J. Calatroni

Transient deformation of liquid surfaces by laser-induced thermocapillarity.

Heavy hydrocarbons are irradiated with a Gaussian laser beam. The time-varying temperature distribution induced in the fluid gives rise to a surface-tension distribution. The later gives rise in turn to an accompanying liquid flow and surface-height distribution. In this paper, the time-varying shape of the induced depression is calculated as a function of the power distribution in the laser beam and of the thermal and mechanical properties of the material. The intensity distribution in the laser beam reflected from the depression is also calculated. Good agreement is found with previous experimental results.

Spectrally-resolved white-light interferometry as a profilometry tool

Phase-shifting interferometry and white-light interferometry are reliable techniques for surface analysis in which the optical path difference has to be changed by some transducer to evaluate the phase. We present here a different procedure in which optical path modulation is completely avoided. This technique is based on the spectral analysis of white-light interferograms. By means of a spectroscopic device, a non-visible interferogram is split into its monochromatic components and absolute, unambiguous values of the phase are obtained along the spectral axis. Only one interferogram is required to obtain the profile of one-dimensional surfaces with nanometric resolution.

Interferometric determination of the surface profile of a liquid heated by a laser beam

Abstract A liquid sample (heavy oil) is heated by an argon laser beam in oblique incidence. The sample takes the place of one of the mirrors in a Michelson interferometer. The resulting time-variations of the surface shape are determined by exploration with a He-Ne laser beam. This allows one to determine the mechanical and thermal properties of petroleum samples.

Visualization and measurement of a stationary thermal lens using spectrally resolved white light interferometry

Abstract The spatial profile of a stationary thermal lens induced by a cw low power laser beam in an absorbing dye solution is directly visualized in real time using spectrally resolved white light interferometry. Spatial characteristics of the lens are measured. Comparison between experimental results and thermal diffusion theory is presented. The accuracy of the measured refractive index in the thermal lens profile is about 10 −6 .

Refractive index distribution measurements by means of spectrally-resolved white-light interferometry

Abstract A new procedure to measure the spatial distribution of the refractive index in transparent media is presented. It is based on the spectral analysis of optical interferograms obtained from a wide, continuous-spectrum light source. The method yields fairly high precision (up to 10-8) in the measurements of local values of differential refractive index, Δn (Δn=n−nref), along a line in the sample. By means of a CCD TV-camera linked to a microcomputer, fast recording and automatic data processing are achieved. As an application, we present an experimental study of a thermal gradient in a liquid sample.

The stationary phase in spectrally resolved white-light interferometry as a refractometry tool

A new method for the analysis of the dispersion behaviour of a transparent medium is presented: the refractive index n is obtained as a function of the wavelength λ: n = n(λ). The procedure is based on the analysis of the hybrid bi-dimensional fringe pattern obtained at the exit plane of a spectrometer which performs the spectral analysis of a white-light interferogram. The phase of the signal depends both on a spatial coordinate and on the chromatic variable wavenumber σ = λ−1. Taking advantage of the dispersion behaviour of the sample, the phase of the signal can be forced to become stationary at certain points of this hybrid plane. The line which joins the stationary phase points stores the parameters of a series expansion of the refractive index as a function of wavenumber. These parameters are experimentally obtained through an appropriate numerical fitting procedure.

Transmission of a surface profile through a single optical fiber

Abstract An entirely optical method which allows the transmission of a 2-D surface through a single optical fiber is presented. The system performs a double encoding of the surface: in a first step a white light interferogram is used to store the relief. In a second step a chromatic encoding is used to introduce, in real time, the 2-D interferogram in a single multimode fiber. The number of transmitted pixels is limited by the luminosity of the image.

Transmission d’image en couleurs dans une seule fibre optique

Two-dimensional color image transmission through a single multimode fiber using only passive spectroscopic components (echelette type diffraction gratings) will be demonstrated experimentally and discussed theoretically with two different performances: (a) nine image pixels, seven distinct colors per pixel; (b) about forty image pixels and three colors per pixel. The present technical limits will be discussed.

Holographic analysis of dispersive pupils in space–time optics

Extension of space-time optics to objects whose transparency is a function of the temporal frequency nu = c/lambda is examined. Considering the effects of such stationary pupils on white light waves, they are called temporal pupils. It is shown that simultaneous encoding both in the space and time frequency domains is required to record pupil parameters. The space-time impulse response and transfer functions are calculated for a dispersive nonabsorbent material. An experimental method providing holographic recording of the dispersion curve of any transparent material is presented.

Multi-channelled white-light interferometry for real-time dispersion measurements

Abstract A white-light source, a two-wave interferometer and a spectrometer are associated in order to analyze the dispersion (refractive index as a function of wavelength) of a prismatic sample. This set-up behaves like a multi-channelled device: simultaneous monochromatic interferograms are stored in a single bi-dimensional interferogram in the hybrid spatial–spectral domain. Real-time, direct determination of refractive index at every resolved wavelength is provided with high precision (up to 10 −6 ).