Transcript Document 7373434

Laser synchronization and timing distribution
through a fiber network using femtosecond modelocked lasers
Kevin Holman
JILA, National Institute of Standards and Technology and University of Colorado,
Boulder, Colorado, USA
Co-workers
Funding
David Jones (UBC)
Jun Ye (JILA)
Steve Cundiff (JILA)
Jason Jones (JILA)
Leo Holberg et al (NIST)
Erich Ippen (MIT)
NIST, NSERC,
ONR-MURI
Why Synchronization?
Desired in next generation light sources
• Synchronize X-rays with beamline endstation
lasers for pump-probe experiments
• Synchronize accelerator RF with electron
bunches
Master clock
laser + RF
• Relative timing jitter of a few fs over ~1 km
FEL seed lasers
Beamline
endstation lasers
Linac RF
Outline
Synchronization of multiple fs lasers
•Underlying technology
–Pulse synchronization
–Phase coherence
•Applications
–Coherent anti-Stokes Raman spectroscopy (CARS)
–Remote optical frequency measurements/comparisons/distribution
...but first how to measure performance of frequency synchronization of two oscillators?
• Allan Deviation
•
Timing jitter
Allan Deviation
Allan Deviation
-typically used by metrology community as a measure of (in)stability
-evaluates performance over longer time scales (> 1 sec or so)
-can distinguish between various noise processes
-indicates stability as a function of averaging time
Phase Lock Loop
Device Under Test
Frequency Counter
15
5
4
10
Allan Deviation
Frequency Deviations (Hz)
Master
Oscillator
5
0
-5
-10
-15
-20
0
50
100
Time (sec)
150
3
2
10
-12
6
5
4
2
3
4
5
6 7 8 9
10
Averaging Time (s)
2
3
4
5
Timing Jitter
Timing jitter
-typically used by ultrafast community
-can be measured in time domain (direct cross correlation)
or frequency domain (via phase noise spectral density of error signal)
-must specify frequency range
Relative timing jitter
leads to amplitude
jitter in SFG signal
Sum frequency
generation
fs laser #1
fs laser #2
Single side band phase noise spectral density
Timing jitter spectral density
Spectrum analyzer
Methods for Synchronization
Radio frequency lock
•Detect high harmonic of lasers’ repetition rates
•Implement phase lock loop
•Able to lock at arbitrary (and dynamically configurable) time delays
Optical frequency lock
•Use very high harmonic (~106) for increased sensitivity
•Can be more technically complex than RF lock
•Can lock to high finesse cavity or CW reference laser
•Similar advantages for arbitrary time delay
Optical cross correlation
•Nonlinear correlation of pulse train
•Use fs pulse’s (steep) rising edge for increased sensitivity
•Small dynamic range…must be used with RF lock
•Time delays are “fixed”
Experimental Setup for RF Locking
SHG
fs Laser 2
Delay
SFG
fs Laser 1
BBO SHG
100
MHz
Sampling
scope
14 GHz
14 GHz
50 ps
Phase
shifter
14 GHz
Loop gain
100 MHz
Loop gain
Phase
shifter
SFG
intensity
analysis
Laser 1
repetition
rate control
1
Top of cross-correlation curve
(two pulses maximally overlapped)
Timing jitter 1.75 fs (2 MHz BW)
Timing jitter 0.58 fs (160 Hz BW)
(two pulses offset by ~ 1/2 pulse width)
Total time (1 s)
0
2
Noise spectrum (fs /Hz)
30 fs
Cross-Correlation Amplitude
Timing Jitter via Sum Frequency Generation
10
10
10
10
0
Locking error signal
-2
Mixer noise floor
-4
-6
0
20 40
60 80 100
Fourier Frequency (kHz)
Ma et al., Phys. Rev. A 64, 021802(R) (2001).
Sheldon et al. Opt. Lett 27 312 (2002) .
Synchronization via Optical Cavity Lock
Optical Cavity
Bartels et al., Opt. Lett. 28 663 (2003).
Synchronization via Optical Cross Correlation
Output
(650-1450nm)
Δt
Cr:fo
Ti:sa
(1/496nm = 1/833nm+1/1225nm).
SFG
Rep.-Rate
Control
SFG
0V
3mm
Fused Silica
Schibli et al Opt. Lett, 28, 947 (2003)
Balanced Cross-Correlator
Output
(650-1450nm)
Δt
Cr:fo
Δt
-GD/2
Ti:sa
(1/496nm = 1/833nm+1/1225nm).
SFG
Rep.-Rate
Control
0V
+
+
-
SFG
3mm
GD
Fused
Silica
Balanced Cross-Correlator
Cross-Correlation Amplitude
Experimental result: Residual timing-jitter
Time [fs]
1.0
-100
0
100
0.8
0.6
Timing jitter 0.30 fs (2.3MHz BW)
0.4
0.2
0.0
0
20
40
Time [s]
60
80
100
The residual out-of-loop timing-jitter measured from
10mHz to 2.3 MHz is 0.3 fs (a tenth of an optical cycle)
Outline cont…
Synchronization of two fs lasers
•Underlying technology
–Pulse synchronization
–Phase coherence
•Applications
–Coherent anti-Stokes Raman spectroscopy (CARS)
–Remote optical frequency measurements/comparisons/distribution
Time/Frequency Domain Pictures of fs Pulses
Time domain
Df
2Df
E(t)
t
1/ frep = t
F.T.
Df = 2p fo/ frep
Frequency domain
I(f)
frep
Phase accumulated in one
cavity round trip
Derivation details:
Cundiff, J. Phys. D 35, R43 (2002)
fo
D. Jones et. al. Science 288 (2000)
nn = n frep + fo
f
Requirements for Coherent Locking of fs Lasers
E(t)
For successful phase locking:
1/ frep1 = t1
fs laser
t
• Pulse repetition rates must be
synchronized with pulse jitter << an
optical cycle (at 800 nm << 2.7 fs)
Pulse envelopes are locked
• Carrier envelope phase must evolve
identically (fo1=fo2)
E(t)
Evolution of carrier-envelope
phases are locked
fs laser
t
fo1
I(f)
fo2
1/ frep2 = t2
frep
f
Experimental Setup
Phase lock: fo1 -fo2 = 0
(Interferometric)
Cross-Correlation
Auto-Correlation
Spectral interferometry
Delay
AOM
SHG
fs Laser 2
SFG
Delay
fs Laser 1
BBO SHG
100
MHz
Sampling
scope
14 GHz
14 GHz
50 ps
Phase
shifter
14 GHz
Loop gain
100 MHz
Loop gain
Phase
shifter
Laser 1
repetition
rate control
Locking of Offset Frequencies
fo1 – fo2
5 MHz
Phase lock
activated
(fo1 – fo2) Hz
1.0
sdev = 0.15 Hz
60 dB
R.B.
100 kHz
(1-s averaging time)
0.5
0.0
-0.5
-1.0
0
200
400
Time (s)
600
800
Spectral Interferometry
(Linear Unit)
Spectral Interferometry
- Laser 1 spectrum
- Laser 2 spectrum
- Both lasers, not phase locked
- Both lasers, phase locked
(a)
700
750
800
Wavelength (nm)
850
900
R. Shelton et. al. Science 293 1286 (2001)
Outline cont…
Synchronization of two fs lasers
•Underlying technology
–Pulse synchronization
–Phase coherence
•Applications
–Coherent anti-Stokes Raman spectroscopy (CARS)
–Remote optical frequency measurements/comparisons/distribution
Coherent Anti-Stokes Raman Scattering Microscopy
•Four-wave mixing process with independent pump/probe and
Stokes lasers (2wp-ws=was)
•First demonstrated as imaging technique by Duncan et al (1982)*
Prepare coherent (resonant)
Convert molecular coherent
molecular state
vibrations to anti-Stokes photon
wp ws
wp
was
n=1
Molecular vibration levels
n=0
•Capable of chemical-specific imaging of biological and chemical samples
*M.D. Duncan, J. Reinjes, and T.J. Manuccia, Opt. Lett. 7 350 (1982).
CARS Microscope
APD
Forward Detection
was
Stokes Laser
Filter
Sample
Pump/Probe Laser
3-D
scanner
NA=1.4
Objective
wp,ws
Dichroic
mirror
was
APD
Epi (Reverse)
Detection
Synchronization Performance
Stokes Laser (Master)
To CARS
microscope
Pump/Probe Laser (Slave)
14 GHz
100 MHz
Feedback Loop
FFT Spectrum
Analyzer
Jitter Spectral Density
30
1/2
locked
unlocked
total jitter
Lasers are Coherent Mira ps
Ti:sapphire lasers
25
20
1
15
10
0.1
5
Noise floor of mixer/amplifiers
0.01
0
7 8 9
10
2
2
3
4
5 6 7 8 9
10
3
Frequency (Hz)
2
3
4
5 6 7 8 9
10
4
Jitter (fs)
Amplitude (fs/Hz )
10
Experimental Setup
Sum Frequency Generation (SFG)
used to measure relative timing jitter
SFG
BBO
Bragg Cells used
to decimate rep. rate
Bragg Cell
Stokes Laser (Master)
Bragg Cell
Pump/Probe Laser (Slave)
Polystyrene beads in
aqueous solution
80 MHz
14 GHz
80 MHz 14 GHz
Loop
Loop
gain
gain
Phase
Shifter
DBM
DBM
3-D
scanner
14 GHz
Phase
Shifter
wp,ws
Dichroic
mirror
was
APD
Relative Timing Jitter
Pulse delay is adjusted to overlap at half-maximum point of cross-correlation
Timing jitter is converted to
amplitude fluctuations
Relative jitter via CARS
1.2
With 80 MHz lock,
rms jitter is ~700 fs
Switching to 14GHz lock,
rms jitter is 21 fs
0.5
0.0
CARS Intensity
Relative Jitter (ps)
Relative jitter via SFG
1.0
SFG
Pump/
Probe
Stokes
1.0
0.8
0.6
0.4
0.2
-0.5
0
5
10
Time (sec)
15
20
Bandwidth is 160 Hz
0
5
10
Time (sec)
15
20
Images of 1mm Diameter Polystyrene Beads
Raman shift = 1600 cm-1
Pump 0.3 mW @ 250 kHz
Stokes 0.15 mW @ 250 kHz
80-MHz lock
~770 fs timing jitter
14-GHz lock
~20 fs timing jitter
Counts
2 mm
100
Counts
100
0
0
Outline cont…
Synchronization of two fs lasers
•Underlying technology
–Pulse synchronization
–Phase coherence
•Applications
–Coherent anti-Stokes Raman spectroscopy (CARS)
–Remote optical frequency measurements/comparisons/distribution
Synchronization of Remote Sources
Compare optical standards for tests of fundamental physics
Increasing stability
Required in next generation light sources
• Synchronize X-rays with beamline endstation lasers for pump-probe experiments
• Synchronize accelerator RF with electron bunches
• Relative timing jitter of a few fs over ~1 km
Telecom network synchronization
• Low timing-jitter: dense time-division multiplexing
• Frequency reference from master clock allows dense wavelength-division multiplexing
Distribution of frequency standards
Optical standard
Optical atomic clock
Noise added by fiber must be
detected and minimized
Optical frequency
standard
1/t
fs Ti:sapphire
comb
1.5-mm transmitting
comb
t
RF standard
Holman et al. Opt. Lett. 28, 2405 (2003)
Jones et al. Opt. Lett. 28, 813 (2003)
End user
Optical fiber network
End user
Degradation of signal during detection minimized
End user
3.45 km fiber link between JILA and NIST
Boulder
Regional
Administrative
Network
Trapped Sr
Iodine clock
JILA
L. Hollberg
C. Oates
Br
oa
dw
ay
J. Bergquist
D. Wineland
Single Hg+ ion
NIST
RF transfer: modulated CW source
RF standard
Counter
3.5 km
1310 nm
laser diode
Modulator
Allan Deviation
10
10
10
10
-12
7 km transmission, no stabilization
Noise floor
-13
-14
-15
J. Ye et al. J. Opt. Soc.
Am. B 20, 1459 (2003)
10
-1
10
0
10
1
10
2
10
3
10
4
Averaging Time (s)
Performance similar to NASA/JPL work on frequency distribution system for radio telescopes
RF transfer: mode-locked laser
Pulses vs. simple sine-wave modulation?
• Easier to transfer optical stability
transmitting laser (all optical)
RF transfer: mode-locked laser
Pulses vs. simple sine-wave modulation?
• Easier to transfer optical stability
transmitting laser (all optical)
• More sensitive derivation of error signal (optical pulse cross-correlation)
RF transfer: mode-locked laser
Pulses vs. simple sine-wave modulation?
• Easier to transfer optical stability
transmitting laser (all optical)
• More sensitive derivation of error signal (optical pulse cross-correlation)
• Time gated transmission (immune to some noise, e.g. spurious reflections)
RF transfer: mode-locked laser
Pulses vs. simple sine-wave modulation?
• Easier to transfer optical stability
transmitting laser (all optical)
• More sensitive derivation of error signal (optical pulse cross-correlation)
• Time gated transmission (immune to some noise, e.g. spurious reflections)
• Simultaneously transmit optical and microwave
1/t
Optical standard
RF standard
RF transfer: mode-locked laser
Pulses vs. simple sine-wave modulation?
• Easier to transfer optical stability
transmitting laser (all optical)
• More sensitive derivation of error signal (optical pulse cross-correlation)
• Time gated transmission (immune to some noise, e.g. spurious reflections)
• Simultaneously transmit optical and microwave
8th harmonic
Frequency / time
domain analysis
End user
8th harmonic
Frequency reference
Mode locked
fiber laser
Local
3.5 km
Transfer with mode-locked pulses
Allan Deviation
10
10
10
10
Modulated CW over 7 km
Mode-locked pulses, noise floor
ML pulses D=12.0 nm
ML pulses D=5.5 nm
-13
-14
-15
-16
1
10
100
1000
Averaging Time (s)
Pulses minimize instability of photodetection:
• Average power ; SNR
but … • Dispersion broadens pulse (~ 1 ns)
maintain SNR
so …
• Reduce bandwidth
• Recompress pulse
more power to
Spectral
Width (nm)
Power
SNR
660 mW
80 dB
12.0 (
160
mW
30 mW
80
85 dB
5.5floor
( )(
Noise
)
30 mW
85 dB
Noise floor (
)
)
Holman et al. Opt. Lett. 29, 1554 (2004)
Use dispersion shifted fiber in link
6
10
-14
-15
)
1/2
Jitter Spectral Density (fs / Hz
10
-13
10
100
Averaging Time (s)
1000
2
100
2
8
6
1
4
8
6
2
4
Integrated jitter
10
2
8
6
Conditions at Receiver
0.1
8
6
4
2
1
4 km of DSF, unstabilized
4 km of DSF, active stabilization
Noise floor
4
4
Photodiode
Power
SNR
40 mW
85 dB
6e-14 ( )
40 mW
85 dB
6e-15 ( )
10
1
10
2
Instability (1s)
2
10
3
10
4
10
5
10
Fourier Frequency (Hz)
• Active stabilization: free-space delay arm in-line with DSF
• Not limited by receiver noise
• Reduce Allan deviation to noise floor
6
10
7
1
8
Integrated Jitter (fs)
Allan Deviation
10
Modulated CW over 7 km
Mode-locked pulses, noise floor
D=5 nm, 4 km of DSF
Active stabilization of fiber length
Summary / Future Work…
Techniques and technology of:
•Synchronization of ultrafast lasers
•Delivering frequency standards over fiber networks
Can be applied to synchronization efforts at next generation light sources
Shorter time scales with < 10 fs jitter at multiple locations will require:
•Optical delivery of clock signal
•Active stabilization of optical fiber network
•Some combination of RF and all-optical error signal generation (depends on
frequency range of interest)
Main message:
No showstoppers on synchronization
(financial or technical)
Compensate dispersion of installed fiber
Dispersion compensation fiber
Frequency reference
3.5 km
Mode locked
fiber laser
End user
Local
3.5 km
81st harmonic
1000
7 km of installed fiber, unstabilized
7 km of installed fiber, active stabilization
Noise floor
10
•
Dispersion compensation
Avg. power ; SNR
•
Eliminate low frequency noise on
installed fiber network
8
6
Integrated jitter
100
4
2
1
10
8
6
4
2
1
0.1
8
6
10
0
10
1
10
2
10
3
10
4
10
5
Fourier Frequency (Hz)
10
6
10
7
Integrated Jitter (fs)
Jitter Spectral Density (fs / Hz
1/2
)
2
Cell Image
• Human Epithelial cell
• Image size is 50 by 50 microns
•Total acquisition time: 8 seconds
• Raman shift = 2845 cm-1
Pump 0.6 mW @ 250 kHz
Stokes 0.2 mW @ 250 kHz
Image taken by Dr. Eric Potma and Prof.
Sunney Xie at Harvard University with
synchronization system commercialized by
Coherent Laser Inc.
Slice
100
80
Counts
5 µm
60
40
20
0
0
5
10
15
20
Distance [µm]
25
30
35
Distribution over Fiber Networks
Optical Fiber Network
Master Clock
End User
Noise added by fiber must be
detected and minimized
End User
Degradation of signal during
detection minimized
Phase Coherent Transmission of Optical Standard
Detection of Roundtrip Signal
3.45 km
fiber
-1 order
corrected
standard
at NIST
JILA I2 Atomic Clock
• Adjustment of AOM 1, shifts center
frequency of Nd:YAG to compensate
fiber perturbations
• AOM 2 differentiates local and roundtrip
signals
Nd:YAG
AOM 1
AOM 2
Transmission of Iodine Standard
Beat Amplitude (dBV)
-20
Fiber phase noise
compensated
-30
20 dB
FWHM:
0.05 Hz
-40
-50
-60
1 kHz
Fiber phase noise
uncompensated
-70
-80
0
20
40
60
Fourier Frequency (kHz)
80
100
30
Fiber phase noise uncompensated
20
10
10
0
-20
sdev (1-s) 5.4 Hz
-30
0
Beat Frequency (Hz)
-13
-10
200
400
600
800
Time (s)
Allan Deviation
Beat Frequency (Hz)
Transmission of Iodine Standard
10
10
30
-14
-15
Digital phase lock
20
10
0
10
-10
Uncompensated
Phase compensated
-16
sdev (1-s) 0.9 Hz
-20
0.1
-30
0
200
400
600
Time (s)
800
1
10
Averaging Time (s)
100
Summary/Future Work…
Techniques and technology of:
•Synchronization of ultrafast lasers
•Delivering frequency standards over fiber networks
can be (easily) applied to synchronization efforts at next generation light sources
Shorter time scales with <10 fs jitter at multiple locations will require:
•Optical delivery of clock signal
•Some combination of RF and all-optical error signal generation (depends on
frequency range of interest)
Self-Referenced Locking Technique
I(f)
0
fo
nm
frep
f
nn = n frep + fo
x2
n2n = 2n frep + fo
fo
•need an optical octave of bandwidth!
D. Jones et. al. Science 288 (2000)
Single Side Band Generator
Outline
• Why transfer highly stable frequency standards?
• Current method for transfer of RF standard
• Mode-locked laser for RF transfer
• Active stabilization of transfer network
Instability of optical amplifier (EDFA)
Jitter spectral analysis (FFT)
8th harmonic
End user
Local
Frequency reference
3.5 km
Mode locked
fiber laser
6
7 km of installed fiber, without EDFA
7 km of installed fiber, with EDFA
Noise floor, without EDFA
4
2
10
1000
Integrated jitter
6
4
2
100
1
6
4
10
2
10
0
10
1
10
2
10
3
10
4
Fourier Frequency (Hz)
10
5
10
6
10
7
Integrated Jitter (fs)
Jitter Spectral Density (fs / Hz
1/2
)
EDFA
Conclusions
•
10x improvement with mode-locked pulses for RF transfer
•
Reducing temporal stretching of pulse
•
Active stabilization implemented
noise floor for frequency transfer
•
EDFA jitter well within stabilization loop bandwidth
optical power ; SNR
instability = measurement