Characterization of optical waveguides in KTiOPO4 by second harmonic spectroscopy

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Abstract
Segmented optical waveguides were fabricated in KTiOPO4 by Rb/Ba ion exchange. The waveguides were characterized by measuring, as a function of input optical wavelength, the Bragg‐reflected power, and the intensity of second harmonic (SH) radiation generated within the waveguides. Seven peaks in the SH spectra are assigned to different interactions involving the two lowest‐order spatial modes at the fundamental wavelength and the three lowest‐order spatial modes at the SH wavelength. ‘‘Combined mode’’ SH interactions are found which sum spectrally degenerate fundamental waves which are spatially nondegenerate. Analysis of the spectra is shown to be a convenient method of determining the effective indices of the propagating modes, and these then determine the refractive index depth profile of the waveguide. The relative intensities of the SH peaks are dependent on the depth of the region within the ion‐exchanged area which has a reversal of the ferroelectric polarization direction (reversed domain). In some waveguides, the spectra imply that the area of reversed domain is buried, and does not extend completely to the surface.