8 research outputs found
Lasers for coherent optical satellite links with large dynamics
We present the experimental realization of a laser system for ground to
satellite optical Doppler ranging at the atmospheric turbulence limit. Such a
system needs to display good frequency stability (a few parts in 10^{-14})
whilst allowing large and well controlled frequency sweeps of +/- 12 GHz at
rates exceeding 100 MHz/s. Furthermore it needs to be sufficiently compact and
robust for transportation to different astronomical observation sites where it
is to be interfaced with satellite ranging telescopes. We demonstrate that our
system fulfills those requirements and should therefore allow operation of
ground to low Earth orbit satellite coherent optical links limited only by
atmospheric turbulence
Determination of the Boltzmann constant by laser spectroscopy as a basis for future measurements of the thermodynamic temperature
In this paper, we present the latest results on the measurement of the
Boltzmann constant kB, by laser spectroscopy of ammonia at 10 ?m. The Doppler
absorption profile of a ro-vibrational line of an NH3 gas sample at thermal and
pressure equilibrium is measured as accurately as possible. The absorption cell
is placed inside a large 1m3 thermostat filled with an ice-water mixture, which
sets the temperature very close to 273.15 K. Analysing this profile, which is
related to the Maxwell-Boltzmann molecular speed distribution, leads to a
determination of the Boltzmann constant via a measurement of the Doppler width
(proportional tosqrt(kBT)). A spectroscopic determination of the Boltzmann
constant with an uncertainty as low as 37 ppm is obtained. Recent improvements
with a new passive thermostat lead to a temperature accuracy, stability and
homogeneity of the absorption cell better than 1 ppm over a day
A coherent optical link through the turbulent atmosphere
We describe the realization of a 5 km free space coherent optical link
through the turbulent atmosphere between a telescope and a ground target. We
present the phase noise of the link, limited mainly by atmospheric turbulence
and mechanical vibrations of the telescope and the target. We discuss the
implications of our results for applications, with particular emphasis on
optical Doppler ranging to satellites and long distance frequency transfer.Comment: version 2, modified following comments from colleagues and reviewer
Measurement of the Boltzmann constant by the Doppler broadening technique at a 3,8x10-5 accuracy level
In this paper, we describe an experiment performed at the Laboratoire de
Physique des Lasers and dedicated to an optical measurement of the Boltzmann
constant. With the proposed innovative technique, determining comes down to an
ordinary frequency measurement. The method consists in measuring as accurately
as possible the Doppler absorption profile of a rovibrational line of ammonia
in thermal equilibrium. This profile is related to the Maxwell-Boltzmann
molecular velocity distribution along the laser beam. A fit of the absorption
line shape leads to a determination of the Doppler width proportional to
sqrt(kT) and thus to a determination of the Boltzmann constant. The laser
source is an ultra-stable CO2 laser with a wavelength . The absorption cell is
placed in a thermostat keeping the temperature at 273.15 K within 1.4 mK. We
were able to measure with a relative uncertainty as small as 3.8x10-5, which
represents an improvement of an order of magnitude for an integration time
comparable to our previous measurement published in 2007 [1
First direct determination of the Boltzmann constant by an optical method
International audienceWe have recorded the Doppler profile of a well-isolated rovibrational line in the ν2 band of 14NH3. Ammonia gas was placed in an absorption cell thermalized by a water-ice bath. By extrapolating to zero pressure, we have deduced the Doppler width which gives a first measurement of the Boltzmann constant, kB, by laser spectroscopy. A relative uncertainty of 2x10-4 has been obtained. The present determination should be significantly improved in the near future and contribute to a new definition of the kelvin
HCOOH high resolution spectroscopy in the 9.18m region
We report on higly accurate absolute frequency measurement against a femtosecond frequency comb of 6 saturated absorption lines of formic acid (HCOOH) with an accuracy of 1~kHz. We also report the frequency measurement of 17 other lines with an accuracy of 2~kHz. Those lines are in quasi coincidence with the 9R(36) to 9R(42) CO laser emission lines and are probed either by a CO or a widely tunable quantum cascade laser phase locked to a master CO laser. The relative stability of two HCOOH stabilized lasers is characterized by a relative Allan deviation of 4.5~10~. They give suitable frequency references for H Doppler free two-photon spectroscopy
Time Transfer by Laser Link - T2L2: Current status and future experiments
International audienceT2L2 (Time Transfer by Laser Link), developed by both CNES and OCA permits the synchronization of remote ultra stable clocks over intercontinental distances. The principle is derived from laser telemetry technology with dedicated space equipment deigned to record arrival time of laser pulses at the satellite. Using laser pulses instead of radio frequency signals, T2L2 permits to realize some links between distant clocks with a time stability of a few picoseconds and accuracy better than 100 ps. The T2L2 space instrument is in operation onboard the satellite Jason 2 since June 2008. Several campaigns were done to demonstrate both the ultimate time accuracy and time stability capabilities. It includes some experiments implemented in co-location to directly compare T2L2 time transfer residuals with the direct link between stations, and some ground to ground time transfer between ultra stable clocks. Important works have been done, between OCA and OP, to accurately compare T2L2 with microwave time transfer GPS and TWSTFT. These comparisons are based on laser station calibrations with a dedicated T2L2 calibration station designed to accurately set the optical reference of the laser station within the PPS reference of the microwave systems. Other experiments are also planned in the future: 3D localization with the lunar space vehicle LRO, T2L2 coverage extension over the Pacific Ocean (Tahiti), DORIS comparison and a third international campaign