Recent Advances of High Power 1 µm Lasers

1. Recent Advances of High Power 1 µm Lasers

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

2. Content

1.
2.
3.
4.
Abstract
Introduction
Beamquality & Brilliance
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. Applications
1. Welding
2. Printing, Engraving and Marking
3. Cutting
Sept. 2009
Manfred Berger - ALT`09 - Antalya
2

3.

Abstract
With the advent of reliable Yb:YAG disk lasers and Ybdoped 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 powerlasers. The advantages and present limitations of 1 µm
solid state lasers will be discussed.
April, 17, 2007
Manfred Berger – PRIMA Industries
3

4. Introduction

/2005
/2%
/41%
43%/38%
/19%
Sept. 2009
Manfred Berger - ALT`09 - Antalya
4

5. Introduction

Evolution of the Beam Parameter Product for Industrial High Power Lasers (2009)
m
Laser Wavelength:1 ≈µ 1µm
Diff. Lim. : 0,34mm mrad
10,6 µm
1 0 .6 µ m
3,7 mm mrad
4 k W N d :Y A G
Lam p P um ped
B P P > 2 0 m m *m ra d
1980
.
2 kW C O 2
S lo w F lo w
B P P : 4 m m *m ra d
.
4 k W N d :Y A G
D io d e P u m p e d
B P P > 1 2 m m *m ra d
.
1990.
.
5 kW C O 2
T r a n s v e r s e F lo w
B P P > 1 5 m m *m ra d
.
4 k W Y b :Y A G D is k
D io d e P u m p e d
B P P : 8 m m *m ra d
4 k W Y b :S iO 2 F ib re
D io d e P u m p e d
B P P : 4 m m *m ra d
2000
.
8 8kW
k W CO2
CO 2
FFast
a s t AAxial
x i a l Flow
F lo w
B BPP:
P P : 46mm*mrad
m m *m ra d
.
0 27kW
. 5 k W Yb:YAG
Y b : Y A GDisk
D is k
S i n g l epumped
M ode
Diode
B BPP:</=1mm*mrad
P P : > 1 m m *m ra d
Sept. 2009
210kW
k W YYb:SiO2Fibre
b :S iO 2 F ib re
SSingle
i n g l e Mode
M ode
B BPP:
P P : 0.4mm*mrad
0 .4 m m * m ra d
.
2009
Manfred Berger - ALT`09 - Antalya
8 kW C O 2
S e a le d S l a b
B P P : 4 m m *m ra d
5

6. Introduction

Efficiencies and Beam Parameter Product of Industrial Laser Systems
HPDL
Disk and Fibre Laser
show very high efficiency
with low beam parameter
product.
HPDL
HPDL
with
with
Fibre
Fibre
Disk
Disk
E fficien cy in %
Diode Lasers show
highest efficiencies with
lowest operating cost.
Fiber
Fibre
DP-Rod
LP-Rod
Beam Parameter Product in mm*mrad
Sept. 2009
Manfred Berger - ALT`09 - Antalya
6

7. Introduction

Beam Quality of Fiber-/Disk-Laser vs CO2-Laser
Sept. 2009
Manfred Berger - ALT`09 - Antalya
7

8. Introduction

Power Levels of DPSS-Lasers and Trend of MOOREs Law
 
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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9. Beam Quality & Brilliance

Beam Quality & Brilliance
Caustic of Gaussian Laser Beam
√2x2wL
√22wL
2w0,G
M2
2W L
0, G
L
BPPL L x1rad xM 2
BPPL
BPPG
F
2
M NA d fibre
f
D
 
Z R 0, G
ZRL
NA sin n12 n2 2
d0
0,G
BPP W0,G
W L L L const
2
2
Sept. 2009
Manfred Berger - ALT`09 - Antalya
B
4 L
K D3
M 2 F
f2
PL
2
2
M x M y L
2
PL
2
BPP L
2
W
m 2 rad
9

10. Solid State 1µm Laser (SSL)

Current Solid State Laser Concepts
Sept. 2009
Manfred Berger - ALT`09 - Antalya
10

11. Nd:YAG Rod Laser

Principle of a Lamp Pumped YAG-Laser
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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12. Nd:YAG Rod Laser

Limits of Rod Lasers, Mechanical Stress
Thermo-Mechanical Parameters limit the max. Thermal Load in a Laser Rod and the
resulting max. Stress is Limited by Tensional Failure (Breakage) of the Laser Rod.
Pv
K (1 )
8
max
l
E
with:
W. Koechner „Solid-State Laser
Engineering“, Springer, 1999
Pv Dissipated Heat
l
Rod Length
K Thermal Conductivity
v
Poisson Ratio
α
Coefficient of Thermal Expansion
E
Coefficient of Elasticity
σmax Max. Permitted Tensile Stress at Rod Surface
The Resulting upper Limit of Thermal
Losses
Dissipated
Heatfor
forYAG
YAGMaterial
Materialisisthen:
then
Pv
W
200
,
l
cm
Sept. 2009
independent of Rod-Diameter
Manfred Berger - ALT`09 - Antalya
12

13. Nd:YAG Rod Laser

Limits of Rod Lasers, Optical Properties
Parameters for Thermo-Optical Effects Result in the Formation of a Thermal Lens with
Focal Length f:
KA 1 dn
r0 ( n0 1)
3
f
C
n
r , 0
Pv 2 dT
l
Fraction of 70%:
due to Index of Refraction
Change with Temperature
with: K
A
Pv
dn/dT
α
1
W. Koechner „Solid-State Laser
Engineering“, Springer, 1999
Fraction of 20%:
due to Index of Refraction
Change with Mechanical
Stress
Thermal Conductivity
Rod Cross-Section
Dissipated Heat
Index of Refraction Change with T
thermal Expansion
Cr,φ
n0
r0
l
Fraction of 10%:
due to change of Optical
Path Length by Variation of
Rod Length
photoelastic Coefficient
On-Axis Index of Refraction
Rod-Radius
Rod-Length
Typical Focal Length (Order of Magnitude) for Rod Lasers with P = 1kW CW-Power:
f 10cm
Sept. 2009
Manfred Berger - ALT`09 - Antalya
13

14. Nd:YAG Rod Laser

Limits of Rod Lasers, Focus-ability
Thermal Lens:
f 10cm
Optical Path Difference between
Axial- and Peripheral Rays :
50µm lopt 100µm
 
Wegunterschied
?
Opt.
Path Difference
temperature
Temperature
Temperatur[a.
? u.]
 
ΔT
Δlopt
radius
[a. u.]
Radius
?
r0
Inhomogeneities of Pump-Intensity and Cooling Efficiency result in Degradation of
Focusability M2, if:
lopt , asph 0,1 0,1µm
For Distortion free Lasing Homogeneity and also Temporal
Stability of the Pump Beam Intensity have to be kept below
0.1%!!!
Focusability Decreases with Laser Power since these Conditions
are increasingly difficult to reach!
Sept. 2009
Manfred Berger - ALT`09 - Antalya
14

15.

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
April, 17, 2007
Manfred Berger – PRIMA Industries
15

16. Yb:YAG Disk Laser

Disk laser, resonator configuration
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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17. Yb:YAG Disk Laser

Pumping Efficiency of a Single Disk
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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18. Yb:YAG Disk Laser

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u p
rtP
e
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(v o
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n
)
n
e
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rki sta
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ü
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-flk
tro
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R
tik
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o
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rth
se
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u sko
A
l-ig
e
p
Cavity design for a multiple-disk laser
Cavity
Mirror
Kavitäten
Pumpeinheiten
Pump Laser
End Mirror
Endspiegel
FaltungsFold
Mirror
spiegel
Partial
Reflector
Auskoppler
Beam Delivery Fibre
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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19. Yb:YAG Disk Laser

Output power scaling with number of disks
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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20. Yb:YAG Disk Laser

Configuration of an 16 kW disk laser
4 Pump modules with Diode bars
4 Disks >4 kW out of 1 Disk
Feedback Power Control
6 Fiber Outputs
September 2009
Manfred Berger – ALT`09 - Antalya
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21. Yb:YAG Disk Laser

Limits of Disk laser, resonator length restriction
Fundamental Mode Operation of a confocal resonator is defined by:
wa 2
NF
1
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:
wa 2
PL
mm 2
50
PL
expected fundamental mode laser power
Consequently the resonator lenght as function of laser power is given by:
wa 2
P
L
6,37xg10 3 L [ 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
Sept. 2009
Manfred Berger - ALT`09 - Antalya
23

22. Yb:YAG Disk Laser

Limits of Disk Lasers, phase distortion and optical path difference
The 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.
OPD
Depending on the design the optical path difference is:
0,1m m l 1µm
This path difference reduces the efficiency of fundamental mode operation.
hG
0,9
0,5
hMM
hG
hMM
Ratio of TEM00- to multimode-efficiency
An adaptive mirror in the disk cavity helps to compensate the phase distortion.
Sept. 2009
Manfred Berger - ALT`09 - Antalya
24

23. Yb:YAG Disk Laser

Limits of Disk-Lasers, amplified stimulated emission (ASE)
Gain Reduction
Lasing within the disk
Remedy possible by:
Disk diameter >> Pump spot diameter
Special edge shaping to supress volume reflexion
ASE limits the maximal power / disk to:
30kW < PL < 1MW
depending on the design
The resulting maximal extractable power density per disk is
therefor:
PL
kW
10 2
A
cm
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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24. Yb:YAG Disk Laser

April, 17, 2007
Manfred Berger – PRIMA Industries
26

25. 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)
April, 17, 2007
Manfred Berger – PRIMA Industries
27

26. Yb:YAG Disk Laser

Yb-Fiber Laser
Emission Spectrum of Fiber Lasers
Ho3+
Nd3+
Pr3+/Er3+/Ho3+/Tm3+
400
800
Yb3+
1200
Pr3+
Er3+
Tm3+
1600
2000
Tm3+
2400
Er3+ Ho3+
Er3+
2800
[nm]
first demonstration of a fiber laser: in the early sixties !
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).
April, 17, 2007
Manfred Berger – PRIMA Industries
28

27.

Yb-Fiber Laser
High power single emitter pumping
HR-Bragg grating
OC-Bragg grating
Spliced-on
passive fiber
April, 17, 2007
Manfred Berger – PRIMA Industries
29

28. Yb-Fiber Laser

Pump Concepts for Fiber Laser
End-on-Pumping by
high power diode laser
Stacks
Pumping by
several small Stacks
Pumping by many
individual Single Mode
Emitters
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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29. Yb-Fiber Laser

Scaling of output power by means of beam combination
2
• incoherent beam
superposition
• total output power scales
with number of modules
• max. theoretical beam
quality scales with P 01/2
• real beam quality
approximately
2 - 5 times lower
(due to losses)
Beam quality : Q w Ptotal
Example :
60 modules,100 W each 6 kW
Max. beam quality 8 * Q0
Sept. 2009
Manfred Berger - ALT`09 - Antalya
31

30. Yb-Fiber Laser

Low NA large mode area fiber design
active core
n
pump core
coating
core <10 µm
core NA = 0.1 – 0.2
absorption length > 20 m
double-clad large-mode-area fiber
active core
n
pump core
coating
increased mode-field diameter
reduced fiber length
Sept. 2009
Manfred Berger - ALT`09 - Antalya
core = 30 .. 40 µm
core NA = 0.06 .. 0.08
absorption length < 10 m
reduced nonlinearity
32

31. Yb-Fiber Laser

Limits of Fiber Lasers, power limitations for active fibers
1. Available power per unit length of fibre
Heat loss within the core material,
Thermal flux through inner and out cladding,
Heat transfer by air or water
Temperature limits laser power per length unit
For water cooling this limit amounts to:
Pumpbeam
PL
W
1200
l
m
Fibrecore
inner
cladding
Laser
For a 40 µm diameter core
the max. extractable volume power
density is therefor:
PL
MW
1
V
cm ³
Sept. 2009
Out cladding
Manfred Berger - ALT`09 - Antalya
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32. Yb-Fiber Laser

Limits of Fiber Lasers,limiting effects of quartz damage threshold and SRS
Power density of fibre core and -endfaces
Damage threshhold of quartz:
1GW
Emax 2
cm
For a fibre core diameter of 40 µm results then:
PL ,max 9kW
Stimulated Raman Scattering SRS (non linear effect) limits the internal power:
Max. power limited by SRS:
PSRS 16 ×
Aeff
Leff g R
PSRS max. power out of the fibre
Aeff
effective area of the fibre core
Leff
effective fibre length
gR
Raman gain coefficient
Power scaling possible by:
• increasing of the core diameter
• Shortening of fibre length
Sept. 2009
Manfred Berger - ALT`09 - Antalya
34

33. Yb-Fiber Laser

Limitations of fundamental mode fiber lasers with present technology
Kerndurchmesser
40µm
Fibre
Core Diameter 40
µm
12
P
o
w
er [kW
]
10
Damage
Threshold
:>/= 1GW/cm2
damage
threshold
8
6
SRS threshold
4
1200 W/m extracted
2
0
0
2
4
6
8
10 12 14 16 18 20 22 24
Fiber length [m]
1010
kW
fundamental
fibre möglich
laser is possible
kW
Grundmode mode
Faserlaser
Sept. 2009
Manfred Berger - ALT`09 - Antalya
35

34. Yb-Fiber Laser

Design of large mode area fibers
air
glass
d
2a
ncore
ncladding
microstructured fiber
n ~ 1·10-4
NA ~ 0.02
September 2009
step-index fiber
n ~ 1·10-3
NA ~ 0.06
Manfred Berger – ALT`09 - Antalya
37

35. Yb-Fiber Laser

Design of multi-kW photonic chrystal fiber
core diameter: 42 µm (~30 µm MFD)
laser core NA: ~0.03
pump core diameter: 500 µm
pump core NA: ~ 0.6
Sept. 2009
Manfred Berger - ALT`09 - Antalya
38

36. 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.
April, 17, 2007
Manfred Berger – PRIMA Industries
39

37. Yb-Fiber Laser

Beam Parameter Product vs Fibercore Diameter
September 2009
Manfred Berger – ALT`09 - Antalya
40

38. Yb-Fiber Laser

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 lamppumped 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
April, 17, 2007
Manfred Berger – PRIMA Industries
42

39.

Applications
Welding results with disk lasers
Sept. 2009
Manfred Berger - ALT`09 - Antalya
43

40. Yb-Fiber Laser

Applications
Welding efficiency of CO2- vs YAG-laser
April, 17, 2007
Manfred Berger – PRIMA Industries
44

41. High Power Diode Laser

Applications
Remote Welding
Sept. 2009
Manfred Berger - ALT`09 - Antalya
45

42.

Applications
Sept. 2009
Manfred Berger - ALT`09 - Antalya
46

43. Applications

Comparison of fiber- andCO2-laser cutting edge quality
Sept. 2009
Manfred Berger - ALT`09 - Antalya
47

44.

Applications
Fiber laser cutting quality for different thicknesses
April, 17, 2007
Manfred Berger – PRIMA Industries
48

45. Applications

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
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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