Abstract
Accurate knowledge of the refractive index profile of optical waveguides is crucial to predicting waveguide performance and in assessing fabrication procedures. Propagation mode near-field methods have recently demonstrated that the refractive index distributions in optical waveguides can be determined from near-field waveguide intensity profiles.1 However the approaches used to date have utilized conventional optics thereby limiting the spatial resolution of the measurements to ./2. The recently developed technique of near-field scanning optical microscopy (NSOM)2 overcomes the diffraction limit allowing optical imaging with a spatial resolution of up to λ/40 or ~12 nm. In NSOM a subwavelength aperture is placed within the near field of the structure to be imaged and thus the spatial resolution is determined by the diameter of the aperture used rather than the wavelength of the light. We use NSOM to measure the near-field intensities of the guided modes in planar waveguides and optical fibers3 and calculate numerically, from the measured intensities, the refractive index distributions.
© 1996 Optical Society of America
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