M. Bertolotti

Light scattered by a liquid crystal: a new quasi-thermal source.

The spatial and temporal coherence properties of laser light scattered by a liquid crystal cell under the application of an external dc field are studied. It is shown that the scattered field constitutes a source with very long coherence time. The coherence time can be changed by changing the value of the applied voltage, and the spatial coherence can be adjusted by varying the dimension of the illuminated spot on the cell. The effects produced when the length of correlation of fluctuations in the liquid crystal is not negligible with respect to the dimension of the spot in the cell are also studied. The scattered field is proposed as a new quasi-thermal source, alternative of the rotating ground glass disk.

Interferometric study of phase fluctuations of a laser beam through the turbulent atmosphere.

Phase fluctuations of a laser beam propagating through a turbulent atmosphere are studied as a function of the distance in a cross section of the beam. An interferometer is described that allows simultaneous measurements of phase fluctuations for pairs of rays that travel in the beam at different distances between each other. Experiments with this method have been performed in an urban center at a distance of 3.5 km and 0.5 km from the source (a helium-neon laser). The results of these measurements are described, and the behavior of the mean square fluctuation of the phase difference is studied. The values of the structure constant C(n) are calculated.

Ultrasonic modulation of light with a liquid crystal in the smectic‐A and nematic phases

An acoustic light modulator, using a liquid crystal (CBOOA) in nematic or smectic phase, is presented. It is shown that with good acoustical matching it is possible, in the nematic phase, to have a modulation of light from very low frequencies up to about 1 MHz, at the least. To the authors’ knowledge this is the first time that modulation has been observed at such high frequencies in the acoustic domain. This behavior is explained as a consequence of the elastic properties of the nematic. Some interesting results are also obtained which follow the behavior of the modulation vs temperature. In the smectic phase it is shown that it is possible to have a modulation only at very high frequencies, as one would expect from the elastic properties of this phase.

Statistical Properties of Light Scattered by Particles Suspended in a Turbulent Fluid

Second‐ and fourth‐order statistical properties of radiation scattered by particles in a turbulent fluid are theoretically investigated. They turn out to be of a non‐Gaussian type so that the measurements of the statistics of the scattered field can furnish information not obtainable by means of a spectrum analysis. In particular, our results are related with the possibility of measuring a Lagrangian type of velocity correlation function, generalized to different fluid elements.

Light scattering by particles suspended in a turbulent fluid

The spectrum of light scattered by particles suspended in a turbulent fluid is investigated theoretically.

On the Possibility of Measuring Optical Visibility by Using a Ruby Laser

The atmospheric attenuation of a ruby laser beam is studied. The possibility of using the laser back scattered radiation to probe optical visibility is investigated. Some experimental measurement support ing this possibility is reported.

Coherence of Light Passing Through a Nematic Liquid Crystal Under a dc Electric Field

A study of spatial coherence properties of light passing through a nematic liquid-crystal (MBBA) cell is given as a function of the external dc electric field applied to the cell and of initial molecular orientation and light polarization. Two different interferometric dispositions have been used. The molecular motion induced by the electric field is also studied.

Propagation properties of a multilayer optical waveguide with an attenuating sheet

A complex multilayer guide composed by a dielectric layer with a thin weakly attenuating sheet, embedded in a dielectric medium, has been theoretically investigated, treating also the limiting case in which the inner dielectric slab is absent. Computations for two typical cases of strong guiding are presented and discussed, from which appear the potentialities of the structure in mode selective guiding and modulation.

Influence of Thermal Turbulence in a Convective Ascending Stream on Phase Fluctuations of a Laser Beam

The effects of thermal turbulence on the phase fluctuations of a laser beam are investigated in laboratory. The turbulent region created by means of a horizontal heated Nichrome grid is made to shift upwards owing to the convective motion. A Mach-Zehnder interference experiment is performed in which two beams from a laser source are superimposed after crossing the turbulent region. The displacements of the fringe pattern allow one to study the temporal decay of the mean square refractive index fluctuation. An interpretation of the results is given on the basis of the theory of an isotropic turbulent scalar field.

Optical Processing of the Phase Correlation Induced by a Turbulent Medium in a Laser Beam

A general method which allows precise measurements of the frequency spectrum of a phase modulated wave down to a few cycles per second is presented. The momenta of the distribution of the instantaneous frequency (time derivative of the phase) can also be easily obtained. An experiment is described in which the study of the phase correlation induced by a laboratory generated thermal turbulence in a laser beam is described.