Approaches and applications WR TWTFT through long-haul duplexed fiber pairs Approaches and applications Jeroen Koelemeij LaserLaB VU University j.c.j.koelemeij@vu.nl 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 Timing is everything 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) Data communication: unidirectional duplexed fiber pairs EDFA optical isolators Rayleigh Location A B

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

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

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, 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) l2 l1 fiber Bragg grating l1 fiber Bragg grating l2 l-selective isolator l1 l2 Isolator for l2 Isolator for l1

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 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

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