Estudio de un conversor óptico fotorrefractivo y sus aplicaciones

Objetivos de la tesis: - estudiar el registro y reconstrucción de distribuciones unidimensionales (redes de Ronchi) de baja frecuencia, registradas incoherentemente en un conversor óptico fotorrefractivo, sin soporte holográfico y en configuración transversal. En particular, se pretende caracterizar el estado de polarización del haz de lectura producido por la modulación de birrefríngencia inducida en el medio. El interés, en este estudio, es controlar el contraste y así optimizar la imagen reconstruida a la salida del sistema. - implementar los efectos Talbot y Lau a partir de una red registrada en un cristal fotorrefractivo. Esto ha permitido visualizar estos efectos con características novedosas, propias de la naturaleza del registro y en esta modalidad efectuar un tratamiento unificado de los fenómenos analizados. A su vez, estas características han motivado la implementación fotorrefractiva de los interferómetros Talbot y Lau.Doctor en Físic

Experimental study of volume speckle in four-wave mixing arrangement

We study a high frequency modulated speckle pattern in a four-wave mixing arrangement. The three-dimensional nature of the speckles is considered in the phase conjugate reflectivity evaluation which implies to analyze its dependence on the average speckle size. Also, the reflectivity is analyzed in terms of the external applied field, the probe beam ratio, and the pupil aperture diameter of the imaging system that generates the subjective speckle beam

Modulated speckle simulations based on the random-walk model

The random-walk model is employed to simulate modulated speckle patterns. We demonstrate that the geometrical image approximation fails to describe the modulated speckle pattern. A new approach to analyzing this phenomenon is proposed. The validity of the approximations employed is verified by comparison of the simulation with the experimental results. Speckle metrological applications and phase measurement techniques could be improved by taking advantage of this model

Speckle photography with different pupils in a multiple-exposure scheme

The use of different multiple aperture pupils for recording each image in speckle photography is proposed. The introduction of suitable spatial frequency carriers, by internally modulating imaged speckles, allows one to selectively isolate or combine the spectral content of different images into spatially separated regions in the Fourier plane. Theoretical and experimental results extend the speckle photography technique to the depic tion of several specklegrams of multiple uniform in plane displacements. In this case, because different pu pils are considered for recording, the cross correlation functions for the amplitudes and intensities in the image plane are calculated on the basis of the statistical properties of the object. Also, the ensemble average inten sity in the Fourier plane is analytically derived, and fringe visibility is investigated

Phase-object analysis with a speckle interferometer

We describe the characteristics of a double-exposure specklegram obtained through a double-aperture system, by introduction of a wedge in front of one aperture in one exposure. It is assumed that a uniform displacement of the diffuser is produced between exposures. The average intensity distribution and visibility of the interferometric fringes in the Fourier plane are analyzed. An alternative interferometric technique for phase-object detection is proposed

Self-imaging through incoherent to coherent conversion

The self-imaging phenomenon is realized by utilizing an incoherent to coherent converter. For this purpose, a photorefractive BSO crystal that becomes uniformly birefringent due to an external applied voltage is employed. Then, an incoherently illuminated one-dimensional grating that locally modulates the induced birefringence is registered. In the readout process, a linearly polarized plane wave from a He-Ne laser is utilized. The ellipticity of the light exiting the crystal will be determined according to the local birefringence. Then, after passing through a polarizer, the coherent output will reproduce the incoherent input. In this way, under free propagation, coherent replicas of the grating will be obtained

Multiplexing encrypted data by using polarized light

We investigate the feasibility of multiplexing, employing polarized light, a set of security encrypted data. The encryption approach is based on the double random pure-phase enciphering method. Phase conjugation operation is conducted in the reconstruction stage with the aid of a photorefractive crystal which stores the encrypted information. When storing each encrypted image, a polarization change is introduced in the system. This induces decorrelation on the speckle patterns inside the storing medium. We apply this approach for multiple image encryption. We show experimental results that confirm our approach

Experimental study of volume speckle in four-wave mixing arrangement

We study a high frequency modulated speckle pattern in a four-wave mixing arrangement. The three-dimensional nature of the speckles is considered in the phase conjugate reflectivity evaluation which implies to analyze its dependence on the average speckle size. Also, the reflectivity is analyzed in terms of the external applied field, the probe beam ratio, and the pupil aperture diameter of the imaging system that generates the subjective speckle beam