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Recent Advances of High Power 1 µm Lasers
1. Recent Advances of High Power 1 µm Lasers
Manfred Berger, II-VI DeutschlandALT`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
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3.
AbstractWith 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
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4. Introduction
/2005/2%
/41%
43%/38%
/19%
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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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 SystemsHPDL
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
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7. Introduction
Beam Quality of Fiber-/Disk-Laser vs CO2-LaserSept. 2009
Manfred Berger - ALT`09 - Antalya
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8. Introduction
Power Levels of DPSS-Lasers and Trend of MOOREs LawSept. 2009
Manfred Berger - ALT`09 - Antalya
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9. Beam Quality & Brilliance
Beam Quality & BrillianceCaustic 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
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10. Solid State 1µm Laser (SSL)
Current Solid State Laser ConceptsSept. 2009
Manfred Berger - ALT`09 - Antalya
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11. Nd:YAG Rod Laser
Principle of a Lamp Pumped YAG-LaserSept. 2009
Manfred Berger - ALT`09 - Antalya
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12. Nd:YAG Rod Laser
Limits of Rod Lasers, Mechanical StressThermo-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
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13. Nd:YAG Rod Laser
Limits of Rod Lasers, Optical PropertiesParameters 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
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14. Nd:YAG Rod Laser
Limits of Rod Lasers, Focus-abilityThermal 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
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15.
Nd:YAG Rod LaserConclusion:
• 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
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16. Yb:YAG Disk Laser
Disk laser, resonator configurationSept. 2009
Manfred Berger - ALT`09 - Antalya
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17. Yb:YAG Disk Laser
Pumping Efficiency of a Single DiskSept. 2009
Manfred Berger - ALT`09 - Antalya
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18. Yb:YAG Disk Laser
ko limu p
rtP
e
lu n
h
stra
g
a ra
P
ie g
lsp
o
b
l
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(v o
s rd
a
L
e
d
n
)
n
e
io
rki sta
se
a
L
l
e r
g
l-fin
h
ü
K
-flk
tro
e
R
tik
p
o
l
rth
se
a
L
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 disksSept. 2009
Manfred Berger - ALT`09 - Antalya
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20. Yb:YAG Disk Laser
Configuration of an 16 kW disk laser4 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 restrictionFundamental 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
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22. Yb:YAG Disk Laser
Limits of Disk Lasers, phase distortion and optical path differenceThe 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
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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, 2007Manfred Berger – PRIMA Industries
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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
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26. Yb:YAG Disk Laser
Yb-Fiber LaserEmission 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
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27.
Yb-Fiber LaserHigh power single emitter pumping
HR-Bragg grating
OC-Bragg grating
Spliced-on
passive fiber
April, 17, 2007
Manfred Berger – PRIMA Industries
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28. Yb-Fiber Laser
Pump Concepts for Fiber LaserEnd-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 combination2
• 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
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30. Yb-Fiber Laser
Low NA large mode area fiber designactive 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
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31. Yb-Fiber Laser
Limits of Fiber Lasers, power limitations for active fibers1. 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 SRSPower 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
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33. Yb-Fiber Laser
Limitations of fundamental mode fiber lasers with present technologyKerndurchmesser
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
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34. Yb-Fiber Laser
Design of large mode area fibersair
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
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35. Yb-Fiber Laser
Design of multi-kW photonic chrystal fibercore 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
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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
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37. Yb-Fiber Laser
Beam Parameter Product vs Fibercore DiameterSeptember 2009
Manfred Berger – ALT`09 - Antalya
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38. Yb-Fiber Laser
High Power Diode LaserConclusion:
• 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
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39.
ApplicationsWelding results with disk lasers
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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40. Yb-Fiber Laser
ApplicationsWelding efficiency of CO2- vs YAG-laser
April, 17, 2007
Manfred Berger – PRIMA Industries
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41. High Power Diode Laser
ApplicationsRemote Welding
Sept. 2009
Manfred Berger - ALT`09 - Antalya
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42.
ApplicationsSept. 2009
Manfred Berger - ALT`09 - Antalya
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43. Applications
Comparison of fiber- andCO2-laser cutting edge qualitySept. 2009
Manfred Berger - ALT`09 - Antalya
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44.
ApplicationsFiber laser cutting quality for different thicknesses
April, 17, 2007
Manfred Berger – PRIMA Industries
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45. Applications
AcknowledgementsI 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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