Methods for data, time and
ultrastable frequency transfer
through long-haul optical fiber links
Jeroen Koelemeij
LaserLaB & Depart. Physics and Astronomy
VU University
Amsterdam, The Netherlands
Outline
• Why time & frequency through optical fiber?
• (Ultra)stable fiber-optical frequency transfer
• Accurate fiber-optical time transfer
• Integration into high-capacity fiber-optical
telecom infrastructure and application to VLBI
• (Ultra)stable fiber-optical frequency transfer
Partners/collaborators in the Netherlands:
Tjeerd Pinkert
VU Amsterdam
Chantal van Tour
VU Amsterdam
Wim Ubachs
VU Amsterdam
Kjeld Eikema
VU Amsterdam
Roeland Nuijts
SURFnet
Rob Smets
SURFnet
Oliver Böll
KVI Groningen
Lorentz Willmann
KVI Groningen
Klaus Jungmann
KVI Groningen
JK
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
Example: 920 km link
PTB group (Braunschweig, Germany): K. Predehl et al., Science 336, 441 (2012)
1840 km link: S. Droste et al., Phys. Rev. Lett. 111, 110801 (2013)
Free-running link
Germany
H-maser
Transport through telecom fiber
• Fiber attenuation: 20 dB/100 km, need amplifiers!
• Issue: bi-directional optical amplifiers needed, but
telecom amplifiers are uni-directional (to avoid lasing)
Scattered
EDFA
• Two approaches:
1. Dark fiber (no other signals, us bi-di amp)
2. Dark channel (bi-di ‘bypass’ amplifier)
optical isolators
(Paris groups, O. Lopez et al., Appl. Phys. B 110, 3 (2012))
Location
A
 Both approaches work
 Both approaches sacrifice telecom capacity
 Approach 2: additional insertion loss
 Telecom operators often reluctant
Location
B
Bidir amp
Part of the solution: out-of-band channels
• Use out-of-band wavelength channels
– C-band: 1530 nm – 1565 nm
erbium-doped fiber amplifier (EDFA) gain spectrum
– Use semiconductor optical amplifiers (SOAs) for signal
amplification <1530 nm
– Ease of wavelength multiplexing with standard
components
… but does it work for optical frequency transfer?
Lab test on 5 km spooled fiber (Amsterdam)
EDFA
SOA
Max. gain [dB]
25-30
20-25
Max. bi-di gain [dB]
<25
<25
Noise Figure [dB]
6-8
8-10
Nonlinearity

 (keep Pin low)
Results
H-maser
5 km link + SOA
5 km link
YES
SOA adds a small amount of noise,
but link stability still far below the
stability of optical clocks (and masers)!
Work in progress: compare performance SOAs with EDFAs
From lab to field: SURFnet optical fiber link
•
•
•
•
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 clocks under development at
both ends of fiber link
• Fiber connects to JIVE Dwingeloo
• Future: bi-directional fiber link
• Accurate fiber-optical time transfer
Partners/collaborators in the Netherlands:
Nikos Sotiropoulos
TU Eindhoven
Chigo Okonkwo
TU Eindhoven
Huug de Waardt
TU Eindhoven
Tjeerd Pinkert
VU Amsterdam
Roeland Nuijts
SURFnet Utrecht
Rob Smets
SURFnet Utrecht
Martin Fransen
VSL Delft
Erik Dierikx
VSL Delft
Henk Peek
NIKHEF Amsterdam
JK
Time transfer – the state of the art
Method
Distance
Accuracy
Ref.
GNSS
>1000 km
3 – 50 ns
TWSTFT
>1000 km
1 ns
T2L2
>1000 km
200 ps expected
White Rabbit (fiber)
(1 Gpbs Ethernet, PTP)
10 km
0.1 - 1 ns
Optical fiber
(20 Mbps PRBS)
540 km
100 - 250 ps
Lopez et al., Appl.
Opt. (2012)
Optical fiber
(20 Mbps PRBS)
73 km
74 ps
Rost et al.,
Metrologia (2012)
Dedicated optical fiber
(10 MHz + 1pps)
69 km
(480 km)
8 ps
(35 ps)
Sliwczynski et al.,
Metrologia (2013)
Fridelance et al.,
Exp. Astr. (1997)
www.ohwr.org
Approach LaserLaB VU – TU Eindhoven
• Collaboration funded by SURFnet, setup at TU Eindhoven
• Find delays via XCOR of 10 Gb/s bit streams through 75 km fiber link
Quasi-bidirectional amplifier
(Amemiya et al., IEEE IFCSE 2005)
25 km
50 km
Two round-trip delays measured:
t12 (l1, l2) and t13 (l1, l3)
Advantages:
• Transmit 10 Gb/s data, no telecom capacity sacrificed
• Time + data transfer
• Compatible with existing telecom methods & equipment
PRBS signals and correlation
50 GS/s
75 km
12.5 GS/s
150 km
Results
OWD - tAB(t) [ps]
Time difference= <OWDestimate> - <OWDdirect>
Estimated accuracy: 4 ps
(agrees with observations)
75 km link
Bit-error rate (BER) below
10-9 :
Error free
communication
at 10 Gb/s
Measurement number
25 255(1)
50 405(1)
75 552(1)
 [ps]
 [ps]
 [ps]
I
3.4
3.4
3.4
DPO time base stability
0.8
1.0
1.7
Fit uncertainty
1.5
1.5
1.0
VOAs
1.0
1.0
1.0
PMD correction
0.6
1.0
0.6
Wavelength measurement
0.2
0.5
0.7
XCOR interpolation
0.3
0.3
0.3
Estimate n′′′
0.05
0.1
0.1
SPM and XPM
<0.1
<0.1
<0.1
Fast fiber length fluctuations
<0.1
<0.1
<0.1
Sourcea
b
Link length uncertainty
Total
<10
-4
4.0
<10
-4
4.1
75 km
-log BER
Link length [m]
25 km
0 km
50 km
<10-4
4.2
Received power [dBm]
Delivery of 10 Gb/s optical data with 4 ps accuracy
over 75 km distance
75 km link
Measurement number
75 km
-log BER
OWD - tAB(t) [ps]
Results
25 km
0 km
50 km
N. Sotiropoulos et al. (submitted)
Received power [dBm]
Time transfer – the state of the art
Method
Distance
Accuracy
GNSS
>1000 km
3 – 50 ns
TWSTFT
>1000 km
1 ns
T2L2
>1000 km
200 ps expected
White Rabbit (fiber)
(1 Gpbs Ethernet, PTP)
Optical fiber
(20 Mbps PRBS)
State-of-the-art
10 km delay determination
0.1 - 1 ns
+
Error-free optical
data
540 km
100 - 250
ps
transfer at 10 Gbps
Ref.
Fridelance et al.,
Exp. Astr. (1997)
www.ohwr.org
Lopez et al., Appl.
Opt. (2012)
Optical fiber
(20 Mbps PRBS)
73 km
74 ps
Rost et al.,
Metrologia (2012)
Dedicated optical fiber
(10 MHz + 1pps)
69 km
(480 km)
8 ps
(20 ps)
Sliwczynski et al.,
Metrologia (2013)
Cross correlation of
10 Gbps optical data
75 km
4 ps
Sotiropoulos et al.
(submitted)
Speed bonus
• Delay determination/synchronization requires a
single shot of 10 Gb/s data lasting less than 1 ms
– For comparison: state-of-the-art methods require
10-100 s of averaging to achieve 4 ps stability
• Integration into high-capacity fiber-optical
telecom infrastructure and application to
VLBI
Use out-of-band wavelengths
integrate time and frequency transfer in
hardware for high-capacity optical telecom
Data out
T&F out
Fiber in
Will require involvment of manufacturers of
optical telecom network equipment and NRENs…
… AND a convincing test case!
eVLBI using fiber-optical synchronization?
Application to eVLBI?
Disclaimer: not necessarily limited to Europe!
• 10 Gb/s channel for antenna signal transport
• Synchronize LO’s at telescope sites through fiber to
4 ps = (1/5) of a 50 GHz cycle
– Useful for initial calibration?
• Phase-lock 10 Gb/s to stable ‘Master clock’ and
distribute through stabilized fiber links
– Phase lock LO to recovered clock
at remote sites
Master
• Use low-noise TCXO/OCXO for short-term
clock stability
• Use recovered clock for long-term stability
– Do away with expensive H-masers?
Special thanks to Paul Boven and Arpad Szomoru of JIVE for insightful discussions about eVLBI
Work in progress…
• Demonstrate time transfer
VSL-VU-SARA-NIKHEF
• Ultrastable frequency transfer
VU – JIVE Dwingeloo – KVI
• Test new techniques that do
not affect/sacrifice telecom
capacity and performance
• Demonstrate an optical
GPS-timing backup system
• Develop terrestrial optical4 ps  2.4 mm accuracy (4D positioning)
wireless positioning with cm
accuracy (with TU Delft Aperture synthesis through mobile handsets?
SuperGPS
Thanks!