1. Approaches and applications
WR TWTFT through long-haul duplexed fiber pairs
Approaches and applications
Jeroen Koelemeij
LaserLaB VU University
Partners
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2. About me
Been active mainly in precision measurements of atoms and molecules using lasers:
Al+ single-ion optical clock at U.S. NIST (Wineland group)
Precision measurements ‘H2+ molecular ion clock’ to determine mass and size of proton & electron (current)
Since 2010 also active in optical fiber TFT because
Timing is everything
Timing through optical fiber is the future

3. Optical TFT activities worldwide
~20 groups in Europe, USA, Japan, China, Australia
Map:
EMRP Joint Research Program s11: “Accurate time/frequency comparison and dissemination through optical telecommunication networks”
Coordinator: Harald Schnatz, PTB Braunschweig (D)

4. Optical TFT in the Netherlands
Netherlands: dense optical fiber network
SURFnet: Dutch research and education network
Fiber link VU University and KVI Groningen through SURFnet: DWDM channel (635 km long)
In progress: dark fiber VU – NIKHEF (Amsterdam region), VU – NIKHEF – KVI
Collaborators/stakeholders:
Amsterdam-The Hague region:
NMI VSL (link to UTC)
ESA-ESTEC?
Groningen region
VLBI community (JIVE, LOFAR)

5. Our interest in WR
Quite general interest: GPS time transfer accuracy limited to 5 – 50 ns
Sub-ns timing accuracy with WR through optical fiber:
Possibility for optical GPS back-up system and ‘SuperGPS’

6. Long-haul optical fiber links
Fiber spans typically 20 – 200 km long
Require optical amplifiers to overcome span losses
(0.2dB/km)
DWDM networks: Erbium-Doped fiber amplifiers often used
(l = 1.5 – 1.6 mm)
EDFAs need to be unidirectional to avoid lasing (Rayleigh backscattered light)
Data communication: unidirectional
duplexed fiber pairs
EDFA
optical isolators
Rayleigh
Location A B

7. Delay asymmetry (DA)  bidirectional amplifier bypass needed
Long-haul duplexed fiber: delay asymmetry >1 ms
Severe limitation for TWTT through WR!
Bidirectional links required
 bidirectional amplifier bypass needed
Location A
Location B
Optical Add-Drop
Multiplexer
Bidir amp

8. Three (possible) solutions
1. Truly bidirectional bypass amp:
2. Quasi-bidirectional bypass amp: l1 l2
3. ‘Interleaved‘ bidirectional link
with unidirectional amps:
DWDM
equipment
fiber
span
amplifier
hut l1 l2
interleaver

9. Truly bidirectional bypass amp
Signal BW
Power
Uplink and downlink bands must
not overlap to avoid cross talk
First developed by Paris groups (U. Paris 13 & SYRTE)
Implemented in long-haul DWDM link (dark channel)
Uses round-trip interferometric method to measure and compensate optical path length fluctuations*
Optical carrier frequency transfer with 19 digits accuracy**
Use l2 slightly offset from l1 (100 MHz)
to distinguish return signal from
back-scattered light
Advantage: small DA due to chromatic dispersion (<1 ps/km)
Disadvantage
must keep gain below 25 dB (lasing threshold)
WR requires larger difference l1-l2 to
avoid cross talk (1.25 Gbit/s)
Brillouin
sidebands l1 l2
* L.S. Ma et al., Opt. Lett. 19, 1777 (1994)
**O. Lopez et al., Opt. Exp. 18, (2010)

10. Truly bidirectional bypass amp
Choose different channels l1 , l2
Install l-selective isolators to create unidirectional paths for each wavelength
Isolators block Rayleigh back-scattered light
High amplifier gain possible
Sacrifice channel l2 on the expense of datacom bandwidth
Added insertion loss
Tolerate some DA
non reciprocal path in isolator (calibrate)
chromatic dispersion (> 10 ps/km) l2 l1
fiber Bragg grating l1
fiber Bragg grating l2
l-selective isolator l1 l2
Isolator for l2
Isolator for l1

11. Quasi-bidirectional bypass amp
Two l channels, max gain, max channel isolation*
Non reciprocal fiber path length inside EDFA ~ 10 m
Amplifier DA can be calibrated (1 cm/c = 50 ps)
Fiber link: DA due to chromatic dispersion (> 10 ps/km) l1 l1 l2 l2
* Amemiya et al., Proc. PTTI p.914 (2005)

12. ‘Interleaved‘ bidir link with unidir amps
ln1 = l1
Insertion loss comparable to OADM (i.e. 1.2 dB for l1, < 1 dB for other l)
Implementation in long-haul DWDM link l1 l2
DWDM
equipment
fiber
span
amplifier
hut
fiber
span
DWDM
equipment

13. ‘Interleaved‘ bidir link with unidir amps
All channels available for data communication! l1 l1
DWDM
equipment
fiber
span
amplifier
hut
fiber
span
DWDM
equipment
ln1
ln1
Compatible with unidirectional amplifier sites
Use amplifiers with calibrated DA
Must deal with DA due to chromatic dispersion (>10 ps/km)

14. Test arrangements Aimed to characterize: Frequency stability (ADEV)
Timing jitter
Timing wander/stability (TDEV)

15. Characterize timing through bidirectional long-haul links
Goal: Test ultimate long-haul timing accuracy using bidirectional links
Procedure:
(1) Create bidirectional link A– A via Location B through dark fiber
(2) Test performance between ‘Virtual locations’ A &B
Location A
Virtual location A
Clock
Dark fiber link
Laser 1 l1
bidirectional
EDFA OC
WR switch Tx Rx
EOM
fiber
10-100km
fiber
10-100km
Laser 2 l2 l1 l2
fiber
10-100km
fiber
10-100km
l2 BPF OC
EOM PD
Location B
bidirectional
EDFA
WR switch Rx Tx
l1 BPF
Link performance
characterization PD
electrical
Virtual location B
optical
Routes A-B-A e.g. NIKHEF-VU v.v. (~ 2 × 20 km) or VU-KVI v.v. (~ 2 × 300 km), through SURFnet
EOM: Electro-optic modulator; OC: Optical Circulator; BPF: optical bandpass filter

16. One-way vs. two-way time transfer
Length-stabilized long-haul fiber links achieve* ADEV(t = 104s) = 1 × (fractional frequency)
 TDEV (t = 104s) = 0.6 fs (!)
Unidirectional (uncompensated) DWDM links
ADEV(t = 104s) < 1 × (noise floor)
 TDEV (t = 104s) < 60 ps
Better than commercial GPS receiver (50 ns) on long time spans (weeks – months –years) ?? requires ADEV(1 yr) = 3 × 10-15
If YES: unidirectional GPS back-up system with infinite holdover possible
*O. Lopez et al., Opt. Exp. 18, (2010)

17. Applications Timing for OPERA ‘Next-generation’ timing and positioning
Optical backbone for sub-ns timing
VU, SURFnet, TU Eindhoven, NIKHEF, VSL, KVI
Optical-to-air interface for timing and positioning of mobile devices
VU, TU Delft

18. Thanks! … and special thanks to:
Roeland Nuijts, Bram Peeters (SURFnet)
Tjeerd Pinkert, Kjeld Eikema, Wim Ubachs (VU)
Oliver Böll, Lorenz Willmann, Klaus Jungmann (KVI)

19. Length-stabilized links