arXiv subjects

Rakesh Kumar Singh

Publications and source records attributed to Rakesh Kumar Singh.

13 recordsLinked to original sources

arXiv2025

Statistics of Non-Rayleigh Speckles Generated from Nonlinear Media

We analytically derive an expression for a speckle field's intensity probability density function (PDF) in a nonlinear medium. The analytically driven results are in good agreement with the numerical outcomes. In a focusing nonlinear medium, the local intensity of the speckle is enhanced as manifested through the longer tail of the PDF. In contrast, the local intensity of speckle is reduced in the presence of a defocusing nonlinearity, and the tail of the probability density function also reduces. This change in local intensity of the speckles arises due to the cubic Kerr nonlinearity, which eventually modifies the second-order statistics. Hence, the intensity correlation is altered as per the nature of the associated nonlinearity while the field correlation remains invariant of both types of the nonlinear conditions.

physics.optics↗

arXiv2023

Low-coherence digital holographic microscope with Fizeau interferometer

We present a new digital holographic microscope (DHM) with a low coherent source for the quantitative imaging of smooth and optically rough objects. The experimental design of the microscope uses an in-line experimental geometry based on the Fizeau interferometer and shows the depth sectioning capability due to the limited longitudinal coherence of the source. A polarization-phase shifting approach is implemented to extract the quantitative and speckle-free image from the experimentally recorded interference fringes. To test and experimentally demonstrate the working of the proposed DHM, we present the results of the quantitative images for different objects.

physics.optics↗

arXiv2023

Randomness assisted in-line holography with deep learning

We propose and demonstrate a holographic imaging scheme exploiting random illuminations for recording hologram and then applying numerical reconstruction and twin removal. We use an in-line holographic geometry to record the hologram in terms of the second-order correlation and apply the numerical approach to reconstruct the recorded hologram. The twin image issue of the in-line holographic scheme is resolved by an unsupervised deep learning(DL) based method using an auto-encoder scheme. This strategy helps to reconstruct high-quality quantitative images in comparison to the conventional holography where the hologram is recorded in the intensity rather than the second-order intensity correlation. Experimental results are presented for two objects, and a comparison of the reconstruction quality is given between the conventional inline holography and the one obtained with the proposed technique.

physics.optics↗

arXiv2022

Reconstructing complex field through opaque scattering layer with structured light illumination

The wavefront is scrambled when coherent light propagates through a random scattering medium and which makes direct use of the conventional optical methods ineffective. In this paper, we propose and demonstrate a structured light illumination for imaging through an opaque scattering layer. Proposed technique is reference free and capable to recover the complex field from intensities of the speckle patterns. This is realized by making use of the phase-shifting in the structured light illumination and applying spatial averaging of the speckle pattern in the intensity correlation measurement. An experimental design is presented and simulated results based on the experimental design are shown to demonstrate imaging of different complex-valued objects through scattering layer.

physics.optics↗

arXiv2022

Ghost diffraction: A spatial statistical approach

We report the reconstruction of a transparency image in Ghost diffraction scheme using a statistical optics approach. This is implemented by using a static diffuser rather than a pseudo thermal light source with a rotating diffuser. The experimental implementation makes use of spatial ergodicity and spatial stationarity for spatially distributed random fields. A strategy to realize the Ghost diffraction scheme through spatial intensity correlation with a phase retrieval algorithm permits reconstruction of the transparency.

physics.optics↗

arXiv2021

Structured transmittance illumination coherence holography

The coherence holography offers an unconventional way to reconstruct the hologram where an incoherent light illumination is used for reconstruction purposes, and object encoded into the hologram is reconstructed as the distribution of the complex coherence function. Measurement of the coherence function usually requires an interferometric setup and array detectors. This paper presents an entirely new idea of reconstruction of the complex coherence function in the coherence holography without an interferometric setup. This is realized by structured pattern projections on the incoherent source structure and implementing measurement of the cross-covariance of the intensities by a single-pixel detector. This technique, named structured transmittance illumination coherence holography (STICH), helps to reconstruct the complex coherence from the intensity measurement in a single-pixel detector without an interferometric setup and also keeps advantages of the intensity correlations. A simple experimental setup is presented as a first step to realize the technique, and results based on the computer modeling of the experimental setup are presented to show validation of the idea.

physics.optics↗

arXiv2021

Measuring obscured OAM spectrum using Stokes fluctuations

A novel technique to measure the orbital angular momentum (OAM) spectrum of the beam obscured by a complex random media is proposed and experimentally demonstrated. This is realized by measuring the complex correlation polarization function (CPCF) with the help of a two-point Stokes fluctuations correlation of the random light. The OAM spectrum analysis is implemented by projecting a complex field of the CPCF into spiral harmonics. A detailed theoretical framework is developed to measure complex amplitude and to decompose different integer OAM states of the obscured beam. The developed theoretical framework is verified by numerical simulation and tested by experimental demonstration of OAM mode compositions of the fractional optical vortex (FOV) beam.

physics.optics↗

arXiv2021

Phase-space interpretation of spatial stationarity for coherence holography

We extend the wide-sense spatial stationarity concept of coherence holography in the regime of phase-space using the wigner distribution function. We focus mainly on the incoherent light source and the Fourier and Fresnel propagation kernels for the optical-field transformation rule (inputoutput relation) and derive the same analogy in WDF. We further show that in phase-space the WDF obtained from the ensemble-averaged and space-averaged coherence functions are the same. Finally, we interpret behaviour of these results through numerical simulations.

physics.optics↗

arXiv2018

Non-invasive tracking of polarization rotation from speckle contrast measurement

A technique based on speckle contrast measurement is proposed and demonstrated to track the polarization rotation of an optical beam behind a scattering layer. It is shown that with the help of a reference speckle pattern with a known linear polarization, it is possible to track the polarization rotation behind a scattering layer in a non-invasive manner by measuring the contrast of the resultant speckle pattern. Working principle and experimental demonstration are presented, and results are compared with the theoretically known results in certain cases. Performance of the proposed technique is quantitatively evaluated by determining the polarization vector rotation behind the scattering layer for two different cases, namely polarization rotation by a half wave plate and sugar solution of different concentrations, and good match are found between the experimental results and theoretical prediction. Issues like resolution of determination of the polarization vector rotation and sugar solution concentration are also discussed in the context of the recovered results. The proposed technique can have potential application in non-invasive sensing and biomedical imaging.

physics.optics↗

arXiv2018

Complex field measurement in a single pixel hybrid correlation holography

We propose a new scheme for recovery of complex valued object in a single-pixel hybrid correlation holography. Idea is to generate an intensity correlation hologram by correlation of the intensity of a single pixel detector and digitally propagated two dimensional intensity patterns. The proposed scheme with theoretical basis is described to reconstruct the objects from a single pixel detector. An experimental arrangement is proposed and as a first step to realize the technique, we have simulated the experimental model for imaging of two different complex objects.

physics.optics↗

arXiv2017

Hybrid correlation holography with a single pixel detector

The correlation holography reconstructs 3D objects as a distribution of two-point correlation of the random field detected by two dimensional detector arrays. Here, we describe a hybrid method, a combination of optical and computational channels, to reconstruct the objects from only a single pixel detector. An experimental arrangement is proposed and as a first step to realize the technique, we have simulated the experimental model for both scalar and vectorial objects. Proposed technique affords background free imaging with 3D capability.

physics.optics↗

arXiv2016

Single-shot optical imaging through scattering medium using digital in-line holography

Non-invasive and single-shot holographic imaging through complex media is technically challenging due to random light scattering which significantly scrambles optical information. Recently, several methods have been presented to address this issue. However, they require complicated measurements of optical transmission matrices, or existing techniques do only retrieve intensity information. Here we propose and experimentally demonstrate a holographic approach for single-shot imaging through a scattering layer based on digital in-line holography in combination with the autocorrelation of the speckle intensity. Using a simple optical configuration and experimental procedure, the proposed method enables to retrieve the complex amplitude image of an object located at arbitrary planes behind scattering media. The technique has potential applications in biomedical imaging, deep tissue microscopy, and 3D imaging through turbid media.

physics.optics↗

arXiv2015

Non-invasive single-shot 3D 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, astronomical telescopy, and spatially multiplex optical communications. Although various approaches for imaging though turbid layer have been recently proposed, they had been limited to two-dimensional imaging. Here we propose and experimentally demonstrate an approach for three-dimensional single-shot imaging of objects hidden behind an opaque scattering layer. We demonstrate that under suitable conditions, it is possible to perform the 3D imaging to reconstruct the complex amplitude of objects situated at different depths.

physics.optics↗