Study of Athermal Waveguides for Temperature Independent Lightwave Circuit

窄带波分复用器和滤波器是下一代密集波分复用光通信系统中不可缺少的器件。然而，存在中心波长随环境温度变化而变化的问题。本研究以实现温度无关的窄带滤光器为目标，研制了一种光程长度与温度无关的无热波导，并在上一财政年度开发了一种用有限元法计算波导光程长度温度系数的数值计算工具，设计了一种三维无热波导。结果，在波长为0.633 μ m时，光路长度的温度系数成功地降低到传统波导的2%。然而，要将这种无热波导应用于实际的光通信波长滤波器，需要研制波长为1.3或1.55 μ m的无热波导。因此，在本财政年度，我们首先用马赫-曾德尔干涉仪测量了NA 45玻璃、SiO_2和聚甲基丙烯酸甲酯薄膜在1.3 μ m波长处的折射率温度系数。接着，根据这些数据设计了一个脊加载三维无热波导，并采用射频溅射和旋涂工艺制作。结果表明，在波长为1.3 μ m时，光路长度的温度依赖性降低到9.6 × 10 ~<-9>（-1）[1/K]，仅为传统波导的0.1%，从而证实了红外波段无热波导的实现。

Narrow band wavelength demultiplexers and filters are indispensable for the next generation of optical communication system using dense wavelength multiplexing. However, there has been aproblem that the central wavelength varies against the temperature change of environment. Inthis study, we aimed at realizing the temperature independent narrow band optical filter, and developed an athermal waveguide of which optical pathlength is independent of temperature.In the last fiscal year, we developed a numerical tool to calcultate the temperature coefficient of optical path-length of waveguides using finite element method, and designed a three dimensional athermal waveguide by using this tool. As a result, the temperature coefficient of optical path-length was successfully reduced to 2% of conventional waveguide at the wavelength of 0.633mum. However, to apply this athermal waveguide to actual wavelength filter for optical communication, it has been needed to develope an athermal waveguide at the wavelength of 1.3 or 1.55mum.Therefore, in this fiscal year, we first measured the temperature coefficient of refractive indeces of NA45 glass, SiO_2, and poly-Methylmethacrylate for their films at the wavelength of 1.3mum using Mach-Zehnder interferometer. Next, a ridge-loaded three-dimensional athermal waveguide was designed from these data, and fabricated using RF sputtering and spin coating. As a result, the temperature dependence of optical pathlength at the wavelenth of 1.3mum was successfully reduced to 9.6x10^<-9> [1/K] , which corresponds to 0.1% of that of conventional waveguide, and thus we confirmed the realization of athermal waveguide at infrared wavelength region.