Overview of photon-counted three-dimensional imaging and related applications
Dodda V.C., Kuruguntla L., Ravichandran N.K., Lee K.-S., Sollapur R., Damodaran M., Kumar R., Anilkumar N., Itapu S., Kumar M., Matoba O., Hennelly B.M., Stern A., Muniraj I.
Review, Optics Express, 2025, DOI Link
View abstract ⏷
Due to its detailed spatial mapping capability, three-dimensional (3D) imaging has became an indispensable tool for numerous scientific applications, including medical diagnostics, industrial inspection, autonomous navigation, environmental monitoring, etc. Conventional 3D imaging techniques often suffer from limitations such as poor resolution,and sensitivity, especially in low-light or photon-starved imaging conditions. A photon-counted imaging (PCI) system is often preferred to overcome this limitation. It is known that PCI systems leverage photon detection techniques to capture high-resolution 3D spatial information with exceptional sensitivity and resolution. To achieve this, single-photon detectors are used to count/detect (typically from single to a few hundred) photons from extremely low light level imaging conditions. This paper provides a comprehensive overview of PCI-based 3D imaging systems aimed at overcoming the limitations of the conventional 3D imaging systems. In addition to this, we also discuss some of the add-on applications of PCI systems, including information security, denoising, and resolution enhancement. This article comprises three major sections with four topics to provide an overview of recent research activities using photon-counted imaging systems.
Coded aperture imaging with helico-conical beams
Vardhan H., Gopinath S., Tiwari V., Sivarajan A.K., Choudhary S., Reddy S.G., Anand V., Kumar R.
Article, Applied Physics B: Lasers and Optics, 2025, DOI Link
View abstract ⏷
Interferenceless Coded Aperture Correlation Holography (I-COACH) has emerged as a powerful computational imaging technique for retrieving three-dimensional information from an object without requiring two-beam interference. In this study, we propose and experimentally demonstrate an I-COACH system employing a Helico-Conical Vortex (HCV) mask. The HCV mask carries orbital angular momentum and features a phase profile with non-separable dependence on both azimuthal and radial coordinates. It is generated by combining helical and conical phase functions, resulting in a spiral-shaped intensity distribution at the focal plane. We compare the performance of I-COACH with the HCV mask against other coded masks (CMs), including random lens, ring lens, spiral axicon, axicon, and spiral lens. Additionally, we evaluate image reconstruction using four widely adopted algorithms: non-linear reconstruction (NLR), Lucy-Richardson algorithm (LRA), Lucy-Richardson-Rosen algorithm (LRRA), and non-linear LRA (NL-LRA). Quantitative analysis is conducted using figures of merit such as entropy, root mean squared error (RMSE), structural similarity index measure (SSIM), and peak signal-to-noise ratio (PSNR). The proposed approach holds promise for advancing incoherent holography and computational imaging applications.
Free space evolution of perfect optical vortices through interferometric analysis
Sivarajan A.K., Vardhan H., Choudhary S., Reddy S.G., Kumar R.
Article, Optics and Lasers in Engineering, 2025, DOI Link
View abstract ⏷
The topological charge (TC) independent annular intensity distributions of perfect optical vortex (POV) beams made them exciting for various applications. In this work, we have experimentally generated POV beams of different TC orders and proposed a modified Mach-Zehnder interferometric arrangement for identifying the TC of the generated beam. We also investigated the free space evolution of the POV beam demonstrated that it naturally evolves into a Bessel-Gaussian (BG) beam with propagation. We confirmed this evolution by analysing the propagation of interference patterns as well as comparing the self-healing property of BG beams with the evolved POV beams. The findings are supported by theoretical and experimental results. The presented analysis will facilitate the application of POV beams in areas such as optical imaging, free space communication, optical trapping, etc.
Scattering of open vortex beams: Applications towards free space optical communications
Ansari M.H., M V.C., Kumar R., Anand V., Prabhakar S., Reddy S.G., Singh R.P.
Article, Optics and Lasers in Engineering, 2025, DOI Link
View abstract ⏷
The topological charge (TC) of optical vortex beams can be measured using various interferometric and non-interferometric techniques in both coherent and partially coherent domains. However, these methods are not suitable for obstructed vortex beams, also known as open optical vortex (OOV) beams. Recently, several methods for studying open optical vortex (OOV) beams, have recently been proposed and demonstrated based on interferometry, phase retrieval, spatial coherence analysis, which limit their applicability in the presence of significant perturbations or long-distance propagation. In this study, we propose and experimentally demonstrate an efficient method for measuring both the magnitude and sign of the topological charge (TC) of OOV beams using the auto-correlation distribution after scattering through a rough surface. We generated the OOV beams using partially blocked computer-generated holograms. Although the rings or zero points present in the auto-correlation are broken, the number of rings is equal to the TC. Further, we have utilized the radius of the first ring and its divergence with propagation distance, which can be easily observed for all orders, for finding the TC of higher orders. We can measure the sign of the topological charge solely through intensity measurements using the rotation of the autocorrelation profile with the help of blocking parameter. Furthermore, we demonstrate that the characteristics of OOV beams derived from our proposed method align well with the propagation characteristics of unobstructed OV beams. The results confirm the efficacy of optical vortex beams for free-space optical communication.
Asymmetric optical cryptosystem using C-point polarization singularity speckles and polar decomposition in 2D-NS-LCT domain
Sivarajan A.K., Vardhan H., Choudhary S., Reddy S.G., Kumar R.
Article, Physica Scripta, 2025, DOI Link
View abstract ⏷
The study introduces a novel asymmetric optical cryptosystem that utilizes bright C-point polarization singularity speckle (BCPSS) patterns as security keys while offering multiuser capabilities. The C-point singular beams, with spatially varying polarization distributions, are created by superposing optical vortex modes of different magnitudes into an orthogonal polarization basis. This complex light beam is then scattered through a rough surface to generate the BCPSS patterns. These generated speckle patterns inherit some unique properties due to the vectorial light field and the randomness of the rough surface, which make them nearly impossible to duplicate. To generate a complex image, the BCPSS phase mask is used to further modify the original image after it has been phase encoded. The final encrypted image is then obtained by processing the intermediate complex image using two-dimensional non-separable linear canonical transform (2D-NS-LCT) and polar decomposition. The 2D-NS-LCT has ten independent parameters which expends the key space, improving its resistance to various attacks. The implementation of polar decomposition in the proposed cryptosystem enables us to have two private keys, helping in multiuser functionality. The proposed method is also validated by testing it against various potential attacks, including contamination and plaintext attacks. Numerical simulations confirm the authenticity and reliability of the proposed cryptosystem.
Theoretical and experimental analysis of coherence vortices and their propagation characteristics
Cris M V., Gangwani P., Ansari MD. H., Kumar R., Gangi Reddy S., Prabhakar S., Banerji J., Singh R.P.
Article, Optics and Lasers in Engineering, 2025, DOI Link
View abstract ⏷
It is well known that the coherence vortices are robust against atmospheric turbulence and can be effectively utilized for communication and imaging applications. In this paper, we study, both theoretically and experimentally, the generation of coherence vortices by the cross-correlation between two scattered optical vortices with different topological orders and how the orders of the input fields affect the size of the generated coherence vortices. We have analyzed the size by considering the coherence vortex at a given plane as a ring with inner and outer radii. The inner and outer radii vary linearly with order and the propagation distance. The slope of radius vs. propagation distance is considered as the divergence by which one will be able to find the order. All the theoretical predictions have been validated with the experimental results. It is also observed that the propagation characteristics are similar to the coherent optical vortices.
Comprehensive survey of plasmonic nano-dendrites: from fabrication to surface-enhanced Raman scattering (SERS) applications
Murugan D., Chithravel A., Shekhawat A.S., Diwan A., Sharma S., Singh N., Kumar R., Shrivastav D., Srivastava T., Saxena S.K., Shrivastav A.M.
Review, Journal of Materials Chemistry C, 2025, DOI Link
View abstract ⏷
Dendrites are beautifully designed branched structures found everywhere in nature, for example, in neurons, snowflakes, and trees. These unique properties of dendritic structures contribute to their applications in tissue integration, light manipulation, energy storage, charge transport, sensing, and other fields. In recent years, plasmonic nanodendrites have been extensively employed for surface-enhanced Raman scattering (SERS) applications, incorporating the highly dense electromagnetic field hot spots at the dendritic tips in addition to the increased surface area. These structures have shown their potential for sensing a wide range of analytes, including explosives, pesticides, bacteria, and viruses. This review provides in-depth information about the fundamentals of the SERS mechanism, fabrication techniques to manipulate dendrite structures for improved SERS performance, and the role of nanodendrite structures in SERS applications. Through an extensive survey, this review compiles the current state-of-the-art technologies for developing plasmonic dendrites and applying them for SERS-based sensing applications. Finally, we present the current challenges and future perspectives of developing such sensors.
Tailoring LRSPR Penetration Depth Employing 2-D Material at Visible and IR Range
Chithravel A., Murugan D., Diwan A., Shekhawat A.S., Sharma S., Srivastava T., Kumar R., Saxena S.K., Shrivastav A.M.
Article, IEEE Sensors Journal, 2024, DOI Link
View abstract ⏷
Surface plasmon resonance (SPR) is a prevalent sensing approach for detecting the additional mass concentrations as small as pg/ml and Refractive Index changes up to the orders of 0.1μRIU. However, SPR possesses a penetration depth of approx. 200 nm limiting its application for microbes such as bacterial cells and viruses. Long-range SPR (LRSPR) possesses more equitable sensing mechanism and deeper penetration depth improving the performance than traditional SPR. In this work, we propose a theoretical design based on transfer matrix method (TMM) for developing LRSPR chip to achieve high penetration depth and figure of merit (FOM). Here, we have used low index prism (CaF2) for high penetration depth with optimized adhesive layer of 1900 and 600 nm for visible and IR wavelengths providing penetration depths of 817 and 1513 nm, respectively. Study revealed that for IR ranges, penetration depth increases comprising the FOM which can further be improved by additional 2-D material between metal-sensing layers. A variety of 2-D materials are used to obtain the best performance of the chip. The inclusion of a monolayer MoS2 improves FOM due to higher absorption of light as it possesses higher imaginary Refractive Index. The simulated findings reveal that the greatest penetration depth of the IR spectrum is 1508.93 nm near surrounding RI of 1.35 aliens with the index of aqueous biomolecular samples. The study demonstrates a potential use of 2-D material with enhanced LRSPR sensor performance with depths comparable to bacterial cells, viruses, and proteins.
Hologram authentication and classification via a Convolutional Neural Network
Ganiga P., Priyadarshini, Dodda V.C., Kumar R., Muniraj I.
Conference paper, Imaging Systems and Applications, ISA 2024 in Proceedings Optica Imaging Congress 2024, 3D, AOMS, COSI, ISA, pcAOP - Part of Optica Imaging Congress, 2024,
View abstract ⏷
Authentication techniques can be used to overcome the hologram counterfeiting problems. Here, we demonstrate an authentication scheme for digital holograms in a raw-complex form that is stored either in the cloud or on the metasurface using a CNN.
Advances in Optical Visual Information Security: A Comprehensive Review
Sachin, Kumar R., Sakshi, Yadav R., Reddy S.G., Yadav A.K., Singh P.
Review, Photonics, 2024, DOI Link
View abstract ⏷
In the modern era, the secure transmission and storage of information are among the utmost priorities. Optical security protocols have demonstrated significant advantages over digital counterparts, i.e., a high speed, a complex degree of freedom, physical parameters as keys (i.e., phase, wavelength, polarization, quantum properties of photons, multiplexing, etc.) and multi-dimension processing capabilities. This paper provides a comprehensive overview of optical cryptosystems developed over the years. We have also analyzed the trend in the growth of optical image encryption methods since their inception in 1995 based on the data collected from various literature libraries such as Google Scholar, IEEE Library and Science Direct Database. The security algorithms developed in the literature are focused on two major aspects, i.e., symmetric and asymmetric cryptosystems. A summary of state-of-the-art works is described based on these two aspects. Current challenges and future perspectives of the field are also discussed.
A symmetric optical cryptosystem based on QZ decomposition and Hermite Gaussian beam speckles
Article, Optical and Quantum Electronics, 2024, DOI Link
View abstract ⏷
In this paper, we propose a new asymmetric optical cryptosystem for phase image encoding with the utilization of speckles generated by scattering the Hermite Gaussian beams (HGBs) through a rough surface. These speckle patterns are unique and almost impossible to clone as one cannot mimic the physical process. The generalized Schur decomposition, named as, QZ decomposition, approach is used to generate unique private keys for decrypting the encoded data. The plaintext image is first phase-encoded and then modulated with the pattern obtained by the convolution of HGBs and random phase masks. The modulated image is then Fresnel propagated for a distance of z1, and the QZ decomposition operation is performed on the complex wavefront to generate the private keys. Afterward, the gyrator transforms with a rotational angle (α), and the phase truncation is used to further process the information. The phase truncation and phase reservation (PT/PR) will result in another phase private key, which will be utilized for decryption. A non-linear power function is introduced to modify the amplitude part after PT/PR operation and the resultant is modulated using an HGB amplitude mask to get an intermediate wavefront. Finally, the encrypted image is obtained by Fresnel propagating the intermediate wavefront with a distance of z2. The effectiveness and validity of the proposed method are tested and verified through numerical simulations. A series of potential attacks such as contamination and plaintext attacks have been tried and tested to further check the robustness of the proposed method. The results confirm the efficacy of the proposed method.
Roadmap on computational methods in optical imaging and holography [invited]
Rosen J., Alford S., Allan B., Anand V., Arnon S., Arockiaraj F.G., Art J., Bai B., Balasubramaniam G.M., Birnbaum T., Bisht N.S., Blinder D., Cao L., Chen Q., Chen Z., Dubey V., Egiazarian K., Ercan M., Forbes A., Gopakumar G., Gao Y., Gigan S., Goclowski P., Gopinath S., Greenbaum A., Horisaki R., Ierodiaconou D., Juodkazis S., Karmakar T., Katkovnik V., Khonina S.N., Kner P., Kravets V., Kumar R., Lai Y., Li C., Li J., Li S., Li Y., Liang J., Manavalan G., Mandal A.C., Manisha M., Mann C., Marzejon M.J., Moodley C., Morikawa J., Muniraj I., Narbutis D., Ng S.H., Nothlawala F., Oh J., Ozcan A., Park Y., Porfirev A.P., Potcoava M., Prabhakar S., Pu J., Rai M.R., Rogalski M., Ryu M., Choudhary S., Salla G.R., Schelkens P., Sener S.F., Shevkunov I., Shimobaba T., Singh R.K., Singh R.P., Stern A., Sun J., Zhou S., Zuo C., Zurawski Z., Tahara T., Tiwari V., Trusiak M., Vinu R.V., Volotovskiy S.G., Yilmaz H., De Aguiar H.B., Ahluwalia B.S., Ahmad A.
Review, Applied Physics B: Lasers and Optics, 2024, DOI Link
View abstract ⏷
Computational methods have been established as cornerstones in optical imaging and holography in recent years. Every year, the dependence of optical imaging and holography on computational methods is increasing significantly to the extent that optical methods and components are being completely and efficiently replaced with computational methods at low cost. This roadmap reviews the current scenario in four major areas namely incoherent digital holography, quantitative phase imaging, imaging through scattering layers, and super-resolution imaging. In addition to registering the perspectives of the modern-day architects of the above research areas, the roadmap also reports some of the latest studies on the topic. Computational codes and pseudocodes are presented for computational methods in a plug-and-play fashion for readers to not only read and understand but also practice the latest algorithms with their data. We believe that this roadmap will be a valuable tool for analyzing the current trends in computational methods to predict and prepare the future of computational methods in optical imaging and holography.
Optical phase image encryption using stokes parameters and singular value decomposition
Vardhan H., Sivarajan A.K., Sakshi, Reddy S.G., Kumar R.
Article, Journal of Optics (United Kingdom), 2024, DOI Link
View abstract ⏷
In this paper, we propose an optical asymmetric phase image encryption method in which the vectorial light field is used to encode the data. In transverse plane, the vectorial light field has spatially varying polarization distributions where we are allowed to have a greater number of degrees of freedom. In this scheme, the input image is first phase encoded and then modulated by a phase encrypting key, synthesized from the speckles obtained by the scattering of Hermite-Gaussian beams. The modulated image is further processed using fractional Fourier transform with a specific order (α). A pixel scrambling operator is utilized to increase the randomness to further enhance the security and singular value decomposition approach is employed to add the nonlinearity in the encryption process. Now, the stokes parameters, i.e. S1 and S2 are calculated using the light intensities correspond to different polarizations. S1 is used as the encrypted image for transmission and S2 is reserved as one of the private decryption keys. The robustness of the proposed technique is tested against various existing attacks, such as known plaintext attack, chosen plaintext attack, and contamination attacks. Numerically simulated results validate the effectiveness and efficiency of the proposed method.
Nonlinear multi-image optical authentication based on QR decomposition and Kramer-Kronig relations
Vardhan H., Sivarajan A.K., None S., Shrivastav A.M., Reddy S.G., Kumar R.
Article, Physica Scripta, 2024, DOI Link
View abstract ⏷
In this paper, a new nonlinear optical multi-image authentication scheme is proposed based on Kramers-Kronig digital holography and orthogonal triangular decomposition or QR decomposition. Here, the complex light field carrying the information of multiple images is modulated by random phase masks and propagated at certain distance. Afterwards, the QR decomposition is applied to the complex wavefront to generate the private keys and to add the non-linearity in the scheme. Next, the product of orthogonal matrix and upper triangular matrix is processed further. The obtained output is modulated by different phase masks and interfered with reference beam to record the encrypted image. For decryption, the Kramer-Kronig relation is utilized to extract the plaintext images directly with only the positive frequency part. A series of numerical simulations are conducted to validate the efficacy and robustness of proposed image authentication scheme.
Phase only CGH and sparse separation based approach to biometric image authentication
Sachin, Yadav R., Kumar R., Singh P.
Article, Journal of Optics (India), 2024, DOI Link
View abstract ⏷
In this manuscript, an image encryption cum authentication method is proposed. The proposed method utilizes the fingerprint-generated phase mask along with the sparse separation approach, and computer-generated hologram to encrypt the information in Fresnel domain. The Phase only computer-generated holography based authentication algorithm provides multiple benefits, including to store substantial information in compact form, improved security, and strong resistance to counterfeiting. Sparse separation is used for authentication with only thirty percent of the data. The proposed authentication algorithm is evaluated through a range of statistical metrics, including mean squared error, information entropy, correlation coefficient along with histogram, and mesh plot analysis. The results affirm that the proposed cryptosystem is secure and resilient against different types of potential threats, including iterative attacks, plaintext attacks, and contamination attacks. Proposed authentication algorithm has vast key space which makes it challenging to breach in real-time with existing computational capabilities.
Cryptoanalysis of a multiple-image encryption scheme based on amplitude and phase truncation in the Fourier domain
Xiong Y., Liu J., Gu J., Kumar R.
Article, Applied Optics, 2024, DOI Link
View abstract ⏷
This paper presents a comprehensive cryptoanalysis of a multiple-image encryption scheme based on amplitude truncation (AT) and phase truncation (PT) in the Fourier domain. In contrast to the conventional single-image cryptosystem based on phase-truncated Fourier transform (PTFT), the enhanced PTFT-based cryptosystem was proposed to encode multiple images efficiently and to augment the security strength by expanding the key space. Nevertheless, we found that the amplitude key exhibits low sensitivity, which has a restricted impact on the security enhancement and makes the scheme vulnerable. Moreover, the two random phase masks (RPMs) employed as private keys are uncorrelated with the plaintexts, which can be recovered through a devised known-plaintext attack (KPA). Once these additional private keys are recovered, the number of unknown keys is reduced to two, making it possible to recover plaintext information encrypted by this advanced PTFT-based cryptosystem using an iterative attack without any knowledge of the private keys. Based on these findings, a hybrid attack consisting of two cascaded KPAs and chosen-ciphertext attacks (CCAs) is proposed to successfully crack the improved PTFT-based cryptosystem. Numerical simulations have been performed to validate the feasibility and effectiveness of the proposed hybrid attack.
Multi-User Nonlinear Optical Cryptosystem Based on Polar Decomposition and Fractional Vortex Speckle Patterns
Mandapati V.C., Vardhan H., Prabhakar S., Sakshi, Kumar R., Reddy S.G., Singh R.P., Singh K.
Article, Photonics, 2023, DOI Link
View abstract ⏷
In this paper, we propose a new multiuser nonlinear optical cryptosystem using fractional-order vortex speckle (FOVS) patterns as security keys. In conventional optical cryptosystems, mostly random phase masks are used as the security keys which are prone to various attacks such as brute force attack. In the current study, the FOVSs are generated optically by the scattering of the fractional-order vortex beam, known for azimuthal phase and helical wavefronts, through a ground glass diffuser. FOVSs have a remarkable property that makes them almost impossible to replicate. In the input plane, the amplitude image is first phase encoded and then modulated with the FOVS phase mask to obtain the complex image. This complex image is further processed to obtain the encrypted image using the proposed method. Two private security keys are obtained through polar decomposition which enables the multi-user capability in the cryptosystem. The robustness of the proposed method is tested against existing attacks such as the contamination attack and known-plaintext attack. Numerical simulations confirm the validity and feasibility of the proposed method.
3D incoherent imaging using an ensemble of sparse self-rotating beams
Bleahu A.-I., Gopinath S., Kahro T., Angamuthu P.P., Rajeswary A.S.J.F., Prabhakar S., Kumar R., Salla G.R., Singh R.P., Kukli K., Tamm A., Rosen J., Anand V.
Article, Optics Express, 2023, DOI Link
View abstract ⏷
Interferenceless coded aperture correlation holography (I-COACH) is one of the simplest incoherent holography techniques. In I-COACH, the light from an object is modulated by a coded mask, and the resulting intensity distribution is recorded. The 3D image of the object is reconstructed by processing the object intensity distribution with the pre-recorded 3D point spread intensity distributions. The first version of I-COACH was implemented using a scattering phase mask, which makes its implementation challenging in light-sensitive experiments. The I-COACH technique gradually evolved with the advancement in the engineering of coded phase masks that retain randomness but improve the concentration of light in smaller areas in the image sensor. In this direction, I-COACH was demonstrated using weakly scattered intensity patterns, dot patterns and recently using accelerating Airy patterns, and the case with accelerating Airy patterns exhibited the highest SNR. In this study, we propose and demonstrate I-COACH with an ensemble of self-rotating beams. Unlike accelerating Airy beams, self-rotating beams exhibit a better energy concentration. In the case of self-rotating beams, the uniqueness of the intensity distributions with depth is attributed to the rotation of the intensity pattern as opposed to the shifts of the Airy patterns, making the intensity distribution stable along depths. A significant improvement in SNR was observed in optical experiments.
An Asymmetric Optical Cryptosystem Using Physically Unclonable Functions in the Fresnel Domain †
Cris Mandapati V., Prabhakar S., Vardhan H., Kumar R., Reddy S.G., Sakshi, Singh R.P.
Article, Engineering Proceedings, 2023, DOI Link
View abstract ⏷
In this paper, we propose a new asymmetric cryptosystem for phase image encryption, using the physically unclonable functions (PUFs) as security keys. For encryption, the original amplitude image is first converted into a phase image and modulated with a PUF to obtain a complex image. This complex image is then illuminated with a plane wave, and the complex wavefront at a distance d is recorded. The real part of the complex wavefront is further processed to obtain the encrypted image and the imaginary part is kept as the private key. The polar decomposition approach is utilized to generate two more private security keys and to enable the multi-user capability in the cryptosystem. Numerical simulations confirm the feasibility of the proposed method.
3D single shot lensless incoherent optical imaging using coded phase aperture system with point response of scattered airy beams
Kumar R., Anand V., Rosen J.
Article, Scientific Reports, 2023, DOI Link
View abstract ⏷
Interferenceless coded aperture correlation holography (I-COACH) techniques have revolutionized the field of incoherent imaging, offering multidimensional imaging capabilities with a high temporal resolution in a simple optical configuration and at a low cost. The I-COACH method uses phase modulators (PMs) between the object and the image sensor, which encode the 3D location information of a point into a unique spatial intensity distribution. The system usually requires a one-time calibration procedure in which the point spread functions (PSFs) at different depths and/or wavelengths are recorded. When an object is recorded under identical conditions as the PSF, the multidimensional image of the object is reconstructed by processing the object intensity with the PSFs. In the previous versions of I-COACH, the PM mapped every object point to a scattered intensity distribution or random dot array pattern. The scattered intensity distribution results in a low SNR compared to a direct imaging system due to optical power dilution. Due to the limited focal depth, the dot pattern reduces the imaging resolution beyond the depth of focus if further multiplexing of phase masks is not performed. In this study, I-COACH has been realized using a PM that maps every object point into a sparse random array of Airy beams. Airy beams during propagation exhibit a relatively high focal depth with sharp intensity maxima that shift laterally following a curved path in 3D space. Therefore, sparse, randomly distributed diverse Airy beams exhibit random shifts with respect to one another during propagation, generating unique intensity distributions at different distances while retaining optical power concentrations in small areas on the detector. The phase-only mask displayed on the modulator was designed by random phase multiplexing of Airy beam generators. The simulation and experimental results obtained for the proposed method are significantly better in SNR than in the previous versions of I-COACH.
Enhanced design of multiplexed coded masks for Fresnel incoherent correlation holography
Gopinath S., Bleahu A., Kahro T., John Francis Rajeswary A.S., Kumar R., Kukli K., Tamm A., Rosen J., Anand V.
Article, Scientific Reports, 2023, DOI Link
View abstract ⏷
Fresnel incoherent correlation holography (FINCH) is a well-established incoherent digital holography technique. In FINCH, light from an object point splits into two, differently modulated using two diffractive lenses with different focal distances and interfered to form a self-interference hologram. The hologram numerically back propagates to reconstruct the image of the object at different depths. FINCH, in the inline configuration, requires at least three camera shots with different phase shifts between the two interfering beams followed by superposition to obtain a complex hologram that can be used to reconstruct an object’s image without the twin image and bias terms. In general, FINCH is implemented using an active device, such as a spatial light modulator, to display the diffractive lenses. The first version of FINCH used a phase mask generated by random multiplexing of two diffractive lenses, which resulted in high reconstruction noise. Therefore, a polarization multiplexing method was later developed to suppress the reconstruction noise at the expense of some power loss. In this study, a novel computational algorithm based on the Gerchberg-Saxton algorithm (GSA) called transport of amplitude into phase (TAP-GSA) was developed for FINCH to design multiplexed phase masks with high light throughput and low reconstruction noise. The simulation and optical experiments demonstrate a power efficiency improvement of ~ 150 and ~ 200% in the new method in comparison to random multiplexing and polarization multiplexing, respectively. The SNR of the proposed method is better than that of random multiplexing in all tested cases but lower than that of the polarization multiplexing method.
Multi-wavelength imaging with extended depth of field using coded apertures and radial quartic phase functions
Dubey N., Kumar R., Rosen J.
Article, Optics and Lasers in Engineering, 2023, DOI Link
View abstract ⏷
This study demonstrates a new multi-wavelength imaging technique with an extended depth of field. The spatial and spectral information of the objects is encoded into a single bipolar function of sparse dots using coded phase masks (CPMs) and a radial quartic phase function (RQPF). Each wavelength produces an intensity response of sparse dots with a unique set of distances between the dots, enabling the decoding of multispectral information. The RQPF is introduced to extend the depth of focus for different wavelengths. A library of point spread holograms (PSHs) is prerecorded with uniquely designed CPMs by placing the point objects of multiple wavelengths at the system input. For reconstruction, the object hologram is cross-correlated with a PSH corresponding to the wavelength of the object. The results are compared with direct imaging and with the conventional technique of interferenceless coded aperture correlation holography. The method is unique because the number of camera shots required for multi-wavelength imaging is the same as for monochromatic imaging.
Collision in a phase-only asymmetric cryptosystem based on interference and phase-truncated Fourier transforms
Xiong Y., Gu J., Kumar R.
Article, Optical and Quantum Electronics, 2023, DOI Link
View abstract ⏷
In this paper, the security strength of a phase-only asymmetric cryptosystem based on interference and phase-truncated Fourier transforms (PTFTs) has been evaluated. Compared to the conventional PTFTs-based scheme where the plaintext is directly encoded into two phase masks (PMs) and the ciphertext, here the plaintext is firstly converted to the phase-only distribution by PTFTs-based encryption process I, and then modulated by the encryption process II with the aid of two masks generated by a carrier image. The security strength of this cryptosystem has been enhanced by an additional secure layer for the output of PTFTs-based structure. Moreover, the four masks generated in the encryption processes I and II are required for the decryption also, it enlarges the key space which further ensures the security strength of the improved cryptosystem. However, we noticed that the carrier image used to generate one of private keys is same as the ciphertext when the input of the encryption is the zero matrix. Thus, the amplitude mask (AM) as the private key could be recovered by the designed chosen-plaintext attack, and then it can be used as the known parameter in the iterative attacks. Employing the recovered mask, two specific attacks with different constraints are designed to break the cryptosystem based on interference and PTFTs successfully. Based on our cryptoanalysis, it is found that most information of the plaintexts were encoded into the AM and the PM in the encryption process I, and silhouette problem would be caused when one of these keys is known. Numerical simulations have been carried out to validate the feasibility and effectiveness of proposed hybrid attacks.
Hybrid plaintext attack for a cryptosystem based on interference and the phase-retrieval technique
Xiong Y., Gu J., Kumar R.
Article, Applied Optics, 2023, DOI Link
View abstract ⏷
In this paper, the security strength of an enhanced cryptosystem based on interference and the phase-retrieval technique is evaluated. The security strength of the optical cryptosystem was improved through the phase-retrieval technique used to generate a phase-only mask (POM) as the ciphertext. Due to the complex mathematical model of the phase-retrieval technique, it seems that a silhouette problem existing in the conventional interference-based scheme was removed. However, we noted that the random phase mask (RPM) regarded as the only private key was fixed in the encryption path, which is not related to the plaintext and makes it possible to be recovered using a known-plaintext attack (KPA). Moreover, we also found that the RPM has high key sensitivity, and it should be recovered precisely to retrieve information of plaintexts during the attack. Thus, a hybrid KPA where three pairs of known plaintexts and their ciphertexts are regarded as the amplitude and phase constraints to obtain the precise estimation of the RPM is proposed. Then, with the help of the estimated private key, information of the original plaintexts encoded using the cryptosystem under study could be retrieved from an arbitrarily given ciphertext without any knowledge of the private key. Our cryptoanalysis shows that the cryptosystem based on interference and the phase-retrieval technique is vulnerable to the proposed attack, and there is a security leak in it. Numerical simulations have been carried out to demonstrate the performance of our proposed attack.
Collision in double-image encryption scheme based on spatial encoding and phase-truncation Fourier transforms
Xiong Y., Gu J., Kumar R.
Article, Applied Optics, 2023, DOI Link
View abstract ⏷
In this paper, the security strength of a double-image cryptosystem using spatial encoding and phase-truncation Fourier transforms (PTFTs) is evaluated. Unlike the conventional PTFT-based cryptosystem, where two random phase masks (RPMs) are used as public keys to provide enough phase constrains in the estimation, in the improved cryptosystem, the RPM generated by a random amplitude mask (RAM) is treated as an unknown parameter. Due to this fixed RAM, the number of constraints in the estimation decreases to achieve high robustness against potential iterative attacks. Moreover, instead of two phase-only masks (POMs), here the two POMs and the RAM are utilized as the private keys in the improved cryptosystem; thus, the key space of the double-image cryptosystem has been enlarged. However, we noticed that the RAM used to encode plaintexts spatially and to generate the phase encryption key is independent of the plaintexts. This could be recovered by a known pair of plaintexts and the ciphertext. Once the information of the RAM is retrieved, the phase key RPM can also be produced making the cryptosystem vulnerable. Based on this finding, new hybrid algorithms, including a known-plaintext attack and a known key attack are proposed to crack the enhanced PTFT-based cryptosystem. The information of the plaintexts can be retrieved from one POM using the proposed algorithms without any knowledge of another POM and the corresponding ciphertext. Numerical simulations have been carried out to validate the information disclosure problem still exists in the double-image cryptosystem based on spatial encoding and PTFTs.
Optical Cryptosystems Based on Spiral Phase Modulation
Kumar R., Xiong Y., Sakshi
Book chapter, Studies in Computational Intelligence, 2023, DOI Link
View abstract ⏷
Optical information security techniques have several advantages over digital counterparts such as ability to process information parallelly, use of physical parameters as security keys, efficient storage capability etc. In last few years, several optical cryptosystems have been designed based on different optical aspects. In this chapter, we discuss optical cryptosystems based on spiral/vortex phase modulation in details. The orbital angular momentum (OAM) associated with a spatially helical phase or vortex beam can be utilized to design enhanced security protocols. Moreover, since the OAM has theoretically unlimited values of topological charges (TCs) and have the orthogonality of OAM modes with different integer TCs, it is an excellent candidate for designing high-capacity secure optical cryptosystems. Here, the spiral phase functions have been first introduced with different TCs and then the 2D spiral phase transform (SPT) and several optical cryptosystems based on it are discussed in detail with possible optical configurations for practice applications. Numerical simulation results for three cryptosystems are discussed showing their feasibility. The security analysis in terms of keys sensitivity and robustness against existing attacks is also performed and discussed for these cryptosystems.
Enhanced Design of Pure Phase Grayscale Diffractive Optical Elements by Phase-retrieval Assisted Multiplexing of Complex Functions
Gopinath S., Bleahu A., Kahro T., Francis Rajeswary A.S.J., Kumar R., Kukli K., Tamm A., Rosen J., Anand V.
Conference paper, Proceedings of SPIE - The International Society for Optical Engineering, 2023, DOI Link
View abstract ⏷
Designing a pure phase multifunctional diffractive optical element (M-DOE) is a challenging task, as the regular summation of multiple pure phase functions results in a complex function. One of the widely used multiplexing methods to design a pure phase M-DOE is the random multiplexing method. In this method, different pure phase functions are multiplied to mutually exclusive binary random functions before summation. However, M-DOEs designed using the random multiplexing method are prone to scattering noise. In this study, a novel approach based on a modified Gerchberg-Saxton algorithm (GSA) has been proposed and demonstrated for the design of pure-phase multifunctional DOEs. In this approach, the complex M-DOE obtained by regular summation is used as a reference, and with suitable constraints, the amplitude component of the complex M-DOE is transported into the phase component, resulting in a pure phase MDOE. This modified algorithm is called Transport of Amplitude into Phase based on GSA (TAP-GSA). This method has been demonstrated on a well-established incoherent digital holography technique called Fresnel incoherent correlation holography (FINCH). In FINCH, it is necessary to multiplex two-phase masks, which can be achieved using random multiplexing or polarization multiplexing, resulting in reconstruction noise and low light throughput, respectively. Under low-light conditions, random multiplexing is a better choice than the polarization multiplexing method. The M-DOE designed using TAP-GSA for FINCH improved the light throughput and exhibited a higher SNR in comparison to the random multiplexing method.
Statement of Peer Review †
Anand V., Jayavel A., Palm V., Gopinath S., Bleahu A., John Francis Rajeswary A.S., Kukli K., Balasubramani V., Kumar R., Smith D., Ng S.H., Juodkazis S.
Editorial, Engineering Proceedings, 2023, DOI Link
4D Imaging Using Accelerating Airy Beams and Nonlinear Reconstruction
Bleahu A., Gopinath S., Kumar R., Arockiaraj F.G., Smith D., Kahro T., Praveen P.A., Ng S.H., Pristy A., Katkus T., Rajeswary A.S.J.F., Kukli K., Tamm A., Juodkazis S., Rosen J., Anand V.
Conference paper, Proceedings of SPIE - The International Society for Optical Engineering, 2023, DOI Link
View abstract ⏷
A 4D computational incoherent imaging technique using accelerating Airy beams (A2-beams) and nonlinear reconstruction (NLR) has been developed. The phase mask was designed as a binary version for the generation of a sparse random array of A2-beams. The imaging process consist of three steps. In the first step a 4D point spread function (PSF) was recorded at different wavelengths and depths. In the next step, a multicolor, multiplane object was loaded and a single camera shot was recorded. Finally, the 4D information of the object was reconstructed by processing the object intensity distribution and 4D PSFs. The simulation results for the imaging concept are presented.
Security analysis on asymmetric optical cryptosystem based on interference and equal modulus decomposition
Article, Optical and Quantum Electronics, 2022, DOI Link
View abstract ⏷
In this paper, the security of an optical cryptosystem based on interference and equal modulus decomposition (EMD) has been evaluated. The security strength of the asymmetric cryptosystem has been enhanced by combining interference and EMD to generate four phase-only masks (POMs). Compared to the previous interference- or EMD-based cryptosystems where only one POM is the private key, the number of private keys in this system increases (i.e., four POMs) consequently increasing the key space of the cryptosystem. Moreover, due to additional private keys as unknown parameters in iterative procedures, this cryptosystem with silhouette removal is immune to special attacks which the EMD-based cryptosystems are vulnerable to. It seems that the security strength of the optical cryptosystem based on interference and EMD has been improved; however, we have noticed that the two POMs used as private key are still related to the ciphertexts and can be potentially utilized to attack the cryptosystem. According to the POMs generation principle, a relationship between private keys and ciphertexts could be derived and used as additional constraints to investigate the iterative attacks. Based on this finding, new iterative ciphertext-only attacks with different constraints have been proposed to crack the cryptosystem. In addition, we found that the POMs P2(u, v) and P4(u, v) which are used as private keys have low key sensitivity, which may lead to information leak. Numerical simulations have been carried out to validate the feasibility and effectiveness of the proposed attacks.
Multiuser optical image authentication platform based on sparse constraint and polar decomposition in Fresnel domain
Sachin, Kumar R., Singh P.
Article, Physica Scripta, 2022, DOI Link
View abstract ⏷
In this paper, a new multiuser optical image encryption and authentication technique is proposed. Sparse multiplexing and polar decomposition are used in the Fresnel domain to obtain the ciphertext of an input image. To enable the multiuser platform, multiple private keys are obtained through polar decomposition during the encryption process. It will allow multiple authorized users to access the secure information simultaneously without having a key distribution problem among them. The proposed method has a large key space and is robust against several attacks, such as contamination attacks (noise and occlusion), brute force attack, plaintext attacks, and special iterative attack. A comparative analysis of the presented technique is also performed with the similar existing techniques. The numerical simulation results demonstrate the robustness and feasibility of the proposed algorithm.
Security analysis on an interference-based optical image encryption scheme
Xiong Y., Gu J., Kumar R.
Article, Applied Optics, 2022, DOI Link
View abstract ⏷
In this paper, the security strength of the improved optical cryptosystem based on interference has been evaluated. Compared to the previous interference-based cryptosystems in which the plaintext is encoded into two phase-only masks (POMs), here the plaintext is encoded into a POM and an amplitude mask (AM). Since the information of the plaintext cannot be recovered directly when one of the masks is released in the decryption process of the improved cryptosystem, it seems that it is free from the silhouette problem. However, we found that the random phase mask (RPM) serving as the encryption key is not related to the plaintext. Thus, it is possible to recover the RPM first by using the known-plaintext attack (KPA). Moreover, the POM and the AM generated in the encryption path only contains the phase and amplitude information, respectively. Thus, these can be utilized as additional constraints in the proposed iterative process. Based on these findings, two kinds of hybrid attacks, including a KPA and the iterative processes with different constraints, are proposed to crack the improved cryptosystem. In the designed KPA with a pair of the known plaintext and its corresponding masks, the RPM is recovered first. With the aid of the recovered RPM, two iterative processes with different released masks are proposed to recover the information of the plaintext without any knowledge of another mask. To the best of our knowledge, this is the first time that the existence of the silhouette problem in the cryptosystem under study has been reported. Numerical simulation has been carried out to validate the feasibility and effectiveness of the proposed hybrid attacks.
Optical incoherent imaging using annular synthetic aperture with the superposition of phase-shifted optical transfer functions
Desai J.P., Kumar R., Rosen J.
Article, Optics Letters, 2022, DOI Link
View abstract ⏷
The optical incoherent synthetic aperture (SA) has wide applications in astronomy and biomedical optics. Superresolution imaging can be achieved through SAs with an effective size several times larger than the size of the physical apertures. In this Letter, we propose a new, to the best of our knowledge, method to realize optical incoherent SA imaging. Instead of scanning the entire area of the full SA, we show that similar imaging performance can be achieved by scanning only along its perimeter. At any given time, only a single pair of sub-apertures located at the SA perimeter is open and reflects the incident light toward an image sensor. For each location of the two sub-apertures, two interfering images are captured. The phase difference between the two sub-apertures is zero for one interfering image and π for the other. The image of the object is reconstructed by superposition of all the interfering images from some of the sub-aperture locations and with the two-phase differences, 0 and π, between the two sub-apertures. Optical experiments have been performed using reflective objects, and the results demonstrate similar imaging capabilities as that of direct imaging with the aperture size of the SA. Furthermore, we have compared the proposed SA imaging results with the results of annular and full aperture direct imaging.
Security analysis on the interference-based optical image cryptosystem with a designed amplitude modulator
Article, Applied Optics, 2022, DOI Link
View abstract ⏷
The security strength of an interference-based optical image cryptosystem with a designed amplitude modulator (AM) is evaluated. Compared to previous improved interference-based cryptosystems in which iterative or post-processing algorithms are involved, to remove the silhouette problem, a pre-designed AM is placed at the image plane to modulate the plaintext directly, which eliminates this drawback. Moreover, the AM is used as an additional private key, which enlarges the key space and further enhances the security system. However, we have noticed that one of the designed parameters in the AM has a low key sensitivity, which is fault-tolerant and is not required for precise reconstruction. Additionally, the AM is plaintext-independent, which means that the AM can be recovered first by using a pair of chosen plaintexts and the corresponding ciphertext. Based on these findings, we propose a hybrid algorithm that includes two chosen-plaintext attacks (CPAs) and a ciphertext-only attack (COA) to break the enhanced cryptosystem. Specially, CPAs with an impulse function and an arbitrarily given plaintext are proposed to retrieve two parameters in the AM. With the retrieved AM, information on the plaintext can be recovered from the corresponding ciphertext by two kinds of COAs without any knowledge of the private keys. To the best of our knowledge, this is the first time that the interference-based cryptosystem with a designed AM has been cracked successfully, which may provide potential evidence for further security improvements. Numerical simulations are carried out to validate the feasibility of the proposed attacks.
Nonlinear Reconstruction of Images from Patterns Generated by Deterministic or Random Optical Masks—Concepts and Review of Research
Smith D., Gopinath S., Arockiaraj F.G., Reddy A.N.K., Balasubramani V., Kumar R., Dubey N., Ng S.H., Katkus T., Selva S.J., Renganathan D., Kamalam M.B.R., Rajeswary A.S.J.F., Navaneethakrishnan S., Inbanathan S.R., Valdma S.-M., Praveen P.A., Amudhavel J., Kumar M., Ganeev R.A., Magistretti P.J., Depeursinge C., Juodkazis S., Rosen J., Anand V.
Review, Journal of Imaging, 2022, DOI Link
View abstract ⏷
Indirect-imaging methods involve at least two steps, namely optical recording and computational reconstruction. The optical-recording process uses an optical modulator that transforms the light from the object into a typical intensity distribution. This distribution is numerically processed to reconstruct the object’s image corresponding to different spatial and spectral dimensions. There have been numerous optical-modulation functions and reconstruction methods developed in the past few years for different applications. In most cases, a compatible pair of the optical-modulation function and reconstruction method gives optimal performance. A new reconstruction method, termed nonlinear reconstruction (NLR), was developed in 2017 to reconstruct the object image in the case of optical-scattering modulators. Over the years, it has been revealed that the NLR can reconstruct an object’s image modulated by an axicons, bifocal lenses and even exotic spiral diffractive elements, which generate deterministic optical fields. Apparently, NLR seems to be a universal reconstruction method for indirect imaging. In this review, the performance of NLR isinvestigated for many deterministic and stochastic optical fields. Simulation and experimental results for different cases are presented and discussed.
Single-shot TIE using polarization multiplexing (STIEP) for quantitative phase imaging
Hai N., Kumar R., Rosen J.
Article, Optics and Lasers in Engineering, 2022, DOI Link
View abstract ⏷
Transport of intensity equation (TIE) is a promising technique for quantitative phase imaging (QPI). Usually, in TIE, two axially defocused intensity distributions are used to recover the phase, which requires the mechanical shift of either the image sensor or tested sample along the optical axis. This paper presents a new method for QPI based on single-shot TIE using polarization multiplexing. The two defocused intensity distributions are recorded simultaneously on a single camera plane. For that, a diffractive optical element is positioned at the Fourier plane of the sample to split the incoming wavefront into two orthogonally polarized wavefronts. These multiplexed wavefronts will result in two axially and symmetrically defocused intensities on the camera plane. Furthermore, the technique is well-suited for both static and dynamic phase measurements, including that of biological samples (e.g., red blood cells). The feasibility of the proposed method is demonstrated with experimental results.
Modified plaintext attacks in a session for an optical cryptosystem based on DRPE with PFS
Sachin, Kumar R., Singh P.
Article, Applied Optics, 2022, DOI Link
View abstract ⏷
In this paper, the security of an optical cryptosystem based on double random phase encoding (DRPE) with perfect forward secrecy (PFS) is analyzed for a particular session. In the cryptosystem, the PFS strategy is utilized to enhance the security and key management of the traditional DRPE scheme. Our analysis reveals that the use of PFS has certain advantages in the key management approach, but the method is still vulnerable against modified plaintext attacks when the attack is performed in the same session. Also, it is noted that the method is safe against conventional plaintext attacks, but it is vulnerable to the modified chosen and known plaintext attacks. The original plaintext can be easily retrieved with the proposed attack algorithms. Numerical simulation results are presented to validate the effectiveness of the proposed attack algorithms.
Resolution-Enhanced Incoherent Imaging Using Annular Synthetic Aperture with Superposition of Phase – Shifted Optical Transfer Functions
Desai J.P., Kumar R., Rosen J.
Conference paper, Optics InfoBase Conference Papers, 2022,
View abstract ⏷
A high-resolution image of an object is reconstructed by superposition of all the interference patterns accumulated from certain locations of two small sub-apertures moving on the perimeter of a synthetic aperture.
Quantitative phase imaging based on single-shot TIE using polarization multiplexing
Kumar R., Hai N., Rosen J.
Conference paper, Optics InfoBase Conference Papers, 2022,
View abstract ⏷
A new method for rapid quantitative phase imaging using TIE based on polarization multiplexing has been developed. The technique is well-suited for different types of phase objects, including biological samples, as demonstrated by experimental results.
Incoherent Digital Holography using Spiral Rotating Point Spread Functions Created by Double-helix Beams
Dubey N., Anand V., Khonina S., Kumar R., Reddy A.N.K., Rosen J.
Conference paper, Optics InfoBase Conference Papers, 2022,
View abstract ⏷
A new incoherent 3D imaging system with a rotating point spread function has been developed. Different computational reconstruction methods such as non-linear reconstruction and the Lucy-Richardson-Rosen algorithm were tested, and their performances were compared.
Three-Dimensional Incoherent Imaging Using Spiral Rotating Point Spread Functions Created by Double-Helix Beams [Invited]
Anand V., Khonina S., Kumar R., Dubey N., Reddy A.N.K., Rosen J., Juodkazis S.
Article, Nanoscale Research Letters, 2022, DOI Link
View abstract ⏷
In recent years, there has been a significant transformation in the field of incoherent imaging with new possibilities of compressing three-dimensional (3D) information into a two-dimensional intensity distribution without two-beam interference (TBI). Most incoherent 3D imagers without TBI are based on scattering by a random phase mask exhibiting sharp autocorrelation and low cross-correlation along the depth axis. Consequently, during reconstruction, high lateral and axial resolutions are obtained. Scattering based-Imaging requires a wasteful photon budget and is therefore precluded in many power-sensitive applications. This study develops a proof-of-concept 3D incoherent imaging method using a rotating point spread function termed 3D Incoherent Imaging with Spiral Beams (3DI2SB). The rotation speed of the point spread function (PSF) with displacement and the orbital angular momentum has been theoretically analyzed. The imaging characteristics of 3DI2SB were compared with a direct imaging system using a diffractive lens, and the proposed system exhibited a higher focal depth than the direct imaging system. Different computational reconstruction methods such as the Lucy–Richardson algorithm (LRA), non-linear reconstruction (NLR), and the Lucy–Richardson–Rosen algorithm (LRRA) were compared. While LRRA performed better than both LRA and NLR for an ideal case, NLR performed better than both under real experimental conditions. Both single plane imaging, as well as synthetic 3D imaging, were demonstrated. We believe that the proposed approach might cause a paradigm shift in the current state-of-the-art incoherent imaging, fluorescence microscopy, and astronomical imaging.
COACH-based Shack-Hartmann wavefront sensor
Dubey N., Kumar R., Rosen J.
Conference paper, Optics InfoBase Conference Papers, 2021,
View abstract ⏷
A new Shack-Hartmann wavefront sensor (SHWS) based on the principle of coded aperture correlation holography is used to achieve higher precision. Coded phase masks are employed to create sparse dot patterns in the sensor plane.
COACH-based Shack-Hartmann wavefront sensor with an array of phase coded masks
Dubey N., Kumar R., Rosen J.
Article, Optics Express, 2021, DOI Link
View abstract ⏷
Shack-Hartmann wavefront sensors (SHWS) are generally used to measure the wavefront shape of light beams. Measurement accuracy and the sensitivity of these sensors are important factors for better wavefront sensing. In this paper, we demonstrate a new type of SHWS with better measurement accuracy than the regular SHWS. The lenslet array in the regular SHWS is replaced with an array of coded phase masks, and the principle of coded aperture correlation holography (COACH) is used for wavefront reconstruction. Sharper correlation peaks achieved by COACH improve the accuracy of the estimated local slopes of the measured wavefront and consequently improve the reconstruction accuracy of the overall wavefront. Experimental results confirm that the proposed method provides a lower mean square wavefront error by one order of magnitude in comparison to the regular SHWS.
Unequal modulus decomposition and modified Gerchberg Saxton algorithm based asymmetric cryptosystem in Chirp-Z transform domain
Sachin S., Kumar R., Singh P.
Article, Optical and Quantum Electronics, 2021, DOI Link
View abstract ⏷
In this paper, we have presented an image encryption method in Chirp-Z transform domain using unequal modulus decomposition (UMD) and modified Gerchberg–Saxton (GS) algorithm. The proposed encryption scheme is highly sensitive to the encryption keys and the modified GS algorithm introduces an additional layer of security. The validity of the proposed method is tested with various grayscale and binary images and the numerical simulation results are demonstrated for ‘Cameraman’, ‘Medical’ and Binary ‘CUH’ images. The presented results confirm the robustness of the proposed method against various existing attacks such as, the noise attack, special attack, statistical attack, and brute force attack. A comparative analysis with existing similar methods is also performed and the enhanced security and efficiency of the proposed method is verified.
Security analysis and modified attack algorithms for a nonlinear optical cryptosystem based on DRPE
Singh P., Kumar R., Yadav A.K., Singh K.
Article, Optics and Lasers in Engineering, 2021, DOI Link
View abstract ⏷
In this paper, security of a nonlinear optical cryptosystem based on double random phase encoding (DRPE) is analyzed. In the cryptosystem, a time-varying nonlinear term is combined with the classical DRPE technique to enhance security. The advantages of the nonlinear term are its easy distribution and no effect on the decrypted image. Also, it is confirmed that the method is resistant to conventional plaintext attacks such as known-plaintext attack, chosen-plaintext attack, and ciphertext only attack. However, our security analysis shows that the method is vulnerable to the modified chosen-plaintext, and known-plaintext attacks. The original image can be recovered successfully using the proposed attack algorithms. Numerical simulation results are presented to validate the proposed attack methods and test effectiveness of our analysis.
Asymmetric color image encryption and compression based on discrete cosine transform in Fresnel domain
Kumari E., Mukherjee S., Singh P., Kumar R.
Article, Results in Optics, 2020, DOI Link
View abstract ⏷
In this paper, a new asymmetric method is presented to encrypt and compress color images using discrete cosine transform in Fresnel domain. First, the input color image is decomposed into three channels: red, green and blue. Each channel image is converted into phase image and bonded with an amplitude mask followed by Fresnel transform to get the complex intermediate image. Phase reservation (PR) and phase truncation (PT) operation are performed on the intermediate image. Thereafter, a random phase mask is bonded with the amplitude part and the phase part serves as of the first private key for decryption. The complex image is Fresnel propagated followed by the second PR and PT operation. Phase part serve as another private key and the discrete cosine transform is applied on the amplitude part of each red, green and blue channel to get the corresponding compressed encrypted images. The final compressed color encrypted image is obtained by combining the encrypted images of all three channels. In the decryption process, encrypted image is decomposed in red, green and blue channel and lost data in each channel is replaced by zeros. Each channel is subject to inverse discrete cosine transform and cascaded PT/PR operations with Fresnel propagation in reverse directions and recombined to get the original image. Numerical simulation results are presented to validate the proposed method. Also, the robustness of the proposed method is checked against various existing attacks and a comparative analysis confirms the effectiveness of our method.
Double image encryption scheme for iris template protection using 3D Lorenz system and modified equal modulus decomposition in hybrid transform domain
Rakheja P., Singh P., Vig R., Kumar R.
Article, Journal of Modern Optics, 2020, DOI Link
View abstract ⏷
In this paper, a double image encryption mechanism for iris template protection using 3D Lorenz chaotic system with modified equal modulus decomposition in the hybrid transform domain is proposed. The hybrid transform is produced by the combination of Walsh transform, Kekre transform and fractional Fourier transform of various orders. Here the fractional orders of the fractional Fourier transform enlarge the key-space of the proposed scheme and improve robustness against brute-force attack. The proposed scheme is asymmetric and non-linear in nature. To validate and authenticate the proposed cryptosystem, the numerical simulations have been performed on grayscale images. Results demonstrate that the proposed cryptosystem has higher robustness against noise and special attacks.
Multiuser optical information authentication using photon counting in spiral phase transform domain
Conference paper, Proceedings of SPIE - The International Society for Optical Engineering, 2019, DOI Link
View abstract ⏷
In the present time, the substantial advances in communication technology have prompted many challenges including secure transmission of sensitive information. Optical techniques have also been studied extensively for information security and validation. In this paper, we present a new optical information authentication techniques using photon counting in spiral phase transform (SPT) domain. SPT is used for the optical propagation. For SPT, a modified spiral phase function is utilized which defines the order of the transform. In the encryption process, an asymmetric approach is used, in which, first the input image is combined with the random phase mask and Fresnel propagated with a distance, d to get the intermediated image. Further, the polar decomposition (PD) is applied to the intermediate image which will give a rotational matrix and two symmetric matrices. The symmetric matrices can be used for authenticating the original information and serve as the private keys. The final encrypted image is obtained by performing the SPT of a particular order on the rotational matrix after PD. The encrypted image is made sparse by randomly retaining few pixels using photon counting approach. For authentication, the nonlinear correlation approach is studied, which offer better correlation peaks with fewer sparse-based complex samples. The numerical simulation results have been presented in support of the validity and effectiveness of the proposed technique. The proposed method deals with the optical encryption, authentication and also overcomes the limitations of data storage issue.
Optical colour image encryption using spiral phase transform and chaotic pixel scrambling
Article, Journal of Modern Optics, 2019, DOI Link
View abstract ⏷
In this study, we propose a new optical colour image encryption technique using spiral phase transform and chaotic pixel scrambling. For encryption, three channels of the colour image i.e. red, green and blue are first separated and modulated with three different structured phase masks. Spiral phase transform (SPT) with a particular order of modified spiral phase function (MSPF) is utilized for further processing. Random modulus decomposition is applied to the complex output after SPT to generate the private key for decryption. The pixels of the image are scrambled by using the chaotic Tinkerbell map for enhanced security. The order of MSPF, three structured phase masks, parameters of Tinkerbell mapping, and the private key generated during the encryption process serve as the security keys. The robustness of the proposed method is checked against various potential attacks. A series of numerical simulation results are presented to validate the proposed colour image encryption method.
RGB laser speckles based 3D profilometry
Zhong F., Kumar R., Quan C.
Article, Applied Physics Letters, 2019, DOI Link
View abstract ⏷
Laser speckle is becoming popular for encoding objects for 3D profile reconstructions. However, the measurement accuracy of laser speckles based stereophotogrammetry is often not high due to the subjective speckles formed on the sensor plane of the camera. In this letter, we propose a method to enhance the measurement accuracy by encoding the surface of the object using red, green, and blue (RGB) laser speckles which are generated when a combined RGB laser beam passes through a ground glass diffuser. A 3CCD color camera is utilized to capture the object from two different perspectives. Interestingly, there is no crosstalk between different channels. Hence, the proposed system is practically equivalent to three synchronized monochrome vision systems having separate red, green, and blue laser speckle projections. Correspondences located by the RGB speckle image correlation are more accurate, because the structured information in the color image is three times that in the monochrome image. Experimental results validate the higher accuracy of the proposed method against the conventional method using monochrome laser speckles. Moreover, the motion robustness of the proposed method is verified on the measurement of the moving objects.
Optical voice information hiding using enhanced iterative algorithm and computational ghost imaging
Kumar R., Zhong F., Quan C.
Article, Journal of Optics (United Kingdom), 2019, DOI Link
View abstract ⏷
Recently, optical techniques have been studied extensively for securing grayscale and colour images. However, at the present time, a lot of sensitive information has also been transmitted in the form of audio/voice signals, thus, the development of secure optical transmission systems for voice signals is also very important. In this paper, we present a new optical cryptosystem for the secure transmission of voice/audio signals. The voice information is distributed among two phase masks and a security image using the enhanced iterative approach. In that, the 2D non-separable linear canonical transform (NS-LCT) is utilized. 2D NS-LCT is defined using ten independent parameters, all of which can be used as the security keys for the cryptosystem. Also, the security image containing the voice information can be further processed through optical encryption systems, which add an extra layer of security. The numerical simulation results are presented to demonstrate the validity and efficiency of the proposed method.
An iris biometric protection scheme using 4D hyperchaotic system and modified equal modulus decomposition in hybrid multi resolution wavelet domain
Rakheja P., Vig R., Singh P., Kumar R.
Article, Optical and Quantum Electronics, 2019, DOI Link
View abstract ⏷
In this paper, a hybrid iris biometric protection scheme using 4D hyperchaotic system by means of coherent superposition and modified equal modulus decomposition in hybrid multi-resolution wavelet domain is proposed. The 4D hyperchaotic framework is employed for producing the permutation keystream for pixel swap-over mechanism. The hybrid multi-resolution is produced by means of Kronecker product of Walsh transform and fractional Fourier transform of different orders. Fractional orders of the hybrid multiresolution wavelet along with the parameters and initial conditions of the 4D hyperchaotic framework, later fill in as additional keys, thus enhancing the key space of the proposed mechanism. The proposed scheme has nonlinear characteristics and has high robustness against brute-force attack attributable to its enormous key-space. Numerical simulations have been carried out on grayscale images to prove the authenticity and efficacy of the proposed cryptosystem. The functioning of the proposed cryptosystem is investigated against various attacks including special attack. Outcomes display that the proposed iris template protection scheme not only has higher robustness against noise attack but is also unassailable to the iterative transform-based attacks.
A cost-effective single-shot structured light system for 3d shape measurement
Zhong F., Kumar R., Quan C.
Article, IEEE Sensors Journal, 2019, DOI Link
View abstract ⏷
Single-shot three-dimensional (3D) shape measurement techniques have attracted extensive researches, as they are suitable for dynamic measurement. In this study, a cost-effective single-shot structured light system (CES-SLS) is proposed for 3D shape measurement by using a color camera and a normal projector. With the aid of two planar mirrors and an isosceles right-angle mirror, a single color camera functions as a binocular vision. A single-shot color random speckle pattern (CRSP) is used to encode the object by the normal projector. The dense corresponding points (DCP) are derived by the spatial-temporal correlation of RGB image subsets instead of the spatial correlation of gray image subsets in the conventional method. An inverse compositional Gaussian Newton (IC-GN) iteration method with the second-order shape function is then introduced to find the sub-pixel corresponding points (CP). Furthermore, a geometrical 3D recovery method is presented to calculate the 3D point by minimizing the re-projection error. The experimental results demonstrate the comparative advantages of the proposed CES-SLS against the system using a single-shot random speckle pattern (RSP) and the curve fitting sub-pixel method in the aspects of measurement accuracy and noise robustness. In addition, the re-projection error of each 3D point from the geometrical method is smaller than that of the conventional mid-point.
Asymmetric multi-user optical cryptosystem based on polar decomposition and Shearlet transform
Article, Optics and Lasers in Engineering, 2019, DOI Link
View abstract ⏷
In this paper, we have proposed an asymmetric optical cryptosystem using polar decomposition and Shearlet transform (ST). Polar decomposition is employed to generate multiple private keys for decryption. Gyrator transform (GT) with angle α, is used for optical propagation. Multi-scale directional property of ST is utilized to get Shearlet coefficients from the complex output after GT. The first coefficient is then replaced by a random phase mask before performing inverse ST. The detailed robustness analysis of the proposed method in terms of various potential attacks, such as, noise, occlusion, brute force, special attack, and plaintext attacks is performed and compared with the traditional asymmetric method. Results are presented to demonstrate the validity and effectiveness of the proposed technique.
Security Analysis on an Optical Encryption and Authentication Scheme Based on Phase-Truncation and Phase-Retrieval Algorithm
Xiong Y., Kumar R., Quan C.
Article, IEEE Photonics Journal, 2019, DOI Link
View abstract ⏷
In this paper, the security of the cryptosystem based on phase-truncation Fourier transform (PTFT) and Gerchberg-Saxton (G-S) algorithm is analyzed. In this cryptosystem, the phase key generated using phase-truncated (PT) operation is bonded with the phase key generated in G-S algorithm to form the first private key, which improves the complexity of the first private key. In addition, since the second private key is generated using the G-S algorithm, the number of known constraints decreases compared to the traditional PTFT-based cryptosystem, which will lead the non-convergence of special attacks. However, it has been found that two private keys generated in the cryptosystem based on PTFT and G-S algorithm are related to one phase key generated in the G-S algorithm, which provides an additional constraint to retrieve the other private key when one private key is disclosed. Based on this analysis, two iterative processes with different constraints are proposed to crack the cryptosystem based on PTFT and G-S algorithm. This is the first time to report the silhouette problem existing in the cryptosystem based on PTFT and G-S algorithm. Numerical simulations are carried out to validate the feasibility and effectiveness of our analysis and proposed iterative processes.
Nonlinear QR code based optical image encryption using spiral phase transform, equal modulus decomposition and singular value decomposition
Kumar R., Bhaduri B., Nishchal N.K.
Article, Journal of Optics (United Kingdom), 2018, DOI Link
View abstract ⏷
In this study, we propose a quick response (QR) code based nonlinear optical image encryption technique using spiral phase transform (SPT), equal modulus decomposition (EMD) and singular value decomposition (SVD). First, the primary image is converted into a QR code and then multiplied with a spiral phase mask (SPM). Next, the product is spiral phase transformed with particular spiral phase function, and further, the EMD is performed on the output of SPT, which results into two complex images, Z 1 and Z 2. Among these, Z 1 is further Fresnel propagated with distance d, and Z 2 is reserved as a decryption key. Afterwards, SVD is performed on Fresnel propagated output to get three decomposed matrices i.e. one diagonal matrix and two unitary matrices. The two unitary matrices are modulated with two different SPMs and then, the inverse SVD is performed using the diagonal matrix and modulated unitary matrices to get the final encrypted image. Numerical simulation results confirm the validity and effectiveness of the proposed technique. The proposed technique is robust against noise attack, specific attack, and brutal force attack. Simulation results are presented in support of the proposed idea.
QR code-based non-linear image encryption using Shearlet transform and spiral phase transform
Kumar R., Bhaduri B., Hennelly B.
Article, Journal of Modern Optics, 2018, DOI Link
View abstract ⏷
In this paper, we propose a new quick response (QR) code-based non-linear technique for image encryption using Shearlet transform (ST) and spiral phase transform. The input image is first converted into a QR code and then scrambled using the Arnold transform. The scrambled image is then decomposed into five coefficients using the ST and the first Shearlet coefficient, C1 is interchanged with a security key before performing the inverse ST. The output after inverse ST is then modulated with a random phase mask and further spiral phase transformed to get the final encrypted image. The first coefficient, C1 is used as a private key for decryption. The sensitivity of the security keys is analysed in terms of correlation coefficient and peak signal-to noise ratio. The robustness of the scheme is also checked against various attacks such as noise, occlusion and special attacks. Numerical simulation results are shown in support of the proposed technique and an optoelectronic set-up for encryption is also proposed.
Nonlinear double image encryption using 2D non-separable linear canonical transform and phase retrieval algorithm
Kumar R., Sheridan J.T., Bhaduri B.
Article, Optics and Laser Technology, 2018, DOI Link
View abstract ⏷
In this paper, we propose a new asymmetric method for double image encryption using the two-dimensional non-separable linear canonical transform (2D NS-LCT) and an iterative phase retrieval algorithm (PRA). First an encryption security key is generated using the PRA. Then two images (2D intensities) are combined to form a complex image. The 2D NS-LCT of this complex valued image is then obtained. A nonlinear phase truncation operation is applied to the transformed image and the amplitude is retained and used as the private key. The phase term is multiplied by the security PRA generated key and the inverse 2D NS-LCT is applied to the result giving the output encrypted image. The robustness of the proposed technique is tested against noise, occlusion and chosen-plaintext attacks. Numerical results are presented demonstrating the security of the proposed technique.
Asymmetric optical image encryption using Kolmogorov phase screens and equal modulus decomposition
Kumar R., Bhaduri B., Quan C.
Article, Optical Engineering, 2017, DOI Link
View abstract ⏷
An asymmetric technique for optical image encryption is proposed using Kolmogorov phase screens (KPSs) and equal modulus decomposition (EMD). The KPSs are generated using the power spectral density of Kolmogorov turbulence. The input image is first randomized and then Fresnel propagated with distance d. Further, the output in the Fresnel domain is modulated with a random phase mask, and the gyrator transform (GT) of the modulated image is obtained with an angle α. The EMD is operated on the GT spectrum to get the complex images, Z1 and Z2. Among these, Z2 is reserved as a private key for decryption and Z1 is propagated through a medium consisting of four KPSs, located at specified distances, to get the final encrypted image. The proposed technique provides a large set of security keys and is robust against various potential attacks. Numerical simulation results validate the effectiveness and security of the proposed technique.
Optical image encryption using Kronecker product and hybrid phase masks
Article, Optics and Laser Technology, 2017, DOI Link
View abstract ⏷
In this paper, we propose a new technique for security enhancement in optical image encryption system. In this technique we have used the Kronecker product of two random matrices along with the double random phase encoding (DRPE) scheme in the Fresnel domain for optical image encryption. The phase masks used here are different than the random masks used in conventional DRPE scheme. These hybrid phase masks are generated by using the combination of random phase masks and a secondary image. For encryption, the input image is first randomized and then the DRPE in the Fresnel domain is performed using the hybrid phase masks. Secondly, the Kronecker product of two random matrices is multiplied with the DRPE output to get the final encoded image for transmission. The proposed technique consists of more unknown keys for enhanced security and robust against various attacks. The simulation results along with effects under various attacks are presented in support of the proposed technique.
Optical image encryption in Fresnel domain using spiral phase transform
Article, Journal of Optics (United Kingdom), 2017, DOI Link
View abstract ⏷
In this study, we propose a new nonlinear optical image encryption technique using spiral phase transform (SPT). First, the primary image is phase encoded and multiplied with a random amplitude mask (RAM), and using power function, the product is then powered to m. This powered output is Fresnel propagated with distance z 1 and then modulated with a random phase mask (RPM). The modulated image is further Fresnel propagated with distance z 2. Similarly, a security image is also modulated with another RAM and then Fresnel propagated with distance z 3. Next, the two modulated images after Fresnel propagations, are interfered and further Fresnel propagated with distance z 4 to get a complex image. Finally, this complex image is SPT with particular spiral phase function (SPF), to get the final encrypted image for transmission. In the proposed technique, the security keys are Fresnel propagation distances, the security image, RPM, RAMs, power order, m, and order of SPF, q. Numerical simulation results confirm the validity and effectiveness of the proposed technique. The proposed technique is robust against noise and brutal force attacks.
Double image encryption in Fresnel domain using wavelet transform, gyrator transform and spiral phase masks
Conference paper, Proceedings of SPIE - The International Society for Optical Engineering, 2017, DOI Link
View abstract ⏷
In this paper, we propose a new technique for double image encryption in the Fresnel domain using wavelet transform (WT), gyrator transform (GT) and spiral phase masks (SPMs). The two input mages are first phase encoded and each of them are then multiplied with SPMs and Fresnel propagated with distances d1 and d2, respectively. The single-level discrete WT is applied to Fresnel propagated complex images to decompose each into sub-band matrices i.e. LL, HL, LH and HH. Further, the sub-band matrices of two complex images are interchanged after modulation with random phase masks (RPMs) and subjected to inverse discrete WT. The resulting images are then both added and subtracted to get intermediate images which are further Fresnel propagated with distances d3 and d4, respectively. These outputs are finally gyrator transformed with the same angle α to get the encrypted images. The proposed technique provides enhanced security in terms of a large set of security keys. The sensitivity of security keys such as SPM parameters, GT angle α, Fresnel propagation distances are investigated. The robustness of the proposed techniques against noise and occlusion attacks are also analysed. The numerical simulation results are shown in support of the validity and effectiveness of the proposed technique.
