Characteristics of Low Coupling Coefficient, Narrow Linewidth,High Order Bragg Grating Based on SiO2 Waveguide

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, SiO2 waveguide material with low transmission loss was selected. The refractive index of the waveguide cladding was 1.444 7, the refractive index of the core layer was 1.455 6, 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 6 mu mx6 mu m. 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 1 mu m to 6 mu m 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 1 550 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 SiO2 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 SiO2 waveguide Bragg grating device with 1 554.053 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.