Transcript SuperGPS through optical networks

WR TWTFT through long-haul
duplexed fiber pairs
Approaches and applications
Jeroen Koelemeij
LaserLaB VU University
[email protected]
Partners
€€€
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
1. Timing is everything
2. Timing through optical fiber is the future
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)
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)
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’
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)
Rayleigh
EDFA
• Data communication: unidirectional
duplexed fiber pairs
optical isolators
Location
A
Location
B
Delay asymmetry (DA)
• 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
Three (possible) solutions
l1
1. Truly bidirectional bypass amp:
l2
l1
2. Quasi-bidirectional bypass amp:
l2
3. ‘Interleaved‘ bidirectional link
with unidirectional amps:
interleaver
interleaver
interleaver
interleaver
l1
l2
DWDM
equipment
fiber
span
amplifier
hut
fiber
span
DWDM
equipment
Truly bidirectional bypass amp
• First
Paris
groups (U. Paris 13 & SYRTE)
Uplinkdeveloped
and downlinkby
bands
must
overlap to avoid
cross talk DWDM link (dark channel)
–notImplemented
in long-haul
– Uses
round-trip
interferometric method to measure and compensate
Signal
BW
Brillouin
Brillouin
optical path
length fluctuations*
sidebands
sidebands
– Optical carrier
frequency transfer with 19 digits accuracy**
Power
• Use l2 slightly offset from l1 (100 MHz)
l1
l2
to distinguish return signal from
back-scattered light
• Advantage:
l1 small DA
l2 due to chromatic dispersion (<1 ps/km)
• Disadvantage
– must keep gain below 25 dB (lasing threshold)
– WR requires larger difference l1-l2 to
* L.S. Ma et al., Opt. Lett. 19, 1777 (1994)
avoid cross talk (1.25 Gbit/s)
**O. Lopez et al., Opt. Exp. 18, 16849 (2010)
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)
l1
Isolator for l2
l2
Isolator for l1
l-selective isolator
fiber Bragg grating l1
l1
l1
l2
l2
fiber Bragg grating l2
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)
‘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
‘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)
Test arrangements
Aimed to characterize:
• Frequency stability (ADEV)
• Timing jitter
• Timing wander/stability (TDEV)
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
Virtual location A
Clock
Dark fiber link
Laser 1
l1
bidirectional
EDFA
OC
WR
switch
Rx
Tx
EOM
l2 BPF
PD
Laser 2
l2
OC
Virtual location B
fiber
10-100km
fiber
10-100km
l1
l2
EOM
Tx
WR
switch
Rx
Link performance
characterization
fiber
10-100km
fiber
10-100km
l1 BPF
bidirectional
EDFA
Location B
PD
electrical
optical
Location A
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
One-way vs. two-way time transfer
• Length-stabilized long-haul fiber links achieve*
ADEV(t = 104s)
= 1 × 10-19 (fractional frequency)
 TDEV (t = 104s)
= 0.6 fs (!)
• Unidirectional (uncompensated) DWDM links
ADEV(t = 104s)
< 1 × 10-14 (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, 16849 (2010)
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
Thanks!
… and special thanks to:
Roeland Nuijts, Bram Peeters (SURFnet)
Tjeerd Pinkert, Kjeld Eikema, Wim Ubachs (VU)
Oliver Böll, Lorenz Willmann, Klaus Jungmann (KVI)
Length-stabilized links