Optical fiber technology has been a key enabler in shaping worldwide high-speed communication networks and connecting different places with enhanced bandwidth, data transmission speeds, and fidelity. The development of optical fiber technology is also vital in proliferating sensing applications. Optical fiber sensors offer favorable advantages such as immunity to electromagnetic interference, lightweight, small size, high sensitivity, large bandwidth, reliable and robust performance, ability to withstand harsh environment, and ease in implementing multiplexed or distributed sensors. To date, optical fiber sensors have been widely used and explored for civil engineering^1-4, environmental monitoring^5-8, agricultural engineering^{9, 10}, biomedical engineering^11-13, etc. Notably, many sensing innovations and applications are proliferated and driven by the inventions and developments of specialty optical fibers, including optical fibers with special structures, special materials, and integration with other technologies to create lab-on-fiber or lab-in-fiber devices.

This review paper aims to provide an overview of the development of specialty optical fiber technology in recent years, highlighting their achievements and future opportunities in advanced sensing applications. The first section will introduce various types of specialty optical fibers including innovations in special structures, special materials, and technology realization of the lab in/on a fiber platform. The second section will provide an application overview of specialty optical fibers in various fields, including an overview on general sensing of physical parameters, followed by wearable sensors, shape sensing, sensing in industrial applications as well as biomedical sensing. The emerging opportunities for future development of specialty optical fiber sensors will be presented in the conclusion section.

Photonic crystal fibers (PCFs) were invented and first fabricated using silica¹⁴ and subsequently extended to other materials, e.g. polymer¹⁵ and chalcogenide¹⁶. The air holes are developed in the core and/or cladding of the fiber structures, giving design flexibility in achieving hollow core guidance, and a wide variety of optical properties not attainable in conventional optical fibers. These novel PCFs provide new features and flexibility in engineering optical properties such as endless single-mode operation, dispersion engineering, and nonlinear properties engineering, to name a few. The PCFs family has many members with different structures. Broadly speaking, PCFs can be categorized based on their guiding mechanisms¹⁷, such as modified index-guiding mechanism, photonic bandgap (PBG)-guiding mechanism, inhibited coupling-guiding mechanism, anti-resonance-guiding mechanism, twist-induced guiding mechanism, and hybrid guiding mechanism with the coexistence of both modified index-guiding and PBG-guiding mechanisms¹⁸. In Fig. 1, four examples of PCF structures are illustrated, including (a) hollow core PCF (bandgap effect, or antiresonance effect), (b) suspended core fiber, (c) solid core PCF, and (d) Bragg fiber.

Furthermore, PCFs have made significant achievements as a versatile platform to provide many novel functionalities and sensing application opportunities¹⁹. PCFs have exhibited many desirable features for physical, chemical, and biochemical sensing applications²⁰. The anti-resonant fibers represent a promising route in achieving low-loss hollow core transmission and an attractive platform for developing sensing applications²¹. Hollow core PCFs are driving the development of surface-enhanced Raman scattering probes²² and fast online Raman spectroscopy with significant performance enhancement²³. For example, the interaction between the optical field and the analyte can be significantly enhanced through engineering the structures of the fibers, e.g. by enhancing the evanescent field to have a greater overlap with the analyte, or bringing the analyte into the hollow core or the air channels in the cladding of the fibers.

PCF-based sensors can be grouped based on their sensing structures and principles, with the possibilities of operating in combinations of several sensing principles for performance enhancements. Firstly, evanescent field based PCF sensors are developed to achieve greater sensitivity by optimizing or modifying the fiber structure to enhance the evanescent field, e.g. through air-suspended silica structures²⁴, tapering²⁵, removing partial cladding²⁶, etc. Secondly, the combination of grating inscription such as fiber Bragg grating and long-period grating into PCFs have provided a powerful platform to achieve various sensing applications^27-29. Thirdly, various interferometry configurations have been demonstrated by PCFs for physical and biomedical sensing applications, including the Fabry-Perot interferometer, Mach-Zehnder interferometer, Michelson interferometer, and Sagnac interferometer^{30, 31}. Fourthly, plasmonic structures are fabricated on PCFs to realize plasmonic sensing, such as thin metal films, nanowires, and nanoparticles-based PCF structures for achieving surface plasmon resonance (SPR) or localized SPR^32-34. A representative illustration of PCF based sensor with the aforementioned sensing principles is shown in Fig. 1.

Besides PCFs, other structural modifications are bringing new possibilities to optical fiber sensors, such as specialty fibers with enhanced scattering are developed for distributed acoustic sensing³⁵, micro/nano fiber^{36, 37}, fibers with whispering gallery mode microresonators³⁸, multicore fibers based sensor for distributed fiber sensing³⁹ and other sensing applications⁴⁰.

Over the long history of optical fibers, the fiber material composition remained to be primarily transparent silica glass and thermoplastic polymers until about two decades ago, when material scientists from MIT put their effort into expanding the material library as well as the functionalities for thermally drawn fibers, carving out a new field called the “multimaterial multifunctional fibers”⁴¹. The multimaterial multifunctional fiber refers to incorporating special material into the fiber core and cladding to realize diverse functions that conventional communication optical fibers cannot attain.

The fabrication of multimaterial multifunctional fibers is similar to that of optical fibers, involving the preparation of the macroscopic preform and the thermal drawing procedure. In this sense, different materials in the same preform should have overlaps in their glass transition temperature (Tg) or melting point to ensure thermal co-drawing. For example, silica cladding can be co-drawn with semiconductors and high melting point metals; polymer cladding can be co-drawn with other polymers, chalcogenide glasses, and low melting point metals. This constraint was overcome by the convergence drawing technique in recent years, enabling the co-drawing of high melting point wires/devices with low Tg cladding materials. With a broad spectrum of functional materials (e.g., bulk glasses, polymers, metals, semiconductors, and nanomaterials), various fiber preforms with complex geometry and material composition can be prepared by proper combined procedures of machining, thin-film rolling technique, extrusion, stack-and-draw approach, and thermal consolidation. The preform is then fed into the furnace of a fiber drawing tower and drawn into fibers with a variety of sophisticated functions. In the last 20 years, significant efforts have been put into this field, and diverse multimaterial fibers and functions such as multilayer photonic bandgap fiber^{41, 42}, photodetecting fiber^{11, 43}, azimuthally polarized radial fiber laser⁴⁴, piezoelectric fiber^{45, 46}, neural probe¹², thermoelectric fiber^{47, 48}, diode fiber^{49, 50}, and biomedical fiber⁵¹. have been successfully developed. In Fig. 2, some representative works are listed to show the development history of the thermally drawn multimaterial multifunctional fibers.

(a) Multilayer photonic bandgap fibers

The multilayer photonic bandgap (PBG) fiber, reported in 2002⁴¹, is regarded as the first proof-of-concept for the multimaterial multifunctional fiber. The fiber consisted of a hollow air core surrounded by a photonic bandgap structure established by the multiple alternating submicrometer-thick layers of arsenic triselenide (As₂Se₃, RI: ~2.8) and poly (ether sulphone) (PES, RI: 1.55). The transmission windows were scaled from 0.75 to 10.6 mm in wavelength by adjusting the layer thickness. Such fibers show advantages over hollow-core negative-curvature photonic crystal fibers for CO₂ laser guiding. An optoelectronic nose was also demonstrated using the multilayer PBG fiber⁴².^.

(b) Photodetecting fibers

The first photodetecting fiber was demonstrated in 2004. The fiber consisted of a 200-μm chalcogenide semiconductor (As₁₀Se₅₀Te₁₀Sn₅) core and two pairs of Sn metal electrodes in contact with the core. External light could be detected by measuring the photocurrent owing to the photoconductive effect¹¹. The signal-to-noise ratio could be effectively improved by decreasing the fiber photodetector’s diameter⁴³.

(c) Azimuthally polarized radial fiber laser

The azimuthally polarized radial fiber laser is a representative work of the multimaterial multifunctional fiber. The fiber integrated a hollow photonic bandgap structure that worked as the laser cavity, the container for the liquid droplet gain medium, and an array of electrically contacted liquid-crystal microchannels surrounding the laser cavity. The emitted laser intensity was dynamically controlled by electrically modulating the liquid crystal window⁴⁴.

(d) Piezoelectric/thermoelectric fibers

The multimaterial piezoelectric fiber, based on the piezoelectric effect, integrated a piezoelectric P(VDF-TrFE) thin film sandwiched between two electrodes and encapsulated within a polymer cladding^{45, 52}. The fiber was exploited for acoustic detection from kilohertz to megahertz frequencies. A recent study further doubled the piezoelectric coefficients by loading the P(VDF-TrFE) film with piezoelectric barium titanate (BaTiO₃) ceramic particles and replacing the rigid cladding with an elastomer cladding¹², showing prospects for wearable health monitoring. Similarly, thermoelectric materials have also been successfully incorporated into multifunctional fibers to realize thermal sensing, positioning, and regulation^{47, 48}.

(e) Neural probes

Simultaneous probing and manipulating neurons during behavioral tasks are critical to understanding neural computation in the brain and nervous system pathologies. In recent years, multimaterial multifunctional fibers have been proven to be a versatile platform for neuron probing. By integrating optical guiding paths, electrodes, and microchannels in the fiber probes, simultaneous optical stimulation, neural recording, and drug delivery in behaving mice over a long term have been realized¹².

(f) Diode fibers

Although various in-fiber optoelectronic devices have been demonstrated, their performance is still inferior to silicon chip-based ones. Alternatively, the convergence drawing technique was developed to integrate chip-based diode devices into the thermally drawn fibers. As a proof of concept, two types of in-fiber devices, including light-emitting and photodetecting p-i-n diodes, were constructed, and their potential for physiological-status monitoring and optical communication was demonstrated⁴⁹. Furthermore, digital sensing and memory devices were integrated into one fiber and operated through a single connection at the fiber edge, providing the ability to measure and store physiological parameters and to operate neural networks for motion pattern recognition⁵⁰. Diode fibers present intriguing opportunities in a variety of applications in wearable electronics.

(g) Biomedical fibers

The multimaterial multifunctional fibers offer a reliable new platform for implantable immune-therapeutics delivery and tumor impedance measurement. By incorporating a hollow channel, a pair of copper electrodes, and a light guiding core inside one fiber device, researchers realized local delivery of ICB antibodies and monitoring of clinical outcomes by tumor impedance measurement over a few weeks⁵¹, demonstrating the immense potential in healthcare.

The intrinsic properties of optical fibers, such as ultrasmall cross-sections, ultrahigh aspect ratios, and curved surfaces, make it difficult for the conventional micro/nano fabrication strategies to be directly applied for fabricating the micro/nano structures as the key components in “lab-in/on-fiber” systems. Various micro/nano fabrication strategies, including focused ion beam (FIB), electrobeam lithography (EBL), interference lithography, femtosecond laser writing, nanoimprinting lithography, two-photon lithography, etc., have been adjusted and applied for micro/nano fabrication using substrates of optical fibers. FIB and EBL technologies could achieve fast fabrication with spatial resolution at the nanoscale, however, at the highest cost and with the limitation in mass production. Nanoimprinting lithography ensures spatial resolution at the nanoscale with the capacity of high-throughput production. Interference lithography and femtosecond laser writing are more cost-effective typically with lower spatial resolution. Recently, multi photon lithography is becoming a very intriguing technique to fabricate unique 3D micro/nano structures in/on optical fibers, which is difficult to achieve with traditional EBL and FIB methods.

Due to the unique advantages of two-photon polymerization (TPP) technology, such as high precision, high flexibility, and true 3D processing, it has become an innovative solution for the integration and fabrication of fiber-optic micro/nano devices. Femtosecond (fs)-laser induced TPP is basically a form of additive manufacturing based on two-photon absorption (TPA) that two photons simultaneously transfer their energy to an absorbing molecule or material⁵³. Since the rate of TPA is proportional to the square of the light intensity, the polymerization can only occur at the center of the fs-laser focus point with high light intensity (~TW/cm²)⁵⁴. Such a nonlinear threshold effect allows the feature size of TPP to easily break the optical limit, currently down to 10 nm⁵⁵. In recent years, a variety of micro/nanostructured devices fabricated by fs-laser induced TPP (Fig. 3), including optical waveguides, microcavities, and micro-optical elements, etc., have been developed and widely reported following the "lab-in/on-fiber" concept, in which the micro/nano structures were often located inside the fibers or on the fiber surfaces.

(a) Optical waveguides

Optical waveguides on fibers fabricated by fs-laser induced TPP enhanced the performances of “lab-in/on-fiber” devices by introducing additional polymer materials to replace the conventional quartz. One type of optical waveguides was located on the surfaces of fibers, such as fiber Bragg grating (FBG). Wang et al. in 2018 first proposed a line-by-line polymer FBG integrated on the surface of a microfiber for measuring the refractive index (RI) of liquids⁵⁶, as shown in Fig. 3(a). This polymer FBG showed a maximum sensitivity of ~207 nm/RIU (RI=1.44). Subsequently, Liao et al. proposed a helical microfiber Bragg grating in 2019, as shown in Fig. 3(b). The device presented a more stable spectrum and higher mechanical robustness as a result of the increased contact area between the FBG and microfiber⁵⁷. The FBG itself on the surface of the fiber could also enhance the mechanical strength of the device. The FBG on the fiber surface exhibited excellent RI responses ranging from 1.345 to 1.395. The other type of optical waveguide was located inside the fiber. Liao et al. fabricated an FBG inside a fiber with reduced width of 653 nm and increased length of 800 μm, which turned out to be a novel fiber-integrated all-optical modulator⁵⁸, as shown in Fig. 3(c). The was tested to be with a short response time of 176 ns and excellent linear modulation of −45.43 pm/mW.

(b) Optical microcavities

Introducing optical waveguides into fibers could construct new types of highly integrated “lab-in/on-fiber” devices with high sensitivity. There are various types of microcavities, among which the Fabry-Perot interferometer (FPI) is the most widely applied one that provides excellent optical localization performance in detecting changes occurring in resonant optical paths or loss and phase changes occurring on reflective surfaces. Recently, Xiong et al. proposed an all-fiber FPI for hydrogen detection based on the fiber-tip microcantilever (Fig. 3(d))⁵⁹. The upper surface of the microcantilever was coated with palladium (Pd) film absorbing hydrogen molecules and deforming the microcantilever, resulting in the change in the cavity length of the FPI. The device was turned to be with a sensitivity of hydrogen detection as high as −2 nm/% and a short response time of 13.5 s under 4% hydrogen. The sensitivity has been further improved by additional optimization of the microcantilever design⁶⁰, as shown in Fig. 3(e). In 2021, Zou et al. first demonstrated a fiber-optic microforce sensor using a similar fiber-tip microcantilever device, as shown in Fig. 3(f), with a sensitivity of 1.51 nm/μN and a detection limit of 54.9 nN⁶¹. The sensor was applied to measure Young's modulus of polydimethylsiloxane (PDMS), butterfly tentacles and human hair. This approach could open new avenues for miniaturized atomic force microscopy (AFM).

Additionally, due to the high accuracy of TPP, it was also applicable for fabricating curved surfaces, such as polymer microdisks. In 2021, Ji et al. fabricated an all-in-fiber polymer microdisk whispering-gallery-mode (WGM) resonator, which was composed of a nanoscale polymer waveguide in conjunction with a polymer microdisk (Fig. 3(g))⁶², with Q factor as high as 2.3 ×10³. The temperature sensitivity was 96 pm/°C in the testing range of 25 °C to 60 °C. The humidity sensitivity was also as high as 54 pm/% RH in the testing range of 30% to 90%. Such microdisk resonator could be applicable for the detection of microorganisms, bacteria and even single molecules.

(c) Micro-optical elements

The polymer material used for TPP is typically transparent with excellent light transmission capability. Since the characteristic size of TPP is smaller than the visible wavelength, it can be used to fabricate the micro-optical elements on optical fibers for optical manipulation, spatial optical coupling, beam shaping, and imaging purposes.

For optical manipulation, Malte et al. implemented ultrathin meta-lens by TPP on the facet of modified single-mode fiber (SMF), as in Fig. 3(h), generating diffraction-limited focal spot with a record-high numerical aperture of up to NA = 0.882⁶³. This work did not require expensive equipment, consisting only of commercial single-mode and multimode fibers. The advantage of diffractive elements over refractive lenses is their ability to produce longer working distances and ease of fabrication. High-precision capturing with high trap stiffness can facilitate single cell/particle microscopy and particle manipulation.

For spatially resolved spectroscopy, Dietrich et al. demonstrated facet-attached lens arrays fabricated by TPP. The lens arrays provided close to 100% fill-fraction along with efficiencies of up to 73% (down to 1.4 dB loss) for coupling of light from free space⁶⁴. Subsequently, Dietrich et al. proposed insitu printing of facet-attached beam-shaping elements, achieving coupling efficiencies of up to 88% between edge-emitting lasers and SMFs. Printed free-form mirrors that simultaneously adapt beam shape and propagation direction were also demonstrated, along with multi-lens systems for beam expansion⁶⁵. The concept paved the way for automated assembly of photonic multi-chip systems with unprecedented performance and versatility.

For generating a focused vortex beam, Yu et al. fabricated a compact all-fiber beam generator, as shown in Fig. 3(i), in which a spiral zone plate (SZP) was integrated on the tip of a composite fiber microstructure⁶⁶. By adjusting the design parameters of the SZP, precise controlling of the output optical field has been achieved. The high divergence during propagation of the vortex beam was effectively overcome. The generator may have potential applications in fiber optic optical wrench, all-fiber stimulated emission depletion (STED) microscopy, and orbital angular momentum (OAM) fiber communications.

For fiber optic endoscopic imaging, Li et al., for the first time, printed multiple micro objective lenses with different fields of view on the end face of a single imaging optical fiber, as shown in Fig. 3(j), thus realizing the perfect integration of an optical fiber and objective lenses⁶⁷. The micro-objective lenses prepared by TPP demonstrated acceptable imaging performances and thus offered a new approach for the realization of ultrathin fiber endoscopes.

Optical fiber sensors are widely applied to the measurement of various physical parameters due to their advantages of anti-electromagnetic interference, corrosion resistance, and small volume. New specialty optical fibers with engineered materials or specially designed structures offer new opportunities to explore novel functions and applications. Different physical parameters, for example, refractive index, temperature, strain, and magnetic, can modulate or perturb the guided light in a specialty optical fiber by means of elastic or thermos optical effects⁶⁸. Specific applications using special optical fibers as sensors to measure refractive index, temperature, and strain are summarized below.

(a) Fiber optic refractive index sensor

Since Cooper et al.⁶⁹ proposed the first liquid refractive index fiber sensor in 1983, with the development of fiber technology, fiber refractive index sensors based on different measurement methods have been widely used⁷⁰. According to the measurement methods, this section is divided into spectral shift, visibility, amplitude modulation and carrier phase tracking.

1)Optical fiber refractive index sensor based on spectral shift measurement

The two FBGs in series to measure temperature and the refractive index of water⁷¹, the LPG written in pure silicon fiber⁷², two large core air clad photonic crystal fibers in series⁷³, as shown in Fig. 4(a), a single-mode non-adiabatic tapered fiber embedded in a fiber-loop mirror⁷⁴ and ultra-wide detection range based on photonic crystal fiber surface plasmon resonance⁷⁵ are used to measure the refractive index through spectral shift measurement.

2)Optical fiber refractive index sensor based on visibility measurement

In 2008, Silva et al.⁷⁶ proposed a Fabry-Perot interferometric refractive index sensor, whose interference wavelength is derived from the reflection of short FBG and the Fresnel reflection at the far end of optical fiber. The sensitivity of 0.59 mV/RIU was obtained by measuring the amplitude of interference fringes using heterodyne technique. In 2012, Gouveia et al.⁷⁷ proposed a refractive index and temperature sensor based on Fabry-Perot interference. Based on the low reflectance Bragg grating inscribed on the panda fiber and the fiber end in contact with liquid (Fresnel reflection), the sensitivities to the refractive index and temperature can be read from the visibility of the fringe pattern the wavelength shift of the grating, respectively. In 2019, Liangtao Hou et al.⁸¹ proposed a new fiber sensor based on a half-tapered single-mode-multimode-single-mode (HT-SMS) fiber structure, in which the Mach-Zehnder interferometer (MZI) was embedded in the multi-mode interferometer (MMI). According to the demodulation method of fringe visibility difference (FVD), the sensitivity of RI is 345.78 dB/RIU based on the relationship between RI and fringe visibility.

3)Optical fiber refractive index sensor based on amplitude modulation measurement

In 2006, Folate et al.⁸² proposed a refractive index sensor based on phase-shifted long-period fiber grating. According to the optical power change caused by the measured object at a specific wavelength, the transmitted power modulation induced by resonance displacement was monitored, and the sensitivity of 146 μW/RIU was obtained. In 2012, Susana et al.⁸³ proposed a refractive index sensor based on reflection multimode interference. The sensitivity of −110 dB/RIU was obtained by measuring the refractive index through the change of the peak amplitude of the interference spectrum.

(b) Fiber optic temperature sensor

According to different operating principles, optical fiber temperature sensors in this section are divided into four categories: bend-loss, microfiber resonators, grating, interferometer and coating⁸⁴.

1)Optical fiber temperature sensor based on bend-loss

Optical loss caused by fiber bending changes with temperature. For the optical fiber temperature sensor based on step-index polymer optical fiber⁸⁵, different thermal and optical coefficients of fiber core and cladding lead to macroscopic bending loss, which affects the numerical aperture by temperature changes, and the maximum sensitivity of 1.92×10⁻³/°C is obtained. The temperature range is 27.2−50.2 °C. Another agarose-coated macro bend optical fiber (AC-MBF) sensor⁸⁶, which is used for 2 μm relative humidity and temperature measurement, can obtain a maximum temperature sensitivity of 5.37 nm/°C and a temperature measurement range of 20−50 °C according to the variation of resonance wavelength with temperature.

2)Optical fiber temperature sensor based on microfiber resonators

Microfibers (MFs) are unique devices formed by heating and stretching standard optical fibers to submicron sizes⁸⁷. The temperature sensors based on microfiber are divided into two categories⁸⁷, namely resonant and non-resonant microfiber temperature sensors. The low temperature sensor based on MF-LPG is coated with a layer of silica film with a thickness of 2 µm on a conical TFLPG whose cycle increases with temperature, and its maximum temperature sensitivity is 62.9 nm/°C⁸⁸. Pure silica based on MFs structure is mainly used to manufacture high-temperature sensors⁷⁸, a sensor based on the principle of dual-beam interference with a maximum measuring temperature of 1000 °C. A sensitivity of 87 pm/°C was obtained by monitoring the dip at 1515.52 nm, as shown in Fig. 4(b).

3)Optical fiber temperature sensor based on grating

Fiber Bragg grating sensors for monitoring physical parameters reviewed in ref.⁸⁹ describe the progress in temperature, refractive index, and strain based on Fiber Bragg grating sensors. An FBG coated with MoS₂ with a 10 nm thick corrosion diameter of 10 μm was used for temperature sensing⁹⁰, and the temperature sensitivity from room temperature to 100 °C was measured to 95 pm/°C. An optical fiber temperature sensor using composite plating method⁹¹, electroless nickel plating followed by electro galvanizing on FBG, can be used for low temperature sensing in the range of 280 K to 16 K. A high-temperature sensor⁹² made by writing an FBG on a sapphire single crystal fiber can measure the maximum temperature up to 1900 °C. A 1-mm long FBG was written directly into microfibers splintered between standard single-mode fibers, with a maximum temperature sensitivity of 479.48 pm/°C under acetal molding.

4)Optical fiber temperature sensor based on interferometry

Optical fiber temperature sensor can use interference structures⁹³, such as the F-P cavity and air cavity based on thin silicon diaphragm⁹⁴ and the single-mode-no-core-single-mode fiber structure embedded in a liquid-sealed capillary⁹⁵ to measure temperature. In order to further measure the high-temperature environment⁹⁶, a FP cavity with pure sapphire structure on the end face of sapphire fiber⁹⁷ and a dual sapphire fiber based on parallel dual waveguide optical path transmission structure⁹⁸ have respectively realized the temperature measurement of 1455 °C and 1080 °C.

5)Optical fiber temperature sensor based on coating

The performance of optical fiber sensors can be improved by using coating materials⁹⁹. The temperature sensors achieve high-sensitivity sensing by embedding in polydimethylsiloxane¹⁰⁰, permeating the UV-cured polymer liquid¹⁰¹ or encapsulating a graphene quantum dot (GQD) solution¹⁰².

(c) Fiber optical strain sensor

In this section, optical fiber strain sensors are divided into three categories: FBG based, multimodal interference and Fabry-Perot cavity.

1)Optical fiber strain sensor based on FBG

The fiber Bragg grating strain sensing technology¹⁰³ such as all solid state Bragg fiber composed of concentric cladding of alternating high and low index rings¹⁰⁴ and etched non-photosensitive single-mode fiber with FBG¹⁰⁵ is reported.

2)Optical fiber strain sensor based on multimodal interference

A strain sensor based on perfluorinated graded-index polymer optical fiber is developed¹⁰⁶. This type of sensor uses strain acting on part of multimode optical fiber¹⁰⁷ and Fresnel reflection principle of MMF remote open end¹⁰⁸, respectively.

3)Optical fiber strain sensor based on Fabry–Perot cavity

One for large strain and high temperature high elastic silicon oxide/polymer hybrid fiber optic sensor⁷⁹, with a continuous polyimide tube instead of the complex three layers of discontinuous glass tube, one of a pair of quartz optical fiber end face of F-P cavity can be free in the PI tube stretching, using phase tracking method, realized the 28 pm/με sensitivity, as shown in Fig. 4(c).

(d) Fiber optical magnetic sensor

Magnetic fluid is a kind of sensing element widely used in magnetic field detection¹⁰⁹. According to the working principle of optical fiber magnetic field sensor, this section can be divided into three categories: grating-based sensors, interference-based sensors, and others.

1)Optical fiber magnetic sensor based on grating

Grating structures with different treatments such as phase-shifted fiber Bragg grating based on magnetic fluid infiltration¹¹⁰, a tilted fiber Bragg grating coated with nanoparticle magnetic fluid¹¹¹ and ferrofluid-infiltrated micro-structured optical fiber long-period grating¹¹² are used as magnetic field sensors.

2)Optical fiber magnetic sensor based on interference

Magnetic field sensors based on different interference principles such as a magnetic fluid-filled FP-FBG structure¹¹³, photonic crystal fiber filled with magnetic fluid⁸⁰, as shown in Fig. 4(d), and amplification of birefringence of magnetic fluid film by Loyt-Sagnac interferometer¹¹⁴ have been developed.

3)Others

Other magnetic field sensors based on magnetic fluid include the combination of a DLUWT-based SPR fiber-optic sensor with a magnetic fluid¹¹⁵, a U-bent single-mode–multimode–single-mode fiber structure with magnetic fluid¹¹⁶ and the SMF-SNS-SMF in a capillary filled with magnetic fluid¹¹⁷.

Research progress related to magnetic field based on different structures and principles have been proposed, such as electromagnetic field measurement based on optical sensors¹¹⁸, magneto-strictive, magneto-optical, magnetic fluid materials optical fiber current sensors, and optical fiber magnetic field sensors¹¹⁹, temperature and magnetic field measurement technology based on magnetic fluids¹²⁰, optical microfiber coupler sensors¹²¹, S-shaped fiber taper¹²², optical microfibers¹²³, Fabry Perot interferometer fiber optic sensors¹²⁴ and core-offset structure¹²⁵.

The spread of the Internet of Things in healthcare facilitates the transition from reactive medicine to predictive medicine. Wearable sensors are in constant development to accommodate the growing demands of healthcare devices. While, with the continuous breakthrough of flexible materials and micro-nano manufacturing processes, more and more demands have been put forward for wearable sensors, such as anti-electromagnetic interference, excellent biocompatibility, and multi-functional integration^{126, 127}. Thus, the further development of wearable sensing technology faces many challenges. The optical fiber sensors provide a potential alternative to overcome the limitations due to their high sensitivity, light weight, electromagnetic immunity, chemical stability, and multiplexing capabilities. To date, substantial effort in optical fiber sensors has been focused on the development of wearable healthcare monitoring, using various types such as optical fiber microstructure-based sensors¹²⁸, optical fiber interferometer-based sensors¹²⁹, and special optical fiber-based sensors¹³⁰. In Fig. 5, representative optical fiber sensors are listed to exhibit the healthcare monitoring of the wearable application.

(a) Optical fiber microstructure-based sensors

Typical fiber microstructure-based sensors are divided into fiber Bragg grating sensors (FBG), tilted fiber Bragg grating (TFBG), and long-period fiber grating sensors (LPG)^{131, 132}. Fiber grating is a spatial phase grating whose refractive index is periodically modulated in the fiber core layer by utilizing the photosensitivity of the fiber material. Various physiological signals, including cardiorespiratory signals, pulse wave signals, body temperature, body posture, and plantar pressure can be monitored by demodulating the light wavelength.

(b) Optical fiber interferometer-based sensors

To employ a low-cost sensor system, significant effort has been devoted to developing fiber interferometer-based sensors such as Mach–Zehnder interferometer (MZI), Sagnac interferometer (SI), Michelson interferometer (MI), and Fabry-Perot interferometer (FPI)^{133, 134}.

The MZI has a sensing arm and a reference arm. Vital signs will change the length and refractive index of the optical fiber sensing arm. For SI, it is possible to detect the phase shifts of the two beams of light transmitted in opposite directions by measuring the interference effect under the external stimulus. The structure of MI is almost identical to MZI. MI uses the reflection mode, which makes it more compact and convenient to use in experiments. FPI is the most popular phase-modulated sensor for wearable applications, and the Fabry-Perot cavity is the sensing area. It is suitable for monitoring heart rate, respiration rate, blood pressure, and body temperature due to its small size. The fiber interferometer-based sensors mentioned above offer the advantages of high sensitivity and low cost.

(c) Special optical fiber sensors

In order to promote the development of portability and integration of wearable devices, special optical fiber sensors such as polymer optical fiber (POF) sensors and micro/nano fiber (MNF) sensors have also drawn a lot of interest in research^{135, 136, 137}, which will be significant in human-computer interaction control, human health monitoring, and movement disorders monitoring.

POF sensors based on soft and stretchable optical waveguides are usually made from hydrogels, biodegradable polymers, and elastomers. The deformation of the soft optical waveguide will cause transmission loss, leading to the intensity variation of transmitted light. Benefiting from the advantages of low Young’s modulus, high flexibility, higher elastic limits, excellent biocompatibility, and wearable physiological monitoring, including body temperature, human motion, speaking, and deep breathing could be realized¹³⁸.

MNF is an optical waveguide with a diameter in the sub-wavelength order and a strong evanescent field. Under external excitation, the guided modes will transit into radiation modes, leading to optical loss¹³⁹. Skin-like wearable sensors based on microfibers have been adopted in healthcare monitoring. In addition, hardness discrimination and slippage monitoring can also be adopted by microfiber tactile sensors¹⁴⁰.

Optical fiber shape sensor (OFSS) is an emerging, rapidly developing technology. Due to the superiority of intrinsic safety, biocompatibility, and flexibility, OFSS has presented great progress and application prospects in areas such as biomedical treatment¹⁴¹, soft robots¹⁴², structural health monitoring¹⁴³, and aerospace engineering¹⁴⁴. The common demands of these application fields are high sensitivity, high accuracy, and real-time tracking of shape position. Currently, the measurement schemes of OFSS mainly rely on Fiber Bragg gratings (FBG)¹⁴⁵, optical frequency domain reflectometry (OFDR)¹⁴⁶, and brillouin optical time domain analyzer (BOTDA)¹⁴⁷. The representative special fiber types, advanced fiber structures as well as application fields can be shown in Fig. 6.

The fiber cable of OFSS is usually composed of multiple fiber cores with the characteristic of symmetrical distribution in space, where the differential strain response between each fiber core can be employed to obtain the shape parameters along the sensing link. Then, by combing with the shape reconstruction model, the three-dimensional shape of the measured object could be deduced without visual aid¹⁴⁸. From the perspective of cross-section, the relative distance and angle between each fiber core and center point should be identical. According to the packaging method of the multiple fiber cores, the special optical fibers for shape sensing can be divided into two categories, namely, self-encapsulated fiber cable and multicore optical fiber.

(a) Basic special fiber types of OFSS

Self-encapsulated fiber cable is usually composed of several ordinary single-mode fibers or Fiber Bragg grating (FBG) array fibers¹⁴¹. Through special manual bundling and packaging, the bare optical fibers are pasted around the substrate such as catheter, needle, or shape memory alloy (SMA) wire or pasted together to form a fiber cluster^{149, 152}. The common arrangement methods include four orthogonal or three optical fibers at an angle of 120 degrees. Due to the large core spacing of self-encapsulation fiber cable, the shape sensor has higher shape resolution and sensitivity. Nevertheless, the spatial position arrangement is greatly affected by the mechanical device and the pasting condition, so the consistency and accuracy of the sensor cannot be guaranteed.

In contrast with self-encapsulation fiber cable, multicore fiber only has one cladding, one coating as well as several cores. Among them, one core is in the center, and other cores are parallelly arranged at equal intervals of about tens of micrometers on the circumference^{146, 147}. For multicore fiber, the structural parameters such as core distance and core angle can be accurately controlled during the fabrication process. Therefore, the reconstruction error caused by geometric parameters is so small to be ignored. While the cost of fan-in and fan-out couplers for multicore fiber is relatively high.

(b) Advanced fiber structures for performance enhancement

It is well known that shape sensing mainly relies on the analysis for strain information, while the strain is obtained by demodulating the inherent scattering information in the fiber. Therefore, the FBG array inscribed on the optical fiber core effectively enhances the scattering magnitude of OFSS. With the improvement of femtosecond laser processing technology, it is possible to directly write fiber grating arrays on multicore fibers^{150, 153}. The grating spacing can reach only several millimeters, the reflectivity can be less than 1%, and the number of gratings can reach several thousand.

Furthermore, the local twist is referred to the localized circumferential angular shift of the fiber away from the neutral axis in OFSS¹⁵⁴. The twist will lead to the degradation of accuracy and stability of shape sensing, and ultimately the failure of shape reconstruction. To solve this issue, the helical structure is developed and applied in both self-encapsulated fiber cable and multicore fiber.

(c) Typical applications

OFSS has shown great potential in the fields of medical treatment consisting of minimally invasive surgery¹⁵⁵, endovascular navigation, epidural administration, ophthalmic as well as cardiac procedures, colonoscopy, and finger gesture recognition¹⁵⁶. Furthermore, OFSS can be embedded in flexible robot links and manipulator arms of continuum soft robots to realize the monitoring of motion status and position¹⁵⁷. Moreover, the OFSS is also an effective real-time nondestructive monitoring method to observe and record the deformation and morphology of the structural behavior¹⁵⁸.

Based on the above two basic special fibers, the combination of fibers inscribed with FBG array and helical structure enable the OFSS to be more powerful, and leads to improved measurement sensitivity and reconstruction accuracy.

(a) Distributed temperature sensing for industrial applications

All fiber Raman-scattering-based distributed temperature sensors (DTSs) have undergone significant improvements in technology and application over the last decades. The anti-Stokes component from spontaneous Raman scattering in fiber is temperature-sensitive and its intensity can be temperature modulated. Conversely, the Stokes component is basically independent of temperature. Therefore, the utilization of Raman scattering and optical time domain reflectometry (OTDR) can achieve the spatial DTS¹⁵⁹. DTS was first used to monitor wellbore temperature in oil wells, enabling continuous and long-term temperature monitoring throughout the well^{160, 161}. In addition, DTS system was applied to measure the temperature along the length of the cable and oil-cooled power transformer^162-164. Fire detection is important for industrial applications. DTS was used to detect fire and localize the seat of the fire within a building with 1-m resolution¹⁶⁵. DTS can be used for long-term real-time temperature monitoring in industry, but it faces the problem of short service life of optical fiber. The development of high temperature and harsh environment optical fiber has attracted increasingly more attention.

(b) Distributed temperature and strain sensor for industrial applications

Distributed Brillouin sensing was first proposed in the late 1980s to measure local attenuation along an optical fiber¹⁶⁹. It has many potentialities for sensing because Brillouin scattering is intrinsically very sensitive to temperature and the deformations experienced by the sensing fiber. Brillouin scattering results from the interaction between pumped incident light and optical phonons. The absorption or release of phonons by the pump incident light results in the Brillouin frequency shift. The Brillouin frequency shift is determined by the acoustic velocity and is a function of fiber temperature and strain. Brillouin-scattering-based distributed temperature and strain sensor (DTSS) can realize the continuous measurement of parameters in space, which has the characteristics of long-distance and high-capacity sensing. DTSS is currently the most studied system in civil structure structural health monitoring². Because of the merit of the extended measurement range, DTSS has great potential in applications where large structures exist, such as dams, pipelines, tunnels, and long-span bridges^{1, 3}. For example, DTSS based on Brillouin sensing was used in the monitoring project of the Andes pipeline and the Italy Rimini gas pipeline to detect the geological hazard around the pipeline and monitor the pipeline deformation and leakage¹⁷⁰. The challenge of Brillouin-scattering-based DTSS is the crosstalk between parameters while performing multi-parameter measurements. In the future, new types of optical fiber and coding technology will be developed to achieve high-resolution temperature and strain sensing over long distances.

(c) Distributed acoustic sensor for industrial applications

The distributed acoustic sensor (DAS) is one of the most attractive and promising fiber sensing technologies in the recent decade. DAS is based on interference of Rayleigh backscattering and optical reflectance measurement technique, which can simultaneously detect and retrieve multiple vibrations over long distances and a high sampling rate. Compared with the well-developed DTS based on Raman scattering, the DAS based on Rayleigh scattering can detect events far from the fiber cable. And compared with the DTSS based on Brillouin scattering, the DAS can keep the coherence of the pump, so its highest strain resolution is orders of magnitude higher than the DTSS¹⁷¹. Together with its advantages like small size, immune to electromagnetic interference, and robust against harsh environment, the DAS is a promising solution in long distance applications like pipeline security, and harsh environments like oil and gas wells¹⁷². DAS can form a large scale and low-cost sensor array to recover the waveform of the mechanical vibration with high fidelity. The most typical industrial application is seismic wave detection. Ajo Franklin et al. used DAS deployed on a regional unlit fiber in dark fiber for broadband seismic monitoring, which showed that the optical fiber network could be effectively used for many earth science observation tasks¹⁷³. In 2019, Lindsey et al. combined DAS with existing dark fiber in subsea cables to observe ocean and solid earth phenomena⁵. In addition to the monitoring of natural geological activities, ambient noise can also be recorded by the DAS for monitoring the Earth’s near surface^174-176. For oil and gas exploration, DAS enables signal acquisition throughout the well and is easy to deploy with lower operating costs. So the DAS processing results outperform a regional and local surface array and will become a powerful tool for long-term dynamic monitoring of oil and gas wells¹⁷⁷. In industrial applications, such as intrusion detection for perimeter and fence, pipeline surveillance system, and railway security, the DAS plays a significant role because of the long working distance, linear configuration, and high sensitivity. The challenge for the DAS industrial applications is pattern recognition in event detection, while machine learning technology has great potential in the realization of high-performance pattern recognition.

(d) Various types and applications of special optical fibers

Fiber Bragg grating (FBG) is the most widely used fiber sensing technology in the industry. FBG can detect temperature, stress, strain, displacement, acceleration, and other multi-parameter information, which should be widely used in structure detection, reservoir detection and fire detection^{178, 179}. Polarization maintaining fiber (PMF) can ensure the linear polarization line in fiber sensing is unchanged and improve the signal-to-noise ratio. PMF is an important part of a fiber optic gyroscope, which is widely used in the inertial navigation of rockets, ships, and aircraft^{180, 181}. Spun HiBi fiber (SHB) is a special type of PMF, which is the main component of fiber optic current transformer and has a wide application prospect in the field of high current measurement¹⁸². Few-mode fiber (FMF) mitigates modal dispersion and has a high nonlinearity threshold in comparison with multi-mode fiber (MMF) and single-mode fiber (SMF). The DTS system based on FMF is expected to reach a temperature measurement distance of 50 km for online temperature monitoring of railways, power lines and long-distance oil and gas pipelines. Multi-core fiber (MCF) has flexible core numbers. It can achieve long-distance distributed bending sensing and performance enhanced distributed sensing, which may find applications in aerospace industry³⁹. Photonic crystal fiber (PCF), characterized by air holes arranged along the fiber length, is regarded as a desirable platform to excite surface plasmon resonance (SPR). PCF-SPR has excellent sensing performance with controllable resonance wavelength and exceptionally high sensitivity, which can realize a new type of highly sensitive gas sensor^{183, 184}. This kind of gas sensor can be widely used in ocean exploration, air environment detection and human respiratory monitoring and other fields. Optical fiber sensing for industry applications can be shown in Fig. 7.

The unique physical and light-transmission properties of optical fibers make them ideal components for biomedical sensors. Various types of optical fibers are finding widespread use in biosensor instrumentation for life sciences-related clinical and research applications. Each optical fiber structure has certain advantages and limitations for specific uses in different spectral bands that are of interest to various biomedical areas¹⁸⁵. The following subsections describe several different biomedical sensing modalities using specialty optical fibers. These are illustrated in Fig. 8 and include (a) biorecognition sensors^{186, 187}, (b) optical coherence tomography (OCT)^188-190, (c) surface-enhanced Raman spectroscopy (SERS)^191-193, (d) surface plasmon resonance (SPR)^{194, 195}, and (e) Michelson interferometry^196-198. Endoscopic methodologies are described in Section Endoscopy and Fig. 9.

(a) Biorecognition sensors

Biorecognition sensors are based on the principle of evanescent electromagnetic field waves in the fiber interacting with materials surrounding the fiber. Evanescent waves are not tightly bound to the fiber core but extend partially into the region surrounding the fiber. This class of evanescent wave-based sensors uses absorbance measurements to detect any variations in the concentrations of substances such as antigens that absorb a specific wavelength of light, as Fig. 8(a) shows. The absorption process of biorecognition material coating on the optical fiber results in a physical-chemical alteration that can affect the characteristic of the evanescent wave in the optical fiber. Changes in the evanescent wave are directly related to concentrations or types of biomaterials in which the sensor emerged. Applications of this methodology have been used for sensing glucose levels, pH levels, oxygen levels, and the presence of antibodies. Specialty fibers employed in this sensing technique include coated multimode and single-mode, photonic crystal fibers, and tapered fibers.

(b) Optical coherence tomography (OCT)

OCT is an optical imaging modality that can capture real-time, 3D images of biological tissues and materials in vivo and noninvasively with a 1-15-μm resolution depending on the specific OCT method used^{188, 189, 190}. OCT is based on an interferometric optical fiber configuration and functions by measuring the intensity and time-of-flight information collected from backscattered light coming from different points within a tissue sample. Conventionally, two fibers are used in the OCT system. In this case, one of the fibers transmits illuminating light to the target specimen and the second fiber collects the reflected scattered light for reconstruction of the image by the detection system. A newer, more advanced version uses a single, multi-clad fiber (such as a double-clad fiber) where the transmission of the source light and the reflected light occurs in a single fiber. In a double-clad fiber the single-mode core transmits light to the specimen and the inner cladding collects the reflected light for the measurement instrument. In addition to the classical interferometric fiber configuration, OCT probes have also been used inside 30-gauge needles to obtain images inside the body. Such a needle probe is shown in Fig. 8(b), which uses optical fibers such as single-mode fibers, hard-clad silica fibers, photonic crystal fibers, side-emitting fibers, and double-clad collection fibers.

(c) Surface enhanced Raman spectroscopy (SERS)

SERS is based on using a large array of efficient scattering molecules, as Fig. 8(c) illustrates^191-193. These collections of molecules can be chemically joined to metallic substrates after which they can produce distinct, optically strong spectra upon illumination by a laser. Through this process, a strong evanescent electromagnetic field is induced on the metal surface. Thereby the Raman modes of molecules that are close to the metallic surface are enhanced dramatically, because the Raman intensity is proportional to the square of the incident electromagnetic field amplitude. One analytical model to describe this effect is to consider an isolated sphere subject to a quasi-static incident electromagnetic field^{192, 193}. This results in the expression

1E2∝E02|ϵm−ϵ0ϵm−2ϵ0|2,

here the parameter E is the electric field magnitude at the surface of the sphere, E₀ is the incident field magnitude, ϵm is the wavelength-dependent dielectric constant of the metal composing the sphere, and ϵ0 is the dielectric constant of the local environment around the sphere. This relation shows that when ϵm=−2ϵ0 , which can be achieved for silver and gold at certain wavelengths in the visible and near-IR, the magnitude of the electric field at the surface of the sphere become very large.

As an alternative to attaching target molecules onto a nanoparticle roughened fiber surface, another technique is to immobilize biomarkers inside the core surface of a photonic crystal fiber. Biochemical samples can be accommodated in holes of fiber in liquid or gaseous form due to the holey nature of hollow-core photonic crystal fiber (HC-PCF), and as a result interaction between the guided light and suspected analytes is very high. ​Compared with the attached target molecules onto a nanoparticle fiber surface, immobilizing biomarkers inside the core surface of a HC-PCF provides better light confinement and a larger interaction length for the guided light and the analyte, resulting in an improvement in sensitivity to detect low concentrations of bio-analytes with only a few nano liter or micro liter analyte samples^200-202. However, the band gap shifts due to refractive index change when the hollow core and air holes are filled with liquid in HC-PCF. As a result, transmitted light may not be well confined in HC-PCF, leading to larger loss. A side channel photonic crystal fiber (SC-PCF) was designed to allow liquid samples to be pumped through a side channel while light is confined and transmitted in the center solid core. By loading the SC-PCF with a mixture of gold nanoparticles and modified cells, and subsequently propagating laser light through the central solid core, strong SERS signal can be obtained. This SERS technique achieved accurate detection of concentration of sialic acid and rapid monitoring of lipid peroxidation derived protein modification in cells^{203, 204}.

(d) Surface plasmon resonance (SPR)

Surface plasmons are electromagnetic waves that propagate along the surface of a metallic-dielectric or a metallic-air interface and are evanescent in all other directions. The surface plasmon resonance effect produces a collective oscillation of electrons when the surface of a solid or liquid is stimulated by polarized light，as illustrated in Fig. 9(a). Because the plasmon waves travel along the surface of a thin metallic layer (on the order of several ten nanometers thickness for visible light), they are very sensitive to any changes in the boundary characteristic at the surface. For example, changes in the boundary characteristic can occur when molecules suspended in a liquid sample are adsorbed on the metal surface. These events will modify the surface plasmon mode thereby causing a shift in the resonance peak of the reflected light, as Fig. 9(b) illustrates. In order to create a more compact miniaturized SPR instrument, specially designed or modified optical fibers have been implemented to replace the much larger prism structure¹⁹⁴. These fiber-based configurations include metallic coatings on a silica or plastic fiber core, tapered fibers, and photonic crystal fibers. One method that has been examined is the coating of the exposed core of a single-mode fiber with a thin layer of gold, thereby creating an SPR configuration¹⁹⁵. The motivation for this type of miniature SPR probe was to work intravenously for real-time detection of circulating tumor cells in bloodstreams. By making use of the SPR effect, biosensors with a high sensitivity can be created for applications such as imaging, medical diagnostics, drug discovery, food safety analysis, and environmental monitoring. Specially designed or modified optical fibers user for SPR include metallic coatings on a silica or plastic fiber core, tapered fibers, and photonic crystal fibers.

(e) Michelson interferometry

Interferometric optical fiber sensors are based on measuring the phase difference between two superimposed light beams that have the same frequency. Commonly deployed interferometer architectures are the Mach–Zehnder, Michelson, and Sagnac interferometers^196-198. As an example, Fig. 8(e) shows an example of Mach–Zehnder interferometer (MZI). Owing to its versatile setup configuration, an MZI is popular for diverse biosensing applications. Various degrees of sensing can be achieved in the center of the MZI by using different types of specialty fibers for fiber 2. These include multimode fibers, fibers with embedded FBGs, or photonic crystal fibers. In order to create a more compact and robust interferometer design, biosensors based on an all-fiber inline MZI scheme have been investigated. The designs include a tapered fiber configuration, a microcavity imbedded in a fiber, the use of fiber Bragg gratings, cascaded segments of different fibers, photonic crystal fibers that are selectively filled with a liquid, and nanofiber probes.

(f) Endoscopy

The field of endoscopy uses a photonic-based instrument containing one or more optical fibers that allow a clinician to examine the inside of a hollow organ or body cavity^{205, 206}. The following units make up a basic endoscope system:

• A flexible tube that encapsulates one or more optical fibers for illumination and viewing functions. The encapsulating endoscope tube also can contain miniature air, water, and suction or biopsy tubes, plus wires for tip control

• An external light source that is coupled to the optical fibers in the encapsulating tube for illuminating the organ, tissue area, or body cavity being diagnosed

• A lens system which collects reflected or fluorescing light from the diagnostic site for transmission via optical fibers to a viewer

• A viewing mechanism such as a simple eyepiece, a monitor, or a camera

New specialty optical fiber types and miniaturized cameras have helped to enable increasingly sophisticated endoscopic capabilities and have resulted in smaller, faster-performing devices. During the evolution of endoscopes, the fibers used in the instrument have included optical fiber bundles and single optical fibers such as multimode fibers, single-mode fibers, double-clad fibers, and hollow-core photonic crystal fibers.

Figure 10 shows six different configurations for using diverse optical fibers and imaging configurations inside of an endoscopic sensing head²⁰⁶. Endoscopes can be classified into side-viewing endoscopes (Fig. 10(a)) and forward-viewing endoscopes (Fig. 10(b)). A side-viewing endoscope is more suited for surveying a large area inside of a hollow organ, while a forward-viewing endoscope is generally more suited for image guidance of biopsies or device placement. Figures 10(c) and 10(d) illustrate the use of a tethered capsule that is used to diagnose and monitor diseases in the digestive tract. The clinical procedure involves swallowing a nominally 11-mm-diameter and 30-mm long rotating viewing capsule that is tethered to an optical fiber link and captures cross-sectional microscopic images as it travels through the digestive tract. The rotation function can be performed either outside of the fiber tether (Fig. 10(c)) or by means of a micro-motor inside of the capsule (Fig. 10(d)). One method for illuminating a tissue area and for collecting the image is the use of an angle-polished ball lens fused to the end of the endoscope fiber as shown in Fig. 10(e) for side viewing. An alternative technique is to metallically coat the fiber end as is shown in Fig. 10(f) for direct end viewing.

Continuous research effort is devoted to pushing the technological advancement of specialty fibers for advancing and fast-growing sensing applications. The innovations in their structures, materials, and technology integration with various sensing platforms are major driving forces in developing novel specialty fibers-based sensing technologies and solutions. This review paper presents an overview of the specialty fiber technologies from three main groups: photonic crystal fibers (PCFs) and optical fibers with other special structures, multifunctional and multimaterial fibers, and lab in/on fibers. The specialty fibers have been widely used in various sensing applications in both research and real-world applications. The potential applications of fiber-based sensors can be shown in Table 1. Compared with traditional optical fiber sensors, sensor performance such as sensitivity could be significantly improved by utilizing specialty fiber-based sensors. Such demonstrations have been made through sensing of physical parameters such as refractive index, strain, temperature, and magnetic field. Wearable sensing technologies have benefitted from the development of specialty optical fibers, ranging from fiber microstructure-based sensors, fiber interferometer-based sensors, polymer optical fiber (POF) sensors, and micro/nano fiber (MNF) sensors. Optical fiber shape sensors based on special fibers and fibers with advanced structures have achieved performance enhancement and attracted growing interest in developing applications in medical treatment, soft robots, and structural behavior monitoring. Besides point sensing applications, distributed fiber sensors have been widely used for industrial applications, such as monitoring of gas pipelines, underwater security, loaded beam structure, earth phenomena, power transformer, and downhole environments. Specialty fibers are also widely used in biosensor instrumentation for life sciences related clinical, and research applications.

In addition, fs laser-based TPP technology is highly accurate and compatible with the micron- and even nano-sized features of the optical fiber itself, allowing the fabrication of microstructures with almost arbitrary designs on the fiber surface, tip, or inside the fiber. Moreover, as a direct-writing process, the substrate surface is not restricted to be flat, but can even be on the arc-shaped outer surface of the fiber. Compared with conventional UV/CO₂-laser based fabrication technologies, TPP not only achieves higher accuracy, but also produces microstructures with much finer surfaces which can achieve more reliable optical performances. On the other hand, beyond the resin-based photoresist, there are more types of new materials being applied in TPP, such as silica nanocomposite, gelatin-based hydrogel, etc. Other types of functionalized composite photoresist that contain magnetic nanoparticles, quantum dots, etc., are also developed for various applications. Thus, we anticipate the micro-structured fiber optic devices based on fs laser based TPP technology could gain wider application perspectives, with finer structures and more diverse functionalities.

Although many promising thermally drawn multimaterial fibers with various advanced functionalities have been achieved, the growth of multimaterial fibers is still in its early stage. More studies are desired to understand further the underlying science, material processing, structure engineering, and system integration. Here are a few exciting directions that can be investigated in the future:

1) Fabrication of semiconductor fibers with other cross-section geometries. Si and Ge fibers fabricated by far have only been cylindrical. However, other shapes of fibers are also favored in specific applications. For example, ribbon fibers or hollow-core semiconductor fibers. Ribbon fibers can be fabricated using the preform with a rectangular hollow-core, while hollow-core semiconductor fibers may be produced with a preform that has a ring-shaped hollow-core. The hollow-core semiconductor fiber can be considered as a semiconductor tube and may have interesting applications. For example, liquid metal can be filled into the semiconductor tube to function as an electrode in an optoelectronic fiber or form an electrode array inside the semiconductor tube to achieve back-illuminated geometry.

2) Optoelectronic fibers with interdigital electrodes by the capillary breakup. For the optoelectronic fibers reported by far, two electrodes are used. For better bandwidth of the devices, more electrodes can be used to decrease the transit time of photogenerated carriers. In planar devices, the interdigital electrode is commonly used. However, in fiber geometry, deposition of the interdigital electrode is complex. Because of the capillary breakup in fibers, a thin layer of material breaks up into several fibers uniformly distributed in a ring. In this manner, a layer of conductive material arranged around the semiconductor fiber could form serval electrodes after experiencing an induced capillary breakup.

3) Fabrication of graphene nanoribbons via the cold drawing method. In the study of tearing graphene via the cold drawing method, the average width of the resulting graphene ribbon fibers is around 900 nm. Further, reducing the width of graphene ribbons to below 100 nm may open the graphene bandgap and enable many potential applications. To date, there is no report of a low-cost, simple, and fast method for producing monolayer graphene nanoribbons. Thus, it could be exciting if the cold drawing method could produce graphene nanoribbons by enhancing the adhesion of the graphene sheet and the polymer substrate.

We are grateful for financial supports from Special Funds for the Major Fields of Colleges and Universities by the Department of Education of Guangdong Province (2021ZDZX1023); Natural Science Foundation of Guangdong Province（No. 2022A1515011434）. Stable Support Program for Higher Education Institutions from Shenzhen Science, Technology & Innovation Commission (20200925162216001); Guangdong Basic and Applied Basic Research Foundation(2021B1515120013); Open Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications，No. IPOC2020A002), The Open Projects Foundation of State Key Laboratory of Optical Fiber and Cable Manufacture Technology (No. SKLD2105).General Program of Shenzhen Science, Technology & Innovation Commission (JCYJ20220530113811026).

H. H. Liu and D. J. J. Hu contributed equally to this work in drafting the manuscript, Q. Z. Sun contributed to review on wearable sensor and shape sensing; L. Wei and K. W. Li contributed to review and prospects on multimaterial multifunctional fiber; C. R. Liao, B. Z. Li, and C. Zhao contributed to review and prospects on lab in/on a fiber; X. Y. Dong and Y. H. Tang contributed to review of general sensing applications; Y. H. Xiao contributed to review of industrial application; G. Keiser contributed to review of biomedical sensing; P. P. Shum supervised the review. All authors contributed to revision on the manuscript.

The authors declare no competing financial interests.