R. V. Vinu

Recovery of complex valued objects from two-point intensity correlation measurement

We propose and experimentally demonstrate a technique for the recovery of the wavefront from spatially fluctuating fields using the two point intensity correlation, i.e., fourth order correlation. Assuming spatial ergodicity and Gaussian statistics for the speckle field, we connect the fourth order correlation to the modulus of the corresponding second order correlation. The idea is to retrieve the complex coherence function and consequently the wavefront using the off-axis holography. Application of this technique is demonstrated in the reconstruction of complex field of the object lying behind a weak scatterer. Experimental results of recovery of the complex field of phase objects “vortices” with three values of topological charges are presented.

Generation of spatial coherence comb using Dammann grating

A new technique to generate a spatially varying coherence field, such as a coherence comb using a Dammann grating, is proposed and experimentally demonstrated. The principle of the technique lies with the vectorial van Cittert-Zernike theorem, which connects vectorial source structure with the coherence-polarization of the light. The Dammann grating is encoded into one of the polarization components of the light to shape the vectorial source structure and, consequently, the coherence-polarization of the light. Experimental results on the generation of a spatial coherence comb by the Dammann grating are presented for different orders.

Polarization modulation for imaging behind the scattering medium

We propose and experimentally demonstrate a technique, based on polarization modulation, for imaging of the polarization discriminating object hidden behind a scattering medium. This is realized by making use of the relation between the complex correlation function of the randomly scattered orthogonal polarization components in the far field and polarized source structure at the scattering plane. Full use of a polarimetric parameter at the scattering plane is realized in the object plane reconstruction behind the scattering medium using a backpropagation approach. To demonstrate application of the technique, imaging of two different objects lying behind the scattering media is presented.

Experimental determination of generalized Stokes parameters

A new technique to determine generalized Stokes parameters by making use of the speckle holographic technique and the two-point intensity correlation is proposed and experimentally demonstrated. Assuming Gaussian statistics and spatial stationarity, a speckle hologram is generated prior to the fourth-order correlation, i.e., the two-point intensity correlation. This measurement technique offers a complete retrieval of complex generalized Stokes parameters. The application of the proposed technique is experimentally demonstrated for two different, random source structures.

Synthesis of statistical properties of a randomly fluctuating polarized field.

An experimental technique for the synthesis of statistical properties of a randomly fluctuating polarized field is investigated and experimentally demonstrated. The technique offers the controlled synthesis of coherence and polarization and subsequent analysis of the synthesized field is carried out by making use of two-point intensity correlation and the speckle holographic technique. The controlled synthesis is achieved by using an aperture of specific size at the source plane and generating a vortex in one of the orthogonal polarization components of the polarized field, thereby producing a singularity in off-diagonal elements of the coherence-polarization matrix.

Holographic imaging through a scattering layer using speckle interferometry

Optical imaging through complex scattering media is one of the major technical challenges with important applications in many research fields, ranging from biomedical imaging to astronomical telescopy to spatially multiplexed optical communications. Various approaches for imaging through a turbid layer have been recently proposed that exploit the advantage of object information encoded in correlations of the random optical fields. Here we propose and experimentally demonstrate an alternative approach for single-shot imaging of objects hidden behind an opaque scattering layer. The proposed technique relies on retrieving the interference fringes projected behind the scattering medium, which leads to complex field reconstruction, from far-field laser speckle interferometry with two-point intensity correlation measurement. We demonstrate that under suitable conditions, it is possible to perform imaging to reconstruct the complex amplitude of objects situated at different depths.

Compressive correlation holography

We propose and demonstrate a compressive sensing (CS) framework for correlation holography. This is accomplished by adopting the principle of compressive sensing and thresholding in the two-point intensity correlation. The measurement matrix and the sensing matrix that are required for applying the CS framework here are systematically extracted from the random illuminations of the laser speckle data. Reconstruction results using CS, CS with thresholding, and intensity correlation are compared. Our study reveals that liminal CS requires far fewer samples for the reconstruction of the hologram and has wide application in image reconstruction.

Investigation of transverse motion of concealed scatterer using correlation of speckled speckles from static surface layer

Speckle-based techniques have noteworthy applications in the field of material science, surface characterization, determining mechanical displacements, biological activity in diffuse layer, imaging through turbid layer etc. The passage of coherent light through a diffuse layer generates a random speckle pattern, which have the inherent feature of carrying information associated with the diffuse layer. Dynamic laser speckle associated with the displacements of scattering surface has prominent impacts in the study of biological activity inside the diffuse layer. Investigations are progressing in the direction of exploring the dynamical properties associated with speckled speckles and its applications in imaging and characterization scenarios. In this work, we theoretically and experimentally study the dynamical properties of speckles through a static scattering layer using intensity correlation. The displacement (transverse or angular) produced in the concealed scatterer generates the dynamic speckle pattern which is observed through a second static diffuser. We expect to find applications of this investigation into the tracking objects hidden in the diffuse layer, measuring biological activity in diffuse layer, displacement measurements, etc.

Effect of speckle pattern illumination on holographic recording and reconstruction

We present an experimental technique to investigate the effect of speckle pattern illumination on holographic recording and reconstruction. In this work, we apply speckle field illumination for digital holography and present our preliminary experimental results. The technique is applied for recording and reconstruction of the complete wavefronts and compare with conventional holographic approach. This technique is expected to play an important role in studying the polarization sensitive materials and opens up a new approach for holographic imaging with high field of view for polarization objects.

Anisotropy imaging using polarization and angular multiplexing

Phase contrast x-ray imaging techniques have shown the ability to overcome the weakness of the low sensitivity of conventional x-ray imaging. Among them, in-line phase contrast imaging, blessed with simplicity of arrangement, is deemed to be a promising technique in clinical application. To obtain quantitative information from in-line phase contrast images, numerous phase-retrieval techniques have been developed. The theories of these phase-retrieval techniques are mostly proposed on the basis of the ideal detector and the noise-free environment. However, in practice, both detector resolution and system noise would have impacts on the performance of these phase-retrieval methods. To assess the impacts of above-mentioned factors, we include the effects of Gaussian shaped detectors varying in the full width at half maximum (FWHM) and system noise at different levels into numerical simulations. The performance of the phase-retrieval methods under such conditions is evaluated by the root mean square error. The results demonstrate that an increase in the detector FWHM or noise level degrades the effect of phase retrieval, especially for objects in small size.