Sanjay Kumar Mishra

Bio: Sanjay Kumar Mishra is an academic researcher from Instruments Research and Development Establishment. The author has contributed to research in topics: Wavefront & Adaptive optics. The author has an hindex of 4, co-authored 18 publications receiving 76 citations. Previous affiliations of Sanjay Kumar Mishra include Defence Research and Development Organisation.

Differential modal Zernike wavefront sensor employing a computer-generated hologram: a proposal

Abstract: The process of Zernike mode detection with a Shack-Hartmann wavefront sensor is computationally extensive. A holographic modal wavefront sensor has therefore evolved to process the data optically by use of the concept of equal and opposite phase bias. Recently, a multiplexed computer-generated hologram (CGH) technique was developed in which the output is in the form of bright dots that specify the presence and strength of a specific Zernike mode. We propose a wavefront sensor using the concept of phase biasing in the latter technique such that the output is a pair of bright dots for each mode to be sensed. A normalized difference signal between the intensities of the two dots is proportional to the amplitude of the sensed Zernike mode. In our method the number of holograms to be multiplexed is decreased, thereby reducing the modal cross talk significantly. We validated the proposed method through simulation studies for several cases. The simulation results demonstrate simultaneous wavefront detection of lower-order Zernike modes with a resolution better than λ/50 for the wide measurement range of ±3.5λ with much reduced cross talk at high speed.

Direct amplitude detection of Zernike modes by computer-generated holographic wavefront sensor: Modeling and simulation

Abstract: A fast holographic wavefront sensor is proposed using a computer-generated hologram (CGH). This CGH is a multiplexed hologram of different Zernike mode–amplitude combinations, and is designed in such a manner as to get the corresponding spots on the detector according to the presence and strength of a particular aberration. Interference between the aberrated wavefront (with a single mode–amplitude combination) and the Fourier transform of an image with single bright pixel (defined as dot image) is numerically calculated for one hologram. Different mode–amplitude combination and corresponding different positions of bright pixels (dots) are taken to compute various holograms and then all the holograms are multiplexed to get the final hologram. When the aberrated wavefront with a particular mode–amplitude combination is incident onto the multiplexed hologram, the corresponding dot is generated in the Fourier plane. A lens performs the Fourier transform in optical domain and provides the instant detection of amplitude of the respective Zernike mode. The main advantage of the scheme is to avoid the need of any computations, which makes it really fast. The simulation results are presented with the cross-talk analysis for few Zernike terms.

Sensitivity of singular beams in the presence of Zernike aberrations

Abstract: Singular beams in the presence of Zernike aberrations create an opportunity for various applications such as trapping and manipulation of micro-particles, atomic optics and atmospheric optics. In the milieu of importance of the role of aberrations, sensitivity of singular beams with Zernike aberrations is studied. In this paper, the effect of various Zernike aberrations on a singular beam is reported in terms of its Point Spread Function (PSF) deformations. The intensity distributions around the focal plane, i.e. PSF, of the singular beam of various topological charges and in the presence of different strengths of Zernike aberrations are theoretically estimated by the Huygens–Fresnel diffraction integral. Experimentally, the singular beams have been generated and known strengths of Zernike aberration introduced in the beam by a phase-only Spatial Light Modulator. Metric Ensquared Energy is used to analyze the PSF of the corresponding intensity distributions of the singular beams. The experimental results have been validated with numerical simulation.

Propagation of optical vortices embedded with multiple wavefront singularities

Abstract: Optical vortices with the embedded wavefront singularities have attracted intensive attentions in many branches of modern physics, due to their important applications in optical tweezers, quantum entangles, optical testing, atmospheric propagations, etc. In this paper, optical vortices are generated by new types of custom designed wavefronts and their propagation in free-space is reported. Huygens–Fresnel diffraction integral is directly solved using the Gauss–Legendre quadrature method to estimate the diffraction pattern at some arbitrary plane. The variation of vorticity is demonstrated under diffraction. Evolution of phase singularities in wavefronts as the wavefront propagate is predicted for various near field distances. Simulations reveal that the exchange of the nature of topological charge occurs at a finite distance. Experimentally, the wavefronts have been generated using the phase-only spatial light modulator and their far-field diffraction patterns are recorded. The experimental result has bee...

Systematic Characterization of Near-Index-Matched Optics Based Atmospheric Turbulence Simulator

Abstract: In the present communication, a systematic characterization of atmospheric turbulence simulator (ATS) based on near-index-matched optics is reported. Characteristics of the propagating laser beam in actual turbulence can be realized in the laboratory by employing such types of turbulence simulators. Such simulators are necessarily required to evaluate the performance of an adaptive optics sensor and compensator modules in the laboratory. Various strengths of atmospheric turbulence can be generated by selecting the different speeds of rotation and the diameters of the beam-interacting area of ATS. A MATLAB-based high-speed video processing method is developed and used for estimating the various turbulence parameters such as angle-of-arrival fluctuations, Fried parameter, Hurst exponent, turbulence frequencies and the scintillation index from the generated turbulence. Also, the maximum transmitted wavefront error produced by this turbulence simulator is measured by an in-house developed Shack–Hartmann wavefront sensor.

Generation and decomposition of scalar and vector modes carrying orbital angular momentum: a review

Abstract: Orbital angular momentum (OAM), one of the most recently discovered degrees of freedom of light beam field has fundamentally revolutionized optical physics and its technological capabilities. Optical beams with OAM have enabled a large variety of applications, including super-resolution imaging, optical trapping, classical and quantum optical communication, and quantum computing, to mention a few. To enable these and several other emerging applications, optical beams with OAM have been generated using a variety of methods and technologies, such as a simple astigmatic lens pair, one-/two-dimensional holographic optical elements, three-dimensional spiral phase plates, optical fibers, and recent entrants such as metasurfaces. All these techniques achieve spatial light modulation and can be implemented with either passive elements or active devices, such as liquid crystal on silicon and digital micromirror devices. Many of these devices and technologies are not only used for the generation of amplitude phase-polarization structured light beams but are also capable of analyzing them. We have attempted to encompass a wide variety of such technologies as well as a few emerging methodologies, broadly categorized into generation and detection protocols. We address the needs of scientists and engineers who desire to generate/detect OAM modes and are looking for the technique (active or passive) best suited for their application.

Response analysis of holography-based modal wavefront sensor.

Abstract: The crosstalk problem of holography-based modal wavefront sensing (HMWS) becomes more severe with increasing aberration. In this paper, crosstalk effects on the sensor response are analyzed statistically for typical aberrations due to atmospheric turbulence. For specific turbulence strength, we optimized the sensor by adjusting the detector radius and the encoded phase bias for each Zernike mode. Calibrated response curves of low-order Zernike modes were further utilized to improve the sensor accuracy. The simulation results validated our strategy. The number of iterations for obtaining a residual RMS wavefront error of 0.1λ is reduced from 18 to 3.

Properties of a strongly focused Gaussian beam with an off-axis vortex

Abstract: The intensity distribution and the phase properties, especially the Gouy phase and the phase singularities are studied in a strongly focused Gaussian beam with an off-axis vortex. The symmetry relation of the focused field is also derived. It is found that the off-axis vortex induces a rotation of the field pattern, the transverse focal shift, and the asymmetric distribution of the phase singularities. Our results also show that the initial position of the off-axis vortex in the incident beam strongly influences the distance of the transverse focal shift, but does not have an effect on the Gouy phase along the central axis.

Differential modal Zernike wavefront sensor employing a computer-generated hologram: a proposal

Abstract: The process of Zernike mode detection with a Shack-Hartmann wavefront sensor is computationally extensive. A holographic modal wavefront sensor has therefore evolved to process the data optically by use of the concept of equal and opposite phase bias. Recently, a multiplexed computer-generated hologram (CGH) technique was developed in which the output is in the form of bright dots that specify the presence and strength of a specific Zernike mode. We propose a wavefront sensor using the concept of phase biasing in the latter technique such that the output is a pair of bright dots for each mode to be sensed. A normalized difference signal between the intensities of the two dots is proportional to the amplitude of the sensed Zernike mode. In our method the number of holograms to be multiplexed is decreased, thereby reducing the modal cross talk significantly. We validated the proposed method through simulation studies for several cases. The simulation results demonstrate simultaneous wavefront detection of lower-order Zernike modes with a resolution better than λ/50 for the wide measurement range of ±3.5λ with much reduced cross talk at high speed.

Practical method for color computer-generated rainbow holograms of real-existing objects.

Abstract: A novel method for computer-generated rainbow holograms (CGRHs) of full-color objects is proposed. First, a new algorithm for fabricating full-color CGRHs of real-existing objects is proposed based on the interrelationship between coding of a CGRH and reconstruction of the hologram. Second, a color rainbow hologram for a real-existing object is generated by combining the proposed algorithm and computer-generated hologram generating system. Finally, the hologram is outputted by an auto-microfilming system. The principle of the algorithm, the process of hologram calculation, and the hologram generating system for real-existing objects and experimental results are presented. The experimental results demonstrate that the new method is feasible.