The Changing Technology of Solid State Lasers
Peter F. MoultonQ-Peak, Inc.
135 South RoadBedford, MA 01730
CLEO 2004San Francisco, CA
May 20, 2004
Introduction
Example of early fiber laser success:Er-doped silica
4F7/2
4S3/2
2H11/2
4F9/2
4I9/2
4I11/2
4I13/2
4I15/2
Pump
2.8 μm
W11
W11
W22
W22
N2
N1
W50
W50
Outline
• Quick review of fiber-laser designs• Diode pump lasers for bulk and fiber lasers• The battle for cw power• The changing boundaries of short-pulse lasers• Driving nonlinear optics• Fiber and bulk lasers working together• Future directions - photonic fibers• Summary
Quick review of fiber-laser designs
Cladding-pumped fiber laser allowsmultimode pumping of single-mode cores
Maurer, U.S Patent 3,808,549 (April 30, 1974)
J. Kafka, U.S. Patent 4,829,529 (May 9, 1989)
Elias Snitzer first described cladding pumped lasers in 1988
Non-circularly symmetric cladding geometries permit effective overlap with laser core
http://www.iap.uni-jena.de/fawl/rdtfawl.html
Absorption length is increased by the ratio of cladding to core areas
End-pumped double-clad requires dichroic mirrors and “bright” pump source
Ytterbium-doped large-core fiber laser with1 kW continuous-wave output power
Y. Jeong, J.K. Sahu, D. N. Payne, and J. Nilsson, ASSP 2004
Diode stack@972 nm, 1 kW
Double-clad Yb-doped fibre II
HT @972 nmHR @~1.1 μm
HT @975 nmHR @~1.1 μm
Signal output@~1.1 μm
HT @975 nmHR @~1.1 μm
Lucent design allowed access to end of fiber and multiple pump ports
D. J. DiGiovanni US patent #5,864,644
Multi-Mode Coupler Approach - IPG Photonics
Multi-Mode Coupler Region
• Multi-Mode coupler is created by fusing under high temperature conditions double-clad doped fiber with multi-mode fiber from pump source
SPI has GTWave Technology
> 70 W per module
V-Groove Side Pumping: Keopsys
double-claddingfiber adhesiveV-groove
micro-lens
broad stripe Laser diode
substrate
Principle:
End viewSide View
TM
High efficiency coupling (>90%) with broad area inexpensive laser diodes & multiple V-grooves
Simple & compact packaging, with large alignment tolerances
No loss of light in core and no need for multiplexer
Embedded mirror design from NRL
J.P. Koplow, S.W. Moore, and D.A.V. Kliner, JQE 39, 529 (2003)
Beyond double-clad designs, further strategies are needed for fiber-laser power scaling
• Eventually, the small area of the mode in the core creates limits:– Optical damage to the fiber faces (more later)– Bulk damage at flaws or defects– Nonlinear optical effects in the bulk of the fiber
• Nonlinear effects include:– Stimulated Brillouin scattering for single-frequency sources,
cw or pulsed > 10 ns• Adds frequency components and can lead to backward wave
generation and catastrophic pulse shortening• Threshold follows (Core area)/(Fiber length)
– Stimulated Raman scattering• Adds frequency components, may limit NL conversion• Threshold follows (Core area)/(Fiber length)
• Larger core/mode size is desirable – lower intensity and shorter fiber length for double-clad designs
Step index fiber - limits for single mode
nc
nc
NAa
oλπ= 2V
22cfstep nnNA −=
maxθ
( )maxsin θ=NA
a is core radius, λ is wavelength
V < 2.405 for single-mode fiber
0
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Core diameter (um)
NA 1.06
1.55
Wavelength(um)
Relation of core diameter to NAfor step-index fiber
Below an NA of 0.06 or so, bend losses are problematic
From Fluke Networks Application Note
Coiling fiber allows single-mode with V > 2.4
J. Koplow, D. Kliner and L. Goldberg, Optics Lett. 25, 442 (2000).
25 um core diameter, NA 0.1 (V=7.4 at 1064 nm)Straight (left) 1.58 cm coil dia. (right)
Other tricks around the core size limits
Complex index profiles
J.A. Alvarez-Chavez et al.,Opt. Lett 25, 37 (2000).
Tapered sections
http://www.orc.soton.ac.uk/hpfl/tapers.php
Or, careful launching of low-temporal coherence,single-mode beam into high-quality, multimode fiber
M.E. Fermann, Opt. Lett. 23, 52 (1998).
Diode pump lasers for bulk and fiber lasers
JDSU 5-W 915-nm diode laser Telcordia-qualified, long-lifetime pump
JDS Uniphase's ultra-reliable 6390 series laser diodes offer 5 W of laser power from a 100 µm fiber into 0.2 NA. The L3 package is a redesign of the existing fiber-coupled L2 package, incorporating telecom design approaches into a commercial product and resulting in a reliability of >200,000 hours MTBF.
Coherent 808nm 30W FAP-BMTTF: 47000hrs(90%CL)
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0 5000 10000 15000 20000 25000 30000 35000Time (hrs)
Nor
mal
ized
Pow
er
233952532528498285002850123864
DARPA SHEDS Program (find CLEO talk ref.)
Attribute Current 18 Mo. 36 Mo.Bar power conversion eff. 50% 65% --
Bar power output 80W
Stack PCE 50% -- 80%
Stack Power 480W
Spectral Width 10-15nm 2nm 2nm
Uniformity of wavelength across the emitting area ±10nm ±0.5nm ±0.5nm
0
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0 20 40 60 80Current (A)
Out
put p
ower
(W)
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icie
ncy
(%)
0%
50%
100%
930. 940. 950. 960.
W avelength (nm)
Inte
nsity
/ Im
ax
High-brightness pump sources
• Apollo Instruments fiber-coupled diode lasers (0.22 NA):
– 35 W from 100 um– 150 W from 200 um– 400 W from 400 um– 500 W from 600 um, 0.22 NA
fiber• Laserlines stacked, beam-shaped
bars– 500 W, 40x50 mrad– 1000 W, 60x80 mrad– 6000 W, 85x400 mrad
• Nuvonyx stacked, beam-shaped bars
– 4000W, focusable to 12.5 x 0.5 mm spot
The battle for cw power
Contenders for high-power industrial solid state lasers
t ransversally pumped rod laser
thin disklaser
INNOSLAB laser
f ibre laserend-pumped
f ibre laser“ Y” -pumped
Toshiba and Shibaura Nd:YAG rod lasers
Shibaura LAL-210/220/230/240/260SERIES 4.5 kW with 600 um fiber
“Toshiba succeeded in obtaining an output power of 12 kW with an efficiency of 23 %, which are, to our knowledge, the highest values for a Nd:YAG laser.”
MELCo March 2002 Press Release
“Mitsubishi Electric recently announced an all-solid-state laser that is the world's most efficient laser of its kind. The new laser converts 23% of the electrical power it receives into light energy. That is more efficient than any other solid-state laser.”
Pcw = 1 kW, Ppulse = 10 kW, focusable to 50 um
Trumpf diode-pumped Nd:YAG rod lasers
Laser device Max. output power (Watts)
Laser power at the workpiece(Watts)*
Beam quality (mm * mrad)
Laser light cable (microns)
HLD 1003 1300 1000 12 300
HLD 3504 4500 3500 16 400
HLD 4506 6000 4500 25 600
Rofin-Sinar diode-pumped Nd:YAG rod lasers
ROFIN DY 022
ROFIN DY 027
ROFIN DY 033
ROFIN DY 044
Excitation Laser diodes
Laser diodes
Laser diodes
Laser diodes
Output power 2200 W 2700 W 3300 W 4400 W
Beam parameter product
12 mm*mrad
12 mm*mrad
12 mm*mrad
12 mm*mrad
Fiber diameter
400 µm 400 µm 400 µm 400 µm
From Giesen
Thermal limits(Evaluated by Brown and Hoffman, IEEE JQE vol. 37, p.
207, 2001)
Fracture: 10 kW/m of heat generation leads to fracture
Thermal beam distortions: 0.2 kW/m leads to Δn= 10-4 over core
Temperature rise: 0.15 kW/m leads to ΔT = 50 K
0.1 kW/m of heating demonstrated in practice without problems
Heat generation in YDFLs ~15% of output power: 150 W/kW
→ ~1 kW/m optical power generation in efficient YDFLs
Enumerate latest fiber-laser data from ASSP, CLEO
• Southampton– 1 KW– SF 240 W
• Tunnerman• IMRA• IPG• NGST
Show Southampton results
IPG Photonics YLR-HP Series: 1-10kWatt Ytterbium Fiber Lasers
•Up to 10 kWatt Output Optical Power
•Over 20% Wall-Plug Efficiency
•Excellent Beam Parameter Product
•>50,000 Hours Pump Diode Lifetime
•Air or Water Cooled Versions
•Maintenance Free Operation
•Up to 200 m Fiber Delivery
•2 Year Warranty
Newest from IPG
May 6, 2004, Burbach Germany- IPG Photonics today demonstrated a revolutionary new model high brightness 5.5 kilowatt Ytterbium fiber laser for remote welding to representatives of the European automotive and heavy industry communities.
This new laser operates at 1070nm, and provides significant features that include a beam parameter product of 4.3 mm milli-radians, a delivery fiber of 100 microns, rapid response modulation, greater than 25% wall plug efficiency, and a new robust industrial interface including digital, analog and tele-control of the laser parameters. This combination of high brightness and small diameter fiber delivery, allows the use of a 1.4 meter focal length to achieve spot sizes required for remote welding applications. In addition, the laser allows long stand off distances on conventional cutting and welding applications, greatly reducing the risk of delivery fiber damage at the output end of the fiber.
This achievement allows IPG to increase the company’s leadership position in offering the highest beam quality available from kilowatt class industrial lasers. These lasers represent a substantially better beam quality than that predicted from the 4kW diode pumped disc lasers scheduled for release in late 2004. .
The Empire Strikes Back: thin-disk lsaers
From Giesen Single-disk results
From Giesen Single-disk results
From Giesen Single-disk results
Trumpf introduces a 4 kilowatt thin-disk laser and a scanner for high-power remote-welding at the Munich show.
The laser, which can be coupled to a 200 µm fiber, provides the highest power available from such a source. The system contains four Yb:YAG disks and is designed for sheet-metal cutting and aluminum welding applications.
1 10 100 1.000 10.000Laserpower P [W]
0.1
1
10
100
1.000
Beam
par
amet
er p
rodu
ct Q
[mm
mra
d]
Print ingtherm. marking
plast icswelding
soldering select ive laser powder remelt ing
t ransformation hardening
melting,cleaningbrazing
θf
F#4 focusingoptics (NA 0,12)P
I2 w 0
metal sheetcut t ing
deep penet rat ion welding metals
Q PIf= ⋅⋅
θπ
diodelasers(2003)
CO -laser2
lamp pumpedNd:YAG-laser
On to higher powers
• Fiber laser bundling can provide > 10 kW• Empire Strikes Back:
– US High-Energy Laser Program (HEL JTO) funding two 25 kW laser demonstrations at Raytheon and TRW
The changing boundaries of short-pulse lasers
Yb fiber lasers not quite ready for NIF
Er fiber lasers not ready for 100-mJ-level eyesafe lidar/ladar sources
0
100
200
300
400
500
1571
-nm
OP
O O
utpu
t
0.2 0.4 0.6 0.8 1 1.2 1064-nm Pump Energy (J)
4.5 W average power
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25
30
35
Pump Power (Watts)
Sign
al P
ower
(Wat
ts)
100-Hz pulse rate 1535-nm OPO
Discussion of limits
Brillouin scattering increases drasticallyfor short pulsewidths
T.J. Kane et al. “3 Watt green and blue sources based on Nd:silica fiber amplifiers”SPRC Annual Meeting, September 17, 2003
LLNL surface damage data for fused silica
B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, and M. D. PerryJ. Opt. Soc. Am. B, 459 (1996).
End-face damage limits (calculated) for different pulsewidths
0.001
0.01
0.1
1
10
0 10 20 30 40 50 60
Mode diameter (um)
Dam
age
puls
e en
ergy
(mJ)
1001010.1
Pulse-width(ns)
Comparison of end-face damage calculationsand (limited) data
1
10
100
1000
0.01 0.1 1 10 100
Pulsewidth (ns)
Dam
age
inte
nsity
(GW
/cm
2)
1
10
100
1000
Dam
age
fluen
ce (J
/cm
2)
SSouthampton data
Fibertek data
Elegant solution to fiber end-face damagefor systems with “side” pumping
IMRA Pat. App. US2004/00369587 A1
7.7 mJ with 60-um Yb-doped multimode core fiber
http://www.orc.soton.ac.uk/hpfl/pulsed.php
Enumerate results to date
Q-Peak’s DPSSL design obtains high efficiency and high beam quality with side-pumping
Diode laser
Diode laser
Lasercrystal
Cylinder lens Laserbeam
Pumpbeam
Multi-Pass Slab (MPS)US Patent 5,774,489
“Gain Module”Applied to Nd:YLF, Nd:YVO4, Nd:YAGYb:S-FAP, Yb:YAG, Tm:YLF, Er:YLF
Nd:YLF CW oscillator performancescales with pump lasers
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Pump Power (W)
Out
put P
ower
(W)
40 W, MM30 W, MM30 W, TEMoo20 W, TEMoo
M2 = 1.05
Short-pulse,AO Q-switched
Nd:YLF laser
30-100 kHz, 8-20 nsec85 W average power at 1047 nm
With harmonic generation at 30 kHz:45 W at 523.5 nm25 W at 349 nm10 W at 262 nm
0
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Repetition rate, kHz
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ower
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SHGTHG
Air-cooled system: fiber-pumped Nd:YVO4laser head and pump assembly
Laser Head
RF
Optical Fiber
Temperature Controller
Micro-Controller
Diode-Laser Current Source
Q-Switch Driver
28 V
28 V
28 V
Current Set Current
Sense
RS-232
Temp. Set
Temp. Sense
28 V
RF Control
0-40 Amp.
Diode Laser
TE Coolers
0-28 V
Diode Temp.
Ext. Trigger
Output Coupler
Q-Switch
Laser Crystal
Semi-Tactical Pump Assembly 10 W @ 1 μm
50 kHz, 250 kW Nd:YVO4 system is candidate for fiber-laser replacement
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Pulse Rate, KHz
Avg
Las
er O
utpu
t, W
02468101214
Puls
ewid
th, n
sec
Meas PwrLS-fi, PwrPWLS-fit, PW
CW Pwr ~11.3-W
Photograph of complete air-cooled laser system
0
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5 10 15 20pump power, W
outp
ut p
ower
, W
Fiber and bulk lasers working together
Micro-VAM system could employYb-doped fiber-preamp stage
Fiber
Nd:YAG/Cr:YAGMicrolaser
TelescopeIsolatorλ/2 plate
Cylindricallens
HR Mirror
Nd:YVO4 Amplifier
λ/2 plate
SHG THG/ 4HG
Diode laser
Results (at 2 kHz):3.2 μJ microchip laser amplifiedto 335 μ J, 370-ps pulses (0.9 MW)SHG, THG, 4HG: 200, 120, 33 μ J
350 W pumping
OC
HR
AOM
Ho:YLF
Tm:YLF laser #1
Tm:YLF laser #2
DM
DM
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Puls
e en
ergy
, mJ
P
E
Future directions - photonic fibers