Rafael Escalona

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.

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 .

Study of axial absorption in liquids by interferometry

Thermoconvective flow induced in a liquid sample by irradiation with a laser beam is studied experimentally. The optical technique employed is a standard interferometric one, but using a novel procedure for measurement of refractive index changes through phase determinations: the wavelet transform (WT). The recent application of WT in interferometry shows robustness, relative speed of calculus and excellent accuracy in phase determinations. Numerical simulations show these performances and confirm experimental results. Both experimental phase and temperature quantitative maps are shown.

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.

Double phase modulation in the hybrid spatial–chromatic domain as a refractometry tool

Abstract Double phase modulation in spatial and chromatic coordinates allows real-time analysis of refractive indexes. The experimental set-up consists of a two-beam interferometer associated to a spectroscopic device illuminated by a white light source. Relative phase stores (in real time) the differential refractive index in its second (crossed) derivative with respect to the chromatic and spatial coordinates. Double modulation applies both to dispersive and non-dispersive media; two different methods for data processing are proposed accordingly. Maximum precision is attained when non-dispersive samples are handled. The method itself suggests the appropriate technique for data processing. Experimental evidence is provided.

From Interferometry to Image Processing: Phase Measurement Vision Method for High Accuracy Position Sensing of Rigid Targets

In this paper, we present a transposition of phase measurements performed usually in interferometry to image processing and intelligent vision for high accuracy position and displacement sensing of moving targets. In that application, fringes are no more generated by some light interaction but result from the observation of adapted strip set patterns directly printed on the target of interest. The moving target is simply observed by a static conventional vision system and usual phase computation algorithms are adapted to that image processing context, in order to sense target position and displacements with a high accuracy. The signal to noise ratio achievable with a standard CCD camera allows a phase resolution equivalent to a lateral resolution of about 10-3 strip pattern period or 10-2 CCD pixel in the image plane. The implementation of the method can be easily adjusted to the desired range of resolution by changing the scale of the strip set patterns as well as the magnification of the vision system. X and Y resolutions of 1 µm were demonstrated for a field of observation of several cm2 with a strip set period of 500 µm. Lateral resolutions of a few manometers are expected from the same experiment based on a strip set period of 2 µm, still observable with diffractive optics. The development of a modified configuration is also carried out for 3D position and displacement sensing. Various applications can be thought, for instance in microrobotics or in nanotechnology but also the position control of large size objects. The main limitations appeared until now are due to image distortions introduced by the vision system.

Absolute distance determination by coherent detection using a frequency-modulated laser diode

An interferometric device for measuring absolute distance is described. A frequency-modulated laser diode is used as the light source. At the output of a Michelson interferometer a beat signal is detected-a process which may be recognized as coherent detection. The optical path difference is determined by numerical phase analysis. Experimental results are presented. The feasability of measuring absolute distance is shown with a submilimeter resolution over a two meter dynamic range.

Frequency to Voltage Converter as a Phase Controller in Phase Shifting Interference Microscopy

A new system for the control of a piezoelectric device is introduced. It works as a phase shifter in an interference optical microscope. This circuit translates controllable constant frequency signals, generated with a standard sound card, to electrical signals with constant voltage through a frequency-to-voltage converter; this full analog conversion allows for producing voltage steps of 16 µv, which is a better resolution than a commercial digital to analog converter working (in the desired voltage range), and hence, results in an arbitrary phase uncertainty under 3%. The calibration method allows for verification of the displacements steadfastness in the arbitrary phase, and for diagnosing of errors that could alter the experimental interferograms. To check the system calibration, it was experimentally and successfully tested on a silicon surface with rectangular holes of known depth.

Adaptive control system for improvement of contrast in interferograms

The present work consists on the development of an adaptive control system to compensate the reduction of the contrast in interferograms caused by mechanical vibrations present in a Mirau based interference microscopy system. The control generates random signals that are injected during the integration time of the camera through a piezoelectric device in order to change the phase of the interferometer. Each obtained image has different control signals injected during the integration time. The contrast is evaluated and if the contrast is improved, the signal injected is adjusted to seek for a better improvement. The best signals collected are added to the control signal, which is applied to the system after the adaptation process is over. The control scheme implemented is capable of finding signals to compensate the main frequency noisy components of the system.

Quantitative phase microscopy for the study of bone cells on multiple roughness surfaces

We present a phase shifting interferometry system for the study of the early adhesion process for osteoblast-like cells, through an interference microscope. Optical phase maps from the cells are obtained experimentally as a function of cell adhesion time. The process is carried out on surfaces of metallic materials relevant to the development of bone implants. The surfaces were subjected to various levels of mechanical polishing and their roughness was measured using the same experimental technique mentioned before. Morphological changes of the cell can be measured over their optical phase maps while the cell adhesion process is accomplished. The experimental technique shows a suitable feature as to the observed time scale, and also shows a high stockiness and precision for the determination of the optical phase.