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The refractive-index profile and dispersion data are computed from the measured far-field exit radiation pattern of the fundamental mode. Instead of Fourier-transforming the far field numerically, the near field is calculated by fitting a series of Gauss-Laguerre functions to the far field, and by further using analytical methods. The procedure is inherently insensitive against random measurement errors. We demonstrate the feasibility of this technique for the first time at a wavelength of 1300nm.