optical synchronization system with femtosecond...
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FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Optical Synchronization Systemwith femtosecond precision
for FLASH and the European XFELHolger Schlarb
MPY
• Motivation• General remarks on synchronization• Sources of electron bunch arrival time jitter• Layout of laser based synchronization system• Summary
2
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Motivation: shorter electron bunch length
Towards shorter electron pulse duration 1 ps = 10-12 s= 1000 fs= 300µm/c0
Synchrotron light sources …BESSY σz = 10mm ~ 20 psLow-alpha Multi Bunch Hybrid Modus ~ 2 ps
Linac driven colliders … SLC σz = 2 mm ~ 6 ps ILC =0.3 mm ~ 1 ps
Free Electron Lasers … XFEL σz = 20 um ~ 60 fs Short pulse operation @FLASH: FEL pulses < 5 fs
Circ = 240 m
t
σt
31km~ 10-3
3
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Motivation: FEL requires high peak currents
Radiation power:
Prad = P0⋅ exp ( z/Lg)
Gain length:
Lg ~ ( γ 3 σ2r / Ipeak)1/3
Ipeak = Peak current ~ 2.5 kAOnly achievable with short bunches
Exp. Growth ⇒Requires very high level of beam control⇒Good understanding of accelerator parameters
Large current ⇒ Driver for collective beam instabilities⇒New field of beam instrumentation⇒Demanding tolerances for sub-systems (RF)⇒High precision synchronization
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FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Motivation: Pump-probe experiments
Atomic / Molecular Physics// Solid state dynamics
Classical setup:Variable delay
pump
probeProbe = flash
Pump
Shot pulses fs ps
Knowledge of time delay and very small time jitter are crucial!
Same source
Pump pulse initiate reaction, probe pulse records current state.
Problem: two different pulse sources: FEL and optical laser
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FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Motivation: Lasers for FELs
Generic layout of single pass FELsinjector pre-linac main linac
chicanechicane
Phot
o-ca
thod
e
Las
er h
eate
r EO
E-S
ASE
Opt
ical
rep
lica
EO
Seed
Seed
Few
cyc
le la
ser
Pum
p-pr
obe
Plas
ma
lase
r
Man
ipul
atio
n
Man
ipul
atio
n
RF RF
Master LaserOscillator
>> 100 m but < 10fs
6
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Motivation: seeding with higher harmonics (sFLASH)
Ti:Sa LaserGas cell 800nm30nm
Dogleg of Collimator
Variable gap undulatorSeparation chicane
100kW>1 GW
Experiment
Transfer line ~ 40m < 10 fs
Electron beam
Temporal overlap
Advantages:• fully longitudinal coherent• intrinsically synchronized• reduced power fluctuation• parasitic operation
Requirements:• longer electron bunches >120 fs• synchronization laser/e- ~ 40 fs Collaboration Uni. HH & DESY(Drescher, Khan, Rossbach)
Gain 104
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FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Motivation: summary
• Accelerator technology advanced from picosecond bunch duration →few 10 fs bunches
to meet the requirements
• Facility length increased from ~100m to 3.5km XFEL & 31km ILC
• Optical laser systems become an integral part of FEL facilities– for the beam generation, – beam diagnostics, – beam conditioning/seeding and – for user experiments
⇒ Changed the requirements on synchronization from ps to fs precision
8
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Synchronization
Electron beam transport through acceleratorE = 1 GeV → Lorenz factor γ = E/m0c2 β≈ 1 - = 0.999999869 1
2γ 2
0 m 1000 m
∆T = 435fs between light pulse and electron beam (T=3.33µs)
Typical energy deviation δE/E < 0.1% ⇒δt < 0.8 fs ☺
∆T
Typical orbit deviation δx < 50µm ⇒δt < 0.04 fs ☺
Light pulseElec. bunch
10 m
Defines start Predicts arrival
⇒ Beam transport over large (straight) distances is no problem!!!
9
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Synchronization
Prediction of arrival using atomic clocks
0 m 1000 m
Desired prediction accuracy ~ 10 fs: Problem: drift during long measurement time ~ 1000sec
∆TElec. bunch
Defines start Predicts arrival
Precision of clock: = ~ 10-17∆tt
∆ff
Resynchronization required
Resynchronization requires constant propagation time of signal
Detector with femtosecond accuracy (short & long term)
Light pulse ?
10
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Synchronization
Laser pulses transported in length-stabilized optical fibersWhat are the requirements for the fiber laser?
0 m 1000 m
∆tElec. bunch
Fiber laser provides excellent stability on short time scales ☺But drifts due to environmental changes over long time scales
tl - tr
PZT
∆tt
∆ffDesired length stabilization ~ 1µm (3fs) ⇒ = ~ 3⋅ 10-10
Round trip time t ~ 10µs (ng=1.5)
x
x
Fiber laser ~ 216MHz Optical fiberMirror
Light pulse ?
Low noiseRF Osc.
RubidiumClock
Stability (1s)∆f/f ~ 10-11
Aging (year)∆f/f ~ 5⋅ 10-10
Requires locking of the fiber laser to an atomic clock (Rb sufficient) ☺
11
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Fiber laser for synchronizationISO
PBS
• /4
• /2
• /4
collimatorcollimator
WDM980 nm Pump
10 cmSMF
50 cmEr doped fiber 10 cm
SMF
10 cmSMF
10 cmSMF
ISO
PBS
• /4
• /2
• /4
collimatorcollimator
WDM980 nm Pump
10 cmSMF
50 cmEr doped fiber 10 cm
SMF
10 cmSMF
10 cmSMF
• Erbium doped soliton fiber laser λ= 1550 nm• Passive mode locked by self-phase polarization rotation • Repetition rate of 216 MHz (=1.3GHz/6)• Amplitude stability ~ 0.004% (1Hz-1MHz)• Output power ~ 100 mW• Pulse duration ~ 100 fs• Timing jitter ~ 12 fs (1kHz-40MHz)
Courtesy of Jeff Chen, MIT/F. Löhl DESY
12
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Link stabilization: Balanced optical cross-correlator
-
From Laser
Link
150fs
Principle of error signal generation [Proposed by MIT]
Reflect
Direct
• background free (type II SHG)• balanced (dA/A suppressed)• robust & reproducible (envelope)• compact & drift free
SNR = 1% ⇒1.5 fs resolution
1560 nm
780 nm
Courtesy of J.W. KIM, MIT/F. Löhl DESY
13
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
40ps
Length variation of fiber
Piezo stretcher
Out-of-loop timing error
25 fs
4.4fs rmstiming jitter over 2 minutes: 4.4 ± 1.1 fs (rms) timing drift over 12 hours: 25 fs measurement bandwidth: 200 kHz
Link stabilization: Results from test bench
Courtesy: F. Löhl
Test bench in acc. Bld 26
fibers
350 m
Syn. Lab.
14
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Sources of timing jitter in accelerator
RF gun Accelerator Undulatorγ
bunch compressor Main Linac
Photo-cathodelaser
Pump-probelaser
• longitudinal and transverse electron beam quality • arrival time jitter at entrance to undulator
Sources of timing jitter (uncorrelated): σt = [Σ (wj σj)2 ]1/2
1. Photo-cathode laser w ~ 40-60%2. RF phase of RF gun (non-relativistic electrons) w ~ 60-40%3. Seed and Pump-probe laser w ~ 100%
Seedlaser
Eacc
Energy chirp + energy dependent path length
t
15
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Sources of timing jitter in accelerator
RF gun Accelerator Undulatorγ
bunch compressor Main Linac
Photo-cathodelaser
Pump-probelaser
• longitudinal and transverse electron beam quality • arrival time jitter at entrance of undulator
Sources of timing jitter (uncorrelated): σt = [Σ (w σt,I)2 ]1/2
1. Photo-cathode laser w < 5%2. RF phase of RF gun (non-relativistic electrons) w < 5%3. Seed and Pump-probe laser w ~ 100%4. RF amplitude and phase w ~ 100%
Seedlaser
Timing jitter behind BC
Gradient
XFEL: 3.3 ps/%FLASH: 5.5ps/%
2 ps/deg 0.05 ps/ps
Phase Incomingcompressionfactor C ~100
A/φ
σ σ
stabilize Ipeak
THz
16
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Sources of timing jitter in accelerator
RF gun Accelerator Undulatorγ
bunch compressor Main Linac
Photo-cathodelaser
Pump-probelaser
• longitudinal and transverse electron beam quality • arrival time jitter at entrance of undulator
Sources of timing jitter (uncorrelated): σt = [Σ (w σt,I)2 ]1/2
1. Photo-cathode laser w < 50%2. RF phase of RF gun (non-relativistic electrons) w < 50%3. Seed and Pump-probe laser w ~ 100%4. RF amplitude w ~ 100%5. RF phase w ~ 100%
Seedlaser
A/φstabilize Ipeak
THz
FBtiming
⇒ Robust system to control the beam requires several measurements⇒ Flexible synchronization scheme for RF & diagnostics
FBIpeak
tiδt
δt δt
stabilize tarrival
17
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
NarrowBand.
EOMs/Seeding
Two color bal. Opt. cross-corr.
End-station
FiberLaser RF Osc.
Laser pulse Arrival beam/laser Klystron
A & φ cavityDesired point-to-point stability ~ 10 fs
<5fs
Direct
Distribution
RF Osc.
Optical link Optical link<5fs <5fs
Optical link
Layout of the synchronization system towards 10fs
EDFL, soliton, ∆t~100fs, f=216MHzadditive mode locked, P = 100mW, phase noise < 10fs (≥3kHz)
Free space distribution
Dispersion comp.,Polarization contr.,Collinear bal. opt. cross-corr.
Other lasers
Development in corporation with F.Kärtner@MIT
18
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Direct use: Bunch arrival-time monitor (BAM)
Single bunch resolution < 30 fs.
Bunches in pulse train
Correlation #2 & #4 bunch
Courtesy: F. Löhl17mm
14.5mm 6.2mm
1.2mm thickAlumina disk
New pickup design &Improved readout
Courtesy: K. Hacker
⇒ resolution < 10 fs
19
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Direct use: Beam position monitor for large apertures
Beam Path
PickupTaperingChannel
BPM
Courtesy K. Hacker
Compact!
-15 -10 -5 0 5 10 15-6
-4
-2
0
2
4
6Beam position (α = 18.0 deg)
delta [%]
po
sitio
n (c
m)
Beam position in dispersive chicane ⇒dE/E
∆t ~ ∆x
tl
tr
-
Measurements with broadband scope
20
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
NarrowBand.
Direct/Interferometer
EOMs/Seeding
Two color bal. Opt. cross-corr.
End-station
Fiber Laser RF Osc.
Laser pulse Arrival beam/laser Klystron
A & φ cavityDesired point-to-point stability ~ 10 fs
<5fs
Direct
Distribution
RF Osc.
Optical link Optical link<5fs <5fs
Optical link
Layout of the synchronization system towards 10fs
EDFL, soliton, ∆t~100fs, f=216MHzadditive mode locked, P = 100mW, phase noise < 10fs (≥3kHz)
Free space distribution
Dispersion comp.,Polarization contr.,Collinear bal. opt. cross-corr.
Other lasers
Development in corporation with F.Kärtner@MIT
21
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
RF generation from optical pulses
Direct conversion with photodetector (PD)– temperature drifts (0.4ps/C°)– AM to PM conversion (0.5-4ps/W)
PD BPFlaser pulses
frep f = n*frep
f = n*frep
~~~
Time domain Frequency domain
T = 5ns = 1/frep
Photo DetectorBandwidth PD
frep
100fs Phase noise
Sagnac loop interferometer– balanced optical mixer to lock RF osc.– insensitive against laser fluctuation – Very low temperature drifts
Results: f=10GHz jitter 12.8 fs (10Hz-10MHz)drifts < 50 fs (limited by detection)
f=1.3GHz jitter 35 fs (1Hz-10MHz) limited by detection
Courtesy F. Ludwig, B. Lorbeer, DESY/J.W. Kim MIT
22
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
NarrowBand.
Direct/Interferometer
EOMs/Seeding
Two color bal. Opt. cross-corr.
End-station
FiberLaser RF Osc.
Laser pulse Arrival beam/laser Klystron
A & φ cavityDesired point-to-point stability ~ 10 fs
<5fs
Direct
Distribution
RF Osc.
Optical link Optical link<5fs <5fs
Optical link
FB
Layout of the synchronization system towards 10fs
EDFL, soliton, ∆t~100fs, f=216MHzadditive mode locked, P = 100mW, phase noise < 10fs (≥3kHz)
Free space distribution
Dispersion comp.,Polarization contr.,Collinear bal. opt. cross-corr.
Other lasers
Main issue: robustness and stability of a laser based synchronization system
Development in corporation with F.Kärtner@MIT
23
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
From the laboratory …
24
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
to a robust design!
First prototype of master fiber laser system
25
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Summary & Conclusion
• Requirements for synchronization of accelerator subsystem have changed
- from the picoseconds to femtosecond accuracy- for large distances ~100m … 3.5 km- where laser systems becoming a key technology and- integrated part of the accelerator facility (as RF, cryogenics, magnets, water,…)
• Laser based synchronization technology is well advance
- fiber laser sources- stabilization of the laser pulse propagation in optical fibers- as efficient and high precision diagnostic tools- to synchronize other laser systems and- the generation of local RF signals
• Availability, robustness and maintainability for accelerator has to be demonstrated
26
FLASH/XFEL
Holger Schlarb, DESYBjorn Wiik Prize, 19.11.2007
Thanks to Synchronization Team
V. Arsov, M. Felber, K. Hacker, B. Lorbeer, F. Löhl, F. Ludwig, K.H. Matthiesen, B. Schmidt, S. Schulz, A. Winter, J. Zemella
Special thanks to: S. Schreiber, P. Schmüser, J. Rössbach & R. Brinkmannthe FLASH/XFEL team for supportand my wife …
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