Laser%20synchronization%20and%20timing%20distribution%20through%20a%20fiber%20network%20using%20femtosecond%20mode-locked%20lasers

1
Laser synchronization and timing distribution
through a fiber network using femtosecond
mode-locked lasers

• Kevin Holman
• JILA, National Institute of Standards and
  Technology and University of Colorado, Boulder,
  Colorado, USA

Co-workers David Jones (UBC) Jun Ye (JILA) Steve
Cundiff (JILA) Jason Jones (JILA) Leo Holberg et
al (NIST) Erich Ippen (MIT)
Funding NIST, NSERC, ONR-MURI
2
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
• Relative timing jitter of a few fs over 1 km

Master clock laser RF
FEL seed lasers
Beamline endstation lasers
Linac RF
3
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

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Allan Deviation
Allan Deviation -typically used by metrology
community as a measure of (in)stability -evaluat
es performance over longer time scales (gt 1 sec
or so) -can distinguish between various noise
processes -indicates stability as a function of
averaging time
Phase Lock Loop
Device Under Test
Master Oscillator
Frequency Counter
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
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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 pulses (steep) rising edge for increased
  sensitivity
• Small dynamic rangemust be used with RF lock
• Time delays are fixed

7
Experimental Setup for RF Locking
SHG
fs Laser 2
SFG
fs Laser 1
BBO
SHG
100 MHz
SFG intensity analysis
Sampling scope
50 ps
Phase shifter
Laser 1 repetition rate control
100 MHz Loop gain
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Timing Jitter via Sum Frequency Generation
1
Top of cross-correlation curve
(two pulses maximally overlapped)
Timing jitter 1.75 fs (2 MHz BW)
30 fs
Cross-Correlation Amplitude
Timing jitter 0.58 fs (160 Hz BW)
(two pulses offset by 1/2 pulse width)
Total time (1 s)
0
Ma et al., Phys. Rev. A 64, 021802(R)
(2001). Sheldon et al. Opt. Lett 27 312 (2002) .
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Synchronization via Optical Cavity Lock
Optical Cavity
Bartels et al., Opt. Lett. 28 663 (2003).
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Synchronization via Optical Cross Correlation
0V
Schibli et al Opt. Lett, 28, 947 (2003)
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Balanced Cross-Correlator

-
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Balanced Cross-Correlator
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Experimental result Residual timing-jitter
The residual out-of-loop timing-jitter measured
from 10mHz to 2.3 MHz is 0.3 fs (a tenth of an
optical cycle)
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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

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Time/Frequency Domain Pictures of fs Pulses
Phase accumulated in one cavity round trip
Df 2p fo/ frep
F.T.
Derivation details Cundiff, J. Phys. D 35, R43
(2002)
D. Jones et. al. Science 288 (2000)
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Requirements for Coherent Locking of fs Lasers
1/ frep1 t1
E(t)

• For successful phase locking
• Pulse repetition rates must be synchronized with
  pulse jitter ltlt an optical cycle (at 800 nm ltlt
  2.7 fs)
• Carrier envelope phase must evolve identically
  (fo1fo2)

fs laser
t

Pulse envelopes are locked
Evolution of carrier-envelope phases are locked
E(t)
fs laser
t

1/ frep2 t2
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Experimental Setup
Phase lock fo1 -fo2 0
(Interferometric) Cross-Correlation Auto-Correlati
on Spectral interferometry
AOM
SHG
fs Laser 2
SFG
fs Laser 1
BBO
SHG
100 MHz
Sampling scope
14 GHz
14 GHz
50 ps
Phase shifter
14 GHz Loop gain
Laser 1 repetition rate control
100 MHz Loop gain
Phase shifter
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Locking of Offset Frequencies
5 MHz
60 dB
fo1 fo2
Phase lock activated
sdev 0.15 Hz (1-s averaging time)
(fo1 fo2) Hz
Time (s)
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Spectral Interferometry
(a)
Spectral Interferometry
(Linear Unit)
900
850
800
750
700
Wavelength (nm)
R. Shelton et. al. Science 293 1286 (2001)
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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

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Coherent Anti-Stokes Raman Scattering Microscopy

• Four-wave mixing process with independent
  pump/probe and Stokes lasers (2wp-wswas)
• First demonstrated as imaging technique by Duncan
  et al (1982)

Prepare coherent (resonant) molecular state
Convert molecular coherent vibrations to
anti-Stokes photon
wp
was
ws
wp
n1
Molecular vibration levels
n0

• Capable of chemical-specific imaging of
  biological and chemical samples

M.D. Duncan, J. Reinjes, and T.J. Manuccia, Opt.
Lett. 7 350 (1982).
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CARS Microscope
Forward Detection
Epi (Reverse) Detection
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Synchronization Performance
Stokes Laser (Master)
To CARS microscope
Pump/Probe Laser (Slave)
14 GHz
100 MHz
Feedback Loop
Jitter Spectral Density
FFT Spectrum Analyzer
Lasers are Coherent Mira ps Tisapphire lasers
Noise floor of mixer/amplifiers
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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
3-D scanner
14 GHz
14 GHz
80 MHz
Phase Shifter
14 GHz Loop gain
80 MHz Loop gain
Phase Shifter
DBM
Dichroic mirror
wp,ws
DBM
was
APD
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Relative Timing Jitter
Pulse delay is adjusted to overlap at
half-maximum point of cross-correlation
Pump/Probe
SFG
Timing jitter is converted to amplitude
fluctuations
Stokes
Relative jitter via SFG
Relative jitter via CARS
With 80 MHz lock, rms jitter is 700 fs
Switching to 14GHz lock, rms jitter is 21 fs
Bandwidth is 160 Hz
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Images of 1mm Diameter Polystyrene Beads
Raman shift 1600 cm-1 Pump 0.3 mW _at_ 250
kHz Stokes 0.15 mW _at_ 250 kHz
80-MHz lock 770 fs timing jitter
14-GHz lock 20 fs timing jitter
2 mm
Counts
Counts
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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

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Synchronization of Remote Sources
Increasing stability
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Distribution of frequency standards
Noise added by fiber must be detected and
minimized
1.5-mm transmitting comb
Optical fiber network
Holman et al. Opt. Lett. 28, 2405 (2003) Jones et
al. Opt. Lett. 28, 813 (2003)
End user
End user
End user
Degradation of signal during detection minimized
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3.45 km fiber link between JILA and NIST
Boulder Regional Administrative Network
Trapped Sr
Iodine clock
L. Hollberg C. Oates
J. Bergquist D. Wineland
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RF transfer modulated CW source
RF standard
3.5 km
1310 nm laser diode
Modulator
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RF transfer mode-locked laser

• Pulses vs. simple sine-wave modulation?
• Easier to transfer optical stability
  transmitting laser (all optical)

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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)

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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)

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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
36
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

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Transfer with mode-locked pulses

• Pulses minimize instability of photodetection
• Average power SNR

Holman et al. Opt. Lett. 29, 1554 (2004)
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Use dispersion shifted fiber in link
Conditions at Receiver
Photodiode Power SNR Instability (1s)
40 mW 85 dB 6e-14 ( )
40 mW 85 dB 6e-15 ( )
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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 lt 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)
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Compensate dispersion of installed fiber

• Dispersion compensation
• Avg. power SNR

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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 _at_ 250 kHz
• Stokes 0.2 mW _at_ 250 kHz
• Image taken by Dr. Eric Potma and Prof.
  Sunney Xie at
  Harvard University with synchronization system
  commercialized by Coherent Laser Inc.

Slice
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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
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Phase Coherent Transmission of Optical Standard
Detection of Roundtrip Signal
3.45 km fiber
1 order
corrected standard at NIST
NdYAG
AOM 1
AOM 2
-1 order

• Adjustment of AOM 1, shifts center frequency of
  NdYAG to compensate fiber perturbations
• AOM 2 differentiates local and roundtrip signals

JILA I2 Atomic Clock
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Transmission of Iodine Standard
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Transmission of Iodine Standard
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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 lt10 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)

48
Self-Referenced Locking Technique
fo
I(f)
nm
frep
f
0
nn n frep fo
x2
n2n 2n frep fo
fo

• need an optical octave of bandwidth!

D. Jones et. al. Science 288 (2000)
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Single Side Band Generator
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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

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Instability of optical amplifier (EDFA)
Jitter spectral analysis (FFT)
8th harmonic
End user
Local
Frequency reference
3.5 km
Mode locked fiber laser
EDFA
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Conclusions

• 10x improvement with mode-locked pulses for RF
  transfer
• Reducing temporal stretching of pulse
  optical power SNR
• Active stabilization implemented instability
  measurement noise floor for frequency transfer
• EDFA jitter well within stabilization loop
  bandwidth