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UBC Theses and Dissertations

An improved transmitted near-field intensity profile method for deriving the refractive index profile of a waveguide Su, Chao-hsiung Paul

Abstract

Refractive index profile determines many physical properties of an optical waveguide. The Transmitted Near-Field Intensity Profile Method (TNFLPM) is a simple and convenient way to measure the refractive index distribution of a waveguide. Despite its simplicity, this technique also presents a lot of challenges and limitations one must overcome to obtain a usable result. In this thesis, we have introduced an improved TNFLPM that focuses on providing more useful results. To begin with, the calculation of the refractive index profile is based on the intensity profile. Therefore, it is important to have an accurate measurement of the intensity profile. However, one of the challenges faced is that the intensity profile itself is most often distorted by diffraction. In this thesis, we have introduced a diffraction-reduced algorithm which, with the gaussian beam equation and an iterative algorithm, produces a precise representation of the intensity profile at the end facet of the waveguide. Besides the diffraction effects, the equipment and spatial noise, if not treated properly, present another error source for standard TNFLPM . We have chosen to use a Savitzky-Golay filter to eliminate the noise in the intensity profile, which provides a good trade-off between noise reduction and the need to preserve the shape of the profile. Although traditionally, waveguides with high frequency components in their index profiles cannot provide accurate results, we have introduced a redundancy test and a morphing algorithm, which searches for the best possible index profile to describe the waveguide under study. In this thesis, the proposed improved method is analyzed and demonstrated for a Ti:LiNbO₃ waveguide.

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