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Abstract
A two-dimensional vector bending sensor that is both compact and simple is proposed and demonstrated, based on an edge-core cladding-type fiber Bragg grating (ECLFBG) inscribed in an edge-core. The ECLFBG is written parallel to the edge-core using a femtosecond laser point-by-point technique. The reflection spectrum of this ECLFBG varies significantly depending on the magnitude and direction of the fiber’s bend. Combining the trend and sensitivity of the wavelength shift and reflection intensity variations of the ECLFBG, the bending magnitude and direction can be measured simultaneously.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last decade, gratings have been intensively developed as devices for optical sensing applications [1–6]. Fiber Bragg gratings (FBGs) have significant advantages in vector bending measurements because of their high sensitivity, light weight, compact structure, and immunity to electromagnetic interference. Moreover, optical fiber vector bending sensors have been extensively studied for a variety of applications, such as intelligent machinery, structural health monitoring, aerospace industry, and shape monitoring [3–6].
One-dimensional bending sensing can be achieved by breaking the cylindrical symmetry of the fiber, such as using tilted FBGs [7–8], off-axis FBGs [9–10], eccentric FBGs [11], and long-period fiber gratings (LPGs) [12]. However, these sensors can only identify one-dimensional positive or negative directions. A straightforward way to realize two-dimensional bending sensing is to combine two or more one-dimensional sensors. Typical sensors include cascaded orthogonal tilted FBGs [13], cascaded orthogonal LPGs [14], cascaded orthogonal eccentric core FBGs [15], and parallel orthogonal cladding FBGs [16]. The multicore FBGs are good solutions for multi-point vector bending sensing [17–18], however the use of fan-in and fan-out devices increases cost and complexity of the system. Although these sensors have been demonstrated to be capable of measuring bending and distinguishing directions, writing multiple gratings into a fiber requires precise alignment operations, and ensuring sensor consistency is difficult. In addition, a two-dimensional bending sensor can also be fabricated by nesting multiple fiber gratings together, which results in significant packaging difficulty and larger volume [19]. Luna developed an optical frequency domain reflectometer for a commercial shape sensing system, however, it requires a high accuracy tunable laser source, resulting in high cost [20]. However, an interference vector bending sensor based on a multicore fiber consisting of strongly coupled cores [21–23], the wavelength shift and intensity variations are used in a single sensor to realize vector bending sensing, but the interferometric structure has poor multiplexing capability.
In this article, we propose a novel vector bending sensing mechanism based on an edge-core cladding fiber Bragg grating (ECLFBG). The femtosecond laser direct writing method was used to inscribe 9 µm offset ECLFBG near the edge-core. The tracking of the wavelength and intensity of the ECLFBG’s reflection spectrum which responds differentially to bend directions enables the determination of the bending magnitude and direction. Using only one grating, the proposed sensing mechanism greatly simplifies the vector bending sensor’s structure.
2. Fabrication and operating principle
Figure 1(a) shows a schematic diagram of the ECLFBG, which was inscribed in the cladding of an edge-core by femtosecond laser point-by-point technique. All inscriptions were performed using a Ti: sapphire laser system at 800 nm (Coherent Inc., Libra-USP-HE), emitting pulses of 50-fs pulse duration, and a variable repetition rate from 10 to 1000 Hz. A quarter-wave plate converts the polarization of the laser beam from linear to circular. After being reflected by the dichroic mirror, the laser beam was focused on the fiber through a microscope objective (ZEISS, 40x/0.75). The focused femtosecond laser can induce a refractive-index modulation spot with a diameter of smaller than 1 µm. To eliminate the cylindrical astigmatism of the fiber during laser processing, the fiber was immersed in an index-matching gel (Cargille 24317). The fiber can be moved arbitrarily in the X and Y directions with the help of two linear electric stages (Newport, XMS100) stacked on top of each other. During the fabrication process, the fiber was imaged by a microscope for real-time observation. When aligning the optical fiber, the focal spot can be precisely controlled to focus on the cladding by adjusting the electric stage. Any reflections from the connector or the end of the fiber must be avoided in all experimental measurements because they will cause changes in the relative amplitude.
The edge-core could be the core of eccentric core fiber or one of the offset outer-core of the multi-core fiber. In this experiment, we used one edge-core of a four-core fiber (9/125 µm) and the core-offset distance (A) from the edge-core to the fiber neutral planes is 32.5 µm. A cladding FBG with a grating length of 2 mm is written 9 µm offset (B) from the center of the edge-core. The ECLFBG period is approximately 1.61 µm, which corresponds to the third-order Bragg wavelength of approximately 1550 nm. From the photomicrograph of ECLFBG shown in Fig. 1(b), it can be observed that the refractive index (RI) modulation area of the ECLFBG is completely located in the cladding. It is monitored in real-time using broadband light from super luminescent diodes (Thorlabs, S5FC1005P) passing through the fiber during the laser exposure and recording either the spectra with an optical spectrum analysis (OSA, Yokogawa, AQ6370D). Figure 1(c) shows the reflection spectra of the ECLFBGs written with offset distances ranging from 6 µm to 10 µm. It can be observed that the peak reflectivity of the ECLFBGs inscribed with otherwise identical inscription parameters decrease with increasing offset from the core center, which exhibits characteristics similar to standard single-mode fiber-based cladding-type fiber Bragg grating (CLFBG) [16]. The orientation-dependent response of the cladding FBG depends on its offset distance. As the offset increases, the asymmetry degree of the structure increases, and the cladding FBG becomes more sensitive to orientation changes. The peak reflectivity of the FBGs inscribed with otherwise identical inscription parameters decreases with increasing offset from the core center. Therefore, we choose the cladding FBG offset by 9 µm as a tradeoff. The reflectivity of the 9 µm offset grating is only about 0.00186 %.
Fig. 1. (a) Schematic diagram of the ECLFBG, (b) photomicrograph of ECLFBG, (c) reflection spectra of ECLFBGs with various offsets.
Figure 2(a) shows a schematic diagram of bending induced wavelength shift of the ECLFBG. When a bend is applied to the ECLFBG, the grating will experience a bend-induced strain owing to its eccentric location. The ECLFBG, similar to multi-core FBGs [17], is compressed when located on the inner side of the bending fiber, resulting in a shift to short wavelengths. On the contrary, the ECLFBG is stretched when it is located on the outer side, resulting in a shift to longer wavelengths. Therefore, the wavelength shift is related to the magnitude and direction of the bending.
To understand the change in the reflected energy of ECLFBG caused by bending, the actual E-field of the bent fiber was simulated using finite element method [11]. As shown in Fig. 2(b), similar to the single-mode CLFBG [16], the fundamental mode of the core is reflected by the ECLFBG and evanescent coupling back to the upstream core. The E-field distribution in the core of a straight fiber is limited to the core-cladding interface, and a small part of the energy leaks into the cladding. When the fiber was bent, the mode field intensity distribution shifted away from the fiber center in the opposite direction of bending. The ECLFBG is located on one side of the cladding, and the mode field deviation determines the total change in the reflection intensity. When the mode field deviates from the ECLFBG, there is a significant decrease in reflected power, whereas the bending fiber in the opposite direction results in a significant increase in the reflected power. Therefore, the ECLFBG reflection intensity is also related to both magnitude and direction of the bending.
Fig. 2. (a) Schematic diagram of the principle of Bragg wavelength shift when the fiber is bent. (b) The E-field distribution of ECLFBG with bending direction at 105$^{\circ }$, 285$^{\circ }$. (c) The reflection spectra responses of ECLFBG to different bending condition.
It is important to note that a single cladding FBG in a standard concentric core fiber cannot be used for vector sensing because it has the same intensity variation in multiple bending directions or different curvatures. Vector sensing can only be realized by using orthogonal cladding FBGs [16]. The unique feature of the sensor in this work is that only one cladding FBG in the edge-core fiber is needed to measure the magnitude and direction of bending simultaneously.
3. Experimental results and discussion
Here we experimentally demonstrated the capability of the ECLFBG to measure the magnitude and direction of the bending. The experimental setup for the bending measurements is shown in Fig. 3(a). The upstream fiber was clamped using a rotatable fiber clamp and the bending direction of ECLFBG sensing part can be rotated and changed. The curvature of the ECLFBG was set by adjusting the height of the Z-axis translation stages. Subsequently, the bending direction of the ECLFBG was set by simultaneously adjusting the rotatable fiber clamp in 5${^\circ }$ increments. In all experimental measurements, any reflections from the end of the fiber must be avoided because they will cause changes in the relative magnitude. The ECLFBG reflection spectra were recorded for each of the different directions and magnitudes. The bending reflection intensity sensitivity of the cladding FBG depends on its offset distance. Here, an ECLFBG with an offset of 9 µm was used to demonstrate the capability of vector bending. The azimuth angle ($\theta$=arctan(A/B)$)$ between ECLFBG and the horizontal is 74.52${^\circ }$.
The sensor was rotated relative to the direction of bending to measure the orientation-dependent responsivity for a range of 0–360${^\circ }$ (with a rotation step of 5${^\circ }$). Fig. 3 shows the reflection spectra responses of the ECLFBG bent in different directions with a curvature range from 0 ${\rm m}^{-1}$ to 15 ${\rm m}^{-1}$. When the bending direction is 0${^\circ }$ (Fig. 3(b)), the reflection spectrum of ECLFBG only exhibits a wavelength blue shift, and the reflection intensity remains constant, whereas at (f) 180${^\circ }$ only the wavelength is red shifted. When the bending direction is (d) 105${^\circ }$, the reflection intensity is significantly weakened, and the wavelength shift slightly, whereas at (h) 285${^\circ }$, only the reflection intensity is significantly increased. When the bending direction is (c) 50${^\circ }$, the wavelength blue shift and the reflection intensity decrease, whereas at (g) 230${^\circ }$, the wavelength red shift and the reflection intensity increase. When the bending direction is (e) 140${^\circ }$, the wavelength red shift and the reflection intensity decrease, whereas at (i) 320${^\circ }$, the wavelength blue shift and the reflection intensity increase. The azimuth angle ($\theta$) of the ECLFBG determines the phase difference between the orientation-dependence of the wavelength shifts and the reflection intensity changes. Therefore, each curvature magnitude and direction correspond to a unique wavelength and reflection intensity value.
Fig. 3. (a) The schematic of experiment setup for bending measurement and cross section of the ECLFBG, and the reflection spectra responses with bend direction at (b) 0${^\circ }$, (c) 50${^\circ }$, (d) 105${^\circ }$, (e) 140${^\circ }$, (f) 180${^\circ }$, (g) 230${^\circ }$, (h) 285${^\circ }$, (i) 320${^\circ }$.
Figure 4 shows the linear responses of the wavelength and the reflection intensity to bend curvatures from 0 ${\rm m}^{-1}$ to 15 ${\rm m}^{-1}$ in different directions. The wavelength sensitivity varies from -60.9 pm/${\rm m}^{-1}$ at 15${^\circ }$ to 59.7 pm/${\rm m}^{-1}$ at 195${^\circ }$. The minimum wavelength sensitivity values at 105${^\circ }$ and 285${^\circ }$ are -2.1 pm/${\rm m}^{-1}$ and 1.8 pm/${\rm m}^{-1}$ respectively. The reflection intensity sensitivity varies from -0.23 dB/${\rm m}^{-1}$ at 90${^\circ }$ and 0.22 dB/${\rm m}^{-1}$ at 270${^\circ }$ respectively. The minimum reflection intensity sensitivity values at 0${^\circ }$ and 180${^\circ }$ are 0.02 dB/${\rm m}^{-1}$ and -0.03 dB/${\rm m}^{-1}$, respectively, which are close to zero. It can be observed that the wavelength and intensity sensitivities of ECLFBG exhibit an independent change. Therefore, the wavelength and reflection intensity can be used as two separate parameters to directly determine the magnitude and direction of the bend.
Fig. 4. Linear response to bending (a) wavelength shifts and (b) reflection intensity of the ECLFBG in different directions.
If we only monitor the wavelength shift or reflection intensity variation, only the vector bending information in a range of 180${^\circ }$ can be demodulated. Each bending direction measurement has a unique sensitivity in that range, and its measurement value (wavelengths, or increase and decrease the reflection intensity) can be unequivocally correlated with the applied bending. However, owing to the symmetry of the fiber, the measured value for the remaining 180${^\circ }$ will be the same as that of the previous 180${^\circ }$ range, resulting in ambiguity in the sensor’s directional response. The two symmetrical bending directions caused the same measurement change in the monitored variable. Therefore, by combining the simultaneous measurement of two variables of wavelength shift and reflection intensity variation, the ambiguity can be resolved, and any bending direction can be identified in 360${^\circ }$ all directions.
Fig. 5 shows the wavelength and intensity sensitivities in different directions ranging from 0 to 360${^\circ }$. The orientation-dependent response sensitivities follow an approximately sinusoidal with a phase difference of approximately 75${^\circ }$, which corresponds exactly to the azimuth angle ($\theta$) of the grating plane of ECLFBG. Based on the response trend of the reflection spectrum (wavelength blue shift or red shift, intensity increase or decrease), the 0-360${^\circ }$ range can be divided into four areas: 0-105${^\circ }$, 105-180${^\circ }$, 180-285${^\circ }$, and 285-360${^\circ }$. Because the wavelength and reflection intensity have a good linear response in the curvature range from 0 to 15 ${\rm m}^{-1}$, linear fitting can be used to determine the exact magnitude and direction of the bend.
In Fig. 6, three elliptic curves are constructed using the wavelength as the horizontal coordinate and the intensity as the vertical coordinate, with curvatures of 3, 6, and 9 ${\rm m}^{-1}$ as examples. Clearly, because the curves do not intersect, each point on the plane in the figure corresponds to a unique magnitude and direction of curvature. The wavelength and reflection intensity detected by the ECLFBG corresponded to a unique magnitude and direction. Three repeated measurements of the ECLFBG, the average relative error is lower than 2.5 % for reconstructed amplitudes and less than 3.3 % for the bending direction. These can be averaged across multiple reconstructions to acquire a more accurate value. The signal-to-noise ratio measured by the demodulator results in a minimum detectable curvature of approximately 0.02 ${\rm m}^{-1}$.
Fig. 5. The wavelength shift and reflection intensity variation sensitivities at each fiber position.
Fig. 6. Wavelength and relative intensity of ECLFBG under the curvatures of 3, 6, and 9 ${\rm m}^{-1}$ versus bending direction.
Table 1 summarizes and compares the configurations and performances of several reported two-dimensional vector bending sensors. Using only one ECLFBG, the proposed sensing mechanism greatly simplifies the vector bending sensor’s structure.
Table 1. Comparison of the two-dimensional vector bending sensor.
4. Conclusion
In summary, a two-dimensional vector bending sensor based on ECLFBG is proposed and demonstrated. The magnitude and direction of the bend can be accurately measured in any direction by simply monitoring the Bragg wavelength and reflection intensity of the reflection spectrum. The unique feature of ECLFBG bending sensor is that only one eccentric core is needed to measure the magnitude and direction of bending with the assistance of a single CLFBG. The proposed sensing mechanism simplifies the vector bending sensor’s structure, paving the way for developing optical fiber 3D shape sensors and other vector sensors. Our device is aimed at applications where optical sensors are preferred over electrical ones, such as real-time structural health and condition monitoring, mechanical engineering of robotic arms, biomedicine and biomechanics that requires the sensors with direction-identification.
Funding
National Natural Science Foundation of China (61735014, 61905160, 61927812, 62075181).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
1. S. H. Badri, S. SaeidNahaei, and J. S. Kim, “Hybrid plasmonic slot waveguide with a metallic grating for on-chip biosensing applications,” Appl. Opt. 60(25), 7828–7833 (2021). [CrossRef]
2. S. H. Badri, “Transmission resonances in silicon subwavelength grating slot waveguide with functional host material for sensing applications,” Opt. & Laser Technol. 136, 106776 (2021). [CrossRef]
3. A. G. Leal-Junior, A. Frizera-Neto, M. J. Pontes, and T. R. Botelho, “Hysteresis compensation technique applied to polymer optical fiber curvature sensor for lower limb exoskeletons,” Meas. Sci. Technol. 28(12), 125103 (2017). [CrossRef]
4. M. Jang, J. S. Kim, S. H. Um, S. Yang, and J. Kim, “Ultra-high curvature sensors for multi-bend structures using fiber Bragg gratings,” Opt. Express 27(3), 2074–2084 (2019). [CrossRef]
5. G. Sun, H. Li, M. Dong, X. Lou, and L. Zhu, “Optical fiber shape sensing of polyimide skin for a flexible morphing wing,” Appl. Opt. 56(33), 9325–9332 (2017). [CrossRef]
6. L. Xu, J. Ge, J. H. Patel, and M. P. Fok, “Dual-layer orthogonal fiber Bragg grating mesh based soft sensor for 3-dimensional shape sensing,” Opt. Express 25(20), 24727–24734 (2017). [CrossRef]
7. P. Kisała, D. Harasim, and J. Mroczka, “Temperature-insensitive simultaneous rotation and displacement (bending) sensor based on tilted fiber Bragg grating,” Opt. Express 24(26), 29922–29929 (2016). [CrossRef]
8. L.-Y. Shao, L. Xiong, C. Chen, A. Laronche, and J. Albert, “Directional bend sensor based on re-grown tilted fiber Bragg grating,” J. Lightwave Technol. 28(18), 2681–2687 (2010). [CrossRef]
9. D. Feng, X. Qiao, and J. Albert, “Off-axis ultraviolet-written fiber Bragg gratings for directional bending measurements,” Opt. Lett. 41(6), 1201–1204 (2016). [CrossRef]
10. F. Chen, D. Su, X. Qiao, and Q. Rong, “Compact vector bend sensor using dual-off-axis innermost cladding-type FBGs,” IEEE Sensors J. 18(18), 7476–7480 (2018). [CrossRef]
11. X. Chen, C. Zhang, D. J. Webb, K. Kalli, and G.-D. Peng, “Highly sensitive bend sensor based on Bragg grating in eccentric core polymer fiber,” IEEE Photonics Technol. Lett. 22(11), 850–852 (2010). [CrossRef]
12. X. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of long-period fiber gratings,” J. Lightwave Technol. 20(2), 255–266 (2002). [CrossRef]
13. W. Bao, C. Wang, Y. Wang, N. Sahoo, and L. Zhang, “2D bending (curvature) recognition based on a combination of a TFBG and an orthogonal TFBG pair,” Appl. Phy. Express 12(7), 072009 (2019). [CrossRef]
14. P. Geng, W. Zhang, S. Gao, H. Zhang, J. Li, S. Zhang, Z. Bai, and L. Wang, “Two-dimensional bending vector sensing based on spatial cascaded orthogonal long period fiber,” Opt. Express 20(27), 28557–28562 (2012). [CrossRef]
15. J. Kong, A. Zhou, C. Cheng, J. Yang, and L. Yuan, “Two-axis bending sensor based on cascaded eccentric core fiber Bragg gratings,” IEEE Photonics Technol. Lett. 28(11), 1237–1240 (2016). [CrossRef]
16. F. Chen, X. Li, R. Wang, and X. Qiao, “Two-dimensional vector accelerometer based on orthogonal Bragg gratings inscribed in a standard single-mode fiber cladding,” Opt. Lett. 46(12), 2992–2995 (2021). [CrossRef]
17. M. Hou, K. Yang, J. He, X. Xu, S. Ju, K. Guo, and Y. Wang, “Two-dimensional vector bending sensor based on seven-core fiber Bragg gratings,” Opt. Express 26(18), 23770–23781 (2018). [CrossRef]
18. J. Cui, D. S. Gunawardena, Z. Liu, Z. Zhao, and H.-Y. Tam, “All-fiber two-dimensional inclinometer based on Bragg gratings inscribed in a seven-core multi-core fiber,” J. Lightwave Technol. 38(8), 2516–2522 (2020). [CrossRef]
19. J. Yi, X. Zhu, H. Zhang, L. Shen, and X. Qiao, “Spatial shape reconstruction using orthogonal fiber Bragg grating sensor array,” Mechatronics 22(6), 679–687 (2012). [CrossRef]
20. A. Beisenova, A. Issatayeva, I. Iordachita, W. Blanc, C. Molardi, and D. Tosi, “Distributed fiber optics 3D shape sensing by means of high scattering NP-doped fibers simultaneous spatial multiplexing,” Opt. Express 27(16), 22074–22087 (2019). [CrossRef]
21. J. Amorebieta, A. Ortega-Gomez, G. Durana, R. Fernández, E. Antonio-Lopez, A. Schülzgen, J. Zubia, R. Amezcua-Correa, and J. Villatoro, “Compact omnidirectional multicore fiber-based vector bending sensor,” Sci. Rep. 11(1),5989 (2021). [CrossRef]
22. J. Villatoro, A. Van Newkirk, E. Antonio-Lopez, J. Zubia, A. Schülzgen, and R. Amezcua-Correa, “Ultrasensitive vector bending sensor based on multicore optical fiber,” Opt. Lett. 41(4), 832–835 (2016). [CrossRef]
23. J. Villatoro, J. Amorebieta, A. Ortega-Gomez, E. Antonio-Lopez, J. Zubia, A. Schülzgen, and R. Amezcua-Correa, “Composed multicore fiber structure for direction-sensitive curvature monitoring,” APL Photonics 5(7), 070801 (2020). [CrossRef]
