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 ‘H 2 + 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 m m) 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 m s 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

l 1 1. Truly bidirectional bypass amp: l 1 2. Quasi-bidirectional bypass amp: 3. ‘Interleaved‘ bidirectional link with unidirectional amps: interleaver interleaver l 1 l

2

DWDM equipment fiber span amplifier hut interleaver fiber span interleaver l 2 DWDM equipment l 2

Truly bidirectional bypass amp

• • • • – Implemented in long-haul DWDM link (dark channel) – optical path length fluctuations* Optical carrier frequency transfer with 19 digits accuracy** Use l 2 slightly offset from l 1 (100 MHz) to distinguish return signal from l 1 l 2 back-scattered light Advantage: small DA due to chromatic dispersion (

1

l

2

< 1 ps/km) Disadvantage – – must keep gain below 25 dB (lasing threshold) WR requires larger difference l 1 -l 2 to avoid cross talk (1.25 Gbit/s) * L.S. Ma et al., Opt. Lett. 19, 1777 (1994) **O. Lopez et al., Opt. Exp. 18, 16849 (2010)

• • • • •

Truly bidirectional bypass amp

Choose different channels – l 1 , l 2 Install l -selective isolators to create unidirectional paths for each wavelength – Isolators block Rayleigh back-scattered light High amplifier gain possible Sacrifice channel l 2 on the expense of datacom bandwidth Added insertion loss Tolerate some DA – non reciprocal path in isolator (calibrate) – chromatic dispersion (> 10 ps/km) l

-selective isolator

fiber Bragg grating l 1 l 1 l 2 l 1 l 2 l 1 l 2 Isolator for l 2 Isolator for l 1 fiber Bragg grating l 2

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) l 1 l 2 l 1 l 2 * Amemiya et al., Proc. PTTI p.914 (2005)

‘Interleaved‘ bidir link with unidir amps

l

n

1 l 1 = Insertion loss comparable to OADM (i.e. 1.2 dB for l 1 , < 1 dB for other l )

Implementation in long-haul DWDM link

l 1 l

2

DWDM equipment fiber span amplifier hut fiber span DWDM equipment

‘Interleaved‘ bidir link with unidir amps

All channels available for data communication!

l 1 l

1

DWDM equipment l

n

1 l

n

1 fiber span amplifier hut fiber span DWDM equipment • • • 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 Laser 1

l 1

Dark fiber link

bidirectional EDFA

OC WR switch

Rx Tx

EOM

l 2

BPF Laser 2

l 2 fiber 10-100km fiber 10-100km fiber 10-100km fiber 10-100km l

1

l

2 OC PD EOM Link performance characterization

Tx

WR switch

Rx l 1

BPF PD

bidirectional EDFA

Location B

Virtual location B

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 = 10 4 s) = 1 × 10 -19 (fractional frequency)  TDEV ( t = 10 4 s) = 0.6 fs (!) • • • Unidirectional (uncompensated) DWDM links  ADEV( t = TDEV ( t = 10 10 4 4 s) s) < < 1 × 10 60 ps -14 (noise floor) 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