Transcript EO measurements at SPPS

EOS at SPPS
Adrian L Cavalieri, David M Fritz, SooHeyong Lee,
Philip H Bucksbaum, David A Reis
(FOCUS Center, University of Michigan)
Holger Schlarb (DESY)
Patrick Krejcik, Jerome Hastings (SLAC/SSRL)
Timing for Ultrafast X-ray Science
• SPPS is an R&D facility for the Linac Coherent Light Source (LCLS)
and is a test bed for future beam diagnostics
50 ps
SLAC Linac
RTL
SPPS
1 GeV
9 ps
0.4 ps
20-50 GeV
<100 fs
3km
• Electron bunches at SPPS are as short as 80fs FWHM, comparable
to the bunches that will drive future XFELs
– EOS delivers shot-to-shot bunch length to machine operators
• 2m undulator after bunch compression delivers 80fs FWHM hard
x-ray pulses
– EOS delivers shot-to-shot arrival time to users
EOS and “Pump-Probe”
• Typical time resolved experiment utilizes intrinsic synchronization
between pump excitation and probe
impulse
system response
S1
S9
S4
S7
S6
S3
S8
S0
S5
• Electro-Optic Sampling (EOS) delivers arrival time to users
– Pump-Probe experiments now possible at XFELs
– Machine jitter exploited to sample time-dependent phenomena
S 2 time
Geography
EOS
undulator
EOS timing applicable
IF optical path lengths
remain constant
osc
amp
X
t
t 
Electro-Optic Sampling
ŷ
• Crystal is affected by applied
DC electric field
ŷ
laser
EEDC
– Principle axes of crystal system
are modified
x̂
– Index of refraction along these
axes changes
• Probe laser field is
decomposed in primed
x̂
coordinate system
• Phase shift between
components can be detected
•   EDC
Electric Field of 30GeV Electron Bunch
Er
•
Approximate field assuming:
–
r    z
– Steady-state
3.4nC
 z  12 m  RMS 
  80 fs  FWHM 
q
•
Er 
•
E peak  5mm  400MV / m
2   0 z r 
•
Accurate calculation requires
numerical analysis
•
Crystal is in a 6” 6-way vacuum
cross so approximation is good
Spatially Resolved Electro-Optic Sampling (EOS)
• Spatially resolved EOS can deliver measurements with high enough
Laser probe earlier
later relative
relative
toto
electron
electron
bunch
bunch
resolution to capture electron bunches at SPPS
k
k
– technique
pioneered using table-top systems by Heinz et. al. in 2000
k
EO Crystal
– spectrally resolved EOS cannot be used due to fundamental bandwidth
k
k
k
limitation
 input chirped
k
• Resolution limit of technique dominated
by EO crystal thickness
k
k
k
k
k
k
28.5GeV
28.5GeV
28.5GeV
28.5GeV
k
28.5GeV
k
Spatially Resolved EOS
(long bunch)
EO Crystal
k
k
k
v
v
v
time
Spatially Resolved EOS
polarizing
beamsplitter
ks  polarized
integrated intensity
time
time; space
klaser
k p polarized
integrated intensity
Arrival time and duration of bunch is encoded on profile of laser beam
SPPS Facility
LINAC
Accelerator
Tunnel
Laser Room
&
X-ray Hutch
Effect of Long Pulse Probe Laser
• Probe pulse longer than e-bunch
– EO signal will be broadened
– If probe pulse shape is very well
known, we should be able to
deconvolve e-bunch shape
– Signal to background problems
k
introduced
• Probe pulse uncompressed
(~10’s of picoseconds or longer)
– Measurement will yield no
spatially dependent signal
v
Ultrafast Laser Transport

2

2


2
Pulse Shaper
f
f
f
f
SLM640-pixel
• 640-pixel SLM can introduce arbitrary dispersion
• Genetic Algorithm used to find configuration that minimizes total
transport dispersion
• frequency doubled throughput provides feedback
• nearly transform limited pulse delivered when combined with grating
pair to compensate for GVD
Ultrafast Laser Transport: 3rd Order Correction
• Data taken with a 110m spool of test fiber at UofM with 128-pixel SLM
• Spectral width of transmitted pulse is ~10nm
Without Pulse Shaper
With Pulse Shaper
FWHM 600fs
FWHM 160fs
Transport Results
• Autocorrelation used for
measurement of laser pulse
135fs (FWHM)
arriving at EO chamber
• Find pulse is not transform limited
• Improvement can be made in
chromatic aberration in pulse
Schematic
shaper
• Fixed phase mask can be used to
PMT
 2 
BBO
increase capacity of pulse shaper
time
Spatially Resolved EOS Data
polarizing
beamsplitter
ks  polarized
integrated intensity
time
time; space
klaser
k p polarized
integrated intensity
Single-Shot Data acquired with 200  m ZnTe
Single-Shot
w/ high frequency filtering
Timing Jitter Data
(20 Successive Shots)
shot
iCCD counts
 ~ 300 fs
time (ps)
color representation
time (ps)
Effect of accelerator parameters on EO signal:
Observation of resolution limit
• Changing Linac Phase detunes electron bunch compressor
Estimate Frequency Response Cut-Off
EO crystal
sign of effect
accumulated effect
1
 2.5THz (for 200 m crystal)
walk - off
for Gaussian THz pulse  176 fs
cut - off 
EO crystal imaging
2f
2f
fiber couple
• EO signal/feature small ~50um extent
• Vacuum
ports and other optics reduce angular resolution
• Object does not lie in a plane perpendicular to optical axis
• High resolution is required over a large depth of field (for
adequate single-shot window)
Effect of Poor Imaging on Single-Shot Data
poor imaging
• Broadening also due to
insufficient resolution in
imaging system
• 100fs feature corresponds to a
30um feature in the beam
reference
• Current chamber limits
achievable resolution
• Broadening is still due
predominantly to crystal
thickness and cut-off frequency
Imaging Solution
45
30

 15
• Depth of field fixed by single-shot window
• Rate of time-sweep fixed by incident angle
• Resolution requirement of imaging system fixed by incident angle
New EO Chamber
• Accepts 15, 30, and 45 degree
angle of incidence
• Thick and thin crystal on
actuator, user chooses which
crystal is used for the EOS
• No internal optics
• Shorter path length to exit port
(use shorter focal length
imaging optics)
The Flowchart
E-beam/X-rays
RF
Pump Laser
(Experiment)
EOS
Optical Path Length Jitter
•
Long term thermal drift caused by:
– Optical fiber transport
– RF reference cable
•
Short term drift caused by vibration
•
Feedback used to stabilize fiber
length and keep signal in single
shot window
•
Tracking changes made in optical
path to keep timing information
valid
EO/Streak Camera Correlation
(no fiber stabilization)
A. MacPhee, LBL
• 5000 shots recorded at 10Hz rate
• EO arrival time accuracy: <30fs
• Streak Camera arrival time accuracy: ~150fs
• Correlation between measurements is .707
EO/Melting Correlation
(no fiber stabilization)
• 30 shots recorded at 1Hz rate
• EO arrival time accuracy: <30fs
• Melting arrival time accuracy: ~50fs
• Agreement between two measurements is 60fs RMS
1010
Is the measurement plausible?
Red line: bunch charge distribution
Expected EO signal
K. Gaffney
– sum of two gaussians, one with
100% amplitude, 80fs width; the
other 20% amplitude, 1ps width
• Instrument function estimated to
be gaussian - 200fs width
– combine laser pusle duration with
imaging imperfections
– returns narrow feature in data
time (fs)
• Blue line is the convolution of the
two – corresponds to expected
EO signal
Is this measurement plausible?
Single-shot data
•
w/ convolution function
Good agreement between test
function and the experimental data
•
Instrument function – reasonable
estimate that returns the sharp
central feature
•
Choice for charge distribution
convolved with instrument function
matches data, but does not match
simulation
Red line: real single-shot data
Blue line: convolution function
Synchronization Using RF Reference
170 fs rms