A stress sensor based on a dye-doped polymeric optical fiber is able to detect stress by simple comparison of two luminescence peaks from a pair of energy transfer organic dyes. Coumarin 540A (donor) and Rhodamine 6G (acceptor) were doped in the core and cladding of the fiber, respectively. For various laser wavelengths, the change in the near-field pattern and visible emission spectrum upon variation in the fiber bending diameter was evaluated. From a comparison with a low-numerical-aperture fiber, it is shown that the sensitivity of the sensor is controllable by optimization of the waveguide parameters

R. Furukawa

S. Kamimura

Applied Physics Letters

English

A stress sensor based on a dye-doped polymeric optical fiber is able to detect stress by simple comparison of two luminescence peaks from a pair of energy transfer organic dyes. Coumarin 540A (donor) and Rhodamine 6G (acceptor) were doped in the core and cladding of the fiber, respectively. For various laser wavelengths, the change in the near-field pattern and visible emission spectrum upon variation in the fiber bending diameter was evaluated. From a comparison with a low-numerical-aperture fiber, it is shown that the sensitivity of the sensor is controllable by optimization of the waveguide parameters.journal articl

Kamimura, S.

Furukawa, R.

C-RECS （Creative Repository of Electro-Communications)

国立大学法人電気通信大学 / The University of Electro-CommunicationsStrain sensing based on radiativeemission-absorption mechanism using dye-dopedpolymer optical fiber著者（英） S. Kamimura, R. Furukawajournal orpublication titleApplied Physics Lettersvolume 111number 6page range 063301year 2017-08-09URL http://id.nii.ac.jp/1438/00008920/doi: 10.1063/1.4998738Appl. Phys. Lett. 111, 063301 (2017); https://doi.org/10.1063/1.4998738 111, 063301© 2017 Author(s).Strain sensing based on radiative emission-absorption mechanism using dye-dopedpolymer optical fiberCite as: Appl. Phys. Lett. 111, 063301 (2017); https://doi.org/10.1063/1.4998738Submitted: 22 May 2017 . Accepted: 02 August 2017 . Published Online: 09 August 2017S. Kamimura, and R. FurukawaARTICLES YOU MAY BE INTERESTED IN High performance organic nonvolatile memory transistors based on HfO2 and poly(α-methylstyrene) electret hybrid charge-trapping layersApplied Physics Letters 111, 063302 (2017); https://doi.org/10.1063/1.4997748On-chip, high-sensitivity temperature sensors based on dye-doped solid-state polymermicroring lasersApplied Physics Letters 111, 061109 (2017); https://doi.org/10.1063/1.4986825 Nonlinearity-induced Laguerre-Gauss modes in organic vertical cavity lasersApplied Physics Letters 111, 063303 (2017); https://doi.org/10.1063/1.4997026Strain sensing based on radiative emission-absorption mechanism usingdye-doped polymer optical fiberS. Kamimura and R. Furukawaa)Department of Engineering Science, Graduate School of Informatics and Engineering, The University ofElectro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan(Received 22 May 2017; accepted 2 August 2017; published online 9 August 2017)A stress sensor based on a dye-doped polymeric optical fiber is able to detect stress by simplecomparison of two luminescence peaks from a pair of energy transfer organic dyes. Coumarin540A (donor) and Rhodamine 6G (acceptor) were doped in the core and cladding of the fiber,respectively. For various laser wavelengths, the change in the near-field pattern and visible emissionspectrum upon variation in the fiber bending diameter was evaluated. From a comparison with a low-numerical-aperture fiber, it is shown that the sensitivity of the sensor is controllable by optimizationof the waveguide parameters. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4998738]Because of their distributed sensor structure, fiber-optic(FO) strain sensors are widely used for structural monitoring.Compared to sensors based on mechanical and electricaltransducers, FO sensors are more reliable and cost effectivefor monitoring building structures in challenging environ-ments and for distributing targets.1Conventional FO strain sensors based on fiber Bragggratings and distributed Brillouin scattering provide theresolution of micro-strains.1 Because they convert straininformation into phase shifts, a spectral analyzing system, awell-tuned light source, and a system for temperaturecompensation are required.Japan where earthquakes occur with high frequencybecause of its geographic location, buildings, and lifelineinfrastructures of various types requires structural monitor-ing.2,3 For example, the degree of damage in houses andinfrastructures before and after an earthquake must be moni-tored. Infrastructures built for the 1964 Tokyo Olympicgames, which do not necessarily satisfy current buildingstandards, are now aged. Regardless of the structure, thedegree of damage needs to be clarified in the order of priorityand be treated efficiently given limited public funds.Conventional FO sensors are not a satisfactory solutionregarding the social demand because of their excessiveinstallation costs and training requirements. Phase-shift sen-sors require electrical engineering and photonics knowledgefor operation of the equipment and for signal analysis.Here, we demonstrate strain detection using a differentmechanism that could be used in a low-cost and simple FOstrain-sensor. The principle is depicted in Fig. 1.4 Donor/acceptor (D/A) dye pairs for the radiative emission-absorption mechanism are doped in the optical fiber core andcladding, respectively. The donor-exciting wavelength kexDis coupled to the core and propagates down the fiber. Thedonor dye in the core emits at the wavelength kemD . At loca-tions where stress is applied to the fiber, the deformationinduces kemD leaks to the cladding. Previously, it was shownthat doping luminescent dye in the optical fiber core enhan-ces light leakage from the core to cladding.5 After absorbingthis leaking light, the acceptor dye in the cladding emitskemA , which is detected at the fiber output end and representsthe extent of the stress load.An advantage of this FO sensor is that it can display thestress information as a large peak shift. Figure 2 plots excita-tion and emission spectra of the Coumarin 540A (donor) andRhodamine 6G (acceptor) pairs used here. The kemD and kemApeaks are about 100 nm apart. This difference can bedetected with simple optical components instead of the spec-tral analysis required in conventional FO sensors. Therefore,because of its simple structure and mechanism, it could beeasily analyzed in various types of situations at low installa-tion and/or operational costs.Polymer optical fibers (POFs) were used because oftheir low processing temperatures, which allow a widerrange of heat-sensitive dye pairs to be used.6 Most organicdyes cannot withstand the processing temperatures neededfor silica fibers.7 Another advantage of using POFs is theirhigh elasticity, which offers a higher stress resolution thanthat expected from materials having low elasticity.Here, we report on the fabrication of POFs with D/Adye pairs doped in the core and cladding, respectively, andanalyze the outputs for stressed/non-stressed situations.Measurements were performed for different kexD and kexAvalues, or either, to demonstrate the sensor principles.Macro-bending of the fiber was used to apply stress to thePOF.8FIG. 1. Principle of the FO strain sensor using the radiative emission-absorption mechanism. kexD ; kemD ; kexA , and kemA are the emission (em) andexcitation (ex) wavelengths of the donor (D) and acceptor (A) dyes. kemDleaks to the cladding because of stress deformation and is detected by theobservation of kemA at the output.a)Electronic mail: fururei@uec.ac.jp0003-6951/2017/111(6)/063301/4/$30.00 Published by AIP Publishing.111, 063301-1APPLIED PHYSICS LETTERS 111, 063301 (2017)Prior to the POF synthesis, the D/A dye concentrationsto be doped in the host polymer were optimized. Coumarin540A and Rhodamine 6G were selected as the donor andacceptor dyes, respectively, to satisfy three factors: (1) largeoverlap of the emD and exA peaks; (2) high quantum yields;and (3) good solubility in the host monomer. Rhodamine 6Gwas selected as the acceptor because it is commerciallyavailable, has a high quantum yield, and was easily dispersedin methyl methacrylate (MMA) to make a POF preform.9–11Coumarin 540A was selected because its emission over-laps the excitation of Rhodamine 6G and it is also soluble inMMA. Its emission peak ranges over 500–547 nm in severalsolvents.12–18The optimum concentration of dyes doped in PMMAwas determined by acquiring fluorescence spectra. Figure 2plots the excitation and emission spectra of 0.001wt. %Coumarin 540A and Rhodamine 6G in bulk PMMA. Majorpeak distortions or shifts that may suggest dye aggregationwere not observed. Considering the pronounced peaks, theconcentration is adequate to obtain high fluorescenceintensity and was thus adopted for POF fabrication.The POFs were fabricated from preforms according tothe interfacial gel polymerization method reported by Koikeet al.19 As was done previously, 11wt. % of diphenyl sulfide(DPS) was added to increase the refractive index of the core.Five types of fibers were fabricated; their characteristics arelisted in Table I. The dyes were dispersed in the monomerbefore polymerization, and the 0.001wt. % concentrationswere at the saturation point without aggregation or precipita-tion. Fiber 1 is the target structure for which we expectedemission from the cladding that was pumped by core emis-sion. Fibers 2 and 3 were fabricated as controls to examineeffects due to the presence of each dye. Fiber 2 lacks bothdonor and acceptor dyes, while fiber 3 lacks the acceptor dyein the cladding. Fiber 4 also has the target structure, but ithas a lower numerical aperture (NA) compared to fiber 1 bydoping with less DPS. Fiber 5 has a lower content ofacceptor dye than fiber 1.As done previously,6,20,21 the polymerized preform washeat-drawn to a core/fiber diameter of 666/1000 lm and 1-mfibers were used for measurements. Figure 3 shows a sche-matic of the apparatus used to obtain the intensity balancebetween the core and cladding during stress application to atest fiber. Pigtail lasers with an approximate mode field of3.2–4.3lm (Precise Gauges LDS1003 series and LDS1004)were used as 406-nm, 532-nm, and 635-nm excitation sour-ces. The 406-nm laser was optimum for efficiently excitingCoumarin 540A in the core (kexD). The 532-nm laser opti-mally excited Rhodamine 6G but weakly excited Coumarin540A (kexA). The 635-nm laser excites neither dye. Each pig-tail end was coupled to the test fiber with a direct contact tothe core center of the test fiber to prevent cladding excitation.The fibers were subjected to various stresses by bendingthem in different loop diameters (1.5 cm, 3.0 cm, and 6.0 cm)created in the middle of the fiber length. The relaxed condi-tion was straight with no loops. The output beam from thetest fiber was analyzed using a near-field pattern (NFP)measuring system (Hamamatsu C9664-01G02) and a spec-trophotometer (JASCO MV-3500).Figure 4 shows NFPs of fibers 1–3 under non-stressedconditions for various excitation wavelengths. From Fig.4(b), it is found that the NFPs of fiber 2 (containing no dyes)had significant intensity fluctuations corresponding tospeckle patterns. The patterns were similar for all excitationwavelengths. In contrast, the NFP of fiber 3 had a homoge-neous pattern only for 406-nm excitation [Fig. 4(c)]. Fiber 3had only the core dye, which is excited by the 406 nm light(kexD).The other data in Fig. 4(c) show coarse speckle pat-terns similar to Fig. 4(b) for 532-nm and 635-nm excitations.This was because the dye was present in the core and theinput wavelength matches its excitation wavelength. Thus,the absorption and emission activities will disturb the modalcondition and its directivity.Fiber 1 had Coumarin 540A doped in the core andRhodamine 6G in the cladding and exhibited homogenousNFPs not only at 406 nm but also at 532-nm excitation aswell [Fig. 4(a)]. Because 532 nm light excites Rhodamine6G, some portion of the cladding material must have beenpresent in the core. Hence, in the process of interfacial gelpolymerization, a small amount of tubular inner surface ofthe cladding dissolved in the core monomer when the coremonomer was in contact.Figure 5 shows the change in the NFPs under variousbending conditions. In each case, 406-nm light was used toexcite Coumarin 540A in the core and the intensity was nor-malized with respect to the maximum intensity of the core.In Fig. 5(a), the patterns of fiber 1 exhibited increasedTABLE I. Test fibers and their compositions.Fiber nameRhodamine 6G(cladding)Coumarin 540A(core)Diphenyl sulfide(core) (wt. %)Fiber 1 0.001 wt. % 0.001 wt. % 11Fiber 2 … … 11Fiber 3 … 0.001 wt. % 11Fiber 4 0.001 wt. % 0.001 wt. % 5Fiber 5 0.0001 wt. % 0.001 wt. % 5FIG. 2. Excitation (dashed curves) and emission (solid curves) spectra ofCoumarin 540A (donor) and Rhodamine 6G (acceptor) in a PMMA host.Each dye was dispersed at a concentration of 0.001wt. %.063301-2 S. Kamimura and R. Furukawa Appl. Phys. Lett. 111, 063301 (2017)cladding intensity as the bending diameter was reduced. Incontrast, there was almost no change in the patterns of fiber3, as seen in Fig. 5(b). This suggests that the presence of theacceptor dye in the cladding realized the principle shown inFig. 1. In Fig. 5(c), the fiber with both dyes and a lower NAexhibited even more cladding intensity as the bending diam-eter was reduced. This suggests that luminescence in thecore was less confined with a lower NA and thus suppliedmore excitation light to the cladding.Figure 6 shows NFPs of fiber 4 (lower NA) for variousmacro-bending conditions at different input wavelengths.The normalized intensity in Figs. 6(a) and 6(b) allowschanges in cladding intensity to be compared clearly amongthese patterns. Figure 6(a) is identical to Fig. 5(c). Both casesshow increased cladding intensity with stress as bendingincreased. The change was more significant in (a) than in (b),suggesting that these NFPs followed the principle of Fig. 1.However, according to that principle, the minor increase incladding intensity seen in Fig. 6(b) was not expected. It mayinstead be attributed to unintentional contamination ofRhodamine 6G in the core suggested earlier in Fig. 4(a).Although it is a small amount, Rhodamine 6G in the corewas excited when 532-nm light was present and was leakedto the cladding as the stress (bend) increased. Figure 6(c)shows the un-normalized pattern obtained when 635-nmlight was used. The coarse speckle pattern suggests that635 nm light did not interact with the dyes. The gradualdecrease in the core intensity with increasing bending indi-cates that the optical power confined in the core was trans-ferred to the cladding. However, because there was noFIG. 4. NFPs of straight (no stress) (a)fiber 1, (b) fiber 2, and (c) fiber 3 forthe input wavelengths of 406 nm(kexD ), 532 nm (kexA ), and 635 nm(excites neither dyes).FIG. 5. Change in NFP of (a) fiber 1,(b) fiber 3, and (c) fiber 4 for differentbending diameters. The excitationwavelength was 406 nm (kexD ).FIG. 6. Change in NFP of fiber 4 fordifferent bending diameters at variousinput wavelengths.FIG. 3. Schematic of NFP apparatusfor dye-doped POF at different bend-ing stresses. The 1-m fiber was eitherstraight (no stress) or bent in a singleloop at the center. The loop diameter(a) was 1.5 cm, 3.0 cm, or 6.0 cm. Apigtail laser with a sufficiently smallmode field was used to prevent clad-ding launches.063301-3 S. Kamimura and R. Furukawa Appl. Phys. Lett. 111, 063301 (2017)Rhodamine 6G excitation, a pronounced change in claddingintensity was not observed.Figure 7 shows output spectra of fiber 5 for differentbending conditions and for a 406-nm input. The intensitywas normalized with respect to the kemD peak to revealchanges in the Rhodamine 6G peak. The increase in the kemApeak was confirmed as the bending increased, which agreeswith the principle depicted in Fig. 1.In conclusion, a stress sensor based on a dye-doped POFwas fabricated, and its ability to sense stress via simple com-parison of D/A emission intensities was demonstrated. A D/A pair of luminescent organic dyes (Coumarin 540A andRhodamine 6G) were doped in the core and cladding of thefiber, respectively. Stress applied to the fiber by the way ofbending could be detected as an increase in cladding inten-sity. Due to this deformation, emission from the core dyeleaks to the cladding and excites the cladding dye. Changesin the NFP upon fiber bending were evaluated using differentinput wavelengths, some of which excite the dyes. The clad-ding intensity increased slightly when the core center waslaunched by kexD . Spectral analysis had shown a continuousincrease in the donor peak as the bending increased.Observations confirmed that the fabricated structure can real-ize the proposed sensing principle. A low-NA fiber exhibitedenhanced sensitivity, suggesting that the resolution of thesensor can be optimized by adjusting the waveguide parame-ters. The materials used here are not optimum for practicaluse. For example, the excitation and emission wavelengthsof the dyes do not necessarily match the optical window ofthe host polymer. Thus, better choices of materials may bepossible in future versions of the proposed fiber structure.This work was supported by the MLIT Program forPromoting Technological Development of Transportation andAccurate maintenance for secure transportation infrastructuresagainst aging and natural hazards, Development of robusttransportation system against natural hazards, and“Development of organic-waveguide strain-sensor for healthmonitoring of shield tunneling process (Project No.: 2015-2).”1B. Glisic and D. Inaudi, Fiber Optic Methods (Wiley, 2007), p. 19.2G. P. Cimellaro, D. Solari, and M. Bruneau, Earthquake Eng. Struct. Dyn.43, 1763 (2014).3I. Matsuda, J. Geogr. 122, 1070 (2013).4R. Furukawa, PCT Application. No. PCT/JP2016/059673, 2016.5R. Furukawa, D. Mizorogi, K. Tsukada, E. Nihei, M. Matsuura, A. Inoue,A. Tagaya, and Y. Koike, “Stress-induced light leakage of rhodamine 6Gdoped polymer optical fiber,” in 23rd International Conference on PlasticOptical Fibers (POF 2014), Kanagawa, Japan, 2014.6R. Furukawa, A. Tagaya, S. Iwata, and Y. Koike, J. Phys. Chem. C 112,7946 (2008).7U. Paek and R. Runk, J. Appl. Phys. 49, 4417 (1978).8R. Furukawa, A. Tagaya, S. Iwata, and Y. Koike, Jpn. J. Appl. Phys., Part1 93, 103303 (2008).9R. F. Kubin and A. N. Fletcher, J. Lumin. 27, 455 (1982).10K. Kuriki, T. Kobayashi, N. Imai, N. T. Tamura, Y. Koike, and Y.Okamoto, Polym. Adv. Technol. 11, 612 (2000).11I. Ayesta, M. A. Illarramendi, J. Arrue, I. Parola, F. Jimenez, J. Zubia, A.Tagaya, and Y. Koike, Polymers 9, 90 (2017).12E. J. Schimitschek, J. A. Trias, P. R. Hammond, and R. L. Atkins, Opt.Commun. 11, 352 (1974).13K. H. Drexhage and G. A. Reynolds, in Quantum Electronics Conference(1974), Paper No. F.1.14J. A. Halsted and R. R. Reeves, Opt. Commun. 27, 273 (1978).15G. Jones, W. R. Jackson, and A. M. Halpern, Chem. Phys. Lett. 72, 391(1980).16F. Bos, Appl. Opt. 20, 1886–1890 (1981).17H. Telle, W. Huffer, and D. Basting, Opt. Commun. 38, 402–406 (1981).18T. C. Eschrich and T. J. Morgan, Appl. Opt. 24, 937–938 (1985).19M. Sato, T. Ishigure, and Y. Koike, J. Lightwave Technol. 18, 952 (2000).20R. A. Furukawa, A. Tagaya, and Y. Koike, Appl. Phys. Lett. 112, 7946(2008).21R. A. Furukawa, A. Tagaya, and Y. Koike, ACS Appl. Mater. Interfaces 7,388 (1982).FIG. 7. Output spectra of fiber 5 for different bending conditions. The fiberwas either relaxed (straightened) or bent in loops with specific diameters.The intensity was normalized with respect to the kemD peak.063301-4 S. Kamimura and R. Furukawa Appl. Phys. Lett. 111, 063301 (2017)

Strain sensing based on radiative emission-absorption mechanism using dye-doped polymer optical fiber

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Strain sensing based on radiative emission-absorption mechanism using dye-doped polymer optical fiber

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