Fig. 1. Schematic diagram of photonics crystal fibers with special structures. (a) Hollow core PCF (bandgap effect, or antiresonance effect). (b) Suspended core fiber. (c) Solid core PCF. (d) Bragg fiber.

Fig. 2. Representative works showing the development history of multimaterial multifunctional fibers. Figure reproduced with permission from: (a) ref.⁴¹, Copyright © 2002 Nature Publishing Group; (b) ref.¹¹, Copyright © 2004 Nature Publishing Group; (g, h) ref.^{50, 51} under the terms of the Creative Commons Attribution License. The insets (c–f) are produced from our published papers in ref.^{44, 52, 12, 47}, respectively.

Fig. 3. Representative works of lab in/on fiber integrating with femtosecond (fs)-laser induced two-photon polymerization. (a) A line-by-line polymer FBG integrated on the surface of a microfiber⁵⁶. (b) A helical microfiber Bragg grating⁵⁷. (c) An all-optical modulator based on FBG inside a fiber⁵⁸. (d) All-fiber FPI for hydrogen detection based on the fiber-tip microcantilever⁵⁹. (e) The optimized fiber-tip microcantilevers⁶⁰. (f) A fiber-optic microforce sensor based on fiber-tip polymer clamped-beam probe⁶¹. (g) An all-in-fiber polymer microdisk WGM resonator⁶². (h) Ultrathin meta-lens on the facet of modified SMF⁶³. (i) An all-fiber beam generator based on a fiber-tip SZP⁶⁶. (j) Multiple micro objective lenses on the end face of a single imaging optical fiber⁶⁷. Figure reproduced with permission from: (h) ref.⁶³ under the terms of the Creative Commons Attribution License.

Fig. 4. Optical fiber sensing structures for different physical parameters.(a) Two air-clad photonic crystal fibers with different dimensions spliced between SMF-28 single-mode fibers⁷³. (b) A Mach-Zehnder interferometer consisting of a thin core fiber sandwiched between two waist-enlarged bitapers⁷⁸. (c) The Fabry–Perot cavity stretching freely in continuous polyimide tube and its test system⁷⁹. (d) Photonic crystal fiber filled with magnetic fluid sandwiched between two single mode fibers⁸⁰. Figure reproduced with permission from: (c) ref.⁷⁹ under the terms of the Creative Commons Attribution License.

Fig. 5. Representative optical fiber sensors for wearable health monitoring.

Fig. 6. The representative special fiber types, advanced fiber structures as well as application fields exhibiting the significant development history of the optical fiber shape sensor. (a) A self-encapsulated fiber cable consisting of three fibers¹⁴¹. (b) A self-encapsulated fiber cable including three fibers and a substrate-SMA¹⁴⁹. (c–e) Multicore fiber with core angles of 120 degrees, 90 degrees, and 60 degrees. (f) Setup for continuous FBG fabrication¹⁵⁰. (g) Schematic diagram of scattering enhancement. (h) Helical multicore fiber with helical pitch of 15.4mm¹⁵¹. (i) Continuous gratings in twisted multicore fiber with UV transparent coating. Figure reproduced with permission from: (a) ref.¹⁴¹, (b) ref.¹⁴⁹, (h) ref.¹⁵¹, under the terms of the Creative Commons Attribution License; (f, i) ref.¹⁵⁰, Copyright © 2022 American Chemical Society.

Fig. 7. Optical fiber sensing for industry applications. (a) DAS in application of protecting gas pipelines against both malicious intrusions and piping degradation⁶. (b) Distributed fiber-optic strain sensor for long-term monitoring of a railway tunnel¹⁶⁶. (c) DAS in application of illuminating earth phenomenon⁵. (d) Optical fiber sensing for detecting the partial discharge of the accessories of a high-voltage power system⁷. (e) Optical fiber sensing for monitoring the real-time status of the surrounding underwater environment¹⁶⁷. (f) Optical fiber sensing for real-time intrusion threat detection on high-speed railway¹⁶⁸. Figure reproduced with permission from: (a) ref.⁶, (b) ref.¹⁶⁶, (e) ref.¹⁶⁷, (f) ref.¹⁶⁸, under the terms of the Creative Commons Attribution License; (c) ref.⁵, Copyright © AAAS.

Fig. 8. Several different biomedical sensing modalities using specialty optical fibers. These include (a) biorecognition sensors, (b) optical coherence tomography (OCT). (c) Surface enhanced Raman spectroscopy (SERS). (d) Surface plasmon resonance (SPR) and (e) Michelson interferometry. Figure reproduced with permission from: (b) ref.¹⁸⁸, (c) ref.¹⁹¹, (d) ref.¹⁹⁹, under the terms of the Creative Commons Attribution License.

Fig. 9. (a) Concept of a biosensor using a surface plasmon resonance effect. (b) Example of the shift in the surface plasmon resonance peak when there is a relative index change from captured biological samples.

Fig. 10. Six different configurations for using different optical fibers inside of an endoscopic sensing head. Figure reproduced with permission from: ref.²⁰⁶ under the terms of the Creative Commons Attribution.

Table 1. The potential applications of fiber-based sensors.