光學同調斷層掃描(Optical coherence tomography, OCT)近年來逐漸成為眼科診察的一項重要儀器，此系統光源需要寬頻光源來達到最佳的縱向解析度，因此利用Cr4+:Forsterite所產生之寬頻光源ASE (Amplified Spontaneous Emission)，其光源頻譜從1050nm~1300nm，不僅對於軟性組織具有較小吸收，也可避開1400nm之水分子吸收，有潛力成為OCT替代光源。
本論文利用雷射加熱基座生長法可生長出直徑為70μm~40μm之Cr4+:Forsterite晶纖，並使用Aluminosilicate與硼酸鹽玻璃包覆為雙纖衣晶體光纖。藉由實驗數據結合理論分析探Cr4+:Forsterite雙纖衣晶體光纖之各項影響因素，包括: 傳輸損耗、吸收係數、側鍍Cr2O3厚度、光纖生長速度、纖衣或纖核激發、與輸入端薄膜製鍍。目前纖核直徑已成功縮小至40μm，有利於日後發展為晶體光纖雷射。
為了提升Cr4+:Forsterite之ASE斜線效率，我們使用以下兩種方法: (1) 輸入端製鍍薄膜，針對激發光波長為高穿透；對ASE光波長為高反射，促使輸入端反射更多ASE光回到纖核中以增加輸出端光功率。結果發現其ASE功率上升幅度與側鍍厚度有關，最佳上升幅度為57.8%，側鍍厚度為38nm。雖然增加側鍍厚度可以增加鉻離子濃度，但由於Cr4+:Forsterite在波長1064nm附近有激發態的吸收(Excited state absorption)，反而使得ASE光最強的波段可能被再吸收，其ASE功率上升比例因此下降。(2) 纖核激發，能提升ASE之斜線效率，與纖衣激發相比，其上升幅度與纖核直徑有關。直徑越小，上升幅度越大，當纖核直徑為40μm時，斜線效率上升幅度可達纖衣激發時的三倍。

Optical coherence tomography (OCT) becomes an important instrument for ophthalmic diagnosis nowadays. It requires wideband broadband light source to achieve good axial resolution. Thus Cr4+: Forsterite, which emits wideband amplified spontaneous emission (ASE) of 1050nm ~ 1300nm in wavelength, not only at minimal absorption for biological soft tissue, but also away from the OH- absorption near 1400nm, is a potential replacement for OCT broadband light source.
In this study we used the laser heated pedestal growth (LHPG) method to make Cr4+: Forsterite crystal fibers of 70μm~40μm in diameter. The crystal fibers were then grown into double-cladded fiber (DCF) with two capillary tubes of aluminosilicate and borosilicate. By means of experiments and theoretical analysis, the affecting factors of Cr4+: Forsterite DCF were studied including the propagation loss, absorption coefficient, thickness of Cr2O3 side deposition, speed of fiber growth, cladding or core pumping, and coating on input facet. We were successful in reducing the core size to 40μmthat would be beneficial for crystal fiber laser development in the future.
In order to increase the slope efficiency of Cr4+: Forsterite, two methods were used as follows: (1) Coating on input facet of DCF which is both anti-reflection (AR) for pump wavelengths and high-reflection (HR) for ASE wavelengths. It helped reflect more ASE light back to the DCF to increase the output power at the output end. We found the magnitude of ASE power increase varied with the thickness of Cr2O3 side deposition. The highest ratio of ASE power increase was 57.8% when deposited with 38nm Cr2O3. Though thicker Cr2O3 deposition would increase the Cr4+ ion concentration, the chance for ASE light at peak wavelength to be re-absorbed also increased due to the excited state absorption near 1064nm, resulting in less ASE power increase. (2) Core pumping increased the slope efficiency of ASE light source when compared with cladding pumping. And the smaller core size, the higher factor of improvement, was obtained. When using 40μm-core fiber, the slope efficiency of ASE was three times as high as cladding pumped.

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