RADIOFREQUENCY

BUT...

BOLOGNA-INAFTORINO-INRIM

19 dB

18 dB

24 dB

20 dB

20 dB

19 dB

12 dB

19 dBBidirectional EDFAs

20 dB

20 dB

The fiber path, with amplifiers and attenuation for each span.

MILANO-CNR

FIRENZE-INFN (LENS)

… towards Europe:Géant PoP - Milano

… towards France:LSM- Modane

ULTRASTABLE LASER 1.5 μm

INRIM

END USER

DEDICATED AMPLIFIERS

Yb CLOCK

Cs CLOCK

ULTRASTABLE LASER 1.5 μm

OPTICAL COMB

650 km FIBER

AOM

MODEM

UTC(IT)PPS INPUT

AOM

OPTICALFREQUENCY

An optical fiber infrastructure for time & frequency dissemination in Italy

- INRIM encodes a PPS signal with pseudo-random modulation in the optical carrier via a modem - The end user decodes it with another modem.

TIME & FREQUENCY DISSEMINATION THROUGH OPTICAL FIBER

The European Laboratory for NonLinear Spectroscopy (LENS) is involved in high resolution spectroscopy and in developing a 88Sr optical clock:-needs optical frequency dissemination at the 10-18 level to carry on absolute characterization of the Sr clock with respect to the SI second.

Sr Clock Bench at LENS

PRELIMINARY RESULTS & OUTLOOKS

The Italian Institute of Metrology (INRIM) has the task of realizing, maintaining and disseminating the second of the International System of Units in Italy. To better perform the dissemination task, INRIM is realizing an optical fiber backbone that will connect some of the major Italian research laboratories.

This work is supported by

THE PARTNERS

C. ClivatiOptics Division, Istituto Nazionale di Ricerca Metrologica, Torino, Italy ; Politecnico di Torino, Torino, Italy;

email: c.clivati@inrim.it

The Medicina Observatory hosts both a 32 meter dish for Very Long Baseline Interferometry (VLBI) and a 20'000 m2

collecting area for Pulsar research:-needs accurate timing for VLBI, now also in real time VLBI, by 1Gb/s fiber.-needs Frequency dissemination at 10-15 level for Pulsar monitoring on long and very long time scales.-offers T&F cross checks by direct comparisons with Pulsar and Quasar, now defining the best possible inertial frame of reference.

Istituto di fotonica e nanotecnologie (IFN-CNR) is interested in high grade frequency dissemination

THE SETUP

PhD theses &PostDoc positions available

TIMING

BOLOGNA-INAF MILANO-CNR

FIRENZE-INO

KEY POINTS➢ Both Time and Frequency distribution on the same infrastructure➢ Strong interaction between Research Centres and Industrial partners➢ Different accuracy and stability grades for multiple partners needs➢ Multiple user distribution

AIMS➢ Remote comparisons of atomic clocks➢ Absolute frequency calibrations for high resolution spectroscopy➢ Remote clocks in replacement of local H-Masers ➢ Timebase distribution and synchronization at 100 ps level➢ Create a distributed network for T&F services on national scale

ITALY & EUROPEThe LIFT project will be developed in tight relation with the European project NEAT-FT, that aims to realize a continental network for clocks comparisons and frequency dissemination. LIFT will provide the connection of the Italian Network to a forthcoming European Network through the cross-borders:-Italy-Switzerland (feasible end 2013) via Milano-Italy-France (feasible end 2013) via Frejus tunnel.

FIRENZE-UNIFI

ACTIVE FIBER NOISE CANCELLATION

UTC(IT)PPS INPUT

TIMING

The Italian National Metrology Institute realizes the SI second in Italy and disseminates it to the user community. INRIM clocks ensemble is composed by two Cs fountains, 5 commercial Cs beam clocks, 3 H-Maser, a POP Rb clock and is developing an Yb clock. INRIM manages a Ti:Sa and a fiber optical comb to compare optical frequencies to the primary clocks and ultrastable laser sources at 1542 nm for the optical link:-will allow Time synchronization at 100 ps level-will enable optical and RF frequencies comparisons at 10-18 stability level

Fiber Optical Comb

Ti:Sa Optical CombCs Fountains

H masers

Ultrastable laser @ 1542 nmYb optical bench

TORINO-INRIM

The Italian Institute of Optics is involved in high resolution molecular spectroscopy -needs high grade optical frequency dissemination

OPTICAL COMB

MODEM

The Radio antenna

✔2. Measure the link phase noise

We measured the phase noise from Torino to each intermediate site (see map). The graph shows the fiber phase noise at someintermediate stations. The noise should scale as the link length, but some of the spans seem much more noisy than others (under investigation).

100 101101

102

103

104

105

106 Torino Bologna Firenze

S ϕ(f) /

rad2 /H

z

frequency /Hz

✔3. Phase compensate the link Torino-Firenze

The graph shows the fiber phase noise of the free running link and of the compensated optical carrier (measured in-loop signal, and expected in Firenze). The control bandwidth is less than 70 Hz (the light needs 6 ms to go to Firenze and come back).

✔1. Compensate the optical lossesThe map (←) shows the optical attenuation of each span. To overcome it we used 9 bidirectional (custom developed) Erbium Doped Fiber Amplifiers. The in-field tests show a degradation of the amplifiers performances compared to those in lab: gain saturation, signal amplitude modulation, ASE, backscattering.. [see point 4.]

✔4. Test alternative amplification techniquesB-EDFA and discrete amplifiers in general are not the best way to amplify weak signals on bidirectional links. We tested alternative technologies such as Distributed Raman Amplification, that enables high gain without the arising of detrimental effect, and is coherent at the 10-19 resolution level. This technology could simplify the setup, making the backbone more reliable, robust and cheaper.

5. Perform the frequency comparison of LENS Sr clock and INRIM IT Cs-F2 fountain clock

The measurement will be performed in the next months, and the whole metrological chain will enable the absolute characterization of the Sr clock at the 10-15 level of accuracy, through a 650 km optical fiber link.

WHY OPTICAL LINKSAtomic clocks are not transportable. Thus, satellite techniques are needed to compare them with other clocks and to calibrate them. However, the resolution of such techniques is only 10-9 at 1 s averaging time: this means that at least 10 days of measurement are needed to perform a comparison between distant clocks. If optical fiber, instead of satellite, is used to transfer such reference signals, the resolution can be improved by 105 folds.

The absolute characterization of the remote clock can be performedThe optical comb enables the generation of others optical or RF frequencies starting from the calibrated one.

FREQUENCY DISTRIBUTION

- INRIM generates a laser source at 1542 nm with a linewidth of 1 Hz.- Its frequency is measured against a primary standard (INRIM Cs fountain IT-CsF2) through an optical fiber comb. - At the same time it is sent to the end user via optical fiber. - The user measures its frequency with respect to the local clock, through another optical comb.

The optical fiber varies its length with temperature, vibration, environmental noise, and this degrades the stability of the transferred carrier, so it must be compensated. To do this:- The radiation from the user-end is reflected back to INRIM- Here it is compared with the source light: the fiber noise can thus be measured- A Phase-Locked Loop corrects the noise by adding the proper frequency shift on the laser, via an Acousto-Optic Modulator- The optical path must be the same in both directions (restrictions on the optical amplifiers)

TIME DISTRIBUTION

Further readings:

I. Coddington,et al., “Coherent optical link over hundreds of metres and hundreds of terahertz with subfemtosecond timing jitter,” Nature Photonics 1, pp. 283-287 (2007).

K. Predehl, et al., “A 920-Kilometer Optical Fiber Link for Frequency Metrology at the 19th decimal place,” Science 4336, pp. 441-444 (2012).

O. Lopez, et al., “Cascaded multiplexed optical link on a telecommunication network for frequency dissemination,” Opt. Expr.16, pp. 16849-16857 (2010).C. Clivati, et al., “Planar-waveguide external cavity laser for an optical link with 10-19 frequency stability” IEEE Trans. Ultrason. Ferroelectr. Freq. Contr. 58, pp. 2582-2587 (2011).

:THE LIFT PROJECT AND PARTNERSLIFT (Italian Link for Frequency and Time) is the future optical fiber network for Time and Frequency dissemination in Italy. LIFT is a project of INRIM, Politecnico di Torino, the Italian Institute of Nuclear Physics (INFN), the University of Firenze (UNIFI) and the Italian Institute of Astrophysics (INAF).A first Torino-Firenze link is expected to be fully operative by the end of 2013. It will enable a remote frequency comparison between the atomic clocks of INRIM and UNIFI-LENS. By 2015, the backbone will be upgraded to the LIFT project providing dissemination to Milano and Bologna as well.

490 km

110 km

0,1 1 10 100 100010-4

10-3

10-2

10-1

100

101

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expected in Firenze

compensated (in loop)

S ϕ(f) /

rad2 /H

zfrequency /Hz

free running