Fig. 1. Schematic of working principle of sapphire fiber Bragg grating (SFBG)

Fig. 2. Coupling efficiency of various modes in sapphire fiber. Effects of diameter of sapphire fiber (a) and dimension of input mode (b) on coupling efficiency^[32]

Fig. 3. Simulated and measured reflection spectra of the multimode fiber Bragg gratings (a)^[39] and simulated transmission spectra of SFBG (b)^[40]

Fig. 4. SFBG inscribed through phase mask method. (a) Experimental setup; (b) Microscope image; Reflection spectrum of (c) single SFBG and (d) SFBG array^{[19, 41]}

Fig. 5. SFBG inscribed through Talbot interferometer. (a) Schematic of experimental setup; (b) Reflection spectrum^[21]

Fig. 6. Experimental setup of SFBG fabricated by femtosecond laser direct writing

Fig. 7. Reflection spectrum and microscope image of (a) single SFBG^[22] and (b) parallel-integrated SFBG^[42] fabricated by femtosecond laser point-by-point method

Fig. 8. (a) Microscope image and (b) reflection spectra of SFBG fabricated by femtosecond laser line-by-line inscription method^[44]

Fig. 9. SFBG inscribed by using femtosecond laser filamentation^[45]. (a) Cross-sectional view; (b) Reflection spectrum of the SFBG array

Fig. 10. High-order mode filtering method and reflection spectrum of SFBG. (a) Taper fiber coupling method^[48]; (b) Fiber lens matching method^[49]; (c) Offset-coupling method^[44]

Fig. 11. Microscopic image and reflection spectrum of micro-SFBG^[54]

Fig. 12. Microscopic image of sapphire derived fiber end face and reflection spectrum of SFBG^[62]

Fig. 13. Microscopic images and reflection spectra of the (a) single-mode helical SFBG and (b) single-mode depressed sapphire waveguide Bragg grating^[69]

Fig. 14. (a) Effect of length of sapphire fiber on the reflection spectrum of SFBG ^[70]; (b) Two methods of end face treatment of sapphire fiber^{[72, 42]}

Fig. 15. High temperature characteristics of SFBG. (a) Reflection spectra at 22-1745 ℃; (b) Temperature response of Bragg wavelength; (c) Temperature cycling test^[41]; (d) Dynamic response^[74]

Fig. 16. Distributed temperature sensing by using SFBGs array. (a) Reflection spectra at 16-1600 ℃; (b) Temperature response of Bragg wavelength^[45]

Fig. 17. Temperature field in furnace measured by using SFBG. (a) Schematic of experimental setup; (b) Temperature distribution ^[74]

Fig. 18. High temperature SFBG sensor. (a) Package diagram; (b) Long term spectral stability and temperature field distribution in boiler^[76]

Fig. 19. (a) High temperature SFBG sensor based on inert gas packaging; (b) Surface morphology of sapphire fiber annealed at 1600 ℃^[77]

Fig. 20. (a) Temperature measurement accuracy and (b) uncertainty of high temperature SFBG sensor^[78]

Fig. 21. High temperature strain sensor based on SFBG. (a) Strain response curve at 1600 ℃^[43]; (b) Structure diagram^[81]