B. Chanteau

Ultra-stable long distance optical frequency distribution using the Internet fiber network.

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.

Cascaded multiplexed optical link on a telecommunication network for frequency dissemination

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.

Quantum cascade laser frequency stabilization at the sub-Hz level

Quantum Cascade Lasers (QCL) are increasingly being used to probe the mid-infrared “molecular fingerprint” region. This prompted efforts towards improving their spectral performance, in order to reach ever-higher resolution and precision. Here, we report the stabilisation of a QCL onto an optical frequency comb. We demonstrate a relative stability and accuracy of 2x10-15 and 10-14, respectively. The comb is stabilised to a remote near-infrared ultra-stable laser referenced to frequency primary standards, whose signal is transferred via an optical fibre link. The stability and frequency traceability of our QCL exceed those demonstrated so far by two orders of magnitude. As a demonstration of its capability, we then use it to perform high-resolution molecular spectroscopy. We measure absorption frequencies with an 8x10-13 relative uncertainty. This confirms the potential of this setup for ultra-high precision measurements with molecules, such as our ongoing effort towards testing the parity symmetry by probing chiral species.

Mid-infrared laser phase-locking to a remote near-infrared frequency reference for high-precision molecular spectroscopy

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 OsO4 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.

Ultra-stable long distance optical frequency distribution using the Internet fiber network and application to high-precision molecular spectroscopy

We report an optical link of 540 km for ultrastable frequency distribution over the Internet fiber network. The phase-noise compensated link shows a fractional frequency instability in full bandwidth of 3×10−14 at one second measurement time and 2×10−18 at 30 000 s. This work is a significant step towards a sustainable wide area ultrastable optical frequency distribution and comparison network. Time transfer was demonstrated simultaneously on the same link and led to an absolute time accuracy (250 ps) and long-term timing stability (20 ps) which outperform the conventional satellite transfer methods by one order of magnitude. Current development addresses the question of multiple users distribution in the same metropolitan area. We demonstrate on-line extraction and first results show frequency stability at the same level as with conventional link. We also report an application to coherent frequency transfer to the mid-infrared. We demonstrate the frequency stabilisation of a mid-infrared laser to the near-infrared frequency reference transferred through the optical link. Fractional stability better than 4×10−14 at 1 s averaging time was obtained, opening the way to ultrahigh resolution spectroscopy of molecular rovibrational transitions.

Multiplexed optical link for ultra-stable frequency dissemination

We transfer the frequency of an ultra-stable laser over a cascaded optical link comprising two compensated links of 150 km and a repeater station. Each link passes through two important nodes of the telecommunication network and includes 114 km of Internet fiber simultaneously carrying data traffic, through a dense wavelength division multiplexing scheme. The metrological signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station is working independently without any remote control. The phase noise on the two links is compensated with the usual round-trip technique. The 300-km multiplexed cascaded link shows an Allan deviation of 3×10−15 at one second and 7×10−20 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.

Quantum cascade laser at Hz-level by use of a frequency comb and an optical link

We present a frequency chain which enables to transfer coherently the stability and the accuracy of an ultrastable laser emitting at 1.54 μm to the mid-infrared region. It includes an optical frequency comb and an ultrastable 1.54 μm frequency signal, referenced to primary standards and transferred from LNE-SYRTE to LPL through an optical link. This system gives us the ability to transfer frequency stability in the range of 10-15 and accuracy in the range of 10-14 from 1.54 μm to a wide spectral range in the mid-infrared domain. With this set-up, we stabilized a Quantum Cascade Laser emitting at 10.3 μm at an unprecedented Hz-level.

Mid-IR frequency control using an optical frequency comb and a remote near-infrared frequency reference

Ultra-high-resolution spectroscopy enables to test modern theories of fundamental physics with molecules as for instance the non conservation of parity or the stability of the electron-to-proton mass ratio. However many of these tests rely on the availability of ultra-stable and accurate laser sources emitting in the mid-infrared (MIR) where molecules exhibit rovibrational transitions. It is thus very challenging to develop a frequency stabilization scheme in the MIR which does not depend on quite rare secondary frequency references.

Quantum cascade laser spectrometer for frequency metrology and high accuracy molecular spectroscopy around 10 μm

Summary form only given. Quantum cascade lasers (QCLs) are an emerging technology [1] suitable for high-resolution spectrocopy and frequency metrology [2] with an incomparable frequency tunability in the mid-infrared range. We are currently developing a new compact, widely tunable QCL-based spectrometer. Such an instrument will broaden the scope of our experimental setups dedicated to molecular spectroscopy-based precision measurements.Eventually, we want QCLs to reach the state-of-the-art metrological stability and accuracy of our existing stabilized CO2 lasers (~10 Hz width, 0.1 Hz frequency instability for 100 s integration time). As recently demonstrated with the CO2 laser, we will lock the QCL to a frequency comb itself referenced via an optical fibre to the atomic foutain clocks in Paris. Stabilizing the laser this way not only provides us with the ultimate frequency accuracy and stability, it also frees us from having to lock the QCL to a molecular transition, making it possible to have a stabilized QCL at any desired wavelength. The use of QCLs will allow the study of any species showing absorption between 3 and 25 μm [1]. We are currently concentrating on QCLs at ~10 μm, which allows us to test them against our CO2 laser. Our first characterization of a free-running continuous-wave mode near-room-temperature distributed-feedback 10.3 μm QCL looks promising. The beat-note with our CO2 laser shows a record ~200 kHz linewidth (see Figure 1). The low level of amplitude and frequency noise, measured using an NH3 linear absorption line as frequency discriminator, should enable spectroscopy with unprecedented levels of precision. Narrowing of the QCL linewidth was achieved by straightforwardly phase-locking the beat-note between the QCL and CO2 laser on a radiofrequency reference. The great stability of the CO2 laser was transferred to the QCL resulting in an expected linewidth of a few tens of hertz.In the future, frequency locking to absolute frequency references consisting of a sub-Doppler molecular lines or to ultra-stable Fabry-Perot cavities, and finally phase-locking to a frequency comb will be investigated. This work will result in a major technological leap that will benefit two of our main projects respectively dedicated to a precise determination of the Boltzmann constant by laser spectroscopy of a molecular gas [3] and to the first observation of parity violation in chiral molecules by ultra-high resolution molecular jet-spectroscopy [4].

Long distance phase-coherent link between near- and mid-infrared frequencies

We present a new 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. We demonstrate the frequency stabilization of a mid-infrared laser with fractional stability better than 4×10-14 at 1 s averaging. 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.