Agnaldo A. Freschi

Analyzing the Total Structural Intensity in Beams Using a Homodyne Laser Doppler Vibrometer

The total structural intensity in beams can be considered as composed of three types of waves: bending, longitudinal, and torsional. In passive and active control applications, it is useful to separate each of these components in order to evaluate their contribution to the total structural power flowing through the beam. In this paper, a twisted z-shaped beam is used in order to allow the three types of waves to propagate. The contributions of the structural intensity, due to these waves, are computed from measurements taken over the surface of the beam with a simple homodyne interferometric laser vibrometer. The optical sensor incorporates some polarizing optics, additional to a Michelson type interferometer, to generate two optical signals in quadrature, which are processed to display velocities and/or displacements. This optical processing scheme is used to remove the directional ambiguity from the velocity measurement and allows nearly all back-scattered light collected from the object to be detect. This paper investigates the performance of the laser vibrometer in the estimation of the different wave components. The results are validated by comparing the total structural intensity computed from the laser measurements, with the measured input power. Results computed from measurements using PVDF sensors are also shown, and compared with the non-intrusive laser measurements.

Formalismo óptico matricial e aplicações

The optical matrix formalism is developed by applying Snell law to describe the refraction of optical rays in a spherical surface. This formalism is then applied to find and justify typical parameters and expressions as focal distance, image condition and lateral magnification for a thin spherical lenses immerse in air. Also, the case of thin spherical lenses immersed in two media with different refraction index is studied, and the corresponding parameters and expressions are found.

Lei de indução de Faraday: Uma verificação experimental

Although there are many relatively simple demonstrations to illustrate Faraday’s law of induction, quantitative experiments on this subject are less common. This paper describes an experimental procedure suitable for use in a basic physics laboratory for investigating the validity of Faraday’s law. The described setup allows recording the electromotive force induced in a coil due to the passage of a neodymium magnet in free fall through it. The analysis of the induced pulses as a function of the speed of the magnet measured on the center of the coil show that the results are consistent with Faraday’s law. We also show that the distance between the voltage peaks induced in the coil is equal to the length of the magnet.

Phases of the Diffracted Waves and Energy Conservation

The energy conservation is used in an experiment in which two incident waves reach the grating at the symmetrical Littrow condition, to analyze the phases of the waves diffracted by a non-absorbing relief grating.

Analyzing the total structural intensity in beams using a homodyne laser Doppler vibrometer

The total structural intensity in beams can be considered as composed of three kinds of waves: bending, longitudinal, and torsional. In passive and active control applications, it is useful to separate each of these components in order to evaluate its contribution to the total structural intensity flowing through the beam. In this paper, a z-shaped beam is used in order to allow the three kinds of waves to propagate. The contributions of the structural intensity due to the three kinds of waves are computed from measurements made over the surface of the beam with a simple homodyne interferometric laser vibrometer. The optical sensor incorporates some additional polarizing optics to a Michelson type interferometer to generate two optical signals in quadrature, which are processed to display velocities and/or displacements. This optical processing scheme is used to remove the directional ambiguity from the velocity measurement and allows to detect nearly all backscattered light collected from the object. This paper investigates the performance of the laser vibrometer in the estimation of the different wave components. The results are validated by comparing the total structural intensity computed from the laser measurements with the measured input power. Results computed from measurements using PVDF sensors are also shown, and compared with the non-intrusive laser measurements.

Stabilized holographic setup for the real-time continuous measurement of surface vibrational mode patterns

We report the operation and performance of a time-averaged holographic interferometry setup for the measurement of vibrational mode patterns in a surface, using the He-Ne laser 632.8 nm wavelength line and a photorefractive Bi12TiO20 crystal as the recording medium. The photorefractive crystal allows the real-time reversible recording and display of the hologram without any (chemical or thermal) development processing. The particular light polarization features of this sillenite family crystal allow the display of vibrational mode patterns and at the same time enable to actively stabilize the holographic setup. Good quality images are obtained even using low power laser beams requiring a long exposure time in comparatively perturbated environment.