Spectral Splitting Sensing Using Optical Fiber Bragg Grating for Spacecraft Lateral Stress Health Monitoring

Abstract

Optical fiber sensing is a promising detection method for spacecraft health monitoring, since optical fiber sensors are lightweight, small in size, easy to integrate and immune to electromagnetic interference. As a significant optical sensor, fiber Bragg gratings (FBG) are widely used for force sensing because of their axial strain characteristics. However, it is necessary to detect not only one-dimensional strain but also plane strain and its deformation in order to comprehensively evaluate the condition of the structure. Therefore, it is very important to analyze the reflection spectrum of FBG under lateral stress. When FBG are subjected to lateral stress, the refractive index of the waveguide in the x and y directions changes, resulting in a birefringence phenomenon. This result causes the reflection spectrum of FBG to split into two peaks. In this paper, a transverse stress detection method based on spectral split sensing for the fiber Bragg grating is proposed, intended for monitoring spacecraft–small particle collisions. The FBG local lateral stress detection system is designed and verified by experiments. The wavelength pressure correlation is established in the experiment by adjusting the number of weights to change the lateral pressure on the FBG. The loading range of FBG lateral pressure is 4.0–7.0 N, the step size is 0.5 N, and round-trip measurement is carried out four times. The wavelengths of the peak and split point of the FBG reflection spectrum are recorded. The experimental results show that FBG’s split point and right peak pressure sensitivities are 16.57 pm/N and 45.14 pm/N, respectively. The spectral splitting phenomenon can be applied in spacecraft structure health monitoring systems and has certain reference value for the simplification of sensor systems.

1. Introduction

Due to the development of space technology in recent years, the service cycle of spacecraft has become much longer in a harsher environment. Long-term operation in such an environment invites numerous serious challenges, including space radiation, micrometeoroid impacts, and impacts from space debris, all of which can cause surface structure wear, buckling, and possibly the extrusion of internal structures. Spacecraft are at risk of being damaged, and it is possible for a space mission to fail as a result. At present, a surveillance network can only monitor debris larger than 10 cm in diameter, and cannot detect tiny fragments smaller than 1 cm. In order to timely discover and prevent potential impacts to the compression state of the internal structure of the spacecraft caused by small debris, it is urgent to carry out spacecraft pressure detection research [1,2,3,4]. In the case of a space station, it is essential for the space station to monitor the structural condition of the load-bearing components, the pressure capsule skin, and other components during service in order to prevent structural fatigue or damage. Additionally, large space structures such as space stations need to be docked and rendezvoused many times during their service, and the impact of docking on the structure needs to be tested. However, for spacecraft, working in the space environment, which contains strong magnetic noise and floating charged particles, poses great challenges to traditional electronic sensors.

Optical fiber sensing technology has been developing along with optical fiber communication technology in recent decades. Fiber Bragg Gratings (FBG) are one of the most widely used fiber sensors due to their optical selectivity. Compared with traditional sensors, FBG have unique advantages in space, such as strong anti-electromagnetic interference performance, small size and wide measurement ranges [5]. FBG can monitor the temperature, strain and other parameters of spacecraft structure with high precision and stability in real time under a complex environment, and sense the state data of a spacecraft structure in an all-round way so as to effectively ensure the normal operation of spacecraft and the life safety of astronauts, and improve the reliability and safety of space missions. In 2022, Li et al. [6] used a wide-range FBG strain sensor with a surface-attached carbon fiber matrix to measure strain and load on aircraft landing gear, and the strain sensitivity was 1.14 pm/με. Okagawa et al. [7] pasted FBG onto a test piece and combined the data collected by the sensor with machine learning to achieve real-time damage classification and structural health monitoring requirements. These were monitored based on FBG axial stress changes.

However, FBG will be affected by lateral stress in practical application. In general, in order to detect all the strain information of the structure, it is necessary to deploy multiple optical fiber sensors inside the material with their vertical axes in the same direction as the stress. Obviously, it is very difficult to lay out these huge fiber Bragg grating sensors, and directly leads to increases in system cost, a more complex structure, and even potentially the weakening of the main material, causing influence or even damage to it [8,9,10,11,12,13]. In addition, this method needs to process a large amount of data when applied to large-scale systems, which greatly limits the application of fiber Bragg gratings in engineering. Therefore, in view of the difficulties in lateral force monitoring of spacecraft, fiber Bragg gratings’ radial stress characteristics are considered for sensing.

Udd et al. [14] proposed the fabrication of overlapping Bragg gratings on polarization-maintaining fiber; that is, two gratings with different center wavelengths are written at the same position to realize lateral strain measurement. Because of the birefringence effect, four Bragg peaks will be output. The demodulation of the two transverse strains, longitudinal strain and temperature can be realized by using four equations. Scholars further confirmed that the polarization-maintaining fiber sensor can simultaneously measure the lateral, axial strain and temperature, and analyze the spectral response of bow tie-type and elliptical cladding-type fiber gratings under lateral load conditions [15,16,17]. Edmon et al. [18] investigated the lateral load sensitivity of different high birefringence (HiBi) FBG. The wavelength reflected by FBG in each polarization eigenmode can be measured independently using a customized demodulation system. The experimental results show that the lateral loading sensitivity of HiBi-FBG obtained by elliptic cladding fiber is the highest, at 0.23 ± 0.02 nm/(N/mm).

Although literature has reported on the measurement of lateral stress with polarization-maintaining fiber gratings, there is still a long way to go before practical use. As a matter of fact, it takes specific skills to deal with the induced polarization effect, and customized devices are strictly required, making them incompatible with conventional single-mode fiber systems. Furthermore, when measuring radial pressure with polarization-maintaining fibers, the angle between the lateral stress and the polarization axes will directly affect the pressure sensitivity [19]. In addition, due to the mismatch between polarization-maintaining fiber and non-polarization fiber, insertion loss and attenuation cannot be neglected.

When the FBG is subjected to uniform lateral pressure, characteristics similar to those of PM fiber also appear [20,21,22]. Rachid Gafsi and Mahmoud A. El-Sheri [23] calculated that the pressure sensitivities are 6.17 pm/N and −0.4332 pm/N in the x and y polarization directions, respectively, within a force range is 0–100 N in the case of plane strain. Bao-Jin Peng et al. [24] realized pressure measurement through the demodulation of fiber Bragg grating peak cracking caused by transverse stress differences in a single-mode fiber core bonded on the outer surface of the cylinder. The measurement sensitivity is estimated to be ~27 pm/MPa, measured in the range of standard atmospheric pressure up to 100 MPa. Madhav et al. [22] used fiber Bragg grating cross-line sensor for strain measurement, with a sensitivity of 41 pm/με. Wada et al. [25] conducted later load tests on polyimide coated and uncoated fibers, and obtained sensitivities of ~0.06 nm/(N/mm) and ~0.08 nm/(N/mm), respectively. Fazzi et al. [26] used a roll-bearing pin to apply lateral load to FBG, and measured the wavelength differences of the Bragg peak to be 0.128 nm, 0.108 nm and 0.34 nm when the force was 2 N, 5 N and 20 N, respectively.

In this paper, a transverse stress detection method based on spectral split sensing using a fiber Bragg grating is proposed, which is for monitoring spacecraft–small particle collisions. A FBG lateral stress sensing system was established and verified, and the lateral detection threshold was analyzed. Its pressure characteristics were experimentally studied in the range of 4.0–7.0 N and error analysis was carried out. The pressure sensitivity was 14.76 pm/N and 32.29 pm/N at the split point and right peak, respectively. This study provides a basis for the selection of lateral stress sensing systems.

2. Lateral Stress Sensing Principle

The spectral peak reflected from the grating is at the Bragg wavelength (${\mathsf{\lambda }}_{\mathrm{B}}$), which is given by:

$${\lambda }_{\mathrm{B}}=2{\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}\mathrm{\Lambda }$$

(1)

where ${\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}$ is the effective refractive index of the core, and $\mathsf{\Lambda }$ is the grating period. According to Equation (1), external factors such as axial strain and temperature variations applied to the FBG can alter the grating period, which would cause the Bragg wavelength to drift.

Since the grating length is much larger than the cladding diameter, the deformation caused by the lateral load on the fiber can be considered planar deformation. Therefore, the stress in each direction of the FBG can be expressed as:

$$\left\{\begin{array}{l}{\mathsf{\sigma }}_{\mathrm{x}}(\mathrm{x},\mathrm{y})=\frac{2\mathrm{F}}{\pi \mathrm{h}\mathrm{D}}\\ {\mathsf{\sigma }}_{\mathrm{y}}(\mathrm{x},\mathrm{y})=-\frac{6\mathrm{F}}{\pi \mathrm{h}\mathrm{D}}\\ {\mathsf{\sigma }}_{\mathrm{z}}(\mathrm{x},\mathrm{y})=\mathrm{v}({\mathsf{\sigma }}_{\mathrm{x}}+{\mathsf{\sigma }}_{\mathrm{y}})\end{array}\right.$$

(2)

The relationship between refractive index change and stress can be expressed as:

$$\left[\begin{array}{l}\mathsf{\Delta }{\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{x}}\\ \mathsf{\Delta }{\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{y}}\end{array}\right]=-\frac{{\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}^3}{2\mathrm{E}}\left[\begin{array}{ccc}{\mathrm{P}}_{11}-2\mathsf{\nu }{\mathrm{P}}_{12}& (1-\mathsf{\nu }){\mathrm{P}}_{12}-\mathsf{\nu }{\mathrm{P}}_{11}& (1-\mathsf{\nu }){\mathrm{P}}_{12}-\mathsf{\nu }{\mathrm{P}}_{11}\\ (1-\mathsf{\nu }){\mathrm{P}}_{12}-\mathsf{\nu }{\mathrm{P}}_{11}& {\mathrm{P}}_{11}-2\mathsf{\nu }{\mathrm{P}}_{12}& (1-\mathsf{\nu }){\mathrm{P}}_{12}-\mathsf{\nu }{\mathrm{P}}_{11}\end{array}\right]\left[\begin{array}{l}{\mathsf{\sigma }}_{\mathrm{x}}\\ {\mathsf{\sigma }}_{\mathrm{y}}\\ {\mathsf{\sigma }}_{\mathrm{z}}\end{array}\right]$$

(3)

where E is the Young’s modulus of the fiber, v is Poisson’s ratio, $P_{ij}$ is the strain-optic coefficient of the elastic-optic tensor, F is the lateral pressure of the FBG, h is the force length, D is the diameter of the fiber, and ${\mathsf{\sigma }}_{\mathrm{x}},{\mathsf{\sigma }}_{\mathrm{y}}$, ${\mathsf{\sigma }}_{\mathrm{z}}$ are the stresses in x, y, z directions, respectively. For ordinary single-mode fiber, the values for each parameter are as follows: $\mathrm{E}=74.52 \mathrm{G}\mathrm{P}\mathrm{a}$, $\mathsf{\nu }=0.17$, ${\mathrm{P}}_{11}=0.121$, ${\mathrm{P}}_{12}=0.270$, ${\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}=1.447$.

Figure 1 shows the relationship between change in refractive index and force when the FBG is subjected to lateral stress with a force length of 4 mm. The solid black and red lines are the refractive index curves in the x and y polarization directions, respectively. It can be seen that when the FBG is subjected to a linear distribution of external forces, there is a good linear relationship between the change in refractive index and the force. The change in refractive index in the x polarization direction is higher than the change in refractive index in the y direction, which is due to the photoelastic coefficient of the fiber ${\mathrm{P}}_{11}<{\mathrm{P}}_{12}$.

According to Equations (1) and (3), the drift formula for the Bragg reflected wavelength is, in the x direction:

$$\mathsf{\Delta }{\mathsf{\lambda }}_{\mathrm{x}}=-\frac{{\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}^3\mathsf{\Lambda }}{\mathrm{E}}\{({\mathrm{P}}_{11}-2\mathsf{\nu }{\mathrm{P}}_{12}){\mathsf{\sigma }}_{\mathrm{x}}+[(1-\mathsf{\nu }){\mathrm{P}}_{12}-\mathsf{\nu }{\mathrm{P}}_{11}]({\mathsf{\sigma }}_{\mathrm{y}}+{\mathsf{\sigma }}_{\mathrm{z}})\}$$

(4)

and in the y direction:

$$\mathsf{\Delta }{\mathsf{\lambda }}_{\mathrm{y}}=-\frac{{\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}^3\mathsf{\Lambda }}{\mathrm{E}}\{({\mathrm{P}}_{11}-2\mathsf{\nu }{\mathrm{P}}_{12}){\mathsf{\sigma }}_{\mathrm{y}}+[(1-\mathsf{\nu }){\mathrm{P}}_{12}-\mathsf{\nu }{\mathrm{P}}_{11}]({\mathsf{\sigma }}_{\mathrm{x}}+{\mathsf{\sigma }}_{\mathrm{z}})\}$$

(5)

The lateral stress on the FBG is shown in Figure 2; z is the axial direction of the fiber, y is the direction of the radial force, and x is perpendicular to y. When a certain section of FBG with length h is subjected to a uniform lateral pressure F, the FBG can be approximately decomposed into three sub-grating series, where ${\mathrm{h}}_2$ is the grating force length, and ${\mathrm{h}}_1$ and ${\mathrm{h}}_3$ are the grating unstressed length.

3. Experimental Setup

The local lateral stress device for the FBG is shown in Figure 3. It includes a test bench, a test fiber, a balance fiber, a pressure rod, a balance rod, an iron thin plate, and weights. The test fiber (with FBG) and the balance fiber (no FBG) are SMF-28e, placed parallel on the test bench and glued at both ends. The pressure rod and balance rod are two parallel hexagonal columns with a side length of 4 mm, perpendicular to both fibers. Notably, the pressure rod directly contacts the FBG area, while the balance rod does not.

The purpose of this design is to ensure that the weights are placed stably on the iron plate, balancing the forces exerted on the two optical fibers. In this way, the gravity of the weight is applied evenly to the two fibers with a total of four stress points. Therefore, the force at each point in the experiment is a quarter of the gravity of the weight, iron plate, pressure rod and balance rod. During the experiment, all materials should be placed slowly and vertically to avoid grating fractures caused by pressure rod displacement or shear stress caused by fiber rotation.

Figure 4 shows a schematic diagram of the experimental setup. The experimental system consists of an ASE light source, a spectral analyzer, an optical circulator and the FBG lateral stress device. Light emitted from the wideband light source (wavelength range 1510–1610 nm, output power −30 dBm) is input to circulator port 1 and is incident into the FBG from port 2 through the optical circulator. The light reflected from the FBG passes through the circulator to port 3 and then enters the spectral analyzer (Yokogawa AQ6375, minimum resolution of 5 pm) for spectral analysis.

The spectra of the light source and the reflection spectra of the original FBG are shown in Figure 5. The light source is relatively flat, in the 1520–1610 nm range. The FBG used in this paper has a center wavelength of 1546.432 nm, a −3 dB bandwidth of 0.18 nm, and an FBG length of 10 mm.

Before applying different weights onto the thin plate, the system needs to be calibrated. The reflective spectra with and without lateral stress are shown in Figure 6. Different pressures (0, 4 N, 7 N) are applied by placing the pressure rod onto the non-FBG section. As can be seen from the figure, it can be seen that lateral pressure does not affect the FBG’s central wavelength, and only has a slight effect on the reflection intensity; thus, it can be neglected in the following experiments.

Figure 7 depicts the experimental system used in this study. After verifying the functionality of the experimental device, move the weights and pressure rod to point B in Figure 3b. The purpose of moving the weights to point B is to position their center of gravity between the pressure rod and the balance rod to ensure even application of force.

4. Results and Discussion

Lateral stress was applied at the central position of the FBG; the pressed length was 4 mm, and the two sides were not stressed. Different pressures (0, 1.5 N, 3 N,4 N) were applied by placing the pressure rod on the FBG, and the results are shown in Figure 8. It can be seen from the figure that the spectrum splitting phenomenon is not obvious when the lateral stress on the FBG is less than 4 N. The reason for this phenomenon is that when the stress is small, the Bragg wavelength drift in the x-polarization direction is small, and the reflected peak intensity is low, which is superimposed with the Bragg peak in the y-polarization direction, where the reflected peak intensity is high. Bragg peaks in the x direction are greatly influenced by those in the y direction, so the split appearance is not obvious. For ease of handling, we consider the split point display threshold for this FBG to be 4 N.

The experiment included lateral stress loading and unloading in a range from 4.0 N to 7.0 N, with a step of 0.5 N. The spectra are shown in Figure 9 and Figure 10, and the data are listed in

Table 1. Experimental results of local lateral forces and left peak, split point, and right peak wavelengths for the FBG.

Force/N	Left Peak $\left({\lambda }_{\mathrm{L}}\right)$/nm			Split Point $\left({\lambda }_{\mathrm{S}}\right)$/nm			Right Peak $\left({\lambda }_{\mathrm{R}}\right)$/nm
	Loading	Unloading	Error	Loading	Unloading	Error	Loading	Unloading	Error
4.0	1546.444	1546.448	0.004	1546.632	1546.648	0.016	1546.752	1546.776	0.024
4.5	1546.440	1546.440	0.000	1546.644	1546.664	0.020	1546.784	1546.808	0.024
5.0	1546.436	1546.444	0.008	1546.648	1546.676	0.028	1546.804	1546.828	0.024
5.5	1546.436	1546.440	0.004	1546.652	1546.676	0.024	1546.816	1546.844	0.028
6.0	1546.436	1546.440	0.004	1546.668	1546.692	0.024	1546.848	1546.872	0.024
6.5	1546.436	1546.436	0.000	1546.672	1546.696	0.024	1546.868	1546.888	0.020
7.0	1546.432	1546.432	0.000	1546.684	1546.684	0.000	1546.892	1546.888	−0.004

. Let the left peak wavelength shift be $\mathsf{\Delta }{\mathsf{\lambda }}_{\mathrm{L}}$, the split point wavelength shift be $\mathsf{\Delta }{\mathsf{\lambda }}_{\mathrm{S}}$, and the right peak wavelength shift be $\mathsf{\Delta }{\mathsf{\lambda }}_{\mathrm{R}}$; the wavelength difference between the left and right peaks is $\mathsf{\Delta }{\mathsf{\lambda }}_{\mathrm{R}-\mathrm{L}}={\mathsf{\lambda }}_{\mathrm{R}}-{\mathsf{\lambda }}_{\mathrm{L}}$.

It can be seen from Figure 9a that the wavelength of the left peak is almost consistent with the central wavelength of the FBG without stress, while the split point and right peak drift to the right as stress increases. As can be seen from the figure, the depth of the split point deepens with increasing stress. This is because as the stress increases, the right peak gradually drifts towards a longer wavelength, while the left peak’s wavelength hardly changes. This results in a decrease in the overlapping area of the two peaks, resulting in a decrease in the intensity of the reflection and an increase in the depth of the split point. The fact that the reflectance of the right peak decreases with increasing pressure can also be explained. There can be seen a similar trend in Figure 10a.

Combined with the data in Table 1, Figure 9b,c and Figure 10 were plotted. It can be noticed that the left peak moves into shorter wavelengths, but the range of movement is small, at about −3.14 pm/N. In contrast, the split point and right peak move into longer wavelengths, with a sensitivity of 16.57 pm/N and 45.14 pm/N, respectively. Furthermore, it can be seen from Figure 9c and Figure 10c that the sensitivity is the largest when using the difference between the two peaks.

Additionally, error analysis of the experimental data was carried out and is recorded in Table 1. It can be inferred from the table and figure that there is hysteresis in the FBG measurement. When the measured pressure is 5.0 N, the split point hysteresis error of FBG is the largest, at 0.028 nm. When the measured pressure is 5.5 N, the right peak hysteresis error of the FBG reflection spectrum is the largest, at 0.028 nm.

The interquartile range (IQR) method was used to analyze the sensor repeatability, and the boxplot results shown in Figure 11. As can be seen from the figure, the wavelength of the right peak fluctuates gradually with increasing pressure, and its stability is slightly lower than that of the split point. As a result, for following research in the future, when conducting the analysis, it is feasible for us to focus primarily on the correlation between pressure and the change in the wavelength of the split point. The return error and hysteresis in pressure measurement were discovered based on analysis of the pressure loading and unloading experiment data for the FBG.

In order to obtain reasonable measurement results, further error analysis was carried out on the data. The arithmetic mean is

$$\overline{\mathrm{k}}=\frac{{\mathrm{k}}_1+{\mathrm{k}}_2+\cdots +{\mathrm{k}}_{\mathrm{n}}}{\mathrm{n}}=\frac{{\displaystyle \sum _{\mathrm{j}=1}^{\mathrm{n}}{\mathrm{k}}_{\mathrm{i}}}}{\mathrm{n}}$$

(6)

where ${\mathrm{k}}_{\mathrm{i}}$ represents the sensitivity corresponding to the i-th measurement. The residual error of each measured value is

$${\mathrm{v}}_{\mathrm{i}}={\mathrm{k}}_{\mathrm{i}}-\overline{\mathrm{k}}$$

(7)

The standard deviation σ of a single measurement is obtained according to Bessel’s formula

$$\mathsf{\sigma }=\sqrt{\frac{{\displaystyle \sum _{\mathrm{j}=1}^{\mathrm{n}}{\mathrm{v}}_{\mathrm{i}}^2}}{\mathrm{n}-1}}$$

(8)

Through further data analysis, the residual distribution of the wavelength drift is shown in Figure 12, and conforms to the 3σ criterion. However, it can be seen from the picture that the maximum wavelength drift residual of the split point is 42.67 pm, while that of the right peak is 58 pm, so the right peak wavelength’s stability is lower than that of the split point.

In addition, the durability of the sensor was also studied. Three more loading–unloading rounds were carried out at 12, 24, and 48 h after the first round. Experimental data are shown in

Table 2. Experimental results of local lateral forces and left peak, split point, and right peak wavelengths for the FBG under different time intervals.

Interval Time (h)	Force/N	Left Peak $\left({\lambda }_{\mathrm{L}}\right)$/nm		Split Point $\left({\lambda }_{\mathrm{S}}\right)$/nm		Right Peak $\left({\lambda }_{\mathrm{R}}\right)$/nm
		Loading	Unloading	Loading	Unloading	Loading	Unloading
12	4.0	1546.424	1546.444	1546.596	1546.636	1546.716	1546.752
	4.5	1546.424	1546.442	1546.596	1546.644	1546.732	1546.784
	5.0	1546.420	1546.440	1546.604	1546.652	1546.748	1546.808
	5.5	1546.416	1546.438	1546.612	1546.656	1546.764	1546.848
	6.0	1546.416	1546.438	1546.624	1546.662	1546.796	1546.852
	6.5	1546.412	1546.436	1546.628	1546.678	1546.812	1546.868
	7.0	1546.420	1546.436	1546.652	1546.692	1546.864	1546.884
24	4.0	1546.428	1546.428	1546.596	1546.596	1546.696	1546.716
	4.5	1546.426	1546.424	1546.600	1546.602	1546.728	1546.728
	5.0	1546.426	1546.424	1546.604	1546.606	1546.744	1546.744
	5.5	1546.422	1546.426	1546.612	1546.612	1546.756	1546.764
	6.0	1546.422	1546.426	1546.624	1546.624	1546.780	1546.792
	6.5	1546.422	1546.422	1546.632	1546.628	1546.808	1546.812
	7.0	1546.424	1546.424	1546.632	1546.632	1546.824	1546.822
48	4.0	1546.431	1546.424	1546.604	1546.600	1546.724	1546.718
	4.5	1546.424	1546.426	1546.598	1546.602	1546.739	1546.738
	5.0	1546.429	1546.426	1546.609	1546.611	1546.753	1546.755
	5.5	1546.425	1546.423	1546.614	1546.619	1546.772	1546.771
	6.0	1546.417	1546.417	1546.625	1546.627	1546.805	1546.799
	6.5	1546.412	1546.420	1546.632	1546.629	1546.813	1546.818
	7.0	1546.422	1546.427	1546.653	1546.652	1546.867	1546.866

and Figure 13.

The sensitivities are recorded in

Table 3. Sensitivity of the left peak, split point and right peak after 12 h, 24 h and 48 h.

Interval Time (h)		Sensitivity (pm/N)
		Left Peak (k1)	Error	Split Point (k2)	Error	Right Peak (k3)	Error
12	Loading	−2.85	−0.56	18.00	2.22	46.57	3.21
	Unloading	−2.71	−0.42	17.57	1.79	43.43	0.07
24	Loading	−1.71	0.58	13.71	−2.07	41.42	−1.94
	Unloading	−1.00	1.29	12.71	−3.07	38.14	−5.22
48	Loading	−1.11	1.18	16.19	0.41	46.22	2.86
	Unloading	−4.38	−2.09	16.49	0.71	44.39	1.03

, and residual distribution is shown in Figure 14. It can be noted that there is a slight yet acceptable fluctuation, meaning that the sensing capability will not degrade over time.

5. Conclusions

In this paper, a lateral stress detection method based on spectral split sensing using a fiber Bragg grating is proposed for the monitoring of spacecraft–small particle collision. A local lateral stress device using FBG was designed and its functionality was verified by experiments. The lateral stress sensing characteristics of the FBG were tested, its sensitivity and linearity were analyzed, and error analysis was carried out. The experiment shows that the reflectance spectrum gradually splits into two peaks with increases in lateral stress. When the lateral stress is less than 4 N, the FBG split point is not obvious in the spectrum. In the 4.0–7.0 N range, the wavelength of the left peak remains almost unchanged, while the split point and right peak gradually drift in the long-wave direction, and the distance between the two peaks is linearly related to the stress. The split point had a pressure sensitivity of 16.57 pm/N, while the right peak had a pressure sensitivity of 45.14pm/N.

It can be seen that the FBG could achieve lateral force detection. According to the FBG’s lateral stress characteristics, combined with the axial strain measurement ability of the grating, FBGs can be further applied as part of a three-dimensional strain sensor network for shape reconstruction. Similarly, they can be combined with temperature sensors to achieve the simultaneous monitoring of spacecraft temperature and strain, so as to meet the requirements of current spacecraft structure health monitoring.