| 摘要: | 本研究主要模擬具有布拉格光柵結構的脊型矽波導，其特徵為能反射一特定波段，而其他波段的光則正常通過波導。該元件常作為光通訊波長分波多工系統的通道訊號擷取元件。為提升此種窄波段的矽波導反射元件之頻譜旁瓣的抑制比性能，本研究考慮使用幾種特定的漸變函數來調控布拉格光柵的結構變化，以達到降低反射頻譜旁瓣的抑制比的目的，並探討不同函數對於穿透殘留量、反射率以及頻譜波形等影響。
研究中使用 GratingMOD 軟體來模擬分析布拉格光柵波導，該軟體的數值模擬方法為模態耦合理論(CMT)。透過文獻中一般型波導與脊型波導的模擬比較結果，得知脊型波導加上布拉格光柵結構後，有相對較窄的頻寬，較容易達到頻譜半高寬之頻寬 0.8 nm的設計目標。因此本研究以脊型布拉格光柵波導為主要方向，並探討光柵結構位於波導側邊與波導頂層的差異，模擬結果中我們得知在相同結構長度與相近頻寬下，側邊型光柵結構損耗較大，但元件體積小；頂層型光柵結構損耗較小，但元件體積大。
透過側邊光柵類型之脊型布拉格光柵波導的參數分析，了解各個結構參數對頻寬與反射率的影響，並將下層矽波導寬度作為主要變動參數，與頻寬進行曲線擬合，從擬合函數中選出符合設計目標的參數組合。
使用漢明函數、修正型漢明函數、升餘弦函數、布雷克曼函數、陳函數以及高斯函數來調變布拉格光柵。將各函數的套用至側邊光柵結構，依照函數圖形的半高寬進行分組探討，最後再比較側邊型與頂層型光柵結構在相近穿透殘留量下的函數性能。在結構長度 3000 m 下，修正型漢明函數的雜訊抑制達-24.9dB、反射率 99.9%為各函數中最高，且頻譜波形最銳利；而陳函數的雜訊抑制程度最高，可抑制雜訊至-95dB，其反射率為 95.2%。雜訊抑制程度越高，損耗就相對越高，然而高斯函數在相近的雜訊抑制程度下，損耗大於其他函數。側邊型與頂層型光柵結構在相近穿透殘留量下，兩結構的函數性能皆相近。;This study mainly simulates a ridge silicon waveguide with Bragg grating structure, which is characterized by reflecting a specific band, while wavelength in other bands passes through the waveguide normally. This component is often used as a signal acquisition component in WDM or DWDM systems, and the modulated Bragg grating structure through a apodization function to reduce the sidelobes. We use several apodization functions to investigate the sidelobe suppression, spectrum waveform, transmission, and reflection of each function.
We use GratingMOD to simulate and analyze Bragg grating waveguides. The numerical simulation method of this software is Coupled-Mode Theory (CMT). According to simulation results of strip waveguide and ridge waveguide in the literature, it is known that the ridge waveguide with the Bragg grating has a relatively narrow bandwidth, and it is easier to achieve the design of a bandwidth FWHM is 0.8 nm. Therefore, this study takes the ridge waveguide with the Bragg grating as the main structure, and explores the difference between the grating structure located on the side of the waveguide and the top layer of the waveguide. From the simulation results, we know that under the same structure length and similar bandwidth, the sidewall grating structure has higher loss, but the component volume is smaller; the top grating structure has less loss, but the component volume is larger.
Through the parameter analysis of the sidewall Bragg grating in ridge waveguide, we can understand the influence of each parameter on the bandwidth and reflection, and use the width of the slab as the main parameter to perform curve fitting with the bandwidth.
We use Hamming function, modify-Hamming function, raised-cosine function, Blackman function, Chen function, and Gaussian function to modulate the Bragg gratings. Apply each function to the side grating structure, group and discuss according to the FWHM of the function, and compare the function performance of sidewall and top grating structures under the similar transmission. When the length is 3000m, the modify-Hamming function has a sidelobe suppression of -24.9dB, and reflection is 99.9%, which is the highest among all functions, and the spectrum waveform is the sharpest; while the Chen function has the highest degree of sidelobe suppression of -95dB, and reflection is 95.2%. The higher the degree of sidelobe suppression, the higher the loss. However, the Gaussian function has higher loss than other functions at similar sidelobe suppression. When the sidewall and top grating structures have similar transmission, the function performances of two structures is also similar. |
