physics of, and requirements for laser crystals blaž kmetec put together by: blaž kmetec prof. dr....

Physics of, and requirements Physics of, and requirements for laser crystalsfor laser crystals

Put together by: Blaž KmetecBlaž Kmetec

Supervisor: prof. dr. Martin Čopičprof. dr. Martin Čopič

Faculty of Mathematics and Physics, Ljubljana

21. 12. 2004

21.12.2004 Physics of, and requirements for laser crystals 2

ContentsContents

Foreword

Introduction

Interactions

Material requirementsmaterial 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 crystalsNd,Cr:GSGG opposed to Nd:YAG

Nd:YAG and Nd:YVO44

Summary


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary


ForewordForeword

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.)

Kyrenaikan gold drachm showing Laser (Silphion) image

Look

At

Source,

Erase

Retina

D a n g e r o u s , i n s t r u c t i v e , c h a l l e n g i n g

Light

Amplification

by Stimulated

Emission of

Radiation

Legal

Amusement

of Students,

Engineers &

Researchers

21.12.2004 Physics of, and requirements for laser crystals: Foreword 3/3 6

Foreword - continuedForeword - continued

Requirements for laser systems

The demand for lower costsimproved reliabilitylong-term durabilityreduced 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


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary


IntroductionIntroduction

Solid-state laser = laser system based on optically active centres (ions) in insulator host materials

Componentslaser crystal = host crystal + active ions

its optical spectroscopic properties are vital to its performance

mechanism of optical pumpingcavity configuration


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary

21.12.2004 Physics of, and requirements for laser crystals: Interactions 1/3 10

InteractionsInteractionsComplex physical processes

static electron-lattice interactionsdetermine the types and position of the electronic energy levels

electron-photon interactionsdetermine the strengths of radiative transitionsdetermine the fluorescence lifetime

electron-phonon interactionsdetermine the rates of nonradiative transitionsdetermine the temperature-dependent widths and shifts of spectral lines

ion-ion interactionscause energy-level splittings and energy transfer between ions

Contributions to photon field in the cavityphotons injected into the cavity by the pump source

photons generated by the optically active ions through spontaneous emission processesstimulated emission processes


Interactions - continuedInteractions - continuedOptical spectral properties of the laser crystal

determined by the electronic transitions of the active ions in the local field environment of the hostTypes of ions that are useful for laser emission:

transition-metal ionsCr3+, Ti3+

rare-earth ionsNd3+, 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


Interactions - continuedInteractions - continued

Efficient emission of pump radiationstrong laser transition at the of the desired laser outputhigh quantum efficiency


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary


Material requirementsMaterial requirements

Material properties are determined bythe 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 astemperaturehumiditystress

thermal effects, thermal lensing

It is possible to put 2 types of ions in the same host materialnonradiative energy transfer from the sensitizers to the activators


Material requirementsMaterial 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


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary

Cr4+:YAG Nd3+:YAG


Material preparationMaterial preparation

Standard techniquespulling from the melt (Czochralski)

melt growth (Bridgman-Stockbarger)

Czochralski


Material preparationMaterial 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.


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary


Nonradiative energy transfer as a result of Nonradiative energy transfer as a result of ion-ion electric dipole interactionion-ion electric dipole interaction

Photon energy absorbed by the sensitizer movesthrough the dipole-dipole interaction

aided by surrounding lattice relaxation to the activator (without radiation exchange).

( )2

sa sa f f i

ion-ion ion-ionf int j j int iion-ion

sa f int ii jresonant interaction

phonon-assisted energy transfer

2;

E E

W E E

H HH

pr

y y y yy y

= =

= + +å

Μ

M

h

K144444244444314444444444444244444444444443

Main interaction = Coulomb interactionmultipole expansion about the sensitizer-activator separation saR

v

( )( )

2EMint

0 sa

2

0 sa a s

2

s a s sa a sa3 20 sa sa

4

4

4

eH

r

eR r r

er r r R r R

R R

pee

pee

pee

= =

= =+

æ ö÷ç= × × × +÷ç ÷çè ø

v

v v v

v vv v v v K

arvsr

v sarv

saRv

( )( )

( )( )

2 2EM:D-Dsa f int i

2 22

s a s sa a sa3 2 a0 sa sa

22 22

s a s sa a sa2 a0 sa

2

s a23

4

4

H

er r r R r R

R R

er r r R

Rrr

RrR

pee

y y

pee

= =

æ ö÷ç

æ ö÷ç= ×

= × × ×÷ç ÷

×

÷çè ø

×÷ç ÷÷çè ø=

M

v vv v v v

1444444444444442444444444444443v vv v vv vv

spatialaverage

21.12.2004 Nonradiative energy transfer as a result of ion-ion electric dipole interaction 3/3 22

EM:D-D 6sa saW Rµ

64EM:D-D 0 0

sa 5 6 sp sp0sa s s sa

3 1 1 1( ) ( )d

64 s a

c RW g

R n Rn s n n

p t n t

¥æ ö æ öæ öæ öæ ö ÷ç ÷ç÷÷ ÷ çç ç ÷= = ÷ç ÷ ç÷ ÷÷ ç ÷ç ç ÷ç ÷è ø ç÷ ÷ç ç÷è øè ø è øè øò

Nonradiative energy transfer as a result of Nonradiative energy transfer as a result of ion-ion electric dipole interaction - continuedion-ion electric dipole interaction - continued

0R

spst

s ( )g n

n

: 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

a ( )s n


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO44

Summary

21.12.2004 Thermal effects 1/3 24

Thermal effects in a crystal Thermal effects in a crystal during laser operationduring 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 diode-pumped solid-state lasers (DPSSL)


Temperature gradients result in optical distortions in the rod, mostly through the refractive index variation attributable to deformations caused by thermal stress (photoelastic effect)

0

0

2

20

2 214

0 2102

1 / ;

0 ;

( (1 2ln ) ) ;

0

/

...................( )

ln ;;

RqR

Rqr

d T dT

dr r dr

q r R

R r R

R r r RT r T

R R r R

q

l

l

l

l+ =

=

ì - <ïïíï £ <ïîìï + - <ï- í

£ <

-

ïïî

0 0.2 0.4 0.6 0.8 1 1.2rR0

0.02

0.04

0.06

0.08

0.1

R2qTrT 0

0 0.5 1 1.5 2 2.5 3rR0

0.05

0.1

0.15

0.2

0.25

R2qTrT 0


Thermal effects - continuedThermal effects - continued

The perturbation is equivalent to the effect of a spherical lens

Optical pump beam cross-section should be larger than resonator beam cross-section

( )( )( )

stress

2

stress

2 2

( ) ( ( ) (0))

1( )

4

1( ) (0) 1

2

T

T

dnn r T r T

dT

q dnn r r

dT

n r n r

l

a

D = -

D =-

= -2

0

1f

n la@

(in the pumped region)

A contribution to lensing power from the end-effects

It is possible to lessen this impact by using composite rods


Foreword

Introduction

Interactions





Nd:YAG and Nd:YVO4

Summary

21.12.2004 Examples of laser crystals 1/3 28

Er:YAG: lases at 2940nm

Examples of laser crystalsExamples of laser crystals

Nd,Cr:GSGG opposed to Nd(,Cr):YAG

Nd:YAG (Nd3+:Y3Al5O12)

YAG host properties:hard, grown by Czochralskihigh 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?idea: a second dopant, like Cr3+

little improvement, however, achievedfurthermore, low laser efficiency for pulsed applications due to (Cr3+ Nd3+ time) (Nd3+ decay time)

High transfer efficiency possible in Nd,Cr:GSGG

CODOPING

YAG=Y3Al2Al3O12


Codoping: Nd,Cr:GSGG and Nd,Cr:YAGCodoping: Nd,Cr:GSGG and Nd,Cr:YAG

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

GSGG YAG

{Gd

1-x Nd

x }3 [(S

c,Ga

)1-y Cr

y ]2 Ga

3 O12


Examples of laser crystals - continuedExamples 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:

21(Nd:YVO4)21(Nd:YAG) higher pulse rates required

for Nd:YVO4

Nd:YAG better for longer pulses

SummarySummary

The requirements for laser crystals

Laser industry The requirements for lasers

Other industries

Physics of laser crystals

Interactions

electrons

light

lattice optical sciences

solid-state theory

physics of deformations

Thank you for your attention!Thank you for your attention!

Which is Nd:YAG and which Ti:sapphire?

A left: Nd:YAG, right Ti:sapphireB left: Ti:sapphire, right Nd:YAGC Nd:YAG and Ti:sapphire spectra are equal

physics of, and requirements for laser crystals blaž kmetec put together by: blaž kmetec prof. dr....

Documents

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laser crystalsput

laser systemsthe demand

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result of ion

nonradiative energy

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