KEYWORDS: Receivers, Design and modelling, Photodiodes, Freeform optics, Visible light communication, Optical surfaces, Internet of things, Data communications, Zemax, Transmitters
A Li-Fi receiver front-end is proposed using monolithic-freeform-Fresnel profile. The modified design replaces four orthogonally oriented optical front-end into a single unit and exhibits 81 dB average SINR with more than 25% reduction in dimension.
The measurement of human skin temperature provides vital physiological information about human health status. In this paper, an application of digital holography has been demonstrated to measure human hand skin temperature. A volume phase holographic grating based digital holographic interferometric system has been used in a lens-less Fourier transform configuration to measure the temperature of human hand skin. The presented system is non-contact, non-invasive, fast, compact, robust, requires lesser number of optical components, simple to align, and is easy to use.
We report the design of a modified angular diversity receiver (ADR) with reduced size and improved communication performance for indoor VLC applications. The proposed design has four freeform and one spherical lens mounted on five co-planer photodiodes. An extended polynomial surface is used to determine the slope of the freeform lens with a quadrilateral off-axis field of view (FoV) (28° × 28°). Four identically designed freeform lenses are placed in a 2×2 format keeping 90° angular separation. Due to the orthogonal slope alignment of the proposed structure, the angular diversity is established. The distinct off-axis FoV of the freeform lenses introduce an uncorrelated channel gain in a multiple-input multiple-output (MIMO) VLC framework. A spherical lens with a narrow FoV (~15°) is also placed at the middle of quadrupole freeform lenses to ensure high SINR of 156 dB when the receiver approaches to the nadir of any transmitter. The compact physical dimension of the designed receiver structure (11.2 mm × 11.2 mm × 6.1 mm) is highly advantageous to facilitate VLC at any handheld gadget. Considering a square indoor environment (5m × 5m × 3m), an excellent average SINR performance of 95.7 dB is observed over the communication floor.
KEYWORDS: Temperature metrology, Digital holography, Holographic interferometry, Diffusion, Holography, Volume holography, Signal to noise ratio, Interferometry, Holographic interferometers, Speckle
Optical measurement techniques are of immense importance for research in engineering, industrial production and biomedical applications. In this paper, an experimental study to measure temperature, temperature distribution, and temperature fluctuations in diffusion flames has been conducted using a volume phase holographic grating (VPHG) based digital holographic interferometric (DHI) system. VPHG made on Dichromated gelatin provides high diffraction efficiency and minimizes stray light and coherent speckle noise. The results obtained from VPHG based DHI system show a good agreement with temperature measured by thermocouple. The proposed system is compact, robust, requires lesser number of optical components, simple to align and easy to implementation.
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°.
The requirement of the complex surface is increasing significantly with growing needs to fulfill the demands of many challenging applications. The advancements in manufacturing technologies attract optical designers to think about more and more complex shapes. The demand for hybrid surfaces i.e. diffractive structures over spherical, aspheric and freeform surfaces is growing for many future applications e.g. hyperspectral imaging, spectroscopy, beam shaping, wavefront coding. The hybrid freeform surfaces are the most complicated and most demanding surfaces for the above applications due to the advantages of the freeform base profile and diffractive structures. The use of these components leads to significant improvement in the performance of these optical systems along with considerable miniaturization and cost reduction. Challenges in fabrication and characterization of diffracto-freeforms are a limiting factor for its realization. This work aims to explore the fabrication and characterization strategy for diffracto-freeform optics. The cubic freeform shape with diffractive structures has been chosen for the study. The slow tool servo machining is utilized for the fabrication of this hybrid surface. The tool path development strategy and the effect of process parameters are discussed. The base shape and diffractive structures are characterized separately by utilizing the optical profiler measurements. The average error in the fabricated diffractive structures height and there radial distance from center is found less than 10% and 15% respectively. The characterization of the fabricated surface and fabrication issues are discussed in this paper. The current study is helpful to understand the various issues involved in the development of such complicated surfaces.
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.
Urine analysis (urinalysis) is a critical component to diagnose urinary tract disease. Microscopic evaluation of the urine provides an insight into potential underlying urinary tract disease, which is used for identification and characterization of both common and much less common formed elements in the urine sample. In this paper, the microscopic urinalysis is presented by using single beam digital holographic microscopy (DHM). This is a common path set up wherein both beams (reference and sample) travel through a similar path providing higher temporal stability. In this paper, phasecontrast three - dimensional imaging of red blood cells (erythrocytes), white blood cells (leukocytes), squamous and nonsquamous epithelial cells, casts and various crystals present in the urine samples, have been demonstrated. The proposed imaging modality for the diagnosis of urinary tract disease is simple, non-contact, non-invasive, and provides higher temporal stability due to its common-path geometry.
Freeform optics is the next generation optics with no rotation symmetry about any axis. The fabrication and metrology of freeform optics are not possible by conventional techniques. Due to non-symmetric nature, it is more critical to align the freeform surface during fabrication and metrology. Fabrication and metrology accuracies of the freeform optics are mainly limited due to alignment errors at all the stages of development process. In this paper, effects of alignment errors on quality of freeform optics during of fabrication and metrology are studied. It is found that alignment errors have significant contribution on quality of freeform optics development. Different types of fiducials and their importance and utilization are discussed. Further, a strategy for effective alignment of freeform optics is proposed.
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 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.
This paper addresses the challenges and limitations involved in the measurement of steep freeform wavefront by using Shack-Hartmann Sensor (SHS). To estimate the slope errors, Zemax simulation tool is used to design a SHS setup including array of lenslets and detector plane with predefined specifications. In first step, error due to approximation of tilted plane wavefront over curved wavefront is simulated. Plane, tilted, curved and tilted-curved wavefronts are defined using appropriate ray source objects. The centroids of the focal spots of lenslets are calculated based on the detector data obtained by using ray tracing method, which is done by an in-plane scanning aperture for segmented local wavefronts sequentially.The scanning aperture is used to block rays from more than one lenslet array. Centroids from the focus spots are calculated and the slopes are estimated with respect to collimated reference wavefront for each ray trace process. Further, matrix of slope errors is used as an input for MATLAB routines for surface reconstruction and error estimation. Based on the simulation data, it is found that the assumption used in Shack-Hartmann wavefront measurement introduce residual errors. For example a 50 wave peak to valley input and 1.19 mm thick lenslet array can give up to 9 waves of residual form error. However, very thin lenslets can have very less residual error.The effect of shift of focal plane, tilted plane wavefront and curve wavefront during the reconstruction using SHS is reported.
In this paper, digital holographic interferometric microscope (DHIM) in conjunction with Fresnel reconstruction method is demonstrated for phase contrast imaging of red blood cells (RBCs). The advantage of using the DHIM is that the distortions due to aberrations in the optical system are avoided by the interferometric comparison of reconstructed phase with and without the object.
The metrology of freeform wavefront can be performed by the use of a noninterferometric method, such as a Shack–Hartmann sensor (SHS). Detailed experimental investigations employing an SHS as metrology head are presented. The scheme is of nonnull nature where small subapertures are measured using an SHS and stitched to give the full wavefront. For the assessment of complex misalignment errors during the spiral scanning, a library of residual slope errors has been created, which makes the alignment process fast converging for minimizing the scanning errors. A detailed analysis of the effects of slope and positioning error on reproducibility is presented. It is validated by null test where a null diffractive optical element has been used in a Mach–Zehnder configuration for compensating the freeform shape. A freeform optics is measured by both measurement schemes, and the results are in good agreement. Further, the nonnull-based scanning subaperture stitching scheme is also validated by performing measurements on an aspheric surface and compared with the measurements from the interferometric method (Zygo Verifire).
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.
An easily implementable illumination optical element enabling collection and redirection of multiple precollimated laser beams to be used as a laser spot illuminator (LSI) is proposed. Unlike the lenses, this element is free from any aberration and focusing/defocusing effects which may arise due to curved surfaces. The nonfocusing nature of beams reduces risk of the thermal instability at the target spot if compared with focused beam, especially for high brightness applications where multiple laser beams are used. Further, the illuminating spot is more uniform and pupil filling is higher which can help in the reduction of speckle contrast for imaging applications. In this research, we present two designs of LSI named biprism and sleek design, suitable for conventional grinding-polishing and molding methods of fabrication, respectively. It is shown that the proposed spot illuminator is suitable for conversion of lamp/LED-based projection engine design into a laser projector engine without any modification in its relay and imaging optics. Display performance parameters such as efficiency and uniformity across the modulator plane are reported and found acceptable for the projection engine designs using light tunnels as well as lenslet arrays for homogenization. Further, a prototype of a biprism laser illuminator for 2×4 laser arrays has been fabricated and evaluated to demonstrate its manufacturability.
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.
The focusing performance of shell optics for the hard X-ray region strongly depends on their axial mid-spatialfrequency-
range figure errors. This paper presents the development of a deterministic computer-controlled polishing
process to minimize these axial figure errors on cylindrical shaped mandrels from which the mirror shells are replicated.
A mathematical model has been developed to simulate the residual surface figure errors due to the polishing process
parameters and the polishing tools used, along with their non-conformance to the mandrel. We present design
considerations of a large-size polishing lap where the experimentally determined process variables have been used for
optimizing the lap configuration and the machine operational parameters. Furthermore, the developed model is capable
of generating a corrective polishing sequence for a known surface error profile. Practical polishing experiments have
been performed to verify the model and to determine its ability to correct known axial figure errors through polishing
machine control.
Future X-ray observatory missions require grazing-incidence X-ray optics with angular resolution of < 5 arcsec half
power diameter. For X-ray mirrors fabricated using replication processes, the achievable resolution depends ultimately
on the quality of the polished replication mandrels. With an aim to fabricate better mirror shells, and also to reduce the
cost/time of mandrel production, a computer-controlled machine is being developed for deterministic and localized
polishing of mandrels. A key component in this is software that predicts the surface residual errors under a given set of
operating parameters and lap configuration. Design considerations of the polishing lap are discussed and the effects of
nonconformance of the lap and the mandrel are presented.
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