gas lasers (atoms, ions, molecules)
Unipolar (quantum cascade) lasers
Dye lasers (liquid)
Free electron lasers
Slide38: 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: Pumping by electric discharge Argon ion laser: Argon ion laser High power, but low efficiency CO2 Laser: CO2 Laser Slide46: Gas lasers exist in nature!
Interstellar medium Solid state lasers: Solid state lasers Nd ions in YAG crystal host Inertial confinement for nuclear fusion : Inertial confinement for nuclear fusion Laser Fusion: Laser Fusion Slide50: D + T ==> 4He + n + 17.6 [MeV] Free electron lasers: Free electron lasers Applications: Applications Industrial applications
Medical (surgery, diagnostics)
Military (weapons, blinders, target pointers,…)
Daily (optical communications, optical storage, memory)