arXiv subjects

Vishwa Pal

Publications and source records attributed to Vishwa Pal.

At least 19 recordsLinked to original sources

arXiv2026

Enhanced abrupt autofocusing and depth-of-focus of circular Airy derivative beams via controlled asymmetry

We demonstrate simultaneous enhancement of abrupt autofocusing and depth-of-focus (DOF) in circular Airy derivative beams through controlled asymmetry. The controlled asymmetry enables multiple discrete autofocusing events to merge into a single elongated focal region, thereby enhancing both the autofocusing ability and the DOF. The DOF is extended by up to 400% with a 32% improvement in autofocusing ability and a ~ 32 {\mu}m focal spot size, whereas the autofocusing ability can be further enhanced by up to 193% with a ~ 56 {\mu}m focal spot size, while maintaining nearly the same DOF. Experimental results show good agreement with numerical simulations. The resulting asymmetric circular Airy derivative beams (ACADBs), combining strong abrupt autofocusing with extreme DOF, offer promising applications in optical trapping, optical communications, microfabrication, and biomedical imaging

physics.optics↗

arXiv2026

Spatially multiplexed concentric discrete optical vortices: Complex topological structures and unconventional rotational dynamics

Precise control over the rotational dynamics of structured lights has become a defining objective in contemporary photonics. It plays a central role in governing the functional distribution of optical energy. Particularly, orbital angular momentum driven intensity rotation and azimuthal energy flow in vortex beams have emerged as crucial degrees of freedom in light-matter interactions. Leveraging this foundation, we establish a discrete optical vortex (DOV) platform comprising concentric rings of phase-locked lasers that enable precise control over vortex-beam rotation. By engineering the spatial distribution of topological charges (TCs) across the concentric rings of DOVs, we realize shape-invariant asymmetric vortex beams with controllable reversal of intensity rotation, without altering the sign of constituent TCs. This result establishes a new framework for controlling energy flow in vortex beams, beyond the conventional paradigm that links vortex rotation solely to the sign of the TC. Such controllable rotational dynamics opens new avenues for programmable beam steering, advanced optical micromanipulation, information multiplexing, and adaptive structured-light systems.

physics.optics↗

arXiv2026

Influence of Refractive Index Distribution on Multimode Soliton Dynamics and Condensation in GRIN-MMFs

Optical solitons propagating through a multimode fiber represents one of the most fascinating class of objects exhibiting peculiar properties, with widespread potential for applications. We theoretically investigate the effect of the core refractive index distribution, characterized by the index exponent $\alpha$, on the evolution of multimode (MM) soliton beams and their peculiar properties in graded-index multimode fibers. Our analysis reveals an optimal range $\alpha$ = 2.04-2.08, within which MM solitons with minimum pulsewidth and characteristic energy are formed, owing to reduced modal walk-off and enhanced intermodal nonlinear interactions. Within this regime, the MM soliton undergoes efficient spatial condensation into the fundamental mode, resulting in a well-defined quasi-Gaussian output intensity profile. Notably, for some particular values of $\alpha$, we observe a reversal of conventional energy flow associated with MM soliton condensation, leading to the net transfer of energy toward higher-order modes, akin to the thermalization of MM optical fields into negative-temperature equilibrium states. Furthermore, we show that the characteristic Raman-induced spectral redshift of MM solitons can be controlled by tailoring the refractive index distribution. Our results highlight the refractive index distribution as a key control parameter governing MM soliton dynamics and their condensation behavior and are expected to be relevant for the design and optimization of MM fiber-based systems where controlled spatiotemporal dynamics are desired.

physics.optics↗

arXiv2026

Feedback-Driven Ground-State Search in Coupled Laser Arrays

Optimisation problems, which appear in numerous fields of science and industry, are challenging to solve even with modern supercomputers. Many such problems can be mapped onto ground-state searches of spin Hamiltonians, implemented on various physical platforms whose intrinsic dynamics are analogous to spin systems. However, the complex energy landscape of spin Hamiltonians often traps the system in local minima, preventing the system from reaching the ground-state (global minimum). We demonstrate an intrinsic feedback-driven annealing mechanism in class-B semiconductor laser arrays arising from the interplay of internal ($\alpha$) and external ($\eta$) coupling. The instantaneous phase configuration self-modulates amplitude fluctuations, which act as an effective temperature, dynamically reshaping the potential and enabling the system to escape from local minima. Using a one-dimensional ring laser array, we analyze defect formation in the $\alpha$-$\eta$ parameter space and identify an optimal regime achieving nearly 100% ground-state probability. Although both $\alpha$ and $\eta$ are essential for the feedback loop, defect suppression results from modifying two competing timescales: amplitude stabilization (t_amp) and phase locking (t_phase), analogous to the Kibble-Zurek mechanism. These timescales can be tuned independently via $\alpha$ or $\eta$. Identical timescale ratios yield identical defect probabilities, confirming that relative timescales, not specific parameters, govern defect formation. Our findings establish internal feedback-driven annealing as a practical route to ground-state search in semiconductor laser arrays, providing a foundation for efficient and scalable laser-based spin simulators for tackling hard optimization problems.

physics.optics↗

arXiv2025

Generating non-diffracting bottle beams with a flat multi-level diffractive lens

We introduce a novel method for creating a high-quality, sharply defined, non-diffracting optical bottle beam by focusing a Bessel beam propagating through a flat multi-level diffractive lens (MDL). This study highlights the impact of the MDL illuminated by a Bessel beam with suppressed sidelobes generated from a binary axicon. The resulting Bessel bottle beam exhibits a series of low- or zero-intensity zones interleaved with high-intensity regions, with variable periods ranging from 0.2 to 1.36 mm along the beam propagation. The transverse intensity profiles of these regions remain shape-invariant over long distances in free space, and thereby the non-diffracting range of the micron-sized optical bottle beam exceeds 5 cm. We also observe that the far-field output from the MDL illuminated by a Bessel beam offers advantages over conventional focusing lenses. Furthermore, this technique can operate on ultrafast timescales (from pico- to femtoseconds) due to the high damage thresholds of the binary axicon and MDL, enabling the generation of high-power optical bottle beams. Ultimately, our experimental approach paves the way for various applications, including high-resolution biological imaging in turbid media, particle manipulation, micromachining, and harmonic generation, leveraging the spatial landscape of the optical bottle beam.

physics.optics↗

arXiv2024

Non-symmetrical vortex beam shaping in VECSEL laser arrays

We propose and numerically test a novel concept for asymmetric vortex beam generation in a Degenerate Vertical External Cavity Surface Emitting Laser (DVECSEL). The method is based on a phase-locking ring array of lasers created inside a degenerate cavity with a binary amplitude mask containing circular holes. The diffraction engineering of the mask profile allows to control the complex coupling between the lasers. The asymmetry between different lasers is introduced by varying the hole diameters corresponding to different lasers. Several examples of masks with non-uniform or uniform circular holes are investigated numerically and analytically to assess the impact of non-uniform complex coupling coefficients on the degeneracy between the vortex and anti-vortex steady-states of the ring laser arrays. It is found that the in-phase solution always dominates irrespective of non-uniform masks. The only solution to make one particular vortex solution dominant over other possible steady-state solutions consists in imprinting the necessary phase shift among neighboring lasers in the argument of their coupling coefficients. We also investigate the role of the Henry factor inherent to the use of a semiconductor active medium in the probabilities to generate vortex solutions. Analytical calculations are performed to generalize a formula previously reported in Opt. Express 30, 15648 (2022) for the limiting Henry factor to cover the case of complex couplings.

physics.optics↗

arXiv2024

Propagation of circular Airy derivative beams in complex media

Controlling light propagation through complex media plays a significant role in a wide range of applications ranging from astronomical observations to microscopy. Although, several advances have been made based on adaptive optics, optical phase conjugation and wavefront shaping, but many of these involve challenges. Recently, controlling light propagation in complex media by simply structuring light has shown promising capabilities. We present experimental and numerical investigations of abruptly autofocusing of circular Airy derivative beams (CADBs) in complex media. We find that up to a relatively high turbulence strength, CADB possesses relatively good abrupt autofocusing, however, efficiency and autofocusing position vary with the strength of turbulence. Further, the spatial distortions in CADB caused by turbulence are quantified by an overlap integral, which shows that CADB possesses reasonably good resilience against the turbulence. The diffraction efficiency of CADB changes by a factor of ~ 1.7 with increasing strength of turbulence from zero to high, indicating good confinement of intensity at autofocusing. The focused beam spot size grows gradually with increasing the strength of turbulence, specifically, it grows by a factor of ~ 2 for a strong turbulence, indicating reasonably good focusing abilities. The results of CADB are compared with a Gaussian beam, and find that CADB possesses superior focusing abilities in turbulent media. We have carried out a detailed analysis of these observations based on Zernike polynomials, which reveals that different kinds of aberrations present in turbulent media leads to distortions in the spatial structure as well as other properties of CADBs. Our results can be used for various applications, such as in biomedical treatment, seismology, optical tweezers and material processing.

physics.optics↗

arXiv2023

Autofocusing and self-healing of partially blocked circular Airy derivative beams

We numerically and experimentally study the autofocusing and self-healing of partially blocked circular Airy derivative beams (CADBs). The CADB consists of multiple rings, and partial blocking of CADB with different kinds is achieved by using symmetric and asymmetric binary amplitude masks, enabling blocking of inner/outer rings and sectorially. The CADB blocked with different types possesses the ability to autofocus, however, the required propagation distance for abrupt autofocusing vary with the amount and types of blocking. The abrupt autofocusing is quantified by a maximum k-value, and how fast it changes around the autofocusing distance ($z_{af}$). In particular, CADB blocked with inner rings (first/two/three) exhibits an abrupt autofocusing, as the k-value sharply increases [decreases] just before [after] $z_{af}$. The maximum k-value always occurs at $z_{af}$, which decreases as the number of blocked inner rings increases. For CADB blocked with outer rings, the k-value gradually changes around $z_{af}$, indicating a lack of abrupt autofocusing. The value of $z_{af}$ increases with the number of blocked outer rings. This suggests that although outer rings contain low intensities, these play an important role in autofocusing. A sectorially blocked CADB possesses an abrupt autofocusing, and maximum k-value depends on the amount of blocking. The CADB blocked with different types possesses good self-healing abilities, where blocked parts reappear as a result of redistribution of intensity. The maximum self-healing occurs at $z_{af}$, where an overlap integral approaches a maximum value. Finally, we have compared ideal CADB and partially blocked CADB having the same radii, and found that an ideal CADB possesses better abrupt autofocusing. We have found a good agreement between the numerical simulations and experimental results.

physics.optics↗

arXiv2023

Probing topological charge of discrete vortices

Discrete vortex, formed by a one-dimensional (1D) ring array of lasers, contains high output power as compared to a conventional continuous vortex, therefore, has attracted considerable interest due to widespread applications in various fields. We present a method for probing the magnitude and sign of the topological charge (TC) of an unknown discrete vortex, by analyzing the interference pattern of a 1D ring array of lasers. The interference pattern of an unknown discrete vortex with TC$\neq 0$ is averaged with the interference pattern of TC= 0, which gives rise to a variation in the fringe visibility as a function of laser number (j) in a 1D ring array. The number of dips observed in the fringe visibility curve is found to be proportional to the magnitude of TC of a discrete vortex. The sign of TC is determined by averaging the interference patterns of unknown discrete vortex (TC$\neq 0$) with known TC= +1. The number of dips in the fringe visibility curve decreases by one for a positive TC, and increases by one for a negative TC. Further, we have verified our method against the phase disorder, and it is found that the phase disorder does not influence an accurate determination of TC of a discrete vortex. The working principle as well as numerical and experimental results are presented for the discrete vortices with TC from small to large values. An excellent agreement between the experimental results and numerical simulations is found. Our method can be useful in the applications of discrete vortices.

physics.optics↗

arXiv2022

Generating asymmetric aberration laser beams with controlled intensity distribution

We present generation of asymmetric aberration laser beams (aALBs) with controlled intensity distribution, using a diffractive optical element (DOE) involving phase asymmetry. The asymmetry in the phase distribution is introduced by shifting the coordinates in a complex plane. The results show that auto-focusing properties of aALBs remain invariant with respect to the asymmetry parameters. However, a controlled variation in the phase asymmetry allows to control the spatial intensity distribution of aALBs. In an ideal ALB containing equal intensity three bright lobes (for $m=3$), by introducing asymmetry most of the intensity can be transferred to any one of single bright lobe, and forms a high-power density lobe. A precise spatial position of high-power density lobe can be controlled by the asymmetry parameter $β$ and $m$, and we have determined the empirical relations for them. We have found that for the specific values of $β$, the intensity in the high-power density lobe can be enhanced by $\sim$6 times the intensity in other bright lobes. The experimental results show a good agreement with the numerical simulations. The findings can be suitable for applications such as in optical trapping and manipulation as well as material processing.

physics.optics↗

arXiv2021

Phase-locking of lasers with Gaussian coupling

A unique approach for steady in-phase locking of lasers in an array, regardless of the array geometry, position, orientation, period or size, is presented. The approach relies on the insertion of an intra-cavity Gaussian aperture in the far-field plane of the laser array. Steady in-phase locking of $90$ lasers, whose far-field patterns are comprised of sharp spots with extremely high power density, was obtained for various array geometries, even in the presence of near-degenerate solutions, geometric frustration or superimposed independent longitudinal modes. The internal phase structures of the lasers can also be suppressed so as to obtain pure Gaussian mode laser outputs with uniform phase and overall high beam quality. The approach could potentially improve the performances of recently developed laser simulators that are used for solving various computational problems.

physics.optics↗

arXiv2021

High-resolution digital control of highly multimode laser

A rapid and efficient method for generating laser beams with controlled intensity, phase and coherence distributions is presented. It is based on a degenerate cavity laser in which a digital phase-only spatial light modulator is incorporated. We show that a variety of unique and high-resolution shaped laser beams can be generated with either a low or a high spatial coherence. We also show that by controlling the phase, intensity and coherence distributions, a laser beam can be efficiently reshaped after propagation.

physics.optics↗

arXiv2019

Rapid fair sampling of XY spin Hamiltonian with a laser simulator

Coupled oscillators such as lasers, OPO's and BEC polaritons can rapidly and efficiently dissipate into a stable phase locked state that can be mapped onto the minimal energy (ground state) of classical spin Hamiltonians. However, for degenerate or near degenerate ground state manifolds, statistical fair sampling is required to obtain a complete knowledge of the minimal energy state, which needs many repetitions of simulations under identical conditions. We show that with dissipatively coupled lasers such fair sampling can be achieved rapidly and accurately by exploiting the many longitudinal modes of each laser to form an ensemble of identical but independent simulators, acting in parallel. We fairly sampled the ground state manifold of square, triangular and Kagome lattices by measuring their coherence function identifying manifolds composed of a single, doubly degenerate, and highly degenerate ground states, respectively.

physics.optics↗

arXiv2019

Dynamics of dissipative topological defects in coupled phase oscillators

The dynamics of dissipative topological defects in a system of coupled phase oscillators, arranged in one and two-dimensional arrays, is numerically investigated using the Kuramoto model. After an initial rapid decay of the number of topological defects, due to vortex-anti-vortex annihilation, we identify a long-time (quasi) steady state where the number of defects is nearly constant. We find that the number of topological defects at long times is significantly smaller when the coupling between the oscillators is increased at a finite rate rather than suddenly turned on. Moreover, the number of topological defects scales with the coupling rate, analogous to the cooling rate in KibbleZurek mechanism (KZM). Similar to the KZM, the dynamics of topological defects is governed by two competing time scales: the dissipation rate and the coupling rate. Reducing the number of topological defects improves the long time coherence and order parameter of the system and enhances its probability to reach a global minimal loss state that can be mapped to the ground state of a classical XY spin Hamiltonian.

physics.optics↗

arXiv2018

Rapid Phase Retrieval by Lasing

Reconstructing an object solely from its scattered intensity distribution is a common problem that occurs in many applications. Currently, there are no efficient direct methods to reconstruct the object, though in many cases, with some prior knowledge, iterative algorithms result in reasonable reconstructions. Unfortunately, even with advanced computational resources, these algorithms are highly time consuming. Here we present a novel rapid all-optical method based on a digital degenerate cavity laser, whose most probable lasing mode well approximates the object. We present experimental results showing the high speed (<100 ns) and efficiency of our method in agreement with our numerical simulations and analysis. The method is scalable, and can be applicable to any two dimensional object with known compact support, including complex-valued objects.

physics.optics↗

arXiv2018

Spatio-Temporal Supermodes:Rapid Reduction of Spatial Coherence in Highly Multimode Lasers

Spatial coherence quantifies spatial field correlations over time, and is one of the fundamental properties of light. Here we investigate the spatial coherence of highly multimode lasers in the regime of short time scales. Counter intuitively, we show that in this regime, the temporal (longitudinal) modes play a crucial role in spatial coherence reduction. To evaluate the spatial coherence we measured the temporal dynamics of speckle fields generated by a highly multimode laser with over $10^5$ lasing spatial (transverse) modes, and examined the dependence of speckle contrast on the exposure time of the detecting device. We show that in the regime of short time scale, the spatial and temporal modes interact to form spatio-temporal supermodes, such that the spatial degrees of freedom are encoded onto the temporal modes. As a result, the speckle contrast depends on the number of temporal modes, and the degree of spatial coherence is reduced and the speckle contrast is suppressed. In the regime of long times scale, the supermodes are no longer a valid representation of the laser modal structure. Consequently, the spatial coherence is independent of the temporal modes, and the classical result, where the speckle contrast is suppressed as the number of spatial modes, is obtained. Due to this new spatio-temporal mechanism, highly multimode lasers can be used for speckle suppression in high-speed full-field imaging applications, as we demonstrate here for imaging of a fast moving object.

physics.optics↗

arXiv2016

Dissipative Topological Defects in Coupled Laser Networks

Topologically protected defects have been observed and studied in a wide range of fields, such as cosmology, spin systems, cold atoms and optics as they are quenched across a phase transition into an ordered state. Revealing their origin and control is becoming increasingly important field of research, as they limit the coherence of the system and its ability to approach a fully ordered state. Here, we present dissipative topological defects in a 1-D ring network of phase-locked lasers, and show how their formation is related to the Kibble-Zurek mechanism and is governed in a universal manner by two competing time scales of the lasers, namely the phase locking time and synchronization time of their amplitude fluctuations. The ratio between these two time scales depends on the system parameters such as gain and coupling strength, and thus offers the possibility to control the probability of topological defects in the system. Enabling the system to dissipate to the fully ordered, defect-free state can be exploited for solving hard combinatorial optimization problems in various fields. As opposed to unitary systems where quenching is obtained via external cooling mostly through the edges, our dissipative system is kept strictly uniform even for fast quenches.

physics.optics↗

arXiv2015

Conversion of out-of-phase to in-phase order in coupled laser arrays with second harmonics

A novel method for converting an array of out-of-phase lasers into one of in-phase lasers that can be tightly focused is presented. The method exploits second harmonic generation and can be adapted for different laser arrays geometries. Experimental and calculated results, presented for negatively coupled lasers formed in a square, honeycomb, and triangular geometries are in good agreement.

physics.optics↗