Applications of FFT and digital filtering in reconstruction of refractive index profile of surface channel waveguides from near-field intensity pattern

Abstract

Refractive index profile of surface channel waveguides can be determined by analyzing the near-field intensity pattern. A mathematical model, the inverse Helmholtz equation, is derived in order to use these data to reconstruct the refractive index profile under consideration. In this work, the measured near-field intensities are preprocessed by means of gamma correction, background noise subtraction, Fast Fourier Transform (FFT), and low pass Finite Impulse Response (FIR) digital filter. Several types of FIR windows are chosen. The results are used to reconstruct the refractive index profile of the waveguide. The results show that the application of low pass FIR digital filter by using Hamming window reduces noises better than other windows. The application of this method in determination of refractive index profile of annealed proton exchange LiNbO 3 channel waveguides is demonstrated.