sensors Article
Orientation-Dependent Displacement Sensor Using an Inner Cladding Fiber Bragg Grating Tingting Yang, Xueguang Qiao *, Qiangzhou Rong * and Weijia Bao Physics Department, Northwest University, Xi’an 710069, China; [email protected] (T.Y.); [email protected] (W.B.) * Correspondence: [email protected] (X.Q.); [email protected] (Q.R.); Tel.: +86-137-0920-0861 (X.Q.); +86-158-0298-0702 (Q.R.) Academic Editors: Christophe Caucheteur and Tuan Guo Received: 18 July 2016; Accepted: 8 September 2016; Published: 11 September 2016
Abstract: An orientation-dependent displacement sensor based on grating inscription over a fiber core and inner cladding has been demonstrated. The device comprises a short piece of multi-cladding fiber sandwiched between two standard single-mode fibers (SMFs). The grating structure is fabricated by a femtosecond laser side-illumination technique. Two well-defined resonances are achieved by the downstream both core and cladding fiber Bragg gratings (FBGs). The cladding resonance presents fiber bending dependence, together with a strong orientation dependence because of asymmetrical distribution of the “cladding” FBG along the fiber cross-section. Keywords: inner cladding-FBG inscription; femtosecond laser; orientation bending
1. Introduction Displacement measurement is one of the critical issues in engineering applications (such as industrial and health monitoring) [1–4]. Fiber Bragg grating, as a smart optical device, has a great performance on monitoring displacement (bending) [5–8]. For instance, FBG inscription within two eccentric cores of a polymer fiber has been applied to measure fiber bending [9]. Besides, a bend sensor based on FBG inscribed in a single-mode fiber within a depressed-index structure has been proposed and experimentally demonstrated [10]. Recently, Villatoro et al. reported a direction-dependent sensor based on a fiber within an asymmetric three-core [11]. For these sensors, fiber-bending variation is retrieved from the wavelength response. Therefore, the temperature perturbation and complex interrogator are inescapable. A power-referenced interrogation technique is a suggested solution to those problems. Especially, cladding modes that are converted from the core mode with the method of core-mismatch [12,13] and post-processing [14,15] significantly lose with fiber bending. Tilted fiber Bragg grating (TFBG) is another typical device based on the coupling of the core mode to the amount of backward-propagating cladding modes for bending sensing [16–18]. In addition, long period gratings (LPGs) have also been widely employed to intrinsically address the coupling of the core-to-cladding mode whose transmission resonant dip is sensitive to bending [19–21]. However, when working with cladding modes, the discontinuity technique needs to be provided for ensuring the coupling of core-to-cladding modes, which may enlarge the light signal-to-noise ratio and make sensors fabrication complex. Another alternative technique based on FBG inscription over fiber cladding using a femtosecond laser side-illumination technique has been demonstrated and successfully utilized for displacement measurement in our previous work [22]. With the high-intensity and ultrashort pulses of the femtosecond laser, the nonlinear light-material interaction involving nonlinear multiphoton absorption and ionization will be induced, and the grating region can be formed in the fiber cladding [22]. The cladding-FBG resonance shows a great response to bending or deflecting on the fiber due to Sensors 2016, 16, 1473; doi:10.3390/s16091473
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cladding [22]. The cladding-FBG resonance shows a great response to bending or deflecting on the fiber due to induced propagation loss and a good orientation dependence because of the asymmetrical distribution of the cladding FBG along the fiber cross-section [23]. Moreover, power induced propagation loss and a good orientation dependence because of the asymmetrical distribution fluctuations (originating the light source, transmission lines, and connectors) be effectively of the cladding FBG alongfrom the fiber cross-section [23]. Moreover, power fluctuations can (originating from canceled out by monitoring the bend-insensitive core-mode reflection. the light source, transmission lines, and connectors) can be effectively canceled out by monitoring the In this paper, we report the performance for a FBG inscription over a fiber core and inner bend-insensitive core-mode reflection. depressed-index cladding in athe section of multi-cladding (MCF)over via the femtosecond laser In this paper, we report performance for a FBG fiber inscription a fiber core and inner side-illumination technique. This construction seems similar to the sensor proposed in the previous depressed-index cladding in a section of multi-cladding fiber (MCF) via the femtosecond laser report [24]. The technique. device hasThis a simple fabrication a great quality. inThe side-illumination construction seems and similar to the spectral sensor proposed the cladding previous mode-assisted coupling can be used to measure the fiber bending with high sensitivity and definite report [24]. The device has a simple fabrication and a great spectral quality. The cladding orientation dependence. mode-assisted coupling can be used to measure the fiber bending with high sensitivity and definite
orientation dependence. 2. Fabrication and Principle of QCF-FBG 2. Fabrication and Principle of QCF-FBG Figure 1 shows the schematic diagram of the FBG inscription system. The proposed grating is Figureusing 1 shows the schematic thelaser FBG outputs inscription system. The proposed fabricated a Ti:sapphire laserdiagram system.ofThe pulses of duration with a grating 1 kHz is fabricated using a Ti:sapphire laser polarized system. The laser outputs pulses of duration with a 1 kHz repetition rate, which emits a linearly light with a central wavelength of approximately repetition which a linearly polarized wavelength of approximately 800 nm. A rate, section of 10emits mm hydrogen-loaded (atlight 60 °Cwith andaacentral H2 pressure of 10 MPa for 15 days) 800 nm. A section 10 mm hydrogen-loaded (at 60 and a Hof pressure of 10 MPa for multi-cladding fiber of (produced by YOFC), with core and◦ C claddings 5 μm and 14 μm, 20 μm,1536days) μm, 2 multi-cladding fiber (produced byand YOFC), with corea and claddings of 5mode µm and µm, using 20 µm, 120 μm, is self-aligned (no-offset) spliced with leading-in single fiber14SMF a 36 µm, 120 µm, is self-aligned (no-offset) and spliced withThe a leading-in single mode fiberofSMF using a commercial compact fusion splicer (Fujikura FSM-60S). optical microscope image quadruple cladding fiber cross-section shown(Fujikura in FigureFSM-60S). 2a. It is seen the MCF has a step refractive commercial compact fusionissplicer Theclearly opticalthat microscope image of quadruple cladding cross-section is shown inofFigure 2a. It or is seen clearly that the thefiber. MCF The has acore stepofrefractive index (RI)fiber profile via the RI difference the dopant material within the fiber is highly with is surrounded by within a deeply cladding. index (RI) doped profile via thegermanium, RI differencewhich of the dopant or material the depressed-index fiber. The core of the fiber is Another cladding higher RIwhich wrapsison the first-layer and two more depressed highly doped withwith germanium, surrounded by acladding, deeply depressed-index cladding. external Another claddings are next to RI it. wraps on the first-layer cladding, and two more depressed external claddings cladding with higher are next to it.
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Figure 1. Schematic diagram of the experimental setup for “cladding” FBG fabrication. Figure 1. Schematic diagram of the experimental setup for “cladding” FBG fabrication.
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The laser beam is precisely focused along one side of the MCF core-cladding interface (~2 μm core offset) before inscription. The average pulse energy of the laser output is fixed at 0.65 mJ (controlled by an optical attenuator), which is optimized by trial and error. The exposure time lasts 60 s (i.e., 60,000 laser pulses) and then a 5 mm grating region in the fiber core and cladding can be achieved simultaneously. As in the zoomed photographic images shown in Figure 2b, the grating inscription region is located along one side of interface of the fiber core and inner cladding. The formation of the uniform periodic patterns is based on nonlinear light-material interactions involving nonlinear multiphoton absorption and ionization because of the high-intensity and Figure 2.2.(a)(a) Microscope image of theofMCF Inset shows theshows refractive cross-section Figurepulses, Microscope image the cross-section. MCF cross-section. Inset theindex refractive index ultrashort which is different from the UV-induced color-center photosensitivity [25,26]. of the MCF. (b) Photomicrograph of the gratings; (c) Schematic diagram of mode-coupling inside fiber. cross-section of the MCF. (b) Photomicrograph of the gratings; (c) Schematic diagram of In addition, those index changes are mediated by a densification from the nonlinear multiphoton mode-coupling inside fiber. ionization that causes local melting and rapid quenching in the dielectric material after The laser beam is precisely focused along one side of the MCF core-cladding interface (~2 µm the optical breakdown. Furthermore, the formation of cladding-FBG is consistent with type-II core Figure offset) 2c before inscription. The average of grating the laser output is fixed at 0.65The mJ shows the schematic diagram pulse of theenergy created structure configuration. damage gratings [25,27]. (controlled anmismatch optical attenuator), which optimized by trial and error. The exposure time lasts 60 s interface ofby the core between theis SMF and QCF is used to forward the core-to-cladding mode coupling and the backward cladding-to-core recoupling. The cladding modes coupled and the core mode will get reflected by the downstream cladding and core FBG, the cladding mode resonance will partially be recoupled back to the upstream SMF, and eventually returns to the interrogation system. Therefore, two well-defined resonances in the reflections have been achieved. What is special is that the inner index-depressed cladding cannot confine the cladding modes well
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(i.e., 60,000 laser pulses) and then a 5 mm grating region in the fiber core and cladding can be achieved simultaneously. As in the zoomed photographic images shown in Figure 2b, the grating inscription region is located along one side of interface of the fiber core and inner cladding. The formation of the uniform periodic patterns is based on nonlinear light-material interactions involving nonlinear multiphoton absorption and ionization because of the high-intensity and ultrashort pulses, which is Figure (a) UV-induced Microscope color-center image of thephotosensitivity MCF cross-section. InsetInshows the those refractive different from2. the [25,26]. addition, indexindex changes cross-section the MCF. from (b) Photomicrograph of the gratings; (c) Schematic diagram of are mediated by a of densification the nonlinear multiphoton ionization that causes local melting mode-coupling inside fiber. and rapid quenching in the dielectric material after the optical breakdown. Furthermore, the formation of cladding-FBG is consistent with type-II damage gratings [25,27]. Figure 2c 2c shows thethe created grating structure configuration. The Figure shows the the schematic schematicdiagram diagramof of created grating structure configuration. interface of the mismatch core between thethe SMF the core-to-cladding core-to-cladding The interface of the mismatch core between SMFand andQCF QCFisisused usedto to forward forward the mode coupling and the backward cladding-to-core recoupling. The cladding modes coupledand andthe the mode coupling and the backward cladding-to-core recoupling. The cladding modes coupled coremode modewill will reflected bydownstream the downstream cladding and core the mode cladding mode core getget reflected by the cladding and core FBG, the FBG, cladding resonance resonance will partially be recoupled back to the upstream SMF, and eventually returns to the will partially be recoupled back to the upstream SMF, and eventually returns to the interrogation interrogation system. Therefore, two well-defined resonances in the reflections have been achieved. system. Therefore, two well-defined resonances in the reflections have been achieved. What is special that the inner index-depressed cannot confine the cladding modes isWhat that is thespecial inner is index-depressed cladding cannotcladding confine the cladding modes well because of well the because of the special RI profile of the MCF. The fiber deformation not only influences the modes special RI profile of the MCF. The fiber deformation not only influences the modes coupling at the couplingjunction at the splicing but also loss the of propagation lossmodes of theincladding in the inner splicing but also junction the propagation the cladding the innermodes cladding. Hence, cladding. Hence,sensor the investigated sensor has ato great to fiber bending.because In addition, because the investigated has a great response fiberresponse bending. In addition, the effective the effective refractive index (RI) difference between the core and inner cladding is 0.032, the refractive index (RI) difference between the core and inner cladding is 0.032, the resonant modes resonant modes present a clear wavelength ofcenter 1.88 nm, and the center of the present a clear wavelength separation of 1.88 separation nm, and the wavelengths of thewavelengths reflection spectra reflection spectra are 1548.97 nm and 1547.09 nm, as shown by the red line in Figure 3. are 1548.97 nm and 1547.09 nm, as shown by the red line in Figure 3.
Figure3.3.Spectra Spectraof ofQCF-FBG QCF-FBGwith withand andwithout withoutbending. bending. Figure
Ingeneral, general,the theRI RIof ofsilica silicamaterials materialsisismodified modifiedby bythe thechange changeof ofthe thegeometrical geometricalcross-section cross-sectionof of In the fiber which is caused by bending-induced anisotropic strain. Therefore, once this configuration the fiber which is caused by bending-induced anisotropic strain. Therefore, once this configurationis bending or or deflecting thethe fiber introduces refractive index variations across the fiber that isachieved, achieved, bending deflecting fiber introduces refractive index variations across the fiber influence the reflection spectrum in several ways: the forward core-to-cladding mode coupling at the that influence the reflection spectrum in several ways: the forward core-to-cladding mode coupling SMF-MCF splicing junction (forward coupling loss); the propagation loss of the cladding modes at the SMF-MCF splicing junction (forward coupling loss); the propagation loss of the cladding between the splicing junctionjunction and downstream FBG (bend and the reflection loss of cladding modes between the splicing and downstream FBGloss); (bend loss); and the reflection loss of modes between the first depressed-index cladding and second cladding because can cladding modes between the first depressed-index cladding and second cladding because bending bending can introduce a strong first-to-second cladding coupling due to the downside of the depressed RI. introduce a strong first-to-second cladding coupling due to the downside of the depressed RI. Finally, Finally, the backward cladding-to-core recoupling at the SMF-MCF splicing junction have a the backward cladding-to-core recoupling at the SMF-MCF splicing junction will have a will significant significant fluctuation. Among these effects, the change in the cladding-to-core mode recoupling at fluctuation. Among these effects, the change in the cladding-to-core mode recoupling at the splicing the splicing junctiontois be thought to be dominant, view of the behavior the recoupled junction is thought dominant, especially especially in view ofinthe behavior that the that recoupled power
decreases and increases around its unbent stage, as shown in Figure 3. In addition, the transverse intensity distribution of the mode in the MCF will be altered as fiber bending [28], caused by the RI
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Sensors 2016, 16, 1473 and increases around its unbent stage, as shown in Figure 3. In addition,4 the of 7 power decreases transverse intensity distribution of the mode in the MCF will be altered as fiber bending [28], caused by the RI change of the fiber. It will reduce the recoupling efficiency from the backward propagating change of the fiber. It will reduce the recoupling efficiency from the backward propagating cladding cladding modes to the core of the upstream fiber. Therefore, the recoupled cladding mode will have modes to the core of the upstream fiber. Therefore, the recoupled cladding mode will have an an extremely high sensitivity to fiber bending. As a result, bending the fiber will induce a strong extremely high sensitivity to fiber bending. As a result, bending the fiber will induce a strong intensity intensity modulation over the recoupled cladding mode but will have no effect on the core mode, modulation over the recoupled cladding mode but will have no effect on the core mode, and thus and thus the power of the reflected core mode can be used as a reference to compensate for the the power of the reflected core mode can be used as a reference to compensate for the unwanted unwanted power fluctuations. In addition, the cladding resonance presents high orientation power fluctuations. In addition, the cladding resonance presents high orientation dependence as the dependence as the asymmetrical distribution of the cladding-FBG over the fiber cross-section. asymmetrical distribution of the cladding-FBG over the fiber cross-section.
3. 3. Experiment ExperimentResults Resultsand andDiscussion Discussion The displacement sensing shown in in Figure Figure 4. 4. The The schematic schematic diagram diagram for for the the displacement sensing system system is is shown The light light from an amplified spontaneous emission (ASE) is launched into the fabricated sensor through from an amplified spontaneous emission (ASE) is launched into the fabricated sensor through aa circulator. circulator. The The reflection reflection light light from from the the sensor sensor is is monitored monitored by by an an optical optical spectrum spectrum analysis analysis (OSA) (OSA) with a wavelength resolution of 0.02 nm. In the experiment, one side of the sensing probe heldonona with a wavelength resolution of 0.02 nm. In the experiment, one side of the sensing probe isisheld arotator rotatoratata afixed fixedstage stagewhich whichcan canchange changethe thebending bendingdirection, direction, and and the the other other free free end end is is fixed fixed to to aa translation μm resolution resolution providing providing displacement displacement along vertical. The translation stage stage with with aa 10 10 µm along the the vertical. The free-fiber free-fiber length downstream is carefully selected to ensure that fiber bending can achieve the maximum length downstream is carefully selected to ensure that fiber bending can achieve the maximum effect effect on the cladding resonance mode power. The power of the reflected cladding mode decrease with on the cladding resonance mode power. The power of the reflected cladding mode decrease with the the increasing fiber bending, bending,while whilethe theresonance resonancewavelength wavelength stays unchanged, as shown by blue the blue increasing fiber stays unchanged, as shown by the line line in Figure 3. The device shows response bending with the highestsensitivity sensitivityofof27.7 27.7dB/mm dB/mm at in Figure 3. The device shows thethe response to to bending with the highest at 60° ranging from −60 to +60 µ m (like an inverted V-shape), as shown in Figure 5. Besides, both the ◦ 60 ranging from −60 to +60 µm (like an inverted V-shape), as shown in Figure 5. Besides, both the intensity the core core mode mode remain remain unchanged. unchanged. intensity and and the the Bragg Bragg wavelength wavelength of of the
Figure Figure 4. 4. Schematic Schematic diagram diagram of of MCF-FBG MCF-FBG as as aa displacement displacement sensing sensing system. system.
Figure 5. Cladding Cladding resonance resonance mode mode power power and wavelength versus displacements. Figure
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the bending orientation dependence of sensor, the fiber is rotated5 of from 7 0° to In 360° with step of 20°,the and the bending-induced intensityofloss is recorded angle. order to acharacterize bending orientation dependence sensor, the fiberatiseach rotated fromThe 0◦ ◦ ◦ order to characterize theorientations bending orientation dependence of sensor, fiber is rotated from bending sensitivity is calculated, and isa recorded strong angular dependence of the to 360 In with a step ofof 20 different , and the bending-induced intensity loss atthe each angle. The bending 0° to 360° with a step of 20°, and the bending-induced intensity loss is recorded at each angle. The bending response has been achieved, as shown in Figure 6. It is caused by the asymmetrical sensitivity of different orientations is calculated, and a strong angular dependence of the bending bending sensitivity of different orientations calculated, andbya the strong angular dependence of of thethe distribution of theachieved, cladding grating over theisfiber cross-section, which is similar to our previous response has been as shown in Figure 6. It is caused asymmetrical distribution bending response has been achieved, as shown in Figure 6. It is caused by the asymmetrical work [23]. The maximum sensitivity is realized thetobending axis is parallel to the grating cladding grating over the fiber cross-section, which when is similar our previous work [23]. The maximum distribution of the cladding gratingresults over the fiber cross-section, which is similar to our previous plates, and the minimum sensitivity from the bending axis being at the orthogonal direction. sensitivity is realized when the bending axis is parallel to the grating plates, and the minimum work [23]. The maximum sensitivity is realized when the bending axis is parallel to the grating It is important to from note that the orientation function not completely symmetrical. This to behavior is sensitivity results the bending axis being at the is orthogonal direction. It is important note that plates, and the minimum sensitivity results from the bending axis being at the orthogonal direction. mainly due to that the fact that the change of the effective RI for the cladding mode caused by the orientation function is not completely symmetrical. This behavior is mainly due to that the fact It is important to note that the orientation function is not completely symmetrical. This behavior is different fiber bending orientations is different from thebyasymmetrical distribution of the that the change the effective RI for thechange cladding mode caused different fiber bending orientations mainly due toofthat the fact that the of the effective RI for the cladding mode caused by cladding-FBG over the fiber cross-section. is different from the asymmetrical distribution of the cladding-FBG over the fiber cross-section. different fiber bending orientations is different from the asymmetrical distribution of the cladding-FBG over the fiber cross-section.
Figure Figure 6. 6. Angular Angular dependence dependence of of the the displacement displacement responsivity responsivity of of the the sensor. sensor. Figure 6. Angular dependence of the displacement responsivity of the sensor.
The temperature response for the sensor is also investigated by placing the MCF-FBG in a The temperature response forfor the the sensor is also investigated by placing the MCF-FBG in a heating The temperature response sensor is also investigated by placing the MCF-FBG in a heating oven with an accuracy◦ of ±0.1 °C. The temperature is varied from 20 to 65 °C. For every ◦ oven with oven an accuracy ±0.1 C.ofThe temperature is varied from 20 tofrom 65 C. every heating with anofaccuracy ±0.1 °C. The temperature is varied 20For to 65 °C. temperature For every temperature point, the temperature is kept constant for 20 min in order to ensure a well-distributed point, the temperature kept constant for 20 min infor order to ensure a well-distributed temperature temperature point, theistemperature is kept constant 20 min in order to ensure a well-distributed temperature around the sensor probe before each record. We plot the cladding resonance temperature around thebefore sensoreach probe before We plot the cladding resonance around the sensor probe record. Weeach plot record. the cladding resonance wavelength as the wavelength as the function of temperature, as shown in Figure 7. It is seen that the wavelength shift wavelength as the function of temperature, in Figure 7. Itwavelength is seen that the function of temperature, as shown in Figureas7.shown It is seen that the shiftwavelength presents ashift linear presents a linear sensitivity ofof6.9 the intensity intensity of of thereflected reflectedcladding cladding mode ◦ C whereas presents a 6.9 linear sensitivity 6.9pm/°C pm/°C whereas whereas the mode is is sensitivity of pm/ the intensity of the reflected claddingthe mode is almost kept unchanged almost kept unchanged with the temperature rising, as shown in Figure 7. Therefore, almost kept unchanged with temperature as shownthein displacement Figure 7. Therefore, thethe with the temperature rising, as the shown in Figurerising, 7. Therefore, measurement displacement measurement is temperature independent, meanwhile giving it a potential displacement independent, measurement meanwhile is temperature independent, meanwhile giving it measurements a potential forfor is temperature giving it a potential for simultaneous of simultaneous measurements of displacement and temperature. simultaneous measurements displacement and temperature.of displacement and temperature.
Figure 7. 7.Temperature of “cladding” “cladding” FBG FBGreflection reflectionresonance resonanceincluded included Figure Temperatureresponse response performances performances of Figure 7. Temperature response performances of “cladding” FBG reflection resonance included wavelength and power wavelength and powerfluctuation fluctuationwith withincreasing increasing temperature. temperature. wavelength and power fluctuation with increasing temperature.
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4. Conclusions In this paper, a novel FBG inscribed in a multi-cladding single-mode fiber over the core and depressed-index cladding by a femtosecond laser is proposed and experimentally demonstrated. A reflection spectrum with two defined resonant modes is obtained corresponding to the core mode and cladding mode. The FBG-based device is employed for displacement measurement, and its sensitivity shows high orientation dependence. Moreover, the construction can provide remote sensing as a reflection probe, and the fabrication is simple and effective, making it a good candidate for structural health monitoring. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Nos. 60727004, 61077060, 61205080), the National High Technology Research and Development Program 863 (Nos. 2007AA03Z413, 2009AA06Z203), the Ministry of Education Project of Science and Technology Innovation (No. Z08119), the Ministry of Science and Technology Project of International Cooperation (No. 2008CR1063), the Shanxi Province Project of Science and Technology Innovation (Nos. 2009ZKC01-19, 2008ZDGC-14). Author Contributions: Tingting Yang and Weijia Bao conducted the experiment. Tingting Yang prepared the paper. Xueguang Qiao, Qiangzhou Rong and Weijia Bao proposed the idea and revised the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Jin, Y.X.C.; Chan, C.; Dong, X.Y.; Zhang, Y.F. Temperature-independent bending sensor with tilted fiber Bragg grating interacting with multimode fiber. Opt. Commun. 2009, 282, 3905–3907. [CrossRef] Baek, S.; Jeong, Y.; Lee, B. Characteristics of short-period blazed fiber bragg gratings for use as macro-bending sensors. Appl. Opt. 2002, 41, 631–636. [CrossRef] [PubMed] Albert, J.; Shao, L.Y.; Caucheteur, C. Tilted fiber Bragg grating sensors. Laser Photonics Rev. 2013, 7, 83–108. [CrossRef] Allsop, T.; Dubov, M.; Martinez, A.; Floreani, F.; Khrushchev, I.; Webb, D.J.; Bennion, I. Bending characteristics of fiber long-period gratings with cladding index modified by femtosecond laser. J. Lightwave Technol. 2006, 24, 3147–3154. [CrossRef] Zhao, D.H.; Chen, X.F.; Zhou, K.M.; Zhang, L.; Bennion, L.; MacPherson, W.N.; Barton, J.S.; Jones, J.D.C. Bend sensors with direction recognition based on long-period gratings written in D-shaped fiber. Appl. Opt. 2004, 43, 5425–5428. [CrossRef] [PubMed] Liu, Y.; Williams, J.A.R.; Bennion, I. Optical bend sensor based on measurement of resonance mode splitting of long-period fiber grating. IEEE. Photonics Technol. Lett. 2000, 12, 531–533. [CrossRef] Bao, W.J.; Qiao, X.G.; Rong, Q.Z.; Hu, N.F.; Yang, H.Z.; Feng, Z.Y.; Hu, M.L. Sensing characteristics for a Fiber Bragg grating inscribed over a fiber core and cladding. IEEE. Photonics Technol. Lett. 2015, 27, 709–712. [CrossRef] Rong, Q.Z.; Qiao, X.G.; Guo, T.; Bao, W.J.; Su, D.; Yang, H.Z. Orientation-dependent fiber-optic accelerometer based on grating inscription over fiber cladding. Opt. Lett. 2014, 39, 6616–6619. [CrossRef] [PubMed] Scarcia, W.; Palma, G.; Falconi, M.C.; de Leonardis, F.; Passaro, V.M.N.; Prudenzano, F. Electromagnetic Modelling of Fiber Sensors for Low-Cost and High Sensitivity Temperature Monitoring. Sensors 2015, 15, 29855–29870. [CrossRef] [PubMed] Smelser, C.W.; Mihailov, S.J.; Grobnic, D. Formation of Type I-IR and Type II-IR gratings with an ultrafast IR laser and a phase mask. Opt. Exp. 2005, 13, 5377–5386. [CrossRef] Mihailov, S.T.; Smelser, C.W.; Grobnic, D.; Walker, R.B.; Lu, P.; Ding, H.M.; Unruh, J. Bragg Gratings Written in All-SiO2 and Ge-Doped Core Fibers With 800-nm Femtosecond Radiation and a Phase Mask. J. Lightwave Technol. 2004, 22, 94–100. [CrossRef] Waltermann, C.; Doering, A.; Köhring, M.; Angelmahr, M.; Schade, W. Cladding waveguide gratings in standard single-mode fiber for 3D shape sensing. Opt. Lett. 2015, 40, 3109–3112. [CrossRef] [PubMed] Guo, T.; Shao, L.Y.; Tam, H.Y.; Krug, P.A.; Albert, J. Tilted fiber grating accelerometer incorporating an abrupt biconical taper for cladding to core recoupling. Opt. Exp. 2009, 17, 20651–20660. [CrossRef] [PubMed] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Orientation-Dependent Displacement Sensor Using an Inner Cladding Fiber Bragg Grating Tingting Yang, Xueguang Qiao *, Qiangzhou Rong * and Weijia Bao Physics Department, Northwest University, Xi’an 710069, China; [email protected] (T.Y.); [email protected] (W.B.) * Correspondence: [email protected] (X.Q.); [email protected] (Q.R.); Tel.: +86-137-0920-0861 (X.Q.); +86-158-0298-0702 (Q.R.) Academic Editors: Christophe Caucheteur and Tuan Guo Received: 18 July 2016; Accepted: 8 September 2016; Published: 11 September 2016
Abstract: An orientation-dependent displacement sensor based on grating inscription over a fiber core and inner cladding has been demonstrated. The device comprises a short piece of multi-cladding fiber sandwiched between two standard single-mode fibers (SMFs). The grating structure is fabricated by a femtosecond laser side-illumination technique. Two well-defined resonances are achieved by the downstream both core and cladding fiber Bragg gratings (FBGs). The cladding resonance presents fiber bending dependence, together with a strong orientation dependence because of asymmetrical distribution of the “cladding” FBG along the fiber cross-section. Keywords: inner cladding-FBG inscription; femtosecond laser; orientation bending
1. Introduction Displacement measurement is one of the critical issues in engineering applications (such as industrial and health monitoring) [1–4]. Fiber Bragg grating, as a smart optical device, has a great performance on monitoring displacement (bending) [5–8]. For instance, FBG inscription within two eccentric cores of a polymer fiber has been applied to measure fiber bending [9]. Besides, a bend sensor based on FBG inscribed in a single-mode fiber within a depressed-index structure has been proposed and experimentally demonstrated [10]. Recently, Villatoro et al. reported a direction-dependent sensor based on a fiber within an asymmetric three-core [11]. For these sensors, fiber-bending variation is retrieved from the wavelength response. Therefore, the temperature perturbation and complex interrogator are inescapable. A power-referenced interrogation technique is a suggested solution to those problems. Especially, cladding modes that are converted from the core mode with the method of core-mismatch [12,13] and post-processing [14,15] significantly lose with fiber bending. Tilted fiber Bragg grating (TFBG) is another typical device based on the coupling of the core mode to the amount of backward-propagating cladding modes for bending sensing [16–18]. In addition, long period gratings (LPGs) have also been widely employed to intrinsically address the coupling of the core-to-cladding mode whose transmission resonant dip is sensitive to bending [19–21]. However, when working with cladding modes, the discontinuity technique needs to be provided for ensuring the coupling of core-to-cladding modes, which may enlarge the light signal-to-noise ratio and make sensors fabrication complex. Another alternative technique based on FBG inscription over fiber cladding using a femtosecond laser side-illumination technique has been demonstrated and successfully utilized for displacement measurement in our previous work [22]. With the high-intensity and ultrashort pulses of the femtosecond laser, the nonlinear light-material interaction involving nonlinear multiphoton absorption and ionization will be induced, and the grating region can be formed in the fiber cladding [22]. The cladding-FBG resonance shows a great response to bending or deflecting on the fiber due to Sensors 2016, 16, 1473; doi:10.3390/s16091473
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cladding [22]. The cladding-FBG resonance shows a great response to bending or deflecting on the fiber due to induced propagation loss and a good orientation dependence because of the asymmetrical distribution of the cladding FBG along the fiber cross-section [23]. Moreover, power induced propagation loss and a good orientation dependence because of the asymmetrical distribution fluctuations (originating the light source, transmission lines, and connectors) be effectively of the cladding FBG alongfrom the fiber cross-section [23]. Moreover, power fluctuations can (originating from canceled out by monitoring the bend-insensitive core-mode reflection. the light source, transmission lines, and connectors) can be effectively canceled out by monitoring the In this paper, we report the performance for a FBG inscription over a fiber core and inner bend-insensitive core-mode reflection. depressed-index cladding in athe section of multi-cladding (MCF)over via the femtosecond laser In this paper, we report performance for a FBG fiber inscription a fiber core and inner side-illumination technique. This construction seems similar to the sensor proposed in the previous depressed-index cladding in a section of multi-cladding fiber (MCF) via the femtosecond laser report [24]. The technique. device hasThis a simple fabrication a great quality. inThe side-illumination construction seems and similar to the spectral sensor proposed the cladding previous mode-assisted coupling can be used to measure the fiber bending with high sensitivity and definite report [24]. The device has a simple fabrication and a great spectral quality. The cladding orientation dependence. mode-assisted coupling can be used to measure the fiber bending with high sensitivity and definite
orientation dependence. 2. Fabrication and Principle of QCF-FBG 2. Fabrication and Principle of QCF-FBG Figure 1 shows the schematic diagram of the FBG inscription system. The proposed grating is Figureusing 1 shows the schematic thelaser FBG outputs inscription system. The proposed fabricated a Ti:sapphire laserdiagram system.ofThe pulses of duration with a grating 1 kHz is fabricated using a Ti:sapphire laser polarized system. The laser outputs pulses of duration with a 1 kHz repetition rate, which emits a linearly light with a central wavelength of approximately repetition which a linearly polarized wavelength of approximately 800 nm. A rate, section of 10emits mm hydrogen-loaded (atlight 60 °Cwith andaacentral H2 pressure of 10 MPa for 15 days) 800 nm. A section 10 mm hydrogen-loaded (at 60 and a Hof pressure of 10 MPa for multi-cladding fiber of (produced by YOFC), with core and◦ C claddings 5 μm and 14 μm, 20 μm,1536days) μm, 2 multi-cladding fiber (produced byand YOFC), with corea and claddings of 5mode µm and µm, using 20 µm, 120 μm, is self-aligned (no-offset) spliced with leading-in single fiber14SMF a 36 µm, 120 µm, is self-aligned (no-offset) and spliced withThe a leading-in single mode fiberofSMF using a commercial compact fusion splicer (Fujikura FSM-60S). optical microscope image quadruple cladding fiber cross-section shown(Fujikura in FigureFSM-60S). 2a. It is seen the MCF has a step refractive commercial compact fusionissplicer Theclearly opticalthat microscope image of quadruple cladding cross-section is shown inofFigure 2a. It or is seen clearly that the thefiber. MCF The has acore stepofrefractive index (RI)fiber profile via the RI difference the dopant material within the fiber is highly with is surrounded by within a deeply cladding. index (RI) doped profile via thegermanium, RI differencewhich of the dopant or material the depressed-index fiber. The core of the fiber is Another cladding higher RIwhich wrapsison the first-layer and two more depressed highly doped withwith germanium, surrounded by acladding, deeply depressed-index cladding. external Another claddings are next to RI it. wraps on the first-layer cladding, and two more depressed external claddings cladding with higher are next to it.
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Figure 1. Schematic diagram of the experimental setup for “cladding” FBG fabrication. Figure 1. Schematic diagram of the experimental setup for “cladding” FBG fabrication.
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The laser beam is precisely focused along one side of the MCF core-cladding interface (~2 μm core offset) before inscription. The average pulse energy of the laser output is fixed at 0.65 mJ (controlled by an optical attenuator), which is optimized by trial and error. The exposure time lasts 60 s (i.e., 60,000 laser pulses) and then a 5 mm grating region in the fiber core and cladding can be achieved simultaneously. As in the zoomed photographic images shown in Figure 2b, the grating inscription region is located along one side of interface of the fiber core and inner cladding. The formation of the uniform periodic patterns is based on nonlinear light-material interactions involving nonlinear multiphoton absorption and ionization because of the high-intensity and Figure 2.2.(a)(a) Microscope image of theofMCF Inset shows theshows refractive cross-section Figurepulses, Microscope image the cross-section. MCF cross-section. Inset theindex refractive index ultrashort which is different from the UV-induced color-center photosensitivity [25,26]. of the MCF. (b) Photomicrograph of the gratings; (c) Schematic diagram of mode-coupling inside fiber. cross-section of the MCF. (b) Photomicrograph of the gratings; (c) Schematic diagram of In addition, those index changes are mediated by a densification from the nonlinear multiphoton mode-coupling inside fiber. ionization that causes local melting and rapid quenching in the dielectric material after The laser beam is precisely focused along one side of the MCF core-cladding interface (~2 µm the optical breakdown. Furthermore, the formation of cladding-FBG is consistent with type-II core Figure offset) 2c before inscription. The average of grating the laser output is fixed at 0.65The mJ shows the schematic diagram pulse of theenergy created structure configuration. damage gratings [25,27]. (controlled anmismatch optical attenuator), which optimized by trial and error. The exposure time lasts 60 s interface ofby the core between theis SMF and QCF is used to forward the core-to-cladding mode coupling and the backward cladding-to-core recoupling. The cladding modes coupled and the core mode will get reflected by the downstream cladding and core FBG, the cladding mode resonance will partially be recoupled back to the upstream SMF, and eventually returns to the interrogation system. Therefore, two well-defined resonances in the reflections have been achieved. What is special is that the inner index-depressed cladding cannot confine the cladding modes well
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(i.e., 60,000 laser pulses) and then a 5 mm grating region in the fiber core and cladding can be achieved simultaneously. As in the zoomed photographic images shown in Figure 2b, the grating inscription region is located along one side of interface of the fiber core and inner cladding. The formation of the uniform periodic patterns is based on nonlinear light-material interactions involving nonlinear multiphoton absorption and ionization because of the high-intensity and ultrashort pulses, which is Figure (a) UV-induced Microscope color-center image of thephotosensitivity MCF cross-section. InsetInshows the those refractive different from2. the [25,26]. addition, indexindex changes cross-section the MCF. from (b) Photomicrograph of the gratings; (c) Schematic diagram of are mediated by a of densification the nonlinear multiphoton ionization that causes local melting mode-coupling inside fiber. and rapid quenching in the dielectric material after the optical breakdown. Furthermore, the formation of cladding-FBG is consistent with type-II damage gratings [25,27]. Figure 2c 2c shows thethe created grating structure configuration. The Figure shows the the schematic schematicdiagram diagramof of created grating structure configuration. interface of the mismatch core between thethe SMF the core-to-cladding core-to-cladding The interface of the mismatch core between SMFand andQCF QCFisisused usedto to forward forward the mode coupling and the backward cladding-to-core recoupling. The cladding modes coupledand andthe the mode coupling and the backward cladding-to-core recoupling. The cladding modes coupled coremode modewill will reflected bydownstream the downstream cladding and core the mode cladding mode core getget reflected by the cladding and core FBG, the FBG, cladding resonance resonance will partially be recoupled back to the upstream SMF, and eventually returns to the will partially be recoupled back to the upstream SMF, and eventually returns to the interrogation interrogation system. Therefore, two well-defined resonances in the reflections have been achieved. system. Therefore, two well-defined resonances in the reflections have been achieved. What is special that the inner index-depressed cannot confine the cladding modes isWhat that is thespecial inner is index-depressed cladding cannotcladding confine the cladding modes well because of well the because of the special RI profile of the MCF. The fiber deformation not only influences the modes special RI profile of the MCF. The fiber deformation not only influences the modes coupling at the couplingjunction at the splicing but also loss the of propagation lossmodes of theincladding in the inner splicing but also junction the propagation the cladding the innermodes cladding. Hence, cladding. Hence,sensor the investigated sensor has ato great to fiber bending.because In addition, because the investigated has a great response fiberresponse bending. In addition, the effective the effective refractive index (RI) difference between the core and inner cladding is 0.032, the refractive index (RI) difference between the core and inner cladding is 0.032, the resonant modes resonant modes present a clear wavelength ofcenter 1.88 nm, and the center of the present a clear wavelength separation of 1.88 separation nm, and the wavelengths of thewavelengths reflection spectra reflection spectra are 1548.97 nm and 1547.09 nm, as shown by the red line in Figure 3. are 1548.97 nm and 1547.09 nm, as shown by the red line in Figure 3.
Figure3.3.Spectra Spectraof ofQCF-FBG QCF-FBGwith withand andwithout withoutbending. bending. Figure
Ingeneral, general,the theRI RIof ofsilica silicamaterials materialsisismodified modifiedby bythe thechange changeof ofthe thegeometrical geometricalcross-section cross-sectionof of In the fiber which is caused by bending-induced anisotropic strain. Therefore, once this configuration the fiber which is caused by bending-induced anisotropic strain. Therefore, once this configurationis bending or or deflecting thethe fiber introduces refractive index variations across the fiber that isachieved, achieved, bending deflecting fiber introduces refractive index variations across the fiber influence the reflection spectrum in several ways: the forward core-to-cladding mode coupling at the that influence the reflection spectrum in several ways: the forward core-to-cladding mode coupling SMF-MCF splicing junction (forward coupling loss); the propagation loss of the cladding modes at the SMF-MCF splicing junction (forward coupling loss); the propagation loss of the cladding between the splicing junctionjunction and downstream FBG (bend and the reflection loss of cladding modes between the splicing and downstream FBGloss); (bend loss); and the reflection loss of modes between the first depressed-index cladding and second cladding because can cladding modes between the first depressed-index cladding and second cladding because bending bending can introduce a strong first-to-second cladding coupling due to the downside of the depressed RI. introduce a strong first-to-second cladding coupling due to the downside of the depressed RI. Finally, Finally, the backward cladding-to-core recoupling at the SMF-MCF splicing junction have a the backward cladding-to-core recoupling at the SMF-MCF splicing junction will have a will significant significant fluctuation. Among these effects, the change in the cladding-to-core mode recoupling at fluctuation. Among these effects, the change in the cladding-to-core mode recoupling at the splicing the splicing junctiontois be thought to be dominant, view of the behavior the recoupled junction is thought dominant, especially especially in view ofinthe behavior that the that recoupled power
decreases and increases around its unbent stage, as shown in Figure 3. In addition, the transverse intensity distribution of the mode in the MCF will be altered as fiber bending [28], caused by the RI
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Sensors 2016, 16, 1473 and increases around its unbent stage, as shown in Figure 3. In addition,4 the of 7 power decreases transverse intensity distribution of the mode in the MCF will be altered as fiber bending [28], caused by the RI change of the fiber. It will reduce the recoupling efficiency from the backward propagating change of the fiber. It will reduce the recoupling efficiency from the backward propagating cladding cladding modes to the core of the upstream fiber. Therefore, the recoupled cladding mode will have modes to the core of the upstream fiber. Therefore, the recoupled cladding mode will have an an extremely high sensitivity to fiber bending. As a result, bending the fiber will induce a strong extremely high sensitivity to fiber bending. As a result, bending the fiber will induce a strong intensity intensity modulation over the recoupled cladding mode but will have no effect on the core mode, modulation over the recoupled cladding mode but will have no effect on the core mode, and thus and thus the power of the reflected core mode can be used as a reference to compensate for the the power of the reflected core mode can be used as a reference to compensate for the unwanted unwanted power fluctuations. In addition, the cladding resonance presents high orientation power fluctuations. In addition, the cladding resonance presents high orientation dependence as the dependence as the asymmetrical distribution of the cladding-FBG over the fiber cross-section. asymmetrical distribution of the cladding-FBG over the fiber cross-section.
3. 3. Experiment ExperimentResults Resultsand andDiscussion Discussion The displacement sensing shown in in Figure Figure 4. 4. The The schematic schematic diagram diagram for for the the displacement sensing system system is is shown The light light from an amplified spontaneous emission (ASE) is launched into the fabricated sensor through from an amplified spontaneous emission (ASE) is launched into the fabricated sensor through aa circulator. circulator. The The reflection reflection light light from from the the sensor sensor is is monitored monitored by by an an optical optical spectrum spectrum analysis analysis (OSA) (OSA) with a wavelength resolution of 0.02 nm. In the experiment, one side of the sensing probe heldonona with a wavelength resolution of 0.02 nm. In the experiment, one side of the sensing probe isisheld arotator rotatoratata afixed fixedstage stagewhich whichcan canchange changethe thebending bendingdirection, direction, and and the the other other free free end end is is fixed fixed to to aa translation μm resolution resolution providing providing displacement displacement along vertical. The translation stage stage with with aa 10 10 µm along the the vertical. The free-fiber free-fiber length downstream is carefully selected to ensure that fiber bending can achieve the maximum length downstream is carefully selected to ensure that fiber bending can achieve the maximum effect effect on the cladding resonance mode power. The power of the reflected cladding mode decrease with on the cladding resonance mode power. The power of the reflected cladding mode decrease with the the increasing fiber bending, bending,while whilethe theresonance resonancewavelength wavelength stays unchanged, as shown by blue the blue increasing fiber stays unchanged, as shown by the line line in Figure 3. The device shows response bending with the highestsensitivity sensitivityofof27.7 27.7dB/mm dB/mm at in Figure 3. The device shows thethe response to to bending with the highest at 60° ranging from −60 to +60 µ m (like an inverted V-shape), as shown in Figure 5. Besides, both the ◦ 60 ranging from −60 to +60 µm (like an inverted V-shape), as shown in Figure 5. Besides, both the intensity the core core mode mode remain remain unchanged. unchanged. intensity and and the the Bragg Bragg wavelength wavelength of of the
Figure Figure 4. 4. Schematic Schematic diagram diagram of of MCF-FBG MCF-FBG as as aa displacement displacement sensing sensing system. system.
Figure 5. Cladding Cladding resonance resonance mode mode power power and wavelength versus displacements. Figure
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the bending orientation dependence of sensor, the fiber is rotated5 of from 7 0° to In 360° with step of 20°,the and the bending-induced intensityofloss is recorded angle. order to acharacterize bending orientation dependence sensor, the fiberatiseach rotated fromThe 0◦ ◦ ◦ order to characterize theorientations bending orientation dependence of sensor, fiber is rotated from bending sensitivity is calculated, and isa recorded strong angular dependence of the to 360 In with a step ofof 20 different , and the bending-induced intensity loss atthe each angle. The bending 0° to 360° with a step of 20°, and the bending-induced intensity loss is recorded at each angle. The bending response has been achieved, as shown in Figure 6. It is caused by the asymmetrical sensitivity of different orientations is calculated, and a strong angular dependence of the bending bending sensitivity of different orientations calculated, andbya the strong angular dependence of of thethe distribution of theachieved, cladding grating over theisfiber cross-section, which is similar to our previous response has been as shown in Figure 6. It is caused asymmetrical distribution bending response has been achieved, as shown in Figure 6. It is caused by the asymmetrical work [23]. The maximum sensitivity is realized thetobending axis is parallel to the grating cladding grating over the fiber cross-section, which when is similar our previous work [23]. The maximum distribution of the cladding gratingresults over the fiber cross-section, which is similar to our previous plates, and the minimum sensitivity from the bending axis being at the orthogonal direction. sensitivity is realized when the bending axis is parallel to the grating plates, and the minimum work [23]. The maximum sensitivity is realized when the bending axis is parallel to the grating It is important to from note that the orientation function not completely symmetrical. This to behavior is sensitivity results the bending axis being at the is orthogonal direction. It is important note that plates, and the minimum sensitivity results from the bending axis being at the orthogonal direction. mainly due to that the fact that the change of the effective RI for the cladding mode caused by the orientation function is not completely symmetrical. This behavior is mainly due to that the fact It is important to note that the orientation function is not completely symmetrical. This behavior is different fiber bending orientations is different from thebyasymmetrical distribution of the that the change the effective RI for thechange cladding mode caused different fiber bending orientations mainly due toofthat the fact that the of the effective RI for the cladding mode caused by cladding-FBG over the fiber cross-section. is different from the asymmetrical distribution of the cladding-FBG over the fiber cross-section. different fiber bending orientations is different from the asymmetrical distribution of the cladding-FBG over the fiber cross-section.
Figure Figure 6. 6. Angular Angular dependence dependence of of the the displacement displacement responsivity responsivity of of the the sensor. sensor. Figure 6. Angular dependence of the displacement responsivity of the sensor.
The temperature response for the sensor is also investigated by placing the MCF-FBG in a The temperature response forfor the the sensor is also investigated by placing the MCF-FBG in a heating The temperature response sensor is also investigated by placing the MCF-FBG in a heating oven with an accuracy◦ of ±0.1 °C. The temperature is varied from 20 to 65 °C. For every ◦ oven with oven an accuracy ±0.1 C.ofThe temperature is varied from 20 tofrom 65 C. every heating with anofaccuracy ±0.1 °C. The temperature is varied 20For to 65 °C. temperature For every temperature point, the temperature is kept constant for 20 min in order to ensure a well-distributed point, the temperature kept constant for 20 min infor order to ensure a well-distributed temperature temperature point, theistemperature is kept constant 20 min in order to ensure a well-distributed temperature around the sensor probe before each record. We plot the cladding resonance temperature around thebefore sensoreach probe before We plot the cladding resonance around the sensor probe record. Weeach plot record. the cladding resonance wavelength as the wavelength as the function of temperature, as shown in Figure 7. It is seen that the wavelength shift wavelength as the function of temperature, in Figure 7. Itwavelength is seen that the function of temperature, as shown in Figureas7.shown It is seen that the shiftwavelength presents ashift linear presents a linear sensitivity ofof6.9 the intensity intensity of of thereflected reflectedcladding cladding mode ◦ C whereas presents a 6.9 linear sensitivity 6.9pm/°C pm/°C whereas whereas the mode is is sensitivity of pm/ the intensity of the reflected claddingthe mode is almost kept unchanged almost kept unchanged with the temperature rising, as shown in Figure 7. Therefore, almost kept unchanged with temperature as shownthein displacement Figure 7. Therefore, thethe with the temperature rising, as the shown in Figurerising, 7. Therefore, measurement displacement measurement is temperature independent, meanwhile giving it a potential displacement independent, measurement meanwhile is temperature independent, meanwhile giving it measurements a potential forfor is temperature giving it a potential for simultaneous of simultaneous measurements of displacement and temperature. simultaneous measurements displacement and temperature.of displacement and temperature.
Figure 7. 7.Temperature of “cladding” “cladding” FBG FBGreflection reflectionresonance resonanceincluded included Figure Temperatureresponse response performances performances of Figure 7. Temperature response performances of “cladding” FBG reflection resonance included wavelength and power wavelength and powerfluctuation fluctuationwith withincreasing increasing temperature. temperature. wavelength and power fluctuation with increasing temperature.
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4. Conclusions In this paper, a novel FBG inscribed in a multi-cladding single-mode fiber over the core and depressed-index cladding by a femtosecond laser is proposed and experimentally demonstrated. A reflection spectrum with two defined resonant modes is obtained corresponding to the core mode and cladding mode. The FBG-based device is employed for displacement measurement, and its sensitivity shows high orientation dependence. Moreover, the construction can provide remote sensing as a reflection probe, and the fabrication is simple and effective, making it a good candidate for structural health monitoring. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Nos. 60727004, 61077060, 61205080), the National High Technology Research and Development Program 863 (Nos. 2007AA03Z413, 2009AA06Z203), the Ministry of Education Project of Science and Technology Innovation (No. Z08119), the Ministry of Science and Technology Project of International Cooperation (No. 2008CR1063), the Shanxi Province Project of Science and Technology Innovation (Nos. 2009ZKC01-19, 2008ZDGC-14). Author Contributions: Tingting Yang and Weijia Bao conducted the experiment. Tingting Yang prepared the paper. Xueguang Qiao, Qiangzhou Rong and Weijia Bao proposed the idea and revised the paper. Conflicts of Interest: The authors declare no conflict of interest.
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