Optical Integrated Waveguides Characterization by Scanning Near Field Optical Microscope
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characterization of waveguides [6]. By means of scanning a sharp optical fiber few nanometers over the surface, the evanescent field is detected and the modal distribution of the optical power inside the waveguide can be elucidated without disturbing the light propagation. As topographic and near-field optical images are acquired simultaneously with a resolution in the nm range, a direct correlation between waveguide structure and light propagation is possible. In this work, we present examples of the propagation of the light in standard and modified Si 3N4 rib waveguides. We are interested in characterizing and understanding the nature of guided modes and whether defects can produce controllable disturbances on light propagation. This would lead to give indications for the design of new structures. For modifying the waveguides, a new technique based in local oxidation of metal films using an Atomic Force Microscope (AFM) is employed [7]. We have chosen this technique because its simplicity and resolution. II. EXPERIMENT II.a Experimental scanning near field optical microscope set-up The microscope follows a stand-alone design; the scanner and the tip are mounted in a screw driven tripod for the coarse approach of the tip over the sample. The whole scanner is placed in the optical set-up where the light coming from a 633 nm laser is coupled into the 37 Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society
waveguide by means of an optical fiber (Figure1), and collected by a photodiode. The whole scanner, 12 cm long x 5 cm high and 8 cm wide, can be easily manipulated over a 10 mm square range with additional micrometer screws. The sample holder is also mounted on a micrometer screw, allowing the study of 3 cm long by 2 cm wide samples. The optical fiber tip can be positioned over this wide scan region with the aid of an optical microscope. Images up to 300jim x 150jm range in the plane of the sample and 4 jim in the vertical direction can be obtained by using piezoelectric bimorph plates. Note that in our set-up the sample is fixed and it is the optical fiber tip which moves over the surface. In this way the images reflect the true propagation field along the waveguide without any change in the light coupling. Fiber
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Figure 1. Experimental set-up: P-photodiode, TF-tuning fork, D-piezoelectric dither, I[V-current voltage converter, PMT-photomultiplier to amplify the evanescent field signal for the optical image.
The tip-sample distance control is based on shear-force detection with tuning fork [8]. This shear-force detection system allows to follow the topography with an accuracy better than I nm in height and a 1Onm in the lateral direction. The signal collected with the tapered optical fiber tip is detected by a photomultiplier (PMT), and monitored with the computer, obtaining the optical image of the sample. Simultaneously, by monitoring the Z voltage applied to the piezo, the topographical image is acquired
II.b Experimental procedures for optical fiber tip and wav
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