Application of Long-Period Fiber Grating Sensors in Structural Health Monitoring: A Review

Abstract

Structural health monitoring (SHM) is crucial for preventing and detecting corrosion, leaks, and other risks in reinforced concrete (RC) structures, ensuring environmental safety and structural integrity. Optical fiber sensors (OFS), particularly long-period fiber gratings (LPFG), have emerged as a promising method for SHM. Various LPFG sensors have been widely used in SHM due to their high sensitivity, durability, immunity to electromagnetic interference (EMI) and compact size. This review explores recent advancements in LPFG sensors and offers insights into their potential applications in SHM.

1. Introduction

Reinforced concrete (RC) finds extensive application in civil infrastructures worldwide, including buildings, bridges, tunnels, and dams, due to the easy availability of its constituents coupled with its numerous advantages in terms of performance and economy. However, many factors may cause deteriorations of concrete structures such as carbonation [1], alkali–silica reaction (ASR) [2,3], and corrosion of rebar [4]. The deterioration issue will shorten the service life of RC structures and even result in significant catastrophes. To solve this, one measure is to increase the durability of structure like using ultra-high-performance concrete (UHPC) to substitute the tradition concrete [5], but a more important and general method is to monitor the deterioration process and provide timely precaution. Structural health monitoring (SHM) was developed based on the urgent need of continuous monitoring of structures and was widely applied for concrete structures in civil engineering [6].

In order to realize the real-time monitoring of concrete structures, many sensors are required in SHM. The commonly used sensors include electrical sensors and optic fiber sensors (OFS). Electrical sensors have good dynamic performance and a wide range of applications, but have some disadvantages such as brittleness, instability, and low-frequency characteristics [7,8]. OFS are ideal for long-term monitoring of concrete structure due to their high sensitivity, durability, immunity to electromagnetic interference (EMI) and compact size [9,10]. OFS can be divided into fiber Bragg gratings (FBG) and long-period fiber gratings (LPFG) according to different grating periods. FBG sensors have a grating period of a few millimeters and use reflective spectrum as the measurand [11]. LPFG sensors have a grating period ranging from 100 μm to 1000 μm and use transmission spectrum as the measurand [12,13]. Compared with FBG sensors, LPFG sensors can sense the refractive index (RI) of the surrounding medium, which has enabled LPFG sensors to be more widely applied.

This paper provides a summary of the latest developments in LPFG sensors for monitoring durability-related parameters of concrete, along with a comprehensive analysis of the advantages and disadvantages of each sensor. The durability-related parameters of concrete include strain, temperature, relative humidity (RH), pH, chloride ion concentration, and corrosion. Strain is an import mechanical property that can indicate the onset of structural issues such as cracking, deformation, or stress concentration, which may affect the integrity and safety of the structure. Temperature variations can cause substantial tensile stress and cracking in massive concrete structures, such as dams [14]. High RH condition can cause or accelerate corrosion and carbonation and promote the ASR [14]. A decrease in pH of the concrete and the penetration of chloride ions can break down the protective passivation layer on rebars and accelerate the corrosion process of rebars [15,16,17]. Corrosion can decrease the stiffness and cross section area of rebars and thus reduce the carrying capacity of the concrete structure. Continuous monitoring of the six parameters mentioned above enables accurate evaluation of the durability of concrete structures.

This paper is organized as follows: firstly, the sensing principle and fabrication of LPFG sensors are introduced. Secondly, the LPFG sensors are categorized into six types according to different measurands: strain, temperature, humidity, pH, chloride, and corrosion. Strain and temperature sensors are introduced together due to the cross-sensitivity issue. Thirdly, existing challenges and future perspectives of LPFG sensors for SHM are discussed. Finally, the introduced LPFG sensors are summarized as conclusions.

2. Long-Period Fiber Grating Sensors

2.1. Sensing Principle

As illustrated in Figure 1, when a broadband spectral light beam passes through the LPFG situated within a single-mode fiber (SMF), the light in the core mode and the cladding modes couple with each other, which results in the creation of several attenuation bands in the transmitted spectrum, occurring at specific wavelengths that fulfill the coupling condition. The resonant wavelength of these attenuation bands can be determined using the expression [13]

$${\lambda }_{res,m}=(n_{eff}^{co}-n_{eff}^{cl,m})\mathsf{\Lambda }$$

(1)

where $\mathsf{\Lambda }$ is the grating period, $n_{eff}^{co}$ is the effective refractive index of core mode which depends on the refractive indices of core $n_1$ and cladding $n_2$, $n_{eff}^{cl,m}$ is the effective refractive index of m-th cladding mode which depends on $n_1$, $n_2$ and the refractive index of the surrounding medium $n_3$, and ${\lambda }_{res}^m$ is the resonant wavelength corresponding to m-th cladding mode.

It can be known from Equation (1) that the resonant wavelength is decided by the grating period and the difference between the effective refractive index of the core mode and cladding mode ($n_{eff}^{co}-n_{eff}^{cl,m}$). Environmental factors may change strain, temperature, and the refractive index of the surrounding medium, and these changes can be reflected in the shift of the resonant wavelength. Therefore, the resonant wavelength shift can be used to measure the changes in the environment.

2.2. Fabrication

Many fabrication methods of LPFG sensors have been proposed in recent decades, and two main methods are laser engraving [18,19,20] and electric arc discharge (EAD) [21,22].

A schematic setup of laser engraving using a CO₂ laser is shown in Figure 2. A computer is used to control the movement of the high-accuracy linear stage and the output of the CO₂ laser, and it is connected to an optical sensing interrogator to monitor the transmission spectrum of the optical fiber when fabricating the LPFG. An SMF is fixed on the linear stage for laser inscription and the linear stage moves at a constant distance (grating period) when the fiber is inscribed by a CO₂ laser with a millisecond-level pulse duration. Thus, an LPFG sensor with a certain cladding mode is fabricated. The specific parameters of the laser vary with different laser devices. For example, C. Guo [23] used a CO₂ laser (Firestar V40, from SYNRAD Inc., Mukilteo, WA, USA) with a tuned output power of 14.4 W and a pulse duration of 100 ms. However, F. Tang [24] used a CO₂ laser (Firestar V20, from SYNRAD Inc., Mukilteo, WA, USA) with a tuned output power of 7.8 W only though the pulse duration is also 100 ms. The common effective length of the LPFG sensors is approximately 4~6 cm, which is the product of the grating period and the grating numbers. For SMF, its outside diameter is approximately 125 µm (like SMF-28e⁺ fabricated by Corning Inc., Corning, NY, USA). The LPFG sensors are often coated with other materials and the coating thickness is within 30 µm. Therefore, the outside diameter of the LPFG sensors is less than 200 µm.

The EAD method uses the electrodes instead of the laser to create the gratings and a schematic setup is shown in Figure 3. A SMF is fixed on the linear stage and the grating area is between the electrodes of a fiber fusion splicer. By controlling the movement of the linear stage and adjusting the discharge intensity and time of electrodes, the LPFG sensor is fabricated. Compared to the laser engraving, the EAD method has the easier fabrication process and lower cost [22]. The commonly used device for the EAD method is the fiber fusion splicer and the specific parameters also vary with different devices. For example, X. Li [21] used a fiber fusion splicer (FITEL S178A, from Furukawa Electric Co., Ltd., Chiyoda City, Tokyo, Japan) with an arc-discharge intensity of 20 and a discharge time of 20 ms. However, C. Du [22] used the same fiber fusion splicer (FITEL, S178A) with an arc-discharge intensity of 5 and a discharge time of 150 ms.

3. Applications

3.1. Strain and Temperature

The temperature effect and strain state are closely related to the structural health of concrete structures [25]. However, cross-sensitivity issues can impact the performance of fiber grating sensors. To solve these issues, a simultaneous measurement of temperature and strain is proposed as an effective way by. The schematic setup for the simultaneous measurement of strain and temperature is shown in Figure 4. Two ends of the optic fiber were fixed on the fixed and translation stages by adhesive, separately. The stage controller can adjust the strain of gratings by moving the translation stage precisely. At one end of the optical fiber, there is a Super-continuum Light Source (SLS) connected, while at the other end, an Optical Spectrum Analyzer (OSA) is connected to collect and analyze the light spectra. The SLS emits light into the fiber, which passes through the gratings to reach the OSA. Additionally, a heating device is positioned to adjust the ambient temperature of the LPFG sensor.

In recent decades, researchers proposed many different LPFG sensors to measure strain and temperature simultaneously and carried out many studies to enhance sensitivities, as shown in

Table 1. Performance of the LPFG sensors for the simultaneous measurement of strain and temperature.

Ref.	Year	Sensor Design/ Fabrication	Fiber Type	Grating Period (μm)	Resonant Wavelength (nm)	Strain Range (με)	Strain Sensitivity (pm/με)	Temperature Range (°C)	Temperature Sensitivity (pm/°C)
[26]	2004	KrF Laser	PMF	480	1565.93	0~1200	1.36	35~90	−36.6
					1602.05		−1.37		129.12
[27]	2014	CO2 Laser	FMF	480	1338.44	0~800	−2.9	30~90	−17.6
			SMF	580	1522.47		−1.47		46.4
[28]	2016	Tapered	SMF	3998.5	1412	0~2392	−1.5	20~225	67
		LPFG		462.3	1280		0		80
[22]	2018	EAD	SMF	500	1425	1000~6000	−0.6	45~75	30.9
					1470		−0.52		68.1
[29]	2018	Modular	SMF+NCF	400 (SMF) 200 (NCF)	1258	0~1200	−1.2	30~170	−15.4
		CO2 Laser	SMF	500	1356		−0.5		58.3
[30]	2019	Tapered	SMF	4800	1540.2	0~900	1.82	30~90	47.9
		CO2 Laser		500	1572.2		8.17		65

.

In 2004, K. J. Han [26] proposed a long-period fiber grating inscribed on a polarization-maintaining fiber (PMF), as known as PM-LPG. The PM-LPG measured the strain and temperature by decoupling spectral signal of two adjacent resonant dips since each resonant dip has different sensitivities for strain and temperature. PM-LPG shows a sensitivity of −1.37 pm/με and 129.12 pm/°C for strain and temperature, respectively.

In 2014, L. Wang [27] proposed a hybrid LPFG of cascading few-mode fiber (FMF) and single-mode fiber (SMF). The FMF-LPFG exhibits the strain and temperature sensitivities of −2.9 pm/με and −17.6 pm/°C, respectively. The SMF-LPFG exhibits the strain and temperature sensitivities of −1.47 pm/με and −46.4 pm/°C, respectively.

In 2016, H. Zeng [28] proposed a cascade structure of tapered and CO₂-laser-notched LPFG based on SMF, which achieves a strain sensitivity of −1.5 pm/με and a temperature sensitivity of 80 pm/°C.

In 2018, C. Du [22] fabricated an LPFG sensor by inscribing a SMF with periodic electric arc-discharge (EAD) technology, which achieves a strain sensitivity of −0.6 pm/με and a temperature sensitivity of 68.1 pm/°C.

In 2018, Q. Yan [29] proposed a cascade structure of notched and modular LPFG. The notched LPFG is manufactured by irradicating CO₂ laser along the SMF while the modular LPFG is fabricated by splicing the SMF and the no-core fiber (NCF) alternately. The sensitivities of the cascade LPFG sensor are −1.2 pm/με and 58.3 pm/°C for strain and temperature, respectively.

In 2019, X. Jin [30] proposed a novel sensor structure fabricated by modulating a tapered LPFG with weak-power CO₂ laser exposure. The tapered LPFG was fabricated using resistive filament heating with a grating period of 4.8 mm, depicting an ultra-long-period fiber grating (ULPFG). The novel sensor structure indicates a strain sensitivity of 8.17 pm/με and a temperature sensitivity of 65 pm/°C.

3.2. Relative Humidity

Significant applications stem from the necessity to monitor structural degradation, frequently associated with the infiltration of water serving as a transport medium for corrosive ions like chloride, sulfate, carbonate, and ammonium [31]. Therefore, the measurement of relative humidity (RH) is crucial for SHM. The traditional RH sensors are electrical RH sensors that measure electrical parameters like resistance and capacitance, but they are prone to be disturbed by electromagnetic interference (EMI) [32]. To overcome this drawback, many optical RH sensors have been proposed such as the FBG-based and LPFG-based RH sensors. The FBG-based RH sensors are fabricated by coating a layer of moisture-sensitive material that will swell or shrink with RH change. The volume change in the coating induces the change in the Bragg wavelength of the FBG, so the FBG sensors can use this principle to measure RH [33]. The LPFG-based RH sensors are also fabricated by coating a layer of moisture-sensitive material, but different from FBG, the LPFG measures the refractive index of the coating. Up to now, many LPFG RH sensors have been explored and demonstrated significant potential in humidity measurement. The performance of some LPFG-based RH sensors is summarized in

Table 2. Performance of the LPFG RH sensors.

Ref.	Year	Coating	Coating Thickness (μm)	RH Range (%)	RH Sensitivity (nm/1%)
[34]	2006	PEO/CoCl2	10	50~77	0.19
				77~95	0.33
[35]	2007	Hydrogel		38.9~100	0.2
[36]	2009	Hydrogel	0.75	35~50	0.42
				50~100	0.47
[37]	2008	PVA	1.5	53~75	0.09
				75~97	0.68
			0.8	53~75	0.81
				75~97	5.71
[38]	2008	PAH/PAA and Al2O3/PSS		50~75	0.44
[39]	2015	PAH/PAA− and Al2O3+/PSS−	0.17	19~25	0.15
				73~90	0.15
[40]	2009	PAH/PSS− and PAH/SiO2-NPs		20~80	0.2
[41]	2019	PAH/SiO2-NPs		35~98	0.53
[42]	2012	Polyimide		20~80	0.1
[43]	2013	Gelatin/CoCl2	1.5	35~90	0.18

. The RH sensors shown in Table 2 have good linearity, but the linearity is segmented, which means the RH sensors have a linear relationship between the RH and the wavelength shift for a given specific measurement range but not for all measurement ranges.

The common schematic setup for the LPFG-based humidity sensors is shown in Figure 5. In this setup, two ends of the optical fiber are connected to a SLS and the OSA, separately. An airtight humidity chamber is used to keep the stable test conditions. The LPFG sensor is inserted into the humidity chamber and fixed on the sensor holders to avoid any bend/strain being imposed on the grating. The relative humidity levels in the chamber are controlled by salt solution with different known concentrations. In addition, a commercial hygrometer is used for RH reference.

In 2006, M. Konstantaki [34] proposed an LPFG humidity sensor with a hybrid coating of poly(ethylene oxide) and cobalt chloride (PEO/CoCl₂). The sensor achieves sensitivities of 0.19 nm/1% and 0.33 nm/1% for 50~77% RH and 77~95% RH, separately, and its response time is in the range of several hundred milliseconds.

In 2007, Y. Liu [35] proposed an LPFG RH sensor coated with the hydrogel, which achieves a sensitivity of 2.0 nm/1% with a linearity of 99.5% when the RH ranges from 38.9% to 100%. In 2009, Y. Liu [36] also used the hydrogel as the coating to fabricate the LPFG RH sensor that utilizes cascaded LPFG to form a Mach–Zehnder interferometer. The sensitivities of the Mach–Zehnder interferometer are 0.42 nm/1% and 0.47 nm/1% for 35~50% and 50~100% RH, separately, which achieved a broader measurement range and higher sensitives compared to Liu’s LPFG RH sensor.

In 2008, T. Venugopalan [37] created an LPFG RH sensor through coating a thin layer of polyvinyl alcohol (PVA). PVA-coated LPFG sensors with two coating thicknesses of 1.5 μm and 0.8 μm were explored and compared. The LPFG sensor with 0.8 μm showed a quicker response time of 50 s and higher sensitivities that are 0.81 nm/1% and 5.71 nm/1% for 53~75% RH and 75~97% RH, separately.

In 2008, J. M. Corres [38] designed an LPFG RH sensor by coating the electrostatic self-assembled alumina (Al₂O₃) and poly(sodium 4-styrenesulfonate, PSS) through electrostatic self-assembly (ESA) method. The proposed sensor has two layers. The inner layer is composed of polycation polyallylamine hydrochloride (PAH⁺) and polyanion polyacrylic acid (PAA⁻), which is used to increase the sensor sensitivity. The outside layer is composed of Al₂O₃⁺ and PSS⁻, which changes reflective index when humidity changes. The ESA method enables uniform coatings with precise thickness control. The experimental results show a sensitivity of 0.44 nm/1% RH when RH ranges from 50% to 75%. In 2015, S. Zheng [39] also proposed an LPFG RH sensor with the composite coating of PAH⁺/PAA⁻ and Al₂O₃⁺/PSS⁻ and proved the excellent thermal stability of the sensor. However, Zheng’s sensor only has a sensitivity of 0.15 nm/1% when RH is within 16~25% and 73~90%. There is almost no wavelength shift when RH ranges from 25% to 73%.

In 2009, D. Viegas [40] proposed a novel LPFG RH sensor by coating a thin film of silica (SiO₂) nanospheres (NPs) through the ESA method. The sensor has two coating layers. The inside layer is four bilayers of PAH⁺/PSS⁻ while the outside layer is 14 bilayers of PAH/SiO₂-NPs. The experimental results show a sensitivity of 0.2 nm/1% when RH is within 20~80% and good thermal stability. In 2019, J. Hromadka [41] proposed an array of two LPFGs where one bare LPFG was used to measure temperature and another was coated with 10 layers of silica nanoparticles to measure RH. The sensitivity of the sensor was 0.46 nm/°C and 0.53 nm/1% for temperature and RH, respectively.

In 2012, L. Alwis [42] proposed an LPFG RH sensor with a tailored layered polyimide (PI) coating on the grating region and a silver mirror coating at the distal end of the fiber probe, as shown in Figure 6. The mirror coating can enable the optical interrogator to acquire the spectrum through reflection mode, just like the FBG sensor, and thus makes the installation of LPFG sensors and the collection of data easier. The sensitivity of the sensor is approximately 0.10 nm/1%RH when RH is within 20~80% and the hysteresis is negligible (<1% RH).

In 2013, Nidhi [43] proposed an LPFG RH sensor coated with a combination of cobalt chloride (CoCl₂) and gelatin, which achieves a sensitivity of 0.18 nm/1% when RH ranges from 35% to 90%.

3.3. pH

In reinforced concrete structures, various factors such as carbon dioxide exposure, acid attack, deicing salt application in winter, and chloride ion ingress can lead to a decrease in concrete alkalinity. The decrease in pH compromises the passivation film on steel bars, leading to corrosion. Therefore, maintaining high alkalinity is crucial for preventing steel corrosion and ensuring concrete durability, which makes the pH monitoring important for SHM. So far, many optical sensors have been used in the pH measurement and most of them are based on LPFG coated with pH-sensitive material. The performance of the LPFG-based pH sensors is shown in

Table 3. Performance of the LPFG pH sensors.

Ref.	Year	Coating	Coating Thickness (μm)	pH Range	pH Sensitivity (nm/pH)	Response Time (s)
[45]	2006	PAH/PAA		4~7	28.3	120
[46]	2007	PAH/PAA	0.4	4~7	28.3	120
				7~8	21
		PB in the PAH/PAA	0.4	4~7	8	60
				7~8	10
[47]	2016	Hydrogel	0.53	2~12	0.66	<2
[48]	2020	PVA/PAA hydrogel		2~7	0.44
[49]	2021	PANI film		2~12	0.152	11~19
[50]	2024	PEI/PAA		5.92~9.23	5.6~6.3	480

.

In 2006, J. M. Corres [45] proposed an LPFG pH sensor with a coating of PAH/PAA by EAS method, which has a sensitivity of 28.3 nm/pH in the 4~7 range of pH units. The experiments show that the response and recovery time are 120 s and 270 separately, when pH changes from 5 to 8 and 8 to 5. In 2007, to increase the response time, Corres [46] proposed a new sensor by incorporating the pigment Prussian blue (PB) in the PAH/PAA coating and compared its performance with the PAH/PAA-coated LPFG sensor without PB. The PAH/PAA sensor with PB achieved a response time of 60 s and a recovery time of 120 s, which is approximately half of the PAH/PAA sensor, but PB decreased the sensitivity of the sensor from 28.3 nm/pH to 8 nm/pH when pH is within 4~7 pH units.

In 2016, S. K. Mishra [47] reported a wide range pH sensor by coating hydrogel on the cladding of the LPFG. The hydrogel is mainly synthesized with acrylamide, bisacrylamide solutions, and methacrylic acid [25]. A pH change alters carboxylic ion levels in the hydrogel, affecting its volume and refractive index. This, in turn, results in a shift of the resonance wavelength of the LPFG as the surrounding solution’s pH changes. The experimental results show a sensitivity of 0.66 nm/pH when pH is within 2~12 and a response time of less than 2 s.

In 2020, Y. Xu [48] proposed an LPFG pH sensor coated with PVA/PAA hydrogel with a sensitivity of 0.66 nm/pH when pH ranges from 1.916 to 7.252).

In 2021, X. Wang [49] proposed a polyaniline (PANI)-coated LPFG pH sensor. PANI is a conductive polymer with high stability and biocompatibility, exhibiting regular changes in refractive index and optical properties within the pH range of 2~12. Though the PANI-coated LPFG sensor has a low sensitivity of 0.152 nm/pH, it has advantages such as fast response speed and high thermal stability.

In 2024, J. M. Pereira [50] fabricated an LPFG pH sensor with two bilayers of polyethylenimine (PEI) and PAA coating through ESA technique. To estimate the coating properties of PEI/PAA, the simulation method was used by employing the transfer-matrix method (TMM) [51] to solve the modified Bragg condition function. The PEI/PAA LPFG sensor has sensitivities of 5.6 nm/pH and 6.3 nm/pH when decreasing and increasing the pH, separately, in the 5.92~9.23 pH range, and a response time of 8 min.

3.4. Chloride

The penetration of chloride ions is one of the main factors responsible for the corrosion of reinforced bars in a marine environment [52]. Thus, early detection of chloride ions is important for the maintenance of the reinforced concrete structures. The commonly available sensors utilize the electrochemical method to detect chloride ions by embedding electrodes into the concrete structures [53,54]. These traditional sensors involve installation issues, low sensitivity, and difficulty in acquiring quantification of chloride concentration. To overcome these drawbacks, many LPFG-based chloride sensors have been proposed in recent decades and their performance is summarized in

Table 4. Performance of the LPFG chloride sensors.

Ref.	Year	Coating	Concentration Range	Sensitivity
[55]	2007	Colloidal gold	0~25 wt.%	0.071 nm/1%
[56]	2007		0~0.232 g/mL	0.001 nm/(g mL−1)
[57]	2009		0~150 g/L	0.00661 nm/(g L−1)
[16]	2009	Gold NPs	0~1.0 M	1.47 nm/M
[58]	2017		50~300 ppm	0.02848 nm/ppm
[59]	2017	CHI/PAA	0.5~0.8 M	36 nm/M
[60]	2019	qP4VP/PAA Hydrogel	0.4~0.8 M	7 nm/M (125.5 pm/‰)
[61]	2022	PDDA/PSS	0~3%	52.2 nm/1%

.

In 2007, J. Tang [55] coated a monolayer of colloidal gold nanoparticles on the surface of the LPFG to detect chloride ion. The sensor was affixed to the surface of a concrete specimen and submerged in saltwater solutions with weight concentrations varying from 0% to 25%. Experimental results indicate that the colloidal gold-coated LPFG sensor has a sensitivity of 0.071 nm/1%, which is much higher than 0.0586 nm/1%, the sensitivity of the bare LPFG.

In 2007, Bey [56] applied a compact non-coated LPFG pair to chloride ion measurement. The LPFG pair underwent testing in sodium chloride (NaCl) solutions with concentrations ranging from 0 to 0.232 g/mL, demonstrating a sensitivity of 0.001 nm/(g mL⁻¹). Moreover, the application of Fast Fourier Transform to the spectra enabled the detection of small quantities of chloride ions (10 ppm).

In 2009, Possetti [57] presented an in-fiber Mach–Zehnder interferometer consisting of two cascaded LPFGs for salinity measurement in a water solution. For NaCl solution within the concentration range from 0 to 150 g/L, the sensor has a sensitivity of 0.00661 nm/(g L⁻¹). The best resolution of the LPFG Mach–Zehnder interferometer was 1.30 g/L (NaCl), approximately 2-fold better than the resolution of the Abbe refractometer, which uses electrical conductivity.

In 2009, Lam [16] proposed a self-interference in a single-LPFG (SILPFG) Michelson interferometer to detect chloride ions in solution. The SILPFG Michelson interferometer can generate differences in the optical path lengths and thus achieve self-interference by coating a layer of silver at one distal end of the fiber as the reflection mirror, as shown in Figure 7. In addition, gold nanoparticles (NPs) are coated on the LPFG part of the SILPFG Michelson interferometer to enhance the sensitivity to chloride ions. Results show that the SILPFG Michelson interferometer coated with gold nanoparticles has a sensitivity of 1.47 nm/M for NaCl solutions with concentrations varying from 0.01 to 1.00 M.

In 2017, L. S. Laxmeshwar [58] showed that the bare LPFG sensor has a sensitivity of 0.02848 nm/ppm when concentration of chloride increases from 50 to 300 ppm.

In 2017, F. Yang [59] demonstrated an LPFG chloride sensor by coating ionic-strength-responsive chitosan (CHI)/poly (acrylic acid) (PAA) polyelectrolyte multilayers via the layer-by-layer (LbL) assembly technique. The CHI/PAA-coated LPFG sensor achieved a sensitivity of 36 nm/M for concentrations of NaCl solution ranging from 0.5 to 0.8 M, which is approximately 17-fold higher than the sensitivity of the bare LPFG.

In 2019, F. Yang [60] demonstrated an LPFG salinity sensor coated with nanoscale overlays of partially quaternized poly(4-vinylpyridine) (qP4VP) and PAA hydrogel via LbL ESA method. The hydrogel-coated LPFG sensor showed a sensitivity of 7 nm/M with a quick response time less than 5 s in NaCl solutions with concentrations ranging from 0.4 to 0.8 M, and its sensitivity is 3-fold higher compared to the bare LPFG sensor.

In 2022, Q. Li [61] enhanced the detection capability of the LPFG to low-concentration salt solutions by assembling salt-containing poly (diallyldimethylammonium chloride) (PDDA) and salt-containing poly (sodium-p-styrenesulfonate) (PSS) nanofilms via the LbL method. The PDDA/PSS-coated LPFG achieved an average sensitivity of 52.2 nm/% for 0~3% salt solution.

3.5. Corrosion

Corrosion can cause the decline in carrying capacity of steel members or rebars and may result in structural failure. Corrosion monitoring enables engineers to take timely precautionary measures to avoid significant economic or life losses. Therefore, corrosion monitoring in steel members or rebars is one of the most important aspects in SHM. At present, the methods of corrosion monitoring include half-cell potential method [62], linear polarization method [63], hyperspectral imaging (HSI) methods [64,65,66], and sensor technology [67,68]. The half-cell potential method and the linear polarization method belong to the electrochemical methods that are susceptible to external interferences so that it difficult to provide an accurate and quantitative evaluation to the corrosion degree [69,70,71]. HSI is a remote sensing technique that utilize the reflectance curve to analyze the chemical composition of the scanned surface [72,73,74]. HSI evaluates the corrosion occurrence indirectly by measuring the chloride ion or rust and cannot provide real-time monitoring [64,65]. Sensor technology is extensively used for corrosion monitoring due to their compact size, low cost, and embeddability [67]. Optical fiber sensors (OFS) are a popular category in corrosion sensors, especially the LPFG sensors due to their high sensitivity. In recent decades, many LPFG corrosion sensors were proposed, and they are introduced below.

In 2012, H. Liu [75] proposed an LPFG corrosion sensor to evaluate corrosion state of rebar in concrete by detecting the liquid rust in phenolic resin, as shown in Figure 8. The rebar and the LPFG sensor are placed inside a stainless-steel base and the LPFG sensor is fixed in the holder to keep straight. The stainless-steel base is filled with phenolic resin and wrapped by a concrete layer. A current of 40 mA is used to accelerate the corrosion speed of the rebar. The sensing principle is that solid rust generated by the rebar can be dissolved by phenolic resin and the LPFG sensor can detect the changes in refractive index caused by the rust diffusion. The experimental results show a good relation between the corrosion rate of the rebar and the resonant wavelength shift of the LPFG spectra.

In 2013, Y. Huang [76] et al. proposed a novel LPFG-based corrosion sensor coated with a thin layer of nano iron/silica particles, as shown in Figure 9. The nano iron and silica particles are dispersed into the polyurethane-acetone solution first and then coated on the surface of the LFPG sensor by dip coating. The nano iron particles undergo a corrosion process similar to that of steel rebar due to their similar chemical composition. The corrosion of nano iron particles can change the refractive index of the coating and consequently cause the shift of the resonant wavelength of the LPFG. Therefore, the wavelength shift can be used to monitor the corrosion process of rebar. The nano silica particles are used to enhance the transparency and robustness of the coating. The experimental results show that the resonant wavelength shift of the corrosion sensor is approximately 0.45 nm for one month test in 3.5 wt% NaCl solution and deploying a parallel bare LPFG sensor can compensate for the coupled effects of temperature and pH. In 2015, Y. Huang [77] calibrated the combination of one LPFG sensor with nano iron/silica coating and one bare LPFG sensor and performed a corrosion test of three steel rebars in 3.5 wt% NaCl solution for 512 h by attaching the sensor combination to each of rebars. The corrosion tests indicate that the resonant wavelength of the LPFG corrosion sensor increases exponentially with immersion time.

Apart from the aforementioned design, there exists another general design of LPFG corrosion sensors based on Fe-C coating [78,79,80], as shown in Figure 10. Each sensor comprises three parts: an LPFG, a conductive layer and a Fe-C layer. The conductive layer is used for Fe-C electroplating and the Fe-C layer is for corrosion sensing due to its similar chemical composition to the steel rebar. The resonant wavelength shift of the LPFG is recorded by an interrogator and the corrosion rate of the Fe-C coating is measured by electrochemical impedance spectroscopy (EIS) [23,65]. Therefore, the resonant wavelength shift of the LPFG sensor can be correlated with the mass loss of the Fe-C coating over time by monitoring the two variables simultaneously. The first application of the design in Figure 10 was proposed by Y. Chen [76] in 2016. Based on Y. Chen’s groundbreaking work, many similar sensors were proposed and tested. These sensors are introduced in the following.

In 2016, Y. Chen [78] proposed an LPFG corrosion sensor with a 0.8 μm thick silver (Ag) coating as the conductive layer and a 20 μm thick Fe-C coating. The corrosion test results demonstrate a sensitivity of 0.0423 nm per 1% mass loss when Fe-C mass loss is less than 80% and 0.576 nm per 1% mass loss for Fe-C mass loss ranging from 80% to 95%. In 2017, Y. Chen [81] explored the mechanism and sensitivity of the Ag based LPFG corrosion sensor with the Fe-C coating using two silver and three Fe-C thicknesses. Experimental results show that the corrosion process of the Fe-C layer can be divided into three stages: (1) fast, (2) slow, and (3) stable, and the shifts in resonant wavelength have a liner relationship with the mass loss of the Fe-C layer within specific ranges.

In 2018, F. Tang [24] employed the Ag based LPFG sensor with the Fe-C Coating to monitor the corrosion process of steel bars under two conditions: (1) direct submersion in 3.5 wt.% NaCl solution and (2) submersion after embedding in mortar cylinders. The LPFG corrosion sensor was fixed to the rubber rings at both ends of the steel bar using adhesive tape. Based on the rate of wavelength change, the corrosion process can be divided into two stages (1) fast drop and (2) slow increase/stabilization. In addition, the test results demonstrate that the LPFG sensor can efficiently monitor steel bar corrosion in both cases and is suitable to monitor the early sate corrosion of RC structures. In 2019, F. Tang [82] performed the corrosion test of the same sensor in saturated calcium hydroxide, Ca(OH)₂, solution for approximately 50 h, and demonstrated that the Fe-C-coated LPFG sensor can effectively monitors passive film growth on steel in aqueous environments.

In 2018, C. Guo [23] improved the sensitivity of the Fe-C-coated LPFG corrosion sensor by using the composite coating of graphene (Gr) and silver nanowire (AgNW) as the conductive layer. The two ends of the Fe-C-coated LPFG sensor were fixed on a glass slide to keep the straight state of the LPFG during the corrosion test. Due to the high optical transparency of the Gr/AgNW coating, the wavelength sensitivity and service life of the Fe-C-coated LPFG sensor increased by over 90% and 110%, respectively, compared to the Ag-based sensor. In 2020, C. Guo [83] performed the corrosion test of the Fe-C-coated LPFG sensors under four strain levels. Strain influences the resonant wavelength shift during the Fe-C corrosion process, showing an initial increase followed by a decrease in two corrosion stages and the resonant wavelength shift can be linearly related to Fe-C mass loss under different strain levels for each stage. Later, based on C. Guo’s research, Y. Zhuo [84,85] applied the probability of detection (POD) methods to analyze the reliability of the Fe-C-coated LPFG sensors.

4. Existing Challenges and Future Perspectives

Although LPFG sensors have been widely applied in SHM in recent decades, they still have some challenges that need to be addressed. These challenges are shown below:

• LPFG sensors are fragile under tension, bending, or impact, which is a general issue for all optical fiber sensors. To use LPFG sensors in practical engineering applications, robust packaging techniques are required to prevent breakage during the process of sensor installation and usage. However, the introduction of packaging may weaken the advantage of optical fiber sensors in terms of small size.
• Compared to other optical fiber sensors, LPFG sensors can measure more parameters due to their unique ability to measure the RI of the surrounding medium, but this brings to more cross-sensitivity issues. For example, temperature variations can introduce interference to the measurement of other parameters, which is typically non-negligible. Although simultaneous measurement can solve this problem for strain measurement, for other parameters like pH and chloride ion concentration, more work is needed for temperature compensation.
• LPFG corrosion sensors can only be used for the early-stage corrosion monitoring and lack of reliable system methods to evaluate the corrosion degree of the structure. The service life of the LPFG corrosion sensors is only a few days, making them currently unsuitable for long-term corrosion monitoring lasting several years. Extending the service life usually decreases the sensitivity of the sensors, and this will induce the trade-off problem between sensitivity and service life. Some measures had been proposed to extend the service life of the LPFG corrosion sensors, like encapsulating the sensors by small steel tubes [86,87], but more research is needed. In addition, LPFG corrosion sensors are focused on measurement of the mass loss of the steel elements, but the strength loss due to corrosion remains unknown due to the unevenness and randomness of corrosion. To solve this issue, other parameters like strain/stress state of the rebars need to be considered. Therefore, reliable system methods are urgent to be developed for the comprehensive corrosion evaluation of the structure.
• An efficient sensing network of LPFG sensors is necessary for large-scale structures such as bridges. To monitor the performance of the large-scale structures, many key locations need to be installed with multiple sensors and these locations are far apart. Therefore, it is important and challenging to develop a reasonable plan to address the installation of sensors and data collection.

5. Conclusions

In conclusion, this paper has reviewed recent developments of LPFG sensors in SHM. A comprehensive summary is provided for the sensing principle, fabrication methods, and performance of six primary categories of LPFG sensors. The existing challenges and future perspectives are also discussed. Based on the above review and discussion, the following conclusions can be made:

• For strain and temperature measurements, the LPFG sensors measure the two parameters simultaneously and can decouple the data since the LPFG sensors have different sensitivities for strain and temperature. The common ranges for strain and temperature measurement are 0~1200 με and 30~90 °C, respectively and the corresponding sensitivities are 1.4~8.2 pm/με and 18~129 pm/°C, respectively. For an ultra-high strain range of 1000~6000 με, the strain sensitivity is only approximately 0.6 pm/με.
• For RH measurement, the LPFG sensors are coated with a layer of humidity-sensitive materials such as PVA, PAH/PAA, hydrogel, gelatin, and polyimide. The common RH measurement range is 35~90% and the sensitivity is 0.1~5.71 nm/1%. Some LPFG sensors have short response time as short as 30 s for RH changes. However, many articles have not studied response time, and more relevant work is needed in the future.
• For pH measurement, the LPFG sensors are coated with a layer of pH-sensitive materials such as PAH/PAA, hydrogel, PEI/PAA, and PANI. When pH ranges from 4~7, the sensitivity is 6~28 nm/pH. When pH ranges from 2~12, the sensitivity is 0.15~0.66 nm/pH. The pH sensitivity of LPFG sensors across a wider pH range is less than 10% of the sensitivity within a smaller pH range.
• For chloride ion concentration measurement, the LPFG sensers use different coatings such as hydrogel and gold nanoparticles to enhance the sensitivity. The LPFG chloride sensors are mainly tested in NaCl solutions with different concentrations. However, it is difficult to compare their sensitivities since researchers used different concentrations units.
• For corrosion monitoring, the LPFG sensors are coated with nano iron/silica particles or Fe-C coatings since these coatings have similar chemical composition with steel bars. The service life and the sensitivity of the LPFG corrosion sensors are dependent on the thickness of the coating, so the trade-off between them needs to be considered. The service life of the LPFG corrosion sensors is only a few days, which means the LPFG corrosion sensors are suitable for early-stage corrosion monitoring.