Strategies for achieving sub-10fs timing in large-scale FELs

R. B. WILCOXG. HUANG, L. R. DOOLITTLE, J. M. BYRD

LBNLJ. C. FRISCH, A. R. FRY

LCLSR. HOLZWARTH

MENLO SYSTEMS

FEL 2012NARA, JAPAN

1

Outline

• Overall system design approach for <10fs • Subsystems

– Clock requirements– Stabilized link– Laser oscillator timing control– Downstream monitors and feedback

• Conclusions

2

<10fs pump/probe experiments drive timing requirements• ≤10fs photon pulses from LCLS, SACLA, FLASH…• Want timing uncertainty ≤ pulse width, otherwise…

– Pulse is statistically widened– Timing range is statistically sampled (then “binned” if measured)– Shots are wasted, reducing effective reprate

3valid data range

pumpprobe

detect timing, “bin” data by time

wastedshots

jitterstatistics

Laser synchronization path

• Carry stability of clock down stream– Minimize jitter added by subsystems

4

opticalX-ray

clock

Transmitterlink

stabil-izer

lasercontrol laser

<10fs 10s to 100s of fs currently

5

opticalX-ray

clock

Transmitterlink

stabil-izer

lasercontrol laser

Laser synchronization path

• Carry stability of clock down stream– Minimize jitter added by subsystems– Feedback at maximum bandwidth for each stage

• X-ray jitter assumed to be low (Byrd et al, TUPD29)

<10fs 10s to 100s of fs currently

• CW carrier : continuous signal (fringe counting), no fiber nonlinearity– Analogous to RF clock, but 106 higher

frequency– Δt = Δφ/2πf = 1rad/(2π*200THz) = 0.8fs

Clock distribution methods

6

pulsedlaser

cross-correlator

inter-ferometer

CWlaser

pulsedlaser

pulsedlaser

cross-correlator

pulsedlaser

inter-ferometer

ø mod.

ø mod.

pulsed:

CW:

Linking CW and pulsed lasers

• Timing always transferred via optical phase detection

7

pulsed laser

lock carrier to envelope pulsed laser

lock carrier to envelope

CW laser

lock CW to carrier

lock carrier to CW

offset

interferometerstabilized link

transmitterclock

offset

reprate

link receiver

∫ ∫

CEP stablepulse train

single frequency

Clock and distribution via links

8

How good does the clock have to be?

• Determined by delay difference tD = tA – tB• High frequency: differential noise with period <2tD• Low frequency: phase delay change • Example: 200m fiber

– tD is 1μS– High frequency noise above 500kHz < 1fs– Long term frequency drift < 10-9

clock experiment

9

Δt = tDΔff

Optical clocks are good enough

• RF and optical frequencies, at exact integer multiples

Menlo Systems

Kubina et al, Opt. Expr. 13, 904 (2005)

~10-15 freq. stability

100MHZ 200THz

opticalRF

frequency

ampl

itude

reprate2 3 4 5... 2e6, 2e6+1...

<0.1fs jitter above 500KHZ

J. A. Cox et al, Opt. Lett. 35, 3522 (2010)

Lock CW to clock• Optical phase-locked loop

– Beat CW with nearest comb line

• 8kHz bandwidth piezo tuner– 1.8fs RMS jitter

11

1.8fs RMS

pulsed laser CW laserjitter spectrum

output

• Optical phase-locked loop– Beat CW with nearest comb line

• 8kHz bandwidth piezo tuner– 1.8fs RMS jitter

• >1MHz EO phase modulator– 320as RMS jitter

12

320as RMS

pulsed laser CW laserEO ø mod. • Fast extracavity

control becomes possible when jitter is <10fs

Lock CW to clock

Interferometric link stabilizer

13

tune

CW laser

opticalfrequencyreference (Rb) RF controller

frequencyshifter

outputmirror

Integrated jitter1.5fs unlocked50as locked

150m

• No moving parts• Similar device demonstrated

over 500km fiber (Science 336, 441 (2012))

• Tracks phase continuously– Nanosecond error range– Sub-fs precision

Link transmission jitter

• Transmission over 100m fiber adds ~400as to optical jitter14

pulsed laser

CW laser

Interferometer

jitter A0.95fs

jitter B1.1fs

100mjitter B-A

pulsed laserpulsed laser

Control of laser timing

15

Lock pulsed laser to CW

• Control reprate based on CW laser phase

• Increase control BW with EO• Pulsed laser not CEP stable

16

CW laser EO ø mod. pulsed laser

piezo

piezo tune only, 0.52rad

EO mod added, 0.34rad

In-loop: 280as RMS

CEP-stabilized fiber laser locked to CW

• 0.138 radian envelope-to-CW phase (113as at 1550nm)– With intracavity electro-optic modulator

• Envelope synced to CW phase with 113as RMS jitter

17

Baumann et alOpt. Lett. 34, 638 (2009)

Cross-correlation of locked lasers

• <8fs RMS difference, 10Hz to 1MHz

• 8fs/√2 = 5.6fs• Most noise below 100kHz• Limited control bandwidth

– Increase BW to 1MHz with EO crystal in cavity

18

crosscorrelator

pulsed CEP stab. laser 2

CW laser

pulsed CEP stab. laser 1

jitter spectrum

integrated jitter

Need to first “coarse” lock at 10s of fs

• RF clock controls remote oscillator • ~10fs is the limit

– 0.01 degree phase error– 10fs at 3GHz

• Currently used in LCLS and Fermi@Elettra

19time, hours

dela

y er

ror,

fs

8.4fs, 20 hours to 2kHz (loop BW)

Out-of-loop resuts:

Rbref

AMCWlaser

FS

RF phasedetect,correct

opticaldelay

sensing

ωRF

transmitter receiver ωRF

Controlling VCXO, 200m fiber

VCO or laser

ωRF

Optimizing RF lock for ti:sapphire laser• Determine open loop transfer function• Add filter to prevent oscillation with high gain (30kHz LPF)

20

Transfer function:

amplitude

phase

39kHzresonance

laser

DAC

stepresponse

ADC

RF locking results with tisaf• In-loop measurement compared with difference between

two externally referenced measuements

21

21fs RMS1Hz to 170kHz

FFT of noise

Jitter spectral densityof laser and reference

control bandwidth26fs RMS

30Hz to 170kHz

Integrated RMS jitter

In-loop:

Out-of-loop:

Downstream monitors and feedback

22

Noise measurement and control depends on repetition (sample) rate• High reprate enables high bandwidth feedback

– Control BW ≈ sample rate/10• Integrated jitter above sample rate is “shot to shot”

23

100kHz

100Hz

Effect of amplifiers on CEP

• CEP thru example optical parametric amp, 240as long term – Measured with a nonlinear interferometer after the amp– Variations due to air turbulence in compressor?

24

Schultze et al,Opt. Exp. 18, 27291 (2010)

3μJ6fs100kHz

88as 240as

Optical pulse timing detector

• Same heterodyne scheme at low reprate• Beat frequency is down at ~20kHz

– More noise, narrow bandwidth 25

100kHzpulse train

choppedCW

100kHztime: optical frequency:

1ns

10μs

1GHz (104 lines)<1kHzlinewidthchop

CW from link pulse trainto be timed

heterodyne beat

Cross correlating with optical radiation

• 2-color or “optical afterburner” concept– Saldin et al, PRST AB 13, 030701 (2010)– Optical synced with X-ray to 30as (10^-4 energy jitter)– Cross correlate with experiment laser, sub-fs uncertainty

• X-ray/optical cross-correlators 5-10fs currently• Use to correct long term drift

– No need for passive thermal stability in other subsystems 26

cross-corr.

expt.laser

Timing system block diagram

freq.converter

exptlaser

informationprocessing

BAMlaser

seedlaser

lock lock

transmitter

clock laser

lockoptional

data binning

linac

lock

RFplants

timing sensor

cathode, heater

27

FEL BAM cross-corr.

Conclusion: robust <10fs sync is likely

• All subsystems capable of ≤1fs short term jitter– Optical phase lock provides fine timing sensitivity– Well developed technology for optical metrology

• Slow drift corrected based on cross-correlation at experiment

• Experiments to further demonstrate capabilities are ongoing

28