Study On Laser Direct Writing Of SiO₂/TiO₂ Sol-Gel Films To Fabricate Strip Optical Waveguides

Strip optical waveguides are essential components in integrated optics technology. In recent years, the strip optical waveguides fabrication technique with simple processing techniques, low cost and flexible patterning have been one important research goal so as to meet the rapid development demands of the optical communication systems. Laser direct writing technology can overcome the shortcomings of traditional processing methods to fabricate optical waveguides based on photolithography, and hence attract wide notice in the manufacturing and research field of optoelectronics parts.In this dissertation, SiO₂/TiO₂ sol-gel films were deposited on SiO₂/ Si surface by dip-coating technology. Strip optical waveguides were fabricated by laser direct writing of films using a continuous Ytterbium fiber laser with a wavelength of 1070 nm and followed by chemical etching to remove the non-irradiated area, which is based on the difference of the chemical etching rates between the irradiated and non-irradiated area of the films. Finally, the transverse mode field distribution and optical propagation losses of the strip optical waveguides were characterized. The following are the main works and corresponding results:Both the synthesis and material properties of the SiO₂/TiO₂ solutions have been examined in detail. The results demonstrate that the design of the synthesis art route is the significant factor that affects the fabrication procesing of sol-gel material. The SiO₂/TiO₂ solutions synthesized by two-step hydrolyzing method with good stabilization can be used in the fabrication of SiO₂/TiO₂ films.The thickness of SiO₂/TiO₂ sol-gel films is influenced evidently by the factors of drawing rate, sol viscosity, sol concentration and the temperature of heat treatment processing. The SiO₂/TiO₂ films can be prepared on SiO₂/ Si surface with a drawing rate of 10 cm/min using a sol with concentration of 40%(V%) and viscosity of 3.0 mPa?s. After being heated at temperature of 200℃for 30 minutes, the surface roughness is 0.31 nm ranging 15μm×15μm in the film, which is low enough for optical waveguides. The refractive index of the films can be controlled accurately through changing the titanium contents in the SiO₂/TiO₂ sols. And the films have high optical transparence in the communication window wavelength ranging from 600 nm to 2500 nm. When the refractive index and thickness of SiO₂/TiO₂ films meet the core layer design of planar optical waveguides in theory, there are propagation modes for the lightwave with a wavelength of 1550 nm. The propagation losses of the SiO₂/TiO₂ planar waveguides at the 1550 nm have been experimentally measured by prism coupling technique. The fabricated planar optical waveguides are low propagation loss at 1550 nm, from 0.34 to 0.75 dB, which decrease with the increase of the thickness of core layer.The effects of the laser processing parameters on the dimensions of densification lines in the film were studied systematically. Furthermore, the densification mechanism of the sol-gel film is analyzed. The initial condensed thresholds (Fc), damaged thresholds (Fd) of the laser power density in processing of the films and the critical beam diameter are defined. The experimental results demonstrate that the width and the shrinkage extent of the densification line in the films increase with the laser power density ranging from Fc to Fd. The available laser power density rangeΔF (ΔF = Fd– Fc) for laser processing increases with the enhancement of the heat treatment temperature of the films. The corresponding critical beam diameter and width of densification lines decrease. Whereas, the difference of the chemical etching rates between the irradiated and non-irradiated area of the films will reduce with the increase of heat treatment temperature.It was found that the energy of laser beam was not absorbed directly by SiO₂/TiO₂ film, but by silicon substrate during the laser direct writing processing. Then the heat conducted from silicon substrate to the SiO₂/TiO₂ film. The nanoscale pores within the film will become smaller or disappear due to the film shrinking for their high surface-to-volume ratio under the laser irradiation.The values of the chemical etching rate difference between the irradiated and non-irradiated area of the films were calculated by observing the transverse profiles of strip optical waveguides etched in HF solution for a given time. The value of the chemical etching rate difference in the films with a heat treatment temperature 500℃for 30 minutes is 4 nm/s, which is much smaller than the value of 19.5 nm/s about the films with a heat treatment at the temperature 200℃for 30 minutes. The surface roughness of strip optical waveguides increases with the chemical etching time due to the un-uniform etching rate about the irradiated area caused by the impurity of the films, which contributes to the surface scattering losses of the SiO₂/TiO₂ strip optical waveguides. The theoritical value of the surface scattering loss for the strip optical waveguide whose core layer has been etched by HF solution for 28 s is 1.43 dB/cm.Strip optical waveguides with a width of 15μm on silicon were fabricated using a fiber lasers with infrared wavelength. Both the simulated optical field obtained by the software FEMLA (Finite Element Modeling Laboratory) and the actual optical field obtained by the experimental measurement demonstrate that the strip optical waveguides can propagate the lightwave with a wavelength of 1550 nm in monomode. The optical propagation losses of the strip optical waveguides at 1550 nm were measured by cut-off method. It is found that the minimum propagation loss of strip optical waveguides is 1.7 dB/cm, which is bigger than that of planar optical waveguides. Higher surface roughness and carbon remains of the core layer are the main factors to increase the propagation loss of the strip optical waveguides.