Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network
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In this work, we demonstrate for the first time that it is possible to transfer simultaneously an ultra-stable optical frequency and precise and accurate timing over 540 km using a public telecommunication optical fiber network with Internet data. The optical phase is used to carry both the frequency information and the timestamps by modulating a very narrow optical carrier at 1.55 μm with spread spectrum signals using two-way satellite time transfer modems. The results in terms of absolute time accuracy (250 ps) and long-term timing stability (20 ps) well outperform the conventional Global Navigation Satellite System or geostationary transfer methods.
In the last 5 years, ultra-stable optical fiber links have been successfully developed enabling precise and accurate frequency transfer with fractional frequency stability in the range of 10−18 after only 3 h of measurement and frequency accuracy of a few 10−19. Recently, ground-breaking frequency transfer has been demonstrated on a record distance of 920 km on dedicated fiber . We extended this technique to public fiber networks with simultaneous data traffic, providing a scalable technique for extension to the continental level [2, 3]. This approach is much more stable and accurate than satellite-based frequency comparisons using the Global Navigation Satellite System (GNSS), or other satellite-based techniques. This opens the way to accurate remote clock comparisons, for modern atomic clocks having already demonstrated accuracy in the range 10−16 to 10−17 [4, 5, 6, 7, 8]. This is a key point in advanced time–frequency metrology and for advanced tests of fundamental physics . Accurate frequency plays also a key role for geodesy, high-resolution radio-astronomy, modern particle physics, and for the underpinning of the accuracy of almost every type of precision measurement. For all these applications, accurate timing is also important and gives the capability to precisely synchronize distant experiments. A salient case is the neutrinos speed measurement from CERN to Gran Sasso. Fiber-optical two-way time transfer methods have been demonstrated on dedicated links with an accuracy of the order of 100 ps or better [10, 11]. Long-distance accurate time dissemination is usually based on GNSS signals, or geostationary telecommunication satellites, with timing accuracy of the order of 1 ns in the best case [12, 13]. In this work, we present a novel method to simultaneously disseminate an ultra-stable optical frequency and accurate timing over a public telecommunication network on a 540-km optical link simultaneously carrying Internet data traffic, using a dedicated “dark” channel.
2 Experimental setup description
Here, we briefly recall the operation of the optical ultra-stable frequency transfer: the frequency signal consists of the frequency of an ultra-stable cavity-stabilized laser (in block A, Fig. 1b), which feeds the optical fiber. The fiber propagation noise is detected and compensated by comparison of the input optical phase with the signal phase after a round trip, thanks to an optical interferometer. Corrections are applied using an acousto-optical modulator (AOM) operating around 40 MHz. For the sake of simplicity, we gather all error signal processing functions into a symbolic box called the optical phase stabilization unit (OPSU). At the far end of the link, the optical signal is regenerated by a narrow linewidth laser (3 kHz) (in block B, Fig. 1b), phase locked on the incoming light and frequency offset by 79 MHz. The optical frequency transfer stability is obtained by measuring the optical beat note between the ultra-stable laser from A and the phase-locked laser in part B.
With regard to time transfer, signals are provided by a pair of two-way satellite time transfer modems . These signals are generated by relating the phase of a pseudorandom noise modulation (20 Mchip/s) on a radio frequency carrier signal (50–80 MHz) to the one-pulse-per-second (1 pps) and 5 MHz reference signals from a common clock. Orthogonal specific pseudo-random codes are allocated to each modem device. This equipment correlates the received signal with a local replica of the signal expected from the transmitting site and measures the time of arrival of the received signal with respect to the local clock. A computer collects the time of arrival of both modems and computes the differential time delays.
We implement an approach similar to the one used in coherent optical telecommunication to encode the time signal on the optical carrier. At each link end, the laser is phase modulated by a fiber pigtailed electro-optical modulator (EOM) with frequency-shifted replicas of the time signal at 400 and 700 MHz, respectively. These frequency shifts are chosen to avoid interference between optical signals and allow efficient filtering of the time comparison signals. The low modulation depth (~1 %) and the spread spectrum nature of the modulating time signals lead to a very pure optical spectrum, allowing the optical frequency carrier transfer to operate without degradation. The time signals are recovered by an optical heterodyne beat note of the local laser with the incoming signal at each link end. After successive frequency mixing and filtering, the spread spectrum time signals are processed by the modems. This step is not trivial considering the frequency width of the pseudo-random codes (~20 MHz) and the large dynamic range of the signals in the detection system, where parasitic signals due to stray reflections are 40 dB larger than the useful signal.
3 Results and discussion
We believe that in the case of slightly lower fiber losses and better distribution of the optical amplification, such a link can be extended over more than 1000 km. This method of accurate time transfer can be extended to segmented optical links by the use of intermediate optical regeneration stations [2, 3], which include RF signal processing techniques and station delay calibration procedures. This method can be improved for example by developing new modems with wider pseudo-random codes. The scheme proposed here can be drastically simplified if one focuses only on time transfer as no ultra-stable laser is then needed. It is important to point out that the use of heterodyne detection allows operation with very large optical losses, for which classical intensity modulation (IM) techniques are ineffective. This method has been proven to be applicable to non-dedicated fiber and installed long-haul fiber links carrying Internet data. This opens the way to frequency and time dissemination on a continental scale, since NRENs together with transnational fiber networks such as GEANT in Europe could be used for that purpose.
This work would not have been possible without the support of the GIP RENATER. We acknowledge the funding support from the Agence Nationale de la Recherche (ANR BLANC 2011-BS04-009-01).
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