Fundamental metrology with laser-cooled atoms and optical frequency combs
Titre du projet
Fundamental metrology with laser-cooled atoms and optical frequency combs
Fundamental metrology with laser-cooled atoms and optical frequency combs Principal investigator: Prof. Pierre Thomann, LTF laboratoire Temps-Fréquence - Université de Neuchâtel Lead : Time is the physical quantity which can be measured with the highest precision, using atomic clocks as measuring sticks. Further improvements are however required in many applications, such as telecommunications and satellite navigation systems, where the performance of yesterday's best clocks is now used on a routine basis. This research focuses on improving the accuracy of existing reference clocks and on new types of atomic clocks. Summary: The precise measurement of time relies on atomic clocks, i.e. clocks whose pendulum is made of atoms oscillating at frequencies ranging from 1010 to 1015 Hz. This research focuses on: 1) improving the stability and the accuracy of an existing reference atomic clock based on an original principle - a continuous fountain of laser-cooled cesium atoms - , and 2) the realization of an ultrastable microwave oscillator based on a laser, stabilized on an optical cavity, and an optical frequency comb. The short-term stability of such an all-optical microwave oscillator is expected to surpass that of active hydrogen masers. Goal: 1) Reference atomic clocks (primary frequency standards) are used in basic metrology, to generate the international atomic time scale (UTC). The laser-cooled fountain clock FOCS-2 is developed in collaboration with - and with the support of - METAS, the Swiss institute of metrology. Its original design is meant to give new insights in the fundamental limits of cold-atom clocks. Other applications of primary clocks are in testing fundamental physical theories. 2) Ultrastable all-optical microwave generators are only beginning to be used in the most critical applications in metrology laboratories. However, their use will soon spread to all present applications of hydrogen masers, e.g. radio-astronomy, geophysics, and space research. Signification: Although time measurements are by far the most precise of all physical measurements, progress in this field is entirely driven by applications: atomic clocks are at the heart of large-scale technological developments such as global, high capacity telecommunication networks and satellite-based navigation and positioning systems (e.g. GPS, Galileo). Thanks to a consistent effort in basic research, applied research and industrial transfer, Swiss industry and academia now make a substantial contribution to the fabrication and use of state-of-the art atomic clocks in such large-scale systems, both in ground and in space applications.
Date de début
1 Avril 2009
Date de fin
31 Mars 2011
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- PublicationAccès libreExperimental Validation of a Simple Approximation to Determine the Linewidth of a Laser from its Frequency Noise SpectrumLaser frequency fluctuations can be characterized either comprehensively by the frequency noise spectrum or in a simple but incomplete manner by the laser linewidth. A formal relation exists to calculate the linewidth from the frequency noise spectrum, but it is laborious to apply in practice. We recently proposed a much simpler geometrical approximation applicable to any arbitrary frequency noise spectrum. Here we present an experimental validation of this approximation using laser sources of different spectral characteristics. For each of them, we measured both the frequency noise spectrum to calculate the approximate linewidth and the actual linewidth directly. We observe a very good agreement between the approximate and directly measured linewidths over a broad range of values (from kilohertz to megahertz) and for significantly different laser line shapes.
- PublicationAccès libreCross-influence between the two servo-loops of a fully-stabilized Er:fiber optical frequency combWe present a study of the impact of the cross-coupling between the two servo loops used to stabilize the repetition rate frep and the carrier-envelope offset (CEO) frequency fCEO in a commercial Er:fiber frequency comb, based on the combination of experimental measurements and a model of the coupled loops. The developed theoretical model enables us to quantify the influence of the servo-loop coupling on an optical comb line, by simulating the hypothetic case where no coupling would be present. Numerical values for the model were obtained from an extensive characterization of the comb, in terms of frequency noise and dynamic response to a modulation applied to each actuator, for both frep and fCEO. To validate the model, the frequency noise of an optical comb line at 1.56 μm was experimentally measured from the heterodyne beat between the comb and a cavity-stabilized ultranarrow-linewidth laser and showed good agreement with the calculated noise spectrum. The coupling between the two stabilization loops results in a more than 10-fold reduction of the comb mode frequency noise power spectral density in a wide Fourier frequency range.
- PublicationAccès libreFrequency noise of free-running 4.6 um distributed feedback quantum cascade lasers near room temperatureThe frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 um range and operated in cw mode near room temperature (277K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz2/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.
- PublicationAccès libreLinewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driverWe report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6 cm^−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ≈1 nA/√Hz allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ≈1.6 MHz/(nA/√Hz) for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several nA/√Hz , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only 350 pA/√Hz current noise, which is suitable to observe the ≈550 kHz linewidth of our QCL.
- PublicationAccès libreNew-generation cryogenic sapphire microwave oscillators for space, metrology and scientific applicationsThis article reports on the characterization of cryogenic sapphireoscillators (CSOs), and on the first test of a CSO in a real field installation, where ultimate frequency stability and continuous operation are critical issues, with no survey. Thanks to low-vibration liquid-He cryocooler design, Internet monitoring, and a significant effort of engineering, these oscillators could bridge the gap from an experiment to a fully reliable machine. The cryocooler needs scheduled maintenance every 2 years, which is usual for these devices. The direct comparison of two CSOs demonstrates a frequency stability of 5 × 10E−16 for 30 s < τ < 300 s integration time, and 4.5 × 10E−15 at 1 day (1 × 10E−14 typical). Two prototypes are fully operational, codenamed ELISA and ULISS. ELISA has been permanently installed the new deep space antenna station of the European Space Agency in Malargüe, Argentina, in May 2012. ULISS is a transportable version of ELISA, modified to fit in a small van (8.5 m2 footprint). Installation requires a few hours manpower and 1 day of operation to attain full stability. ULISS, intended for off-site experiments and as a technology demonstrator, and has successfully completed two long-distance travels.