本研究旨在探討以雙光束干涉法記錄於 PQ-PMMA 感光材料中的體積布拉格光柵(Volume Bragg Grating, VBG)，其繞射光譜在記錄後隨時間演變的行為。為達成此目的，本研究結合了數值模擬與實驗驗證。首先，基於耦合波理論與傳遞矩陣法，建立了一套數值模擬系統，用以分析折射率調變、平均折射率、光柵週期與材料厚度等結構參數，對繞射效率、光譜半高全寬(FWHM)及布拉格波長等光譜參數的影響。
實驗部分，透過雙光束干涉法在 2 mm 與 5 mm 厚的 PQ-PMMA 樣品中寫入光柵，並系統性地量測在不同曝光條件下，繞射光譜隨時間的演變。研究結果顯示，熱效應是造成光譜不穩定的關鍵因素。曝光後材料的非均勻冷卻收縮以及光熱效應，會導致光譜出現飄移、變寬甚至分裂成多個波峰的現象。其中，布拉格波長的飄移方向取決於熱光效應 (????/????) 所造成的折射率上升與光柵週期 (Λ) 收縮之間的競爭結果。實驗發現，在入射光強度為 2.45×103 W/m2、曝光 195 秒的條件下，對應到布拉格波長為 844.91 nm 的光柵結構，可獲得最高的繞射效率。
本研究成功透過一個結合熱效應與折射率變化的模型，對觀測到的光譜演變趨勢提出合理解釋，為優化 VBG 元件的穩定性提供了重要依據。;This study investigates the time-dependent evolution of the diffraction spectrum in Volume Bragg Gratings (VBGs) recorded in PQ-PMMA photosensitive material via two beam interference. To this end, the research combines numerical simulation with experimental verification. A numerical simulation system was first developed based on coupled wave theory and the transfer matrix method to analyze how structural parameters—such as refractive index modulation, average refractive index, grating period, and material thickness—influence spectral parameters like diffraction efficiency, full width at half maximum (FWHM), and the Bragg wavelength.
Experimentally, gratings were written into 2 mm and 5 mm thick PQ-PMMA samples using the two-beam interference method, and the temporal evolution of the diffraction spectrum was systematically measured under various exposure conditions. The results indicate that thermal effects are a key factor causing spectral instability. Non-uniform cooling shrinkage and thermo-optic effects in the material post-exposure lead to spectral shifts, broadening, and even splitting into multiple peaks. The direction of the Bragg wavelength shift is determined by the competition between the refractive index increase caused by the thermo-optic effect and the shrinkage of the grating period (Λ). It was experimentally found that under an incident intensity of 2.45×103 W/m2 and an exposure time of 195 seconds, the highest diffraction efficiency was achieved for a grating structure corresponding to a Bragg wavelength of 844.91 nm.
This research successfully explains the observed trends in spectral evolution through a model that combines thermal effects and refractive index variations, thereby providing an important basis for optimizing the stability of VBG components.
