Experimental study of volume speckle in four-wave mixing arrangement

Abstract

We study a high frequency modulated speckle pattern in a four wave mixing arrangement. The three-dimensional nature o f the speckles is considered in the phase conjugate reflectivity evaluation which implies to analyze its depen dence 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 o f the imaging system that generates the subjective speckle beam. waves through scattering media such as the at mosphere.The first experience of phase conjugation in a photorefractive material was done by Huignard et al. [4]. Applications of phase conjugation to nondestructive testing, image processing, and imaging through phase disturbing media have been already demonstrated [5 9]. In [10] a real time double exposure holographic interferometric method of testing phase object by using degenerate four-wave mixing DFWM with photorefractive Bii2GeC>20 (BGO) crystals has been reported.As it is well known, when a diffusing surface is illuminated by coherent light, speckles appear in front of the surface. The speckle patterns have important applications in the field of metrology

Influence of the volume speckle on fiber specklegram sensors based on four-wave mixing in photorefractive materials

“…The reduction of the pupil aperture diameter produces a reduction of the speckle intensity beam too. However, the effect was observed even without controlling the probe beam ratio of the incident intensities [18]. This behavior allowed recovering the autocorrelation functions for fringe patterns associated to displacements that previously led to decorrelation.…”

“…In this way, the recording of a speckle pattern in a volume media such as photorefractive crystals results to be useful to extract as much information as possible from the tridimensional characteristics of the speckles. In previous papers, the diffraction properties of volume gratings of spatially modulated speckle produced by a diffuser element and recorded in photorefractive crystals of the sillenite family have been analyzed [14][15][16][17][18][19]. In this work, we take advantage of the high photosensitivity and carrier mobility of the sillenite crystal family [20,21] to achieve a real time double exposure holographic FSS by using a four-wave mixing (FWM) arrangement in a BSO (Bi 12 SiO 20 ) crystal.…”

Improvement of the dynamic range of a fiber specklegram sensor based on volume speckle recording in photorefractive materials

“…In principle, this statistical behavior must be taken into account. In several works [18][19][20][21][22][23][24][25] it has been shown that Kogelnik's theory [31] can adequately predict the experimental behavior of the diffraction efficiency for specklegrams in photorefractive materials if, as a first approximation, an average light modulation is considered, and if the interaction length, i.e. the crystal thickness, is replaced by the average of the speckle length when the latter is shorter than the crystal depth.…”

“…The imaged speckle pattern and a reference beam were mixed in a 10 mm  10 mm  10 mm BSO crystal. The average size of the speckle grains was controlled varying a pupil aperture adjacent to the lens of the speckle imaging system [17][18][19][20][21][22][23][24][25]. For a particular pupil aperture, several fringe patterns were obtained for different micro displacements of the fiber output end.…”