【作者】 李爱魁；

【导师】 曾晓雁；

【作者基本信息】 华中科技大学 ， 物理电子学， 2007， 博士

【摘要】 条形光波导是光电子器件的基本元器件。近年来,随着集成光学的飞速发展,迫切需要发展工艺简单、成本低、柔性化程度高的条形光波导制备技术。激光直写技术克服了传统的以光刻技术为核心的光波导制作技术的种种弊端,成为光电子器件制造领域的热点研究课题。本文采用浸渍提拉法在SiO₂/Si表面制备了疏松多孔的SiO₂/TiO₂溶胶-凝胶薄膜。利用波长为1070 nm连续掺镱光纤激光器直接处理SiO₂/TiO₂薄膜,使得薄膜的致密度发生改变,然后采用化学腐蚀溶液对薄膜进行湿法腐蚀,利用薄膜致密化区域与未致密化区域具有腐蚀速率差的特点,去除薄膜的未致密化区域,形成SiO₂/TiO₂条形光波导,对所制备的条形光波导的模场分布和光传输损耗进行了分析测试。主要研究内容和结果总结如下:系统研究了SiO₂/TiO₂二元溶胶-凝胶体系的合成工艺以及相应的材料性能。结果表明,合成工艺路线的设计是影响制备优良性能的溶胶-凝胶材料的重要因素。采用两步水解法制备的SiO₂/TiO₂二元溶胶-凝胶体系性能稳定,可用于激光直写光波导薄膜的制备。在SiO₂/TiO₂溶胶-凝胶薄膜的提拉制备工艺中,提拉速度、溶胶浓度、溶胶粘度以及干燥热处理温度是影响薄膜厚度的主要因素。采用浓度为40%（V%）,粘度为3.0 mPa?s的溶胶,以10 cm/min的提拉速度提拉制膜,在200℃下保温30分钟的干燥热处理,制备的薄膜表面平整,在15μm×15μm范围内的粗糙度数值为0.31 nm。薄膜折射率大小可通过调节溶胶体系内的钛含量精确控制,在波长为600 nm - 2500 nm的窗口内有较高的透光率。当SiO₂/TiO₂溶胶-凝胶薄膜的折射率和厚度达到平板光波导的设计要求时,对于波长为1550 nm的光波存在导模。棱镜耦合法测试结果显示平板光波导的光传输损耗随着波导芯层薄膜的厚度增加而降低,所制备的SiO₂/TiO₂平板光波导的光传输损耗最小值为0.34 dB/cm。系统研究了激光直写工艺对薄膜致密化线条的影响因素以及作用机理。提出了激光起始收缩阈值（Fc）、烧蚀损伤阈值（Fd）、临界离焦量和临界光斑直径的概念。当用于直写薄膜的激光功率密度范围（ΔF）在Fc和Fd之间时,随着激光功率密度的增大,得到的薄膜致密化线条宽度增加,收缩率增大。提高溶胶-凝胶薄膜的热处理温度,可以增大ΔF,减小临界光斑直径,进而减小致密化线条的宽度。但是同时降低了薄膜致密化区域与未致密化区域的腐蚀速率差,不利于条形光波导的腐蚀成型。薄膜的激光直写机理是利用单晶硅衬底对激光能量的吸收,然后以热传导形式将能量从衬底传导至表面的SiO₂/TiO₂多孔薄膜,使溶胶-凝胶薄膜内纳米孔隙受热收缩致密化。通过对薄膜致密化线条在氢氟酸溶液中腐蚀不同时间的截面轮廓的表征,计算了不同温度热处理的薄膜经激光处理后的致密化区域与未致密化区域的相对腐蚀速率差。结果表明,随着溶胶-凝胶薄膜热处理温度的提高,薄膜致密化区域与未致密化区域的相对腐蚀速率差减小,当热处理温度由200℃提高到500℃时,相对腐蚀速率差由19.5 nm/s减少到了4 nm/s。由于SiO₂/TiO₂薄膜内存在不与氢氟酸反应的其它组分,因此在条形光波导腐蚀成型过程中除了薄膜的腐蚀溶解之外,还有未反应组分的物理脱附过程,由此造成条形光波导芯层薄膜的表面粗糙度的增加,进而增加了SiO₂/TiO₂条形光波导在光传输时的表面散射损耗。在氢氟酸溶液中腐蚀28 s成型的条形光波导,表面散射损耗的理论值为1.43 dB/cm。在国内外首次利用光纤激光器在硅基衬底表面制备了最小宽度为15μm的条形光波导。采用FEMLAB（Finite Element Modeling Laboratory）中电磁场功能模块进行的光场模拟和实际通光测试的近场光斑均显示制备的条形光波导可以实现对波长为1550 nm光波的单模传输。采用截断法对SiO₂/TiO₂条形光波导的光传输损耗进行的测试结果显示,相对于平板光波导,条形光波导的光传输损耗大幅度增加,而且光波导芯层薄膜厚度越小,增加幅度越大。文中利用激光直写技术制备的条形光波导在传输1550 nm波长光时的传输损耗最小值为1.7 dB/cm。分析认为,体系中的残余碳以及化学腐蚀工艺造成的芯层表面粗糙度的增加是条形光波导的光传输损耗增加的主要原因。更多还原

【Abstract】 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.更多还原