advanced optical sensing techniques using novel fibre gratings.pdf

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Advanced optical sensing techniques using novel fibregratings

Lin Zhang, Xuewen Shu, Ian BennionPhotonics Research GroupElectronic Engineering, Aston University, Aston Triangle, Birmingham, B4 7ET, UK

email: [email protected]. uk

Abstract - We describe some recently developed fibre grating sensing devices andapplications with emphasis on simultaneous measurement of multiple measurands usingcombinational grating structures, and the realisation of ultra-high sensitivity sensorsutilising the quadratic dispersion of long-period grating structure.1. INTRODUCTIONThe last decade has witnessed an upsurge in fibre grating-based smart opticalsensing techniques [11. In-fiber grating sensors have many advantages overconventional electrical and altemative fibre optic sensor configurations. Theyare relatively straightforward and inexpensive to produce, immune to EMinterference and interruption, light in weight and small in size, and they are self-referencing with a (usually) linear response. More importantly, they are suitablefor wavelength-, time- and spatial-multiplexing; tens of gratings arranged inseveral fibres can be formed into structurally integrated quasi-distributed sensorsystems, which are much in demand for smart structural engineeringapplications.

Despite the relative maturity of fibre grating sensing technologies, novel andsophisticated grating-based sensor concepts and devices are still beingdeveloped. In this paper, we focus on two new device types offering,respectively, dual-parameter and ultrasensitive performance. The first type isimplemented using combinations of fibre Bragg grating (FBG) structures [2-41,and the second exploits long-period fibre gratings (LPFGs)with phase matchconditions close to the dispersion-tuming-points [5-71.

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2. DUAL-PARAMETER SENSORS BASIED ON COMBINATIONALGRATING STRUCTURESThe cross-sensitivities of FBGs and LPFGs to multiple measurands has beenproblematic in optical sensing as their presence complicates and limitsindependent measurement procedures for any one of the co-existingenvironmental parameters. To overcome cross-sensitivity, a number of dual-grating techniques have been proposed and investigated [l]. However, most ofthese techniques are far from convincing for practical use, due to eitherinsufficiently large differentiation between the thermal and strain coefficients ofthe dual components, or over-complex signal interrogation and processingrequirements. The two schemes which we have recently developed forsimultaneous measurement of straidtemperature and refractive-indedtemperature using combinational grating structures show significantlyimproved performance.2.1The sampled fibre Bragg grating (SFBG), whiclh is formed by superimposing alow-frequency spatial variation on a high-frequency modulation of the corerefractive index of a fibre, has two elements to its spectral response. An FBG-like response, produced by the high spatial frequency modulation, appears asmulti-harmonic narrow resonances with a central Bragg wavelength satisfyingthe phase match condition h, =2n:fA,, wheire n z s the effective index of thecore mode and A~. the period of the high-frequency modulation. There is alsoan LPFG-like component in the response, created by the low spatial frequencyvariation, that exhibits several broad transmission loss peaks with resonancewavelengths at h =( n z - $,,)ALPFG,where n z m s the effective index of the m*cladding mode and A~~~~the period of the low-frequency variation. The coremode coupling mechanism renders the FBG response sensitive to temperaturebut not to the surrounding refractive index (SRI). In contrast, core-to-claddingmode coupling in the LPFG results in its response being sensitive to bothtemperature and SRI. Thus, the SFBG facilitates simultaneous measurement oftemperature and SRI, with the former measured from its FBG response and thelatter extracted from the total response of the LI'FG.Fig.1 shows the distinctive thermal and SRI behaviour of the SFBG. Both theFBG and LPFG resonances shift towards longer wavelengths with increasingtemperature (fig. la), whereas only the LPFG response reacts to a change of SR I(fig.lb). The spectral responses for both components are plotted againsttemperature for a range from 10C to 60"C,ancl against SRI from n=1 to n =1.46, respectively, in figs. I C and Id. Both components exhibit linear thermal

Sampled Fibre Bragg Grating for Simultaneous Refractive Index andTemperature Measurement

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responses, but the LPFG achieved a temperature sensitivityofA h, / A T =7 8 . 8 p m / "C some six times higher than the AhFBG A T =12.2pm/" Cof the FBG. In fig. Id, the FBG is totally unaffected by changes in the SRI,whereas the LPFG exhibits a two-part sensitivity behaviour, less sensitive in therange from n =1 to n =1.3 than from n=1.3 to n=1.46.

Temperature. O Ci I

l 1 0 l& ls ao 1;80 l s mVUmlength,nm 10 1 1 12 13 1 4Surrounding refractive index

Figure. 1 -Spectral evolution of the SFB G &&(a) temperature and (b) SRI. Spectralresponses of FBGand LPFG to (c) temperature and (d) SRI. I

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Another type of dual-parameter sensor has been realised utilising the differentialsensitivity properties between different types of FBG. Recently, a new type ofFBG was identified and designated Type IA [3,4] (The previously known typesof FBG are Type I, I1 and IIA). The newly discovered Type IA grating, which isregenerated in hydrogenated fibre after erasure of the normal Type I grating, hasa much reduced thermal coefficient in comparison with the Types I, I1 and IIAgratings; however, its mechanical properties remain similar to those of the othergrating types. Thus, multi-parameter sensors in a dual-grating structure are

Simultaneous Temperature and Strain Measurement Using a Dual-Grat ing Sensor

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readily conceivable that exploit the unusual combination of similar strain anddissimilar thermal properties between gratings of different types.Fig. 2 shows the thermal coefficients measureld for Type I and IIA gratings inhydrogen-free fibres, and Type I and IA gratings in hydrogenated fibres. Thevalues for hydrogen-free fibres - (a) and (lb) - are higher than those forhydrogenated fibres- c) and (d). Of the four types of grating, Type IIA yieldsthe highest temperature coefficient of 1O.80pndoC, while Type IA exhibits thelowest at 6.93pdOC. The 55 % difference between these two is substantiallylarger than in all previously reported dual-complonent sensors.

Type I I Ain hydrogen-free fibreType I inhydrogenated fibreType I inhydrogen-free ibre

C

Type IA in hydrogenated fibre

11o^Q 10--zllI.-80 8-ect 7 - daE8 6-

SamDle numb ers

Figure. 2 Distribution of the temperature coefficient measured for different types ofFBG inscribed in B/Ge codoped fibre.Fabrication of a dual-grating sensor in a single fibre incorporating Type IA andType IIA gratings was conducted in several steps. A Type IA grating was firstUV-inscribed in a hydrogen-loaded B/Ge fibre, which was then annealed toremove residual hydrogen. Then, a Type IIA grating with a slightly differentcentre wavelength was inscribed in the fibre selparated from the Type IA gratingby lmm. The fibre was annealed once more to stabilise the final structure.The transmission spectra of the dual-grating sensor at two temperatures (2 1"Cand 120C) and two strain conditions (OPE and 2000ps) are shown in fig. 3b and3c, respectively. Clearly, the constituent grating peaks shift at a similar rate withstrain, but at different rates with changing temperature. The large differentiationbetween the thermal coefficients is illustrated in fig. 3d. Quantitative analysisgives thermal coefficients of 7.37pdOC for the Type IA grating and10.02pm/C for the Type IIA: the difference between the two peaks is some

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36%. The strain sensitivities are 1.074pm/p& and 1 . 0 7 5 p d p ~ or the twogratings, respectively, a difference of less than 0.1%.

(a) FBGl : Type IAgrating FBG2:Type IIA grahng\ \pedfiber

7 7 *I

..1552 1553 1554 1555 1556 1557 1558Wavelength (nm)

15645

Figure 3. (a) Schematicof a dual-grating sensor employing concatenated Type IA nd Type II A gratings; Spectral response changes in thedual-grating sensor induce d by (b)temperature and (c) strain; Re sponse of the dual-gratingsensorto (d ) temperature, and (e) strain.With regard to practical applications, these dual-grating sensors can bewavelength-multiplexed to form an effective, quasi-distributed sensing systemwith low splicing losses and high mechanical strength, thereby offering theadvantages of single light source operation and robustness that elude most otherconfigurations.3. ULTRA-HIGH SENSITIVITY SENSORS UTILISING THEDISPERSION TURNING POINT CHARACTERISTICS OF TH EThe current popularity of LPFGs as fibre-optic devices or applications intelecommunications and optical sensing is due largely to their low-costfabrication and low back-reflection. Most of the LPFG-based devicesdemonstrated so far utilise the positive dispersion property of the loworder cladding modes of gratings produced with relatively large periods(300pm-500pm). Recent studies have succeeded in characterising theentire dispersion of LPFGs as a quadratic distribution f6J.As discussedbelow, the dispersion-turning-point and dual-resonance state inherent in

LONG-PERIOD GRATING

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the quadratic distribution is of great importance to the realisation ofLPFG sensors intrinsically ultra-sensitive to external perturbations.3.1Fig.4 depicts the quadratic distributionof the LPFG waveguide dispersionexhibited by the phase curves for thirty coupled cladding modes CfromLP;' to LPZJ calculated for a set of LPFGs with grating periods ranging

from 50pm to 400pm in a conventional B/Ge co-dopedfibre. The slopedirectionsof the phase curves change rom low-order modes to high-orderones, turning at the points where I& 1dAl+ 03. The curves in the regionsbelow these points have positive dispersion 1d A >o), while those abovehave negative dispersion / d A


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3.2 Ultra-High Sensitivity Sensors Utilising the Dual-ResonanceProperty of the LPFGThree dual-resonance LPFGs were evaluated for optical sensing bymeasuring the perturbation-induced spectral separations of the dualpeaks. Fig. 5a, 6a and 7a illustrate their respective spectral evolutions as

they were subjected to increasing changes in temperature, strain andSRI, respectively. The dual peaks separated with temperature or SRIchanges, as shown in fig. 5a and 7a, but moved closer with increasingstrain, as indicated in f ig. 6a. In general, the spectral trends of the dualpeaks are determined by whether the phase match condition is above orbelow the dispersion turning-point. The temperature and strainsensitivities achieved by the dual-resonance LPFGs are estimated fromfig. 5 b and 6b to be 3.4nm/"C and 33.6/1000p, respectively. Thesevalues are two orders of magnitude higher than those of conventionalLPFGs. The dual-resonance refractive index sensor also demonstratedan exceptionally high sensitivity: with SRI increasing from 1 to 1.44, thetotal separation between the blue-shifting and the red-shifting peaksexceeded a value of 470nm, as shown in fig. 7b.

Figure 5. (a) Temperature induced spectral evolution; (b) the shifts of the red-

Figure 6. (a) Strain induced spectral evolution; (b) the separation of the dual peaksagainst strain.71


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4 i 1

1400

13X

1.0 1.1 12 13 1.4Surrounding Refractive Index

1~3XIZ W

1.0 1.1 12 13 1.4Surrounding Refractive Index

Figure 7. (a) SRI induced spectral evolution;(b) Shifts of the dual peaks against SRI.

3.3 Dispersion Turning-Point LPFG Sensors Based on IntensityMeasurementIn general, both conventional and dual-resonance LPFG sensors rely ona measurand-induced change of resonant wavelength. However, analternative sensor scheme exists: for LPFGs designed to operate at the

dispersion turning-point, but a little off-resonance, external perturbationsinduce only a variation in the coupling efficiency, and the maximumcoupling wavelength remains constant. Thus, an optical sensing schemecan be effected by measuring the measurand-induced intensity change.Intensity measurement-based optical sensors are attractive for practicalapplications since they can be interrogated using low-cost signalprocessing systems that do not require costly passive or active tuneableor dispersive filters.Fig. 8a and 9a illustrate the spectral resporises of two LPFGs which weresubjected to changes of temperature and strain, respectively. These twogratings were designed with coupled cladding modes exactly on thedispersion-turning points. It is clear that the increases of the temperatureor the strain have caused no distinct wavelength shifts, but only changesin the coupling strengths and, therefore, in the correspondingtransmissions of the gratings.Fig. 8b and 9b plot the maximum transrriission losses against appliedtemperature and strain, respectively. The sensor responses for largetemperature and strain ranges are notably nonlinear, as indicated in fig.9b and lob, but for the temperature range from 14C o -24C and thestrain range from 5 O O p to 2500p,he linearity improves. The averagesensitivities for these ranges are 6.67% transmission per"C nd 0.025%per PE , respectively. With a detector capable of resolving 0.01% changein transmitted intensity, these two LPFG sensors could yield 0.002"Ctemperature and0 . 4 ~ ~train resolutions, respectively.

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88 . . . . . . . . . . . . . . . . . . . . . . . . .

10 , . , . , . , . , . , . ,

2 0 4F

Wavelength (nm) Temperature("C)

Figure 8. (a) Temperature (fiom 12OC to 26OC) induced spectral evolution;(b) Plot of thepeak value of the transmission against temperature.4. CONCLUSIONSIn this paper, we have reviewed recent developments in optical sensingtechniques utilising novel fibre gratings of the Bragg and long-periodtypes. Dual-parameter sensors based on two types of combinationalgrating structure have been shown to offer significantly improvedperformance in comparison with previously reported schemes. LPFGsensors with phase-match condition close to the dispersion turning-pointhave been successfully exploited to implement two sensor configurationsbased, respectively, on dual-resonance and intensity measurement: bothexhibit ultra-high sensitivity to external perturbations. We anticipate thatthese newly developed low-cost, high-efficiency fibre grating sensordevices will find roles in a number of industrial applications.

Figure 9. (a) Strain (from 0 to 3000pe) induced spectral evolution; (b) Strain responseto the peak value of the transmission.

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REFERENCESL. Zhang, W. Zhang and I. Bennion, In-fiber Grating Optical Sensors,Chapter 4 in Fiber O ptic Senso rs, Marcel Dekker Inc., 2002.X. Shu, B.A.L. Gwandu, Y. Liu, L. Zhang and I. Bennion, Sampled fiberBragg grating for simultaneous refractive index and temperaturemeasurement,Opt. Lett. 26, pp. 774-776, 2001.Y. Liu, J. A. R. Williams, L. Zhang, I. Bennion, Abnormal spectralevolution of fibre Bragg gratings in hydrogenated fibres, Opt. Lett., 27,2002.X. Shu, Y. Liu, D. Zhao, B. Gwandu, F. Floreani, L. Zhang and I.Bennion, Dependence of Temperature ;and Strain Coefficients on FiberGrating Type and Application to Simultaneous Temperature and StrainMeasurement, to appear in Opt. Lett., 2002.X.. Shu, L. Zhang, I. Bennion, Sensitivity Characteristics of Long PeriodGratings,J . Lightwave Technol.,20(2), pp. 255-266,2002.X. Shu, L. Zhang, I. Bennion, Sensitivity Characteristics near thedispersion turning points of long-period fiber gratings in B/Ge codopedfiber, Opt. Lett., 26, pp. 1755-1757,2001.X. Shu, T. Allsop, B. Gwandu, L. Zhang and I. Bennion, HighTemperature Sensitivity of Long-Period Gkatings in B/Ge CO-doped Fibre,ZEEE Photon. Technol.Lett., 13, pp. 818-820,2001.

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advanced optical sensing techniques using novel fibre gratings.pdf

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