We report here an implementation of high-spatial resolution distributed sensors employing compact-grating-array structure, offering advantages of free designing dynamic range and simple interrogation by utilising established wavelength-division-multiplexing technique

Bennion, I.

Chisholm, K.

Gwandu, B. A L

Liu, Y.

Zhang, L.

English

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PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieCompact fbg grating array structurefor high spatial resolution distributedstrain sensingB. A. L.  GwanduB. A. L.  Gwandu, "Compact fbg grating array structure for high spatialresolution distributed strain sensing," Proc. SPIE 4185, FourteenthInternational Conference on Optical Fiber Sensors, 41853S (9 November2000); doi: 10.1117/12.2302279Event: Fourteenth International Conference on Optical Fiber Sensors, 2000,Venice, ItalyDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/8/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-useP3-19Compact FBG grating array structure for high spatial resolution distributed strainsensingB.A.L. Gwandu, L. Zhang, K. Chisholm, Y. Liu and I. BennionPhotonics Research Group, Electronics Engineering and Applied Science, Aston UniversityAston Triangle, Birmingham, B4 7ET, UK.Tel: +44 (0)121 3593611; Fax: +44 (0)121 3590156ABSTRACTWe report here an implementation of high- spatial resolution distributed sensors employing compact- grating-array structure, offering advantages of free designing dynamic range and simple interrogation by utilisingestablished wavelength- division- multiplexing technique.1. INTRODUCTIONThe past few years have seen intensive research and development of fibre Bragg grating (FBG) sensingtechnologies for smart structure applications. In comparison with conventional sensors, FBGs for optical sensingoffers a number of advantages including sensing a wide range of measurands, immunity to EM interference andinterruption, light weight and miniature size, embeddable to composite materials and most importantly, intrinsicmultiplexing capability. FBG sensing systems based on wavelength- and spatial- multiplexing grating arrayinterrogation have been developed for smart structure applications across a wide range of industrial engineeringprojects. However, grating sensor arrays in these systems are used for quasi- distributed strain sensing, offeringspatial resolution of typical values from few centimeters to several meters. There are some applicationsrequiring short -gauge but high- spatial- resolution distributed sensors. FBG based high spatial resolutiondistributed sensors have been reported by Volanthen and LeBlanc et al [1,2,3], however, the interrogationtechniques which they use are cumbersome, and limiting to the practical implementation for real applications.In this paper, we report an implementation of high spatial resolution distributed sensing utilising compact -grating -array structure. This configuration permits directly employing the well established wavelength- division-multiplexing (WDM) FBG sensor array interrogation technique, therefore, offers advantage of highpracticability for real applications. The additional advantage is that the dynamic range and spatial resolution areindependently designable to be application- specific. Furthermore, high spectral resolution can also be achievedby apodising the gratings.2. CONCEPT OF COMPACT -GRATING -ARRAY FOR DISTRIBUTED SENSINGThe concept of a compact -grating -array is illustrated in fig. I. The element gratings with distributedwavelengths are concatenated along a fibre without any physical gaps. The spatial resolution and sensingdynamic range for distributed sensing from such a structure are simply determined by the physical length of theelement grating and the spectral spacing between the adjacent gratings respectively. The spatial resolution canhe as high as 1 mm since now it is possible to produce fibre gratings of such short length but still withsufficiently high reflectivity. However, there are some difficulties in realisation of free -tailoring wavelengths,high spectral resolution and incorporating large number of element gratings.Figure. 1. Concept of a compact -grating -array- 756 - t  f r i  s atial resolution distributed strainse sii  . i, l tr ics gineering and pplied Science, Aston Universityni int ti  of high- i  istri uted sensors employing compact-grating-array structure, offering advantages of free designing dyna ic range and si ple interrogation by utilisingestablished avelength-division- ultiplexing technique. t t r  licati ns. In co parison with conventional sensors, FBGs for optical sensingt  i l i  sensing a ide range of easurands, immunity to EM interference andt  i i t re size, e beddable to co posite materials and most importantly, intrinsicultiplexing capability.  sensing systems based on wavelength- and spatial-multiplexing grating array l  f r s art structure applications acro s a wide range of industrial engin ering. ,  r rr s i  these syste s are used for quasi-  t i  si , fferingt-  spatial-resolution distributed sensors. F  base  i  i t i t     t   l t   l  t !  I ,2,3], however, the inte rogation  r  rs e, and li iting to the practical implementation for real a plications.I  t is a er, e re rt a  i le e tati  f high spatial resolution distributed sensing utilising compact-grating-array structure. his configuration per its directly e ploying the well established wavelength-division-lti l i  ( ) FBG sensor array i terrogation technique, therefore, offers advantage of highti ilit   l li ti .  iti l t  i  t t t  i    ti l l ti  i specific. rt r , hby apodising the gratings.. P  F P -ARRAY FOR DISTRIBUTED SENSINGpt of a compact-grating-a ray is i trated  . 1 . T element gratings with distributedt  i l a s. he spatial resolution and sensing  t t r  r  si ly deter ined by the physical length of the tr l i  et een the adjacent gratings respectively. The spatial resolution canb     1m  since now it is pos ible to pro  i  rt length hut sti l with  iffi lties i  realisation of freei       r f l e t gratings.HHHIHII1IIIII1IUIIIIII-1 2 I I Ii r . 1. Concept of a compact-grating-Fourteenth International Conference on Optical Fiber Sensors, edited by A. G. Mignani, H. C. Lefèvre, Proc. of SPIE Vol. 4185, 41853S · © (2000) 2017 SPIE · CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2302279Proc. of SPIE Vol. 4185  41853S-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/8/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-useP 3-19At Aston University in UK, the sophisticatedly designed and fully programmable phase mask scanning fibregrating fabrication system is capable of arbitrarily positioning and scanning the UV beam of relative complexpatterns. In addition, the system incorporates a computer- controllable fibre- straining unit which provides amethod to free change grating wavelength even using one -period -pattern dedicated standard phase mask. Thus,the fabrication of compact -grating- arrays of arbitrary dynamic range and spatial resolution is realisable by thissystem.3. COMPACT -GRATING -ARRAY FABRICATIONTwo designs of compact -grating- arrays were fabricated using the above described grating fabrication system.The phase mask used in the fabrication contains a group of five 5mm -long concatenated pitch -patterns. A directUV beam scan across this phase mask will allow produce a compact- grating -array consisting of five gratings ofBragg wavelengths from 1546nm to I558nm with a -3nm spectral spacing. Fig.2a shows a typical transmissionspectrum of such a grating array structure. This structure can be used for distributed sensing offering an intrinsicspatial resolution of 5mm and a total sensing gauge of 2.5cm. In order to increase the spatial resolution,compact -grating- arrays comprising ten 2.5mm -long gratings have been fabricated using the same phase mask.This was achieved by a combination of straining fibre and programming UV beam position techniques. Prior tothe UV exposure, the fibre was strained by an amount which would blue -shift Bragg wavelength by -1.5nm.Then, the UV beam scan procedure was programmed in an on- and -off fashion along the phase mask with a 50%duty cycle. As a result, five 2.5mm -long gratings with 2.5mm physical gap and -3nm spectral spacing betweenthem were produced along the fibre. The fibre was then released from the strain and the UV beam start -positionwas off -set by 2.5mm in order to write the first grating from the first gap. The second scan was also followed the50% duty cycle procedure and produced another five 2.5mm -long gratings situated exactly in the five gaps,resulting in a final compact -grating -array structure formed by ten 2.5mm -long gratings with --1.5nm spectralspacing. In comparison with the five -grating -array structure, this sensor has improved the spatial resolution by afactor of two.Figure. 2. (a) Transmission spectra of the (a) five- grating -array and the (b) ten -grating -array structures with(dotted -line) and without (continuous -line) distributed strain.- 757 -t t  i r it  i  , t  i ti t l  i   f ll  rogra able phase ask scanning fibreti  i ti  t  i  l   it il  iti i   i  t     l ti  ltt .  iti , t  t  i corporates a computer- tr lla le fibre- t i i  it i  r vides ati  a elength even using one ri r  ase ask. Thus,t t- r ti ar ays of arbitrary d i   i  r alisable by thissyste .-  - r ti ar ays were fabricated si  t   ri ation syste . t  f rication contains a group of five 5mm-long concatenated pitch- atterns.  directv beam scan across t i    c pact- -array c si ti le ths fro  1546n  to 1  it   —3nm spectral spacing. Fig.2a shows a typical transmi sion t t r   e sed for distributed sensing o fering an intrinsic          . .       ,-gratin -ar ays comprising te  . -long grati s  sing the same phase mask. i   r ra ing  bea  position techniques. Prior tot  s strained by an amount which would blue-shift Bragg wavelength by -1 .Snm. r re as programmed in an on-a d  se ask ith a 50%t  l .   lt, i  . -long gratings with 2.5mm physical gap and —3nm spectral spacing betw en t  fi r . e fibre as then released from the strain and the UV beam start- osition-set by 2.5  i  r r t  it     ls  f llo ed the t  l    r  a ther five 2.5 l  ti  it t  tl  i  t  fi  s,  i l pact -array str t r  -l  r ti —l.Sn  spectral ris n ith the five - rr   i r ed the spatial resolution by a.1563 . ( ) rans i sion spectra of the (a) five-grating-a ray and the (b) ten-grating  str ctures with-li  ti us t  strain.-  (a)-550C0U)U). -65U)CU)I-(b):!_-55C0U)U)EU)C -60U)I-I--651543 1548 1553 1558Wavelength (nm)Proc. of SPIE Vol. 4185  41853S-2Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/8/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-useP 3-194. DISTRIBUTED SENSINGThe distributed strain sensing experiment was carried out by applying linearly distributed strain to both the five -and ten -grating -array configurations. A four -point bend system was employed to create distributed strain. Fig. 3shows the geometric configuration of this system. The fibre containing 2.5cm -long compact -grating -array wasattached to the one side of a O.5mm thick, 2Omm wide and 15Omm long metal bar. In order to eliminate theslippage effect, the fibre was glued onto the metal bar. The bar was bent by depressing the centre with amicrometer driver. According to the geometric configuration, the bend -induced strain distribution in the metalbar is approximately linear in the sections close to the ends of the bar.E ! Compact- grating -arrayFigure. 3. Geometric configuration of a four -point bend system used for generate linear distributed strain.When the grating -array was subjected to the distributed strain, the Bragg wavelength of each element gratingshifts an amount according to its physical position along the bending metal bar. The transmission spectra of thetwo grating -arrays under distributed strain are also presented in fig.2 as dotted lines. It can be seen clearly fromthe figure that the transmission peaks in both structures shift accordingly so that the spectral spacings reflect thelinear distribution of the strain. Fig. 4a and b plots the wavelength shift against the grating order for linearstrains with increasing gradients. Both five- and ten -grating -array sensors exhibit a good linear response,(b)2 3Grating Order 4 53 Grating order 9Figure.4. Wavelength shift against grating orders (a) for five- grating -array and (b) for ten- grating -arraystructures under increasingly linearly distributed strains.- 758 -I I i  ri e t as carried out by applying linearly distributed strain to both the five--grating-ar ay configurations.  f -point bend s st   t  reate distributed strain. Fig. 3tric configuration of this system. The fibre containing 2.5cm-long compact-grating    si e of a 0 , 0 0  l  t  t  l  t  t e etal bar. he bar as bent by depre sing the centre with ai  t  t  etric configuration, the bend  t i ti  i  t e etal   t  s f t e ar.i r ti  f a f r  i tri t  strai .-arra  as s j t  t    l t rati g l  t  i g etal bar. The trans i sion spectra of the-arrays un er istri t  t     learl  froi i  s i  th structures shift accordingly so that the spectral spacings reflect theli  i t i ti   t  t i . i .    l t  t  l t  i t i t t  ti    li i r si g gradients. Both five- and ten-grating  i it a good linear response,i r . . l gth shift against grating orders (a) for five-grating-array and (b) fort  t i .-  ten-grating-array(a)EC.CC) IC.1.o1rati( )11r tiProc. of SPIE Vol. 4185  41853S-3Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/8/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-useP 3-19demonstrating the effectiveness for distributed strain sensing. The slippage effect did occur to the seventhgrating in the ten -grating -array sensor. As clearly shown in fig.4b, the strain response of this grating did not fitwell with the rest of gratings possibly due to the loose glue effect.Although the distributed strain sensing using compact -grating -array structures was implemented only forlinearly distributed strain, the proposed structure should be capable of arbitrary local strain measurement withinits defined dynamic range. It should be pointed out that for some applications where -4 -5mm spatial resolutionis adequate, the grating apodisation technique can be employed in the grating -array fabrication to narrow thespectral response effectively increasing the spectral resolution.5. CONCLUSIONSTo summarise, we have proposed and demonstrated a concept of high -spatial- resolution distributed opticalsensing employing a compact -grating -array structure. Two configurations of five- and ten -grating -array havebeen fabricated and successfully used to implement distributed strain sensing. In comparison with other reportedsensor structures and techniques, the compact -grating -array structure approach provides a simple and practicablesolution for real applications as the WDM FBG sensing interrogation method can be directly applied. Inaddition, this structure also permits independent design for dynamic range and spatial resolution, adding moreadvantages for producing practical devices.REFERENCES1. M. Volanthen, H. Geiger, M.J. Cole and J. P. Dakin, "Measurement of arbitrary strain profiles within fibregratings ", Electron. Lett., 32, pp. 1028 -1029, 19962. M. LeBlanc, S. Huang, M. Ohn, R.M. Measures, A. Guemes and A. Othonos, "Distributed strainmeasurement based on a fibre Bragg grating and its reflection spectrum analysis ", Opt. Lett., 21, pp. 1405-1407, 19963. M. LeBlanc and A.D. Kersey, "Distributed, intra grating sensing by Fabry -Perot wavelength tuned low -coherence interferometry ", Proceeding of 12th International Conference on Optical Fibre Sensors, pp. 52-55, 1997- 759 --gratin -ar ay sensor. As clearly sho  i  fi .  t i  rating did not fit l  l  ffect.lt  t  i tri t  tr i  s sing using compact- r ti - rr  tr t r   i l t  l  forli  l   a le of arbitrary local strain measurement within—4-5mm spatial resolutioni i ti  technique can be employed in the grating-a ray fabrication to na row thespectral response effectively increasing the spectral resolution.o cLusio s s  and de onstrated a concept of high-spatial- i t i t  optical  c pact - rr  fi urations of five- and ten-grating  a et i tri t  strain sensing. In co parison with other reported- rati -ar ay structure approach provides a si ple and practica lei  i t rr ti  ethod can be directly applied. Initi , t i  t t  l  it  i t i  f r a ic range and spatial resolution, adding moreadvantages for producing practical devices.I . i , s r e t of arbitrary strain profiles within fibreti ", l t . tt., , . 1028 , 96 M. LeBlanc, S. Huang, M. Olin, R.M. Measures, A. Guemes and A. Othonos, " istri t    i r  r  rating and its reflection spectrum analysis t. tt., , . 5-. M. LeBlanc and AD. Kersey, "Distributed, intra grati  r l th tuned low-coherence interfero etry", roceeding of 12" International Confere   ti l i r  s rs, pp. 52--  Proc. of SPIE Vol. 4185  41853S-4Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/8/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Compact FBG grating array structure for high spatial resolution distributed strain sensing

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Compact FBG grating array structure for high spatial resolution distributed strain sensing

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