Attention is drawn to the properties of Ta2O5 film which make it attractive as a new dielectric medium for use in integrated optical circuitry. The Ta2O5 film discussed was formed by thermally oxidizing sputtered , β-tantalum films in an oxygen atmosphere. Measurements showed that these films will support propagation of coherent red (6328-Å.) light with a loss of 0.9 dB cm-1 and coherent blue (4880-Å) light with a somewhat higher loss of 4.1 dB cm-1. From measurements of the coupling angle at which propagation of the various modes takes place, refractive indices for the TE and TM modes at 6328 Å, 5145 Å, and 4880 Å were obtained. The loss mechanisms are discussed, and it is shown that most of the loss is due to scattering during the internal reflections at the air-oxide and oxide-substrate film surfaces. A small fraction of the loss appears to occur in the interior of the film. With Ta2O5 it is demonstrated for the first time that optical circuit components can be formed for use in integrated optics by employing existing thin-film technology. The basic process steps include the pattern generation of the β-Ta followed by thermal oxidation of the pattern. Experimental studies of propagation in straight and curved waveguides are described. It was found that the losses in pattern generated waveguides are of the order of 5 dB cm-1 to 7 dB cm-1, with the excess loss due to scattering from the waveguide edges. It is concluded that improvements in the smoothness of the boundaries of circuit elements are desirable to reduce transmission losses in pattern generated film.
D. H. Hensler, J. D. Cuthbert, R. J. Martin, and P. K. Tien, "Optical Propagation in Sheet and Pattern Generated Films of Ta2O5," Appl. Opt. 10, 1037-1042 (1971)