A new multiplexing scheme for monitoring fiber optic Bragg gratings in the coherence domain has been developed. Grating pairs with different grating distances are distributed along a fiber line, and interference between their reflections is monitored with a scanning Michelson interferometer. The Bragg wavelength of the individual sensor elements is determined from the interference signal frequency

Dakin, J.P.

Ecke, W.

Rothardt, M.

Schauer, J.

Usbeck, K.

Willsch, R.

English

Southampton (e-Prints Soton)

1 Optoelectronics Research Centre, University of Southampton, UKFig. 1  Evaluation scheme of our new coherence domain multiplexed fiber grating sensor based on pairsof Bragg gratings and scanning readout interferometer.New Multiplexing Scheme for Monitoring Fiber Optic Bragg Grating Sensorsin the Coherence DomainJ.P.Dakin, Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UKW.Ecke, M Rothardt, J.Schauer, K.Usbeck, R.WillschInstitute for Physical High Technology (IPHT Jena), Helmholtzweg 4, D-07743 Jena, GermanyAbstractA new multiplexing scheme for monitoring fiber optic Bragg gratings in the coherence domain has beendeveloped. Grating pairs with different grating distances are distributed along a fiber line, andinterference between their reflections is monitored with a scanning Michelson interferometer. The Braggwavelength of the individual sensor elements is determined from the interference signal frequency.IntroductionMany methods have been devised for interrogating optical path difference (OPD). When illuminatedin-fiber Bragg gratings, including scanned filters by a broadband source of short coherence length,(e.g., Fabry-Perot [1], acousto-optic [2], and each interferometric sensor in a chain shows fringestretched-fiber grating [3]), passive filters using visibility only when interrogated via an opticalwavelength selective couplers [4], readout interferometer having an OPD matching that of thespectrometers [5,6] and stretched-fiber sensor, to within the source coherence length. Theinterferometers [7]. The passive wavelength- method is sometimes termed "white lightselective filters and the acousto-optic tunable interferometry", as it is possible, in principle to usefilters have the potential for fastest response, but white light when path differences are very small.the stretched fiber interferometer has, so far,demonstrated the greatest wavelength measurementprecision. The latter can also be used in principleto read multiple arrays of gratings using Fouriertransform spectrometry [8], but this usually In this paper, we shall report the first work on arequires significant processing time and also, in coherence multiplexing scheme which allows linearorder to avoid distortions, a good quality signal, arrays of gratings to be interrogated by a scannedfree of polarization fading. It also requires all the receiving interferometer.gratings to have different wavelengths, limiting thenumber possible within a given spectral range of In our new interrogation system (Fig. 1) thethe source. gratings in the array are arranged as a number ofThe method of coherence multiplexing [9] has beenused to separate out sensors according to theirPrinciple of Bragg grating multiplexing in thecoherence domainin-line matched pairs with different spacings nc L2 nc L12 Optoelectronics Research Centre, University of Southampton, UKFig. 2 Schematic view of the interference visibilityof two grating pairs P  and P  during scanning (in 1    2each direction) the armlength imbalance in thereadout interferometer through their spacings, L1and L  (n  - effective refractive index of fiber core). 2  cbetween each pair. A pathlength-scanned readoutinterferometer determines, in turn, the centroidwavelengths of each grating pair from the period of The multiplexing scheme has been testedeach group (burst) of visible interference fringes, experimentally in the arrangement of Fig. 1 with aformed sequentially as the scanned-interferometer 1300nm fiber-pigtailed SLD (SLD561) and a 1MSpathlength passes through the path difference of transimpedance receiver PRM56/4 (both fromeach grating pair. Superlum Ltd.). The Bragg grating strain sensorHowever, the grating pairs act as a reflective filter,increasing the coherence length of the light- The Michelson readout interferometer had onesource/grating-pair combination to approximately fiber endface mirrored by silver coating. Thethat of the length of the gratings. In order to avoid optical path length of the second interferometerundesirable overlap of observed fringes from arm could be periodically varied: the fiber endfacedifferent pairs of gratings, the spacings of each was reflection-free polished under an angle of 8°,pair must differ by a "decoherence" length, the collimated light beam being reflected by asubstantially more than the coherence length spherical retro-reflector, driven in alternating linearrepresented by the reduced spectral width. directions with a p-p amplitude L =12.5mm by aUnfortunately, the necessary differences increase if push-pull solenoid pair. the spectrum of the broadband light source is non-ideal, e.g., with residual narrow mode spectra with Separate addressing of the two sensors washigh-gain superluminescent diodes (SLDs). performed by adjusting, sequentially, the readoutThus, the readout interferometer selects grating corresponding to an imbalance close to each of thepairs, sequentially, by a process of coherence optical pathlengths n qL  and n qL .multiplexing, and simultaneously measures theircentroid reflective wavelengths 8  from the The mean velocity of the moving mirror was Btemporal fringe spacing, observed as it scans adjusted to v =L /*t=0.55m/s. The resultingthrough. If the readout interferometer varies its mean frequency of interference signal was typicallypathlength linearly with time, at constant velocity f =2v /8 =850kHz. Small deviations *8  fromv , then the temporal fringe spacing represents an 8  during mechanically straining (or heating) of a Mapproximately constant frequency f  of fringes grating pair lead to incremental frequency changes Mf =2v /8 . *f =-(*8 /8 )qf . For noise reduction, the M M BExperimental resultsarray consisted in the first laboratory test of twofiber grating pairs P  and P  , formed by UV 1    2inscription in standard single mode fiber. Thelength of each grating was 4mm, the reflectivityR=4..5%, the half-power bandwidth was*8=0.5nm at  a mean wavelength 8 =1305nm. L B0   dof the Bragg grating reflections, with bandwidth*8=0.5nm, was expected to be L =4q8 ²/*8 d B0=14mm [10]. The spacing between the gratingswas L =51mm for P  and L =71mm for P . 1     1    2     2However, in practice, the optical path differences2n qL  (n  - effective refractive index of fiber core, c 1  cn ~1.47) as well as their increments 2n q(L -L ) c             c 2 1were chosen to be much longer than thedecoherence length L  of light reflected from the dfiber gratings (for reasons see below).The fiber section between the grating pairs was setto be greater than 2m, to guarantee that thecorresponding reflection signals would no longercontribute to interference.Minterferometer to a mean retro-reflector positionc 1    c 2M MM M B0        BB0M B B0 Minterference signal at frequency f  was selectively Mamplified within a bandwidth of 50kHz. Thefrequency changes *f  were determined by M3 Optoelectronics Research Centre, University of Southampton, UKFig. 3 Interference visibility of a single Bragggrating reflection, as a function of readoutinterferometer armlength imbalance.The length scale is nonlinear and is given onlyapproximately. The distance between maxima ofcoherence is 2.7mm, corresponding to the opticalpath length of the SLD chip and to the ripple periodof 0.3nm in the SLD spectrum.counting the interference fringes at maximum The absolute frequency shift was 790HzqN  tensileinterference visibility during the retro-reflector force, or 0.7Hzqµ, . The equivalent temperaturescan. Interval duration of 3ms (pass length of sensor would show a frequency shift of 6HzqK .moving retro-reflector 1.7mm) was chosen, wherethe interference visibility was maximum and thevelocity of the retro-reflector was nearly constantin the experiments. We expect later to employ ahigher amplitude mirror scanner, to enable all A new multiplexing scheme of fiber Bragg gratingsgrating pairs to be interrogated in one sweep.  has been developed and experimentally realized. ItIn contrast to the simplified signal shown in Fig. 2, a fiber line and with interrogation of  theirthe real system showed full decoherence only at reflections using a scanning Michelsonoptical path differences >50mm. This was interferometer to separate their returns in theexplained by residual ripple (relative height about coherence domain. The actual Bragg wavelength of10%, period 0.3nm) in the spectrum of the SLD. each of the sensor elements can be simultaneouslyThis caused an undesirable non-monotonous determined during this scanning process bybroadening of the coherence, leading in practice to conventional frequency counting of the interferencethe envelope of the interference signal departing fringes. from the ideal Gaussian shape. A long-periodicstructure (Fig. 3) appears in the coherence function Using this technique, a resolution of strain <1µ,due to the few (typically 2 or 3) residual SLD and temperature <0.1K is possible at measuringFabry-Perot modes which overlap the 0.5nm times ~1s. Similar accuracy of Bragg wavelengthhalfwidth Bragg grating reflections. determination from the frequency of interferenceThe multiplexing scheme with two Bragg grating measurements using a reference laser wavelengthpairs was tested as a two-point strain sensor array. (as, for example, proposed in Ref. 8) and byTensile strain was applied immediately to the fiber reducing the polarization and SLD-spectrum-leads of either grating pairs. At force levels induced errors discussed below. F=0.3N, the fractional strain induced frequencychange was 9q10 /N. From this value, a Bragg This new multiplexing scheme of Bragg gratings in-4wavelength shift coefficient of 1.2nm/N was the coherence domain needs no optical filters orcalculated, in reasonable agreement with that of fast pulsed optoelectronics, but it can be combined1.17nm/N from direct spectral measurement [6]. with known wavelength and time domain-1-1-1Conclusionuses pairs of gratings with different spacings alongfringes is expected in future by calibrating themultiplexing methods in order to increase thenumber of sensors possible in the network.The spacings of the gratings in each of theindividual measuring points had to differ by amuch greater amount (about 20mm) than predictedby simple theory. This was caused by the residualFabry-Perot mode spectrum of the SLD used in theexperiment. The use of a broadband light sourcewith a smoother spectral distribution (e.g.,fluorescent fiber light source) would allow us todecrease the distance steps to <5mm, asdetermined from the coherence lengthcorresponding to a 0.5nm wide reflection spectrumof the Bragg gratings, and, therefore, wouldshorten the overall length of each sensor point. The undesirable modal structure of the SLD causesalso an alternating error of  Bragg wavelengthdetermination (observable by Fouriertransformation of the SLD spectrum to have aperiod equal to its mode spacing of 0.3nm).4 Optoelectronics Research Centre, University of Southampton, UKCalculation, numerical compensation, and 6. W.Ecke, H.Bartelt, G.Schwotzer,experimental verification of this non-linearity, as K.Usbeck, R.Willsch, S.Birkle,well as the influence of small Bragg wavelength T.Bosselmann, and P.Kraemmer, "Low-differences within one grating pair  will be reported Cost Optical Temperature and Strainlater, after mechanically improving the sensor Sensing Networks Using In-Line Fiberinterrogation arrangement. Gratings", European Symposium onThe interference visibility is influenced by drifts of Munich 16-20 June 1997, Proc. SPIE Vol.the birefringence in the optical fiber between thefiber coupled Bragg grating pairs, as well as in thefiber optic readout interferometer. Use ofpolarization scrambling or diversity will increasestability of the interrogation technique.References1. A.D.Kersey, T.A.Berkoff, andW.W.Morey, "Multiplexed fiber Bragggrating strain-sensor system with a fiberFabry-Perot wavelength filter", OpticsLett., Vol. 18, pp. 1370-1372 (1993)2. M.G.Xu, H.Geiger, J.-L.Archambault,L.Reekie, and J.P.Dakin, "Novelinterrogating system for fibre Bragggrating sensors using an acousto-optictunable filter", Electron. Lett., Vol. 29, pp.1510-1511 (1993)3. D.A.Jackson, A.B.Lobo Ribeiro,L.Reekie, and J.-L.Archambault, "Simplemultiplexing scheme for a fiber-opticgrating sensor network", Optics Lett., Vol.18, pp. 1192-1194 (1993)4. R.M.Measures, S.Melle, and K.Liu,"Wavelength demodulated Bragg gratingfiber optic sensing systems for addressingsmart structure critical issues", SmartMaterials and Structures Vol. 1, pp. 36-44(1992)5. C.G.Askins, M.A.Putnam, andE.J.Friebele, "Instrumentation forinterrogating many-element fiber Bragggrating arrays", Smart Sensing, Processingand Instrumentation, Proc. SPIE Vol.2444, pp. 257-266 (1995)7. A.D.Kersey, T.A.Berkoff, andW.W.Morey, "High resolution fibre-grating based strain sensor withinterferometric wavelength-shiftdetection", Electron. Lett. Vol. 28, pp.236-238 (1992)Lasers and Optics in Manufacturing,3099 (1997)8. M.A.Davis and A.D.Kersey, "Applicationof a Fiber Fourier Transform Spectrometerto the Detection of Wavelength-EncodedSignals from Bragg Grating Sensors", J.of Lightwave Technology Vol. 13, pp.1289-1295 (1995)9. J.L.Brooks, R.H.Wetworth,R.C.Youngquist, M.Tur, B.Y.Kim, andH.J.Shaw, "Coherence multiplexing offiber optic interferometric sensors", IEEEJ. Lightwave Technology Vol. 3, pp.1062-1072 (1985)10. H.Lefevre, "The fiber-optic gyroscope",Artech House, Boston-London (1993)

and E.J.Friebele, &quot;Instrumentation for interrogating many-element fiber Bragg grating arrays&quot;,

and W.W.Morey, &quot;Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter&quot;,

Application of a Fiber Fourier Transform Spectrometer to the Detection of Wavelength-Encoded Signals from Bragg Grating Sensors&quot;,

High resolution fibregrating based strain sensor with interferometric wavelength-shift detection&quot;,

Novel interrogating system for fibre Bragg grating sensors using an acousto-optic tunable filter&quot;,

R.C.Youngquist, M.Tur, B.Y.Kim, and H.J.Shaw, &quot;Coherence multiplexing of fiber optic interferometric sensors&quot;,

Simple multiplexing scheme for a fiber-optic grating sensor network&quot;,

The fiber-optic gyroscope&quot;, Artech House,

Wavelength demodulated Bragg grating fiber optic sensing systems for addressing smart structure critical issues&quot;,

New multiplexing scheme for monitoring fiber optic Bragg grating sensors in the coherence domain

New multiplexing scheme for monitoring fiber optic Bragg grating sensors in the coherence domain

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