Cladding Technologies for Sapphire Single-Crystal Fiber: Evolution and Prospects

doi:10.16553/j.cnki.issn1000-985x.2026.0057

Abstract

Abstract: Sapphire single-crystal fiber is an promising candidate material for optical sensing in extreme environments due to its excellent properties such as high-temperature resistance， corrosion resistance， radiation resistance and wide spectral transmission. However， most of the existing sapphire single-crystal fibers are non-cladding structures， which have problems such as weak mode confinement， high transmission loss and strong environmental sensitivity， which restrict their practical engineering applications. By constructing a low-refractive-index optical confinement structure outside or inside the sapphire single-crystal fiber， the cladding processing improves its transmission performance， improves the device level and expands the application scenarios. In this paper， the research progress in cladding technologies for sapphire single-crystal fiber is systematically reviewed. These technologies can be classified into three categories： external additive cladding techniques， internal modification cladding techniques， and micro-structured cladding techniques. The implementation principles， fabrication processes， representative studies， and performance characteristics of these technologies are summarized. The development status， technical features， and application potential of different cladding routes are also comparatively analyzed. On this basis， the development direction of sapphire single-crystal fiber cladding technology is prospected for extreme environment transmission and sensing applications.

Key words: sapphire single-crystal fiber; cladding technology; extreme-environment sensing; external additive cladding technique; internal modification cladding technique; micro-structured cladding technique; fiber device

CLC Number:

TN253

LI Qian, ZHOU Feng, LIU Xianming, LEI Xiaohua, ZHANG Peng, XU Hengyi. Cladding Technologies for Sapphire Single-Crystal Fiber: Evolution and Prospects[J]. Journal of Synthetic Crystals, 2026, 55(7): 1022-1043.

Figures/Tables 19

References 95

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Technique	Year	Institution	Advantage	Disadvantage
Suspension dip-coating-sintering method	1993	Drexel University & NASA Lewis Research Center^［26］	Simple； low-cost； thickness-tunable； suitable for thick coatings	Prone to pores/cracks； sensitive to slurry and sintering conditions
	1994	Drexel University & NASA Lewis Research Center^［27］
	1995	NASA^［28］
Solution dip-coating and staged curing	1994	Rutgers University^［29］	Simple； high efficiency； scalable coating	Limited temperature resistance； thermal mismatch； poor long-term stability
Sol-gel dip-coating and subsequent thermal treatment	1994	Rutgers University^［29］	Composition-tunable； uniform coating； mature process	Multi-cycle deposition； porous defects； limited high-temperature stability
	1994	NASA Lewis Research Center^［31］
	1997	NASA Lewis Research Center^［32］
	2000	Zhejiang University & Fang Yuan technical Incorporation^［33］
	2008	General Electric Company^［34］
	2022	NETL^［35］
Surface reaction- conversion method	2005	UT-Battelle， LLC^［37］	Strong bonding； in situ cladding； large NA reduction	Very high temperature； poor control of thickness； core distortion risk
Polymeric precursor method	2013	University of Cincinnati & Missouri University of Science and Technology^［38］	Good precursor homogeneity； low formation temperature； stable coating	Low efficiency； repeated cycles required； thermal stability still limited
Molten-aluminum dip-coating and subsequent anodization	2015—2016	Stevens Institute of Technology & Academy of Sciences of the Czech Republic^{［39⁃40］}	In situ oxide conversion； useful for functional coatings	Poor uniformity control； rough/porous interface； higher scattering loss
UV-assisted curing and forming technique	2024	Harbin Engineering University^［41］	Good shape control； thick cladding	Eccentricity， pores， and cracks likely； stability not fully verified
PVD	1993	University of California & Max-Planck-Institut für Metallforschung & Pratt & Whitney Company^［30］	High purity； precise thickness control； suitable for thin functional films	Slow； expensive； limited uniformity on curved surfaces
PVD	2021	Iowa State University & SSAB Americas^［42］		Slow； expensive； limited uniformity on curved surfaces
CVD	1990	Virginia Polytechnic Institute and State University^［43］	Dense and uniform coatings； controllable thickness； multilayer capable	Complex equipment； high process requirements； high-temperature degradation
	1993	University of California & Max-Planck-Institut für Metallforschung & Pratt & Whitney Company^［30］
	2008	General Electric Company^［34］
	2019	Jilin University^{［44⁃46］}
	2019	Northwestern Polytechnical University & Southwest University^［47］
	2020	Northwestern Polytechnical University & Changcheng Institute of Metrology and Measurement & Southwest University^［48］
	2024	Northwestern Polytechnical University & AECC Sichuan Gas Turbine Establishment^［49］
	2025	Shandong University^［50］
Melt extrusion	1993	Rutgers University^［10］	Simple； fast； continuous thick coating	Poor thermal resistance； mainly suitable as a protective layer
Melt extrusion	1994	Rutgers University^［29］	Simple； fast； continuous thick coating
LHPG-based hybrid crystal-glass fiber fabrication	2016	National Dong Hwa University & National Taiwan Ocean University & National chi nan university^［51］	Direct core-cladding formation； good interface quality	Glass cladding limits ultrahigh-temperature use； process is complex
Liquid sapphire single-crystal fiber cladding	2022	Wuhan University of Technology^［52］	Simple； refractive index tunable	Poor thermal stability； difficult for practical continuous cladding
Crystalline-core/ crystalline-clad fiber	2025	The Pennsylvania State University & General Opto Solutions， LLC^［54］	All-crystalline structure； best thermal match； high-temperature stability	Complex； low efficiency； limited length； coupling is difficult

Technique	Year	Institution	Advantage	Disadvantage
Suspension dip-coating-sintering method	1993	Drexel University & NASA Lewis Research Center^［26］	Simple； low-cost； thickness-tunable； suitable for thick coatings	Prone to pores/cracks； sensitive to slurry and sintering conditions
	1994	Drexel University & NASA Lewis Research Center^［27］
	1995	NASA^［28］
Solution dip-coating and staged curing	1994	Rutgers University^［29］	Simple； high efficiency； scalable coating	Limited temperature resistance； thermal mismatch； poor long-term stability
Sol-gel dip-coating and subsequent thermal treatment	1994	Rutgers University^［29］	Composition-tunable； uniform coating； mature process	Multi-cycle deposition； porous defects； limited high-temperature stability
	1994	NASA Lewis Research Center^［31］
	1997	NASA Lewis Research Center^［32］
	2000	Zhejiang University & Fang Yuan technical Incorporation^［33］
	2008	General Electric Company^［34］
	2022	NETL^［35］
Surface reaction- conversion method	2005	UT-Battelle， LLC^［37］	Strong bonding； in situ cladding； large NA reduction	Very high temperature； poor control of thickness； core distortion risk
Polymeric precursor method	2013	University of Cincinnati & Missouri University of Science and Technology^［38］	Good precursor homogeneity； low formation temperature； stable coating	Low efficiency； repeated cycles required； thermal stability still limited
Molten-aluminum dip-coating and subsequent anodization	2015—2016	Stevens Institute of Technology & Academy of Sciences of the Czech Republic^{［39⁃40］}	In situ oxide conversion； useful for functional coatings	Poor uniformity control； rough/porous interface； higher scattering loss
UV-assisted curing and forming technique	2024	Harbin Engineering University^［41］	Good shape control； thick cladding	Eccentricity， pores， and cracks likely； stability not fully verified
PVD	1993	University of California & Max-Planck-Institut für Metallforschung & Pratt & Whitney Company^［30］	High purity； precise thickness control； suitable for thin functional films	Slow； expensive； limited uniformity on curved surfaces
PVD	2021	Iowa State University & SSAB Americas^［42］		Slow； expensive； limited uniformity on curved surfaces
CVD	1990	Virginia Polytechnic Institute and State University^［43］	Dense and uniform coatings； controllable thickness； multilayer capable	Complex equipment； high process requirements； high-temperature degradation
	1993	University of California & Max-Planck-Institut für Metallforschung & Pratt & Whitney Company^［30］
	2008	General Electric Company^［34］
	2019	Jilin University^{［44⁃46］}
	2019	Northwestern Polytechnical University & Southwest University^［47］
	2020	Northwestern Polytechnical University & Changcheng Institute of Metrology and Measurement & Southwest University^［48］
	2024	Northwestern Polytechnical University & AECC Sichuan Gas Turbine Establishment^［49］
	2025	Shandong University^［50］
Melt extrusion	1993	Rutgers University^［10］	Simple； fast； continuous thick coating	Poor thermal resistance； mainly suitable as a protective layer
Melt extrusion	1994	Rutgers University^［29］	Simple； fast； continuous thick coating
LHPG-based hybrid crystal-glass fiber fabrication	2016	National Dong Hwa University & National Taiwan Ocean University & National chi nan university^［51］	Direct core-cladding formation； good interface quality	Glass cladding limits ultrahigh-temperature use； process is complex
Liquid sapphire single-crystal fiber cladding	2022	Wuhan University of Technology^［52］	Simple； refractive index tunable	Poor thermal stability； difficult for practical continuous cladding
Crystalline-core/ crystalline-clad fiber	2025	The Pennsylvania State University & General Opto Solutions， LLC^［54］	All-crystalline structure； best thermal match； high-temperature stability	Complex； low efficiency； limited length； coupling is difficult

Technique	Year	Institution	Advantage	Disadvantage
Two-step laser melting technique	1977	Bell laboratories^［55］	Monolithic crystalline structure	Parameter-sensitive； prone to surface defects； limited structural tunability
Hydrogen ion implantation	2013	State University of New York^{［58⁃59］}	Controllable cladding location； strong optical confinement	Parameter-sensitive； low efficiency； limited cladding uniformity
⁶Li（n，α）³H reaction-based ion implantation	2017	The Ohio State University^［60］	High-temperature stability； potential for distributed sensing	Harsh experimental conditions； complex process； limited higher-order-mode suppression
⁶Li（n，α）³H reaction-based ion implantation	2022	The Ohio State University & Luna Innovations Inc. & Idaho National Laboratory^{［61⁃62］}
Type I directly written waveguides	2022	Beijing Institute of Technology^［66］	Direct core writing； simple structure； suitable for local waveguide fabrication	Narrow process window； limited thermal stability； poor repeatability
Type II depressed-cladding waveguides	2022	University of Oxford & Rolls-Royce Plc^［69］	Flexible design； depressed-cladding structure； potential for single-mode guidance	High propagation loss； crack-prone； narrow process window and high cost
	2024	University of Oxford & Rolls-Royce Plc & Shandong University & Jiangsu Jingying Optoelectronics Technology Co. Ltd.^［70］
	2025	Chongqing university^［72］
	2025	Chongqing university^［73］
	2021	Jilin University & Changchun Institute of Optics， Fine Mechanics and Physics， Chinese Academy of Sciences & Tsing-Hua University^［74］	Integrated cladding and grating； combined mode control and sensing； high device integration	Mostly short local devices； low efficiency； difficult long-length consistency and low-loss coupling
	2023	Huazhong University of Science and Technology^［75］
	2023	Zhejiang Lab^［76］
	2023	Shenzhen University^［77］
	2025	Shenzhen University & Hiwing Technology Academy of CASIC^［78］
	2026	North University of China & Xiamen University^［79］

Technique	Year	Institution	Advantage	Disadvantage
Two-step laser melting technique	1977	Bell laboratories^［55］	Monolithic crystalline structure	Parameter-sensitive； prone to surface defects； limited structural tunability
Hydrogen ion implantation	2013	State University of New York^{［58⁃59］}	Controllable cladding location； strong optical confinement	Parameter-sensitive； low efficiency； limited cladding uniformity
⁶Li（n，α）³H reaction-based ion implantation	2017	The Ohio State University^［60］	High-temperature stability； potential for distributed sensing	Harsh experimental conditions； complex process； limited higher-order-mode suppression
⁶Li（n，α）³H reaction-based ion implantation	2022	The Ohio State University & Luna Innovations Inc. & Idaho National Laboratory^{［61⁃62］}
Type I directly written waveguides	2022	Beijing Institute of Technology^［66］	Direct core writing； simple structure； suitable for local waveguide fabrication	Narrow process window； limited thermal stability； poor repeatability
Type II depressed-cladding waveguides	2022	University of Oxford & Rolls-Royce Plc^［69］	Flexible design； depressed-cladding structure； potential for single-mode guidance	High propagation loss； crack-prone； narrow process window and high cost
	2024	University of Oxford & Rolls-Royce Plc & Shandong University & Jiangsu Jingying Optoelectronics Technology Co. Ltd.^［70］
	2025	Chongqing university^［72］
	2025	Chongqing university^［73］
	2021	Jilin University & Changchun Institute of Optics， Fine Mechanics and Physics， Chinese Academy of Sciences & Tsing-Hua University^［74］	Integrated cladding and grating； combined mode control and sensing； high device integration	Mostly short local devices； low efficiency； difficult long-length consistency and low-loss coupling
	2023	Huazhong University of Science and Technology^［75］
	2023	Zhejiang Lab^［76］
	2023	Shenzhen University^［77］
	2025	Shenzhen University & Hiwing Technology Academy of CASIC^［78］
	2026	North University of China & Xiamen University^［79］

Technique	Year	Institution	Advantage	Disadvantage
Sapphire photonic crystal fiber	2010	Virginia Polytechnic Institute and State University^［83］	High design flexibility； effective mode and dispersion control	Complex fabrication； poor hole stability at high temperature； difficult structural control
	2015	Xidian University^［84］
	2024	Huazhong University of Science and Technology^［85］
Segmented cladding sapphire single-crystal fiber	2015	Virginia Polytechnic Institute and State University^［87］	High structural flexibility； large-core single-mode potential； effective mode filtering； strong dispersion engineering capability	Mostly theoretical； high machining precision required； poor long-length consistency
	2016	Virginia Polytechnic Institute and State University^［88］
	2018	Graduate University of Advanced Technology， Department of Physics and Photonics， Kerman， Iran & Yazd University^［89］
	2021	Wuhan University of Technology & Quaid-i-Azam university^［90］
	2022	Beijing Institute of Technology^［93］