A low loss optical waveguide of $P_2O_5-SiO_2$ on Si substrate is produced by using the chemical vapor deposition method of $SiO_2$ thin films used in Si technology. Propagation loss of the waveguide layer reduces to 1/16 after 1000℃ heat treatment. $P_2O_5-SiO_2$ waveguide core is flowed at 1100℃ to reduce the scattering loss due to the roughness introduced during the lithography of waveguide patterns. The cross section of the resulting waveguide core is half-disk-shaped and the surface is smooth. This waveguide shows a low loss of 0.03 dB/cm at 0.6328μm and 0.04 dB/cm at 1.3μm wavelength.
Low loss $Si_3N_4-SiO_2$ optical waveguides with a loss of less than 0.3 dB/cm in the wavelength range of 1.3 - 1.6 μm are fabricated. This waveguide consist of high refractive index $Si_3N_4$ waveguide core and $SiO_2$ cladding on Si substrate. In this study, IR transmission of the waveguide was first measured and we show that there exist two absorption peaks at 1.40 μm and 1.53 μm due to the absorption of hydrogen in $SiO_2$ and $Si_3N_4$ layer. Such absorption peaks were almost removed by annealing at 1100 - 1200℃.
The refractive indices of thermal oxide, $P_2O_5-SiO_2$ and $Si_3N_4$ films are precisely measured in the wavelength range of 0.6 - 1.5 μm by the extrapolation of mode index data with an accuracy of 1 x 10$^{-4}$. The refractive indices of these films are compared according to the materials of chemical vapor depositions, heat treatment, and content of $P_2O_5$. Annealing study of $P_2O_5-SiO_2$ films show that films containing 2% or more $P_2O_5$ reached their equilibrium refractive index after annealing at 800℃, while undoped silica films require annealing at 1100℃ to reach equilibrium.
A theory of Bragg reflector with generalized structure which devides Bragg reflector into N sections and assumes uniform grating and uniform gain in each section is developed. Fabricating method of holographic grating on thick waveguide layers with uniformity and low background scattering is established. It is found that the phase fluctuation of light in holographic exposure due to the fluctuation of air density has significant effects for grating image and this effects are removed. By these improvements Bragg reflectors are made reproducibly with uniformity over 4 inch on wafer and with high diffraction efficiency.
Added loss of waveguide associated with the formation of Bragg reflector with 15 A reflection bandwidth is 0.2 dB/cm. Several attenuation peaks on the short wavelength side of Bragg reflection peak are found in the transmission measurement of Bragg reflector. This originates from the coupling between the $Si_3N_4$ waveguide mode and the modes associated with cladding $SiO_2$ layer or $Si_3N_4$ layer without mesa. Measured wavelength of the attenuation peaks agreed well with the calculations. Narrow band Bragg filter of 2 A bandwidth is fabricated and the causes which affects the minimum bandwidth are examined. This filter has the smallest bandwidth among the Bragg filters yet reported.
Transmission waveguide resonator which consists of two Bragg reflectors on a waveguide and resonant optical reflector which utilizes coupling between a waveguide and a transmission waveguide resonator are demonstrated. In all cases considered, the measurements agreed well with the calculations. The bandwidth of this resonator is 0.35 A and is consistent with the waveguide loss of 1 dB/cm. Bragg reglector which has a reflection grating buried in waveguide layers is fabricated. Method of $Si_3N_4$ etching with extremely shallow depth as 10 A is developed. This type of grating device is immune to the contamination of waveguide surface and can be used for fabrication of tunable Bragg filter.
