Progress in femtosecond timing distribution and synchronization for ultrafast light sources

1
Progress in femtosecond timing distribution and
synchronization for ultrafast light sources

• John Byrd
• Lawrence Berkeley National Laboratory

2
Acknowledgements

• John Staples, LBNL
• Russell Wilcox, LBNL
• Larry Doolittle, LBNL
• Alex Ratti, LBNL
• Franz Kaertner, MIT
• Omar Illday, MIT
• Axel Winter, DESY
• Paul Emma, SLAC

• John Corlett, LBNL
• Mario Ferianis, ST
• Jun Ye, JILA
• David Jones, U of B.C.
• Joe Frisch, SLAC
• Bill White, SLAC
• Ron Akre, SLAC
• Patrick Krejcik, SLAC

3
A great intro to fsec lasers
Femtosecond Optical Frequency Comb Principle,
Operation and Applications Jun Ye (Editor),
Steven T. Cundiff (Editor)
4
Synchronicity

• Next generation light sources require an
  unprecedented level of remote synchronization
  between x-rays, lasers, and RF accelerators to
  allow pump-probe experiments of fsec dynamics.
• Photocathode laser to gun RF
• FEL seed laser to user laser
• Relative klystron phase
• Electro-optic diagnostic laser to user laser

Master
5
Overview

• Motivation LCLS example
• Ultrastable clocks
• Stabilized distribution links
• Synchronizing techniques
• Measuring synchronization

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Lots of FEL activity
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Small things

• 100 femtoseconds
• 100x10-15 sec
• 30 microns
• 0.8 mrad_at_1.3 GHz
• 0.045 deg_at_1.3 GHz
• 1.8 mrad_at_2856 MHz
• 0.1 deg_at_2856 MHz
• (10 TeraHertz)-1
• 20(1.5 micron)

8
Motivation LCLS
Critical LCLS Accelerator Parameters

• Final energy 13.6 GeV (stable to 0.1)
• Final peak current 3.4 kA (stable to 12)
• Transverse emittance 1.2 mm (stable to 5)
• Final energy spread 10-4 (stable to 10)
• Bunch arrival time (stable to 150 fs)

P. Emma
(stability specifications quoted as rms)
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Electron Bunch Compression
d
d ? DE/E
d
under-compression
szi
chirp
z
z
z
sz
sdi
Dz R56d
V V0sin(kz)
P. Emma
RF Accelerating Voltage
Path-Length Energy- Dependent Beamline
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Compression Stability
d
z
P. Emma
11
LCLS Machine Schematic
6 MeV ?z ? 0.83 mm ?? ? 0.05
250 MeV ?z ? 0.19 mm ?? ? 1.6
4.30 GeV ?z ? 0.022 mm ?? ? 0.71
13.6 GeV ?z ? 0.022 mm ?? ? 0.01
135 MeV ?z ? 0.83 mm ?? ? 0.10
Linac-X L 0.6 m ?rf -160?
rf gun
Linac-1 L ?9 m ?rf ? -25
Linac-2 L ?330 m ?rf ? -41
Linac-3 L ?550 m ?rf ? 0
2?3-m
Linac-0 L 6 m
undulator L 130 m
21-1b 21-1d
X
21-3b 24-6d
25-1a 30-8c
...existing linac
BC1 L ?6 m R56? -39 mm
BC2 L ?22 m R56? -25 mm
DL1 L ?12 m R56 ?0
LTU L 275 m R56 ? 0
1 X-klys.
1 klystron
26 klystrons
45 klystrons
3 klystrons
SLAC linac tunnel
research yard
P. Emma
12
Phase, Amplitude, and Charge Sensitivities
parameter DE/E0 0.1 DI/I0 12 Dtf 100 fs unit
Dti 1.6 4.4 1.5 psec
DQ/Q0 46 5.2 24
Df0 3.5 0.65 5.9 deg-S
DV0/V0 0.32 0.24 0.95
Df1 0.32 0.17 1.0 deg-S
DV1/V1 0.29 0.25 0.78
DfX 5.5 1.4 7.6 deg-X
DVX/VX 2.0 1.2 6.3
Df2 0.54 0.21 0.084 deg-S
DV2/V2 1.1 1.0 0.13
Df3 0.35 24.8 15 deg-S
DV3/V3 0.15 5.7 8.6
P. Emma
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Optical metrology

• A revolution is going on in optical metrology due
  to several coincident factors
• development of femtosecond comb lasers
• breakthroughs in nonlinear optics
• wide availability of optical components

2005 Nobel Prize in Physics awarded to John L.
Hall and Theodor W. Hänsch "for their
contributions to the development of laser-based
precision spectroscopy, including the optical
frequency comb technique"
This technology is nearly ready for applications
in precision synchronization in accelerators
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A brief history of timekeeping

• 1949 Ramsey's separated oscillatory field
  technique
• 1955 First caesium atomic clock
• 1960 Hydrogen maser
• 1967 Redefinition of the second in terms of
  caesium
• 1975 Proposals for laser cooling of atoms and
  ions
• 1978 Laser cooling of trapped ions
• 1980s GPS satellite navigation introduced
• 1985 Laser cooling of atoms
• 1993 First caesium-fountain clock
• 1999 First optical-frequency measurement with
  femtosecond combs
• 2001 Concept of an optical clock demonstrated

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Mode-locked Lasers
Locking the phases of the laser frequencies
yields an ultrashort pulse.
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Locking modes
Intensities
17
Femtosecond combs
diode detection
18
ExampleTiSapph MLL
Repetition rate given by round trip travel time
in cavity. Modulated by piezo adjustment of
cavity mirror. Passive mode locking achieved by
properties of nonlinear crystal Modern commercial
designs include dispersion compensation in
optics Comb spectrum allows direct link of
microwave frequencies to optical frequencies
19
Self-referencing stabilizer
CEO frequency can be directly measured with an
octave spanning spectrum and stabilized in a
feedback loop. This allows direct comparision
(and or locking) with optical frequency
standards.
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Master Oscillator Passively Mode-Locked
Er-fiber lasers
Ippen et al. Design Opt. Lett. 18, 1080-1082
(1993)

• diode pumped
• sub-100 fs to ps pulse duration
• 1550 nm (telecom) wavelength for fiber-optic
  component availability
• repetition rate 30-100 MHz

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Master Oscillator Timing Jitter
Agilent Signal Analyzer 5052a
f01 GHz

• Scaled to 1 GHz
• Limited by photo
• detection
• Theoretical limit 1 fs

Very stable operation over weeks !
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Why fiber transmission?

• Fiber offers THz bandwidth, immunity from
  electromagnetic interference, immunity from
  ground loops and very low attenuation
• However, the phase and group delay of single-mode
  glass fiber depend on its environment
• temperature dependence
• acoustical dependence
• dependence on mechanical motion
• dependence on polarization effects
• These are corrected by reflecting a signal from
  the far end of the fiber, compare to a reference,
  and correct fiber phase length.
• Two approaches CW and pulsed

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Stabilized fiber link
Frequency-offset Optical Interferometry
Principle Heterodyning preserves phase
relationships 1 degree at optical 1 degree
RF 1 degree at 110 MHz 0.014 fsec at
optical Gain 105 leverage over RF-based systems
in phase sensitivity
Technique used at ALMA 64 dishes over 25 km
footprint, 37 fsec requirement
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Detailed configuration

• Phase errors,drifts in 110 MHz RF circuits
  insignificant
• Reflections along fiber don't contribute only
  frequency-shifted reflection beats with outgoing
  laser line to produce error signal
• Low power cw signals, linear system, commodity
  hardware

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Drift Results
Compare phase at the end of fiber with reference
to establish stability. Measure slow drift (lt1
Hz) of fiber under laboratory conditions Compensa
tion for several environmental effects results in
a linear drift of 0.13 fsec/hour and a residual
temperature drift of 1 fsec/deg C.
Lab AC cycle

• Environmental factors
• Temperature 0.5-1 fsec/deg C
• Atmospheric pressure none found
• Humidity significant correlation
• Laser Wavelength Stabilizer none
• Human activity femtosecond noise in the data

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Laser Standard Clock

• Laser provides absolute standard for length of
  transmission line
• Narrow-line (2 kHz) Koheras Laser (coherence
  length gt 25 km)
• For single fringe stabilization over 150 m, laser
  frequency must be stabilized to better than 1108
• Use frequency lock with acetylene cell

Frequency lock loop on acetylene (C2H2) 1530.3714
nm absorption line
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Thermal control of critical components
Peltier Coolers
Baseplate
Aluminum Chamber
Some components
Complete
Insulating Jacket
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RF signal transmission
RF (S-band) may be modulated directly onto the
optical carrier with a zero-chirp Mach-Zehnder
modulator and recovered directly at the far end
of the fiber. Any modulation pattern is
acceptable.
Critical to minimize added phase noise at
demodulation. Modulation of CW carrier has signal
S/N advantages over pulsed modulation.
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An advantage of AM
pulse train spectrum
RF out
optical in
f
t
150ps
100MHz
T
two methods
3GHz
1/f

• Diode has an average current limit before
  saturation
• At saturation, high frequencies drop in power
• Diode bandwidth is chosen to be equal to RF
  frequency, and pulse width is 1/bandwidth
• For t150ps, T10ns and f3GHz, AM has 15db more
  power in the transmitted frequency

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Group and Phase Velocity Correction
Interferometric technique stabilizes phase delay
at a single frequency . At a fixed T, simple a
1.6 correction for 1 km cable. Possible fixes
measure group velocity from the differential
phase velocity at two frequencies. Correction can
be applied dynamically or via a feedforward
scheme.
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Pulsed distribution system
Low-noise microwave oscillator
low-bandwidth lock
1
4
3
fiber couplers
Optical to RF sync module
Master laser oscillator
2
stabilized fibers
Optical to RF sync module
Low jitter modelocked laser
5
low-level RF
Optical to optical sync module
Laser
Demonstration of complete link with 50fs jitter
(1-4) and 20fs jitter from (2-4)
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Stabilized Fiber Links pulsed
PZT-based fiber stretcher
SMF link 500 km
5050 coupler
Master Oscillator
isolator
OC
coarse RF-lock
lt50 fs
fine cross- correlator
ultimately lt 1 fs
Optical cross correlator enables sub-femtosecond
length stabilization, if necessary
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RF-Transmission over Stabilized Fiber Link

• Passive temperature stabilization of 500 m
• RF feedback for fiber link
• EDFL locked to 2.856 GHz Bates
  master oscillator

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RF-Synchronization Module
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Summary so far
RF Jitter Dtrms10Hz,1MHz
Drift Dtp-pgt8hours
Optical Jitter Dtrms10Hz,1MHz
Characteristic CW Pulsed
RF-RF Transmission Jitter lt13fs 10Hz-1kHz Drift lt50 fs over 24h Jitter 50fs Drift lt50fs up to 10s
Link Stability Jitter 0.2 fs Drift 1fs/8 hours (Phase stability) Jitter lt22fs Drift lt 2fs up to 10s Opt. X-Corr lt 0.5fs gt 12 hours
Comparison of RF phase over independent
transmission lines now in progress for CW and
pulsed approaches
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RF transmission design

• RF transmission has looser requirements on jitter
• LLRF system can integrate between shots to reduce
  high frequency jitter

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Synching mode-locked lasers
ML Laser
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Idealized example
80 th harmonic
Achieved 4.3 fsec jitter over 160 Hz BW for 10
seconds.
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Two-frequency synch scheme
Lock two frequencies within the frequency comb
separated by 5 THz. For a 1?degree error in
phase detection, temporal error is lt0.6 fsec
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Two-frequency synch layout
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Direct seeding laser systems
Amplification to high energy at low repetition
frequency a) All fiber 1 ?J _at_ 1550 nm b)
Grating compressor 10 ?J _at_ 1550 nm c) OPCPA
100 ?J 1mJ _at_ 1550 nm
pump coupler
air-core photonic crystal fiber (lt 1 uJ)
input pulse
a) 1 uJ, 100 fs
Er-doped fiber
stretcher fiber
b) 10 uJ, 100 fs
975 nm pump diode
bulk grating compressor (high energy)
c) 100mJ-1mJ, 20 ps
OPCPA
1 um, 1mJ, 20ps Regen. Ampl.
PPLN
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Conceptual system design

• Laser synch for any popular modelocked laser
• RF transmission via modulated CW, and
  interferometric line stabilization
• RF receiver is integrated with low level RF
  electronics design

43
Details, details

• Actual performance depends on many technical
  details
• thermal and acoustic environment of cable layout
• design of feedback loops
• gain limited by system poles (i.e. resonances in
  the system)
• multiple audio BW feedback loops suggests
  flexible digital platform
• feedback must deal with drift and jitter
  (separate loops?)
• AM/PM conversion in photodiode downconversion

44
Example Menlo EDFL
old
new
amplitude

• Piezo driven cavity end mirror controls reprate
• Was a 10mm long piezo on a light Al plate
• Replaced with 2mm piezo on steel plate

phase
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AM-to-PM conversion in a photodiode
var. atten.
CW laser
modulator
var. delay
EDFA
1.1Vpp
power meter
network analyser

• Measured at 3GHz using a network analyser
• Modulation was 100 AM on 1530nm CW carrier
• From 1mW to 0.5mW on a 15GHz photodiode, phase
  shift was 87fs/mW
• In this test, phase noise from 10Hz to 3kHz was
  92fs p-p. The noise was averaged over 100ms to
  determine AM/PM shift
• CW power stability through 100m fiber lt10 p-p
  variation over 16h (low polarization dependent
  loss)
• This variation results in 8.7fs p-p
• Conclusion for RF transmission, AM-to-PM is not
  an issue

46
Measurement techniques

• How do we characterize the achieved
  synchronization on the electron or photon beam?
• Use classic approaches
• time to angle or position
• time to frequency
• time to amplitude
• Deflecting cavity
• Electro-optic sampling
• Streak camera
• Laser tagging
• X-ray/laser cross correlator

47
Time to Position
Electron bunch measurements using a transverse RF
deflector
P. Emma
S-band
V0 ? 20 MV sz ? 50 mm, E ? 28 GeV
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EO Sampling
Electro-Optic Sampling encodes electron pulse
shape on a laser pulse
A. Cavalieri
EO Crystal
49
time
polarizing beamsplitter
integrated intensity
time space
time
integrated intensity
50
EOS data from SPPS
A. Cavalieri
Timing Jitter Data (20 Successive Shots)
Single-Shot w/ high frequency filtering
iCCD counts
shot
time (ps)
color representation
time (ps)
51
X-ray streak camera
View from side
Photocathode
Magnetic lens
Streaked image
2-D Detector
Time
X-rays
Sweep
Space
Sample
Anode Mesh)
View from top
Voltage gradient on deflector 5V/psec
Space
Time
Electron guns with a twist! Convert time to
vertical deflection

• Deflection triggered by synchronous laser.
• Each image uses 3rd harmonic laser fiducial.

52
SPPS SC and EO Measurements
SC and EO sampling measurements show good
correlation. Measurement of centroid can be done
to higher resolution than separating time events.
Good for relative timing measurement.
53
Laser tagging
imprint optical pattern on beam
allows adoption of many optical pulse
characterization techniques FROG, GRENOUILLE,
SPIDER, etc.
54
Attosecond measurements!
R. Kienberger, et al., Nature 427, 26 February
2004
Optical field modifies energy spectrum of ionized
electrons Requires very fine synchronization of
x-rays and laser. Techniques like these are the
Rosetta stone for understanding FEL performance.
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Summary

• Accelerators are ready to take advantage the
  revolution in optical metrology
• femtosecond lasers can be synchronized to RF
  oscillators
• distribution links can be (optically) stabilized
  to fsec level
• results expected soon in synching remote
  mode-locked lasers
• Fiber-based systems under development
• TTF-DESY
• LCLS
• FERMI/Sincrotrone Trieste
• All subsequent 4th generation light sources
• Applications for large machines (ILC)
• Synchronization diagnostics have a bright future

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If we share this nightmare Then we can
dream Spiritus mundi If you act as you think The
missing link Synchronicity We know you, they
know me Extrasensory Synchronicity A star fall,
a phone call It joins all Synchronicity It's so
deep, it's so wide You're inside Synchronicity Ef
fect without cause Sub-atomic laws, scientific
pause Synchronicity
With one breath, with one flow You will
know Synchronicity A sleep trance, a dream
dance A shaped romance Synchronicity A
connecting principle Linked to the
invisible Almost imperceptible Something
inexpressible Science insusceptible Logic so
inflexible Causally connectable Yet nothing is
invincible
Thank you for your attention