Approximation of reflection coefficients for rapid real-time calculation of inhomogeneous films

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Fast computer algorithms for calculating the reflection coefficients of inhomogeneous films are developed. The applications of such algorithms are aimed at real-time monitoring and control of the growth of gradient index optical coatings. Inhomogeneous films with a refractive index varying along the direction normal to the surface are generally treated by dividing the film into a large number of homogeneous layers and using an iterative or transfer-matrix method to calculate reflectivities. We develop alternative methods. We show that the elements of the transfer matrix and the reflection coefficients can be written in terms of single, double, and higher-dimensional integrals over the films. In most cases it is sufficient to include only the first term, i.e., the single integrals, and the calculation of reflection coefficients is then between 1 and 2 orders of magnitude faster than that for the transfer-matrix and iteration methods. This gives the possibility of real-time determination of optical parameters of inhomogeneous layers for growth monitoring and opens up feedback control.