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.
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°.
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 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.
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.
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 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.
Silicon is widely used in IR optics, X-Ray optics and electronics applications. These applications require Silicon of
optical quality surface as well as good form accuracy. To get the desired finish and dimensional accuracy, diamond
turning is preferable. Taylor-Hobson Nanoform-250 diamond turning equipment is used to machine flat Silicon mirror.
Negative rake diamond tool is used with a tool nose radius of 1.5 mm. A series of SPDT machining operations are
performed in the sequential combinations of tool feed rate, Spindle Speed and depth of cut. In order to find out the effect
of machining parameters on the Surface Roughness during turning, Response Surface Methodology (RSM) is used and a
prediction model is developed related to average Surface Roughness (Ra) using experimental data. The surface quality is
analyzed in terms of arithmetic roughness (Ra) and Power Spectral Density for uniform evaluation. In addition, a good
agreement between the predicted and measured Surface Roughness is observed.
A stable optical resonator comprising a spherical rear mirror with a phase step of λ/2 at its center and a flat output coupler promises to produce a beam with very low diffraction. Based on this premise, a phase step mirror is fabricated on oxygen-free high-conductivity copper substrate using single-point diamond turning method for use in kilowatt-class CO 2 laser. The fabrication technique and subsequent metrology measurements of the optical component, e.g., surface profile error and surface finish, are discussed.
The one dominant characteristic of our times is the significant increase in the density of communication. Popularity of WWW has skyrocketed and so has the need for the bandwidth. The advent of fiber optics and WDM technology has appeared as a savior for solving these problems. In this paper, an overview of the WDM technology and its marriage with IP networking will be presented.
In this note, we present our endeavors to segment same cross-sections of the human brain obtained from the two modalities -- x-ray computed tomography (CT) and magnetic resonance imaging (MRI) -- using the fuzzy c-means technique developed by Bezdek. The two advantages of the technique are that it is unsupervised and is robust to missing and noisy data. Attempts at integrating the images from these two modalities are also mentioned.
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