A Review of Nanostructure Coating Techniques to Achieve High-Precision Optical Fiber Sensing Applications

Abstract

Optical fiber sensors have emerged as a critical sensing technology across various fields due to their advantages, including high potential bandwidth, electrical isolation that is safe for utilization in electrically hazardous environments, high reliability, and ease of maintenance. However, conventional optical fiber sensors face limitations in achieving high sensitivity and precision. The integration of nanostructures with advanced coating technology is one of the critical solutions to enhancing sensor functionality. This review examined nanostructure coating techniques that are compatible with optical fiber sensors and evaluated etching techniques for the improvement of optical fiber sensing technology. Techniques such as vapor deposition, laser deposition, and sputtering to coat the nanostructure of novel materials on the optical fiber sensors are analyzed. The ability of optical fiber sensors to interact with the environment via etching techniques is highlighted by comparing the sensing parameters between etched and bare optical fibers. This comprehensive overview aims to provide a detailed understanding of nanostructure coating and etching for optical fiber sensing and offer insights into the current state and future prospects of optical fiber sensor technology for sensing performance advancement, emphasizing its potential in future sensing applications and research directions.

1. Introduction

The advancement of optical fiber sensing technology has revolutionized numerous sensing applications, including those in wireless communication [1], biomedical engineering [2], and environmental monitoring [3,4]. To achieve enhanced performance of optical fiber sensors (OFSs), promising approaches involve the deposition of the nanostructure and etching on fiber.

Experiments have been carried out to investigate nanostructure coating on thin film substrates. The results showed that coating techniques have significantly enhanced the inherent properties to meet production requirements in layer thickness, optoelectronic properties, and surface roughness [5,6,7,8], enabling applications of coated thin films in renewable energy monitoring, environmental control, and electronic device manufacturing [9,10,11].

Leveraging the coating techniques on sensing region of OFSs, as illustrated in Figure 1, is important in improving the sensitivity and selectivity of the sensing device by increasing the interaction between the evanescent field of the guided light and the surrounding medium [12,13]. The coating on the cladding of optical fiber structures can enhance the sensing capabilities, as this approach enhances interaction between the cladding and the coating, increasing the sensitivity to changes in the surrounding medium [14,15].

The coating on etched regions of OFS enables precise control of surface morphology and aids the engineering of refractive index profiles of fibers, enabling greater interaction with the evanescent field for higher accuracy and sensitivity [16,17,18]. However, the etching approach might impact the sensor performance due to exposure to external disturbances and environmental changes [19]. Optimizing the etching dimension and coating thickness is crucial to balance the trade-off between the sensitivity and the potential optical losses to achieve the desired performance characteristics [20].

Coating nanostructures on the fiber tip led to localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS) effects, which can significantly improve sensing by modulating the optical properties [21]. The local electric field enhancement on the fiber tip also creates a SERS-active platform, increasing the sensitivity in the detection of the target molecule. The fiber tip configuration with nanostructure coatings increases the analyte loading capacity and enables remote and in vivo sensing, enhancing the overall performance of the OFS [22]. Figure 2 shows a cross-sectional side view of the OFS and illustrates the coating on different regions for sensing performance improvement.

This review aims to provide a comprehensive overview of the latest developments in nanostructure synthesis techniques, discuss, in detail, coating techniques that are compatible with optical fiber sensors, and examine etching techniques for improving the overall sensing performance. By identifying current challenges and limitations in the OFS and highlighting its advancements via nanostructure coating, this review provides future research directions for the development of high-precision optical fiber sensing technologies.

Figure 1. OFS coating on etched cladding. Author’s own work adapted from [23,24].

Figure 1. OFS coating on etched cladding. Author’s own work adapted from [23,24].

Figure 2. Cross-sectional side view of OFS with coating applied on (a) unetched cladding, (b) etched cladding, and (c) fiber tip. Author’s own work adapted from [25].

Figure 2. Cross-sectional side view of OFS with coating applied on (a) unetched cladding, (b) etched cladding, and (c) fiber tip. Author’s own work adapted from [25].

2. Optical Fiber Sensors (OFSs)

OFSs consist of a fused silica (SiO₂) cylindrical core at the center of the fiber with a cladding layer encircling it. OFSs operate based on the modulation of light propagating through optical fibers, enabling precise measurement of various physical parameters, including temperature, pressure, strain, and chemical concentrations [26].

2.1. Optical Fiber Sensors Classification

The most common operating mechanisms of OFSs include scattering modulation, phase modulation, wavelength modulation, intensity modulation, and polarization modulation [27,28]. A Raman sensor is one of the scattering-based sensors, utilizing the phenomenon of Raman scattering to measure changes in the properties of the optical fiber, such as temperature, strain, and pressure. By analyzing the shifts in the scattered light spectrum, these sensors offer distributed sensing capabilities over long distances.

The Mach–Zehner interferometer operates based on phase modulation by splitting an input light beam into two paths so that the signal propagates through a reference arm and a sensing arm. A phase shift occurs due to a change in the sensing arm length caused by external perturbations. The phase shift between the two light beams is recombined at the detector to form an interference pattern, which is measured by the interferometer for the detection of parameters such as strain, temperature, and refractive index [29].

OFSs can be divided into three categories: interferometric, distributed sensors, and grating-based. The Fabry–Perot interferometer is one of the interferometric sensors, which consists of two reflective surfaces that can be constructed inside or outside the fiber. The operating mechanism relies on two interfering light beams reflected by the two reflective surfaces. The external perturbations change the length of the cavity, leading to alterations in the transmission and reflection spectra. By detecting the change in the transmission or reflection spectrum, multiple parameters can be measured [27].

The optical time domain reflectometer (OTDR) is a type of distributed sensor with a working principle based on the scattered light propagating through the fiber, where a short pulse of light is injected into the system to achieve spatial resolution. By analyzing the backscattered or reflected light, the region where the scatter originated is identified. The frequency or arrival time of the scattered light enables detection of the measurand amplitude and location [30].

Fiber Bragg grating (FBG) is categorized under grating-based sensors, where Bragg grating is written into the fiber core to create a periodic refractive index perturbation along the axial direction of the fiber. The refractive index perturbation leads to the reflection of light, where FBG will reflect light that has a wavelength corresponding to twice its period, multiplied by the effective refractive index, for which a Bragg condition is satisfied, given by

$${\lambda }_B=2\Lambda n_{eff}$$

(1)

where λ_B is the Bragg wavelength that will be reflected, n_eff is the effective refractive index of the fiber core, and Λ is the period of the grating. Light at other wavelengths will be transmitted without significant attenuation [31,32,33].

The OFS emerges as a powerful tool and an alternative to mechanical- and electrical-based sensors due to the multiple drawbacks encountered by both mechanical- and electrical-based sensors. Past generations of mechanical sensors have faced significant robustness constraints, and mechanical wear and tear has led to inconsistent readout. Additionally, the reduction in sensor displacement driven by the rigid body presents challenges when operating in hazardous environments. An electrical-based sensor can be complex due to multiple components, circuits, and connections, thus requiring complex signal processing techniques prone to errors, thereby reducing the sensor’s accuracy. Furthermore, these sensors are susceptible to external perturbations such as static electricity, electromagnetic interference (EMI), and radio frequency interference (RFI) [34,35].

2.2. Current Limitations and Challenges of Optical Fiber Sensing Technology

Compared to the conventional electrical- or mechanical-based sensor [36,37], the OFS possesses advantages including electrical isolation, safety for utilization in electrically hazardous environments, high reliability, and ease of maintenance [29]. However, OFSs often face limitations in measurement range, sensitivity, resolution, and precision to meet industry standards [38,39]. The integration of surface plasmon resonance (SPR) with the coating of nanostructures onto sensor surfaces has the potential to strengthen the interactions between analyte molecules and metal surfaces and enhance the sensitivity, resolution, and precision of sensors [40].

The experimental results showed that coating the FBG with titanium dioxide (TiO₂) enhanced the interaction between the evanescent field and the sensing medium, leading to improved sensitivity and detection limits for chemical sensing [41]. Coating OFSs with metal has been proven to be able to achieve a wide measurement range and exhibit good repeatability, as the metal ions induce refractive index perturbations, leading to wavelength shifts that are captured by the sensing device [42].

OFSs often face challenges of birefringence, which is a common phenomenon where a single light beam splits into two beams with polarization states that travel at different speeds, caused by thermal expansion and contraction when exposed to varying temperatures, which lead to differences in refractive indices for light polarized in different directions within the fiber. The birefringence drift results in time-varying polarization mode dispersion (PMD), limiting high-speed data transmission over long distances in optical networks [43,44]. Research showed that polyimide coatings on optical fiber demonstrated thermal stability, where a low coefficient of thermal expansion and low birefringence can be obtained simultaneously [45].

The robustness of OFSs for specified applications is another challenge; poor mechanical robustness limits the ability of OFSs to withstand bending, twisting, and other mechanical forces, which may lead to fiber breakage or the degradation of optical performance [46]. To enhance the mechanical robustness, an experiment was carried out with UV resin-coated FBG. The results demonstrated higher temperature sensitivity for coated FBG compared to uncoated FBG, and the sensor also exhibited the ability to withstand higher strains [47]. Polymer optical fibers (POFs) are a type of OFS made from polymeric materials instead of silica glass [48]. The sensors exhibit extremely high flexibility and strain tolerance compared to conventional OFSs, making them a good candidate for applications with stringent robustness requirements.

OFSs also face difficulties when exposed to a high radiation environment where radiation-induced attenuation (RIA) can affect the Bragg peak, which leads to the performance degradation of FBG [49]. The RIA can also alter the structure of the silica glass of OFSs, inducing an error in the measurement. Past research focusing on OFDR sensors reported that coatings including polyimide and polyimide with a carbon layer provide stability to the OFDR sensor to withstand high temperatures and high radiation exposure [50].

Most of the limitations and challenges can be addressed through the coating of nanostructures. Metal or metal oxide nanostructures improve the limitations in the sensitivity, detection limits, measurement range, and repeatability of OFS; UV resin nanostructures provide flexibility and robustness, whereas polymer or polyimide nanostructures improve thermal stability to minimize the birefringence phenomenon and overcome measurement error under high radiation exposure. By optimizing the nanostructures and respective coating techniques, high-precision OFS sensing can be achieved.

3. Nanostructures and Synthesis Methods

3.1. Classification of Nanostructures

Nanostructures are usually defined as materials or structures that have at least one dimension between the 1–100 nm size range and exhibit a higher surface-to-volume ratio compared to their bulk counterparts [51,52]. It can be classified into four categories, including zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanostructures [53]. Zero-dimensional nanostructures, including carbon dots [54,55] and metallic nanoparticles [56], are characterized by having all three dimensions confined so that electrons can move freely in 0 directions [57]. The quantum confinement effects of 0D nanostructures lead to discrete quantized energy levels, resulting in size-dependent optical and electronic properties [58].

One-dimensional nanostructures have one direction for electrons to move freely, provide a high aspect ratio, and exhibit excellent photocatalytic activity [59,60]. Two-dimensional nanostructures such as quantum wells [61] and multimetallic nanosheets [62] have electrons confined in one direction, providing high surface energy that allows extensive interaction and high carrier mobility [63]. All dimensions of a 3D nanostructure are outside the nanometer range or greater than 100 nm; thus, it is also called the bulk nanostructure or bulk nanocrystalline materials [64].

3.2. Synthesis Methods of Nanostructures

Nanostructures can be synthesized via top-down and bottom-up approaches. Top-down approaches involve breaking down bulk materials into nanoscale structures [65]. One of the top-down approaches to nanostructure synthesis is the lithography technique, which utilizes patterning to create nanoscale structures on surfaces [66]. This method provides high precision and control over dimensions [67], but with the limitations of being expensive and low-throughput [66,68]. A simple mechanical process of the top-down approach is the milling technique, which involves grinding and stirring the raw materials through the rotation or vibration of the mill to crush the bulk material into nanoparticles, creating nanocomposites and nanoalloys [69,70]. Laser ablation is another top-down approach, referring to the method of breaking down one part of the material from the surface under high-intensity laser irradiation [71]. It is a simple and rapid synthesis method, creating nanostructures with tunable surface properties [72].

Bottom-up approaches involve the assembly of smaller atoms or molecules to form larger nanostructures [65]. An approach such as the sol–gel technique, involves the transition of a system from solid particles in a liquid “sol” phase into an interconnected network of solid “gel” phases, allowing the formation of solid materials through the gelation of solutions [73]. The hydrothermal technique is another bottom-up approach offering advantages such as high purity, controlled morphology, narrow particle size, and lower energy requirements [74,75,76]. The self-assembly bottom-up synthesis involves a combination of non-covalent interactions, including electrostatic forces, hydrogen bonding, hydrophobic interactions, and van der Waals forces, to assemble small molecules into nanostructures [77].

4. Nanostructure Coating Techniques for Optical Fiber Sensors

The synthesis of nanostructures and the coating of the nanostructures in ordered arrays for enhanced functionality are crucial to achieving high-precision OFSs and broadening their applications. A wide range of coating techniques are available, especially for thin film applications [78]. These techniques can be broadly categorized based on the method of application, such as chemical, physical, or electrochemical [79,80,81,82]. However, some of the techniques might interfere with the optical properties and performance of the OFS due to various reasons, such as processing temperatures, mechanical stress, or unstable current density during the process.

The thermal spray technique involves heating nanostructures such as wire, rod, or powder and propelling them onto a target substrate to form a coating. It exhibits advantages in the deposition of a wide range of materials onto various substrates with improved corrosion resistance [83]. However, the extremely high temperature and mechanical stress of the process can potentially degrade the integrity of the optical fiber and its protective coating [84,85].

Applying an electroplating technique to a non-conductive substrate or substrate with low conductivity could potentially lead to a high-roughness coating surface and a high-porosity nanostructure [86]. Additionally, the instability of current density during the process can introduce thickness variations, which adversely affect the optical properties of the fiber [87]. The electron beam technique requires a high annealing temperature to improve the crystallinity and grain size of the nanostructure [88]. The increase in annealing temperature influences the stress state of the films. A change in stress from compressive to tensile will influence the structural and optical properties of the OFS [89].

The spin coating technique involves applying a liquid coating solution to the surface of the substrate and then spinning the sample at high speeds using a spin coater. Through the spinning process, the solvent in the coating solution evaporates, allowing the formation of a thin and uniform nanostructure across the substrate surface [90]. The coating thickness can be controlled by adjusting parameters such as the spin speed, processing time, solution concentration, and viscosity [91]. However, this technique is mostly applied to flat substrates, as the centrifugal force to spread the coating evenly cannot be effectively applied to OFS substrates. Modification is necessary to hold the OFS for evenly distributed coating.

After reviewing the challenges faced by the coating of nanostructures on OFSs, it was determined that only a few techniques, such as vapor deposition, dip coating, and drop coating, are suitable for OFS coating, as listed in

Table 1. Comparison of coating techniques and their mechanisms, advantages, and challenges.

Coating Technique	Mechanism	Advantages	Challenges
MOCVD	(1) Gaseous precursors are delivered into the chamber through a controlled flow system (2) Precursors undergo thermal decomposition or react with each other at the heated substrate surface (3) Growth of nanostructure on the substrate (4) Unreacted precursor molecules and reaction by-products desorb from the surface and are removed by the carrier gas	(1) Allow coating of wide variety of semiconductor materials and their alloys (2) High throughput (3) Low manufacturing cost (4) Formation of highly oriented single-crystalline structure	(1) Complicated growth mechanism (2) Produce unwanted by-products (3) High coating temperature led to high tensile stress and crystalline lattice defects
PECVD	(1) Diffusion of precursors through a boundary layer to the substrate (2) Adsorption of precursors onto the substrate surface (3) Surface reactions lead to the growth of nanostructure	(1) Allow low-temperature coating (2) Achieve high-aspect-ratio nanostructures	(1) Deposited device can be sensitive to annealing temperature, which may require careful optimization
ALD	(1) Exposure of the substrate to the first precursor gas (2) Purging of the reaction chamber (3) Exposure of the substrate to the second precursor gas (4) Purging of the reaction chamber	(1) Better control over materials thickness and composition compares to conventional CVD (layer-by-layer growth) (2) Highly conformal deposition technique (3) Allows the coating of metal oxide nanostructure with tunable structural properties	(1) Slow deposition rate (2) Limited choice of precursors
Magnetron Sputtering	(1) Utilizing magnetic field to trap electrons near the target surface (2) Positive ions from the plasma will be accelerated toward the negatively biased target (3) Ejection of target atoms towards substrate	(1) Enhance ionization efficiency for high deposition rate (2) Uniform and dense film forming with good reproducibility (3) Accurate control of film thickness (4) Allow different composition coating (5) Produce nanostructures with good mechanical performance (6) Good mechanical stability and adhesion of the nanostructures to the substrate (7) Ease of control of the film structure and composition	(1) Limited to conductive targets (2) Nanostructures may exhibit surface oxidation
DC Sputtering	(1) Insulating layer form on the target surface (2) Positive pulsed DC power helps discharge the insulating layer (3) Reactions between target atoms and reactive gas cause the target to be uniformly distributed over substrate surface	(1) Mitigate issue of debris generation or arcing, better plasma stability (2) Sputtering targets can be adapted to the surface of a substrate such as a cone or sphere	(1) Limited to conductive targets (2) Lower deposition rate than magnetron sputtering
RF Sputtering	(1) High-frequency alternating current applied between target and substrate (2) Oscillating electric field ionizes the sputtering gas and accelerates the ions toward the target (3) Target deposition on substrate surface	(1) Arc suppression as oscillating electric field helps prevent buildup of charge on the target surface (2) High sputtering yield (3) Higher plasma and electron densities leading to increased ionization (4) Ability to sputter insulating materials	(1) Lower deposition rate than magnetron and DC sputtering
PLD	(1) Excess energy from the laser pulse breaks down the local bonds at the irradiated zone (2) Eject particles from the target surface (ablation) (3) Ejected particles interact with the laser pulse and form a plasma plume (4) Plasma plume expands directionally toward the substrate (5) Ablated species in the plasma plume then condense and deposit on substrate	(1) Promote smoother surfaces (2) Allow nanostructure growth to greater thicknesses without breakdown (3) High-density coating at low temperatures and low pressure (4) Ability to deposit in reactive atmospheres (5) Effective for doped and multilayer coatings, offering layer-by-layer control (6) Allow coating of semiconductors and superconductors on different substrates	(1) Potentially produce particulates or droplets, causing quality degradation
Thermal Evaporation	(1) Target materials heated to a certain temperature (2) Low chamber pressure to ensure transportation of vapor from the source to the substrate without collisions (3) The atoms or molecules condense on the substrate (4) Nanostructures nucleate and grow on substrate	(1) Allow coating at room temperature and low chamber vacuum level (2) Suitable for coating a wide range of materials, including metals, alloys, and compound materials (3) Easy and precise control of deposition rate, enabling the coating of dense and uniform nanostructure (4) Reproducible process due to slow reaction and crystallization rate	(1) High temperature, high chamber vacuum level, and fast deposition rate are generally required to prevent oxidation but can slightly decrease the sensitivity (2) Require precise control of the flow rate of ambient gas to form desired nanostructure morphology
Dip Coating	(1) Immerse substrate into precursor solution (2) Nanostructure deposited on the substrate surface (3) Withdraw substrate from precursor solution (4) Solution evaporates and formation of nanostructure on substrate	(1) Allow multilayer coating with different materials (2) Offers flexibility and customization functionalities	(1) Time-consuming for multilayer coating, not suitable for large-scale or high-throughput manufacturing
Drop Casting	(1) Droplets of precursor solution released onto substrate with a pipette (2) Evaporation of solution (3) Formation of nanostructure	(1) Ultra-low-cost and scalable technique (2) Simple equipment setup (3) Compatible with various substrates	(1) Non-uniformity of nanostructure

. It is crucial to understand the mechanisms and influencing factors of each technique to ensure optimal performance and reliability in advancing OFS technologies.

4.1. Chemical Vapor Deposition (CVD)

In CVD, as illustrated in Figure 3, the vaporized precursor species undergo chemical reactions to deposit the desired material on the substrate, producing high-quality nanostructures [92], allowing scalability that is suitable for large-area deposition, and requiring relatively low processing temperatures compared to other techniques [93]. The CVD process involves surface reactions, diffusion or mass transfer reactions, and desorption reactions [94], and can be categorized into metal–organic chemical vapor deposition (MOCVD) and plasma-enhanced chemical vapor deposition (PECVD).

4.1.1. Metal–Organic Chemical Vapor Deposition (MOCVD)

MOCVD is a technique that deposits materials such as semiconductors on solid substrates using organic metallic compounds as source materials [95], typically in gaseous form. The gaseous precursors are delivered into the chamber through a controlled flow system. Inside the chamber, the precursors undergo thermal decomposition or react with each other at the heated substrate surface, growing the nanostructure on the substrate. Unreacted precursor molecules and reaction by-products desorb from the surface and are removed by the carrier gas [96].

In 2023, MOCVD was utilized for the deposition of tin (IV) oxide (SnO₍₂₎ on a chemically etched SMF-28 single-mode OFS [97]. The transmission spectrum of the optical path was recorded with a change in evaporation temperature from −20 °C to 20 °C. The nanostructure obtained at a higher temperature and flow rate demonstrated high sensitivity, achieving 3800 nm/RI in the RI range from 1.35 to 1.41. In addition, the experiment concluded that with a lower deposition rate of SnO₂, the chemical resistance can be increased to achieve high stability. The effects of chemical etching on the sensing performance of SMF-28 single-mode OFSs have been studied in a separate experiment by the same group of researchers [98]. It was concluded that chemical etching can be used to produce fiber tapers with a reduced diameter, leading to an increase in the interaction between the evanescent field and the external environment. By optimizing the diameter of the OFS via chemical etching, an OFS with high sensitivity and low optical loss can be obtained.

4.1.2. Plasma-Enhanced Chemical Vapor Deposition (PECVD)

The mechanism of PECVD is similar to MOCVD, where the process starts with the diffusion of precursors through a boundary layer to the substrate, followed by the adsorption of precursors onto the substrate surface, where the surface reactions lead to the growth of nanostructure [99]. MOCVD exhibits improved crystallinity with increasing deposition temperature, transitioning from polycrystalline to a highly oriented single-crystalline structure, whereas PECVD allows deposition at a relatively low temperature [100], where high-aspect-ratio nanostructures can be achieved by adjusting the RF power [101].

A study in 2022 demonstrated that the deposition of a thicker top oxide nanostructure up to 4 µm on a Mach–Zehnder interferometer (MZI) was possible by utilizing PECVD [102]. This high-quality, uniform, and thick oxide cladding achieved via PECVD helped minimize the mode interaction of the silicon substrate with the metal heater, which is important for reducing optical loss. From the experiment results, a high extinction ratio of up to 61.2 dB and an insertion loss of 1.5 dB were reported. The interferometer was also observed to exhibit more than 60 nm of bandwidth. By achieving this extinction ratio, the MZI can improve optical routing and optical modulation, improving the overall performance of the photonic circuit.

Figure 3. Illustration of CVD coating. Author’s own work adapted from [103,104].

Figure 3. Illustration of CVD coating. Author’s own work adapted from [103,104].

4.2. Atomic Layer Deposition (ALD)

ALD, as illustrated in Figure 4, offers the potential for atomic layer-by-layer control and enables the coating of a variety of materials [105]. In CVD, the film growth depends on the deposition time, while, in ALD, it is controlled by the number of deposition cycles; thus, this technique provides better control over material thickness and composition compared to conventional CVD [105,106]. The mechanism of ALD involves the sequential exposure of a substrate to two or more volatile precursor gases, which react with the surface in a self-limiting manner to deposit thin films in a layer-by-layer growth. A purging of the reaction chamber is required after every exposure of precursors to remove any by-products or unreacted precursors [107,108].

In 2015, a novel fiber-optic Fabry–Perot interferometer for temperature measurement using a thin zinc oxide (ZnO) layer fabricated by ALD was introduced [110]. The 310 nm thick ZnO layer acted as an active medium in the Fabry–Perot interferometer, providing a greater contact area with the surrounding medium and allowing free access to one of the interferometer mirrors, making the sensor more versatile. With the ALD technique, the properties of the ZnO layer, such as thickness, grain size, chemical composition, and surface morphology, can be precisely controlled. This allowed the optical, mechanical, and other fundamental properties of the ZnO layer to be tailored, optimizing the sensor’s performance. The interferometer was characterized in the temperature range of 50–300 °C with a resolution of 1 °C. The output signal was analyzed by measuring the shift in the maxima in the spectral pattern, and the sensor showed a sensitivity of about 0.05 nm/°C with good linearity.

Later, in 2021, the fabrication and characterization of a novel OFS based on a microsphere coated with a thin layer of ZnO were presented [111]. The sensor was fabricated using a standard single-mode optical fiber with a microsphere structure at the end and a 100 nm ZnO layer coated by ALD. The ALD allowed the coating of uniform and conformal thin films on complex structures. The sensor’s performance was investigated over the temperature range of 100–300 °C, with a resolution of 10 °C. An interferometric signal was used to monitor the integrity of the microstructure, while the spectral shift in the reflected signal was used to measure temperature changes. The ALD coating of ZnO exhibited a linear response to changes in temperature, achieving a sensitivity of 0.019 nm/°C.

4.3. Physical Vapor Deposition (PVD)

PVD can be categorized into sputtering, thermal evaporation, and pulse laser deposition (PLD), where the process involves the vaporization of precursor material and deposition on the substrate without any chemical reactions taking place, as shown in Figure 5 [112]. The deposited material is identical to the precursor [113], producing a denser coating with a higher ion content, allowing the deposition of interlayers of more noble metals between the coating and substrate [114], and enabling multilayer coatings with alternating different compositions [115]. The coated nanostructures also exhibit high hardness, high elastic modulus, high fracture strength, oxidation resistance, and thermal resistance [116].

4.3.1. Magnetron Sputtering

The sputtering technique usually utilizes ion bombardment to vaporize the target material and deposit it onto the substrate, offering advantages such as direct application to polymer substrates, high throughput, and the ability to tune defect densities and crystalline domain sizes [112]. Magnetron sputtering is a plasma-based PVD technique utilizing a magnetic field to trap electrons near the target surface, leading to enhanced ionization efficiency [117]. During the process, positive ions from the plasma will be accelerated towards the negatively biased target, causing sputtering and the ejection of target atoms.

An etched FBG coated with a Pd/Ag composite film was successfully carried out via magnetron sputtering in 2013 for hydrogen sensing [118]. The original diameter of the FBG was 125 µm; etching was carried out to reduce the FBG diameter for sensitivity improvement. A sample with a 38 µm and 20.6 µm diameter was prepared by dipping the FBG in HF solution for 37 and 46 min, respectively. All three samples were coated with a 110 nm thick Pd/Ag nanostructure to overcome the hysteretic effect during hydrogen absorption and desorption, which was an issue with previous FBG hydrogen sensors. By controlling the sputtering pressure, power, and deposition rates, magnetron sputtering allowed the deposition of the desired composition and thickness of nanostructures, forming cubic phase nanostructures in an orderly arrangement. When exposed to 4% hydrogen concentration, the wavelength shifts in FBGs with diameters of 125 μm, 38 μm, and 20.6 μm are 8 pm, 23 pm, and 40 pm, respectively, indicating that a significant improvement of sensitivity by up to 400% can be achieved with etched FBG compared to unetched FBG. The FBG hydrogen sensors also demonstrated a linear response for 1.5–4% hydrogen concentration.

In recent years, Mo and Cu targets were used for FBG coating via magnetron sputtering [119]. A two-component gradient structure was sputtered between the adhesive layer Mo and the conductive layer Cu, with Mo selected as the adhesive layer due to its low thermal expansion coefficient and Cu chosen as the conductive layer because of its good conductivity and larger thermal expansion coefficient. The gradient structure reduced the residual stress and thermal stress by continuously changing the composition and eliminating the interface between metal layers. The bare FBG was fixed on a special rotating fixture to achieve a uniform metallic coating on the FBG cladding. The different composition ratios were achieved by controlling the sputtering rate ratio of the two targets, whereas the sputtering rate was controlled by adjusting the sputtering power. The thermal shock resistance test confirmed that the Mo/Cu coating enhanced the thermal stability of FBG, reduced thermal stress by 5.5 kPa, and achieved a temperature sensitivity of 15.27 pm/°C.

Figure 5. Illustration of PVD coating. (a) PLD and (b) sputtering. Author’s own work adapted from [120,121,122,123].

Figure 5. Illustration of PVD coating. (a) PLD and (b) sputtering. Author’s own work adapted from [120,121,122,123].

4.3.2. DC Sputtering

DC sputtering uses a direct-current power source instead of a magnetic field. During DC sputtering, an insulating layer may form on the target surface, leading to charge buildup. Positive pulsed DC power helps discharge the insulating layer. The reactions between target atoms and the positively charged reactive gas cause the target to be ejected and uniformly distributed over the whole substrate surface [124]. Compared to magnetron sputtering, DC sputtering helps mitigate the potential issue of debris generation or arcing that can occur in the unsputtered target area in conventional cylindrical magnetron designs, ensuring a more stable plasma process [125]. However, magnetron sputtering offers several advantages, including higher deposition rates, better target utilization, and improved deposition uniformity [126].

To allow ethanol detection in the pharmaceutical and food industries, the DC sputtering technique was used to coat the etched FBG sensor with a thin layer of gold (Au) in 2024 [127]. For sample preparation, the FBG cladding was immersed in HF acid for etching, and the etched fiber was immersed in deionized water for 5 min to stop the etching process and remove the residual contamination of HF acid. From the FESEM image, the etched FBG was observed to have a smooth and uniform surface without cracks or distortions. The smooth surface improved the uniformity and consistency of the Au coating. The pure Au target was placed in the target holder and coated over the etched FBG. The sputter coater settings were 0.1 mbar gas pressure and 50 mA sputter current, and the FBG was rotated at 180° to have a uniform coating over the sensing region. To achieve the coating thickness of 45 nm, the sputtering time was adjusted accordingly. This Au coating serves as the sensing layer to improve the evanescent field around the fiber, resulting in better interaction with the ethanol. It also added stiffness to the etched sensor, reducing the risk of bending, which could potentially cause unwanted wavelength shifts. The sensor was tested by exposing it to different ethanol concentrations in aqueous solutions, from 0% to 100%. A sensitivity of 20 pm/RIU was reported from the experiment.

4.3.3. RF Sputtering

In RF sputtering, a high-frequency alternating current is applied between the target and the substrate, generating an oscillating electric field that ionizes the sputtering gas and accelerates the ions toward the target, allowing the sputtering process to occur at lower pressure [126,128], and enabling the coating of insulating or low-conducting materials by overcoming the charge buildup on the target surface [129]. Compared to DC sputtering, the power consumed for depositing ceramic or insulator materials is much lower in RF sputtering, but the deposition rate is also lower [130].

A refractometer was proposed in recent years based on Ge-Sb-Se-Te (GSST)-coated tilted FBG for measuring the transformer oil of high-voltage power transformers [131]. The GSST with a thickness of 130 nm was coated on the TFBG via RF sputtering with double-sided deposition; the deposition thickness was controlled based on the deposition duration. The GSST, which has a higher refractive index than silica, increased the effective refractive index of the cladding modes and created two groups of cladding mode resonances with different polarization dependency and sensitivity to the surrounding refractive index, which allowed the sensor to have an extended refractive index detection range beyond the cut-off point of the cladding modes. A good linear sensitivity of 9.952 dB/RIU for the refractive index range of 1.472–1.484 RIU was reported when tested with transformer oil samples.

4.3.4. Pulse Laser Deposition (PLD)

Pulsed laser deposition (PLD) is another type of PVD in which a laser with a high-power density and narrow frequency bandwidth is used as a source for vaporizing the desired material [132]. In PLD, targets were irradiated and vaporized in a vacuum environment with the pulsed laser. The excess energy from the laser pulse breaks down the local bonds at the irradiated zone, ejecting particles from the target surface in a process known as “ablation” [133]. The ejected particles interact with the laser pulse and form a plasma plume, which then deposits onto a nearby substrate. PLD is better suited for electrical insulators, as the higher surface reflectivity and thermal conductivity of metals and other conductors can result in less energy absorption and rapid heat dissipation [112,134]. It is also an effective technology for depositing doped and multilayer coatings [120].

In 2022, graphene-coated tilted FBGs with improved sensitivity to external environmental changes, particularly strain, were developed [121]. PLD was used to coat graphene on the surface of the sensor. The coated FBG was subjected to axial tension, and an increase in strain sensitivity from 0.3 nm/mε to 0.48 nm/mε was observed. It was also reported that the coated sensor had almost the same temperature sensitivity as the bare sensor.

Another experiment carried out in 2024 suggested coating titanium dioxide (TiO₂) on plastic OFSs with PLD [135]. The PLD technique allows the coating of sensing material directly on the optical fiber surface, ensuring strong adhesion and preventing delamination, which is crucial for the sensor’s stability and reliability. The surface morphology and chemical composition of the coated sensor were analyzed, and the results showed that the TiO₂ coating had a porous and granular structure with an average grain size of 20–30 nm, providing a high surface area for enhanced sensing performance. The optical absorption spectra of the TiO₂ nanostructures indicated that a denser nanostructure was obtained through PLD. The differences in size and morphology of the TiO₂ nanostructures are highly dependent on the properties of the produced plasma on the target’s surface. However, the optical properties can be tuned by adjusting parameters such as laser energy and pulse count, allowing the optimization of the sensor’s response to the target analyte. When the sensor was tested using a chromium chloride solution, the wavelength shift was reported at a maximum sensitivity of 54.38 nm/RIU. The study also reported that increasing laser energy and pulse count can increase the coating thickness; the sensitivity decreased to 8.76 nm/RIU as the coating thickness increased.

4.3.5. Thermal Evaporation

Thermal evaporation with low to moderate deposition rates is regarded as one of the conventional and commonly used PVD techniques [136]. The process requires the target materials to be heated to a certain temperature. Thermal energy causes the atoms or molecules to break free from the surface of the material, transitioning into the gaseous state [137]. The low pressure of the chamber ensures vapor transport from the source to the substrate without collisions. The vaporized atoms, or molecules, condense on the cooler substrate for nucleation and growth. The formation of different nanostructure morphologies can be attributed to the change in ambient flow rate inside the chamber, which determines the nucleation and crystal growth rates [135]. Therefore, precise control of the flow rate is required to tailor the nanostructure for different applications.

An experimental study on a SPR-based fiber optic with a thin ZnO layer was conducted in 2013 for gas sensing [138]. The ZnO layer was deposited on top of the copper (Cu) layer via the thermal evaporation technique to support surface plasmons at the metal–dielectric interface. This coating technique enabled the deposition of metal oxide nanostructures on the unclad portion of the optical fiber at room temperature. The unclad portion allowed the evanescent field of the light propagating through the fiber to interact with the coatings deposited on the fiber surface, enhancing the excitation of surface plasmons and the sensing of the target gas. In addition, varying nanostructure thicknesses were made available by the coating technique for sensor performance optimization. The sensor was exposed to concentrations of hydrogen sulfide gas from 0 to 100 ppm. The wavelength interrogation mode was used to characterize the sensor by launching an unpolarized light source from one end of the fiber. The corresponding SPR spectrum showed a shift in the resonance wavelength with a change in gas concentration. At lower hydrogen concentrations, the sensor possesses a maximum sensitivity of around 0.65 nm/ppm with a ZnO thickness of 10 nm.

In 2023, aluminum (Al) SPR sensors prepared via thermal evaporation were introduced to achieve a high-purity Al nanostructure [139]. The effects of different vacuum levels and deposition rates on the quality of Al thin film were examined by measuring the SPR curves under different NaCl solution concentrations. The resonance angle, reflectance at the resonance angle, and full width at the half-maximum (FWHM) of each SPR curve were also analyzed. A sensitivity of 149.9 °/RIU was reported, and the study concluded that a high vacuum level (1 × 10⁻⁴ Pa) is mandatory to prevent oxidation of the Al film by the residual gas during the coating process. A low vacuum level (7 × 10⁻⁴ Pa) and low deposition rate can result in a high-quality Al nanostructure with low surface roughness.

4.4. Dip Coating

Dip coating is a widely used technique to deposit coatings on various substrates. The process starts with immersing the substrate in the coating solution for nanostructure deposition. As the substrate is withdrawn from the solution, the solution evaporates, and nanostructure is formed on the surface as shown in Figure 6 [140]. The coating thickness, surface morphology, and wetting properties of the deposited nanostructure are influenced by various factors, such as the dipping length, withdrawal speed, solution viscosity, surface tension, and substrate characteristics [141,142]. Multilayered dip coating can be used to increase the thickness or improve the uniformity of the coating and offers the customization of functionalities to enable different sensor applications [143]. This technique also allows the coating of different materials in successive layers for improved electrochemical performance. In addition, the self-centering effect of dip coating can improve sensor performance by ensuring a uniform cladding thickness.

In 2010, the fabrication and characterization of a humidity sensor based on an optical fiber coated with indium tin oxide (ITO) with a dip coating technique was presented [144]. The cladding of the OFS was removed in the experiment to allow the deposition of a 115 nm thick ITO coating directly onto the exposed optical fiber core. The ITO coating acted as the resonance supporting layer for the humidity sensor, enabling resonance in the infrared region. The coating process consisted of a 10-layer dip coating with a constant withdrawal speed of 4 cm/s. After coating each layer, a 1 h annealing process at 550 °C was performed, followed by a 3 h annealing step after the coating of the final layer. The dip coating process enables precise control of the coating thickness, which is essential for tuning the sensor to operate with maximum sensitivity, while the annealing process improves the quality and properties of the ITO nanostructure. The dynamic response of the sensor with an RH range of 20% to 90% was obtained, with a reported sensitivity of 1.2% RH/nm, exhibiting a fast response and good repeatability of the sensor.

Later, in 2021, a SPR-based relative humidity (RH) sensor fabricated with a polymer OFS was developed for human breath monitoring [145]. The SPR-based sensor was coated with a gold surface; the sensor was then cleaned and immersed in a prepared polyvinyl alcohol (PVA) solution for a single dip coating on top of the gold layer. The experiment varied the PVA concentration from 0.5% to 2% to study the correlation between coating thickness and sensitivity. Upon drying, a homogeneous PVA nanostructure was observed on the sensor surface from the SEM image. The sensor was exposed to an RH range of 40% to 90%, and an average sensitivity of 4.98 nm/%RH was reported. The sensor prepared with a 2% PVA concentration and a higher coating thickness exhibited the highest sensitivity compared to other samples prepared with a lower concentration. The PVA film deposited using the dip coating technique showed excellent hygroscopic expansion characteristics and swelled significantly during moisture absorption. The swelling effect resulted in a redshift in the resonance wavelength when humidity increased, contributing to the high sensitivity of the sensor. A linear response between 75% and 90% RH with a sensitivity of 10.15 nm/% RH was reported, demonstrating the high stability of the sensor. The response of the sensor to human breathing was also tested; wavelength shifts of approximately 228 nm with response and recovery times of around 0.44 s and 0.86 s, respectively, were recorded.

Figure 6. Illustration of dip coating. Author’s own work adapted from [146].

Figure 6. Illustration of dip coating. Author’s own work adapted from [146].

4.5. Drop Casting

Drop casting is an ultra-low-cost and scalable technique, as illustrated in Figure 7. To allow deposition, droplets of solution will be released onto a substrate with a pipette with high positioning accuracy. Samples will then be dried and subjected to annealing to complete evaporation of the solution and achieve controllable crystallinity [147,148]. The multilayer drop coating approach allows the buildup of the desired coating thickness and improves the optical properties and sensitivity of sensors [149].

An SPR-based single-mode FBG was coated with a thin gold (Au) and graphene oxide (GO) nanostructure in 2016 for a sensitivity enhancement of ethanol sensing [150]. The Au layer was coated via the sputtering technique, while the GO layer was coated with drop casting, followed by 30 min of annealing at 70 °C. With these combined techniques, a 45 nm thick Au layer and a 400 nm thick GO layer were observed from the FESEM images. The large surface area of the GO layer improved the interaction with the ethanol analyte, leading to improved sensor sensitivity and accuracy. When exposed to different concentrations of ethanol, the Au-coated sensor was reported to have a sensitivity of around 200 nm/RIU, whereas the sensor with an additional GO layer demonstrated a sensitivity 2.5 times higher than the Au-coated sensor, which is approximately 500 nm/RIU.

In recent years, coating long-period grating optical fiber with silk fibroin as a chemo-sensing transducer for methanol vapor was introduced [151]. The aqueous solution of silk fibroin was drop cast on the grating region to form a 400 nm thick nanostructure. The coating enabled the sensor to respond to different pressures of methanol vapor. A wavelength shift was observed with increasing pressure, reaching a maximum 4 nm shift at 100 mbar. The sensor was reported with a sensitivity of 0.22 nm/mbar in the range of 80 to 100 mbar.

5. High-Precision Optical Fiber Sensing with Nanostructure Coating

5.1. Key Coating Parameters

In CVD, coatings deposited at different chamber flow rates and chamber pressure impact the deposition rate of the nanostructure, leading to variation in surface morphology [97]. The coating temperature is another critical parameter that influences the crystallinity and optical properties of the deposited nanostructure, whereas the RF power is a key parameter that strongly influences the optical band gap [100,101,152]. An increase in crystallinity that affects the optical properties, such as the optical band gap, can also occur by increasing the post-coating annealing temperature [101].

The ambient gas use during the ALD process can influence the composition, crystallinity, and properties of the deposited coatings, whereas the substrate temperature can influence the thickness and energy band gap of the deposited nanostructure [153]. In addition, the residual stress, elastic modulus, hardness, and adhesion of the deposited coatings are dependent on coating temperature, with higher temperatures enabling the use of precursors that require higher temperatures to react.

The ambient gas concentration and chamber pressure of sputtering affect the optical properties of the deposited nanostructure [154]. The applied sputtering power during the coating process affects the reflectance and surface morphology of the nanostructure, where higher sputtering power generally results in coatings with higher reflectance and smoother surfaces compared to lower power. The thickness of the nanostructure coating can be varied by changing the sputtering time, whereas the combination of the sputtering parameters, such as the sputtering current, gas pressure, and rotation of the OFS, determines the thickness and uniformity of the nanostructure coating on the sensor [127]. The crystallinity of the nanostructure improved with the increase in substrate temperature and an increase in grain size [155].

In PLD, the laser fluence, or the energy per unit area of the laser pulse, can impact the ablation rate and composition of the deposited nanostructure [133,156]. Variations in the laser fluence can lead to non-uniformity of the nanostructure and can affect the overall quality of the nanostructure. Pressure in the PLD chamber can influence the plume dynamics and the kinetic energy of the ablated species, affecting the microstructure and properties of the deposited nanostructure. The deposition rate, which is influenced by the laser parameters and the target–substrate distance, also has an influence on the microstructure and properties of the deposited nanostructure. The substrate temperature can affect the gas density and the kinetics of nanostructure formation, impacting the final thickness, composition, crystallinity, and adhesion of the nanostructure to the substrate [133,157]. Additionally, the surface condition of the target material, such as its microstructure and roughness, can impact the particulate generation during the ablation process, leading to an impact on the morphology and uniformity of the deposited nanostructure.

The deposition rate, which is controlled by the evaporation power of thermal evaporation, can affect the surface morphology, composition, and structure [139]. The temperature of the substrate during the coating process can influence the growth, crystallinity, and adhesion of the nanostructure. The vacuum level or chamber pressure can impact the mean free path of the evaporated species, affecting the uniformity and purity of the nanostructure [137]. The surface condition of the substrate can influence the nucleation and growth of the deposited film. The coating time or duration can determine the final film thickness and properties. In addition, rotating the substrate during deposition can improve the uniformity and coverage of the coating [158].

In dip coating, the withdrawal speed of the substrate from the coating solution has a significant impact on the thickness and surface morphology of the deposited nanostructure [143]. The concentration of the coating solution affects the viscosity and density of the solution, leading to changes in the surface energy and wetting properties of the nanostructure. Higher solution concentrations or higher viscosity solutions generally result in thicker deposited films for better sensing performance, as it slows the drainage of the coating solution [140,159]. The surface tension of the coating solution affects the capillary forces that drive the coating process. The temperature of the coating solution can impact the evaporation rate and capillary flow, which also affects the final coating thickness. The duration of the dip coating process can influence the final coating thickness. Longer coating times tend to produce thicker deposited nanostructures as more material is accumulated on the substrate surface [160]. The properties or conditions of the substrate, such as porosity and surface roughness, can also affect the coating outcome and the interaction between the coating solution and the substrate.

The concentration of the coating solution can affect the uniformity, thickness, and optical properties of the nanostructure deposited by drop casting, where a higher concentration can lead to more even coverage and higher plane spacing of the nanostructure [150]. Careful control of speed and angle of dropping the solution can increase the uniformity of the coating. The drying conditions, such as temperature and humidity, can also influence the uniformity of the deposited layer. Controlled drying can help prevent the formation of cracks or uneven drying patterns. Additionally, the surface properties of the substrate, such as wettability and surface roughness, can impact the uniformity and surface morphology of the drop-cast layer [161].

Table 2. Comparison of coating techniques for OFS technology advancement.

	PVD	ALD	Dip Coating	Drop Casting
Cost	Cost-effective technique	Low cost	Ultra-low cost	Ultra-low cost
Scalability	Can be scaled up for large-scale industry production except for PLD, which is only suitable for small-scale substrates	Can be scaled up for larger substrate surface, allowing multiple fibers to be coated simultaneously	Can be scaled up for larger substrate surface, allowing multiple fibers to be coated simultaneously	Can be scaled up for larger substrate surface
Complexity	Highly mature coating technique, simple and straightforward except PLD, which requires complex setup	Simple and reproducible process	Straightforward mechanism	Straightforward mechanism
Uniformity of Nanostructure	Excellent thickness and uniformity, precise control over composition	High conformity, high uniformity, and large substrate coverage	High uniformity achievable	High uniformity achievable
Deposition Rate	Fast deposition rate	Slow deposition rate	Fast deposition rate but repeated process required for multilayer coating	Fast deposition rate
Technology Maturity	Mature technology	Mature technology	Existing technology	Existing technology
Advancement in OFS	High-precision and uniform coating for coating thickness control	Offers layer-by-layer deposition, suitable for multilayer coating	Varying coating parameters to create specified surface morphology and wetting properties	Enables tailoring of the optical properties and interlayer spacing

provides a comparison between recommended coating techniques and the contributions of these techniques toward high-precision OFSs, whereas

Table 3. Key parameters of coating techniques.

Coating Techniques	Key Process Parameters	Impact on Nanostructure
CVD	(1) Chamber flow rate, chamber pressure (2) Coating temperature (3) RF power	(1) Surface morphology (2) Crystallinity, optical properties (3) Optical properties
ALD	(1) Ambient gas (2) Substrate temperature (3) Coating temperature	(1) Composition, crystallinity (2) Thickness, energy band gap (3) Residual stress, elastic modulus, hardness, adhesion to substrate
Sputtering	(1) Ambient gas concentration, chamber pressure (2) Sputtering power (3) Sputtering time (4) Sputtering current, gas pressure, rotation of sensor (5) Substrate temperature	(1) Optical properties (2) Reflectance, surface morphology (3) Thickness (4) Thickness, uniformity (5) Crystallinity
PLD	(1) Laser fluence (2) Deposition pressure, deposition rate (3) Target surface condition (4) Substrate temperature	(1) Uniformity (2) Microstructure (3) Surface morphology, uniformity (4) Thickness, composition, crystallinity, adhesion to substrate
Thermal Evaporation	(1) Deposition rate (2) Coating temperature (3) Chamber pressure (4) Substrate condition, temperature (5) Coating time (6) Rotation of substrate	(1) Surface morphology, composition, structure (2) Nanostructure growth, crystallinity, adhesion to substrate (3) Uniformity, purity (4) Nucleation, nanostructure growth, crystallinity (5) Thickness, nanostructure properties (6) Uniformity, coverage
Dip Coating	(1) Withdrawal speed, coating duration (2) Concentration, viscosity, surface tension, temperature of coating solution	(1) Thickness, surface morphology (2) Thickness, surface energy, wetting properties
Drop Casting	(1) Concentration of coating solution (2) Substrate condition and properties (3) Deposition speed and angle (4) Drying condition	(1) Uniformity, thickness, optical properties, interlayer spacing (2) Uniformity, surface morphology, adhesion to substrate (3) Uniformity, thickness (4) Formation of defects

summarizes the key parameters of each coating process and the impact of the parameters on the coated nanostructure.

5.2. Advancement of Optical Fiber Sensing

In recent years, advancements in OFS technologies have focused on improving sensing performance, adaptability for extreme environments, and advanced sensing for a wide range of measurands and parameters [31,162]. OFSs based on nanomaterials have been widely developed for various applications, including structural health monitoring of concrete structures [163], real-time tracking of vital signs and health parameters [164], and monitoring the life cycle of aerospace composite structures [165]. One of the strategies to enhance sensor performance and expand applications is the integration of nanomaterials by optimizing the nanostructure, coating thickness, and coating layer [166,167,168].

5.2.1. Modification of Dimensional Nanostructure

The application of various 0D, 1D, 2D, and 3D nanomaterials in different sensing principles enables the development of OFS technologies. One-dimensional nanostructures improve the electrical properties, and semiconductor nanowires with controlled electrical properties can be altered to offer electronically tunable nanoscale components [169]. Carbon nanotubes provide a large number of active sites to promote ionic reactions and shorten the ion transfer distance [170]. Metallic nanorods are highly anisotropic, enhancing electrical conductivity along the axial axis of the nanostructures [171]. Two-dimensional nanostructure provides high electron transfer efficiency, in which transition metal dichalcogenides (TMDs) showed high compatibility with any substrates due to the van der Waals (vdW) force bonding character [172]. The growth of high-quality, relaxed 2D layers of TMDS for large-area synthesis of vdW heterostructures is possible as the thermal and lattice mismatches between TMDs and substrates are insignificant [173].

Three-dimensional nanostructures enhance mechanical strength, where the porosity of nanoporous structures such as zeolite and metal–organic frameworks allow for tunable pore sizes, facilitating charge transfer [174,175]. The high surface areas enhance the catalytic activity and provide high thermal stability and conductivity of graphene to enable efficient heat management [175,176]. The nanocomposite is another 3D nanostructure that can be categorized into metal matrix, ceramic matrix, and polymer matrix nanocomposites [177,178]. Ceramic matrix nanocomposites improve mechanical properties such as durability and strength; metal matrix nanocomposites have high ductility and toughness, whereas polymer matrix nanocomposites provide thermal stability [179].

5.2.2. Optimization of Coating Thickness and Coating Layer

The performance and properties of the OFS can be optimized by fine-tuning the coating thickness and total layers of coating. Thinner coatings enhance the interaction between molecules of the target analyte and the sensor, improving the response times of the sensor. However, a thin layer of coating limits the interaction of the evanescent field with the coating, impacting the sensitivity of the sensor [167]. With the increase in coating thickness, the crystallization of nanostructures can be further improved. A higher crystallinity of the thicker nanostructure led to a higher band gap and lower defect concentration, which is important for optoelectronic applications in the IR and mid-IR regions [180].

The multilayer coating provides good adhesion between the layers of the fiber, facilitates the fabrication of highly controllable nanostructures on substrates of various sizes or shapes, enables the customizable composition at the molecular level, and allows the incorporation of various nanomaterials to further enhance the functionality and performance of OFSs [181,182]. Incorporating a wide range of nanomaterials into the multilayer structure led to the development of advanced functional coatings for sensing applications, which change the sensing properties in response to the analyte, enabling wavelength-dependent sensing with higher robustness to perturbations and improving the response time, selectivity, and sensitivity of optical fiber sensors compared to single-layer coated sensors [168,183].

6. Recommendations and Future Works

This paper reviews the challenges and limitations faced by OFSs and suggests coating the sensing region with nanostructures as one of the approaches for sensing performance improvement. Coating techniques such as PVD, ALD, and dip coating are recommended as effective approaches for nanostructure deposition on OFSs. PVD, including sputtering and thermal evaporation, are highly mature techniques that produce high-quality nanostructures with precise control over the composition and thickness. RF sputtering might incur a slower deposition rate compared to other PVD techniques, but it allows the deposition of insulators, whereas other PVD techniques only allow the deposition of conductors or semiconductors. It is challenging to achieve large-area uniform deposition with PLD, but this technique produces high-quality coatings for small-scale substrates.

ALD is a coating technique with good repeatability that provides a layer-by-layer coating with precise conformity. Further exploration is crucial for their potential to provide atomic-level composition control in coating applications. Dip coating and drop casting are both extremely low-cost techniques with a simple equipment setup that is suitable for small-scale substrates and small sample size experiments. To allow large-scale industry production, further improvements to achieve real-time monitoring and automation are necessary for better control of parameters such as dipping length, withdrawal speed, and solution concentration for dip coating and dropping speed, dropping angle, and drying conditions for drop casting [184].

The etching approach has also been examined, as the experimental results have demonstrated the capability to enhance the interaction of the evanescent field with the external environment and aid in the precision control of the surface morphology. Combining the etching with the right coating technique allows the deposition of high-quality nanostructures that enhance sensitivity, accuracy, and repeatability.

For further optimization of optical fiber sensing, multilayer coatings with different materials, such as catalytic materials with excellent electrocatalytic performance, improve the interaction between the sensor and the analyte [185], leading to increased sensitivity of the sensor. High-temperature environments could lead to the potential performance degradation of uncoated OFSs; therefore, it is crucial to explore low-pressure and low-temperature coating techniques such as low-pressure CVD, plasma-enhanced CVD, and atomic layer deposition (ALD) to reduce thermal expansion issues without compromising the uniformity and quality of the coated nanostructure [186,187].

From past research, increasing the surface area of nanostructures improves the interaction of the sensor with the target analyte and leads to improved accuracy, sensitivity, and thermal stability. Thus, further research is necessary to optimize the surface morphology and increase the surface area for the availability of reaction sites on the substrate surface [188]. The incorporation of additives can be one of the solutions to achieve a higher surface area, adjust the catalytic activity, and increase the electron exchange rate, altering the physical/chemical properties of nanostructures to improve the sensor interaction with the analyte [189]. The advancements in coating and etching techniques for the deposition of nanostructures [190] are crucial to achieving high-precision optical fiber sensing applications. By achieving the desired uniformity of nanostructure, reducing the complexity of the deposition process, and increasing the consistency of coating quality, enhanced sensor performance can be achieved.