Methods for data, time and ultrastable frequency transfer through long-haul optical fiber links Jeroen Koelemeij LaserLaB & Depart. Physics and Astronomy.

Methods for data, time and ultrastable frequency transfer through long-haul optical fiber links

Jeroen KoelemeijLaserLaB & 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 AmsterdamChantal van Tour VU AmsterdamWim Ubachs VU AmsterdamKjeld Eikema VU AmsterdamRoeland Nuijts SURFnetRob Smets SURFnetOliver Böll KVI GroningenLorentz Willmann KVI GroningenKlaus Jungmann KVI GroningenJK

Optical path length stabilization

Laser frequency outpo

wer

Noise detection+compensation

Optical fiber(~ 100 km)

1.5 mmclock laser

Clock laser + noise

Laser frequency in

pow

er

Partial reflector

roundtripcontains 2× noise!

Laser frequency outpo

wer

Compensation of frequency fluctuations due to length fluctuations*:

PLL

*L.-S. Ma, P. Jungner, J. Ye, J.L. Hall, Opt. Lett. 19, 1777(1994)

Example: 920 km linkPTB group (Braunschweig, Germany): K. Predehl et al., Science 336, 441 (2012)

H-maser

Germany

Free-running link

Stabilized link

1840 km link: S. Droste et al., Phys. Rev. Lett. 111, 110801 (2013)

Both approaches work Both approaches sacrifice telecom capacity Approach 2: additional insertion loss Telecom operators often reluctant

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)• Two approaches:

1. Dark fiber (no other signals, us bi-di amp)2. Dark channel (bi-di ‘bypass’ amplifier)

(Paris groups, O. Lopez et al., Appl. Phys. B 110, 3 (2012))

LocationA Location

B

EDFA

optical isolators

Scattered

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

5 km link + SOA

5 km link

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

YES

H-maser

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 EindhovenChigo Okonkwo TU EindhovenHuug de Waardt TU EindhovenTjeerd Pinkert VU AmsterdamRoeland Nuijts SURFnet UtrechtRob Smets SURFnet UtrechtMartin Fransen VSL DelftErik Dierikx VSL DelftHenk Peek NIKHEF AmsterdamJK

Time transfer – the state of the art

Method Distance Accuracy Ref.

GNSS >1000 km 3 – 50 ns

TWSTFT >1000 km 1 nsT2L2 >1000 km 200 ps expected Fridelance et al.,

Exp. Astr. (1997)White Rabbit (fiber)(1 Gpbs Ethernet, PTP)

10 km 0.1 - 1 nswww.ohwr.org

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)

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

Advantages:• Transmit 10 Gb/s data, no telecom capacity sacrificed• Time + data transfer• Compatible with existing telecom methods & equipment

25 km 50 km

Two round-trip delays measured:t12 (l1, l2) and t13 (l1, l3)

Quasi-bidirectional amplifier(Amemiya et al., IEEE IFCSE 2005)

PRBS signals and correlation

75 km 150 km

50 GS/s 12.5 GS/s

ResultsTime difference= <OWDestimate> - <OWDdirect>

-log

BER

Received power [dBm]

75 km

50 km25 km

0 km

Estimated accuracy: 4 ps(agrees with observations)

Measurement number

OWD

- t AB(t

) [p

s]

75 km linkBit-error rate (BER) below

10-9 :Error free

communication at 10 Gb/s

Link length [m] 25 255(1) 50 405(1) 75 552(1)

Sourcea [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

XCORb 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

Link length uncertainty <10 4 <10 4 <10 4

Total 4.0 4.1 4.2 aFor link configuration with FRMs; bXCOR: cross correlation.

Results

-log

BER

Received power [dBm]

75 km

50 km25 km

0 km

Measurement number

OWD

- t AB(t

) [p

s]

75 km link

Delivery of 10 Gb/s optical data with 4 ps accuracy over 75 km distance

N. Sotiropoulos et al. (submitted)

Time transfer – the state of the artMethod Distance Accuracy Ref.

GNSS >1000 km 3 – 50 ns

TWSTFT >1000 km 1 nsT2L2 >1000 km 200 ps expected Fridelance et al.,

Exp. Astr. (1997)White Rabbit (fiber)(1 Gpbs Ethernet, PTP)

10 km 0.1 - 1 nswww.ohwr.org

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(20 ps)

Sliwczynski et al., Metrologia (2013)

Cross correlation of 10 Gbps optical data

75 km 4 ps Sotiropoulos et al. (submitted)

State-of-the-artdelay determination

+Error-free optical data

transfer at 10 Gbps

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 wavelengthsintegrate time and frequency transfer in hardware for high-capacity optical telecom

Will require involvment of manufacturers ofoptical telecom network equipment and NRENs…… AND a convincing test case!

eVLBI using fiber-optical synchronization?

Fiber in

Data outT&F out

Application to eVLBI?

• 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

• Use low-noise TCXO/OCXO for short-term stability• Use recovered clock for long-term stability

– Do away with expensive H-masers?

Masterclock

Special thanks to Paul Boven and Arpad Szomoru of JIVE for insightful discussions about eVLBI

Disclaimer: not necessarily limited to Europe!

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 optical-wireless positioning with cm accuracy (with TU Delft - SuperGPS

4 ps 2.4 mm accuracy (4D positioning)

Aperture synthesis through mobile handsets?

Thanks!