Fig. 1. Annual search of academic papers on application of optical fiber technology in medical field in past 20 years. (a) Number of papers published on application of optical fiber technology in medical field; (b) number of papers published on applications of optical fiber technology in biological field

Fig. 2. Annual search of patents for application of optical fiber technology in biomedical field in past 20 years. (a) Number of patents for applications of optical fiber technology in medical field; (b) number of patents for applications of optical fiber technology in biological field

Fig. 3. Cross-sectional diagrams and 3D refractive index distributions of dual-core fibers with air holes. (a) Cross-sectional diagram of single-hole dual-core fiber and refractive index distribution^[4]; (b) double-hole dual-core fiber and its refractive index distribution^[15]

Fig. 4. Microflow laser biosensing method based on hollow core fiber^[17]

Fig. 5. Schematic diagrams of TD-OCT, SD-OCT, and SS-OCT systems

Fig. 6. Typical structures of direct-view and side-view fiber OCT probes

Fig. 7. Schematic diagrams of applications of fiber optic photoacoustic endoscope in vivo. (a) Schematic diagram of working mode and structure of fiber optic photoacoustic endoscope; (b) imaging results of rectal blood vessel distribution and oxygen saturation in healthy rats^[68]

Fig. 8. Schematic illustration of fiber-optic detection and therapy integration^[81]

Fig. 9. Fiber optic thermal in vivo therapy of mouse model^[81]. (a) Cross-sectional scheme of fiber in needle; (b) therapeutic approach for tumor-bearing mouse (for suitable treatment time investigation); (c) real-time thermal IR images of a tumor-bearing mouse after treatment by the needle under 980 nm laser irradiation (160 mW); (d) heating dynamic curve on the tumor surface in the treatment process; (e) body weight changes of mice during the treatments with various time; (f) relative volume changes of tumors during the treatments with various time; (g) periodic acid Schiff (PAS) images of tumor tissues of mice during treatments with various time; (h) therapeutic approach for tumor-bearing mice (for the tumor inhibition effect investigation); (i) grayscale thermal images of tumor-bearing mice after treatment with the needle under 980 nm laser irradiation (160 mW, 30 min); (j) body weight changes of mice in the treatment and control groups; (k) relative volume changes of tumors in the treatment group and control group; (l) representative photographs of mice in the treatment group and control group during the treatment process; (m) photographs of tumor tissue obtained from mice in the treatment group and control group after 15 d of treatment; (n) relative tumor inhibition rate of mouse after treatment by the needle

Fig. 10. Schematic diagram of fiber optic biosensor for specific detection based on biological body fluids^[107]

Fig. 11. Interaction of excitation light with biological tissues in bulk spectroscopic detection technique by optical fiber

Fig. 12. Different fiber tip structures for in vivo optogenetics. (a1) Flat-end fiber diagram^[180]; (a2) fluorescence distribution and normalized intensity distribution of light field after flat-end fiber is implanted in cerebral cortex section^[180]; (a3)(a4) bright field, fluorescence field and normalized intensity distributions of cone fiber implanted in gray matter and striatum brain sections, respectively^[180]; (b1)‒(b3) microscopic images of tip of cone fiber with window numbers of 2, 3 and 7, respectively^[178]; (b4)‒(b12) output radiant light field of cone fiber end with three different window quantities under different input angles (imaging in fluorescent liquid)^[178]

Fig. 13. Drawing process of glass and metal composite fiber and results of drawing fiber^[187]. (a) Hot process preparation of multi-material integrated fiber; (b)‒(d) end optical images of multi-material integrated fiber with one, two and four metal electrodes, respectively; (e)‒(p) electron microscope scanning images and element distribution maps of these three kinds of fiber (the scale in the figure is 100 μm); (q) photo of multi-material fiber drawn; (r) structural diagram of implantable multi-material glass fiber probe; (s) physical diagram of multi-material glass fiber probe

Fig. 14. Cross-sectional images of different multifunctional fibers^[190]

Fig. 15. Cross sections of several typical core-hole combination special fibers

Fig. 16. Quartz capillary and multi-cladding fiber are embedded in porous quartz prefabricated parts, realizing hybrid integration of fan-in fan-out device matched with special fiber

Fig. 17. Schematic diagram of porous quartz capillary prefabricated component prepared by stack method

Fig. 18. Special fiber obtained by integrated drawing and its fan-in fan-out device. (a) Components are integrated and tapered to realize natural interconnection of fiber and fan-in fan-out device; (b) schematic diagram of refractive index profile of double-cladding fiber; (c) cross-sectional diagram of double-cladding fiber

Fig. 19. Schematic diagrams of mode field control of tapered double-cladding fiber. (a) Waveguide structure diagram after tapered double-cladding optical fiber; (b) simulation results of optical field transmission changes during tapering of double-cladding optical fiber; (c) refractive index profile and fundamental mode field distribution diagram before and after tapering of double-cladding optical fiber

Fig. 20. Refractive index profile structure designs and microscopic images of two kinds of transition optical fibers. (a) Refractive index profile structure designs of optical fiber; (b) microscopic images of fiber end face

Fig. 21. Schematic diagrams of working principle of DIY preparation of special fiber and corresponding laboratory preparation system. (a) Preparation process of special optical fiber preforms; (b) special fiber drawing process; (c) photo of miniature fiber drawing machine system; (d) photo of microprefabricated rod preparation system

Fig. 22. End micrographs of porous quartz capillaries prepared by stack method (top) and its corresponding multi-core fiber (bottom)

Fig. 23. Starting from different application scenarios and needs, the DIY methods for specialty optical fibers are provided, and explorations are conducted around three dimensions of optical fiber technology application innovation