An experimental and theoretical investigation of waveguide scatter, with applications to solution-deposited silica-titania planar waveguides
A theoretical and experimental investigation of scatter from surface roughness and core refractive index fluctuations in planar waveguides was performed, with an emphasis placed on applications in solution-deposited SiO₂-TiO₂ (silica titania) planar waveguiding systems. A perturbation theory was used to model TE₀ mode scattering from surface and volume microstructure, and to predict attenuation when provided with the necessary waveguide and scattering parameters. Final forms for the equations predicting surface and volume scatter losses into the cover and substrate regions of the waveguides were provided. The rather complex perturbation theory model of surface scatter was compared to a very simple, intuitive model based on the Rayleigh criterion. The two models were shown to predict surface induced attenuation values which were in very close agreement when the guided mode propagation angle approached 90°. Thus, the simple model was shown to be adequate for predicting TE₀ mode surface scattering losses for waveguides which were very thick, and/or possessed a low refractive index. Considerable emphasis was placed on providing a simple, physical picture of guided mode scattering, utilizing rays to represent the scattered light. Following the development of this technique, it was utilized to explain the origins of interference peaks in surface scattered radiation at certain values of film thickness. Solution chemistry and processing methodologies for 50:50 mol% and 35:65 mol% SiO₂-TiO₂ sol-gel films, yielding high quality, amorphous, glass waveguides, were discussed. Attenuation in the 50:50 mol% films was 1-2 dB/cm, while attenuation in the 35:65 mol% films was 0.3-0.5 dB/cm, at λ = 0.6328 μm. Absorption in these films was negligible. Waveguide losses were measured by transferring the scattered streak to a remote image plane (using a coherent fiber bundle) and scanning it using an automated, stepper-motor controlled, apertured photomultiplier tube. Testing and calibration techniques were described in detail. We found that surface-induced scattering was the dominant loss mechanism in the 35:65 mol% SiO₂-TiO₂ films. Surface roughnesses of the sol-gel films, measured using Atomic Force Microscopy, ranged from about 2-5 A rms, with correlation lengths from about 0.05-0.75 μm.
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