Fig. 1. General scheme of the FBG inscription setup^[13]

Fig. 2. Three individual cores of seven-core fiber are selected for FBG inscription^[13]. (a) FBG distribution on fiber cores;(b) grating inscription results for three cores

Fig. 3. Three cores of the seven-core fiber are selected for simultaneous TFBG inscription^[14]. (a) Schematic diagram of TFBG inscription; (b)‒(d) spectra of three cores

Fig. 4. Effect of different focus positions on selective inscription^[17]. (a) Schematic diagram of laser focus position;(b)‒(d) spectra of three inscription schemes

Fig. 5. FBGs were fabricated using the SSDUW technique^[18]. (a) SSDUW setup;(b) schematic diagram of grating location in multicore fiber and resulting optical reflection spectrum of each core

Fig. 6. Selective inscription based on femtosecond laser and phase mask^[21]. (a) Schematic of femtosecond laser writing system; (b) spectrum of core A

Fig. 7. Femtosecond laser inscription scheme of Donko's team^[23]. (a) Femtosecond laser grating writing device; (b) microscope image of 7-core MCF cross section

Fig. 8. Point-by-point FBG inscription structure proposed by Wolf's team^[24]

Fig. 9. Femtosecond laser selective inscription based on plane-by-plane inscription.(a) Schematic of setup for plane-by-plane grating fabrication^[26]; (b) schematic of six FBG arrays inscribed in 7-core optical fiber; (c) spectra of FBG array measured for one of cores of 7-core optical fiber^[27]

Fig. 10. FBGs selectively inscribed in seven-core spun fiber by femtosecond laser^[28]. (a) State diagram of femtosecond laser pulse picker and velocity profile of fiber when longitudinal FBG array is inscribed; (b) reflection spectra of seven-core inscribed with a fixed period in a single pass

Fig. 11. Schematic of the light trace of a seven-core fiber with full-cores FBG inscription^[29]

Fig. 12. Schematic of reel-to-reel array inscription apparatus for inscribing gratings in twisted multicore fiber^[31]

Fig. 13. Schematic diagram of a full-core inscribed grating based on a three-core plane^[13]. (a) FBG distribution on fiber cores;(b) results for gratings inscriptions

Fig. 14. RFBG fabrication scheme^[32]. (a) Schematic of full-core inscribed FBG by phase-mask method; (b) RFBG spectra in four cores

Fig. 15. Before-after comparison of grating spectra with modified core photosensitivity and adjustment of position^[35]

Fig. 16. Full-core FBG inscription of DPAMCF. (a) Cross-section of DPAMCF; (b) transmission spectra of DPAMCF-FBGs when plane containing six cores is parallel to phase mask^[36]

Fig. 17. Full-core inscription scheme of Emma Lindley's team. (a) Before-after comparison of UV power distribution in a seven-core fiber using polished capillaries; (b) transmission spectra of gratings inscribed in seven-core fiber after using a capillary^[37]

Fig. 18. Full-core inscription based on defocusing phase mask technology^[38]. (a) Schematic experimental setup of TFCF FBGs inscription system; (b) transmission spectra of four cores

Fig. 19. Spectra of four-core fiber with core position presents rhombus and square distribution^[17]

Fig. 20. Schematic diagram of cross-section of a seven-core fiber surrounded by air holes^[39]

Fig. 21. DTG's full-core inscription scheme. (a) Schematic diagram of DTG fabrication setup; (b) reflection spectrum of DTG array in four cores^[40]

Fig. 22. Seven-core FBGs full-core inscription using a modified Talbot interferometer^[41]. (a) Modified Talbot interferometer; (b) transmission spectra of the FBGs inscribed in a seven-core fiber

Fig. 23. Full-core inscription based on CO₂ laser^[43]. (a) Schematic diagram of HLPG inscription; (b) spectrum of the fourth HLPG sample and spectrum of the main couplings of the other HLPGs with various pitches

Fig. 24. Full-core inscription based on the electrodes arc discharges method^[44]. (a) Schematic of LPFG fabrication and monitoring platform for programmable electrodes arc discharges method based on a fiber optic fusion splicer;(b)‒(h) transmission spectra of LPFGs in seven cores; (i) crosstalk measured in outer cores

Table 1. Characteristics of FBGs inscribed in four fiber cores^[23]

Table 2. Comparison of the selective inscription performance of two optical sources

Table 3. Grating characteristics of six samples^[34]

Table 4. Effective refractive index (n_eff), Bragg wavelength (λ_B), and its strain and temperature sensitivities according to numerical simulations and experiments in all of the 7 cores (where R1‒R6 are the external cores and R7 is the central core) of the MCF^[39]