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A scanning near-field optical microscopy study is undertaken on photonic crystal tapers. These tapers are used to couple a 3 microm wide multimode dielectric waveguide to a single-line defect monomode photonic crystal waveguide. Two kinds of taper, with or without a localized defect, are compared. Higher transmission efficiency is obtained when a defect is utilized. The near-field study at 1550 nm shows experimentally that this defect prevents leaky resonant states to appear in the taper and thus permits a decrease in out-of-plane losses. These observations are supported by band diagram calculations.

We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.