Recent Advances of High Power 1 µm Lasers Manfred Berger, II-VI Deutschland ALT`09 – Antalya -...

Recent Advances of High Power 1 µm Lasers

Manfred Berger, II-VI DeutschlandALT`09 – Antalya - Sept. 09

Sept. 2009 Manfred Berger - ALT`09 - Antalya 2

Content

1. Abstract

2. Introduction

3. Beamquality & Brilliance

4. Solid State 1 µm Laser (SSL)1. Nd:YAG Rod Laser

2. Yb :YAG Disk Laser

3. Yb-Fiber Laser

4. High Power Diode Laser (HPDL)

5. Applications1. Welding

2. Printing, Engraving and Marking

3. Cutting

April, 17, 2007 Manfred Berger – PRIMA Industries 3

Abstract

With the advent of reliable Yb:YAG disk lasers and Yb-doped fiber lasers the industry is now adopting these novel sources in their laser material processing systems. Not only superior beam quality and brightness in comparison to conventional technological high power lasers, but also the simplified handling via multi-kW-fibers open up new high performance industrial applications. Recent results underline the importance of 1 µm wavelength high power-lasers. The advantages and present limitations of 1 µm solid state lasers will be discussed.


Introduction

/2005

/41%

/19%

43%/38%

/2%


Introduction

1 µm

0 .5 kW Y b :Y A G D iskS ing le M ode

B P P : > 1 m m *m rad

4 kW Y b :Y A G D iskD iod e P um p ed

B P P : 8 m m *m rad

2 kW Y b:S iO 2 F ib reS ing le M ode

B P P : 0 .4 m m *m rad

4 kW Y b:S iO 2 F ib reD iod e P um p ed


4 kW N d :Y A GD iod e P um p ed

B P P > 1 2 m m *m rad

4 kW N d :Y A GL a m p P um p ed


10 .6 µm

8 kW C O 2S ea le d S lab


8 kW C O 2F a st A xia l F low


5 kW C O 2T ran sve rse F low


2 kW C O 2S low F low


Laser Wavelength: ≈ 1µm 10,6 µmDiff. Lim. : 0,34mm mrad 3,7 mm mrad

1980

.

.

.1990.

.

.

2000

.

.

.

2009

27kW Yb:YAG DiskDiode pumped

BPP:</=1mm*mrad

10kW Yb:SiO2FibreSingle Mode

BPP: 0.4mm*mrad

8kW CO2Fast Axial Flow

BPP: 6mm*mrad

Evolution of the Beam Parameter Product for Industrial High Power Lasers (2009)


Introduction

Disk and Fibre Laser show very high efficiency with low beam parameter product.

Diode Lasers show highest efficiencies with lowest operating cost.

Eff

icie

nc

y in

%

Beam Parameter Product in mm*mrad

Fibre

Disk

HPDL withFibre

DP-Rod

LP-Rod

HPDL with Fibre

Disk

Efficiencies and Beam Parameter Product of Industrial Laser Systems

Fiber

HPDL


Introduction

Beam Quality of Fiber-/Disk-Laser vs CO2-Laser


Introduction

Power Levels of DPSS-Lasers and Trend of MOOREs Law


Beam Quality & Brilliance

√2x2wL

LG,0

GRZ ,0

2w0,G

√22wL

2WL

ZRL

Caustic of Gaussian Laser Beam

fibredNAM 2

22

21sin nnNA

D

fF


Solid State 1µm Laser (SSL)

Current Solid State Laser Concepts


Nd:YAG Rod Laser

Principle of a Lamp Pumped YAG-Laser


Nd:YAG Rod Laser

T h e rm o -M e ch a n ica l P a ra m e te rs lim it th e m a x . T h e rm a l L o a d in a L a se r R o d a n d th ere su ltin g m a x . S tre ss is L im ite d b y T e n s io n a l F a ilu re (B re a ka g e ) o f th e L a se r R o d .

m a x

(1 )8vP K

l E

W . K o e ch n e r „S o lid -S ta te L a se r E n g in e e rin g “, S p rin g e r, 1 9 9 9

w ith : P v D iss ip a te d H e a tl R o d L e n g thK T h e rm a l C o n d u c tiv ityv P o isso n R a tioα C o e ffic ie n t o f T h e rm a l E xp a n s io nE C o e ffic ie n t o f E la s tic ityσ m a x M a x . P e rm itte d T e n s ile S tre ss a t R o d S u rfa ce

T h e R e su ltin g u p p e r L im it o f T h e rm a l L o sse s fo r Y A G M a te ria l is th e n :

2 0 0 ,vP W

l cm in d e p e n d e n t o f R o d -D ia m e te r

Limits of Rod Lasers, Mechanical Stress

Dissipated Heat for YAG Material is then


Nd:YAG Rod Laser

P a r a m e t e r s f o r T h e r m o - O p t i c a l E f f e c t s R e s u l t i n t h e F o r m a t i o n o f a T h e r m a l L e n s w i t hF o c a l L e n g t h f :

3 0, 0

1

0 ( 1 )1

2 rv

K Af

rd n

l

n

d TC n

P

W . K o e c h n e r „ S o l i d - S t a t e L a s e r E n g i n e e r i n g “ , S p r i n g e r , 1 9 9 9

3 0, 0

1

0 ( 1 )1

2 rv

K Af

rd n

l

n

d TC n

P

W . K o e c h n e r „ S o l i d - S t a t e L a s e r E n g i n e e r i n g “ , S p r i n g e r , 1 9 9 9

w i t h : K T h e r m a l C o n d u c t i v i t y C r , φ p h o t o e l a s t i c C o e f f i c i e n tA R o d C r o s s - S e c t i o n n 0 O n - A x i s I n d e x o f R e f r a c t i o nP v D i s s i p a t e d H e a t r 0 R o d - R a d i u sd n / d T I n d e x o f R e f r a c t i o n C h a n g e w i t h T l R o d - L e n g t hα t h e r m a l E x p a n s i o n

F r a c t i o n o f 7 0 % : d u e t o I n d e x o f R e f r a c t i o n C h a n g e w i t h T e m p e r a t u r e

F r a c t i o n o f 2 0 % :d u e t o I n d e x o f R e f r a c t i o n C h a n g e w i t h M e c h a n i c a l S t r e s s

F r a c t i o n o f 1 0 % :d u e t o c h a n g e o f O p t i c a lP a t h L e n g t h b y V a r i a t i o n o f R o d L e n g t h

T y p i c a l F o c a l L e n g t h ( O r d e r o f M a g n i t u d e ) f o r R o d L a s e r s w i t h P = 1 k W C W - P o w e r :

1 0f c m T y p i c a l F o c a l L e n g t h ( O r d e r o f M a g n i t u d e ) f o r R o d L a s e r s w i t h P = 1 k W C W - P o w e r :

1 0f c m

Limits of Rod Lasers, Optical Properties


Nd:YAG Rod Laser

1 0f c m T h e r m a l L e n s :

O p t i c a l P a t h D i f f e r e n c e b e t w e e n A x i a l - a n d P e r i p h e r a l R a y s :

I n h o m o g e n e i t i e s o f P u m p - I n t e n s i t y a n d C o o l i n g E f f i c i e n c y r e s u l t i n D e g r a d a t i o n o f F o c u s a b i l i t y M 2 , i f :

, 0 , 1 0 , 1o p t a s p hl µ m F o r D i s t o r t i o n f r e e L a s i n g H o m o g e n e i t y a n d a l s o T e m p o r a l S t a b i l i t y o f t h e P u m p B e a m I n t e n s i t y h a v e t o b e k e p t b e l o w0 . 1 % ! ! !

F o r D i s t o r t i o n f r e e L a s i n g H o m o g e n e i t y a n d a l s o T e m p o r a l S t a b i l i t y o f t h e P u m p B e a m I n t e n s i t y h a v e t o b e k e p t b e l o w0 . 1 % ! ! !

F o c u s a b i l i t y D e c r e a s e s w i t h L a s e r P o w e r s i n c e t h e s e C o n d i t i o n s a r e i n c r e a s i n g l y d i f f i c u l t t o r e a c h !F o c u s a b i l i t y D e c r e a s e s w i t h L a s e r P o w e r s i n c e t h e s e C o n d i t i o n s a r e i n c r e a s i n g l y d i f f i c u l t t o r e a c h !

Weg

unte

rsch

ied

?

Δ l o p t

Weg

unte

rsch

ied

?

Δ l o p tΔ l o p t

tem

pe

ratu

re [

a.

u.]

r a d i u s [ a . u . ]R a d i u s ?

Tem

pera

tur

?

Δ T

r 0

tem

pe

ratu

re [

a.

u.]

r a d i u s [ a . u . ]

tem

pe

ratu

re [

a.

u.]

r a d i u s [ a . u . ]R a d i u s ?

Tem

pera

tur

?

Δ T

r 05 0 1 0 0o p tµ m l µ m

Tem

pera

ture

Opt

. P

ath

Diff

eren

ce

Limits of Rod Lasers, Focus-ability


Nd:YAG Rod Laser

Conclusion:• The maximum extractable volume power density is limited to ≤ 100 Wcm-3 (typically ≤1kW/rod and for slabs - depending on size - </= 10 kW/slab).

• The typical high BPP (20 to >50 mm*mrad) of multi-rod kW-class Nd:YAG-lasers and the even higher BPP (30 to >80 mm *mrad) for Nd:YAG slabs results in poor focusability

• The Wall Plug Efficiency of rod- and slab-lasers is very low (≤3% for lamp pumped lasers to approx.10% for Diode pumped systems)

• Rod- and slab-lasers (lamp- or Diode-pumped) suffer from thermal problems due to radial temperature gradients (ΔT ≈ 50 K)

• The maximum dissipated heat in a laser rod is limited to ≈ 200 W/cm length and independent of the rod diameter

• Focusability decreases with increased laser power


Yb:YAG Disk Laser

Disk laser, resonator configuration


Yb:YAG Disk Laser

Pumping Efficiency of a Single Disk


kollimierte Pumpstrahlung(von den Laserdioden)

Retro-reflektor-optik

Kühl-finger

Laserkristall

Auskoppel-spiegel

Parabolspiegel

Laserstrahl



Kühl-finger

Laserkristall

Auskoppel-spiegel

Parabolspiegel

Laserstrahl



Kühl-finger

Laserkristall

Auskoppel-spiegel

Parabolspiegel

Laserstrahl

Faltungs-spiegel

Endspiegel

Auskoppler

Kavitäten

Pumpeinheiten



Kühl-finger

Laserkristall

Auskoppel-spiegel

Parabolspiegel

Laserstrahl



Kühl-finger

Laserkristall

Auskoppel-spiegel

Parabolspiegel

Laserstrahl



Kühl-finger

Laserkristall

Auskoppel-spiegel

Parabolspiegel

Laserstrahl

Faltungs-spiegel

Endspiegel

Auskoppler

Kavitäten

Pumpeinheiten

Cavity Mirror

End Mirror

Fold Mirror

Partial Reflector

Beam Delivery Fibre

Pump Laser

Cavity design for a multiple-disk laser

Yb:YAG Disk Laser


Yb:YAG Disk Laser

Output power scaling with number of disks

September 2009 Manfred Berger – ALT`09 - Antalya 22

Yb:YAG Disk Laser

4 Pump modules with Diode bars

4 Disks >4 kW out of 1 Disk

Feedback Power Control6 Fiber Outputs

Configuration of an 16 kW disk laser


2

1aF

wN

L

NF Fresnel-number

Wa beam radius at the disk

L resonator-length

For a max. achievable power density of 50 W/mm2 at the disk the beam radius is:

Consequently the resonator lenght as function of laser power is given by:

2

36,37 10a Lw PL mm

Resonator lenght scales with laser power and reaches 30 m at 5 kW laser power

That long resonators are mechanically and optically not very stable

Fundamental Mode Operation of a confocal resonator is defined by:

2 2

50L

a

Pw mm

PL expected fundamental mode laser power

Limits of Disk laser, resonator length restriction

Yb:YAG Disk Laser

x


OPDThe temperature gradient at the edge of the pump-spot results in a phase distortion since the index of refraction in hot Yb:YAG material is different to the cooled outer part of the disk.

Depending on the design the optical path difference is:

0,1 1m l µm

This path difference reduces the efficiency of fundamental mode operation.

An adaptive mirror in the disk cavity helps to compensate the phase distortion.

0,9 0,5G

MM

Ratio of TEM00- to multi-mode-efficiency

G

MM

Yb:YAG Disk Laser

Limits of Disk Lasers, phase distortion and optical path difference


Gain Reduction

Lasing within the disk

Remedy possible by:

Disk diameter >> Pump spot diameter

Special edge shaping to supress volume reflexion

The resulting maximal extractable power density per disk is therefor:

210LP kW

A cm

ASE limits the maximal power / disk to:

30 1LkW P MW depending on the design

Yb:YAG Disk LaserLimits of Disk-Lasers, amplified stimulated emission (ASE)


Yb:YAG Disk Laser


Yb:YAG Disk Laser

Conclusion:

• The maximum extractable volume power density per disk ≈ 1 MW/cm3 resulting in extractable power density of 10 kW/cm2: …100 kW/disk possible

• The low BPP of industrial multi-disk kW-class laser results in good focusability

• High Wall Plug Efficiency (e.g. >/= 27% at the work piece)

• Conventional (folded) Resonator allows for tailoring of the BPP and very compact design (important features for material processing)

• Low resonator power-density (back-reflex insensitivity)

• Effective pumping (≈65% optical efficiency)


Yb-Fiber Laser

Er3+

Nd3+

Er3+Ho3+Er3+Tm3+Tm3+

Ho3+

Pr3+Pr3+/Er3+/Ho3+/Tm3+

400 800 1200 1600 2000 2400 2800 [nm]

Yb3+

E. Snitzer, “Neodymium glass laser,” Proc. of the Third International conference on Solid Lasers, Paris, page 999 (1963).C.J. Koester and E.Snitzer, “Amplification in a fiber laser,” Appl. Opt. 3, 10, 1182 (1964).

first demonstration of a fiber laser: in the early sixties !

Emission Spectrum of Fiber Lasers


Yb-Fiber Laser

Spliced-on

passive fiber

OC-Bragg gratingHR-Bragg grating

High power single emitter pumping


End-on-Pumping by high power diode laser Stacks

Pumping byseveral small Stacks

Pumping by many individual Single Mode Emitters

Yb-Fiber Laser

Pump Concepts for Fiber Laser


Scaling of output power by means of beam combination

• incoherent beam superposition

• total output power scales with number of modules

• max. theoretical beam quality scales with P 0

1/2

• real beam quality approximately 2 - 5 times lower (due to losses)

0Q * 8 quality beam Max.

kW 6 each W 100 modules, 60 :Example

:quality Beam

totalPwQ

2

Yb-Fiber Laser


core <10 µm core NA = 0.1 – 0.2

absorption length > 20 m

n

n

active core

pump core

coating

double-clad large-mode-area fiberactive core

pump core

coating

core = 30 .. 40 µm core NA = 0.06 .. 0.08

absorption length < 10 m

increased mode-field diameterreduced fiber length reduced nonlinearity

Yb-Fiber Laser

Low NA large mode area fiber design


1. Available power per unit length of fibreHeat loss within the core material,Thermal flux through inner and out cladding, Heat transfer by air or water

inner

cladding

Out cladding

Fibre-

core

Pump-

beam

Laser

Temperature limits laser power per length unit

For water cooling this limit amounts to:

1200LP W

l m

For a 40 µm diameter core the max. extractable volume power density is therefor:

1³

LP MW

V cm

Yb-Fiber Laser

Limits of Fiber Lasers, power limitations for active fibers


Power density of fibre core and -endfaces

Damage threshhold of quartz: max 2

1GWE

cm

For a fibre core diameter of 40 µm results then:

,max 9LP kW

Power scaling possible by:• increasing of the core diameter• Shortening of fibre length

Stimulated Raman Scattering SRS (non linear effect) limits the internal power:

16 effSRS

eff R

AP

L g

PSRS max. power out of the fibre

Aeff effective area of the fibre core

Leff effective fibre length

gR Raman gain coefficient

Max. power limited by SRS:

Yb-Fiber Laser

Limits of Fiber Lasers,limiting effects of quartz damage threshold and SRS


0 2 4 6 8 10 12 14 16 18 20 22 240

2

4

6

8

10

12

1200 W/m extracted

damage threshold

SRS threshold

P

ow

er

[kW

]

Fiber length [m]

Kerndurchmesser 40µm

10 kW Grundmode Faserlaser möglich

Fibre Core Diameter 40 µm

10 kW fundamental mode fibre laser is possible

Yb-Fiber LaserLimitations of fundamental mode fiber lasers with present technology

Damage Threshold :>/= 1GW/cm2


Yb-Fiber Laser

microstructured fiber

air

glass d

step-index fiber

2a

ncore

ncladding

n ~ 1·10-4

NA ~ 0.02n ~ 1·10-3

NA ~ 0.06

Design of large mode area fibers


core diameter: 42 µm (~30 µm MFD)

laser core NA: ~0.03

pump core diameter: 500 µm

pump core NA: ~ 0.6

Design of multi-kW photonic chrystal fiber

Yb-Fiber Laser


Yb-Fiber Laser

Conclusion:

• The maximum extractable volume power-density may reach 1 MW/cm3:…>/=10 kW/fiber with fundamental mode possible

• The very good low BPP of a multiple-fiber kW-class Yb-fiber laser results in very good focusability (depending on design: < 10 mm*mrad)

• The Wall Plug Efficiency of fiber lasers is very high (i.e. ≈ 30%)

• Power scaling by incoherent beam superposition

• With the availability of Large Mode Area (LMA) fibers multi-kW fundamental mode lasers offer highest beam quality and brilliance.


Yb-Fiber Laser

Beam Parameter Product vs Fibercore Diameter


High Power Diode Laser

Conclusion:

• High power diode lasers offer highest wallplug efficiency (e.g. 40-50%)

• The beam quality of multi-kW Diode lasers at present is comparable to lamp-pumped YAG-lasers

• Lifetime of Diodes, stacks and bars has increased considerably (>50.000h)

• Wavelength combining of several high power Diode modules possible

• Fiber Optic delivery is state of the art


Applications

Welding results with disk lasers


Applications

Welding efficiency of CO2- vs YAG-laser


Applications

Remote Welding


Applications


Applications

Comparison of fiber- andCO2-laser cutting edge quality


Applications

Fiber laser cutting quality for different thicknesses


Acknowledgements

I am very grateful for help and advise I received from the following organisations:

- University of Stuttgart – IFSW- University of Jena – IAP- Fraunhofer Institute IOF Jena- Fraunhofer Institute ILT Aachen- Fraunhofer Institute IWS Dresden- Rofin Sinar Laser- IPG Photonics- Trumpf

Recent Advances of High Power 1 µm Lasers Manfred Berger, II-VI Deutschland ALT`09 – Antalya -...

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