Silicon Nitride Interlayer Coupling Interference Ring for Integrated Optical Gyroscope
Objective Integrated optical gyroscopes offer a significant solution for achieving high reliability,miniaturization,and cost-effectiveness at various levels of precision.Because all the optical components are integrated into a single chip,these gyroscopes belong to the category of all-solid-state gyroscopes without any moving parts.They exhibit a strong resistance to shocks and vibrations.Among them,interferometric integrated optical gyroscopes have gained increasing attention in recent years owing to their effective suppression of nonreciprocal noise generated by the light source in the gyroscope;however,the maximum detection accuracy of interferometric integrated optical gyroscopes is limited by the length of the critical device,that is,the waveguide interferometric ring.Increasing the lengths of planar structures leads to a significant increase in the volume of the waveguide ring,which hampers the miniaturization of integrated optical gyroscopes.Moreover,silicon nitride waveguide materials have found extensive applications owing to their advantages,such as their ultra-low loss,large transparent wavelength range,and their compatibility with complementary metal-oxide semiconductor(CMOS)processes.Hence,this study proposes and designs an integrated optical gyroscope based on silicon nitride waveguides using interlayer coupled interferometric ring structures.High aspect ratio silicon nitride waveguides are utilized to reduce the losses caused by sidewall scattering in the waveguide interferometric ring.Theoretical simulations are performed to design interlayer coupled interferometric ring structures with lengths reaching hundreds of meters,with the aim being to replace fiber-based rings.The successful design of interlayer coupled interferometric rings using silicon nitride is important for reducing the volume of integrated optical gyroscopes and enabling their miniaturization,integration,and low-cost implementation to thereby promote engineering applications.Methods This study utilizes the finite difference time domain(FDTD)module of the Lumerical simulation software package to perform numerical calculations on the essential parameters of an interlayer coupled waveguide interferometric ring.Initially,the cross-sectional dimensions of the high-aspect-ratio silicon nitride waveguide are determined,and its transmission characteristics are analyzed.Subsequently,the bending radius of the interlayer coupled interferometric ring,horizontal and vertical distances between the waveguides,and key parameters of the tapered vertical coupler are determined.The effects of the different parameters on the performance of the gyroscope are analyzed.Based on these analyses,a structural scheme for a hundred meter scale waveguide interferometric ring is established.Finally,a fabrication process is designed to obtain low-loss interlayer coupled interferometric rings using silicon nitride.Results and Discussions In this study,interlayer-coupled interferometric rings are fabricated using silicon nitride waveguides with ultrahigh aspect ratios.The cross-sectional dimensions of the waveguides are 3.0 μm×0.1 μm(Fig.1).Initially,the bending radius of the silicon nitride waveguide interferometric ring is determined to be 5 mm via simulations(Fig.5).Furthermore,a coupling model of the silicon nitride interferometric ring is established by using the Lumerical MODE simulation software package,and the coupling coefficients of the waveguides at different separations are simulated.To ensure that there is no cross-coupling interference in the fabricated interferometric rings,the waveguide separations in the horizontal and vertical directions are determined to be 7 μm and 3 μm,respectively(Figs.6 and 8).The coupling efficiency between the upper-and lower-layer waveguides is further validated for tapered vertical waveguide couplers with different end-face sizes(narrow-end sizes)[Figs.9(a)and(b)].The simulation results indicate that the taper waveguide with an end-face size of 0.8 μm achieves the highest interlayer coupling efficiency.Subsequently,the length of the taper waveguide is determined to be 300 μm to meet the requirements of adiabatic evolution[Fig.9(c)].Additionally,calculations based on Equations(2)-(4)reveal that within a bending radius range of 5-10 mm,a single-layer interferometric ring has a length of approximately 7.8 m.By stacking approximately 13 layers,silicon nitride interlayer-coupled interferometric rings with lengths on the scale of hundreds of meters can be achieved.Finally,by analyzing the major factors affecting the transmission loss of silicon nitride waveguides,it is determined that high-performance interlayer-coupled interferometric rings can be fabricated using processes such as thermal oxidation,low pressure chemical vapor deposition(LPCVD),tetraethoxysilane-low pressure chemical vapor deposition(TEOS-LPCVD),tetraethoxysilane-plasma enhanced chemical vapor deposition(TEOS-PECVD),and electron beam lithography(Fig.10).Conclusions This study proposes and designs an integrated optical gyroscope based on silicon nitride waveguides with ultrahigh aspect ratios by using an interlayer-coupled interferometric ring structure.The important parameters of the interferometric ring are simulated using the Lumerical MODE software package.The bending radius of the interlayer-coupled interferometric ring is 5 mm.To prevent coupling crosstalk between the waveguide rings and to minimize additional losses,the horizontal separation between the waveguide interferometric rings is set to 7 μm,and the vertical separation is set to 3 μm.The upper and lower layers of the waveguide interferometric rings are coupled by using tapered vertical couplers.Furthermore,within a bending radius range of 5-10 mm,a single-layer interferometric ring with a length of 7.8 m is obtained.By stacking multiple layers,a silicon nitride interlayer-coupled interferometric ring structure with lengths in the range of hundreds of meters is achieved.Finally,a low-loss fabrication process for silicon nitride interlayer coupled interferometric rings is discussed and designed.High-performance silicon nitride interlayer coupled interferometric rings can be fabricated by employing processes such as thermal oxidation,LPCVD,TEOS-LPCVD,TEOS-PECVD,and electron beam lithography.The successful design of silicon nitride interlayer coupled interferometric rings in this study contributes to the volume reduction,miniaturization,integration,and cost-effectiveness of integrated optical gyroscopes,thereby laying a solid foundation for their engineering applications.
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