Multiphysics Simulation-Based Study on Single Crystal Fiber Growth via Laser-Heated Pedestal Growth Method

doi:10.16553/j.cnki.issn1000-985x.2026.0062

Abstract

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

CLC Number:

TN253

WANG Sichao, GAO Xibao, HAO Xingyue, TANG Qilong, WANG Qinglan, LIU Bo. Multiphysics Simulation-Based Study on Single Crystal Fiber Growth via Laser-Heated Pedestal Growth Method[J]. Journal of Synthetic Crystals, 2026, 55(7): 1111-1119.

Figures/Tables 6

Fig.1 Schematic of single crystal fiber （a） and computational mesh （b）

Table1 Material physical parameters for simulation

Material	Variable	Value	Reference
Alumina solid	Thermal conductivity， K/（W·m^-1·K^-1）	25	［14］
	Specific heat capacity， C_p /（J·kg^-1·K^-1）	745+0.95T-7.11×10^-4T²
	Density， ρ/（kg·m^-3）	3 720
Alumina liquid	Dynamic viscosity， μ/（mPa·s）	μ（T）=3.2exp［43.2×10³/（R·T）］ 2 190 K≤T≤2 500 K R=8.31 J/（mol·K）	［15］
	Specific heat capacity， C_p /（J·kg^-1·K^-1）	880	［16］
	Thermal conductivity， K/（W·m^-1·K^-1）	2.741 6	［17］
Air	Thermal conductivity， K/（W·m^-1·K^-1）	2.6×10^-2	［18］
	Density， ρ/（kg·m^-3）	7.561×10^-4
	Specific heat capacity，C_p /（J·kg^-1·K^-1）	1 005

Fig.2 Heat flux input （a） and melt pool temperature response （b） under different laser power fluctuation modes

Fig.3 Temperature distribution and isotherm diagrams of the molten zone for drawn single crystal fibers with 200 μm diameter （a） and 300 μm diameter （b）， and their axial temperature and absolute temperature gradient distribution curves along the molten zone axis （c），（d）

Fig.4 Coupled thermal and flow fields in the molten zone with only natural convection for 200 μm diameter （a） and 300 μm diameter （b）， and their coupled thermal and flow fields in the corresponding molten zone with the Marangoni effect superimposed （c），（d）

Fig.5 Coupled temperature distribution and fluid flow characteristics in the molten zone under conventional pulling speed， for 200 μm diameter （a）， and 300 μm diameter （b）， and their coupled temperature distribution and fluid flow characteristics in the molten zone under higher pulling speed （c），（d）

References 23

[1]	HARRINGTON J A. Single-crystal fiber optics： a review［J］. Solid State Lasers XXIII： Technology and Devices， 2014， 8959： 895902.
[2]	MU X D， MEISSNER S， MEISSNER H， et al. High efficiency Yb∶YAG crystalline fiber-waveguide lasers［J］. Optics Letters， 2014， 39（21）： 6331-6334. DOI URL
[3]	原东升. 微下拉设备研制、单晶生长及功能晶体TbCOB的制备和性能研究［D］. 济南：山东大学， 2016.
	YUAN D S. Equipment development and single crystal growth of micro-pulling-down， and the synthesis and investigations of functional crystal TbCOB［D］. Jinan： Shandong University， 2016 （in Chinese）.
[4]	YANG Y L， YE L H， BAO R J， et al. Growth and characterization of Yb∶Ho∶YAG single crystal fiber［J］. Infrared Physics & Technology， 2018， 91： 85-89.
[5]	LIU B， KARKI D， BERA S， et al. Fabrication and application of single crystal fiber via laser heated pedestal growth system［C］// Fiber Optic Sensors and Applications XVII. April 12-17， 2021. Online Only， USA. SPIE， 2021： 2.
[6]	LAN C W， KOU S. Heat transfer， fluid flow and interface shapes in floating-zone crystal growth［J］. Journal of Crystal Growth， 1991， 108（1/2）： 351-366. DOI URL
[7]	CHANG C L， HUANG S L， LO C Y， et al. Simulation and experiment on laser-heated pedestal growth of chromium-doped yttrium aluminum garnet single-crystal fiber［J］. Journal of Crystal Growth， 2011， 318（1）： 674-678. DOI URL
[8]	LO C Y， CHEN P Y. Three-dimensional simulation and experiment on micro-floating zone of LHPG with asymmetrical perturbation［J］. Journal of Crystal Growth， 2013， 362： 45-51. DOI URL
[9]	LIU B， YU Z H， HILL C， et al. Sapphire-fiber-based distributed high-temperature sensing system［J］. Optics Letters， 2016， 41（18）： 4405-4408. DOI PMID
[10]	LIU B， YU Y， BERA S， et al. Study of molten zone profile and defect formation during laser heated pedestal growth［C］// Micro- and Nanotechnology Sensors， Systems， and Applications XI. April 14-18， 2019. Baltimore， USA. SPIE， 2019： 91.
[11]	YIN Y Q， ZHANG N， ZHANG J， et al. Thermal optimization of single crystal fiber manufacturing based on heat loss compensation［J］. Applied Thermal Engineering， 2022， 201： 117741. DOI URL
[12]	WANG S C， HAO X Y， WANG Q L， et al. Optimization of thermal field and control algorithms in laser heated pedestal growth systems［C］// 8th Optics Young Scientist Summit （OYSS 2025）. September 26-29， 2025. Qingdao， China. SPIE， 2025： 14.
[13]	GAO X B， WANG S C， LIU B， et al. Machine learning enhanced single crystal fiber fabrication via laser heated pedestal growth［J］. IEEE Photonics Technology Letters， 2025， 37（18）： 1017-1020. DOI URL
[14]	张广路，张文涛，孙一强，等. 高功率CO₂激光加热氧化铝样品均温低应力设计［J］. 光学学报， 2025， 45（11）： 200-210.
	ZHANG G L， ZHANG W T， SUN Y Q， et al. Uniform temperature and low stress design of alumina samples heated by high power CO₂ laser［J］. Acta Optica Sinica， 2025， 45（11）： 200-210 （in Chinese）.
[15]	PARADIS P F， ISHIKAWA T. Surface tension and viscosity measurements of liquid and undercooled alumina by containerless techniques［J］. Japanese Journal of Applied Physics， 2005， 44（7R）： 5082. DOI
[16]	ZHOU X L， ZHOU Y F， DENG Y， et al. Molecular dynamics study on structure， vibrational properties， and transport coefficients of liquid alumina［J］. Materials， 2022， 15（23）： 8370. DOI URL
[17]	VORA H D， SANTHANAKRISHNAN S， HARIMKAR S P， et al. One-dimensional multipulse laser machining of structural alumina： evolution of surface topography［J］. The International Journal of Advanced Manufacturing Technology， 2013， 68（1）： 69-83. DOI URL
[18]	吕搏闻，武珈羽，张晗旭，等. 基于多物理场仿真的YIG薄膜液相外延生长工艺优化的研究［J］. 人工晶体学报， 2026， 55（1）： 29-36. DOI
	LYU B W， WU J Y， ZHANG H X， et al. Optimization of LPE growth process of YIG films based on multi-physics field simulation［J］. Journal of Synthetic Crystals， 2026， 55（1）： 29-36 （in Chinese）. DOI
[19]	YANG H D， ZHANG S. Numerical simulation of temperature field and stress field in fused deposition modeling［J］. Journal of Mechanical Science and Technology， 2018， 32（7）： 3337-3344. DOI
[20]	LIU J， LU W Q. Preliminary study of non-isothermal phase change phenomena in vertical Bridgman crystal growth［J］. Chinese Science Bulletin， 2007， 52（5）： 701-710. DOI URL
[21]	ZHANG N， LIU D. Numerical simulation of MHD oscillatory mixed convection in CZ crystal growth by Lattice Boltzmann method［J］. Results in Physics， 2018， 10： 882-890. DOI URL
[22]	YAO M W， MATTHIESEN D H， CHAIT A. Numerical simulation of heat transport and fluid flow in directional crystal growth of GaAs［J］. Numerical Heat Transfer， Part A： Applications， 1996， 30（7）： 685-701.
[23]	施宇峰，王鹏飞，穆宏赫，等. 尺寸效应对坩埚下降法生长氟化钙晶体影响机制的数值模拟分析［J］. 人工晶体学报， 2024， 53（6）： 973-981.
	SHI Y F， WANG P F， MU H H， et al. Numerical simulation investigation of size effect on calcium fluoride crystals grown by vertical bridgman method［J］. Journal of Synthetic Crystals， 2024， 53（6）： 973-981 （in Chinese）.