Fabrication and Characterization of Sapphire-Derived Fibers by Molten Core Method

doi:10.16553/j.cnki.issn1000-985x.2026.0082

Abstract

Abstract: Sapphire-derived fibers （SDFs） have attracted increasing attention as promising optical fiber platforms for sensing applications in extreme environments， owing to the excellent thermal stability and chemical inertness inherited from sapphire-based core materials. Nevertheless， the controllable fabrication of SDFs with designed core diameters and stable optical performance remains a key challenge for their practical use in high-temperature sensing. Most previous studies have focused on material characterization or device-level demonstrations， whereas the relationship among preform design， drawing parameters， core-diameter controllability， and transmission quality has not been sufficiently clarified. In this work， we demonstrate the controllable fabrication of SDFs using millimeter-scale sapphire rods as core preforms in combination with the molten core method （MCM）. The aim is to realize stable continuous drawing of SDFs with tunable core diameters and to verify their feasibility for high-temperature optical sensing.During fabrication， a sapphire rod was used as the core material and was thermally processed within a silica cladding during high-temperature fiber drawing. By adjusting the drawing parameters， SDFs with different core diameters were obtained while maintaining a standard outer cladding diameter of 125 μm. Three representative SDF samples were selected for systematic characterization， with core/cladding diameter configurations of 21.0/125 μm， 18.1/125 μm， and 14.1/125 μm for samples Ⅰ， Ⅱ， and Ⅲ， respectively. The results demonstrate that the MCM-based drawing process enables effective regulation of the SDF core geometry， providing a feasible route for preparing SDFs with controllable structural parameters. Optical transmission measurements show that the as-drawn SDFs exhibit a low propagation loss of 1.08 dB/m in the near-infrared wavelength region， indicating that favorable optical quality can be maintained during the drawing of sapphire-derived core fibers.To further evaluate the sensing potential of the fabricated SDFs， a Fabry-Pérot interferometer （FPI） high-temperature sensor was constructed using the SDF with a 14.1 μm core diameter. The high-temperature sensing performance of the device was investigated over a wide temperature range from 100 ℃ to 1 100 ℃. The experimental results show that the SDF-based FPI sensor achieves a temperature sensitivity of 16.8 pm/℃ and exhibits good linear response characteristics， with a coefficient of determination （R²） higher than 0.997. In addition， the sensor maintains good repeatability during heating and cooling cycles， confirming the stability of the SDF-based interferometric structure under the present high-temperature testing conditions.This study demonstrates that the combination of millimeter-scale sapphire core preforms and the MCM drawing process provides an effective approach for the controllable fabrication of SDFs with different core diameters. The results clarify the influence of preform design and drawing-parameter regulation on the geometrical structure and optical transmission properties of SDFs. More importantly， the successful demonstration of an SDF-based high-temperature FPI sensor verifies the feasibility of the fabricated fibers for optical sensing in high-temperature environments. This work therefore provides both a material-processing basis and a device-level validation for the development of SDF-based high-temperature fiber sensors.

Key words: sapphire-derived fiber; molten core method; optical fiber fabrication; core-diameter adjustment; fiber-optic temperature sensing; Fabry-Pérot interferometer

CLC Number:

LIU Xuecheng, WANG Zhifeng, ZHANG Liang, WEI Heming, ZHU Mengshi, LU Yarong, YANG Xiaorong, HAO Wenjuan, LIU Guanghe, PANG Fufei. Fabrication and Characterization of Sapphire-Derived Fibers by Molten Core Method[J]. Journal of Synthetic Crystals, 2026, 55(7): 1143-1153.

Figures/Tables 12

Fig.1 Schematic diagram of the SDFs drawing （a）， and photograph of the SDFs （b）

Table 1 Drawing process typical parameters of SDFs

Draw parameters	Sample Ⅰ	Sample Ⅱ	Sample Ⅲ
Preform feed rate/（mm·min^-1）	0.38	0.39	0.41
Draw temperature/℃	2 150	2 150	2 180
Draw speed/（m·min^-1）	9.7	9.9	10.6
Core diameter/μm	21.0	18.1	14.1

Fig.2 Micrograph of the SDF FPI cavity length characterization （a）， and SDF FPI reflection spectrum （b）

Fig.3 Schematic diagram of the high-temperature test setup for the SDF FPI sensors

Fig.4 Fiber end-face microscope images with different core diameters

Fig.5 Refractive index profile of fiber sample Ⅲ. （a） Two-dimensional refractive index profile；（b） one-dimensional profiles of relative refractive index difference along the horizontal and vertical directions of the optical fiber cross-section

Table 2 Comparison of geometric parameters and refractive index profile characteristics of SDF samples

Sample ID	Core diameter/μm	Cladding diameter/μm	$∆ m a x$ /%
Sample Ⅰ	21.0±1.0	125.0±1.5	5.5
Sample Ⅱ	18.1±1.0	125.0±1.5	4.8
Sample Ⅲ	14.1±1.0	125.0±1.5	4.4

Table 2 Comparison of geometric parameters and refractive index profile characteristics of SDF samples

Sample ID	Core diameter/μm	Cladding diameter/μm	$∆ m a x$ /%
Sample Ⅰ	21.0±1.0	125.0±1.5	5.5
Sample Ⅱ	18.1±1.0	125.0±1.5	4.8
Sample Ⅲ	14.1±1.0	125.0±1.5	4.4

Fig.6 Transmission loss test of fibers with different core diameters

Fig.7 Transmission spectra of sample Ⅲ SDFs

Fig.8 Spectra of the SDF FPI sensor during thermal cycling

Fig.9 Response and linear fitting of the sensor during thermal cycling （heating ： 100~1 100 ℃， cooling：1 100~300 ℃）

Fig.10 High-temperature stability test of the SDF FPI sensor. （a） Reflection spectra at room temperature， 1 100 ℃， and after cooling back to room temperature；（b） characteristic wavelength shift during 4 h holding at 1 100 ℃

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