A novel bending vector sensor based on spatial cascaded orthogonal long period fiber gratings (SCO-LPFGs) written by high-frequency CO2 laser pulses has been proposed, and two-dimensional bending vector sensing characteristics based on the simple SCO-LPFGs have been experimentally demonstrated. A three-dimensional orthogonal sensing coordinate system has been established, and the measurement results of the proposed SCO-LPFGs sensor based on the above coordinate system is given, and furthermore both of curvature and bending-direction could be intuitively solved according to the three-dimensional orthogonal sensing coordinates. The research work presented in this paper would be helpful to improve the practicability of fiber vector sensors due to the distinguished characteristics such as simple structure, low-cost, ease of fabrication.
©2012 Optical Society of America
In the past fifteen years, some direction fiber bending sensors capable of recognizing positive and negative directions of single-axis have been presented through writing long period fiber grating (LPFG) or fiber Bragg grating (FBG) in the eccentric core fiber [1, 2], or D-shaped cladding fiber [3–5]. However these fibers are difficult to be spliced with ordinary single-mode fibers (SMFs). Moreover, a few direction bending sensors based on asymmetric refractive index modulation were also reported through writing LPFG or tilted FBG in SMF or photonic crystal fiber (PCF) [6–12], which unfortunately could still only determine positive and negative directions of single-axis. In 2012, a fiber-optic directional bending sensor based on Mach–Zehnder interferometer exploiting lateral-offset and up-taper is reported , but it could still only determine positive and negative directions of single-axis.
In 2000, M. J. Gander, D. Macrae, and E. A. C. Galliot et al. firstly employed a four-core optical fiber to measure the bending characteristics as to two orthogonal axes simultaneously , and on this basis, in 2003, a two-axes curvature sensor was presented through writing FBG into four-core optical fiber . Actually, the above study extended the curvature measurement from single-axis to two-axes. Although only two orthogonal directions were measured, curvature measurement was fulfilled from one-dimension to two-dimension applications. In 2002, W. G. Zhang et al. have achieved two dimensions measurement of stress or displacement by bonding two different period FBGs onto the two adjacent orthogonal side faces near the fixed end along the neutral axis of a cantilever beam . However, this scheme requires two sensors placed at significantly difference areas of a bending apparatus. In 2005, Y. P. Wang and Y. J. Rao reported an LPFG sensor for measurement of curvature and bending direction [17, 18], with one UV-laser-fabricated LPFG (LPFGC) in charge of curvature measurement and two high-frequency-CO2-laser-fabricated LPFGs (LPFGA and LPFGB) for bend-direction recognition.
On the basis of the above work, a spatial cascaded orthogonal LPFGs (SCO-LPFGs) sensor that enables two-dimensional bending vector sensing is proposed in this paper. Relative to the afore-mentioned structures reported in reference 16 and 17, the proposed novel SCO-LPFGs sensor has several advantages such as simple configuration, compact size, high integration, and ease of fabrication. Besides, a three-dimensional orthogonal sensing coordinate system (3D-OSC) is established, and the measurement results of our proposed SCO-LPFGs sensor based on the 3D-OSC is given, which demonstrates a high-precision, intuitive, monotonic, and universally applicable interrogation method. The proposed SCO-LPFGs and interrogation method are also applicable for other vector sensing occasions, which expected to expand the functionality and applications of fiber vector sensor.
2. Experimental setup and principle
Figure 1(a) illustrates the experimental setup for the Corning-SMF-28-based SCO-LPFGs fabrication system. Figure 1(b) illustrates one part of the experimental setup (other parts are the same as Fig. 1(a)) for testing bending characteristics of the SCO-LPFGs. The steel beam will be placed in the position as shown only for testing bend characteristics. Figure 1(c) illustrates a partial enlarged detail of the rotation setup. One segment of SMF is placed inside a 300μm-diameter capillary tube and then fixed by two copper pillars and copper flakes which inserted through the slots of dial-1 and dial-2, respectively, as shown in the photograph of Fig. 1(c), the copper pillar-1 is fixed by screwing the bolt. The presence of slits in the slots could ensure that the copper flakes could move slightly along the slit with copper pillars. Therefore, the copper pillar-2 could move slightly along the axis of the disks to eliminate the bending-induced axial strain over the LPFG, and 50-g mass is attached at the end of the fiber in order to keep a constant applied tension. And on the other hand, both of the two copper pillars could rotate when the two dials rotate synchronously.
As two gratings with different periods are written in one same fiber, in order to avoid resonance peak superimposition, we select two gratings with periods of 500μm and 700μm, respectively. In our experiment, the transmission spectrum is monitored by an optical spectrum analyzer (OSA) (AQ6317B, ANDO) with a resolution of 0.02nm. The LPFGΛ = 700 is firstly fabricated, and then the optical fiber is rotated by 90° through the two dials. Finally, the LPFGΛ = 500 is fabricated.
As shown in Fig. 2 , the blue, black and red line represent the transmission spectra of LPFGΛ = 700, LPFGΛ = 500 and SCO-LPFGs, respectively. The transmission amplitude of LPFGΛ = 500 is close to 0 dB for a wavelength range of 1550 nm to 1640 nm, and thus in this band, the interaction between LPFGΛ = 500 and LPFGΛ = 700 is very weak, so dip A and B are selected for bending measurement. For bending characteristics test, the flexible tube is slowly moved to the position of the SCO-LPFGs, and then the flexible tube is pasted onto a 337.5-mm-long steel beam clamped between a translation stage and a fixed stage.
As shown in Fig. 1(b), when the distance specified as L between the two stages is reduced, steel beam and LPFGs will bend accordingly. Curvature obtained for certain translation is estimated by approximating the bent steel beam and fiber as an arc of a circle. The bending radius could be obtained from initial length L0 and the reduced length L between the two stages by using R·sin(L0/2R) = L/2. Figure 3 illustrates cross sections of the two orthogonal LPFGs, the darker color corresponds to greater refractive index modulation area. According to previous experimental and theoretical studies for high-frequency-CO2-laser- and UV-laser-fabricated LPFG in reference [3, 7, 17, 18], maximum and minimum bending-sensitivity for LPFGΛ = 500 and LPFGΛ = 700 are marked as max and min in Fig. 3, respectively.
In our experiment, the bending characteristics of the LPFGs are firstly tested for 0°, and the position for 0° is marked in Fig. 3. Every time we reduce the 0.35 mm distance by adjusting the stage. When finishing testing each group of data, the fiber is turned along its axis with an angle step of 45°, and then the above process is repeated.
3. Result and discussion
As shown in Fig. 4, resonance wavelength is insensitive to bending for the curvature range of 0~0.5 m−1, while it shifts rapidly and linearly against bending for the curvature range of 0.5~1.98 m−1. In addition, dip A is bend-insensitive at 0° and 180°, while it is ultra bend-sensitive at 90° and 270°; dip B is bend-insensitive at 90° and 270°, while it is very bend-sensitive at 0° and 180°. The bending sensitivity for other orientations is moderate.
In order to acquire accurate bending orientation information, resonance wavelengths for dip A and B are defined as the x-axis and y-axis to establish a new coordinate system. And curvature gradually increases outward from the center in this new coordinate system. As shown in Fig. 5 , once the resonance wavelengths of the dip A and B (for example: λ1 and λ2) are given, the accurate bending orientation information could be uniquely determined. The case of multiple solutions does not exist in the new coordinate system. And the value of curvature could be solved from Fig. 4 based on the determined resonance wavelength and bending orientation.
From the above analysis, the information on orientation and curvature could be interrogated through three coordinate systems (Fig. 4(a), 4(b) and Fig. 5). In order to make the proposed interrogation method more intuitive, as shown in Fig. 6 , a 3D-OSC for vector sensing is established, in which the resonance wavelengths of dip A and dip B respectively correspond to x and y axes that reflect the orthogonal nature of the SCO-LPFGs. In addition, the curvature is described by z axis, and thus curvature and bending orientation could both be intuitively solved based on the new 3D-OSC. For example, assume the measured resonant wavelengths are λ1 and λ2, respectively. Figure 6 demonstrates the simple and intuitive process to solve the curvature and the bending direction specified as C and θ, respectively.
In order to verify and further demonstrate the interrogation method and evaluate the sensing ability of the proposed sensor, under constant room temperature, the bending responses of the proposed sensor has been measured again for several directions among these eight directions at time t1, t2, t3, t4, t5. Curvature and the bending direction could be determined according to the measured wavelength values (λA, λB,) and the proposed interrogation method. As shown in Table 1 , the measured λA, λB, and the information on determined and actual values of curvature and bending direction are listed.
As shown in Table 1, the proposed SCO-LPFGs sensor is able to determine the curvature and the bending direction with the single-value properties, and the determined values of the curvature and the bending directions are basically in accordance with actual values. Obviously, curvature and bending direction identification precision of the proposed SCO-LPFGs sensor depends on the number of prior measured curvature and bending directions. Therefore, on some occasions which require high-resolution curvature and bending direction measurement, the number of prior measured reference values need to be increased.
A novel SCO-LPFGs bending vector sensor written by high-frequency CO2 laser pulses has been proposed. We integrated our previous proposed complex, bulky and heavy vector sensing system  into a single optical fiber, and have achieved a miniaturized and integrated fiber vector sensor. Compared with others structures, this novel LPFGs sensor has several advantages such as simple structure, compactness, and no need of expensive UV laser, femtosecond laser, phase mask, photosensitive fiber, and hydrogen-loaded process in the fabrication process. Another important merit of our scheme is that the two and three-dimensional orthogonal sensing coordinates systems have been proposed and demonstrated, and the measurement results of the proposed sensor based on the 3D-OSC system is given. And moreover, a high-precision, intuitive and universally applicable interrogation method with single-value properties is proposed based on the characteristics of the SCO-LPFGs and 3D-OSC system. The proposed SCO-LPFGs and interrogation method are also expected to be promising for other vector sensing applications.
This work was jointly supported by the National Natural Science Foundation under Grant Nos. 11274181, 10974100, 11274182, 10674075, 11004110, the Tianjin Key Program of Application Foundations and Future Technology Research Project under Grant No.10JCZDJC24300, and the Fundamental Research Funds for the Central Universities.
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