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Examples of Specific Laser Systems
Gas Lasers CO2 200+ kW
Solid-State Lasers Nd:YAG (15 kW)
Fiber Lasers Yb3+ (5+ kW) Dye Lasers
Chemical Lasers COIL (7+kW), MIRACL (>1 MW !!)
Semiconductor Lasers
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6.5 Active media and spectral ranges
wavelength
100 nm 1 mm 10 mm 500 nm
Ultraviolet Visible Infrared
dye lasers
semiconductor lasers excimer lasers
solid-state lasers
molecular gas lasers
atomic gas lasers
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Proprietary Data University of New Mexico
A B
A B
A B
electronic transitions
vibrational transitions
rotational transitions
emission
VIS, UV
NIR, IR
FIR
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Section 6.5, p.2
Proprietary Data University of New Mexico
Typical laser efficiencies : Argon - ion < 0.1% CO2 laser < 20% Excimer < 20% GaAlAs (diode laser) < 40% HeNe < 0.1% Nd:YAG < 10%
powerinput electrical
poweroutput
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Gas Lasers
The excitation mechanism in most gas lasers is via electric discharge
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The first Gas Laser: He-Ne
Ali Javan, et al. (Bell Labs, 1962)
Section 11.1 p.2
• The second working LASER system to be demonstrated.
• The first gas LASER to be produced.
• The first LASER to produce a continuous output beam
• The active laser medium is a gaseous mixture of He & Ne atoms, in a roughly 10:1 proportion
• The gas is enclosed in a cylindrical quartz DISCHARGE tube
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Laser Type Linear Power Density
W/m
Maximum Power
W
Power Efficiency percent
He-Ne 0.1 1 0.1
Argon 1-10 50 0.1
CO2 60-80 >104 15-20
Comparison of Gas Lasers
Section 11.1 p.3
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C. K. N. Patel, "Continuous-Wave Laser Action on Vibrational Rotational Transitions of CO2," Physics Review, Vol. 136 A, (Nov., 1964) P. 1187
CO2 Lasers (9-11 micron)
Applications (pealing peanuts to star wars)
•Industrial (cutting, welding, material processing)
•Military (range finding, targeting, remote sensing, sensor blinding, destroying …)
•Medical (cutting, skin resurfacing)
•…..
Section 11.1 p.4
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•Transitions are between molecular vibrational-rotational levels.
C O O
Modes of vibrations:
•Symmetric stretch
•Asymmetric stretch
•Bending mode
11.2 Molecular Vibrations and Rotations
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Simple Harmonic Oscillator (Quantum Mechanics):
1 2 3 1 1 2 2 3 3( , , ) ( 1/ 2) ( 1/ 2) ( 1/ 2)E n n n h n h n h n
C
O O C O O C O O
(000)
(001)
(020)
(010)
(200)
(100)
10.6 mm
9.6 mm
J=50
…..
J=18
….
J=1
Rotational states
Section 11.2 p.2
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(mm)
P- branch
JJ-1
P(50) P(20)
001100 001020
10.4 10.6 9.4
R- branch
JJ+1 P- branch
JJ-1 R- branch
JJ+1
R(17) P(19)
CO2 Laser Transitions
Tuning:
diffraction grating =9-11(mm)
Section 11.2 p.3
CO2:N
2:He
H.V.
CO2:N
2:He
H.V.
CO2:N
2:He
H.V.
CO2:N
2:He
H.V.
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Effect of Gas Mixtures: CO2+N2+He
C
OOC OOC OO
(000)
(001)
(020)
(010)
(200)
(100)
10.6 mm
9.6 mm
C
OO
C
OOC OO C OO C OOC OO C OO
(000)
(001)
(020)
(010)
(200)
(100)
10.6 mm
9.6 mm
(200)
(100)
(1)
N NN N
(0)
Metastable
discharge excitation
(200)
(100)
He N2
•Nitrogen helps populating the upper laser level in a discharge
•Helium helps to depopulate the lower laser level by collisions
Other possible additions to the gas mixture: CO, H2
Section 11.2 p.4
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Typical Co2:N2:He Gas Ratios Recommended by Laser Manufacturers
CO2
N2
He
Laser Power Rating W
1
1
1
1
1
1
1
3
1.5
1.5
1.35
8
6.7
2.3
17
9.3
9.3
12.5
23
30
17
20
50
100
275
375
525
1000
Section 11.2 p.5
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11.3 Gas Discharge Phenomena
+H.V.
e- e+
E/P (V/cm/torr)
1 10 100 1000
100
80
60
40
20
Per
cen
tag
e o
f to
tal
po
wer
CO2:N
2:He =1:2:3
CO2 (001) + N2 (v=1)
ionization
electronic
E/P (V/cm/torr)
1 10 100 1000
100
80
60
40
20
Per
cen
tag
e o
f to
tal
po
wer
CO2:N
2:He =1:2:3
CO2 (001) + N2 (v=1)
ionization
electronic
•Electrons emitted from cathode get accelerated by the electric field
•The energetic electrons excite the vibrational modes of the gas molecule via inelastic collisions
cathode anode
Example:
L=1 meter and P=25 torr
Need V=25 kV for optimum operation
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11.4 Specific Types of CO2 Lasers
High Power CW Operation
DC-Discharge
•Longitudinal discharge (High Voltage: 10-100 kV)
•Pressure: 10-100 torr
•Multistage discharge tubes can be used to produce kilowatts of output power
CO2:N
2:He
IR Brewster Windows (ZnSe, NaCl , KCl)H.V.
CO2:N
2:He
IR Brewster Windows (ZnSe, NaCl , KCl)H.V.
CO2:N
2:He
RF (10KHz-100 MHz)
CO2:N
2:He
RF (10KHz-100 MHz)
RF-Discharge
•In practice waveguides are used.
•High discharge stability, high pulsing frequency (up to 100 kHz)
•Expensive RF generator and requires EMI shielding
0.2 W/cm in a waveguide laser
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Example: A 250 W CW CO2 Laser
Active Length meters
Output Power watts
Gas Mixture CO2:N2:He
Gas Flow Rate liters/min
Power/ Length W/m
Water Flow Rate liters/min
1 2 5 6 9 18
50 100 275 375 525
1000
1:1.5:9.3 1:1.5:9.3 2:1.35:9.3 1:8:23 1:6.7:30 1:2.35:17
1.15 1.15 4.01 4.26 4.23 14.35
50 50 55 62.5 58.3 55.6
2 2 10 10 10 15
Operating Parameters of Commercial Class I CO2 Lasers.
Section 11.4 p.2
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Laser Hardened Materials Evaluation Laboratory (LHMEL) WP-AFB, Dayton, OHIO
Electric Discharge Coaxial Laser (EDCL)
Section 11.4 p.3
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Gas-Dynamic Lasers Basov & Oraevskii (1963)
Principle: Population inversion by rapid expansion (supersonic flow) of s super-heated gas
CO2+N2+H2
T=1000-3000 K
P=1-20 atm. Inversion region
v= 105 cm/sec.
•cw powers up to 1 MW have been obtained from gas-dynamic CO2 lasers !!
Section 11.4 p.4
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HELEX
Gas-Dynamic Lasers
Large scale 135 Kilowatt gasdynamic laser at Avco Everett Research Lab.
High Energy Laser Experimental Germany, 1970’s
Section 11.4 p.5
C2N2 or CO
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•Pulsed CO2 Lasers
Most Common: Transversely Excited Atmospheric (TEA) CO2 Lasers
CO2:N2:He
Pulsed H..V
•Flowing or sealed systems
•Pulsewidths from 50 ns to 300 ns
•Repetition rates: 1Hz. to 1 kHz.
•Pulse energy: 50 mJ to 10 J (amplified)
Low pressure gain cell
(for single longitudinal operation)
Section 11.4 p.6
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Capacitor bank
Example
Section 11.4 p.7
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Terra Watts Pulsed CO2 Lasers
Section 11.4 p.8
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Picosecond TW CO2 Laser at BNL Section 11.4 p.9
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Proprietary Data University of New Mexico
Section 6.5, p.4
Excimer lasers:
molecules exist only in the excited state XeCl 308 nm KrF 248 nm ArF 193 nm F2 156 nm
A
B
excitation
A
B*
A
B
emission of a photon
applications in lithography and eye surgery
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10. Solid-state lasers
(a) Nd:YAG (yttrium aluminum garnett crystal doped with Nd atoms) = 1.064 mm, 1.331 mm (b) Nd:glass (glass doped with Nd:atoms) = 1.062 mm (silicate glass) = 1.080 mm (fused silica) (c) Ti:sapphire = 0.7 - 1.1 mm (d) Hm:YAG (holmium atoms doped into a YAG crystal) = 2.1 mm (e) color centers (intentionally created defects in a crystal) = 1.5 - 3.5 mm (in different hosts)
Examples:
The lasing atoms are fixed in a solid (crystal, glass). Solid-state lasers can operate in continuous (cw) or various pulsed modes.
elementary cell of the host crystal
dopand
host crystal
10.1 Introduction
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Optical Science & Engineering
University of New Mexico The 4f-4f transitions in Rare-Earths Ions:
Orbital Radius
En
erg
y
Orbital Radius
En
erg
y
Xe
4f
5S
6S
4f
Yb (Xe)4f136s2
Yb3+ = (Xe)4f12
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end mirror
power supply
outcoupler
lamp pump (flashlamp, arclamp, laser diodes)
laser head (reflective walls to concentrate the pump light)
Laser rod (solid host material doped with the atoms of the active medium)
coolant in
coolant out
dual elliptical reflector laser rod lamps
10.2 Layout of a solid-state laser
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Maiman’s Ruby Laser
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10.3 Nd:YAG laser
The most common solid-state laser is based on Nd atoms as dopands.
ground state
absorption bands
70
0 n
m
80
0 n
m
energy is transferred to the crystal (heating)
lasing
Energy diagram of Nd:
By changing the host material the laser wavelength and the thermal properties can be changed.
thermal effects:
temperature profile
refractive index profile
Output (Nd:YAG) cw: 1000 W pulsed: pulse energy 1 Joule Q-switched - 10 ns pulse duration modelocked - 100 ps pulse duration
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11. Fiber lasers
Optical fiber
cladding n1
core n2
L … 103 km
Light can be guided (confined) in the core of optical fibers over great distances. This allows for large interaction lengths of light with an active medium that is doped into the fiber core.
condition: n2 > n1
fiber laser
lens duct (unfolded resonator)
laser with resonator (many passes through the active medium)
Realizing large gain
11.1 Introduction
Total reflection
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11.2 Example: erbium-doped glass fibers
500 1000 1500 nm
absorption
emission
pumping with diode lasers is possible
pu
mp
emission absorption
The wavelength of about 1550 nm is particularly interesting for applications in telecommunication.
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11.3 Fiber-optic Communications
Why? The carrier frequency of light ( 1014 Hz) and subsequently the transmitted bandwidth is much larger than what can be achieved by electronics.
wavelength (mm) 1.5 1.0
atte
nu
atio
n c
oef
fici
ent
(db
/km
)
1
0.3
Rayleigh scattering
infrared absorption
silica-glass fibers
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Fiber transmission line
signal processing
transmitter signal
processing receiver
amplifier/ repeater
input signal
6000 km
• transatlantic US - UK • 560 Mb/s per fiber pair • 80000 simultaneous voice channels • repeaters 100 km apart
InGaAsP diode laser
Section 11.3, p.2
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12. Chemical Lasers
F + D2 DF* + D DF 3.5 - 4.1 mm Cl + HI HCl* + I HCl 3.5 - 4.1 mm H +Br2 HBr* + Br HBr 4.0 - 4.7 mm F + H2 HF* + H HF 3.5 - 4.1 mm
- population inversion is produced by a chemical reaction A + BC AB + C* - electrical power supply is not needed - airborne lasers - first chemical laser: 1964
Examples:
chemical reaction:
- exothermic - generation rate must be large enough to overcome spontaneous emission and collisional relaxation
reaction active medium wavelength
I + O2* I* + O2 I 1.31 mm
molecules in an excited vibrational state
atoms in an excited electronic state
12.1 Introduction
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12.2 The chemical oxygen-iodine laser
chemical reaction: O2(1 ) + I O2(3 ) + I*
O2(1 )
I
energy transfer
O I
O2(3 )
I*
1. generation of singlet oxygen Cl2 + H2O2 +2NaOH O2(1 ) + 2H2O + 2NaCl 2. production of excited iodine O2(1 ) + I O2(3 ) + I*
3. lasing of excited iodine
steps:
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schematic diagram of a chemical iodine laser parameters
• MW ouput power
• wavelength 1.315 micron
• pulsed and cw
atmospheric absorption
Section 12.2, p.3
singlet oxygen
iodine
mixer supersonic nozzle
laser output
expanding gas (cooling)
1 2 3 4 5 0.0
0.2
0.4
0.6
0.8
1.0
1 km propagation in atmosphere
ab
so
rptio
n
wavelength ( m m) wavelength (mm)
abso
rpti
on