Proceedings of IBIC2012, Tsukuba, Japan

TUPA44STATUS OF THE LCLS EXPERIMENT TIMING SYSTEM*

Abstract
X-ray / optical laser pump - probe experiments are used
for a significant fraction of the scientific work performed
at LCLS[1]. The experimental laser systems are locked tothe timing of the electron beam through a combination of
RF and optical fiber based systems. The remaining ~100
femtosecond RMS jitter of the X-rays relative to theoptical laser is measured shot-to-shot by both a RF timing
detector, and by direct X-ray to optical cross-correlation,and the result is used to correct the experiment timing to10s of femtoseconds. We present the present status of thesystem and plans for future upgrades.

SYSTEM OVERVIEW
The experiment timing system locks the RF reference
for the experimental laser (typically used as a pump inpump / probe measurement) to the average beam timefrom the accelerator. The timing system also measures theshot-to-shot electron beam time relative to the RFreference and provides this data to the experiment foroffline jitter correction. In addition, where practical adirect X-ray to laser cross correlator is used to measure
the relative beam times for offline correction. A simplified
block diagram of the system is shown in Figure 1.

ܲis the RF power, BW the system bandwidth ܲ௡ሺଵு௭ሻ isthe noise power in a 1 Hz bandwidth, ߱ the RF frequency
in rad/sec and οܶ the RMS timing jitter. Most RF systems
can operate near (a few dB) the thermal noise limiṯͶ ൈ ͳͲିଶଵ ܹȀ‫ݖܪ‬ଵȀଶ with transmission powers of a few
millwatts.
For fiber systems the receiver noise is typically̱ͳͲିଵଵ ܹȀ‫ ݖܪ‬ଵȀଶ optical (limited by the noise in the preamplifier)[2]. Since the detector output voltage typicallyscales linearly with optical power, the phase sensitivity
varies inversely with optical power (rather than inverse
square root for RF systems).
Oscillators have phase noise that increases withdecreasing frequency since they are measured relative toan absolute clock. Most high quality commercial
oscillators are based on quartz resonators, however there
exist some lower phase noise (but more expensive)
oscillators based on microwave sapphire resonators.
A comparison of the phase noise of RF systems isgiven in Figures 2 and 3. Figure 2 shows the phase noise
density in femtoseconds /Hz 1/2, Figure 3 shows the phasenoise integrated down from 10KHz (a typical feedbackbandwidth) in femtoseconds. Phase noise for 2 fiber
system is also shown, representing examples of simple
and high performance fiber systems.

Figure 1: System Overview.

ACCELERATOR TIMINGThe LCLS operates at a repetition rate of 120Hz,
typical for room temperature accelerators. Since the RFfields completely decay between pulses there issignificant pulse-to-pulse timing jitter that cannot becorrected with feedback; for LCLS this is on the order of100fs RMS. Experiments which require better timing
resolution rely on measuring the beam time on each pulse
and correcting the data offline.

Figure 2: Phase noise in fs/Hz1/2 for various RF and fibersystems.

RF SYSTEMS FOR TIMING
RF systems provide a convenient method for providing
timing synchronization. The timing noise of an RF system
is given approximately as: οܶ ൌට஻ௐή௉೙ሺభಹ೥ሻൗ௉
ఠ. Where____________________________________________* Work Supported by DOE Contract DE-AC02-76SF00515

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○J. Frisch, C. Bostedt, R. Coffee, A. Fry, N. Hartmann, J. May, D. Nicholson, S. Schorb, S. Smith,SLAC, Menlo Park, CA 94309, USA

TUPA44

Proceedings of IBIC2012, Tsukuba, Japan
would be possible to use the optical phase rather than theenvelop modulation to carry timing information, possibly
allowing sub-femtosecond stability[6].

BEAM TIMING MEASUREMENTFigure 3: Integrated phase noise in femtoseconds from
10kHz for various RF systems.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○From the indicated curves, it is clear that the theoreticalnoise limits for both RF and fiber systems are suitable forfemtosecond timing systems. Timing drift of the systems
is generally the more difficult problem, with typical RF
cables and fibers changing delay by 30fs/m/ºC.

FIBER TIMING DISTRIBUTION
Precision timing systems, including the LCLS system,
typically use a fiber for long distance timing transmission.
The fiber length is measured and the signal timing iscorrected by a feedback. Three different types of systems
are in use at accelerator laboratories:
RF reflectometer: The optical signal is modulated at RF
and detected at the far end of the fiber. A fiber mirror
returns some of the light to the transmission end where a
directional coupler samples the reflected signal. The
phase of the RF modulation of the reflected signal ismeasured and the fiber length adjusted to stabilize theround trip length. This system is simple and robust but
has only demonstrated 100s of femtosecond long termstability[3].
Optical interferometer: The optical signal is modulatedwith RF to transmit timing in the same manner as in theRF reflectometer. An optical interferometer measures thelength of the optical fiber and the RF phase is shifted tocompensate for the calculated delay time. The use of an
optical interferometer provides <<1fs resolution on thefiber length. The overall stability of this systems is ~20femtoseconds[4].
Pulsed laser: A ~picosecond pulsed laser is used tocarry the timing information on the fiber. The timing ofthe pulses reflected from the fiber end is used to measure
the round trip delay, and the fiber length is corrected. The
high bandwidth and high power optical pulses allow
direct optical phase detection with ~10 femtosecond
stability[5].
There is also ongoing R&D on a fiber system based on“comb” lasers where the envelope modulation of the laser
is locked to the optical phase. For this type of system it

The LCLS uses two S-band RF phase cavities tomeasure the beam time relative to a RF reference with aRMS noise of ~10 fs RMS and a drift of ~100fs/day.
Other accelerator projects have used electro-optical beam
pickups to measure with <10fs RMS noise[7].
The most serious limitation of both cavity and electrooptical pickups is that they measure the electron beamtime, not the X-ray time. In normal operation the X-ray
timing can jitter relative to the electron beam by asignificant fraction of the bunch length, and when theLCLS is operated in ultra-short bunch mode using theslotted foil[8], the timing jitter of the X-rays relative tothe electrons can be several times the X-ray bunch length.

LASER LOCKING
The mode-locked oscillators for the LCLS lasers arelocked to the reference time from the fiber system using
photodiodes and RF phase detectors with timing jitters of
50-100fs RMS. There is no direct measurement of thetiming drift from the oscillator or of additional jitter anddrift introduced by the laser amplifier and pulse
compression system.
Pulsed fiber distribution systems offer the possibility ofusing optical cross correlation to directly measure theamplified and compressed pulses from the experimentlaser.

X-RAY / OPTICAL CROSSCORRELATION
Pump – Probe experiments are sensitive to the relative
timing of the optical pulse and the X-ray pulse.
Techniques are being developed to measure this directly.
The techniques at use at SLAC are based on the X-ray
beam changing the real and imaginary parts of the index
of refraction of a material, and detecting those changes
with the experiment laser. Two different techniques basedon this principal are under development:

Spatial Cross Correlation[9]The X-rays and laser cross at an angle in a thin
(100nm) Si3N4 Film. The crossing angle causes therelative arrival times of the beams to vary across the film.
The reflected optical beam is then imaged onto a camera.

Figure 4: Spatial Cross Correlator.

ISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

Figure 5: Spatial cross-correlator images.

based noise of the fit is a few femtoseconds, but there is
not yet data available for comparison with an independent
timing measurement.

Figure 7: Spectral cross-correlator, change in transmission
vs. optical frequency in PHz. Emperical fit.
The changes in index of refraction of the film incombination with interference between the front and backsurfaces create ripples in the spectral transmission. Workis underway to compare the data with a physical model ofthe interaction.

Limitations of cross-correlators

Figure 6: Comparison of Nitrogen ionization data withspatial cross-correlator.
The change in reflection from the thin film is believed
to be primarily due to X-ray induced changes in the realpart of the index of refraction and subsequent changes in
the interference between the front and back surfaces ofthe film.

Spectral Cross Correlation[10]The amplified and compressed laser is passed through acontinuum generator to create a very broadband (~550650nm) pulse. This pulse is then frequency dispersed andtransmitted through a thin Si3N4 film. The dispersion
maps time onto optical wavelength and the resulting stepin transmission can be read in the spectrum. Figure 7
shows the spectral step after dividing out a background(no X-ray) pulse. The full spectral width with thedispersion represents approximately 1 picosecond. Thefit is an empirical fit to the edge of the step. The statistics

For both cross-correlator designs the change in opticalsignal may not be linear in the X-ray power, possibly
resulting in an incorrect measurement of the centroid of
the X-ray pulse. This can also generate intensity related
errors on the timescale of the pulse width, or film
response timescale. Fortunately for the majority ofexperiments determining the arrival time of the X-rays toless than the pulse width is sufficient.
Both cross-correlators rely on the X-rays interactingwith a solid film. In some experiments the target is
opaque to X-rays and the film must be located upstream.
The X-ray transmission of a 100nm Si3N4 film decreases
from 90% at 1000eV to 50% at 400eV, limiting its
usefulness for soft X-ray experiments.
Both cross-correlators are X-ray pump / optical probe
systems and cannot work with very low X-ray intensities.
Unfortunately the shortest pulses in LCLS and other
XFELS are expected to be produced with low X-ray
fluxes. The minimum flux required for the crosscorrelators has not yet been determined.

NEW LCLS TMING SYSTEM
The LCLS timing system is currently being modified toprovide improved performance and reliability. The new
design has two key features:x Primary timing is derived from the X-ray /optical cross correlator.
x Modular design to allow upgrades of individualcomponents.

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The resulting images for different time delays are shownin Figure 5,and a comparison with Nitrogen ionization
data is shown in Figure 6 [10]. The nitrogen ionization
data has an 83fs FWHM, consistent with the 50fs width ofthe laser pulse and the expected 50-100fs X-ray pulsewidth.

TUPA44

TUPA44

Proceedings of IBIC2012, Tsukuba, JapanThe overall design is based on using an X-ray /optical cross correlator as the most direct way tomeasure relative timing of the X-rays and pump laser.
When this data is available for offline data correction,
there is minimal advantage to improving theperformance of the rest of the timing system below the~50-100fs RMS jitter of the accelerator. The
subsystems are shown in Figure 8.

Fiber Subsystem
In the new LCLS design the fiber system is used toimprove the timing stability of the RF from the cablestabilization system over long timescales. Initially theLBNL interferometer fiber system will be used, but othersystems can be substituted as the technology develops.
A simplified RF based fiber system is being tested at
SLAC for this application since the use of the crosscorrelator reduces the long term stability requirements on
the fiber system – and may even allow operation with justthe RF system.

Red=RF, Cyan = fiber, Green = laser, Blue = X-ray
Figure 8: Timing Subsystems.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○RF Subsystem
The phase cavity design is essentially unchanged fromthe existing LCLS system[7]. Two 2805MHz cavities areused to measure the electron beam time relative to the RFreference.
RF at 476MHz is transmitted from the phase cavity
system to the experimental hutches. The design has been
modified relative to the previous LCLS design toeliminate the phase locked loop between the phase cavity
signal and the hutches by integrating its function into thecable system.
The cable system operates by locking a VCO at thereceive end of the cable to the phase measured at thetransmit end. At the receive end the average of the phaseof the forward and reflected signals in the cable is thenfirst order independent of cable length. Laboratory tests
indicate a long term stability of a few picoseconds for thissystem. The hardware block diagram is shown in Figure 9.

Figure 10: SLAC simplified fiber system.

Figure 11: Simplified fiber system: 1 week test 600fs pkpk for 1.8 ºC pk-pk, 500M fiber.

Laser Locking System
The laser locking system uses a photodiode to monitor
the mode-locked oscillator and phase compare against theRF reference. Recent results at LBNL with high linearity
diodes and an improved feedback algorithm havedemonstrated 25fs RMS noise on the laser system[6].
Commercial laser locking systems can also be used ifthey can meet the stability and noise requirements.

FUTURE DEVELOPMENTS

Figure 9: RF subsystem.

We expect the “conventional” timing for the LCLS; thebeam pickups, fiber system and laser locking to develop
into a simple, high reliability system to provide jitter andstability below the ~100fs uncorrectable timing jitter ofthe accelerator. This should be sufficient to keep the beam
within the dynamic range of the X-ray / Optical crossISBN 978-3-95450-119-9
4Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

REFERENCES[1] J. M. Glownia et al. “Time Resolved Pump-Probe
Experiments at the LCLS”, Optics Express 2011.
[2] Electro-optics technology.
www.eotech.com/store/uploads/fck/file/10GHz_Amp_Detectors.pdf[3] J. Frisch, D. Brown, E. Cisneros, “Performance of theprototype NLC RF Phase and Timing DistributionSystem”,SLAC-PUB-8458 (2000).

[4] R. B. Wilcox, J. M. Byrd, L. R. Doolittle, G. Huang, J.
W. Staples, “A 20fs Synchronization Sysetm for
Lasers and Cavities in Accelerators and FELs”,
Proceedins of SPIE,vol. 7581 (2010).
[5] J. A. Cox, F. X. Kartner, “A femtosecond-precisionfiber-optic timing transfer system with long-term
stable, polarization maintaining output” 978-1-42446051-9/11 IEEE. (2011).
[6] R. B. Wilcox, Private Communication.
[7] A. Brachmann et al. “Femtosecond Operation of theLCLS for User Experiments”. TUPE066, IPAC
2010.
[8] P. Emma, Z. Huang “Attosecond X-ray Pulses in theLCSL Using the Slotted Foil Method”, SLAC-PUB10712, (2004).
[9] S. Schorb et al. “X-Ray-Optical Cross-Correlator for
Gas-Phase Experiments at the Linac Coherent Light
Source Free-Electron Laser” Appl. Phys. Lett. 100
121107 (2012).
[10] M. R. Bionta et al. “Spectral Encoding of X-ray /Optical Relative Delay”, Optics Express 21855, Oct
2011.
[11] Y. Ding et al. Phys. Rev. Lett, 102, 254801 (2009).
[12] E. Goulielmakis et al, “Single-Cycle Nonlinear
Optics”, Science 20, vol 320, 5883 June 2008.

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○correlators which will be used for offline correction of thedata.
The existing spatial and spectral cross-correlator
concepts will be engineered to require minimal
intervention by the experimenters. Data analysis systems
are under development to provide near real-time timing
information to allow experimenters to debug experiments
during their beam time.
Ongoing developments in X-ray FELs suggest that
pulse lengths below 5fs FWHM are readily available(though not yet directly measured)[11], and fewfemtosecond optical pulses have been demonstrated at anumber a laboratories [12]. In combination these could
allow pump-probe experiments on few-femtosecond
timescales. The existing X-ray / Optical cross correlators
are expected to have single femtosecond resolution andnew techniques will be needed for the ultra-fast regime.

TUPA44