Optical Synchronization System with femtosecond...

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


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


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

4

FLASH/XFEL


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

5

FLASH/XFEL


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


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

7

FLASH/XFEL


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


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


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


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


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


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


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


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





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





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


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


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


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


NarrowBand.

Direct/Interferometer

EOMs/Seeding


End-station

Fiber Laser RF Osc.



<5fs

Direct

Distribution

RF Osc.


Optical link





Other lasers


21

FLASH/XFEL


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


NarrowBand.

Direct/Interferometer

EOMs/Seeding


End-station

FiberLaser RF Osc.



<5fs

Direct

Distribution

RF Osc.


Optical link

FB





Other lasers

Main issue: robustness and stability of a laser based synchronization system


23

FLASH/XFEL


From the laboratory …

24

FLASH/XFEL


to a robust design!

First prototype of master fiber laser system

25

FLASH/XFEL


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


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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