关键词: 单晶光纤; 蓝宝石光纤; 激光加热基座法; 马兰戈尼效应; 温场-流场耦合

Abstract: Laser-heated pedestal growth （LHPG） method is a well-established， crucible-free technique for fabricating micrometer-scale single crystal optical fibers， in which a localized molten zone is generated by laser heating of a feed rod and directional crystal growth is realized through precisely controlled feeding and seed-crystal pulling. As a fibrous “quasi-one-dimensional” functional crystal， the single crystal fiber combines the large aspect ratio of conventional glass fibers with the superior physical and chemical properties of bulk crystals， making it highly attractive for high-power lasers， high-energy radiation detection， high-temperature sensing， and optical communication. Owing to its extremely high heating temperatures， freedom from crucible contamination， and rapid solidification rates， LHPG is widely regarded as the only viable route for growing single crystal fibers at the 100 μm diameter scale. However， the high-temperature molten zone is governed by strongly coupled multiphysical interactions—including heat conduction， thermal radiation， melt convection， phase change， and surface-tension-driven flow—whose pronounced nonlinearity， together with the inability of conventional experiments to observe internal thermal and flow fields in real time， leaves the mechanisms governing process stability and interface evolution insufficiently understood and limits further improvements in crystal quality and dimensional uniformity. To address this gap， a two-dimensional axisymmetric multiphysics model was established on the COMSOL Multiphysics platform， coupling geometric-optics-based laser energy absorption， solid-liquid heat transfer， and fluid dynamics that incorporates the Marangoni effect； the molten-zone boundary was fitted from experimental images of stably growing sapphire fiber. Using a 1 mm diameter α-Al₂O₃ feed rod， the study systematically investigated the coupled influences of laser-power fluctuation mode， fiber diameter， Marangoni convection， and pulling speed on the thermo-fluid characteristics of the molten zone. The results show that the form of laser-power fluctuation （5% amplitude） strongly affects thermal stability： square-， sine-， and triangular-wave modulation produced molten-zone temperature swings of 88， 80， and 61 K， respectively， with the triangular wave generating the weakest thermal shock and the smoothest temperature response， identifying it as the preferred reference signal for power-stabilization loops. Increasing the fiber diameter from 200 μm to 300 μm lowered the axial temperature gradient and yielded a more uniform thermal distribution； in both cases the gradient displayed a characteristic edge-high， center-low pattern， peaking near the laser-irradiated region and the solid-liquid interface， which indicates that small-diameter growth is inherently more difficult to stabilize. With a bond number far below unity， Marangoni convection was confirmed as the dominant flow mechanism： introducing surface-tension gradients raised the maximum melt velocity from the order of 10^-5 m/s （buoyancy-driven natural convection alone） to 0.07~0.1 m/s and reconstructed the flow field from a single vortex into a complex double-vortex structure. Finally， whereas a baseline pulling speed of 1.28×10^-4 m/s preserved both thermal and flow stability， raising it by three orders of magnitude destroyed the coupled steady state， necessitated a doubling of laser power， severely distorted the temperature field， and produced jet-like high-speed flow conducive to growth interruption. Overall， this study clarifies the strongly coupled thermo-fluid mechanisms governing the LHPG process and the regulatory roles of key processing parameters in molten-zone stability. The findings provide a comprehensive theoretical foundation for optimizing growth conditions and practical guidance for improving the quality， uniformity， and structural integrity of single crystal optical fibers， particularly sapphire and other advanced functional crystalline materials.

Key words: single crystal fiber; sapphire fiber; laser-heated pedestal growth method; Marangoni effect; thermal-fluid coupling