Transcript Physics of, and reqiurements for laser crystals

Physics of, and requirements
for laser crystals
Put together by: Blaž Kmetec
Supervisor: prof. dr. Martin Čopič
Faculty of Mathematics and Physics, Ljubljana
21. 12. 2004
Contents
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
2
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
3
Foreword
Laser inter eximia naturae dona numeratum plurimis
compositionibus inseritur
 The Laser is numbered among the most miraculous gifts of
nature and lends itself to a variety of applications.
 Plinius, Naturalis historia, XXII, 49 (first century A.D.)
21.12.2004
Kyrenaikan gold drachm showing
Laser (Silphion) image
4
Look
Light
Legal
At
Amplification
Amusement
Source, by Stimulated of Students,
Erase
Emission of
Engineers &
Retina
Radiation
Researchers
Dangerous,
instructive,
challenging
Foreword - continued
Requirements for laser systems
The demand for lower costs
improved reliability
long-term durability
reduced operating costs
The demand for improved beam quality
The demand for shorter wavelengths
the need for UV laser sources in the semiconductor chip industry
The demand for shorter pulses
Solid-state lasers
high power output at relatively low power consumption and with
high beam quality
High stability and long life expectancy
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Physics of, and requirements for laser crystals: Foreword 3/3
6
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
7
Introduction
Solid-state laser = laser system based on optically active
centres (ions) in insulator host materials
Components
laser crystal = host crystal + active ions
its optical spectroscopic properties are vital to its performance
mechanism of optical pumping
cavity configuration
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Physics of, and requirements for laser crystals
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Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
9
Interactions
Complex physical processes
static electron-lattice interactions
determine the types and position of the electronic energy levels
electron-photon interactions
determine the strengths of radiative transitions
determine the fluorescence lifetime
electron-phonon interactions
determine the rates of nonradiative transitions
determine the temperature-dependent widths and shifts of spectral lines
ion-ion interactions
cause energy-level splittings and energy transfer between ions
Contributions to photon field in the cavity
photons injected into the cavity by the pump source
photons generated by the optically active ions through
spontaneous emission processes
stimulated emission processes
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Physics of, and requirements for laser crystals: Interactions 1/3
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Interactions - continued
Optical spectral properties of the laser crystal
determined by the electronic transitions of the active ions
in the local field environment of the host
Types of ions that are useful for laser emission:
transition-metal ions
Cr3+, Ti3+
rare-earth ions
Nd3+, Er3+
Efficient absorption of pump radiation
 strong absorption transition at the  of the pump radiation
pump source can have broad or narrow emission spectra
Generally the terminal state of the absorption is not the level from
which laser emission occurs
 transition absorbing the pump energy must result in populating the
metastable state of the laser transition 
 requires efficient radiationless relaxation to the desired level
without loss of excitation energy to other emission transitions
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Physics of, and requirements for laser crystals: Interactions 2/3
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Interactions - continued
Efficient emission of pump radiation
strong laser transition at the  of the desired laser output
high quantum efficiency
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Physics of, and requirements for laser crystals: Interactions 3/3
12
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
13
Material requirements
Material properties are determined by
the properties of the host material,
the properties of the optically active ions,
and the mutual interaction between the host and the dopant ions
The most fundamental requirement for a laser material is
 that it can be easily and economically produced with high
quality in large amounts and different sizes
Stability with respect to local environmental changes such as
temperature
humidity
stress
thermal effects, thermal lensing
It is possible to put 2 types of ions in the same host material
nonradiative energy transfer from the sensitizers to the activators
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Physics of, and requirements for laser crystals
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Material requirements
TOTAL SYSTEM
Economic production and fabrication in large size
Ion-host compatibility:
Valence and size of substitutional ion similar to host ion
Uniform distribution of optical centres in the host
HOST MATERIAL
Stable with respect to operational environment
Chemical stability against thermal, photo, and mechanical changes
Mechanical:
High stress-fracture limit
High threshold for optical damage
Hardness for good polishing
Optical:
Minimum scattering centres
Minimum parasitic absorption at lasing and pump wavelengths
OPTICALLY ACTIVE CENTRES
Efficient absorption
of pump radiation
Efficient radiative emission
at the laser wavelength with high quantum efficiency
Low absorption
at the lasing wavelength
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Physics of, and requirements for laser crystals
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Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
Cr4+:YAG
Nd3+:YAG
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
16
Material preparation
Standard techniques
pulling from the melt (Czochralski)
melt growth (Bridgman-Stockbarger)
Czochralski
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Physics of, and requirements for laser crystals
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Material preparation
Even if the conditions for ideal crystal growth are known,
accurate control of these conditions may be difficult
Any variations in growth conditions can result in pieces with
bubbles,
multiple phases,
and other defects that
 scatter or distort optical beams passing through the crystal.
Providing for optically active centres:
should be uniformly distributed throughout the host crystal, otherwise
spatial variations in lasing properties throughout the crystal occur
Accurately knowing the dopant concentration and spatial
distribution is one of the major challenges in characterising
solid-state materials.
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Physics of, and requirements for laser crystals
18
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
19
Nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Photon energy absorbed by the sensitizer moves
through the dipole-dipole interaction
aided by surrounding lattice relaxation
 to the activator (without radiation exchange).
2p
2
Wsa =
Μsa r f (Ef = Ei ) ;
h
ion-ion
ion-ion
y
H
y
y
H
yi
f
int
j
j
int
ion-ion
Msa = y f H int
yi + å
+K
1444442 444443
E  Ej
resonant interaction
144444444444442i 44444444444443
phonon-assisted energy transfer
Main interaction = Coulomb interaction
v
multipole expansion about the sensitizer-activator separation Rsa
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Physics of, and requirements for laser crystals
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H
EM
int
e2
= 
v =
4pee0 rsa
v
rsa
v
rs
2
e
= 
v
v v =
4pee0 Rsa + ra  rs
v
ra
v
Rsa
æv v  v v v v ö
e2
çrs ×ra  2 (rs ×Rsa )(ra ×Rsa )÷
= 
+K
÷
3 ç
÷
4pee0 Rsa çè
Rsa
ø
2
Msa = y f H
EM:D-D
int
yi
2
=
2
2
æ e2
ö v v
v
v

v
v
÷
= çç
r
×
r

r
×
R
r
(
)
(
÷
s
a
s
sa
a a ×Rsa )
2
çè4pee0 Rsa3 ÷
Rsa
ø
1444444444444442 444444444444443
2
2
æ e2 ö v v
v
v

v
v
÷
= çç
÷ rs × ra  2 ( rs ×Rsa )( ra a ×Rsa ) =
çè4pee0 R ÷
Rsa
ø
spatial
average
2
3
v
rs
2
v
ra
2
Nonradiative energy transfer as a result of
ion-ion electric dipole interaction - continued
WsaEM:D-D
t ssp
g s (n )
s a (n )
n
R0
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6
4
æ
ö
¥
æ
ö
æ 3 öæ 1 öç 1EM:D-D æc0 ö
1 ç R0 ÷
÷

6
÷
÷
÷
çç 6 ÷çWsasp ò ççµ Rsa÷ g s (n )s a (n ) dn ÷
= çç
= sp ç ÷
÷
5
÷
÷
ç
è64p øçè Rsa ø
÷èt s 0 çèn n ÷
÷ t s èç Rsa ø
ø
ø
: radiative decay time of the sensitizer metastable level
: line-shape function of the sensitizer emission
: absorption cross-section of the activator
: refractive index of the host crystal
: Förster radius; for good overlap, of range 2 nm – 4 nm
Nonradiative energy transfer as a result of ion-ion electric dipole interaction 3/3
22
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
23
Thermal effects in a crystal
during laser operation
Diode-pumping of solid-state lasers has greatly reduced
the proportion of wasted pump energy which is deposited as
heat in the crystal
end pumping (longitudinal)
side pumping (transversal)
Diode laser prices decline  high pump power available 
thermal distortion is again a critical issue in designing diodepumped solid-state lasers (DPSSL)
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Thermal effects 1/3
24
0.1
T0
T0
d 2T 1 dT ìïï - q/l 0 ; r < R
+
=í
2
dr
r dr ïïî - 0q/l ; R £ r < R0
ìï 14 lq ( R 2 (1 + 2ln RR0 ) - r 2 ) ; r < R
T ( r ) - T0 = ïí q 2 R
ïïî 12 l R ln r0 ...................; ; R £ r < R0
0.2
R2q T r
0.08
R2q T r
0.25
0.06
0.04
0.02
0
0
0.2
0.4
0.6
r R
0.8
1
1.2
0.15
0.1
0.05
0
0
0.5
1
1.5
r R
2
2.5
3
Temperature gradients result in optical distortions in the rod,
mostly through the refractive index variation attributable to
deformations caused by thermal stress (photoelastic effect)
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Thermal effects 2/3
25
Thermal effects - continued
dn
D n ( r )T = (T ( r ) - T (0))
dT stress
1 q dn
D n ( r )T = r 2 (in the pumped
4 l dT stress
region)
1 2 2
n ( r ) = n (0) 1 - a r
2
( )
( )
(
)
A contribution
to lensing
power from the
end-effects
It is possible to lessen this impact by
using composite rods
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Thermal effects 3/3
The perturbation
is equivalent to
the effect of a
spherical lens
f @
1
a 2 n0 l
Optical pump
beam crosssection should
be larger than
resonator beam
cross-section
26
Foreword
Introduction
Interactions
Material requirements
material preparation
Representative calculation: nonradiative energy transfer as a result of
ion-ion electric dipole interaction
Thermal effects in a crystal during laser operation
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd:YAG
Nd:YAG and Nd:YVO4
Summary
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Physics of, and requirements for laser crystals
27
Examples of laser crystals
Nd,Cr:GSGG opposed to Nd(,Cr):YAG
Nd:YAG (Nd3+:Y3Al5O12)
YAG host properties:
 hard, grown by Czochralski
 high thermal conductivity
optically isotropic (cubic lattice)
doping: Y3+ is substituted by Nd3+
the radii differ by 3%
strains occur at high doping
how to increase the pump efficiency?
Er:YAG: lases at 2940nm
idea: a second dopant, like Cr3+
 little improvement, however, achieved YAG=Y3Al2Al3O12
furthermore, low laser efficiency for
High transfer efficiency
pulsed applications due to
possible in Nd,Cr:GSGG
(Cr3+  Nd3+ time)  (Nd3+ decay time)
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Examples of laser crystals 1/3
28
Codoping: Nd,Cr:GSGG and Nd,Cr:YAG
YAG
GSGG
{Gd1-xNdx}3[(Sc,Ga)1-yCry]2Ga3O12
 Nd and Cr ions are separated by only 1 nm in Nd,Cr:GSGG
 mostly usable with flashlamp pumping
 Nd,Cr:GSGG exhibits stronger thermal focusing and stress birefringence
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Examples of laser crystals 2/3
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Examples of laser crystals - continued
Nd:YAG and Nd:YVO4
Nd:YVO4
 large stimulated cross section
 the highest efficiency TEM00
performance ever demonstrated
naturally birefringent
 less sensitive to diode T:
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21(Nd:YVO4)21(Nd:YAG
)
 higher pulse rates required
for Nd:YVO4
Nd:YAG better for longer
pulses
Examples of laser crystals 3/3
30
Summary
Other industries
Laser industry
The requirements for lasers
The requirements for laser crystals
Physics of laser crystals
electrons
physics of deformations
lattice
Interactions
light
optical sciences
solid-state theory
Thank you for your attention!
Which is Nd:YAG and which
Ti:sapphire?
A left: Nd:YAG, right Ti:sapphire
B left: Ti:sapphire, right Nd:YAG
C Nd:YAG and Ti:sapphire spectra are equal