Advanced Fiber Sensing Technologies

Surface plasmon waves are coupled electron–photon modes at the metal–dielectric interface. They can significantly enhance light–matter interactions that are favorable in many applications including nanophotonics, data storage, microscopy, solar cells, and sensing. Compared with the prism-based coupling configuration, optical fiber-based plasmonic devices offer more compact and robust configuration for exciting the plasmon modes. Photonic crystal fibers (PCFs) are a special class of optical fibers in which the presence of holey structures or periodic microstructures of refractive index modulations can provide a wide scope of flexibility in the control and engineering of the optical properties, thus opening up the potential for many new applications and scientific explorations. Notably, PCFs are a desirable platform to incorporate plasmonic structures for the excitation of surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR). Three main types of plasmonic PCF structures have been developed and reported in the literature, including metal nano-/microwire-filled plasmonic PCF, metal-coated plasmonic PCF, and nanoparticle-deposited/filled plasmonic PCF. This chapter provides a comprehensive review on the recent progress of these reported plasmonic PCF structures in terms of design and applications. Firstly, the operating principles based on surface plasmon polaritons and localized surface plasmon polaritons are presented. Secondly, the experimental studies of plasmonic PCF structures for various application areas are reviewed, including refractive index sensing, biosensing, temperature sensing, polarization, and birefringent devices. Lastly, design considerations and challenges are discussed.

With the increasing demand of achieving comprehensive perception in every aspect of life, optical fibers have shown great potential in various applications due to their highly sensitive, highly integrated, flexible and real-time sensing capabilities. Among various sensing mechanisms, plasmonics-based fiber-optic sensors provide remarkable sensitivity benefited from their outstanding plasmon–matter interaction. Therefore, surface plasmon resonance (SPR) and localized SPR (LSPR)-based fiber-optic sensors have captured intensive research efforts. Conventionally, SPR or LSPR-based fiber-optic sensors rely on the resonant electron oscillations of thin metallic films or metallic nanoparticles functionalized on fiber surface. Coupled with the new advances in functional nanomaterials as well as fiber structure design and fabrication in recent years, new solutions continue to emerge to further improve the fiber-optic plasmonic sensors performance in terms of sensitivity, specificity and biocompatibility. For instances, 2D materials like graphene can enhance the surface plasmon intensity at metallic film surface so as the plasmon–matter interaction. 2D morphology of transition metal oxides can be doped with abundant free electrons to facilitate intrinsic plasmonics in visible or near-infrared frequencies, realizing exceptional field confinement and highly sensitivity detection of analyte molecules. Gold nanoparticles capped with macrocyclic supramolecules show excellent selectivity to target biomolecules and ultralow limit of detection. Moreover, specially designed microstructured optical fibers are able to achieve high birefringence that can suppress the output inaccuracy induced by polarization crosstalk meanwhile deliver promising sensitivity. This chapter aims to reveal and explore the frontiers of such hybrid plasmonic fiber-optic platforms in various sensing applications.

Microstructured optical fibers (MOFs), which have a holey structure in the cladding/core region, exhibit enhanced sensing sensitivity and performance for liquid/gas samples. In MOFs, the presence of sensing samples in the holey cladding/core region increases mode-field overlap and effective interaction length between the samples and the optical signals, resulting in a deep modulation on the optical signal. Moreover, in places of a bulky chamber for hosting liquid/gas samples in conventional fiber-based sensing configurations, the tiny voids in MOFs save the volume of sensing samples and avoid contaminations, making the sensing scheme more compact for in-line sensing applications. In this chapter, we first introduce the structures of MOFs and lightwave guiding mechanisms in MOFs, including index-guiding mechanism, photonic bandgap guiding mechanism, and antiresonance guiding mechanism. Then, we present MOF fabrication methods for different fiber structures and materials. Last but not least, several kinds of MOF-incorporated sensing configurations, including fiber gratings, Fabry–Pérot interferometers, Mach–Zehnder interferometers, and Sagnac interferometers, and surface-enhanced Raman scatterings, are discussed with theoretical analysis and cutting-edge achievements in a few application scenarios.

Optical microfiber is a class of specialty fibers, which is featured with wavelength scale diameters. With such small dimensions, the optical microfiber offers large fractions of evanescent fields and high surface field intensities, making it highly sensitive to disturbances in the surrounding medium. Thus, the optical microfiber is an ideal building block for high-performance photonics sensing devices. In this chapter, recent progress in optical microfiber-based sensors is reviewed. It starts with a brief introduction of the fundamental optical properties of optical microfibers and the well-developed fabrication techniques. Then, a brief summarization of the well-established microfiber-based refractive index sensing schemes is given, including working principles and sensing performances. Following this section, the latest progress on new effects and strategies for sensing enhancement is reviewed. In the last, the conclusions and an outlook are presented.

With the maturity of optical fiber sensing technology, its related applications have penetrated into various fields. Fiber-based acoustic sensors (FAS) are one of the most important research fields in fiber optic sensors. So far, it has been widely applied in natural disaster warning, medical diagnosis, geological exploration, and even battlefields. In recent years, natural disasters such as earthquakes have occurred frequently, and modern naval warfare technology has developed rapidly. Due to the special demand on acoustic detection in these fields, FAS have developed in the direction of high precision, low frequency, low cost, and miniaturization. Aiming at the low frequency or infrasound detection requirements for special applications and taking the advantages of fiber interferometric sensing structure, this chapter proposes a composite diaphragm-type fiber (external Fabry–Perot interferometer) EFPI infrasound sensor. The circular composite film is formed by combining a polymer film and an aluminum foil; then, an EFPI interferometer structure is formed between the composite film and the end face of the fiber FC joint. By theoretically simulating and optimizing the materials, sizes, and structures of the transducer, the relevant parameters are selected according to the detecting requirements. Then, the fiber EFPI infrasound sensor is well fabricated and packaged. In the simulated infrasound field environment, a comparison test was performed with a standard B&K infrasound sensor. The experimental results show an acoustic sensitivity of up to −138.3 dB re 1 V/μPa (~121 mV/Pa) in the infrasound frequency range of 1–20 Hz, which is higher than the commercial acoustic sensor used for comparison.

Aiming at the high sensitivity, high precision, small size, and low cost acoustic detection requirements, a novel fiber sensor for curvature and acoustic wave measurement based on a thin core ultra-long period fiber grating (TC-ULPFG) has been proposed in this chapter. By tracking the power variation of different resonant wavelength caused by TC-ULPFG, high curvature sensitivity of 97.77 dB/m⁻¹ is achieved, to best of our knowledge, which is highest than other structures at the same measurement range. Thus, the desired curvature property of the TC-ULPFG is used for acoustic measurement. The polyethylene terephthalate (PET) film is selected as a transducer, on which TC-ULPFG is tightly pasted. The acoustic pressure sensitivity of 1.89 V/Pa is two orders higher than other structures based on the diaphragm transducer, and the noise-limited minimum detectable pressure is 1.94 mPa/Hz^1/2 at 200 Hz. In addition, the frequency fluctuations are nearly ±0.4 dB from 70 to 200 Hz and ±0.2 dB from 1 to 3 kHz, respectively. Therefore, the proposed optical fiber acoustic sensor (OFAS) has a flat frequency response in relatively lower frequency. The TC-ULPFG shows many advantages including high sensitivities of curvature, high acoustic pressure sensitivity, easy fabrication, simple structure, and low cost.

With the rapid development of human technology, we are continuously facing new problems and challenges. On the one hand are the limited natural resources, and on the other hand are the increasing demands for convenient lifestyle. It is not a good idea to sacrifice one aspect to satisfy the other, but we should seek a balance between sustainable development and the comfortable living concept. Thus, it becomes a very crucial task to explore the low-cost, large-scale, and environmentally friendly fabrication methods for wide applications. Electrospinning is a top-down method in which polymeric or melt components are drawn out from a solution system onto a collector by electrostatic force. In comparison with other methods, including drawing, template synthesis, chemical vapor deposition, and so on, electrospinning offers some attractive features. One of the most important advantages is its industrial scalability, which makes it possible to directly transfer the results from laboratory research to the industry. In general, the unique advantages of electrospun nanostructures, including high spatial interconnectivity, high porosity, and large surface-to-volume ratio, make it a promising fabrication method in a wide range of applications. This chapter covers aspects of information relating to electrospinning nanofibers, including the materials for nanofiber fabrication, processing mechanism and parameters of electrospinning techniques, special electrospinning techniques, and potential applications of electrospun nanofibers.

This chapter reports a comprehensive review of the nanofiber gas sensor for enabling fast, relatively inexpensive, and minimal monitoring of the target gas concentration. The front part provides detail information on the sensing mechanism, evaluation criteria and application fields of the nanofibers-based gas sensor. The sensing mechanism mainly divided into the ionosorption model and oxygen-vacancy model, which can determine the fundamental factor for the evaluation of sensing performance. The main application fields focus on public safety, food processing, environmental monitoring, and disease diagnosis so far. The rear section then discusses the effect of the composition and morphology of nanofibers for the sensing performance in detail. Construction of multiple heterojunction components and various dopants, including the noble metal and the rare-earth, can effectively improve the sensing performance of the nanofiber-based gas sensor, which can provide more oxygen-vacancy and active sites. Meanwhile, the control of morphology provides a larger specific surface area and more gas diffusion channel. This chapter might bring further development and evolution of sensors based on nanofibers for the detection of various analytes.

The optical fiber being optical sensors exhibit several advantages including the immunity to the electromagnetic interference, simple structure, easy to carry, and large dynamic range. Especially, the possibility of working under harsh environment marks the fiber sensors that are superior to the electrical sensor counterparts. To achieve new breakthroughs in high-temperature sensing technology, exploring high-temperature-resistant optical fibers with good mechanical properties is necessary. The sapphire-derived fiber (SDF), a high-concentration alumina-doped silica fiber based on a single-crystal sapphire rod, has good mechanical strength, high-temperature resistance, etc. Such SDF shows great potential in high-temperature sensing and distributed strain sensing. This chapter mainly introduces the SDF for high-temperature sensing application. Firstly, the fabrication and the characterization of SDF have been introduced. Secondly, the SDF-based optical devices have been discussed including Fabry–Perot interferometer, Bragg grating, Mach–Zehnder interferometer, long-period fiber grating. Then, the advantages of SDF fiber in the field of high temperature are proved by introducing the performance of various sensors based on SDF.

Flexible thermoelectrics enables a direct and green conversion between heat and electricity to power or refrigerate flexible and wearable electronics. Organic polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity, but low thermoelectric performance limits their development. The other promising alternatives of inorganic-based flexible thermoelectric materials that have high energy-conversion efficiency, large power output, and stability at relatively high temperature, yet it is impeded by inferior flexibility. Hence, researchers propose a paradigm shift in material research as flexible thermoelectrics requires the material of which the device is made to simultaneously have inorganic semiconductor-like high thermoelectric performance and organic material-like mechanical flexibility. In this chapter, this dilemma is tackled, on both material level and device level, by introducing a new kind of flexible thermoelectric fibers, which overcomes the problems that thin film-based thermoelectrics can only be bent in one direction and lack essential wearable properties such as air permeability. Herein, the state-of-the-art in the development of flexible thermoelectric fibers and devices is summarized, including organic conducting polymer thermoelectric fibers, fully inorganic flexible thermoelectric fibers, and inorganic TE materials hybridized with organic polymer fibers. Finally, the remaining challenges in flexible thermoelectric fibers are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future of fiber-based flexible thermoelectric devices in the energy market.

In recent years, multifunctional multimaterial fibers based on the thermal drawing process have made considerable development, which enables various practical fiber devices with optoelectronics, photonics, acoustics, biomedicine, and energy harvesting functionalities. The future development of multifunctional fibers requests highly integrated ingenious in-fiber structures and excellent material properties. To meet these challenges, the technologies of using fluidic instabilities induced in-fiber breakup phenomena are presented, allowing us a way to modify the traditional axially invariant in-fiber structure and to achieve in-fiber material engineering. The post-drawing thermal treatment can soften the selective part of the functional fibers, induce perturbations on the interface between materials, and eventually break the continuous fiber inner structures to fabricate in-fiber functional structures. The in-fiber breakup process enables the fabrication of in-fiber structured particles by a variety of materials with a wide range of processing temperatures from 2400 to 400 K and material viscosity ratio of 10 orders. Moreover, the in-fiber breakup process provides a useful tool to form fiber-based functional devices. On the other hand, the fundamental understanding of the in-fiber breakup phenomena shall be contributing to optimizing the fiber thermal drawing process. By selecting suitable materials and suppressing the in-fiber breakup phenomena, the designed structure of the fiber preforms will be preserved in maximum. This chapter covers aspects of (1) the introduction and theory of thermal treatment induced in-fiber fluidic instabilities, (2) the in-fiber fabrications based on in-fiber breakup process, (3) potential applications of in-fiber breakup process, and (4) the future research directions.

Cold drawing is a well-established manufacturing process for tailoring properties of polymers in polymer industries. It refers to the phenomenon that necks formed during the tensile drawing of polymer fibers, films, or any other shapes. The necking reduces the height and width of the polymer sample being stretched and elongates it in length. The cross-sectional area in the necking region becomes smaller and keeps in a constant value with traveling to the remaining non-necking regions until the non-necking regions disappeared. Fundamentally, cold drawing is the movements and orientations of entangled polymer chains under external forces. It happens both for amorphous and for semicrystalline polymers in specific conditions. Though the cold drawing process has long been used in industrial applications, it has been rediscovered as a method of nano- and micro-structuring of materials recently. Materials used for structuring are wrapped in polymer fiber to form a core-shell structure or attached to the surface of the polymer substrate, then experience ordered fragmentations during the cold drawing of the composite structure, and finally formed regular patterns in nano- and micro-scale. This robust nano- and micro-structuring process is easy to perform and can be conducted using bare hands in some situations. Further, this method can be applied to various materials and is suitable for large-scale fabrication. This chapter discusses nano- and micro-structuring of materials including crystal, glass, and polymer with the core-shell structured fiber and film-based flat geometries.

With the fast-growing demand for flexible and wearable electronics, fibers show great application prospects in this multidisciplinary area due to its breathability, washability, flexibility, and lightweight. Combining its advantages with the newly developed triboelectric nanogenerator technology, wearable biomotion energy harvesting, and multifunctional self-power sensors play an increasingly important role in this coming intelligent era. This technology is to utilize low frequency mechanical energy from dailylife mechanical motions to large-scale seawave energy. Based on the common triboelectric effect, electricity would be generated out of physically manipulating two materials with opposite surface charges. Simply by walking or running, the output current is enough to power sensors, microcontrollers, memories, arithmetic logic units, displays, and even wireless transmitters. Fiber TENGs are expected to both work as convenient energy harvesters to supply wearable electronics and directly work as self-powered real-time wearable sensors to monitor personal healthcare and bio-motions. Through textile method, fibers are also fabricated to into fabric and textile TENGs which makes the harvesting energy a higher amount level with more efficient area and could perform as a multifunctional cloth itself. In this chapter, we will introduce this emerging technique from the basic working principles, and the thoughts of device designs, to the versatile applications as energy harvesters and self-powered sensors. For remaining challenges to face both for energy harvesting textile TENG and TENG sensors, we will discuss and provide some practical insights and viewpoints.

With the rapid development of science and technology, portable and wearable electronic devices presented a prominent technological trend for future lifestyles and they have brought enormous convenience to our daily life. In order to realize the wearability of the whole equipment, it is necessary to develop matching high flexible, lightweight and small volume energy supply devices. The volume of planar flexible energy-storage device is too large to be integrated into the fabric, so it is difficult to give full play to the advantages of energy storage device. The fiber-shaped energy storage devices with their unique advantages of tiny volume, high flexibility and remarkable wearability have triggered wide attention. Thus, developing high-performance fiber-shaped energy storage devices is recognized as a promising strategy to address the above issues. This chapter discusses the design principles and device performance of fiber-shaped energy storage devices. In the first section, design principles of fiber-shaped energy storage devices with fiber electrode, electrolyte and device configurations are presented. In the next section, the development of fiber-shaped energy storage devices, including supercapacitors, nonaqueous and aqueous batteries, are comprehensively summarized, with particular emphasis on electrochemical and mechanical properties. The existing challenges and future directions are finally discussed to provide some useful insights from the viewpoint of practical applications.

A single longitudinal mode (SLM) Brillouin fiber laser (BFL) with cascaded ring (CR) Fabry–Pérot resonator, a SLM triple ring (TR) BFL with a saturable absorber ring (SAR) resonator and a stable multiwavelength (MW) SLM dual ring BFL (MW-SLM-DRBFL) are proposed and demonstrated. By optimizing the CR length of the single-mode fiber cavity at 100 m (or 50 m) and 10 m, stable SLM operation is obtained with 0.41 kHz (or 3.23 kHz). TR-BFL with approximately 65-Hz linewidth and 185 linewidth-reduction ratio is composed of a 1-km-long single-mode fiber (SMF) ring, a 100-m-long SMF ring, and an SAR with 8-m-long unpumped Erbium-doped fiber (UP-EDF), respectively. 7 stable SLM lasing wavelengths with DR configuration of 100 and 10 m length SMF are obtained with 0.084 nm wavelength spacing and 15 dB average optical signal-to-noise ratio (OSNR) through the cascaded stimulated Brillouin scattering (cSBS) and four-wave mixing (FWM). A MW SLM Brillouin–Erbium fiber laser (BEFL) sensor with ultrahigh resolution is proposed and demonstrated and Iezzi et al. proposed and investigated experimentally a distributed higher order Stokes SBS temperature fiber sensor. The one short common cavity of MW-SLM-BEFL with 50 m of SMF as the fiber under test (FUT) and 100 m of SMF as the reference realize 3.104 MHz/°C sensitivity and approximately 10⁻⁶ °C ultrahigh resolution in the short term of the third-order Stokes wavelength. While maintaining a fairly normal spatial resolution over a few kilometers of sensing length using time gating technology, sensitivity is increased by several folds to over 4 MHz/°C.