Voici les éléments 1 - 10 sur 20
  • Publication
    Accès libre
    Absolute frequency referencing in the long wave infrared using a quantum cascade laser frequency comb
    (2022-4-4) ;
    Gianella, Michele
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    Jouy, Pierre
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    Kapsalidis, Filippos
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    Shahmohammadi, Mehran
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    Beck, Mattias
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    ; ;
    Hugi, Andreas
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    Faist, Jérôme
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    Emmenegger, Lukas
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    ;
    Optical frequency combs (OFCs) based on quantum cascade lasers (QCLs) have transformed mid-infrared spectroscopy. However, QCL-OFCs have not yet been exploited to provide a broadband absolute frequency reference. We demonstrate this possibility by performing comb-calibrated spectroscopy at 7.7 µm (1305 cm−1) using a QCL-OFC referenced to a molecular transition. We obtain 1.5·10−10 relative frequency stability (100-s integration time) and 3·10−9 relative frequency accuracy, comparable with state-of-the-art solutions relying on nonlinear frequency conversion. We show that QCL-OFCs can be locked with sub-Hz-level stability to a reference for hours, thus promising their use as metrological tools for the mid-infrared.
  • Publication
    Accès libre
    Coherently-averaged dual comb spectrometer at 7.7 µm with master and follower quantum cascade lasers
    (2021-6) ; ;
    Terrasanta, Giulio
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    ; ;
    Gianella, Michele
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    Jouy, Pierre
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    Kapsalidis, Filippos
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    Shahmohammadi Mehran, Mehran
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    Beck Matthias, Matthias
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    Faist, Jérôme
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    Emmenegger, Lukas
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    Hugi, Andreas
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    We demonstrate coherent averaging of the multi-heterodyne beat signal between two quantum cascade laser frequency combs in a master-follower configuration. The two combs are mutually locked by acting on the drive current to control their relative offset frequency and by radio-frequency extraction and injection locking of their intermode beat signal to stabilize their mode spacing difference. By implementing an analog common-noise subtraction scheme, a reduction of the linewidth of all heterodyne beat notes by five orders of magnitude is achieved compared to the free-running lasers. We compare stabilization and post-processing corrections in terms of amplitude noise. While they give similar performances in terms of signal-to-noise ratio, real-time processing of the stabilized signal is less demanding in terms of computational power. Lastly, a proof-of-principle spectroscopic measurement was performed, showing the possibility to reduce the amount of data to be processed by three orders of magnitude, compared to the free-running system.
  • Publication
    Accès libre
    Compact rubidium-stabilized multi-frequency reference source in the 1.55-μm region
    Combining light modulation and frequency conversion techniques, a compact and simple frequency-stabilized optical frequency comb spanning over 45 nm in the 1.56-μm wavelength region is demonstrated. It benefits from the high-frequency stability achievable from rubidium atomic transitions at 780 nm probed in a saturation absorption scheme, which is transferred to the 1.56-μm spectral region via a second-harmonic generation process. The optical frequency comb is generated by an electro-optic modulator enclosed in a Fabry–Perot cavity that is injected by the fundamental frequency stabilized laser. Frequency stability better than 2 kHz has been demonstrated on time scales between 1000 s and 2 days both at 1560 nm, twice the rubidium wavelength, and for a comb line at 1557 nm.
  • Publication
    Accès libre
    Methods and evaluation of frequency aging in distributed-feedback laser diodes for rubidium atomic clocks
    Distributed-feedback laser diodes emitting at 780nm have been evaluated, with respect to the aging of the injection current required for reaching the rubidium D2 resonance line. Results obtained for lasers operating in air and in vacuum for 9 months are reported. When operated at constant temperature, the laser current required for emission at the wavelength of the desired atomic resonance is found to decrease by 50 to 80 uA per month. The impact of this result on the lifetime and long-term performances of laser-pumped rubidium atomic clocks is discussed. © 2011 Optical Society of America OCIS codes: 140.2020, 350.4800.
  • Publication
    Accès libre
    Evaluation of the frequency stability of a VCSEL locked to a micro-fabricated Rubidium vapour cell
    (2010)
    Di Francesco, Joab F.
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    ; ; ; ; ;
    Salvadé, Y.
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    de Rooij, Nicolaas F.
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    Petremand, Y.
    We present our evaluation of a compact laser system made of a 795 nm VCSEL locked to the Rubidium absorption line of a micro-fabricated absorption cell. The spectrum of the VCSEL was characterised, including its RIN, FM noise and line-width. We optimised the signal-to-noise ratio and determined the frequency shifts versus the cell temperature and the incident optical power. The frequency stability of the laser (Allan deviation) was measured using a high-resolution wavemeter and an ECDL-based reference. Our results show that a fractional instability of ≥ 10-9 may be reached at any timescale between 1 and 100'000 s. The MEMS cell was realised by dispensing the Rubidium in a glass-Silicon preform which was then, sealed by anodic bonding. The overall thickness of the reference cell is 1.5 mm. No buffer gas was added. The potential applications of this compact and low-consumption system range from optical interferometers to basic laser spectroscopy. It is particularly attractive for mobile and space instruments where stable and accurate wavelength references are needed.
  • Publication
    Accès libre
    Frequency of Boundary-Layer-Top Fluctuations in Convective and Stable Conditions Using Laser Remote Sensing
    (2010)
    Martucci, Giovanni
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    Mitev, Valentin
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    Richner, Hans
    The planetary boundary-layer (PBL) height is determined with high temporal and altitude resolution from lidar backscatter profiles. Then, the frequencies of daytime thermal updrafts and downdrafts and of nighttime gravity waves are obtained applying a fast Fourier transform on the temporal fluctuation of the PBL height. The principal frequency components of each spectrum are related to the dominant processes occurring at the daytime and nighttime PBL top. Two groups of cases are selected for the study: one group combines daytime cases, measured in weak horizontal wind conditions and dominated by convection. The cases show higher updraft and downdraft frequencies for the shallow, convective boundary layer and lower frequencies for a deep PBL. For cases characterized by strong horizontal winds, the frequencies directly depend on the wind speed. The temporal variation of the PBL height is determined also in the likely presence of lee waves. For nighttime cases, the main frequency components in the spectra do not show a real correlation with the nocturnal PBL height. Altitude fluctuations of the top of the nocturnal boundary layer are observed even though the boundary layer is statically stable. These oscillations are associated with the wind shear effect and with buoyancy waves at the PBL top.
  • Publication
    Accès libre
    Laser offset-frequency locking up to 20 GHz using a low-frequency electrical filter technique
    A simple, easy-to-implement, and robust technique is reported to offset lock two semiconductor lasers with a frequency difference easily adjustable up to a couple of tens of gigahertz (10 and 19 GHz experimentally demonstrated). The proposed scheme essentially makes use of low-frequency control electronics and may be implemented with any type of single mode semiconductor laser, without any requirement for the laser linewidth. The technique is shown to be very similar to the wavelength modulation spectroscopy method commonly used for laser stabilization onto molecular absorption lines, as demonstrated by experimental results obtained using 935 nm laser diodes.
  • Publication
    Accès libre
    Unprecedented evidence for deep convection hydrating the tropical stratosphere
    (2008)
    Corti, T.
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    Luo, B. P.
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    de Reus, M.
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    Brunner, D.
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    Cairo, F.
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    Mahoney, M. J.
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    Martucci, G.
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    Mitev, Valentin
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    dos Santos, F. H.
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    Schiller, C.
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    Shur, G.
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    Sitnikov, N. M.
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    Spelten, N.
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    Vössing, H. J.
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    Borrmann, S.
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    Peter, T.
    We report on in situ and remote sensing measurements of ice particles in the tropical stratosphere found during the Geophysica campaigns TROCCINOX and SCOUT-O3. We show that the deep convective systems penetrated the stratosphere and deposited ice particles at altitudes reaching 420 K potential temperature. These convective events had a hydrating effect on the lower tropical stratosphere due to evaporation of the ice particles. In contrast, there were no signs of convectively induced dehydration in the stratosphere.
  • Publication
    Accès libre
    Morphology of the tropopause layer and lower stratosphere above a tropical cyclone: a case study on cyclone Davina (1999)
    (2008)
    Cairo, F.
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    Buontempo, C.
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    MacKenzie, A. R.
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    Schiller, C.
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    Volk, C. M.
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    Adriani, A.
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    Mitev, Valentin
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    Di Donfrancesco, G.
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    Oulanovsky, A.
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    Ravegnani, F.
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    Yushkov, V.
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    Snels, M.
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    Cagnazzo, C.
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    Stefanutti, L.
    During the APE-THESEO mission in the Indian Ocean the Myasishchev Design Bureau stratospheric research aircraft M55 Geophysica performed a flight over and within the inner core region of tropical cyclone Davina. Measurements of total water, water vapour, temperature, aerosol backscattering, ozone and tracers were made and are discussed here in comparison with the averages of those quantities acquired during the campaign time frame.
    Temperature anomalies in the tropical tropopause layer (TTL), warmer than average in the lower part and colder than average in the upper TTL were observed. Ozone was strongly reduced compared to its average value, and thick cirrus decks were present up to the cold point, sometimes topped by a layer of very dry air. Evidence for meridional transport of trace gases in the stratosphere above the cyclone was observed and perturbed water distribution in the TTL was documented. The paper discuss possible processes of dehydration induced by the cirrus forming above the cyclone, and change in the chemical tracer and water distribution in the lower stratosphere 400–430 K due to meridional transport from the mid-latitudes and link with Davina. Moreover it compares the data prior and after the cyclone passage to discuss its actual impact on the atmospheric chemistry and thermodynamics.
  • Publication
    Accès libre
    Comparison between Backscatter Lidar and Radiosonde Measurements of the Diurnal and Nocturnal Stratification in the Lower Troposphere
    (2007)
    Martucci, Giovanni
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    Mitev, Valentin
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    Richner, Hans
    A collection of boundary layer heights has been derived from measurements performed by a ground-based backscatter lidar in Neuchâtel, Switzerland (47.000°N, 6.967°E, 485 m ASL). A dataset of 98 cases have been collected during 2 yr. From these data, 61 are noon and 37 are midnight cases. The following two different schemes were used to retrieve the mixed layer depth and the height of the residual layer from the measurements: the gradient and variance methods. The obtained values were compared with those derived from the potential temperature profiles as computed from radiosonde data. For nocturnal cases, the height of the first aerosol layer above the residual layer was also compared to the corresponding potential temperature value. Correlation coefficients between lidar and radiosonde in both convective and stable conditions are between 0.88 and 0.97.