Examples of Specific Laser Systems

Gas Lasers
CO2 (9-10 m), High Power
He:Ne (632 nm) , Ar (ion) Laser (520 nm)
Excimer (UV) Lithography

Solid-State Lasers
Nd:YAG , Ti:Sapphire (Ultrafast Revolution)

Fiber Lasers (Rare-Earth Doped Fiber Lasers)

Yb3+ (5+ kW), Tm3+ , Ho3+ (2 m), Er3+ (1.5 m)

Dye Lasers

Chemical Lasers
COIL (7+kW), MIRACL (>1 MW !!)

Semiconductor Lasers
Laser Market and Applications
 Ultraviolet Visible Infrared

dye lasers

excimer
lasers semiconductor lasers

solid-state lasers

molecular gas lasers

atomic gas lasers

wavelength

100 nm 500 nm 1 m 10 m
 Optical Transitions
2 2
discrete levels OR energy bands
1 1

emission
electronic transitions VIS, UV
A B

vibrational transitions A B NIR, IR

rotational transitions A B FIR

Proprietary Data
University of New Mexico
 output power
Typical laser efficiencies  : 
electrical input power

Argon - ion < 0.1%

CO2 laser < 20%
Excimer < 20%
GaAlAs (diode laser) < 40%
HeNe < 0.1%
Nd:YAG < 10%

Proprietary Data
University of New Mexico
 Gas Lasers (Examples: HeNe, Ar+, Excimer, CO2,..)

The excitation mechanism in most gas lasers is via electric discharge

 The first Gas Laser: He-Ne

Ali Javan, et al. (Bell Labs, 1962)

• 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
 Comparison of Gas Lasers

Laser Type Linear Power Maximum Power Power Efficiency

Density %
W/m W

He-Ne 0.1 1 0.1

Argon 1-10 50 0.1

CO2 60-80 >104 15-20

 CO2 Lasers (9-11 micron)

C. K. N. Patel, "Continuous-Wave Laser Action on Vibrational Rotational Transitions of CO2," Physics

Review, Vol. 136 A, (Nov., 1964) P. 1187

Applications (pealing peanuts to star wars)

•Industrial (cutting, welding, material processing)
•Military (range finding, targeting, remote sensing, sensor blinding, destroying …)
•Medical (cutting, skin resurfacing)
•…..
 Molecular Vibrations and Rotations

•Transitions are between molecular vibrational-rotational levels.

O C O

Modes of vibrations:
•Symmetric stretch
•Asymmetric stretch
•Bending mode
Simple Harmonic Oscillator (Quantum Mechanics):

E (n1 , n2 , n3 )  h 1 (n1  1/ 2)  h 2 (n2  1/ 2)  h 3 ( n3  1/ 2)

Symmetric Stretch Bending Asymmetric Stretch

O C O O O
O C O
C

(200)


J=50

(001) …..
J=18 Rotational
10.6 m states
….
J=1

9.6 m
(100)
(020)

(010)

(000)
 Section 11.2 p.3
CO2 Laser Transitions

(m) 10.6 10.4 9.4

001100 001020

P- branch R- branch P- branch R- branch

JJ-1 JJ+1 JJ-1 JJ+1


P(50) P(20) R(17) P(19)

Tuning: CO 2:N 2:He

diffraction grating =9-11(m)

H.V.
 Section 11.2 p.4
Effect of Gas Mixtures: CO2+N2+He

He O C O O

C
O
O C O N N N2
(200) (200) (200)

(001) (1) Metastable

10.6 m
9.6 m
(100) (100) (100)
(020)
discharge
excitation
(010)

(000) (0)

•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

Typical Co2:N2:He Gas Ratios Recommended by Laser Manufacturers

CO2 N2 He Laser Power Rating W

1 3 17 20
1 1.5 9.3 50
1 1.5 9.3 100
1 1.35 12.5 275
1 8 23 375
1 6.7 30 525
1 2.3 17 1000
 11.3 Gas Discharge Phenomena
cathode
anode

e- e+

+H.V.

•Electrons emitted from cathode get accelerated by the electric field

•The energetic electrons excite the vibrational modes of the gas molecule via inelastic collisions

CO2:N2:He =1:2:3
100
Percentage of total power

electronic Example:
CO2 (001) + N2 (v=1)
80
L=1 meter and P=25 torr
60 Need V=25 kV for optimum operation

40

ionization
20

1 10 100 1000
E/P (V/cm/torr)
 11.4 Specific Types of CO2 Lasers

High Power CW Operation

 DC-Discharge CO2:N2:He

H.V. IR Brewster Wi ndows (ZnSe, NaCl , KCl)

•Longitudinal discharge (High Voltage: 10-100 kV)

•Pressure: 10-100 torr
•Multistage discharge tubes can be used to produce kilowatts of output
power

RF (10KHz-100 MHz)

 RF-Discharge
CO2:N2:He

•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

 Laser Hardened Materials Evaluation Laboratory (LHMEL) Section 11.4 p.3
WP-AFB, Dayton, OHIO

Electric Discharge Coaxial Laser (EDCL)

 Section 11.4 p.4
 Gas-Dynamic Lasers Basov & Oraevskii (1963)

Principle: Population inversion by rapid expansion (supersonic flow) of a super-heated gas

CO2+N2+H2 v= 105 cm/sec.

T=1000-3000 K

P=1-20 atm.
Inversion region

•cw powers up to 1 MW have been obtained from gas-dynamic CO2 lasers !!

 Section 11.4 p.5
Gas-Dynamic Lasers

Large scale 135 Kilowatt gasdynamic laser at Avco Everett Research Lab.

C2N2 or CO

HELEX
High Energy Laser Experimental
Germany, 1970’s
 Section 11.4 p.6
•Pulsed CO2 Lasers

Most Common: Transversely Excited Atmospheric (TEA) CO2 Lasers

Pulsed H..V

CO2:N2:He

Low pressure gain cell

(for single longitudinal operation)

•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)
 Section 6.5, p.4
Excimer Lasers
Nikolai Basov*, V. A. Danilychev and Yu. M. Popov, at the Lebedev Physical Institute

Applications: lithography, micromachining and eye surgery

molecules exist only in the excited state (Excimer =Excited Dimer)

XeCl* 308 nm
KrF* 248 nm
ArF* 193 nm
F2* 156 nm

Pulsed, Typically 10-50 ns, 5-20 mJ

A A
A

B*
B B
excitation
emission
of a photon
Proprietary Data
*Nobel Prize, Physics (1964), Shared with Townes and Prokhorov University of New Mexico
 Excimer Lasers

Eye surgery

Lithography
 Argon Ion Laser
488 and 514 nm

Ar+
Organic Dye Lasers
Example: Rhodamine 6G
 Solid-State Lasers
• The lasing atoms (ions) are fixed in a solid (crystal, glass).
• Solid-state lasers can operate in continuous (cw) or various pulsed modes.
• The active ions are most commonly either a rare-earth or transition metal elements

host crystal

dopant

Examples:
(a) Nd3+ :YAG  = 1.064 m, 1.331 m

(b) Nd3+ :glass  = 1.062 m (silicate glass),  = 1.080 m (fused silica)

(d) Hm3+ :YAG, Tm3+ :YAG  = 2.1 m

(e) Yb3+ : YAG,  = 1.03-1.05 m

(f) Ti3+ :sapphire,  = 0.7 - 1.1 m

(g) Fe2+ :ZnSe  = 3.7 – 5.5 m

 Optical Science & Engineering
The 4f-4f transitions in Rare-Earths Ions: University of New Mexico

4f

Energy
Xe

Orbital Radius
Yb (Xe)4f136s2

Yb3+ = (Xe)4f12
4f
6S

5S
Layout of early (flash-lamp or arc-lamp pumped) solid-state lasers

Laser rod (solid host material doped

with the atoms of the active medium)

outcoupler

end
mirror

laser head
(reflective walls to
concentrate the
lamp pump pump light)
(flashlamp,
arclamp, power supply
laser diodes)

coolant coolant
in out
dual elliptical reflector
lamps laser rod
Maiman’s Ruby Laser
 Example: Nd:YAG laser
YAG: Yttrium Aluminum Garnet (Y3Al5O12)

Energy diagram of Nd3+:

energy is transferred
to the crystal (heating)
absorption
bands
800 nm
700 nm

lasing
Output (Nd:YAG)
cw:  1000 W
ground
pulsed: pulse energy  1 Joule
state
Q-switched – 5-20 ns pulse duration
modelocked – 10-100 ps pulse duration
 DPSS : Diode-Pumped Solid-State Laser

Similar to one used in our Optics Lab

 Diode Lasers for Pumping
Semiconductor lasers will be covered later in more detail

• Efficient (current injection)

• High power
• Designer Wavelength
• Poor Beam Quality
• Broadband (3-5 nm)

High Power (Diode Bar) Fiber Coupled

DPSS : Diode-Pumped Solid-State Laser
Example Nd:YVO4 (Vanadate)
From laser pointers (5-10 mW) to 100W lasers
 McCumber Relation
D. E. McCumber, “Einstein relations connecting broadband emission and absorption spectra”, Phys. Rev. 136 (4A), A954 (1964),

Vibronic Transitions

Yb-doped germanosilicate glass

 Nobel Prizes

John L. Hall and Theodor W. Hänsch“

“for their contribution to the development of
laser-based precision spectroscopy, including
the optical frequency comb technique” (2005)

Gérard Mourou and Donna Strickland

“for their method of generating high-intensity,
ultra-short optical pulses” 2018

Ahmed H. Zewail ”for his studies of the transition

states of chemical reactions using femtosecond
spectroscopy” 1999 (Chemistry)
High Power Scaling: Heat is such a nuisance!

Increase surface-to-volume ratio (fibers, disks)

Fiber Lasers
 Fiber Lasers
Single-Mode Fiber Multimode Fiber
6 µm core / 125 µm cladding 50 µm core / 125 µm cladding
+ High Beam Quality + Higher Power Pumping
- Lower Power Pumping + Inexpensive Pumping
- Expensive Pumping - Poor Beam Quality

Optical Fiber Modes

Graphics: courtesy of Colin Diehl & Jered Richter
 Example: Erbium-Doped fiber Lasers
The wavelength of about 1550 nm is particularly interesting for applications in telecommunication.

emission
absorption

pump
emission absorption

500 1000 1500 nm

pumping with diode lasers

is possible

EDFA: Erbium Doped Fiber Amplifier

 Fiber-optic Communications

attenuation coefficient (db/km)

silica-glass fibers
1

Rayleigh
scattering
infrared
0.3 absorption

1.0 1.5 wavelength (m)

 Fiber transmission line

input
signal signal amplifier/ signal
transmitter receiver
processing repeater processing

6000 km

InGaAsP
diode laser

• transatlantic US - UK
• 80000 simultaneous voice channels
• repeaters 100 km apart
Erbium-Ytterbium Co-Doped Fiber

High Power
 Yb Silica Fiber Lasers

Diode Pumped at 940-980 nm

Laser: 1020-1050 nm
 High Power Fiber Lasers
Double-Clad Fiber
• Laser light propagates in single-mode core
• Pump light propagates in inner cladding
100 kW (current record)!
 Thin Disk Lasers
Yb:YAG
pump radiation
indium
Output- >10 kW CW
Couple
r laser beam
D
D
d
heat sink
o.c. mirror
thin disk
d pump radiation
Pump-Beam

Requires multipass pumping

 12. Chemical Lasers
12.1 Introduction

- population inversion is produced by a chemical reaction

chemical reaction:
A + BC AB + C*

- electrical power supply is not needed - exothermic

- airborne lasers - generation rate must be large
- first chemical laser: 1964 enough to overcome spontaneous
emission and collisional relaxation

Examples:

reaction active medium wavelength

F + D2  DF* + D DF 3.5 - 4.1 m

molecules in an excited
Cl + HI  HCl* + I HCl 3.5 - 4.1 m vibrational state

H +Br2  HBr* + Br HBr 4.0 - 4.7 m

F + H2  HF* + H HF 3.5 - 4.1 m

atoms in an excited
I + O2*  I* + O2 I 1.31 m
electronic state
 The chemical oxygen-iodine laser (COIL): MW CW Power !!

Singlet oxygen has a >40 min lifetime.

chemical reaction: O2(1 ) + I  O2(3 ) + I*

O2(1 ) O2(3 )

energy transfer

OI

I I*

steps:

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

schematic diagram of a chemical iodine laser
parameters

iodine
mixer supersonic • MW ouput power
nozzle
• wavelength 1.315 micron
expanding
gas (cooling) • pulsed and cw

singlet oxygen

laser
output
atmospheric absorption
1 km propagation in atmosphere
1.0

0.8
absorption
absorption

0.6

0.4

0.2

0.0
1 2 3 4 5
wavelength (m) (m)
wavelength
Free Electron Laser (FEL)

wiggler period

𝐾2
𝜆𝑤 (1 + )
𝜆𝑟 ≈ 2
2𝛾 2

1
𝛾= > 104
1 − 𝛽2

𝐾 ∝ |𝐵|
wiggler strength
(often <1)
Semiconductor Lasers

Demonstrated by 3 Groups in 1962:

• GE (Robert Hall)
• MIT Lincoln Laboratory (Ted Quist)
• IBM (Marshal Nathan)

GaAs
Laser Market and Applications
Direct-Gap vs. Indirect-Gap
E(energy)

k (momentum)
Optical Interactions in a Direct-Gap Semiconductor
 A Brief Introduction to Semiconductors
Energy Bands

E Conduction band
(empty)

Eg (bandgap energy)

1 0 Valance band
1
f FD  (full)
 E  Ef 
1  exp  
 kT  Equilibrium
Fermi-Dirac Statistics

http://britneyspears.ac/lasers.htm !!!???
 Conduction band

Eg

Valance band

Nonequilibrium Electron-Hole Injection

104 cm-1

Example
Gain

GaAs
Absorption or

Eg=1.4 eV
(g=850 nm)
0

hw
E g
High Gain!

Example: InGaAsP
 p-n junctions
Doping with Impurities
n-type p-type

Ef
Eg Eg
Ef

Examples: GaAs doped with Br Examples: GaAs doped with Zn

Si doped with P Si doped with Al
 Semiconductor junction lasers

Forward-Biased p-n Junction (LED)

n+-GaAs p+-GaAs

+V

Inversion (Gain) Layer

N e ,h ed
Current Density Threshold J th 
r
Recombination time
 Section 12.2 p.2
Edge-Emitting Homojunction Laser Diodes

+v Polished
facets

p-GaAs
N e ,h ed
n-GaAs J th 
r
Lg100 m

•Waveguide Modes

Homojunction Lasers have very high current threshold mainly because.

•Electrons and holes are free to diffuse and therefore dilute the gain (no carrier confinement)
•Optical mode has poor overlap with gain (no optical confinement or guiding)
 Heterojunction Lasers Diodes

A fortunate coincidence: n when Eg

electrons

n+ p+
AlGaAs GaAs AlGaAs Energy Carrier confinement

holes
holes
100 nm

Index of
Refraction Mode confinement

Jth=5kA/cm2, RT, Pulsed (1969) Jth=0.5 kA/cm2, RT, Pulsed (1975)

 Edge-Emitting Heterojunction Laser Diodes

Examples:

Edge-Emitting Buried Heterojunction Laser Diodes

 Cleave d refle cting surfa ce
W

L
Stripe electrod e

Oxide insulator
p-GaAs (Contactin g laye r)
p-AlxGa 1-xAs (Confinin g laye r)
p-GaAs (Active layer)
n-AlxGa 1-xAs (Confinin g laye r) 2 1 3
Current
Subs trate
n-GaAs (Subs trate )
Substrate
paths
Electrod e

Ellipt ical Cleave d refle cting surfa ce

laser
Active region where J > Jth.
beam
(Emiss ion region)

Schematic illustration of the the structure of a double heterojunction stripe

contact laser diode
© 1999 S.O. Kasap,Optoelectronics (Prentice Hall)
 Quantum Well Lasers
Multiple Quantum Well (MQW) Lasers

+v

n+ p+
10 nm p+-AlGaAs
AlGaAs AlGaAs

n+-AlGaAs

n-GaAs Substrate

mode confinement

Epitaxial Growth
 Epitaxial Growth
Compound Semiconductors (Lattice Matching)
Example: MBE
 High Power Diode Bars

•P>100 W (cw)
•Diode-pumping solid-state lasers (DPSS)
•Material Processing
•…
 Vertical Cavity Surface Emitting Lasers (VCSEL)

Mirror

MQW (gain)

Mirror

•Good mode quality couples to fiber efficiently for telecom applications

•Single mode operation
•2-D structures cam be made
•Low power
VECSEL
Single Mode
 Section 12.4 p.4
Laser Diodes Cover the Spectrum

Compound Spectral Region Notes

AlxGa1-xN uv
GaN uv (350 nm)
InxGa1-xN blue (480-400 nm) data storage, display

GaxI1-xP (x=0.5) 670 nm display

GaxAl1-xAs (x=0-0.45) 620-895 nm
GaAs 904 nm diode pumping solid-state and
InxGa1-xAs (x=0.2) 980 nm fiber lasers.

InxGa1-xAsyP1-y 1100-1650 Telecom

(x=0.73, y=0.58) 1310 nm
(x=0.58, y=0.9) 1550 nm

PbSSe 4200-8000 nm cryogenic

PbSnTe 6300-29,000 nm cryogenic
 Quantum Cascade Lasers

Faist, J; Capasso, F; Sivco, DL; Sirtori, C. ; Hutchinson, Al; Cho, AY "Quantum Cascade Laser"
Science 264, 553-556 (1994) Bell Labs
Section 12.5 p.2
•DFB QCLs in Thermoelectrically Cooled
•Center Wavelengths Between 8.00 and 10.00 µm (1250 and 1000 cm -1)
•100 mW Typical Output Power
•Custom Wavelengths, Packages, and Output Powers
THE END !