基于SiO_(2)波导的低耦合系数、窄线宽、高阶布拉格光栅特性研究

Characteristics of Low Coupling Coefficient,Narrow Linewidth,High Order Bragg Grating Based on SiO_(2)Waveguide

推导了布拉格光栅三维耦合系数公式,构建了一种新型的波导型布拉格光栅三维数值模型,并基于此设计制备出一种基于SiO2平面波导结构的低耦合系数、窄线宽、高阶布拉格光栅。从理论设计和试验验证两方面系统分析了布拉格光栅刻蚀深度及占空比对光栅耦合系数和线宽的影响,并最终设计制备出了中心波长为1554.053 nm,反射率为-8.5 dB,峰值半高宽为89 pm的SiO2波导结构布拉格光栅器件。本文设计制备的低耦合系数高阶布拉格光栅器件工艺简单,成本低,在滤波器、传感器及外腔窄线宽激光器领域中有广阔的应用前景。

As an important frequency selective component,Bragg grating is widely used in the fields of lasers,sensors and filters.Especially in the field of narrow linewidth lasers,Bragg grating is an important component for narrowing the linewidth of lasers.The linewidth and reflectivity of Bragg grating itself have a decisive influence on the performance and reliability of narrow linewidth lasers.The narrower the linewidth of the Bragg grating itself is,the greater the mode competition between the cavity mode of the gain chip and the longitudinal mode of the grating will be improved,and the temperature stability of the wavelength will also be improved accordingly.In this paper,SiO_(2)waveguide material with low transmission loss was selected.The refractive index of the waveguide cladding was 1.4447,the refractive index of the core layer was 1.4556,and the refractive index difference was 0.75%.Using the single-mode condition simulation of the waveguide transmission mode,the cross-sectional size under the single-mode condition of the waveguide was calculated to be 6μm×6μm.Under this single mode condition,starting from the wavelength equation of Bragg grating satisfying Bragg reflection condition,the paper mainly analyzed the coupling between TE-TE modes.Through the derivation of the refractive index change formula and the normalization equation,the three-dimensional numerical model of the coupling coefficient of Bragg grating was finally deduced,and the coupling coefficient variation relationship corresponding to the change of the grating structure was simulated when the grating etching depth increases from 1μm to 6μm and the duty cycle increases from 0.5 to 0.8.On this basis,the paper also further analyzed the numerical relationship between the etching depth,duty cycle of waveguide Bragg gratings,the reflectivity and FWHM of the gratings,thus establishing a high-precision theoretical model for the design of waveguide Bragg gratings,and designing a series of waveguide Bragg grating devices under this model.The grating waveguide was prepared by contact ultraviolet exposure process with large process tolerance and Inductive Coupled Plasma(ICP)etching,then the waveguide Bragg grating device wafer was prepared by Plasma Enhanced Chemical Vapor Deposition(PECVD)growing the upper cladding with the same refractive index as the lower cladding.Then,the waveguide Bragg grating device designed in this paper was finally prepared by cutting,polishing,and other back-end processes.In the paper,a waveguide Bragg grating test platform was built using a 1550 nm broadband spectrum light source,a circulator,a spectrum analyzer and a singlemode fiber.The fabricated devices were tested and analyzed in detail.The final test results show that the data relationships between the etching depth,duty cycle of the SiO_(2)Bragg grating prepared in this paper and the coupling coefficient,reflectivity and FWHM of the device were completely consistent with our theoretical model.Finally,a SiO_(2)waveguide Bragg grating device with 1554.053 nm center wavelength,-8.2 dB reflectivity and 89 pm FWHM was designed and fabricated.The low coupling coefficient,narrow linewidth and high-order Bragg grating devices designed and fabricated in this paper were simple in process and low in cost,and have broad application prospects in the fields of filters,sensors and external cavity narrow linewidth lasers.