Abstract:We report an optical link of 540 km for ultrastable frequency distribution over the Internet fiber network. The stable frequency optical signal is processed enabling uninterrupted propagation on both directions. The robustness and the performance of the link are enhanced by a cost effective fully automated optoelectronic station. This device is able to coherently regenerate the return optical signal with a heterodyne optical phase locking of a low noise laser diode. Moreover the incoming signal polarization variation are tracked and processed in order to maintain beat note amplitudes within the operation range. Stable fibered optical interferometer enables optical detection of the link round trip phase signal. The phase-noise compensated link shows a fractional frequency instability in 10 Hz bandwidth of 5×10 -15 at one second measurement time and 2×10-19 at 30 000 s. This work is a significant step towards a sustainable wide area ultrastable optical frequency distribution and comparison network.

High-precision measurements with molecules may refine our knowledge of various fields of physics, from atmospheric and interstellar physics to the standard model or physics beyond it. Most of them can be cast as absorption frequency measurements, particularly in the mid-infrared 'molecular fingerprint' region, creating the need for narrow-linewidth lasers of well-controlled frequency. Quantum cascade lasers provide a wide spectral coverage anywhere in the mid-infrared, but show substantial free-running frequency fluctuations. Here, we demonstrate that the excellent stability and accuracy of an ultra-stable near-infrared laser, transferred from a metrological institute through a fibre link, can be copied to a quantum cascade laser using an optical frequency comb. The obtained relative stability and accuracy of 2 × 10 −15 and 10 −14 exceed those demonstrated so far with quantum cascade lasers by almost two orders of magnitude. This set-up enables us to measure molecular absorption frequencies with state-of-the-art uncertainties, confirming its potential for ultra-highprecision spectroscopy.M olecules are increasingly being used in precision tests of physics thanks to progress made in controlling molecular degrees of freedom 1,2 . They are now being used, for example, to test fundamental symmetries 3-5 and to measure fundamental constants 6-8 and their possible variation in time 9-11 . Most of these experiments are spectroscopic precision measurements and are often in the mid-infrared (MIR) domain where the molecules exhibit intense and narrow rovibrational transitions. This creates a need for efficient MIR laser sources, prompting efforts to develop ultra-stable and accurate continuous wave (c.w.) lasers as well as MIR frequency combs (refs 12 and 13 for instance). Quantum cascade lasers 14,15 (QCLs) are promising c.w. sources-they are available anywhere in the 3-25 µm MIR range, and each QCL can be tuned over several hundreds of gigahertz. However, their freerunning linewidth of tens to thousands of kilohertz makes their frequency stabilization challenging [16][17][18][19][20][21][22][23][24][25] .Common references used for frequency stabilization in the MIR region include molecular rovibrational absorption lines 3,26 . However, molecular degrees of freedom cannot be controlled as efficiently as atomic ones, leading to limited frequency reproducibility and accuracy. Attempts to develop MIR ultra-stable cavities have been made, but their performances are far from those reported in the near-infrared (NIR) or visible regions 27,28 . It is thus appealing to use the best ultra-stable lasers as frequency references. As these are predominantly in the NIR region, it is necessary to bridge the gap between the NIR and MIR domains. This is possible using an optical frequency comb (OFC). The MIR frequency is locked to a high harmonic of the OFC repetition rate using sum-or difference-frequency generation processes. This not only provides ultimate stabilities of lasers locked to state-of-the-art ultra-stable cavities 29 , but ...

We demonstrate a cascaded optical link for ultrastable frequency dissemination comprised of two compensated links of 150 km and a repeater station. Each link includes 114 km of Internet fiber simultaneously carrying data traffic through a dense wavelength division multiplexing technology, and passes through two routing centers of the telecommunication network. The optical reference signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station operates autonomously ensuring noise compensation on the two links and the ultra-stable signal optical regeneration. The compensated link shows a fractional frequency instability of 3 x 10(-15) at one second measurement time and 5 x 10(-20) at 20 hours. This work paves the way to a wide dissemination of ultra-stable optical clock signals between distant laboratories via the Internet network.

The distribution and the comparison of an ultra-stable optical frequency and accurate time using optical fibres have been greatly improved in the last ten years. The frequency stability and accuracy of optical links surpass well-established methods using the global navigation satellite system and geostationary satellites. In this paper, we present a review of the methods and the results obtained. We show that public telecommunication network carrying Internet data can be used to compare and distribute ultra-stable metrological signals over long distances. This novel technique paves the way for the deployment of a national and continental ultra-stable metrological optical network. RésuméLa distribution et la comparaison d'étalons de fréquence optique ultra-stables et d'échelle de temps ont été grandement améliorées depuis dix ans par l'emploi de fibres optiques. La stabilité de fréquence et l'exactitude des liens optiques fibrés surpassent les méthodes bien établies fondées sur les communications satellitaires. Dans cet article, nous présentons les méthodes et les résultats obtenus pendant cette décade. Nous montrons que les réseaux de télécommunication publics transportant des données internet peuvent être utilisés pour comparer et distribuer des signaux métrologiques sur de grandes distances. Ceci ouvre la voie au déploiement d'un réseau métrologique à l'échelle nationale et continentale.

We present a method for accurate mid-infrared frequency measurements and stabilization to a near-infrared ultra-stable frequency reference, transmitted with a long-distance fibre link and continuously monitored against state-of-the-art atomic fountain clocks. As a first application, we measure the frequency of an OsO 4 rovibrational molecular line around 10 µm with an uncertainty of 8 × 10 −13 . We also demonstrate the frequency stabilization of a mid-infrared laser with fractional stability better than 4 × 10 −14 at 1 s averaging time and a linewidth below 17 Hz. This new stabilization scheme gives us the ability to transfer frequency stability in the range of 10 −15 or even better, currently accessible in the near infrared or in the visible, to mid-infrared lasers in a wide frequency range.

We demonstrate a dispersion scan (d-scan) pulse characterization scheme employing cross-polarized wave (XPW) generation as a nonlinear optical process. XPW generation is a degenerate four-wave mixing process with no phase-matching limitations. Therefore, its implementation in the d-scan method is a good choice for the characterization of few-cycle pulses in remote spectral regions. We fully characterize 5-10 fs pulses delivered through a hollow-core fiber in the near-IR region and compare the results with the second-harmonic generation (SHG) frequency-resolved optical gating and SHG d-scan characterization methods.

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We have built a frequency chain which enables to measure the absolute frequency of a laser emitting in the 28-31 THz frequency range and stabilized onto a molecular absorption line. The set-up uses an optical frequency comb and an ultrastable 1.55 µm frequency reference signal, transferred from LNE-SYRTE to LPL through an optical link. We are now progressing towards the stabilization of the mid-IR laser via the frequency comb and the extension of this technique to quantum cascade lasers. Such a development is very challenging for ultrahigh resolution molecular spectroscopy and fundamental tests of physics with molecules.I.