Transcript slides

Putting optical-fiber
frequency links to work
Jeroen Koelemeij
LaserLaB VU University
[email protected]
Partners
Funding
Atomic clocks & network synchronization
• Atomic clocks:
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–
–
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Define International Atomic Time (TAI) and UTC
Stratum 1 frequency reference in telecom networks
Global Positioning System (GPS)
Scientific clocks
• Important: network synchronization
– Telecom, power grid, electronic financial transactions, …
Outline
1. Atomic clock
– Principle of operation
– State of the art atomic clocks
2. (Optical) time and frequency transfer
– Clock comparisons through optical networks
– Applications
Optical network synchronization
Geodesy
Positioning
Atomic clock: principle of operation
• Traditional (cesium) atomic clocks: based on microwaves
• Accuracy 15-16 digits (commercially 10-14 digits)
• NEW: ‘optical’ clocks based on lasers
1. Stable oscillator
(microwave source)
2. Atoms
(fixed oscillation
frequency)
3. Counter
(convert oscillator frequency to
useful quantity)
Optical clock: principle of operation
Atoms: single-ion trap (NIST)
Oscillator: ultrastable laser
(<1 Hz linewidth)
Image courtesy W. Oskay (NIST)
~10 mm
*NIST Al+ clock:
17+ digits accuracy
C.W. Chou, D.B. Hume,
J.C.J. Koelemeij, D.J. Wineland,
T. Rosenband,
PRL 104, 070802 (2010)
intensity
Counter: frequency comb laser
frep
CCD camera image
d
clock
laser
f0
frequency
How accurate are 17 digits accuracy?
• Relativistic effects at ‘human scale’ are ~10-17
= Time dilation running @ 2 m/s
= Gravitational redshift over 10 cm height difference
• Effects visible when moving/lifting two Al+ clocks!*
Optical clocks = ‘Einstein sensors’
2 m/s
10 cm
* C.W. Chou, D.B. Hume, T. Rosenband, D.J. Wineland, Science 329, 1630 (2010)
Clock comparisons: scientific value
• Atomic clock frequency depends on value of
fundamental constants
e2
• Example: fine structure constant  
4 0 c
(electromagnetic coupling strength)
• Atomic clocks based on different elements depend
differently on 
• If value of  is changing over time(!), clocks based on
different elements drift apart over time
Current best test of variation 
• Comparison of Al+ and Hg+ optical clocks at NIST Boulder, CO
(USA)
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27Al+
:
199Hg+ :
little dependence on 
large dependence on 
• Constraint on variation  : (-1.6 ± 2.3) × 10-17/year
T. Rosenband et al., Science 319 1808 (2008)
• Europe: number of optical
clocks (under development),
but located in laboratories
>>100 km apart
• Example: Netherlands
Amsterdam
(27Al+)
Groningen
(223Ra+)
Comparing clocks at >100 km distance
‘Long-haul time and frequency transfer’:
• GPS (traditionally state of the art)
• Works well for traditional atomic clocks, but orders of
magnitude insufficient for optical clocks…
Development of atomic clock accuracy over time
Digits of accuracy
‘traditional’ atomic clocks
‘optical’ atomic clocks
GPS accuracy
Year
?
Comparing clocks at >100 km distance
‘Long-haul time and frequency transfer’:
Digits of accuracy
• GPS (traditionally state of the art)
• Works well for traditional atomic clocks, but orders of
magnitude insufficient for optical clocks…
Development of atomic clock accuracy
over
time
Limitations
GPS
accuracy:
• Based
traditional atomic clocks
‘traditional’ atomic clocks ‘optical’
atomiconclocks
• Atmospheric disturbances
• Relatively weak signals
• Fixed signal format
GPS accuracy
?
Year
Optical clock through optical fiber?
• ‘Ticking’ signal of optical clock is a laser
• Use optical fiber for T&F transfer?
Telecom-wavelength optical clock
Frequency comb laser spectrum
intensity
1 GHz
Telecom wavelengths
(1.5 mm)
Clock laser
1 121 015
123…
frequency
• This example: fclock laser = 1 121 015 × (1 GHz)
• ‘Copy’ optical clock laser to frequency comb (PLL)
 all comb teeth become replicas of the clock laser!
Clock comparisons through fiber
Location B
Clock laser B
Telecom
wavelengths
(1.5 mm)
B
Location A
frequency
Telecom
wavelengths
(1.5 mm)
Clock laser A
A
frequency
But what about optical path
length fluctuations in the fiber?
(acoustic, thermal)
Test: SURFnet optical fiber link VU - KVI
Frequency
comb
Frequency
comb
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•
•
•
Link part of SURFnet DWDM network
Length 317 km, round trip 635 km
Single l-channel (1559.79 nm)
Fiber carrying live data traffic
Optical link frequency stability test 1
KVI Groningen
SURFnet
optical fiber link
Clock laser B
Telecom
wavelengths
(1.5 mm)
B
VU Amsterdam
frequency
Telecom
wavelengths
(1.5 mm)
Clock laser
GPS
comparison
A
frequency
GPS
comparison
Optical link frequency stability test 1
Result:
• Identical laser frequencies measured in Amsterdam and
Groningen (to within GPS accuracy, 12 digits)
Stability: Allan deviation
vs. averaging time t
Combined
inaccuracy
GPS links
Optical link frequency stability test 2
• Bypass GPS altogether: compare optical frequency
before and after 635 km round trip
• Signal: optical beat note send and return light
Optical link frequency stability test 2
• Resulting frequency stability (Allan deviation)
6×10-13 @ 1 s
1×10-14 @ 1 200 s (20 min)
2×10-15 @ 20 000 s (5.5 hr)
Performance better than
state-of-the-art GPS!
Stability (ADEV)
Limiting factor: fiber optical path length fluctuations (likely)
Averaging time
Ultrastable frequency links
• Standard network fiber links provide 14-15 digits
accuracy in frequency transfer
• BUT optical clocks require 17+ digits!
• Solution: active fiber length compensation
Optical path length stabilization
Compensation of frequency fluctuations due to length fluctuations*:
*L.-S. Ma, P. Jungner, J. Ye, J.L. Hall, Opt. Lett. 19, 1777(1994)
1.5 mm
clock laser
roundtrip
contains
2× noise!
Partial
reflector
Clock laser + noise
power
power
power
Noise detection
PLL +compensation
Optical fiber
(~ 100 km)
Laser frequency in
Laser
Laser frequency
frequency out
out
Ultrastable fiber frequency transfer
• Does this work in, say, urban dark telecom fiber?
• YES - tests on links 50 – 900 km in Germany, France, USA, UK *:
• w/o compensation:
• compensated link:
14-15 digits accuracy
19 digits accuracy!
• Does this work in, say, live telecom networks?
• YES - France, Paris metro area**
• w/o compensation:
13-14 digits accuracy
• compensated link:
19 digits accuracy!
• Daisy-chaining of ~100 km segments demonstrated
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•
GPS accuracy:
Can we get rf/microwave instead of optical carrier too?
12-15 digits
YES – Use AM, 14 digits w/o compensation, 18 digits with*
* 2008-2011 PTB, NIST/JILA, NPL
** 2010 Observatoire de Paris/U. Paris Nord
Optical T&F transfer activities Europe
EMRP Joint Research Program
NEAT-FT:
“Accurate time/frequency
comparison and dissemination
through optical
telecommunication networks”
Coordinator: Harald Schnatz,
PTB Braunschweig (D)
General issue:
Getting access to
(dark) fiber is difficult
and/or expensive
Applications
• Fundamental scientific research
• ‘Relativistic geodesy’
Clocks & fibers for height measurement
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•
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Compare clocks through fiber to sense height differences
Clocks ‘measure’ geoid (sea level)
GPS position w.r.t. to ellipsoid
Complementary method to GPS geodesy
– Leveling & geodesy:
geoid/sea level is relevant
– frame transformation
ellipsoid  geoid:
~2-3 cm inaccuracy
– This decade: clocks
< 1 cm accuracy
Model of the Earth
Digging for (black) gold
• Gravimeters (accelerometers):
– Measure g ~1/R2 (R = distance to center of the Earth)
– Sensitive to density variations in Earth crust
(oil, mineral reserves) AND to height variations R + h
• Fiber-coupled clocks:
– Measure gravitational potential U ~1/R
– Barely sensitive to density or g variations,
measure primarily R + h
GPS data:
inaccuracy
2-3 cm
• Combination: improve gravimeter sensitivity to
hidden oil and mineral reserves
Applications
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Fundamental scientific research
‘Relativistic geodesy’
Surveying and oil/mineral exploration
With accurate time and frequency available everywhere
in the network you can do whatever GPS can do –
and better!
Digits of accuracy
A vision of ‘SuperGPS’ through optical networks
Development of atomic clock accuracy over time
‘SuperGPS’ accuracy
GPS accuracy
Year
A vision of ‘SuperGPS’ through optical networks
• Enhanced Terrestrial Positioning
Systems (ETPS)
Very high-accuracy positioning,
indoors and on busy highways
• Verify timing in ‘superluminal
neutrino’ experiment CERN-LNGS
• ‘Einstein sensors’ for relativistic
geodesy (monitor sea level, dike
conditions at cm level)
• ‘Innovation beyond imagination’
Thanks!
… and special thanks to:
Roeland Nuijts, Bram Peeters (SURFnet)
Tjeerd Pinkert, Kjeld Eikema, Wim Ubachs (VU)
Oliver Böll, Lorenz Willmann, Elwin Dijck,
Klaus Jungmann (KVI)