單模摻鐿釔鋁石榴石晶體光纖之製備與模擬

Abstract

近年來，高功率固態雷射技術的發展中，以三價稀土離子摻雜氧化物晶體為主體的雷射材料，其中摻鐿釔鋁石榴石是種大有可為的光纖雷射增益介質，搭配InGaAs雷射二極體能獲得高效率螢光，且由於其能階結構，能有效阻止上能階轉換、激發態吸收、濃度淬滅等現象，使其在高摻雜濃度下依舊有很長的螢光生命期；然而，傳統以塊狀晶體作為增益介質的雷射系統受限於散熱能力，容易產生熱效應問題，導致光束輸出品質受影響；相較之下，晶體光纖因其高表面積體積比，能有效提升散熱效率，解決傳統塊狀晶體散熱效率差的問題；同時，傳統摻鐿釔鋁石榴石塊狀晶體作為雷射增益介質使用時，其在高功率操作下端面溫度極高，直接進行鍍製抗反射膜時，膜層容易因熱累積造成剝離或損壞，進而影響雷射輸出穩定性；為解決此問題，商業化中普遍採用undoped–doped–undoped的結構設計，使膜層可鍍製於外層未摻雜的YAG區域，以降低鍍膜區域受熱能的影響，提升鍍層附著性；此類結構則多以擴散鍵合法（Diffusion bonding）製備，透過高溫高壓條件促進晶體間原子擴散與鍵合，可形成undoped–doped–undoped的結構。 基於上述概念，本研究中嘗試將undoped–doped–undoped結構應用於晶體光纖中，能使摻雜區域集中於晶體光纖纖心中間區域，藉由兩側未摻雜之YAG區域進行熱傳導，可有效降低增益區的熱堆積，提升系統穩定性；同時，在摻鐿釔鋁石榴石晶體光纖上難以直接進行抗反射膜之鍍製，因其易受熱累積造成剝離或損壞損壞；而若透過此結構設計，將鍍膜位置轉移至外層未摻雜YAG區域，由於端面的溫度較低，能提升鍍層附著性與穩定性，藉此提升晶體光纖作為增益介質的品質。 本研究採用雷射加熱基座生長法（Laser heated pedestal growth）結合電子束蒸鍍法，製作具undoped–doped–undoped結構之摻鐿釔鋁石榴石晶體光纖；再於表層側鍍YAG作為纖衣，並經高溫處理促使膜層的固態生長，使其轉換為單晶之YAG纖衣；最後，研究中透過SEM、EBSD與EPMA進行結構及元素分佈分析，以驗證晶體光纖纖心中Yb元素的擴散行為以及晶體光纖纖衣中YAG固態生長的結果，並藉此評估製程設計對摻雜濃度與固態生長之影響。

Conventional bulk Yb:YAG lasers are limited by their poor heat dissipation during high-power operation, which easily induces thermal effects that degrade the beam quality and output stability. In contrast, crystal fibers offer a much higher surface-to-volume ratio, which improves heat dissipation and alleviates the thermal management issues faced by bulk crystals.When Yb:YAG bulk crystals are used as gain media, the end facets can reach extremely high temperatures under high-power operation. The anti-reflection coatings applied to the end surfaces often experience thermal delamination, leading to instability and efficiency loss. To address this problem, commercial Yb:YAG bulk lasers commonly adopt an undoped–doped–undoped structure, where the AR coatings are deposited on the outer undoped YAG regions. This configuration effectively reduces the thermal load on the coated surfaces and improves coating adhesion and reliability. Such structures are typically fabricated by diffusion bonding, in which undoped and doped YAG layers are joined under high temperature and pressure through atomic diffusion and bonding. Based on this concept, this study aims to apply the undoped–doped–undoped design to Yb:YAG crystal fibers, concentrating the doped region at the fiber core while allowing the surrounding undoped YAG regions to conduct heat away efficiently. This configuration minimizes thermal accumulation in the gain region, thereby enhancing overall system stability. Moreover, since AR coatings are difficult to deposit directly on Yb:YAG fibers due to thermal stress–induced damage, transferring the coating position to the undoped YAG end sections can improve coating adhesion and thermal stability, further enhancing the performance of the crystal fiber as a laser gain medium. In this work, a Yb:YAG crystal fiber with an undoped–doped–undoped structure was fabricated using a combination of LHPG and E-beam evaporation.Yb₂O₃ film was first deposited on a pure YAG crystal fiber, followed by regrowth using LHPG to dope Yb³⁺ ion into core. Subsequently, a YAG layer was deposited on the fiber surface as the cladding, and annealing process was performed to induce solid-state growth, forming a single-crystal YAG cladding. The structural and compositional properties of the fabricated crystal fibers were analyzed using SEM, EBSD, and EPMA, confirming the Yb diffusion behavior within the core and evaluating the effects of processing parameters on doping concentration and solid-state growth of the YAG cladding.