Photonic lanterns are novel photonic devices initially developed for an astro-photonics application, but later found immense potential in the field of space-division multiplexing and novel optical communication interconnects. They are adiabatically tapered structures, thus having large device lengths which scale quadratically with the number of cores. Adiabaticity is exploited to design a mode-selective photonic lantern. The Shortcuts to adiabaticity (STA) protocol is used to design an optimum quasi-adiabatic taper profile corresponding to a certain device length and a measure of adiabaticity. By homogenizing adiabaticity, a framework was developed to obtain an optimum taper profile for any required device length, which will have a corresponding measure of quasi-adiabaticity. This measure relates to the coupling losses suffered by the system digressing from absolute adibaticity. We have shown that the optimum taper profile significantly reduces the device length without compromising on adiabaticity. Photonic lanterns have also been examined for the generation of Orbital angular momentum modes in fibers. Here results for a three core photonic lantern structure for the generation of OAM1 mode have been discussed.
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.
Simulation and analysis of photovoltaic cells require both optical modelling and semiconductor device modelling. Often, both are very involved and therefore, the simulations or analyses of the photovoltaic cells make compromises in either modelling.
Here, we present a simple coupled model, where the optical modelling is done using finite difference beam propagation method and the electronic modelling is done using the drift-diffusion model. Starting from the semi-classical Boltzman transport equation, we make valid approximations to arrive at the drift-diffusion equation which is further discretized by the Scharfetter-Gummel Scheme. For optical modelling, we start with Maxwell`s equations, apply successive valid approximations and arrive at the finite- difference beam propagation method. We then solve both the equations simultaneously, the input flux at every step used for calculation of carrier transport in the drift diffusion equation is supplied by the solution of the optical beam propagation. This coupled model would give us a fast and simple method to simulate and analyze novel optical structures for photovoltaic cells.
Initial results of both the independent models are very encouraging. Equilibrium and non-equilibrium transport properties have been successfully described by the electronic modelling, similarly for low contrast photonic structures, FD-BPM is a long-proven method. We would present an integrated approach to arrive at a very fast, simple yet efficient model for simulation of optical structures for photovoltaic applications.
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.
Climate change has become a cause of concern as well as the challenge of this century. Himalayan mountain ranges with high snow fields and numerous valley glaciers may bear the brunt of such changes already being reported including Intergovernmental Panel on Climate Change (IPCC). Gangotri is one of the most prominent snow-fed catchments of Indian Himalayan Region (IHR) due to origin of river Ganga situated within it. Spatio-temporal changes in snow covered area of this basin were examined for melting seasons of the years 2006 to 2010 and a latest reference year of 2012 as a special test case. Standard snow data products (MOD10A2) of Moderate Resolution Imaging Spectroradiometer (MODIS)-Terra sensor with spatial resolution of 500 m were used. For all the years of reference, snow covered area percentage was derived for respective months representing usual ablation or melting periods. Snow depletion curves (SDCs) were generated for such periods of the respective years. CARTOSAT digital elevation model (DEM) was used for topographic information of terrain. Relationship of SDCs with the land surface temperatures (LST) of the basin was worked upon using MODIS-Terra LST (MOD11A2) product (version 5) with 1 km resolution at 8-day interval for the day time temperature for respective months of above reference years. Thereafter, interpolation and simulation of snow covered areas was carried out on the basis of LST data. The study thus produced snow cover maps for the years of reference as well as their relationship with LST for climate change inferences.
The mode-field expander (MFE) is a microstructured optical fiber (MOF) based device that enlarges the modal field distribution and can couple light from large mode area (LMA) fibers into small core fibers or vice-versa and other optical waveguides. Using our earlier developed analytical field model, we studied the mode-field expansion in MOFs having triangular lattice, and low-loss splicing of MOFs to standard single-mode fibers (SMFs), based on the controlled all airhole collapse method, which leads to an optimum mode-field match at the joint interface of the MOF-SMF. Comparisons with available experimental and simulation results have also been included.
An analytical approximation for the modal field of a single mode index-guiding microstructured fiber is developed
further to obtain the evolution of the far-field pattern of these fibers. Comparison with available experimental results is
also included.
In numerical wave propagation methods, the perfectly matched layer (PML) boundary condition is employed to prevent spurious reflections. However, PML takes additional resources in number of computation points and time. In this study, the PML performance is examined with change in the distribution of sampling points and PML absorption profile with a view to optimizing its efficiency. We have used the collocation method in our examples. We have found that equally spaced field sampling points give better absorption of beams under both optimal as well as non-optimal conditions for lower PML widths. While at higher PML widths, unequally spaced basis points may be more advantageous. The behavior of different absorption profiles varies with point spacing. For numerical tests, Gaussian beam propagation in a homogeneous medium is considered. Comparing different profiles, we find that a new profile sinp with p=4 and quartic profiles are best in equally spaced points, sin2 and square profiles are best in unequally spaced points.
A new method for solving the wave equation is presented, which, being non-paraxial, is applicable to wide-angle beam propagation. It shows very good stability characteristics in the sense that relatively larger step-sizes can be used. It is both faster and easier to implement. The method is based on symmetrized splitting of operators, one representing the propagation through a uniform medium and the other, the effect of the refractive index variation of the guiding structure. The method can be implemented in the FD-BPM, FFT-BPM and collocation schemes. The method is stable for a step size of 1 micron in a graded index waveguide with accuracy better than 0.001 in the field overlap integral for 1000-micron propagation. At a tilt angle of 50°, the method shows an error less than 0.001 with 0.25-micron step. In the benchmark test, the present method shows a relative power of ~0.96 in a 100 micron long waveguide with 1000 propagation steps and 800 sample points, while FD-BPM with Pade(2,2) approximation gives a relative power of 0.95 with 1000 sample points and 2048 propagation steps. Thus, the method requires fewer points, is easier to implement, faster, more accurate and highly stable.
Collocation method has been developed as a method which can treat the modal field computation as well as the total field propagation with equal ease and efficiency. The method, based on the orthogonal collocation principle, converts the Helmholtz equation into a matrix ordinary differential equation. This equation termed as the collocation equation can be solved either by using direct numerical techniques or by special techniques based on the matrix operator algebra. In the present paper, we outline the basic principle of the method and the implementation of the perfectly matched layer technique to overcome the reflections from the edge of the numerical window. In addition, we discuss its applications to Bragg waveguides and to soliton propagation which ave lately assumed great significance.
We present the reflection characteristics of Bragg gratings using the collocation method and the effect of various parameters such as grating profile, duty cycle of periodic variation, index change and strain on the spectra. Comparisons with the coupled mode theory have also been included.
A new method for obtaining mode cutoffs for asymmetric planar waveguides is developed. The method is based on defining the wave equation for modes at cutoff and solving it using the Galerkin method. The method has been used to obtain cutoffs of a variety of asymmetric waveguides with arbitrary profiles including exponential, traingular, parabolic and other power law profiles. Comparisons with available results show that very good accuracies can be obtained with moderate matrix sizes.
The paper presents the formulation of nonlinear TE waves guided by waveguides having second order polynomial in the film with nonlinear cover medium. These waveguides are usually obtained in glass by silver-sodium ion exchange technique. It is shown in this paper that the formation leads to an exact dispersion relation and an exact expression for power flow.
An efficient numerical method for the analysis of periodic waveguides has been presented. The method is based on the collocation method which we have earlier developed for propagation of waves through uniform and nonuniform (e.g., tapers) waveguides. In this method, one converts the Helmholtz equation, which is a partial differential equation, into a set of total (ordinary) differential equations. In our method, the wave is to be propagated only through one period of the waveguide, after which analytical solutions for arbitrary lengths can be obtained. We have also presented an example to show the effectiveness of our method.
A simple and reliable method for obtaining the index profile and the value of (Delta) n of diffused planar and channel waveguides from the near-field scan data is presented. The method involves fitting of the Hermite-Gauss functions to the near-field scan data; the implicit assumption in case of channel waveguides being the separability of the field in its dependence of the two transverse coordinates (x and y). The coefficients of expansion are then used to obtain two 1D index profiles. These two profiles are then suitably combined to give the index profile and the value of (Delta) n of channel waveguides. The validity of the procedure is first tested on planar waveguides. Our simulations show that the method is very stable with respect random errors up to 15 - 20% in the intensity, and accurate estimated for (Delta) n can be reliably obtained. For diffused channel waveguides also, our simulation studies show that the method yields very good estimates for (Delta) n with typical errors being less than a few percent.
Fiber optic evanescent wave absorption sensors based on four different designs are described. The launching condition and the geometry of the sensing region are found to be the main parameters that can influence the sensitivity of the sensor. High sensitivity is achieved by launching the selected rays into the fiber having tapered sensing regions. The experimental results obtained are qualitatively explained using a simple model based on meridional rays.
A new numerical technique for the propagation of optical pulses through nonlinear dispersive media is presented. The method is based on the collocation principle and is numerically more efficient than the split-step Fourier method. The accuracy and efficiency of the new method is demonstrated and several numerical examples are presented.
We present a new method to obtain nuerica1 solutions of the Generalised Nonlinear Schrodinger Equation (GNLSE) which describes nonlinear pulse propagation through optical fibers. The aethod based on the collocation principle is more efficient than the conventionally used FFT based method as we show in this paper through numerical examples. I .
We present an approxiMate aethod for the analy8is of diffused channel waveguides. The aethod is iterative requiring only 3-4 iterations and is based on the variational principle. We have also applied the Rethod to study the diffused channel waveguide directional couplers. I
We present a method , based on the variatiml principle , for Obtaining equivalent
1-D profile for the given 2-D profile of a veguide. The method leads to a better
equivalent profile in ccniparison to other existing methods.
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