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Fiber Bragg gratings (FBGs) in a hollow eccentric fiber (HEF) have been proposed and demonstrated experimentally. The single-core and two-core HEF FBGs have been inscribed successfully using KrF excimer laser (248 nm), respectively. The temperature and axial strain sensing properties of the two samples have been measured. The experimental results indicate that the temperature and axial strain sensitivities of the two samples are similar, but they are smaller than that of conventional SMF-FBGs. Furthermore, the bending characteristics of the two-core HEF-FBG strongly depend on the bending direction due to the asymmetry of the fiber. Therefore, the proposed two-core HEF-FBGs facilitate temperature-compensated vector-bending sensing by measuring the difference between peak shifts of the two gratings. In addition, the two-core HEF-FBG can be a promising candidate for achieving two-channel filter since the signal crosstalk between the two cores can be largely eliminated by the central air hole.
© 2017 Optical Society of America
1. Introduction
In recent years, fiber Bragg gratings (FBGs) have attracted numerous interests in sensing and communication applications because of their versatile advantages such as their high sensitivity, electromagnetic immunity and compact size. The first permanent grating in an optical fiber was fabricated using the internal writing method by Hill et al. [1]. Meltz et al. [2] proposed the holographic technique to overcome the internal writing limitation. At present, these two methods have been replaced by the phase mask technique [3], which has an advantage of greatly simplifying the FBG’s manufacturing process, yielding a grating with high performance. Besides single-mode fibers (SMFs), plenty of FBGs have been written in special fibers, such as doped fibers, optical microfibers [4,5], polymer optical fibers [6,7], photonic crystal fibers [8–10], holey fibers [11,12]. The FBGs in these special fibers usually offer an opportunity to improve the sensitivities of the temperature, strain, bending or refractive index. Recently, the holey fibers (HFs) have received much attention as a strong evanescent field can generate due to their air hole. It is readily filled with fluid, therefore facilitating the detection of ambient change. Han et al. fabricated the FBG [13] and the long-period grating (LPG) [14] in a six-hole fiber and investigated the effect of the air hole’s size on the properties of the gratings. Jewart et al. [15] presented a FBG-based pressure sensor in two-hole microstructured fiber and studied the shift and split of the FBG resonant peak for the external hydrostatic pressure change. A double-layered Fabry-Perot resonator was formed by a hollow core fiber and used as an all-fiber magnetic field sensor [16]. However, there are few literatures to report the FBGs based on HFs with a big air hole and few cores.
The good performance and simple fabrication of the HEF allow important applications in in-fiber microfluidic and sensing devices [17, 18]. In this work, we have accomplished FBGs in single-core HEF and two-core HEF, respectively. The proposed FBGs have been fabricated with phase-mask technique by using 248 nm KrF excimer laser. We investigate the dependences of the resonant peaks on the temperature and axial strain for both single-core and two-core HEF-FBGs. Especially, the bending characteristics of the two-core HEF-FBG are experimentally studied. The measured results indicate that the FBG fabricated in the proposed fiber is suitable for distinguishing bending direction due to strong direction-dependent bending sensitivities.
2. Hollow eccentric fibers
Figure 1 shows the microscope images of the cross section of single-core and two-core HEFs. The HEF comprises a large central air hole, a quasi-elliptical core and an annular cladding. The geometrical parameters of single-core HEF and two-core HEF are somewhat different. For the single-core HEF, as shown in Fig. 1(a), the diameters of the central air hole and the cladding are 70.04 and 119.68 μm, respectively. The quasi-elliptical core has a major axis and a minor axis with their respective length 7.82 and 3.57 μm. For the two-core HEF displayed in Fig. 1(b), the diameters of the central air hole and the cladding are 71.43 μm and 117.79 μm, respectively. The lengths of the major and minor axes in the quasi-elliptical core are 7.68 and 3.53 μm, respectively. Both the single-core and two-core HEFs have a thin cladding between the air hole and the core and its thinnest section is about 1.85~2 μm. The difference of the effective refractive index between the core and cladding is about 0.0049.
3. FBGs inscribed in hollow eccentric fibers
The setup of the grating fabrication is shown in Fig. 2. Firstly, the ultraviolet (UV) light beam launched by KrF excimer laser (German TuiLaser Company, Bragg Star 200) passes through an aperture stop, which has a standard size of 5 mm × 5 mm. Then the cylinder lens with a focal length of 10 cm is used to focus the UV light upon the HEF behind the phase-mask. The period of the phase mask used in the experiment is 648nm. The HEF is supported by two coaxially rotatable fiber clamps with a resolution of 1°. One end of the HEF is butted with the single mode pigtail of Super-K supercontinuum laser (NKT Photonics). In order to couple the broadband light into the core of the HEF, two multi-axis platforms are used to precisely adjust the relative position between fibers. The optical spectrum analyzer (OSA) is connected with the other end of the HEF and applied to record the real-time transmission spectra. The inscription processes of single-core HEF-FBG and two-core HEF-FBG are identical.
To enable the inscription process successfully, before the UV exposure, the fibers were placed in a hydrogen chamber under 10 MPa high pressure for 7 days to increase its photosensitivity. During the grating inscription process, firstly, we observed the position of the fiber core by using a microscope. Then we rotated the two coaxially rotatable fiber clamps to acquire the required position of the fiber core as shown in the inset of Fig. 3. For the single-core HEF, we adjusted the fiber core as close to the phase mask as possible, as a result the writing efficiency will be maximized. For the two-core HEF, when the plane including two cores is perpendicular to the UV light, the two fiber cores will be irradiated by the same exposure dose. By this way, the two FBGs written in two cores are almost the same and can serve as a reference with respect to each other. The frequency and output energy of the UV-laser are 60 Hz, 4.0 mJ for single-core HEF and 60 Hz, 5.0 mJ for two-core HEF, respectively. Under those conditions, a deepest transmission dip is observed when the fibers are exposed for about 3-5 minutes.
Fig. 3 The measured transmission spectra of the two samples. (a) Single-core HEF-FBG and (b) two-core HEF-FBGs. The insets illustrate the exposure direction of the HEFs.
The measured transmission spectra of gratings in the single-core and two-core HEFs are shown in Fig. 3. Figure 3(a) shows the transmission spectrum of single-core HEF-FBG (sample I). Obviously, this grating has a resonant wavelength of ~942.4 nm and a dip of ~17 dB in the straight state. The resonant peak reveals a half bandwidth of ~0.07 nm. The transmission spectra of two-core HEF-FBGs (sample II) are shown in Fig. 3(b). It is noted that the spectra of two FBGs are slightly different. The most probable cause is that the core size of core 1 is slightly larger than that of core 2 due to the fabrication imperfection during the fiber drawing process. In detail, the dark blue transmission curve (FBG 1) has a resonant dip with its amplitude of ~8.1 dB and half bandwidth of ~0.09 nm at ~942.1nm, while the red one (FBG 2) has a resonant dip with its amplitude of ~8.3 dB and half bandwidth of ~0.12 nm at ~941.9nm. The amplitude of two-core HEF-FBGs are nearly half of the single-core HEF-FBG, resulting from the fact that the single-core fiber is as close to the exposure direction as possible while the two-core fiber is symmetrically exposed as shown in the insets of Fig. 3. In addition, in core 2 another small sharp spectral resonance is observed at the short wavelength side of the grating center line.
4. Sensing properties of the FBGS in HEF
In order to fully characterize the properties of HEF-FBGs, we studied their sensing performance experimentally. For sample I, the dependences of the measured resonant wavelength on temperature and axial strain are shown in Figs. 4(a) and 4(b), respectively. Experimental results show that the temperature and axial strain sensitivities are 5.2 pm/°С and 0.6 pm/με when the temperature range is 40 - 160 °С and the strain range is 0 - 1190 με, respectively. For sample II, Fig. 5(a) reveals the responses of the two-core HEF-FBGs to the temperature. The resonant wavelengths of two-core HEF-FBGs show a linear red shift with a sensitivity of 5.7 pm/°С for core 1 and 5.8 pm/°С for core 2. Meanwhile, the dependences of the resonant peaks on the axial strain are shown in Fig. 5(b). When the axial strain changes from 0 to 952 με, the two resonant peaks of the two-core HEF-FBGs shift about 0.5 nm (0.57 pm/με) for core 1 and 0.51 nm (0.58 pm/με) for core 2, respectively. As a consequence, the temperature and axial strain sensitivities of the two-core HEF-FBGs are almost identical and similar to that of single-core HEF-FBG. However, both the temperature and axial strain sensitivities of our two samples are slightly lower than that of conventional SMF-FBGs [19, 20]. The embedded core of HEF is supported only by a thin cladding with its thinnest section of about 1.85~2 μm, and besides the thin cladding is the air. According to the previous reference [21], the thin cladding fiber has low sensitivities for temperature and strain when the fiber core is surrounded by the medium (air) with a small thermooptic coefficient compared with the fused silica cladding. In addition, the sensitivities of the two cores in sample II are not completely identical, both the temperature and axial strain sensitivities of core 2 are slightly larger than those of core 1, which is mainly caused by a subtle structure discrepancy between the two cores.
Fig. 4 The measured resonant wavelength of the single-core HEF-FBG as a function of (a) the temperature and (b) the axial strain.
Fig. 5 The measured resonant wavelength of the two-core HEF-FBGs as a function of (a) the temperature and (b) the axial strain.
Furthermore, the responses of the resonant wavelength of the two-core HEF-FBGs to the curvature at three different directions were investigated. Prior to the bending measurement we need to confirm the initial position of the fiber core by a microscope. The experimental setup for measuring the bending characteristics is shown in Fig. 6(a). A micrometer caliper was used to induce a curvature in the gratings. The bending orientation of the sample can be adjusted accurately by rotating the fiber clamp, which is a 360° rotator with a precision of 1°. To remove the influence of the temperature and strain, all the bending experiments were carried out at room temperature of 20 °С, and a 5g weight was hung on the fiber to keep a constant strain. In the experiments, the bending properties with respective to three directions (0°, 90° and 180°) for each core were investigated individually. The bending directions are illustrated in the Fig. 6(b). The 0° bending direction of core 1 corresponds to the 180° bending direction of core 2 and vice versa. When the measurement for one direction was completed, the micrometer was restored back as before to make the FBG in the straight state, then both fiber clamps were rotated by 90° simultaneously. We repeated the same steps in the rest bending directions, and obtained the curvature dependences of the resonant wavelengths for different bending directions.
Fig. 6 (a) Schematic of the experimental setup for testing bending characteristics. (b) Illustrations of the bending direction angles.
For each direction, the shift of the resonant peak is observed for a bending curvature range of 0 - 4.759 m−1. As shown in Fig. 7(a), when the core 1 of sample II is bent at 180° orientation, the resonant peak of core 2 moves to the short wavelength whereas the resonant peak of core 1 exhibits a red shift simultaneously. The resonant wavelength dependences on curvatures for three orientations are experimentally demonstrated in Fig. 7(b). It is obvious that all the bending properties show good linear responses and the bending sensitivities are obviously different. For core 1, the gratings are sensitive to the bending curvature for the directions of 0° and 180°, where the gratings and the fiber central axis lie in the bend plane. Moreover, the bending responses along 0° and 180° directions are opposite. The blue shift and red shift of the resonant peaks correspond to the bend-induced compression and extension of the core, respectively. The resonant wavelength of the FBG undergoes much larger shift in the bending direction of 180° than that in the bending direction of 0°. Similar to the previous reference [22], the two-core HEF without FBGs is sensitive to the bending angle of 180°, but in the opposite bending direction (0°), the fiber has a strong antibending capacity due to the big air hole and the asymmetric distribution of the cladding around the core. For 90° bending direction, the plane including gratings and the fiber central axis is perpendicular to the bending plane. Both the gratings in core 1 and core 2 shift hardly and show a bend insensitive property, due to weak compression or extension in this direction. The maximum shift of the resonant peaks does not exceed 0.01 nm under the bending curvature of 4.76 m−1.
Fig. 7 (a) The measured transmission spectra for different curvatures when the core 1 is in the bending direction of 180°. (b) The resonant wavelength dependence on the curvatures for the bending directions of 0°, 90° and 180°.
As a consequence, the bending response of the two-core HEF-FBGs has a sensitivity up to 33 pm/m−1 and is relatively low comparable to the sensors based on eccentric core fiber reported by Kong et al [23] and Chen et al [6]. The fiber used in our experiment has a big air hole in the center, leading to a weak fluctuation of the effective refractive index of the core mode. Therefore, we can observe a relatively low bending sensitivity of HEF-FBGs. In fact, the bending sensitivity can be doubled by measuring the difference between peak shifts of the two gratings in two-core HEF.
5. Conclusion
In conclusion, we proposed and experimentally demonstrated novel FBGs inscribed in HEF. The high-quality single-core HEF-FBG and two-core HEF-FBG were fabricated successfully, respectively. The responses of the two samples to temperature and axial strain were investigated. The temperature and axial strain sensitivities of the two samples are similar, and they are slightly lower than that of conventional SMF-FBGs due to the large air hole and the thin cladding in the HEF. The measured bending characteristics of the two-core HEF-FBGs show that bending directions strongly affect the bending properties. For the direction of 90° both the FBGs in core 1 and core 2 are immune to the bending. While for the directions of 0° and 180°, the FBG resonant wavelengths exhibit blue and red shifts, respectively. Therefore, the HEF-FBGs have a capability to measure bending curvature and distinguish bending direction simultaneously. Moreover, since the temperature and axis strain sensitivities of the two gratings in two-core HEF are the same, two-core HEF-FBG can be applied as a temperature-compensated vector-bending sensor by measuring the peak wavelength difference of the two gratings. In addition, the signal cross talk between two cores in the two-core HEF can be prevented as the two cores were separated by the large air hole. The proposed two-core HEF-FBGs will be promising to realize two-channel filters and fluidic sensors.
Funding
National Natural Science Foundation of China (NSFC) (U1231201, 61675054, 61275094, 61411130152); Natural Science Foundation of Heilongjiang Province in China (LC201424, A2015014); Foundation for University Key Teacher by Heilongjiang province (1254G014); 111 project to the Harbin Engineering University (B13015).
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