We report the recording of nearly 35% diffraction efficiency holograms of photorefractive nature in Bi2TeO5 crystals, using 633 nm wavelength laser beams. Holographic techniques showed that a slow and a fast holograms arise, the latter based on electrons and the former based on positively charged carriers. We also measured the quantum efficiency for photoelectron generation and the characteristic diffusion (LD) and Debye (ls) lengths of these holograms and verified that LD and ls are modified as the light intensity onto the crystal increases, in agreement with previously reported theoretical prediction and experimental results but on other photorefractive material.
Photo-electromotive-force effect induced by an oscillating speckle pattern of light onto photorefractive CdTe: V and
Bi12TiO20 is studied. The light pattern is produced by the reflected laser beam on a vibrating surface. As a result of the
photo-emf effect, a photocurrent signal in the material arises that is related to the vibrating surface amplitude and
frequency. This electric signal is first pre-amplified by an op-amp in transimpedance regime and then the first temporal
harmonic component is measured using a phase selective frequency tuned lock-in amplifier. This harmonic shows a
maximum value occurring for certain value of the normalized vibration amplitude. This technique is been developed to
be used for the measurement of transverse mechanical vibrations.
We report the use of photo-electromotive-force effect in a speckled pattern of light onto a photorefractive Bi12TiO20 crystal to measure the amplitude of transverse vibrations. Our theoretical model shows that, for vibrations much faster than the photorefractive response time, the photo-emf signal exhibits a maximum at a characteristic vibration amplitude value that may be used to calibrate the photo-emf sensor.
We report the selective erasure of a previously recorded 100% diffraction efficiency grating in a photorefractive lithium niobate crystal. Both the recording and the selective erasure are carried out using self-stabilized holographic recording techniques. The recording and the erasure are shown to be in excellent agreement with the proposed corresponding theoretical model. After erasure, a grating of less than 5% diffraction efficiency remains in place. Almost no sensible light scattering was detected.
Fringe-locked running holograms are produced in the volume of photorefractive crystals and the characteristic speed υ and diffraction efficiency η are simultaneously measured as a function of the externally applied electric field Eo. These data have the corresponding η versus υ and Eo three-dimensional theoretical function fit to them. From this fitting three relevant theoretical material's parameters (diffusion length LD, Debye screening length 1s and quantum efficiency Φ) and an experimental parameter (effective field coefficient inside the sample ξ) are univocally and straightforwardly obtained.
We report a mathematical formulation that successfully describes the holographic photocurrent produced in photo-emf experiments, with large oscillation amplitudes, in strongly absorbing photorefractive materials. The large amplitude produces a sensible enhancement of the photocurrent signal and in this way facilitates measurements. Accounting for bulk light absorption of the sample is essential in order to adequately describe the experiment. We measure pure and doped potorefractive Bi12TiO20 (BTO) crystals and show that these data are in excellent agreement with theory. From data fitting we are able to determine some material's parameters.
Feedback experiments by the Brazilian authors in lithium niobate [1, 2, 3] have shown that for a wide range of intensity ratios a grating with diffraction efficiency i = i can be achieved which seemingly does not change in time. Also holographic scattering is reduced in such experiments.
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.
We describe the latest improvements in holographic interferometry that enable the real-time measurement of vibrational modes and static deformations in surfaces using low power laser illumination and a photorefractive Bi12TiO20 crystal as the recording medium. An efficient setup has been developed where the most critical elements have been optimized: target illumination and backscattered light collection, distribution of light between the object and reference beams, and stabilized system operation. Experimental results for vibration and deformation measurements are reported.
We report the use of actively stabilized recording techniques to achieve 100% diffraction efficiency holograms in Fe-doped LiNbO3 crystals exhibiting a considerable reduction in the scattered light.
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.
We describe the use of a self-stabilized setup for timeaveraged holographic interlerometry recording of a vibrating surface in a perturbed environment. The hologram is recorded with the 632.8-nm wavelength of an He-Ne laser on a Bi12TiO20 crystal. The performance of the setup is discussed and experimentally evaluated.
We describe the use of a self-stabilized setup for time-average holographic interferometry recording of a vibrating surface in a perturbated environment. The hologram is recorded with the 632.8 nm wavelength of an He-Ne laser on a Bi12TiO20 crystal. The performance of the setup is discussed and experimentally evaluated.
Fringe-locking is a convenient technique for recording running holograms in photorefractive crystals (1) where the phase shift i between the recorded interference fringes and
the resulting hologram is adaptively fixed to ψ = 90°. ψ = 90° means that the real part
of the complex expression for the spatial charge arising electric fields modulation
Esc (2) is fixed to zero. This condition allows computing the expression for the hologram
velocity for different applied electric field E, and compare it to experimental data from
fringe-locking experiments.
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