Carlos Perez-Lopez

Interferometric measurement of a diffusion coefficient: comparison of two methods and uncertainty analysis

Two methods to measure the diffusion coefficient of a species in a liquid by optical interferometry were compared. The methods were tested on a 1.75 M NaCl aqueous solution diffusing into water at 26 °C. Results were D = 1.587 × 10−9 m2 s−1 with the first method and D = 1.602 × 10−9 m2 s−1 with the second method. Monte Carlo simulation was used to assess the possible dispersion of these results. The standard uncertainties were found to be of the order of 0.05 × 10−9 m2 s−1 with both methods. We found that the value of the diffusion coefficient obtained by either method is very sensitive to the magnification of the optical system, and that if diffusion is slow the measurement of time does not need to be very accurate.

Transient deformation measurement with electronic speckle pattern interferometry by use of a holographic optical element for spatial phase stepping

We have used a computer-generated holographic optical element (HOE) with electronic speckle pattern interferometry to calculate the interference phase corresponding to the deformation of a test object from a single TV frame. The HOE is a modified crossed phase grating that introduces a known phase change between the +/-1 diffracted orders, without being translated. The progressive propagation of transient mechanical waves was measured with an rms precision of 2pi/30.

Study of transient deformations with pulsed TV holography: application to crack detection

We report the measurement of reproducible transient deformations with single-pulsed-subtraction and double-pulsed-addition TV holography with both in-plane and out-of-plane optical configurations. An original synchronization system (controlling mechanical excitation of the object, firing the laser pulses, and image acquisition) used to obtain the subtraction and the addition correlation fringes is described. Experimental results are presented for the propagation of mechanical shock waves in a cantilever specimen and their subsequent diffraction by a crack in the specimen. These preliminary results show that the technique may be used for crack detection.

Very high speed cw digital holographic interferometry

It is reported for the first time the use of a very high speed camera in digital holographic interferometry with an out of plane sensitivity setup. The image plane holograms of a spherical latex balloon illuminated by a cw laser were acquired at a rate of 4000 frames per second, representing a time spacing between holograms of 250 microseconds, for 512 x 512 pixels at 8 bits resolution. Two types of tests were accomplished for a proof of principle of the technique, one with no constrains on the object which meant random movements due to non controlled environmental air currents, and the other with specific controlled conditions on the object. Results presented correspond to a random sample of sequential digital holograms, chosen from a 1 second exposure, individually Fourier processed in order to perform the usual comparison by subtraction between consecutive pairs thus obtaining the phase map of the object out of plane displacement, shown as a movie.

Transient deformation analysis by a carrier method of pulsed electronic speckle-shearing pattern interferometry

The introduction of a pulsed laser into an electronic speckle-shearing pattern interferometer allows high-speed transient deformations to be measured. We report on a computerized system that permits automatic data reduction by introducing carrier fringes through the translation of a diverging lens. The quantitative determination of the phase map that is due to deformation is carried out by the spatial synchronous detection method. Experimental results obtained for a metal plate transiently deformed by an electromagnetic hammer illustrate the advantages of the proposed system.

Pulsed digital holographic interferometry for dynamic measurement of rotating objects with an optical derotator

A method for measuring dynamic deformations of rotating objects with pulsed digital holography is described. An optical derotator is used to compensate for the rotation. A CCD camera is used to record two holograms with a short time separation (20 mus). Results of deformations between the recordings are obtained after subtraction of the phase distribution between the two digital holograms. Fringe phase maps of the phase subtraction of two holograms compensated by the derotator and recorded with a Q-switched double-pulsed ruby laser are presented. A flat disk and the blades of a fan were investigated. We used an optical arrangement that allowed us to improve laser illumination and energy efficiency. Experimental results on quantitative evaluation of dynamical out-of-plane deformations are presented.

Detection of surface strain by three-dimensional digital holography

Three-dimensional digital holography with three object-illuminating beams has been successfully used for the detection of surface strain in metallic objects. The optical setup that uses illuminating beams to irradiate the object from three directions means that all three object surface displacement components, x, y, and z, can be independently calculated and used to find the strain gradients on the surface. The results show the conversion of the complete surface displacement field into a surface strain field. The method is capable of measuring microstrains for out-of-plane surface displacements of less than 10 microm.

Low-frequency harmonic vibration analysis with double-pulsed addition electronic speckle pattern interferometry

We describe the control of a pulsed laser for measuring har- monic vibrations with double-pulsed addition electronic speckle pattern interferometry (ESPI). The basic requirement is to synchronize the laser pulses both to a known position of the object vibration cycle (to obtain a stationary fringe pattern) and to a charge coupled device (CCD) camera (to obtain one double pulse per field.) It is shown theoretically and ex- perimentally that at low object-vibration frequencies (typically ,500 Hz) these synchronization requirements can cause instabilities in the laser output. A synchronization scheme to overcome these problems is de- scribed and experimental results presented at several low object- vibration frequencies. Addition fringe visibility is improved in real-time with an electronic filter.

Fringe visibility enhancement and phase calculation in double-pulsed addition ESPI

Abstract We report the calculation of the optical interference phase directly from double-pulsed electronic speckle pattern interferometry (ESPI) addition fringes using phase-stepping techniques, for an object undergoing harmonic vibration. The optical phase is used to calculate the harmonic-vibration amplitude and phase. This involves shifting the laser double pulses with respect to the object-vibration phase. Two techniques to improve the addition fringe visibility have been proposed in the literature: subtraction of a reference interferogram recorded when the object is stationary from an addition fringe pattern, and subtraction of addition patterns. We present a mathematical analysis of these two techniques and comment on some misconceptions that have arisen in the literature. These two techniques offer less immunity to environmental disturbances than addition ESPI, but improve fringe visibility. Phase calculation for the latter case (subtraction of addition fringes) is presented.

Modal vibration analysis of a metal plate by using a laser vibrometer and the POD method

The proper orthogonal decomposition (POD), also known as Karhunen–Loeve expansion, is applied to the modal vibration analysis of a metal plate. The metal plate was subject to vibrations with an electrodynamical shaker in a range of frequencies from 100 to 5000 Hz. The data were obtained from the measurements with a laser vibrometer. The plate vibration measurements were used to calculate the eigenfunctions and the eigenvalues. It was found that a large fraction of the total energy of the vibrations is contained within the first four POD modes. The essential features of the vibrations are thus described by only the first four eigenfunctions.