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Use of near-field scanning optical microscopy (NSOM) to characterize optical channel waveguide structures

Near field scanning optical microscopy (NSOM) has been used to investigate the guided mode intensity distribution in channel waveguides, directional couplers, and Y-junctions. The intensity profile above the sample surface and transverse to the waveguide propagation direction has been measured using a tapered optical fiber to probe the guided evanescent field. The fiber probe was maintained at a constant height above the sample surface using feedback provided by performing these near field scanning measurements simultaneously with shear force microscopy topography measurements. Single mode channel waveguides were formed by etching a ridge in planar Si3N4/SiO2 structures and were excited with light of a wavelength of 830 nm. Measurements transverse to a channel waveguide revealed a cosine squared variation of intensity above the ridge and an exponential decay away from the ridge, as expected. Considerations for characterizing AlGaAs waveguides in this manner also are discussed. Multiple scans along the two waveguides of a directional coupler provided a detailed view of optical power transfer from one waveguide to the other and were in agrement with beam propagation method calculations. We anticipate that this type of measurement will provide a more detailed understanding of a central photonic structure, the channel waveguide, and its incorporation into a variety of device configurations.