S. Avrillier

High stability CW waveguide CO 2 laser for high resolution saturation spectroscopy

We describe a stable waveguide CO 2 laser for spectroscopic applications. The short term spectral width is less than 10 kHz and a 10-12flicker floor is reached for the Allan variance. A study of the oscillation frequency versus resonator length demonstrates the major influence of the anomalous index of refraction of the medium. Saturation resonances with a 20 kHz peak-to-peak width have been obtained with a very good signal-to-noise ratio. A complete spectrum of SF 6 for P (16) at 947.74 cm-1is given as an example to illustrate the 500 MHz bandwidth of the laser.

Supersonic beam spectroscopy of low J transitions of the ν3 band of SF6: Rabi oscillations and adiabatic rapid passage with a CW laser

Abstract The P(3) and P(4) manifolds of the ν3 band of SF6 have been observed in a supersonic beam with a bolometric detection. The influence of the laser beam divergence on the excitation efficiency has been studied. Rabi oscillations are observed when the wavefront is flat in the interaction region whereas only adiabatic rapid passage occurs when the molecules see a curved wavefront.

Real time optical coefficients evaluation from time and space resolved reflectance measurements in biological tissues

Quantitative and accurate determination of turbid media optical coefficients was achieved using time and space resolved measurements of diffuse backscattered light. The global analysis of the reflectance map registered on a streak camera was based on the diffusion approximation. This fast and practical method was tested on suspensions of latex microspheres in water, and the results were in good agreement with the Mie theory. Experiments were then performed on biological tissues. The total time required for the determination of the optical coefficients of a sample, essentially limited by the instrument integration time, consisted of a few seconds.

A new and easy way to perform time-resolved measurements of the light scattered by a turbid medium

We demonstrate that a low cost interferometric set-up and the analysis of the speckle fluctuations due to a wavelength modulated continuous source can be used to perform time-resolved measurements of the light scattered by a turbid medium. This method was tested on a solid scattering media with known optical coefficients. Our experimental results are in good agreement with Monte Carlo simulations.

Light propagation near turbid–turbid planar interfaces

The aim of this paper is to solve the diffusion equation in the case of two semi-infinite homogeneous scattering and absorbing media separated by a flat interface when a light source S is placed in one of these media. We present here a new approach which allows us to work directly in real space and to establish efficient analytical expressions for space and time resolved average intensity distributions in both media. The results obtained by this method are compared with Monte Carlo simulations. This new approach permits very efficient numerical processing, which is a decisive criterion for the study of light propagation in layered turbid media.

High Resolution Saturation Spectroscopy with CO2 Lasers. Application to the v3 Bands of SF6 and OsO4

Saturation spectroscopy of vibration-rotation transitions started in 1967 with the first observations of molecular Lamb dips in H2O and CO2 lasers [1, 2]. One year later the inverted Lamb dip technique of LEE and SKOLNICK was beautifully applied to a vibration-rotation line of CH4 by J.L. HALL and R. BARGER [3]. With the large output power available from CO2 lasers it appeared very quickly that the absorbing gas could be removed outside the laser resonator with a much greater flexibility in the experiments. For example it became possible to use the saturation chopper method in which only the signal corresponding to the non-linear resonances was amplified and recorded [4]. Many spectra corresponding to numerous molecules were obtained by this technique but most experiments used SF6 as the absorbing gas.