Synthesis of diffractive optical bar codes
Abstract
A new type of diffractive optical bar code (DOBC) is proposed. Rather than scan the bar pattern directly, the DOBC is coherently illuminated and the first diffraction order is sensed. The spacing between the bars is chosen so that the thresholded diffraction pattern yields a specified binary code. Two approaches are investigated for synthesis of the DOBC; phase shaping and a gradient-based, nonlinear, constrained optimization technique. The two design methods are compared based on numerical results; and the validity of the overall design approach is verified by optically sensing the diffraction patterns for a number of fabricated DOBC's. We are engaged in a research effort to design modulated ribbon gratings that are composed of a number of parallel, dielectric cylinders joined by flat sections of dielectric material. As an important first step in this work, we have solved for the multiply scattered field from a planar array of N parallel dielectric cylinders. The multiple scattering model takes into account all contributions to the excitation of a particular cylinder by the radiation scattered from the remaining cylinders. A common approach to calculating the field scattered by two or more parallel cylinders is to express the multiple scattering linear equations in matrix form and then to solve the matrix expression for the multiple scattering coefficients by an iterative method or matrix inversion. However, when the expansion of the matrix expression is divergent, the iterative method is also divergent. In addition, the matrix inversion approach fails for ill-conditioned matrices. We describe a nonlinear programming approach to solve the multiple scattering matrix expression for an arbitrary planar array of N parallel dielectric cylinders. To our knowledge, no calculations have been made for multiple scattering by more than two parallel dielectric cylinders. We present numerical examples of scattering from eight unequally spaced, parallel dielectric cylinders.
Degree
Ph.D.
Advisors
Allebach, Purdue University.
Subject Area
Electrical engineering
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