Olivier Lopez

Long-distance frequency dissemination with a resolution of 10(-17).

We use a new technique to disseminate microwave reference signals along ordinary optical fiber. The fractional frequency resolution of a link of 86 km in length is 10(-17) for a one day integration time, a resolution higher than the stability of the best microwave or optical clocks. We use the link to compare the microwave reference and a CO2/OsO4 frequency standard that stabilizes a femtosecond laser frequency comb. This demonstrates a resolution of 3 x 10(-14) at 1 s. An upper value of the instability introduced by the femtosecond laser-based synthesizer is estimated as 1 x 10(-14) at 1 s.

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.

A widely tunable 10-μm quantum cascade laser phase-locked to a state-of-the-art mid-infrared reference for precision molecular spectroscopy

We report the coherent phase-locking of a quantum cascade laser (QCL) at 10-μm to the secondary frequency standard of this spectral region, a CO2 laser stabilized on a saturated absorption line of OsO4. The stability and accuracy of the standard are transferred to the QCL resulting in a line width of the order of 10u2009Hz, and leading to the narrowest QCL to date. The locked QCL is then used to perform absorption spectroscopy spanning 6u2009GHz of NH3 and methyltrioxorhenium, two species of interest for applications in precision measurements.

Frequency measurement of an Ar/sup +/ laser stabilized on narrow lines of molecular iodine at 501.7 nm

A spectrometer for ultrahigh-resolution spectroscopy of molecular iodine at wavelength of 501.7 nm, near the dissociation limit is described. Line shapes about 30 kHz wide half-width half-maximum (HWHM) were obtained using saturation spectroscopy in a pumped cell. The frequency of an Ar/sup +/ laser was locked to a hyperfine component of the R(26)62-0 transition and the first absolute frequency measurement of this line is reported.

Bimodality as a signal of a liquid-gas phase transition in nuclei?

We use the heavy-ion phase-space exploration model to discuss the origin of the bimodality in charge asymmetry observed in nuclear reactions around the Fermi energy. We show that it may be related to the important angular momentum (spin) transferred into the quasiprojectile before secondary decay. As the spin overcomes the critical value, a sudden opening of decay channels is induced and leads to a bimodal distribution for the charge asymmetry. In the model, it is not assigned to a liquid-gas phase transition but to specific instabilities in nuclei with high spin. Therefore, we propose to use these reactions to study instabilities in rotating nuclear droplets.

Two-way optical frequency comparisons at 5*10^-21 relative stability over 100-km telecommunication network fibers

By using two-way frequency transfer, we demonstrate ultra-high resolution comparison of optical frequencies over a telecommunication fiber link of 100 km operating simultaneously digital data transfer. We first propose and experiment a bi-directional scheme using a single fiber. We show that the relative stability at 1 s integration time is 7 10^18 and scales down to 5 10^21. The same level of performance is reached when an optical link is implemented with an active compensation of the fiber noise. We also implement a real-time two-way frequency comparison over a uni-directional telecommunication network using a pair of parallel fibers. The relative frequency stability is 10^15 at 1 s integration time and reaches 2 10^17 at 40 000 s. The fractional uncertainty of the frequency comparisons was evaluated for the best case to 2 10^20. These results open the way to accurate and high resolution frequency comparison of optical clocks over intercontinental fiber networks.

Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network

A two-way method to simultaneously transfer an ultra-stable optical frequency and an accurate timing over a fiber links is presented. The ultra-stable laser acts as frequency standard and as carrier for timestamps. Timestamps signals are encode by phase modulation with a spread spectrum pseudorandom modulation at 20 Mchip/s, provided by a pair of two-way satellite time transfer modems, one for each direction.

Cascaded optical link on a telecommunication fiber network for ultra-stable frequency dissemination

The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. Optical fiber links have been intensively studied for a decade and brought the potential to transfer frequency with a very high accuracy and stability thanks to an active compensation of the propagation noise. We are currently developing an optical metrological network using the fibers of the French National Research and Education Network. Using the so-called dark-channel approach, the ultrastable signal is copropagating with data traffic using wavelength division multiplexing. Due to significant reflections and losses along the fibers, which cannot be compensated with amplifiers, we have developed some repeater stations for the metrological signal. These remotely-operated stations amplify the ultrastable signal and compensate the propagation noise. The link is thus composed of a few cascaded spans. It gives the possibility to increase the noise correction bandwidth, which is proportional to the inverse of the fiber length for each span. These stations are a key element for the deployment of a reliable and large scale metrological network. We report here on the implementation of a two-spans cascaded link of 740 km reaching a relative stability of a few 10-20 after 103 s averaging time. Extension to longer links and alternative transfer methods will be discussed.

Towards large scale metrological fibre network

In the frame of the REFIMEVE+ project, aiming at disseminating an optical frequency standard to more than 20 laboratories in France, we present the progress made towards a metrological fibre wide-area network.

Bi-directional optical amplifiers for long-distance fibre links

Bi-directional optical frequency links for phase-stabilized frequency transfer require bi-directional amplifiers. Here we present some results from the development and test of two types of bidirectional amplifiers, Er+-doped fiber amplifiers (EDFA) and fibre Brillouin amplifiers (FBA). to be deployed along a fiber link connecting SYRTE/France and PTB/Germany.