Laser Technology for Compact X

Laser Technology for Compact X-ray Sources
Franz X. Kärtner
Ultrafast Optics and X-rays Division
Center for Free-Electron Laser Science, DESY and
Department of Physics, University of Hamburg, Hamburg, Germany2!
The Hamburg Center for Ultrafast Imaging
and
Department of Electrical Engineering and Computer Science
and Research Laboratory of Electronics, MIT, Cambridge MA, USA
www.cfel.de
www.rle.edu
Doha CLS-Workshop April 4-5, 2014
•  Research center in the Helmholtz Association
•  2200 staff
•  3000 visiting scientists/year
Hamburg. City of Light
• 
• 
• 
New High Brilliance X-ray Sources
Research Labs / Inhouse
Application Labs
DESY
CFEL Building, September 2012
4
Acknowledgement (DESY, MIT)!
CFEL @ DESY!
MIT
Emilio Nanni, Hua Lin, Luis Zapata and Krishna Murari
5
Compact X-Ray Light Source
6
Overall Laser Layout
ICS laser
Seed source
200 MHz, 10 nJ,
2 W, 200 fs
Single 2 mJ pulse at 1 kHz
Pulse
picker
Yb-fiber laser front end
Burst mode photocathode laser
AOM selects 100
pulses at 1 kHz
rep.rate
Stretcher (~300 ps)
Yb:KYW regen
Single 100 mJ
3 ps pulse at 1 kHz
Multi-pass
Yb:KYW
Gain 100
20 W,
Yb:KYW
short pulse
100 W, cryoYb:YAG
Compressor
FHG to UV
in BBO
100 pulses, 250 nm
20 µJ, 150 fs
Compressor
50 mJ 2 ps 515 nm pulse in 200 MHz
ring down cavity for ICS
100 pulses 2 µJ
100 pulses
200 µJ, 600 fs
Diode
pump
940 nm
SHG in linear cavity with
dichroic mirrors
Multi-pass Yb:KYW
Gain 400
Diode
pump
980 nm
Cryo-Yb:YAG
amplifier
RF gun
50 mJ: Ringdown cavity!
7
Interaction Point
8
Linear Image Relay Cavity
Lens
Fused Silica
Mirror
SHG Crystal
LBO
IP
Dichroic Mirror
θ
f1
f1
IR
2f2
Electron
Beam
9
Key Laser Parameters
§ 
§ 
§ 
§ 
§ 
§ 
IP beam waist is 3 microns
Green pulse length is 2 ps
Target conversion efficiency – 50%
IR pulse energy 100 mJ
Desired repetition rate for cavity is 5 ns or LTot ~ 1.5 m
Damage threshold – 0.28 J/cm2 for 2 ps pulse
10
Cavity Loss
§  Lens, mirrors: <0.2% (single element)
§  SHG crystal: 0.25%~1%
§  Total cavity loss 0.5%~2%
11
Stability
2
§  Stable if
§  For:
§ 
§ 
§ 
§ 
⎛ A + D ⎞
g = ⎜
⎟ ≤ 1
2
⎝
⎠
f1
34.5 cm
f2
3 cm
L1
34.5 cm
L2
34.5 cm
L3
6 cm
g=1 cavity is marginally stable
Cavity round-trip length 150 cm
Gaussian beam
Analyze waist for some deviations.
Aq + B
q' =
Cq + D
1 1
λ
= −i
q R π w2
12
Stability vs Pass Number
§  What if L2 and L3 vary?
Marginal Stability
L2 + L3 = f1 + 2 f 2
Stable
L2 + L3 = f1 + 2 f 2 − 0.2 µ m
Unstable
L2 + L3 = f1 + 2 f 2 + 1µ m
13
Additional Cases
Stable
L2 + L3 = f1 + 2 f 2 − 0.5µ m
Stable
L2 + L3 = f1 + 2 f 2 − 1µ m
14
Timing Requirement
§  The accumulated phase difference between the cavity
round trip time and the electron beam repetition rate
should be held to a minimum.
§  Maximum acceptable 0.5 ps of accumulated offset
§  For 100 pump pulses the stability requirement is 1.5 µm
Lock cavity to stable single-frequency laser
< 1nm precision
15
B-Integral Calculation
ICS cavity rep
rate (MHz)
Cavity length
L (mm)
Focal length
f (mm)
WSHG
(mm)
W0
(um)
θ
(deg)
200
750
181
6
5
1.88
LBO crystal:
Laser parameter:
Length (mm)
n0 @ 515nm
n2 (cm2/W)
3.15
1.6
3E-16
1030 nm input
energy (mJ)
Pulse
duration (ps)
SHG to 515 nm
efficiency
Cavity
loss
100
2
50%
0.5%
BSHG = 3
Btotal = 5
16
OXALIS Simulations - Input Source
§ 
§ 
§ 
§ 
§ 
E=100mJ
Wavelength=1064nm
Peak fluence=124 J/m2
Peak intensity=4.1e13 W/m2
Grid Size: Time(50pts),
Space(128x128)
Far Field
Near Field
17
OXALIS Simulations
Intensity of beam at the focus
Far
Field
The shape
remains same
after each round
trip
Near
Field
18
Cryogenic Yb:YAG operation - spectroscopy
Energy Levels in Yb:YAG
Yb:YAG Absorption Spectrum*
Energy
Laser:
1030 nm
Pump:
940 nm
3kBT @ 300K,
9kBT @ 100K
§ 
§ 
§ 
§ 
Absorption Coefficient (cm–
1)
10
8
Laser
Wavelength
77 K
6
Pump
Array
4
2
300 K
0
900 920
940 960 980 1000 1020 1040
Wavelength (nm)
4-level laser with small quantum defect
Higher absorption coefficient ! ease of pumping
Lower saturation fluence (~1.8 J/cm2) ! efficiency and low damage risk
Improvement of thermo-optic-mechanical properties by about 100!
CRYOGENIC OPERATION
Enables efficient high-power lasers
with near-ideal beam quality
LN2 use:
6l / hour for
1 kW output
spectroscopic properties measured by MITLL
Pioneered by T. Y. Fan MIT LL
19
494-W Power Oscillator w. Rod-type Module
Near-Field Profile
at 275 W
FiberCoupled
Pump
Laser
LN2
Dichroic Dewar
Mirror
Yb:YAG Crystals
Laser
Output
Polarizers
High
Reflector
Output
Coupler
Output Power (W)
500
400
300
200
100
0
0
100
200
300
400
500
600
700
• 
• 
• 
• 
494-W average power
71% optical-optical efficiency
M2 ~ 1.4 @ 455 W (wavefront sensor)
OC reflectivity = 25%, L = 1 m,
Near-flat-flat resonator
• 
Performance limited by available
pump power
Incident Pump Power (W)
20
Cryogenic Composite Thin – Disk
Heat
Laser
21
Cryogenic Composite Thin – Disk
Cryogenic
Yb:YAG CTD
Angled
kaleidoscope
Heat
Laser
Features: • 
• 
• 
• 
• 
• 
Vacu
um te
lesco
pe / s
patia
l
Out " Strict image relay Smooth every transit Passive switching High gain High energy High power filter
Mirror
switchyard
Thin-Film Polarizer
" In
22
12-pass amplification architecture - key components
Features: •  Strict image relay •  Smoothing with every transit •  Passive polariza;on switching Vacu
um te
lesco
pe / s
patia
l
filter
TFP
23
Cryogenic disks / hardware
24
ASE-control and gain hold-off
“Bare” disk
◊  C7
§ CTD1
C7
Composite disk
with fashioned edges
DRAMATIC INCREASE IN
GAIN-HOLD REALIZED
CTD1
25
Experimental results are close to expectations
Gain-narrowing
damage
Beam quality at 60 mJ, 200 Hz !
•  Damage sustained ~45 mJ
•  200 ps pulse duration
26
Conclusion
§  Design, validation of high energy ring down
cavity to meet CLS goals
§  Alternatively coherent built-up cavity in burst
mode to reduce B-integral further
§  Cryogenic Yb:YAG are ideal ICS – lasers,
becoming a mature technology
§  Cryogenic composite thin disk laser: High gain,
high energy and power delivers easily 100 mJ,
1kHz pulses; Scalability to 1J, 1kW and beyond
27
Detailed Parameters
Cavity Length (m) 1.50 Focal Length (cm) - f1 34.50 Focal Length (cm) -f2 3.00 3 Green w0 (microns) Green w1 on Lens (mm) 18.85 Green w1 on Mirror (mm) 3.28 Green Peak Surface Intensity (GW/cm^2) Green Peak Surface Field (GV/m) Green Energy (mJ) 50 Green Pulse width (ps) 2 Red w1 (mm) 26.66 Red Peak Surface Intensity (GW/cm^2) 26.15 Red Peak Surface Field (GV/m) 0.44 Red Energy (mJ) 100 Red Pulse width (ps) 2.83 SiO2 Lens Thickness (mm) 2.8 SHG - LBO Total Thickness (mm) 3.15 Surface Safety Factor 1.91 B-integral (SHG) 2.37 B-integral (Total) 5.29 Number of Passes
100 Loss
1% 73.9678 0.53 28
Optic Size
§  Lens will be 3 cm in radius with a notch for the x-ray
beam
§  Angle of incidence (cut includes 10 mrad of clearance
for x-rays):
Notch width 7 mm
§  40 mrad – 1.38 cm
§  65 mrad – 2.24 cm
§  90 mrad - 3.1 cm
Edge Cut
29
ABCD Model
f2
f1
L2=f1
L1=f1
L3=2f2
Unwrap
L2
L1
f1
L3
L3
L2
f2
L1
f1
Dashed lines represent periodic boundary condition
0 ⎤ ⎡1 L ⎤ ⎡1 L ⎤ ⎡ 1
⎡1 L1 ⎤ ⎡ 1
2
3
⎢
⎥
⎢ 1
M = ⎢
⎥
⎢
⎥
⎢
⎥
1
⎣0 1 ⎦ ⎢⎣ f1 1 ⎥⎦ ⎣0 1 ⎦ ⎣0 1 ⎦ ⎢⎣ f 2
0 ⎤ ⎡1 L ⎤ ⎡1 L ⎤ ⎡ 1
0 ⎤ ⎡1 L ⎤
3
2
1
⎥ ⎢
⎢
⎥
1 ⎥ ⎣0 1 ⎥⎦ ⎢⎣0 1 ⎥⎦ ⎢ 1
1 ⎥ ⎢⎣0 1 ⎥⎦
⎦
⎣ f1
⎦
30
OXALIS Simulations - Beam at the output
Far Field
Near Field
31
OXALIS Simulations
Pulse evolution in first round trip
Pulse parameters
Output
Energy at 515 nm (mJ)
38
LBO B-Integral (per pass)
0.0022
Peak Fluence(J/m2)
2e9
Average fluence(J/m2)
2.6e8
Peak Intensity (W/m2)
8.9e20
FWHM(µm) at focus
4
FWHM (mm) at the flat mirror
21.63
FWHM (mm) at the curved
mirror
3.75
32
Overall Laser Layout
Seed source
ICS laser
Yb-fiber laser (200 MHz)
1030 nm, 100 fs, 1nJ
Fiber
stretcher
~100 ps
Single 2 mJ pulse
at 1 kHz
200 MHz
2W
Yb-fiber
preamp 2W
Pulse
picker
Photo-cathode laser
Stretcher (~300 ps)
Yb:KYW regen
Single 100 mJ 3
ps pulse at 1 kHz
AOM selects 100
pulses at 1 kHz
rep.rate
Diode
pump
980 nm
100 pulses
200 µJ, 600 fs
Diode
pump
940 nm
Compressor
SHG in linear cavity with
dichroic mirrors
Multi-pass Yb:KYW
Gain 400
100 pulses
2 µJ
Cryo-Yb:YAG
amplifier
50 mJ 2 ps 515 nm pulse
rings down in 200 MHz cavity
for ICS
Multi-pass
Yb:KYW
Gain 100
Compressor
FHG to UV
in BBO
100 pulses, 250 nm
20 µJ, 150 fs
RF gun
33
Wavefront quality was excellent at high power
34
Laser design demonstrated successfully
•  High gain from single
disk
ü Composite design
ü ASE rejecting shape
Heat
•  Beam quality at power
ü Imaged multipass
architecture
ü Single 4-mm aperture
ü 60mJ @ 200 Hz
Laser
35
Thermobond, post-process inspection
Version 1.0
Version 1.3
36
ELI Secifications and Deployment
Phase 1: 1 mJ, 100 kHz, sub-7 fs (Quarter 3, 2015)
Building ready in Oct. 2015, installation Q1, 2016 in Szeged
Phase 2: 5 mJ, 100 kHz, sub-5 fs (Quarter 1, 2016)
installation Q3, 2016 in Szeged
Assuming a conservative conversion efficiency of 50% to
green and 10% OPA/OPCPA efficiency to IR requires
Phase 1: 20 mJ, 100 kHz, 2-5 ps, 2 kW pumps
(Quarter 1, 2015)
Phase 2: 100 mJ, 100 kHz, 2-5 ps, 10 kW pumps
(Quarter 4, 2015)
37
Face-pumped composite-thin-disk
• 
• 
• 
• 
• 
1-D thermal distribution
Low wavefront distortion
Shape rejects ASE
92% absorption in double pass
Simple gain-element fabrication
inde
x
38
Regenerative Amplifier
To cryoamp.
FM
1&2
Compressor
G1
TFP
M8
Eout
Regenerative
amp.
G2
λ/2
FI
RM
M7
λ/4 PC
TFP
M6
M3
M4
M2
M
5
M1
S
L DC XTAL
Eseed
Stretcher
CFBG3
C3
FA2
CFBG2
C2
FA1
XTAL DC L
CFBG1
LD
λ/2
PBS
L
CFBG4
Osc
C1
Eosc
39
Experimental results: Cavity-dumped
Output energy at 1kHz [mJ]
5.5
5
4.5
4
3.5
3
80
90
100
110
120
Incident pump power [W]
130
140
Cavity dumped cavity
λ = 1025 nm; Δλ = 2.3 nm
GatePumpe = 450 µs
frep = 1 kHz
40
Spectra and Autocorrelation
Seeder
Cavity dumped
Regenerative amp.
Intensity [A.U.]
1
0.8
FWHM
FWHM
0.6
FWHM
= 4.9 nm
= 5.7 nm
= 3.6 nm
0.4
0.2
0
1020
1025
1030
Wavelength [nm]
1035
1040
A.-L. Calendron et al., UP 2014 (submitted).
41
Regenerative Amplifer Stability
42
Cryo-YAG Power Amplifier Development
Cryogenic Rod-Type Yb:YAG Laser Development at
MIT – DESY Collaboration
and its use for the Phase I effort
43
287 W Cryogenic Yb:YAG Amplifier
CVBG stretcher
30-mW Yb-fiber
preamplifier
LN2 Dewar
λ/4
PBS
Yb-fiber
oscillator
(78 MHz)
0.8 mW
370 ps
λ/4
λ/2
CVBG
compressor
L1
L2 DM
Yb:YAG crystals
DM L2
Fiber-coupled
pump laser
L1
F1029
FI
λ/2
CM
L3
λ/4
Telescop
e
10-W Yb-fiber a
mplifier
λ/4
TFP
PBS
287 W, 5.5 ps, 3.7 µJ@78 MHz
λ/2
FR
λ/2 PBS
K.-H. Hong, et al, Opt. Lett., 33, 2473 (2008)
K.-H. Hong, et al. Opt. Express. 17, 16911 (2009)
44
287-W Picosecond Amplifier Layout
Yb:YAG Crystal
LN2 Dewar
~1.5 L capacity
Conflat sealed
Ion pump for P<10-6 T
2% Yb:YAG Crystals
M=4x
22.5o AR 940 nm
HR 1030 nm
350 W
400 µm
Thin Film
Polarizers
287 W, 5.5 ps,
3.7 µJ, 78 MHz
• 
• 
• 
λ/4
Mirror
4-W seed input; 700 W of pump power at 940 nm
1.5-mm diameter seed beam at crystals
Double-pass amplification
45
High-energy ps cryogenic Yb:YAG laser at 1-2 kHz
(c) Yb:YAG multipass amplifier
LN2 Dewar
L3
Yb:YAG crystals
(b) RGA
Yb:YAG crystal in
LN2 dewar
L1
L2
LD (350 W)
L4 DM
DM L4
L3
DM
PC
TFP
TFP
Telescope
FI
TFP
(1)
Telescope
seed
PBS
(d) Multi-layer dielectric
grating compressor
G
fs, Yb-fiber
oscillator λ/4
Telescop
e
(2)
(3)
40 mJ@ 2kHz
8.2 mJ@ 2kHz
Telescope
FI
λ/4
TFP
G
LD (100 W)
(1) ~15 ps, 6.5 mJ @ 2 kHz
from RGA
(2) ~32 mJ expected
λ/4
CFBG stre
tcher
(a) Fiber seed
0.1 mW
>400 ps
FI
λ/2
λ/4
10-mW Yb-fiber prea
mplifier (1029 nm)
K.-H. Hong, et al. Opt. Lett. 35, 1752 (2010)
46
Advanced Cryo-YAG Power Amplifiers
Cryogenic Composite-Thin-Disk Yb:YAG Laser
- Monolithic Array of Gain Cells for scaling
Luis E. Zapata1,2 & Franz X. Kaertner1,2
Huseyin Cankaya2, Anne-Laure Calendron2, Hua Lin1, Kyung-Han Hong1
1Department
2Center
of Electrical Engineering and Computer Science and Research Laboratory of Electronics,
Massachusetts Institute of Technology, Cambridge, USA
for Free-Electron Laser Science, Deutsches Elektronen Synchrotron, Hamburg, Germany.
47
1 Joule laser driver – Layout
Cryogenic Yb:YAG
12-pass power amplifier
Multi-layer-dielectric
Compressor
Cryogenic Yb:YAG
12-pass preamplifier
Yb:fiber
master oscillator
Amplitude
Yb:KYW
regenerative
amplifier
Grating Stretcher
1.5 me
ter 48
Problem reduced to one-dimension
•  Boltzman distributed energy levels
•  In equilibrium with the diode pumping…
•  …and their own radiation trasport
Boundary conditions:
• 
1) x=0, Ip=0
• 
2) at x=L/2, Ip = Im
• 
Transcendental equation can be solved
49
Average gain-length, average seed In
ΣCTD
Iseed
QD· Iabs· t 2
⎛⎜ ⌠ π + δ ⌠ λ
⎞⎟
= ‒‒‒‒‒‒‒‒‒‒
· Ω( δ2, λ ) ≡ ⎮
⎮ sin( θ) ⋅ cos ( θ) dθ dϕ
⎜ ⌡
⎟
⌡
4·∙π ·∙ ∆Σ ⎝
π −δ
−λ
2
⎠
ΣTD
Solid angle: λ is the longitude and δ the latitude
50
The composite-thin-disk advantage
Laser Laser ü  Added volume
dilutes ASE
Enables:
Ø  Larger
aperture
ü  Resilience to
n.a.
deformations
Ø  Higher gain
ASE go ( cm-‐1 ) go ( cm-‐1 ) 51
Gain narrowing after 6 passes
Measurements vs. calculation
2
Dl (nm)
1.5
calculation
1
0.5
Low DF data
0
5
10
15
20
Multiplicative signal gain G
52
Bandwidth expected at average power:
Temperature distribution in operation
(500 W peak diode power, 20-30% DF )
2
Dl (nm)
1.5
150 K
1
Expectation
…
0.5
100 K
Low DF Data
0
20
40
60
80
100
Multiplicative signal gain G
53
Beam quality
•  Beam profile after 6-passes shows modulations
Ø  concern !
Ø  corrosion of coating during thermalcon may be the cause
•  Beam quality measurements with HASSO show good beam quality
output
Ø HASSO wavefront sensor does not detect high spatial frequencies
Input amplitude
beam
After 6th pass
(un-amplified)
54
Benefits of Cryogenic Cooling
100-K
Yb:YAG
300-K
Nd:YAG
κ (in W/mK)
47
11
dn/dT(ppm/K)
0.9
7.9
α (ppm/K)
2.0
6.2
Relative FOMd
87
1
(300-K Nd:YAG = 1)
(~100 K) Yb:YAG: improved power scalability,
beam quality, and efficiency
87x increased power output per unit of
wavefront distortion
for power scalability with good beam quality
Material thermo-mechanical and thermo-optical
properties are better at 100 K than at 300 K
High electrical-to-optical efficiency, low optical
damage risk, four-level laser at 100 K
High gain, low saturation intensity/fluence
•  note: modest LN2 requirements
(requires ~ 6 l /hr at 1 kW )
55
The cryogenic composite thin-disk laser improves the state
of the art, and is based on advanced technologies
fluence 1.8 J /cm2
Diode stack!
Nonimaging
concentrato
r
PUMP
1.  Cryo-Yb3+:YAG Laser
relay imaged, smooth 2. 
multi-pass extraction
ASE
Composite thin-disk
ASE
PUMP
heat flow
3.  Diffusion bonded-”CAP”
4.  Bright diode stacks
5.  Low thermal-impedance
HR coating
backplane chiller 6. 
*Heat-spreader+boiling heat transfer in liquid
nitrogen is convenient and sufficient for many
applications (~100 W /cm2 heat rejection)
Higher average power (repetition rate)
is only limited by the backplane cooler
56
Off-axis 4·f telescope,
multiplexed geometry
High performance coolers
The composite-thin-disk advantage
ü  Resilience to
deformations
ü  Added volume
dilutes ASE
57
57
The composite-thin-disk advantage
ü  Resilience to
deformations
Enables:
Ø  Larger aperture
ü  Added volume
dilutes ASE
Ø  Higher gain
58
58
Aspect ratio determines ASE performance
All else remains 1-Dimensional
59
Cryogenic composite thin-disk technology
60

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