LOSSES IN TITANIUM-DIFFUSED LITHIUM-NIOBATE CHANNEL WAVEGUIDES DUE TO DIRECTIONAL CHANGES

• Main Author: Hutcheson, Lynn Donald
• Other Authors: Burke, James J.
• Language: en_US
• Published: The University of Arizona. 1980
• Subjects: Optical wave guides.; Integrated optics > Equipment.
• Online Access: http://hdl.handle.net/10150/282027

The optical transmission characteristics of dielectric optical waveguides having directional changes is considered in this study. Experimental and theoretical loss results are presented for two types of waveguide bends. First is the corner bend where two straight waveguides are joined together at some angle. The second type is a curved waveguide having some radius of curvature. The loss mechanism is different for each type of bend. The corner bend is basically a scattering loss due to a mismatch of the modes of the two joining waveguides. The loss in the curved waveguide is due to radiation of the energy away from the waveguide as it propagates around the bend. The waveguides were fabricated by diffusing 3 μm wide 200 Å thick titanium strips into LiNbO₃. All of the curved portions of waveguide were joined by straight waveguides at the input and output of the curved waveguides. Rayleigh scattering and absorption loss was measured in the straight waveguides to determine their optical quality which yielded about 1.4 dB/cm. The loss due to corner bends was measured for angles from 0.1° to 3.0° in steps of 0.1°. The loss ranged from about 0.1 dB to 23 dB for 0.1° and 3.0° respectively. The results are slightly dependent upon the polarization of the light and the orientation of the LiNbO₃ crystal. The curved waveguides were fabricated in two different geometries. The first geometry was a straight waveguide joined by a curved portion and then joined by another straight waveguide. The second geometry is different from the first by another curved portion joining the first curved portion in between the two straight waveguides. The two curved portions are equal but have opposite curvature which have an S shape. The radiation loss was measured for radius of curvatures from 1.0 cm to 3.0 cm. The results ranged from 41 dB/cm to 1 dB/cm for 1.0 cm and 3.0 radius of curvatures respectively. At each of the straight to curved and curved to curved junctions there exists a mode mismatch loss. The straight to curved mode mismatch loss was 1.65 dB for R = 1 cm and 0.5 dB for R = 3 cm. The curved to curved mode mismatch loss was 6 dB for R = 1 cm and 0.5 for R = 3 cm. The results for the corner bends and the curved bends were used to study the constraints on integrated optical devices. In many integrated optical devices, it is necessary for two different straight portions of a single mode channel waveguide to be connected with a given amount of transverse offset. The experimental and theoretical results showed that for small transverse offsets the corner bend approach yields smaller loss. The curved bend (S bend) approach was better for larger transverse offsets. Theory was developed for this study of bending loss in titanium diffused LiNbO₃ waveguides. In general all of the experimental results agreed quite well with the theoretical predictions.