Chandra Shakher - Academia.edu (original) (raw)

Papers by Chandra Shakher

Hybrid solar system converts the incident solar radiation into electrical energy by utilizing com... more Hybrid solar system converts the incident solar radiation into electrical energy by utilizing complete solar spectrum. Such a system uses spectrum splitting solar concentrator for separation of solar spectrum into light and heat component and focusses on high-efficiency multi-junction solar cell and heat receiver. In this paper, chirped volume holographic grating (CVHG) is investigated as solar concentrator cum spectrum splitter. The grating is designed to separate the visible light of solar spectrum from the infrared wavelength band. The main advantage of CVHG over normal volume gratings are high diffraction efficiency, large bandwidth separation and lightweight. Chirp rate, period of grating, modulation index, thickness of grating, and grating profile are the critical parameters to be optimized. CVHG using photopolymer as a recording medium was designed with a dimension of 200x200x100µm3. The grating was simulated using rigorous coupled wave analysis for the incident solar spectrum wavelength ranging from 0.3 to 3.0 µm. CVHG designed with hyperbolic index profile showed concentration ratio of 15x and spectrum separation at 1.0µm.

The subaperture stitching technique requires the registration of freeform subapertures into globa... more The subaperture stitching technique requires the registration of freeform subapertures into global coordinate frame before stitching in order to compute entire freeform wavefront. A scanning Shack-Hartman Sensor (SHS) utilizes translation stages to scan the freeform surface in XY plane and measure the slope data of various subapertures. The measured slope data is then integrated using weighted cubic spline (WCSLI) based integration method to compute the phase data. The positioning error during scanning causes misalignments between the measured subapertures and their nominal values. The least square based subaperture stitching methods are not capable to minimize lateral misalignment errors of freeform subapertures and therefore degrade the performance of subaperture stitching process. In this work, we have utilized fiducial added planes for correction of angular and rotation misalignments of an extended cubic phase plate. An intrinsic surface feature (ISF) based registration method is used for lateral misalignment corrections. Gaussian curvature is used as an intrinsic pattern which can be defined as one of the fundamental second order geometric properties of a surface. Any shift in the peaks of the Gaussian curvature of reference and measured subaperture corresponds to lateral misalignments in X and Y directions and need to be minimized before registration of subaperture into global frame of reference. After precise registrations, all the subapertures are stitched with consistent overlapping area by using least square fitting method. A numerical validation of the proposed scheme is carried out which demonstrates the effectiveness of the proposed method to improve the subaperture stitching accuracy.

The paper presents the development of Shack-Hartmann sensor as in-situ metrology technique that c... more The paper presents the development of Shack-Hartmann sensor as in-situ metrology technique that can be integrated to the machining platforms and helps in the development process of aspheric and freeform optics within the required tolerances.

The paper presents a comparative study of different metrology techniques for the measurement of u... more The paper presents a comparative study of different metrology techniques for the measurement of ultra-precision diamond turned freeform optics. Their limitations and advantages are being discussed and compared. The experimental results are presented.

To suppress the Fresnel reflection loss and enhance light transmission, subwavelength structured ... more To suppress the Fresnel reflection loss and enhance light transmission, subwavelength structured surfaces are used as antireflector. In this work, rigorous coupled wave (RCWA) analysis-based design approach has been adopted to simulate nano-sized moth-eye like structures on silicon substrate for wide-angle, short-wave infrared (SWIR) antireflector. Large reflection loss due to high refractive index of silicon is detrimental to optical performance. The proposed surface relief moth-eye structures introduce gradient refractive index to the surface depending on substrate materials, structure geometry, height, periods. Optimum selection of these parameters during design and fabrication are essential steps for the effective quelling of undesirable reflection from air-surface boundary and enhance transmission. Due to subwavelength nature, maintaining accuracy of all design parameters during fabrication on silicon is challenging. Careful tradeoff is required to fix tolerance of each parameter depending on priority to overall performance. In this work, with help of Taguchi optimization techniques, optimum combination of the structure height, periods, and top surface area of moth-eye structure are selected for antireflector. Analysis of variance (ANOVA) approach has been opted to identify the contribution of individual design parameters to performance. This performance model based on RCWA design, Taguchi optimization techniques and ANOVA analysis acts as a tool to predict the performance trend and fix tolerance of design parameters. For wavelength range (700nm -3000nm), with optimized height 600 nm, period 200 nm, and flat top diameter 70 nm of tapered moth-eye structures, the obtained reflectance is less than 1 % for the incidence angle up-to 45°.

Proceedings of SPIE, Jun 26, 2017

The increased range of manufacturable freeform surfaces offered by the new fabrication techniques... more The increased range of manufacturable freeform surfaces offered by the new fabrication techniques is giving opportunities to incorporate them in the optical systems. However, the success of these fabrication techniques depends on the capabilities of metrology procedures and a feedback mechanism to CNC machines for optimizing the manufacturing process. Therefore, a precise and in-situ metrology technique for freeform optics is in demand. Though all the techniques available for aspheres have been extended for the freeform surfaces by the researchers, but none of the techniques has yet been incorporated into the manufacturing machine for in-situ measurement. The most obvious reason is the complexity involved in the optical setups to be integrated in the manufacturing platforms. The Shack-Hartmann sensor offers the potential to be incorporated into the machine environment due to its vibration insensitivity, compactness and 3D shape measurement capability from slope data. In the present work, a measurement scheme is reported in which a scanning Shack-Hartmann Sensor has been employed and used as a metrology tool for measurement of freeform surface in reflection mode. Simulation studies are conducted for analyzing the stitching accuracy in presence of various misalignment errors. The proposed scheme is experimentally verified on a freeform surface of cubic phase profile.

Proceedings of SPIE, Sep 16, 2002

The authors have developed an image processing system and a wavelet transform (Symlet Wavelet) ba... more The authors have developed an image processing system and a wavelet transform (Symlet Wavelet) based processing technique to find the repeat texture of fabric without any priori information. The method is also capable of characterizing the texture of fabric not having an obvious repeat pattern. The proposed methodology is able to measure the warp and weft diameter, and spacing per unit length per yarn and percentage of their coefficient of variation (CV%).

The International Commission for Optics, May 16, 2017

The freeform optical surfaces are the advanced optical elements being used in the optical systems... more The freeform optical surfaces are the advanced optical elements being used in the optical systems ranging from the illumination system, head up display and ophthalmic systems. So far the metrology is not well established for freeform surfaces.There are interferometric, profilometry, deflectometry and slope measurement techniques used to measure the freeform surfaces. Due to non-rotationally symmetric nature of freeform surfaces, slope measurement systems like Shack Hartman Sensors (SHS) are being explored for the measurement of freeform wavefronts. The spatial resolution of Shack Hartmann sensor is limited by the size of the lens lets used in the sensor which is typically 100 μm to 200 μm. The self-imaging based sensing uses a periodic structure which can be replicated under collimated illumination at certain distance known as Talbot distance. If there is a wavefront other than collimated light, the deviation in self-imaging pattern is observed, and this deviation can be utilised for wavefront measurements. Being a smaller pitch of the periodic structure, a high resolution data is obtained. In the present study, we have proposed a high resolution system for measurement of freeform surface using self-imaging based technique, which is having advantage of higher spatial data as compared to Shack Hartman Sensor. A simulation study is carried out and demonstrated the improved performance of the proposed sensor as compared to SHS.

Indian Journal of Health and Wellbeing, May 28, 2015

Orphanhood is generally related to a traumatic experience of loss and grief; it may include physi... more Orphanhood is generally related to a traumatic experience of loss and grief; it may include physical and sexual abuse. The orphan becomes emotionally, socially, physically, and economically dependent on others. He has a feeling of loneliness and dissatisfaction with his life which largely affects their mental health. The present study consists of 100 participants out of which 50 participants were orphan, and 50 participants were nonorphan children. Mental health scale (2002) by Sharma and difficulty in emotion regulation scale by Gratz and Roemer (2004) were used.The results showed a significant difference in mental health and emotion regulation of orphan and nonorphan children.

The paper presents a non-null metrology technique for the shape measurement of freeform wavefront... more The paper presents a non-null metrology technique for the shape measurement of freeform wavefront. A stitching algorithm for freeform shapes has been developed and is applied to sub-apertures measurements taken by Scanning Shack Hartmann Sensor.

Frontiers in Optics / Laser Science, 2018

This paper presents an improved stitching technique for freeform metrology. The lateral misalignm... more This paper presents an improved stitching technique for freeform metrology. The lateral misalignments are minimized using modified iterative closest point based registration. The method is applied to freeform measurement using Shack-Hartmann Sensor.

Proceedings of SPIE, Apr 26, 2016

In this paper, moisture induced deformation and shrinkage behaviour of deodar wood during convect... more In this paper, moisture induced deformation and shrinkage behaviour of deodar wood during convective drying is experimentally investigated by using digital holographic interferometry. There induces dimensional changes in wood due to the moisture absorption and desorption. Lensless Fourier transform digital holographic interferometry (LLFTDH) is used to study the moisture induced deformation and strain distribution in deodar wood. The proposed technique having high sensitivity and enables the observation of deformation and strain distribution during the variations of moisture content in the deodar wood.

Proceedings of SPIE, Aug 24, 2015

In the present work, application of digital speckle pattern interferometry (DSPI) was applied for... more In the present work, application of digital speckle pattern interferometry (DSPI) was applied for the measurement of mechanical/elastic and thermal properties of fibre reinforced plastics (FRP). Digital speckle pattern interferometric technique was used to characterize the material constants (Poisson’s ratio and Young’s modulus) of the composite material. Poisson ratio based on plate bending and Young’s modulus based on plate vibration of material are measured by using DSPI. In addition to this, the coefficient of thermal expansion of composite material is also measured. To study the thermal strain analysis, a single DSPI fringe pattern is used to extract the phase information by using Riesz transform and the monogenic signal. The phase extraction from a single DSPI fringe pattern by using Riesz transform does not require a phase-shifting system or spatial carrier. The elastic and thermal parameters obtained from DSPI are in close agreement with the theoretical predictions available in literature.

Optics and Lasers in Engineering, Nov 1, 2004

In this paper the study of out-of-plane or transverse vibrations in a square plate using digital ... more In this paper the study of out-of-plane or transverse vibrations in a square plate using digital speckle pattern interferometry (DSPI) is presented. To improve the measurement accuracy, we have implemented a new filtering scheme based on combination of average/median filtering and Symlet wavelet filtering which enhances the signal to noise ratio in the speckle interferogram obtained from DSPI. A large number of fringe patterns are shown for square plate with two different boundary conditions. Experimentally obtained resonance frequencies for the square plate for the boundary condition one edge fixed and other edges being free, the resonance frequencies obtained from DSPI show good agreement with that obtained from classical theory for thin plates.

Applied optics, Feb 15, 1987

Use of a two hololens imaging system in speckle metrology is studied in detail. Theoretical analy... more Use of a two hololens imaging system in speckle metrology is studied in detail. Theoretical analysis and experimental investigations reveal that an almost diffraction-limited performance in speckle metrology can be achieved over the entire object field that can be imaged by this hololens configuration.