Fiber Grating Hydrogen Sensor: Progress, Challenge and Prospect

Hydrogen is a kind of clean and sustainable energy source and arises wide attention in industrial and commercial scenarios. The utilization of hydrogen in safety is eternal theme, thereby it is important to detect hydrogen quickly and accurately in production, use and storage stage. This review focuses on the fiber grating hydrogen sensor, which depicts a great promising for industrial application, introduces the up‐to‐date progress and provides new insight for scale use. Starting with an overview of the sensing mechanism of hydrogen‐sensitive material, then this paper briefly introduces the working principle of fiber grating hydrogen sensor. After that, design and optimization methods for fiber grating hydrogen sensor are emphatically summarizes from viewpoints of sensor structure, hydrogen‐sensitive material, and sensor durability. Last, the challenges of fiber grating hydrogen sensor for practical applications are demonstrated and the future prospects are also put forward, namely the depth investigation in structure, material, key fabrication technology and environmental adaptability of sensor.


Introduction
Hydrogen energy, solar energy and nuclear energy are deemed as the three major new energy in the new century, which possesses a wide application in variable scenes including automotive fuel, aerospace power, etc. [1] In addition, hydrogen plays the irreplaceable role in petrochemical industry (ammonia, methanol and et als), metallurgy, soldering, electroplate, medicine and the manufacturing industry of rocket fuel because of the great reducibility. [2]However, hydrogen possesses the high diffusion coefficient (0.16 cm 2 s −1 ) and combustion heat (285.8kJ mol −1 ), low ignition energy (0.018 mJ), wide explosion concentration range (4%−75%), belonging to the flammable and explosive gas, DOI: 10.1002/adsr.202300088   which induces the problem of leakage and explosion during production, use and storage.For example, the three Mile Island accident happened in 1979 [3] and the Fukushima accident happened in 2011 [4] results in serious consequences.Therefore, it is important to supervise concentration of hydrogen in production, use and storage stage.
Nowadays, the popular hydrogen sensors include the catalytic combustiontype, [5,6] thermal conductivitytype, [7,8] electrochemical-type, [9,10] and semiconductor-type, [11,12] etc.These kinds of sensors are classified into the electrical sensor, and part of sensors have low gas selectivity.Optical fiber hydrogen sensor has attracted wide attention due to the characteristics of intrinsic safety, high stability, small size, anti-electromagnetic interference and easy networking, and illustrates important scientific significance and application value.
In principle, the hydrogen-sensitive material deposited on optical fiber could react with hydrogen, which leads to detectable changes of optical signals.The concentration of hydrogen can be identified by calculating the variation in intensity, phase, wavelength and polarization of optical signals.Optical fiber hydrogen sensors are mainly divided into micro mirror-type, [13] evanescent field -type, [14] interferometer-type, [15] and fiber Bragg grating (FBG)-type, [16] etc. Micro mirror-type optical fiber hydrogen sensor possesses simple structure and manufacturing technology, low cost and convenient using.Micro-mirror sensor classified into light intensity sensor is merely suitable to point measurement because of the sensibility to optical path system, and the reuse ability is greatly limited.Evanescent field hydrogen sensor is sensitive to hydrogen accompanied by a quick response speed, while the research is still in lab-scale because of complex manufacturing technology and huge attenuation after series.Interferometer-based hydrogen sensor is sensitive to external environmental factors (temperature), which leads to a low-test accuracy.Fiber grating hydrogen sensor is one of the most promising optical fiber hydrogen sensors.On the one part, fiber grating hydrogen sensor has large capacity detection capability.It is possible to realize the multipoint detection by writing FBG with different wavelengths onto a single fiber.On the other hand, FBG is an excellent passive optical device for temperature measurement.Using reference grating for temperature compensation can greatly reduce the influence of temperature on measurement accuracy.At present, some FBG-type optical fiber hydrogen sensors have been able to achieve high sensitivity, high precision and wide range of hydrogen concentration detection, but most of them are in the experimental platform and have not yet entered the practical application stage.The possible reason is that the sensor manufacturing process and environmental adaptability of the sensor have not yet met the requirements of mass production and practical application, and it is necessary to achieve breakthroughs in the key process and environmental adaptability of sensor.
In this work, we focus on the most promising fiber grating hydrogen sensor and introduce the progress of development, providing some new ideas for its practical application.The organizational framework of this work is displayed as follows: in second part, sensing mechanism of hydrogen-sensitive material is introduced.In third part, working principle of fiber grating hydrogen sensor is presented.In fourth part, design and optimization methods for fiber grating hydrogen sensor are summarized from sensor structure, hydrogen-sensitive material and sensor durability perspective.In fifth part, the challenges of sensor structure, sensing material, key manufacturing process and environmental adaptability of sensor in practical application are analyzed, and the future prospects are also presented.In the sixth part, we draw a brief conclusion.

Hydrogen-Sensitive Materials
The basic method for hydrogen sensing technology is the use of materials that can react with hydrogen because the spectral absorption method cannot detect hydrogen molecules.Hydrogensensitive materials are essential for preparing hydrogen sensors.The main developing trends of fiber grating hydrogen sensors are based on two kinds of sensitive materials, the first kind is palladium (Pd)-based sensitive materials which can produce volume expansion or optical constant change during hydrogen response, the other is tungsten oxide (WO 3 )-based sensitive materials performing exothermic or aerochromic reactions in the hydrogen atmosphere.

Pd-Based Sensitive Materials
Palladium is known for its high affinity with hydrogen, making it a suitable material for hydrogen storage and sensing applications.As a hydrogen reactant, it can absorb 900 times its equivalent volume of H 2 at room temperature. [17]When hydrogen appears near the Pd samples, absorption of hydrogen by the Pd structure is observed.First H 2 gas molecules move to the Pd surface, where they interact with Pd atoms through van der Waals forces.The potential energy of gas molecules shows a minimum at a distance of about a molecular radius, resulting in the adsorption of H 2 gas on the metal surface. [18]The adsorbed gas molecules are then dissociated into H atoms and diffused in metal structures. [18,19]The absorption of hydrogen by palladium is exothermic and follows the reversible equation (1) at equilibrium. [20]+ From a physical point of view, the permittivity of the metal can be changed by inserting hydrogen atoms into the metal lattice and performing corresponding volume expansion.Specifically, after the formation of hydride (PdH x ), the volumetric density of free electrons decreases, resulting in the variation of Pd permittivity according to the Drood model, which can be expressed as: where  Pd (c%) is the permittivity of Pd sample with a hydrogen concentration of c%,  Pd (0%) is the permittivity of pristine Pd sample in the absence of hydrogen, and h (c%) is a coefficient nonlinearly related to c%.It takes values of h (c%) are 1.0 and 0.8 for hydrogen concentration of 0% and 4%, respectively. [21]y coating Pd-based sensitive materials on fiber surface, the variation of hydrogen concentration can alter the volume and permittivity of Pd, which will cause changes in the optical signal in terms of wavelength, intensity, wavelength, or phase depending on the sensor configurations.By monitoring the change of optical signals to infer changes in hydrogen concentration. [22]2.WO 3 -Based Sensitive Materials WO 3 and hydrogen can chemically interact, causing the color of WO 3 to change from greenish yellow to blue.This gasochromic effect allows WO 3 to detect hydrogen.In addition, when WO 3 is exposed to hydrogen, its optical properties, such as transmittance, reflectance, absorption and refractive index are modulated, therefore, WO 3 is an ideal candidate for hydrogen detection.[23] However, because WO 3 can also chemically interact with other gases, such as acetylene and hydrogen sulfide and acetylene, WO 3 materials are not selectively sensitive to hydrogen, which limits their use for hydrogen sensors.In order to solve this problem, a common approach is to modify WO 3 by doping metal catalysts, such as platinum (Pt), Pd, and Aurum (Au), which can dissociate hydrogen molecules into atoms, thus reducing the activation energy of the reaction.[24,25] At present, platinum (Pt) is most commonly used as catalyst, [26,27] the chemical reaction between Pt-loaded WO 3 and surrounding hydrogen can be expressed as the following equation: [28] Pt-loaded WO 3 can drastically react with hydrogen and continuously generate heat, by indirectly measuring the variation of temperature caused by the exothermic reaction, hydrogen concentration can be calculated.Conversely, when the hydrogen concentration is reduced, the intermediate WO 3-x will be oxidized to reform WO 3 .Therefore, based on above reactions, WO 3 -based hydrogen sensors can be used for repeated measurements.

Working Principle
Fiber grating hydrogen sensors are based on the Bragg grating with a designed periodic structure, and monitor the central wave- length shift of the reflected Bragg spectrum through the broadband optical signal entering the fiber.There are two types of Fiber grating hydrogen sensors according to the difference of modulation period, one is FBG with a period smaller than 1 μm, the other is called long period fiber grating (LPFG) featuring in a period of 100-1000 μm (Figure 1).

FBG-Based Hydrogen Sensor
The sensing mechanism of the FBG-based hydrogen sensor depends on the variation of strain or temperature caused by the chemical interaction between sensitive materials and hydrogen to change the Bragg wavelength.According to the grating equation, the relationship between the reflected wavelength ( B ), effective refractive index (n eff ), and grating period (Ʌ) can be expressed as: When the sensitive film is coated on the fiber Bragg grating, the volume expansion of Pd-based sensitive material or exothermic reaction of WO 3 -based sensitive material under the exposure of hydrogen will cause the strain or temperature change of the underlying fiber Bragg grating, which then change the n eff and Ʌ of the FBG, finally resulting in the Bragg wavelength shift of the FBG, as shown in Figure 1a.Therefore, the hydrogen concentration can be detected by measuring the wavelength shift of FBG.The wavelength shift can be denoted as: The first FBG-based hydrogen sensor was reported by Sutapun et al. in 1999. [29]The typical structure of the sensor is shown in Figure 1a, FBG with cladding of 35 μm was evaporated with 560 nm of pure Pd film for hydrogen characterization.The stress in the Pd coating stretched and shifted the Bragg wavelength of the FBG.Results demonstrate that the sensor showed linear wavelength shift when the hydrogen concentration (in N 2 atmosphere) ranged from 0.3% to 1.8% (in volume), with sensitivity of 1.95×10 −2 nm % −1 .When the hydrogen concentration was over 1.8%, the hydrogen sensor deteriorated and became irreversible, mainly due to the poor stability of pure Pd film.However, due to the significant difference in volume between the sensitive film and the optical fiber, the induced wavelength shift of FBG is relatively small, which results in a relatively low measurement sensitivity of hydrogen concentration.Therefore, since then, a number of methods have been proposed to improve the measurement sensitivity of FBG-based hydrogen sensors.

LPFG-Based Hydrogen Sensor
In LPFG sensors, the essence of the sensing process is the interaction between the cladding modes and the evanescent waves at the cladding/sensing film interface, so the output spectrum presents multi-mode characteristics, as shown in Figure 1b.The wavelength shift of LPFG is mainly caused by the change of effective refractive index, which can be written as: The interaction between the sensitive film and hydrogen will change the effective index difference between the core mode and the cladding mode.Finally, the variation of resonance wavelength in the LPG transmission spectrum can be used as an effective parameter of hydrogen concentration sensing.
In 2006, A. Trouillet et al. [30] proposed the first LPG-based hydrogen sensor which departed from a long period grating coated with a 50 nm-thick Pd film on one side of the fiber, through a 3 cm length mask centered on the grating.When hydrogen concentration is 4% in volume ratio, a wavelength shift of 5 and 7 nm are obtained for the two wavelength resonance peaks at room temperature for the long-period grating, the sensitivity is enhanced by a factor up to 500 compared to the FBG-based sensor with similar structure.Latter in 2010, Y. H. Kin et al. [31] used highorder cladding modes which are quite sensitive to the external refractive index change for enhancing the spectral shift.The LPG has 400 μm of grating period and 40 mm of grating length, a hydrogen-induced spectral shift of 7.5 nm is achieved when exposed to 4% hydrogen.

Design and Optimization
Fiber grating hydrogen sensors have been widely studied in recent years due to their many advantages.In response to the practical needs of optical fiber hydrogen sensors, researchers have proposed many methods to design and optimize fiber grating sensors, and the following mainly introduces the research carried out in three aspects: sensor structure, hydrogen-sensitive material, and sensor durability.

Modification of the Sensor Structure
Strain can be directly measured by using FBG, however, due to the significant difference in volume between the sensitive film with a thickness of tens to hundreds of nanometers and the untreated optical fiber with diameter of 125 μm, the induced wavelength shift of FBG is relatively small, which results in a relatively low measurement sensitivity of hydrogen concentration.Therefore, many methods have been proposed to improve the measurement sensitivity of FBG-based hydrogen sensor, modification of the sensor structure is a widely used strategy to enhance the sensitivity.In the FBG sensors, the deformation of FBG can be magnified by reducing the fiber diameter, which enhances the stress generated from the sensing film expansion.The main processing techniques include side polishing, [32][33][34] etching, [35][36][37] grooving, [38][39][40] and tapering [41,42] (Figure 2).
The first one, perhaps the most common, is side-polished FBG, as shown in Figure 2a.In 2009, K. Schroeder et al. [32] demonstrated a side-polished FGB sensor coated with a Pd film, based on the refractive index change through evanescent wave interaction instead of the deformation of FBG.Compared with traditional FBG sensors, the sensor showed an improved sensitivity of 33 pm/% at hydrogen concentration below 4%.In 2011, J. Dai et al. [33] developed a side-polished FBG hydrogen sensor with WO 3 -Pd composite film of 110 nm.For polishing depth of 59 μm, when hydrogen concentrations are 4% and 8% in volume percentage, maximum wavelength shifts of side-polished FBG are 25 and 55 pm, respectively.Similarly, in 2015, J. Jiang et al. [34] also demonstrated a side-polished FBG hydrogen sensor with a residual thickness of 20 μm and sputtered with Pd/Ag, for monitoring dissolved hydrogen concentration in power transformer oil.
As for the etched FBG, in 2012, J. Dai et al. [35] demonstrated a greatly etched FBG hydrogen sensor with the fiber diameter of 17 μm and coating with Pd/Ni composite film as the sensing material, as depicted in Figure 2b.In the hydrogen response experiment, the central wavelength shift of etched fiber Bragg grating increases linearly with the increase of hydrogen concentration, and the linear sensitivity increases significantly to 15 pm/%, but the response rate slows down because of the oxidation of Ni.To further enhance the sensitivity of above proposed sensor, then in 2014, the authors adopted a flexible polypropylene substrate due to its good stability and low Young's modulus, as shown in Figure 3a. [36]Due to the extra extension generated from the polypropylene substrate, the sensitivity of sensor is significantly improved, showing 146 pm wavelength shift towards 4% hydrogen.In 2015, L. Coelho et al. [37] presented a hydrogen sensor with self-temperature compensation by cascading two etched FBGs, as shown in Figure 3b.Copyright 2014, the authors, published by Elsevier.b) Illustration of etched FBG hydrogen sensor with two cascaded and etched FBGs.Reproduced under the terms of the Creative Commons Attribution CC-BY license. [37]Copyright 2015, the authors, published by Optica.
Besides the side-polished FBG and etched FBG, in 2015 J. M. Karanja et al. [38] proposed a micro-structured femtosecond laser assisted FBG hydrogen sensor, as illustrated in Figure 2c.The sensing head was fabricated by 3D laser machining on the fiber cladding to form multiple periodic micro-grooves and sputtered with Pd/Ag film of 520 nm thickness.The sensitivity of the hydrogen sensor is 16.5 pm/%, additionally, demonstrating the hydrogen sensitivity improved with the increased number of microgrooves.X. Zhou et al. [39] also studied the hydrogen sensing properties of Pt-WO 3 films prepared by hydrothermal method on spiral microstructured fiber Bragg grating, the spiral microstructure FBG was fabricated using femtosecond laser, as shown in Figure 4a.Spiral microstructure FBG hydrogen sensor can detect hydrogen concentration from 0.02% to 4% at room temperature, and the response time is greatly reduced from a few minutes to 10-30 s.Double spiral microstructure at pitch 60 μm and sputtered with 2 μm Pt-WO 3 film reached hydrogen sensitivity of 522 pm/%.In 2016, M. Zou et al. [40] proposed a FBG hydrogen sensor with a composite microstructure consisting of a femtosecond laser, straight-trenches and spiral micro-pits, as illustrated in Figure 4b.A Pd-Ag film was sputtered as hydrogen sensing transducer on the surface of the laser processed FBG single mode fiber.The experimental outcome demonstrates the sensor has a high sensitivity of 26.3 pm/% and a response time of 140 s, which offers great potential in engineering applications.
In addition, in 2013 S. Silva et al. [41] proposed that the tapered FBG could be used for hydrogen measurement, as illustrated in Figure 2d.The sensitivity becomes higher by reducing the fiber diameter, because the evanescent-field interaction is stronger for thinner microfiber.The sensing head with 150 nm thickness Pd film is able to respond to hydrogen concentration from 0% to 1% at room temperature, reaching a maximum sensitivity of 81.8 pm/%.Then Z. P. Yu et al. [42] further studied the tapered FBG, a microfiber FBG was fabricated in multimode fiber with a diam-  [39] Copyright 2017, the authors, published by Optica.b) Illustration of FBG hydrogen sensor with a composite microstructure fabricated by femtosecond laser ablation.Reproduced under the terms of the Creative Commons Attribution CC-BY license. [40]Copyright 2016, the authors, published by MDPI.
eter of only 3.3 μm.In principle, the finer the fiber diameter, the higher sensitive the hydrogen sensor will be.However, it should be mentioned that excessive reduction of the fiber diameter can reduce the mechanical stability of the sensor.
To improve sensor sensitivity, in addition to the method of thinning the fiber cladding thickness, in 2018, J. Yu et al. [43] employed tilted fiber Bragg grating (TFBG) to eliminate the cross-sensitivity caused by the external stress.In 2022, C. Zhang et al. [44] proposed a TFBG hydrogen sensor based on Pd/Au (25 nm/35 nm) composite nanofilms, as shown in Figure 5a-c.The experimental results show that when hydrogen concentration changes by 1.02%, the intensity of cladding mode resonance wavelength (such as 1558.4nm) decreases by 1.628 dB, and the sensitivity of hydrogen concentration is 1.597 dB/%, ≈16 times that of the current similar hydrogen sensor.The average response time and recovery time are 37 and 49 s, respectively.Then, C. Shen et al. [45] proposed a reflective hydrogen sensor based on TFBG coated with Au-Pd nanofilms, as shown in Figure 5d-f.By cascaded and amplified conical structures upstream of the TFBG, higher intensity cladding modes can be coupled to the fiber core in the reflection spectrum, which is conducive to the formation of reflective TFBG sensors.Experimental results show that the maximum sensitivity of the sensor is 4.83 dB/%, the response time of the proposed sensor and the calculated detection limit are 26 s and 0.07%, respectively.
The structures and performances of the FBG hydrogen sensors mentioned before are presented in Table 1.The highly industrialized fabrication of FBGs makes FBG hydrogen sensors easier to be commercialized.FBG-based hydrogen sensors show many unique advantages, such as: more suitable for the distributed measurement, favorable for remote sensing, and easy for temperature compensation.The measurement accuracy of FBG-based hydrogen sensor is relatively good, however, it is also needed to reduce the external diameter of the fiber to improve the measurement sensitivity.Fiber processing including polishing and tapering has been widely used as effective methods to improve the sensitivity of FBG-based sensors, but increase the vulnerability to mechanical disturbances and render impracticability of the sensor in practical applications.Therefore, it is necessary to find better methods to optimize the sensor structure for practical application.By contrast, LPFG-based hydrogen sensor have relatively high sensitivity, transmit demodulation capabilities and do not require fiber post-processing. [30,31,46]However, LPFG sensors are not suitable for remote sensing and require complicated measurement equipment to demodulate spectral signals, so it is not specifically covered in this paper.

Development of the Sensitive Material
The deposition process of hydrogen-sensitive material is the key to the composition and proportion control of hydrogen sensitive materials and the formation of hydrogen sensitive film form, which determines the quality and life of hydrogen sensitive materials.The commonly used preparation methods of hydrogensensitive films are mainly divided into magnetron sputtering, solgel method and vacuum coating method.For optical fiber hydrogen sensor based on Pd film, in the process of circulating hydrogen absorption and desorption, Pd film vulnerable to lattice expansion caused by mechanical damage, such as crack, blister, layering, etc.This so-called embrittlement effect negatively affects the stability and sensitivity of fiber hydrogen sensors based on Pd films.Recent studies have shown that sputtering composite films, such as Pd/Ni [35,36] and Pd/Ag, [40,47] can alleviate  c) TFBG hydrogen sensor coated with Pd/Au composite nanofilms.Reproduced with permission. [44]Copyright 2021, Elsevier.d-f) hydrogen sensor based on enlarged taper cascaded with tilted fiber grating.Reproduced with permission. [45]Copyright 2022, IEEE.
phase transition to a certain extent, weaken the brittleness effect, and thus improve the mechanical stability and repeatability of hydrogen sensors.However, in the doping process, sensitivity, responsiveness and other issues need to be considered comprehensively.The performances of some typical FBG hydrogen sensors based on Pd sensitive materials are summarized in Table 2. [29,31,32,35,36,40,43,[47][48][49][50][51] In addition to the traditional hydrogen-sensitive material preparation method, M. Fisser et al. [52] proposed a new process of pasting large area Pd foil on the surface of FBG.As a result of the new process, the response of pure Pd foil is 72% higher than that of the previously reported sensor, [53] with a sensitivity of 0.062 pm ppm −1 .The sensitivity of the Pd/Ag alloy foil sensor is 17 times higher than that of the pure Pd foil sensor, reaching 0.77pm ppm −1 .The proposed sensor with Pd/Ag alloy is the most sensitive Pd alloy FBG hydrogen sensor available, capable of detecting hydrogen below 100 ppm, but with a response time of about 100 h.The slow response and low sensing range make these sensors suitable for only a few applications.However, the ongoing relaxation process possibly induced by creeping at the adhesive interface needs to be addressed before practical applications.In addition, the sensitivity of the sensor to vibration, the absolute size and complexity of the sensor may be key for practical applications.MoO 3 has been widely investigated as hydrogen sensing material due to its excellent electrical and opti1cal properties.Yang Minghong's group at Wuhan University of Technology has made outstanding achievements in fiber optic hydrogen sensing technology based on WO 3 sensitive materials.In 2018, M. H. Yang et al. [54] reported FBG hydrogen sensor coated with mesoporous WO 3 , which was synthesized by using SBA-15 and KIT-6 silica templates and encapsulated Pt nanoparticles in the mesoporous WO 3 channel, as shown in Figure 6a.At the room temperature, the sensor with KIT-6 mesoporous WO 3 moved ≈35pm at 1500 ppm H 2 /Air and the lowest detection limit was 100 ppm, only one-fourth of that in the control group without mesoporous structure, as depicted in Figure 6b.In addition, hydrogen sensing cycle tests showed that the repeatability of the mesoporous WO 3based sensor was significantly improved by an order of magnitude.However, the response time of the sensor is long, and the response speed needs to be further improved.Subsequently, M. H. Yang et al. [55] proposed an ion intercalated method to improve the stability of FBG hydrogen sensor based on MoO 3 , successfully intercalating Na + and K + ions into MoO 3 nanobelts, as illustrated in Figure 6c, and studied the influence of the intercalated ions' amount on the sensing performance.Compared with MoO 3 , ion-intercalated MoO 3 nanoribbons showed significant improvements in stability and cycling performance, as shown in Figure 6d.It has been shown that intercalated ions between the intercalations can enhance the layer structure and inhibit  Reproduced with permission. [54]Copyright 2018, IEEE.c) Schematic illusion of the reaction between H 2 with MoO 3 and Na (K)-MoO 3 , and d) the sensing performance of Pt/MoO 3 , Na(1)-Pt/MoO 3 , and K(1)-Pt/MoO 3 .Reproduced under the terms of the Creative Commons Attribution CC-BY license. [55]Copyright 2018, the authors, published by Elsevier.e) Schematic illusion of the reaction between H 2 with WO 3 and TBAOH-WO 3 , and d) the detection range and repeatability of the TBAOH-Pt/WO 3 sensor.Reproduced under the terms of the Creative Commons Attribution CC-BY license. [56]opyright 2022, the authors, published by Elsevier.
the phase transition.This ion intercalation strategy may provide a new perspective for improving the stability of sensors based on layer molybdenum trioxide.Recently, Yang's group [56] proposed a polymer intercalation method to achieve high performance fiber optic hydrogen sensor, aiming at the poor response performance of FBG sensors based on WO 3 , and synthesized tetrabylammonium hydroxide (TBAOH) molecular intercalation WO 3 , as illustrated in Figure 6e.The experimental results show that the response time of the sensor based on TBAOH Pt/WO 3 is significantly improved.In addition, the FBG hydrogen sensor based on the new material can detect the hydrogen concentration from 300 ppm to 12 000 ppm, showing excellent stability and repeatability, and has a broad application prospect in the future, as showed in Figure 6f.WO 3 -based FBG hydrogen sensors with high sensitivity, has great advantage in low concentration of hydrogen detection, however, it is still necessary to further improve the sensor responsiveness, the stability of hydrogen-sensitive materials and the repeatability of the preparation process for commercial application.

Enhancement of the Sensor Durability
Environmental adaptability is a key problem for the practical application of optical fiber hydrogen sensor.Environmental adaptability includes the cross interference of ambient temperature, Reproduced with permission. [41]opyright 2013, IEEE.b) Schematic diagram of fiber grating hydrogen sensor with a controlled optical heating system.Reproduced with permission. [58]opyright 2022, IEEE.
pressure and other sensing gases, among which the ambient temperature is the main factor affecting the performance of optical fiber hydrogen sensor.In recent years, researchers have proposed a number of ways to enhance the durability of sensors.
For the effect of temperature, the common methods are to add temperature sensitive units and use temperature insensitive materials.In 2013, S. Silva et al. [41] used femtosecond laser to continuously etch two Bragg gratings on single-mode fiber, one of which was plated with Pd-based hydrogen sensitive material as a hydrogen sensitive unit and the other grating as a temperature compensation unit, as illustrated in Figure 7a.In 2015, Y. Yang et al. [57] proposed a hydrogen sensor based on birefringent photonic crystal fiber, which can better suppress the temperature error.In 2021, C. Zhang et al. [44] proposed an optical fiber hydrogen sensor with tilted grating as the sensing unit, which realized the self-compensation of wavelength shift caused by temperature by using the difference calculation of fiber core mode and cladding mode.In 2022, M. H. Yang et al. [58] proposed a compact fiber op-tic hydrogen sensing system using a self-referential structure and controlled light heating technologies, as shown in Figure 7b, and experimentally proved that the system has ultra-high sensitivity and wide dynamic range of hydrogen concentration detection.
In addition, CO, [59] NH 4 [60] and water vapor [61] are often present in the actual detection environment of hydrogen.The presence of these gases will seriously affect the detection accuracy and sensitivity of hydrogen.A. Hosoki et al. [62] proposed a kind optical fiber hydrogen sensor with Pt film as coating featuring layers of Au/Ta 2 O 5 /Pd for long-term hydrogen detection (Figure 8).Primarily used as a catalyst for hydrogen sensors based on WO 3 and other oxide materials, Pt has a greater ability to break down hydrogen molecules into hydrogen atoms, contributing to achieving practical and stable hydrogen sensors for long-term measurements.The sensor's hydrogen response persisted after 1and 2-month storage at room temperature in air conditioning.Szilágyi et al. [63] creatively proposed the idea of using a metalorganic framework (MOF) thin film as a protective coating, and Figure 8. Illustration of fiber grating hydrogen sensor with metal (Pt or Au) coating.Reproduced with permission. [62]Copyright 2019, IEEE.demonstrated that homogeneous and continuous MOF coatings can indeed be deposited on Pd surfaces without severely affecting the sensor response.In addition, coating polymer films such as poly(methylmethacrylate) (PMMA) and polytetrafluoroethylene (PTFE) for gas separation is a feasible method to fabricate selective gas sensors. [64,65]Research shows that hydrogen sensitive film will react with oxygen or moisture in the air, resulting in deterioration of sensor performance. [66,67]The moisture resistance and selectivity are determined by the sensor material, which directly affects the life of sensor, and can be enhanced by protective coatings such as metal films or polymer films.However, there are only a few reports on the hydrogen sensitive film protection of optical fiber hydrogen sensor, which is one of the key problems that must be solved in the practical application of optical fiber hydrogen sensor.

Existing Challenges and Future Prospects
Optical fiber hydrogen sensing technology has been rapidly developed in recent years, and some fiber grating hydrogen sensors have been able to achieve high sensitivity and high precision for hydrogen detection.However, most of them are still in the laboratory and have not entered the stage of practical application.There are still some problems to be solved, and breakthroughs need to be made in sensor structure, hydrogen sensitive material and environmental adaptability.The following aspects will be the focus of future development.

Sensing Head Structure
The future goals of the sensor head structure can be summarized as simplification, miniaturization and reuse.Simple construction can reduce the cost and manufacturing complexity of sensors.Preliminary results show that the FBG-based sensor can be designed as a cascade structure to detect hydrogen concentration in multiple different regions.Single-mode gratings can obtain higher sensitivity by etching and other methods to reduce fiber diameter, but attention should be paid to maintain the robustness and machinability of fiber gratings.Multimode gratings have the potential of multi-point hydrogen concentration monitoring or temperature compensation.The refractive index sensitivity of the etched FBGs in multimode fiber is higher than that of the etched FBGs in single-mode fiber.However, expensive spectrum analyzer is usually needed to analyze the corresponding spectral changes of the multimode gratings.In the future, there is still a great need to further develop low-cost, miniaturized and multi-point sensing structure to build hydrogen monitoring network.

New Sensitive Materials
The innovations in new hydrogen sensitive materials and new parameter transfer materials are also needed to obtain efficient and practical optical fiber hydrogen sensors.The composition ratio of hydrogen-sensitive material affects the sensitivity, response time, service life and other important indexes of optical fiber hydrogen sensors.Future goals can be designed to develop the hydrogensensitive films with fast response, long-term stability and good selectivity.At present, Pd alloys have been used to inhibit the embrittlement of Pd film, thereby reducing response time.In addition, transition metal oxides, such as TiO 2 , NiO x , and WO 3 , have been used to provide good repeatability of changes in circulating hydrogen, and enhance sensor stability.In the future, the research of hybrid hydrogen sensitive materials that composed Pd and transition metal oxide can be carried out to give full play of the advantages of the two materials.At the same time, further research on new sensitive materials is needed to improve the selectivity and response speed of hydrogen sensors.
The optical fiber is quartz material with a cylindrical structure, it is hard to construct the sensitive coating film with high firmness and stability feature.Research shows that the stability of sensitive coating film could be optimized by adding a transition layer between optical fiber and hydrogen sensitive material.The common material of transition layer includes Ti, Ni, and so on.Furthermore, nanofilms with large surface area such as graphene will be considered as the substrate to enhance the adhesion between optical fiber and hydrogen-sensitive material.

Coating Technology
The coating process is the key to the composition and proportion control of hydrogen-sensitive material and the morphology of sensitive film, which determines the quality and life of hydrogen-sensitive material.The standardization and maturation of key technology is a crucial step for the application of fiber grating hydrogen sensor.[70] However, several problems including complex operation, high cost and instability are still involved in the preparation of hydrogen-sensitive film, besides, fabrication parameters and processes need to be further standardized.In this regard, there is still a huge space for the progress of coating technology of hydrogen-sensitive material.In addition, the combination of theoretical research on hydrogen response of hydrogen-sensitive film and characterization of hydrogen-sensitive film is a necessary strategy to obtain a hydrogen sensitive film with good stability, long life and high response rate.

Environmental Adaptability of Sensor
At present, most of the verification of fiber grating hydrogen sensor is still in lab-scale, namely the experimental environment is ideal.How to evaluate the concentration of hydrogen in a complex environment is the existing problem for the industrial application of fiber grating hydrogen sensor.The multi-parameters measurement, and the repeated utilization of sensor are one of the strategies for this challenge.In addition, the applicable demodulation technology of signals is still needed.Research shows that the hydrogen-sensitive film could react with oxygen and moisture within air, which leads to the degeneration in performance of sensors.In the further, the research of packaging film material should be promoted in the hydrogen sensor.The degeneration in the performance of fiber grating hydrogen sensor would be remarkably relieved as well as the lifetime would be largely prolonged by packaging a protective film on hydrogensensitive material.
Optical fiber hydrogen sensor has the advantages of intrinsic safety, small size, anti-electromagnetic interference, and distributed measurement, which is especially suitable for flammable and explosive workplaces.In the future, the main applicable scenarios for optical fiber hydrogen sensor are as follows: 1) In the field of electric power, the operating status of oil-filled electric equipment is monitored by measuring the dissolved hydrogen content in the insulating oil using the optical fiber hydrogen sensor.2) In the field of chemical industry, optical fiber hydrogen sensor can realize safety monitoring of oil and gas in the mining and transmission processes.3) In the field of hydrogen energy, the distributed measurement advantage of optical fiber hydrogen sensors is used for real-time monitoring to avoid hydrogen leakage.4) In the field of energy storage, optical fiber hydrogen sensor can detect the hydrogen produced by the battery and realize the early warning of the battery thermal runaway.In addition to the above fields, optical fiber hydrogen sensor has unique advantages in the medical industry, automotive industry, aerospace industry, etc., with a huge market scale and a very broad application prospect.

Conclusion
In this paper, we focused on the great promising fiber grating hydrogen sensor, briefly introduced the mechanism of hydrogensensitive material and the working principle of sensor, and mainly summarized the design and optimization methods of fiber grating hydrogen sensor in recent years.The challenges of fiber grating hydrogen sensors for practical applications were analyzed, and the future prospects were put forward, which requires in-depth and comprehensive research in the practical applications of the structure, material, key fabrication technology, and environmental adaptability of sensor.A review of the reported work shows that fiber grating sensor plays a critical role in the field of hydrogen measurement, displaying a great industrial value and broad application prospect.The purpose of this paper is to give all readers a better knowledge of the working principle and performance optimization method of fiber grating hydrogen sensor, providing some new inspirations for the practical application of the sensor.

Figure 1 .
Figure 1.Illustrations of the sensor structure of a) FBG hydrogen sensor and b) LPFG hydrogen sensor.

Figure 2 .
Figure 2. Illustrations of FBG hydrogen sensors with a) a side-polished fiber, b) a greatly-etched fiber, c) a micro-structured femtosecond laser assisted fiber and d) a tapered fiber.

Figure 3 .
Figure 3. a) Illustration of etched FBG hydrogen sensor with a polypropylene substrate.Reproduced under the terms of the Creative Commons Attribution CC-BY license.[36]Copyright 2014, the authors, published by Elsevier.b) Illustration of etched FBG hydrogen sensor with two cascaded and etched FBGs.Reproduced under the terms of the Creative Commons Attribution CC-BY license.[37]Copyright 2015, the authors, published by Optica.

Figure 4 .
Figure 4. a) Illustration of FBG hydrogen sensor with a spiral microstructured FBG.Reproduced under the terms of the Creative Commons Attribution CC-BY license.[39]Copyright 2017, the authors, published by Optica.b) Illustration of FBG hydrogen sensor with a composite microstructure fabricated by femtosecond laser ablation.Reproduced under the terms of the Creative Commons Attribution CC-BY license.[40]Copyright 2016, the authors, published by MDPI.

Figure 6 .
Figure 6.Material structure diagrams and hydrogen response performances of sensors based on WO 3 materials.a) Schematic illusion of mesoporous Pt/WO3 , sensor manufacturing process and hydrogen response performance; b) the hydrogen response performance of K-Pt/WO 3 , S-Pt/WO 3 , and C-Pt/WO 3 .Reproduced with permission.[54]Copyright 2018, IEEE.c) Schematic illusion of the reaction between H 2 with MoO 3 and Na (K)-MoO 3 , and d) the sensing performance of Pt/MoO 3 , Na(1)-Pt/MoO 3 , and K(1)-Pt/MoO 3 .Reproduced under the terms of the Creative Commons Attribution CC-BY license.[55]Copyright 2018, the authors, published by Elsevier.e) Schematic illusion of the reaction between H 2 with WO 3 and TBAOH-WO 3 , and d) the detection range and repeatability of the TBAOH-Pt/WO 3 sensor.Reproduced under the terms of the Creative Commons Attribution CC-BY license.[56]Copyright 2022, the authors, published by Elsevier.

Figure 7 .
Figure 7. a) Schematic diagram of fiber grating hydrogen sensor with an added temperature compensation unit.Reproduced with permission.[41]Copyright 2013, IEEE.b) Schematic diagram of fiber grating hydrogen sensor with a controlled optical heating system.Reproduced with permission.[58]Copyright 2022, IEEE.

Table 1 .
Performances of FBG hydrogen sensors based different sensor structures.

Table 2 .
Performances of FBG hydrogen sensors based different sensitive materials.