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International audience; This paper presents the development, the operating principle, and the experimental results of an innovative technique for the measurement of acoustic pressures in the range of 40 mHz up to 5 Hz. This new measurement technique is based on the use of a Fabry Perot refractometer in which the air density variations related to an acoustic wave can be tracked and determined by measuring the optical frequency variations of a laser locked on a longitudinal mode of the Fabry Perot cavity. This approach covers a wider range in the low and infrasonic frequencies compared to conventional sensors.

We present our first results in the realization of an absolute distance meter that will bring well-defined and metrologically traceable measurements for distances of several kilometers. The purpose is to achieve an accuracy of 1 mm up to 5 km and beyond. To this end, the developed prototype operates at two different wavelengths, 780 nm and 1560 nm. For each of these wavelengths, the measured distance is determined from the phase accumulated by a radio-frequency carrier propagated in air by a laser beam modulated in intensity. Thus, using the dispersion relation between these two measurements, a compensation of the refractive index of air can be applied, i.e. no measurement of temperature, pressure and CO2 content of the air is required. In this paper, the principle of the two-wavelength absolute distance meter is explained, and preliminary results are presented, for each wavelength and with air refractive index compensation.

International audience; The accuracy of length measurements performed in harsh industrial environments by optical systems, such as laser trackers and absolute distance meters, for the dimensional inspection of large mechanical parts, strongly depends on the air refractive index. The latter is usually calculated as a function of ambient conditions i.e. temperature, pressure, humidity and CO2 content using the Edlen formula. Its value should be carefully controlled during length measurements. In industrial environments, monitoring all these parameters, especially temperature, with sufficient accuracy can be most difficult. Indeed, the industrial workspaces can be subjected to temperature gradients of few degrees, and an uncertainty of 1°C in the temperature estimation leads to an uncertainty of 1 μm/m in distances measured optically. The aim of this work is to develop and optimize an acoustic thermometer able to perform real-time measurements of the average air temperature along an optical pathway. This apparatus is destined to be integrated into a laser absolute distance meter to improve its performances. The acoustic thermometer presented in this paper is based on the measurement of the time of flight of acoustic pulses between two measuring heads. The requirements for such a thermometer, for measurements in industrial environments, make the technical choices and practical realization challenging. Indeed, the working frequency range should be in the ultrasonic region, which implies a strong attenuation of the acoustic waves in air and correspondingly a low signal-to-noise ratio. Moreover, the whole measurement system should remain compact, with wireless communication between the measuring heads.

International audience; Accurate absolute distance measurements over several kilometres are of great interest for several applications such as the surveying of geological faults or of large sites: dams, colliders, tunnels, etc… In this framework, we have developed a new telemeter: Arpent.

We will present at the workshop the development of a telemeter based on an intensity-modulated laser diode. It has the advantage to be more robust than classical fringe counting interferometers, especially for long-distance outdoor propagation, and less expensive than superheterodyne synthetic-wavelength interferometers [1]. For such a system, we aim a relative resolution of 10-7, i.e. some micrometers at short range or 100 ?m over 1 km.

International audience

International audience; Ce papier présente la caractérisation du couplage amplitude phase ayant lieu dans un système de réception optique (photodiode, mixeur et amplificateur). Il est montré qu?en jouant sur la tension de polarisation de la photodiode, il est possible d?annuler ce couplage.

We describe a transportable system based on synthetic wavelength interferometry for absolute distance measurement. The synthetic wavelength is generated using two frequency-doubled Nd:YAG lasers. Main feature is the elimination of polarization cross-talk issue using a setup where spatial separation is preferred to polarization separation. A superheterodyne technique has been implemented to detect the synthetic phase, enabling a fringe interpolation of ~2 pi/5600. A resolution of 700 nm over 60 s of integration time is demonstrated for a synthetic wavelength of 7,5 mm. An accuracy better than 10 μm is demonstrated over 25 m, based on an indoor comparison with a classical fringe counting interferometer at VSL (Netherlands)., article based on oral presentation at the conference MacroScale 2011 "Recent developments in traceable dimensional measurements", Bern-Wabern (Switzerland), 04-06, October, 2011

A spectrometer for ultra high-resolution spectroscopy of molecular iodine at wave length 501.7 nm, near the dissociation limit is described. Line shapes about 30 kHz wide (HWHM) were obtained using saturation spectroscopy in a pumped cell. The frequency of an Ar+ 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.