We describe the characterisation of long period gratings written in elliptical core fibre, which yield a discriminatory sensor for curvature and temperature with a resolution ±0.05m-1 for curvature and ±0.9 °C for temperature

Allsop, Thomas D.P.

Bennion, Ian

Floreani, Filip

Webb, David J.

English

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PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieThe bending and temperaturecharacteristics of long period gratingswritten in elliptical core step-indexfibreT. Allsop, F. Floreani, D. J. Webb, Ian BennionT. Allsop, F. Floreani, D. J. Webb, Ian Bennion, "The bending and temperaturecharacteristics of long period gratings written in elliptical core step-indexfibre," Proc. SPIE 5855, 17th International Conference on Optical FibreSensors,  (23 May 2005); doi: 10.1117/12.623422Event: 17th International Conference on Optical Fibre Sensors, 2005, Bruges,BelgiumDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 08 Nov 2019  Terms of Use: https://www.spiedigitallibrary.org/terms-of-useThe Bending and Temperature characteristics of Long Period Gratingswritten in Elliptical Core Step-Index fibreT. Allsopa, F. Floreani, D.J. Webb, I. BennionAuthor’s Affiliations:T. Allsop, F. Floreani, D.J. Webb, I. Bennion (Photonics Research Group, Aston University, AstonTriangle, Birmingham, B4 7ET, U.K).ABSTRACTWe describe the characterisation of long period gratings written in elliptical core fibre, which yield a discriminatorysensor for curvature and temperature with a resolution ?0.05m-1 for curvature and ?0.9 oC for temperature.INTRODUCTIONA fibre long period grating (LPG) is an axially periodic refractive index variation inscribed in the core of a photosensitivesingle-mode optical fibre by ultra-violet irradiation, which couples light from the core of the fibre into the fibre claddingmodes at discrete wavelengths. The index modulation change within the core of a single mode optical fibre isapproximately 10-4 and has a period typically between 100-600 ?m. The index modulation produces a set of attenuationbands seen in the transmission spectrum of the optical fibre core. The study of the LPG attenuation bands has yieldedmany potential applications in the field of sensing through their sensitivities to strain (?), temperature (T), the refractiveindex of the surrounding medium (ns) and bending1-4. The LPG's sensitivity to these parameters can manifest itself in twoways. Firstly the central wavelength of the attenuation band can shift in the spectral domain, which will be referred to asspectral sensitivity and secondly a change in the spectral transmission profile of the attenuation band may occur. Thespectral shift of the attenuation band arises from the phase matching condition of the LPG1.Recently LPGs have been inscribed into various types of optical fibres with different materials and variousgeometries with a view to optimising the sensitivity to specific measurands or reducing the cross-sensitivity to othermeasurands. A few papers have been presented concerning LPGs written in asymmetric optical fibres3 for bend sensing;this approach can lead to problems with polarisation dependence5,6 due to birefringence in the fibre. This paper presents a study of the spectral characteristics and polarisation dependence of LPGs written into anelliptical core step-index fibre (E-core fibre) to form bend sensors. The evolution of the LPG's attenuation bands underincreasing curvature differs from normal step-index fibre. In normal step-index fibre a splitting of attenuation bands isobserved due to induced birefringence at 2m-1 curvature or over. In the E-core fibre we observed splitting for bothpolarisation states of the LPGs attenuation bands for curvatures from 0.2m-1 to 2m-1. We found that the wavelengthseparation between adjacent bend-induced attenuation bands and the normal attenuation bands is approximately linear asa function of curvature. Finally, a normal LPG attenuation band and a bend induced attenuation band are used todiscriminate between temperature and bending effects, giving a well-conditioned sensitivity matrix that is comparable toother discrimination approaches7,8.FABRICATION AND CHARACTERISATION OF LONG PERIOD GRATINGS This elliptical core step-index fibre has an inner core composed of GeO2/SiO2 with a major semi-axis of 5 ?m and aminor semi-axis of 3 ?m with a cladding radius of 62.5 ?m, which is assumed to be pure SiO2. The E-core fibre is notspecifically designed to be photosensitive and so its photosensitivity was increased by hydrogenation at a pressure of 120Bar for a period of 2 weeks. The LPGs were fabricated using a frequency doubled argon ion laser at a wavelength of 244nm with a point-by-point writing technique. Several grating periods were used from 200 ?m to 450 ?m with a gratinglength of 4 cm. The characterisation of the attenuation bands was carried out by illuminating the LPG using a broadbandlight source and observing the transmission spectrum with an optical spectrum analyser (OSA) with an accuracy of 0.05nm.The use of a polariser and a polarisation controller working in conjunction with the broadband light sourcerevealed a roughly 20nm separation between the attenuation bands associated with the two orthogonal polarisation states. 17th International Conference on Optical Fibre Sensors, Marc Voet, Reinhardt Willsch,Wolfgang Ecke, Julian Jones, Brian Culshaw, eds., Proceedings of SPIE Vol. 5855(SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.623422711Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 08 Nov 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-useTwo examples are shown in figure 1 and figure 2, the former corresponding to an LPG with a period of 200?mand the latter an LPG with a period of 400?m.Figure 1 Examples of the transmission spectra of LPGs written in the elliptical core step-index fibre (a) period of 200?m and (b)period of 400?m, both having a length of 4cm (fast axis (      ), slow axis (      )).The two spectra shown in figure1 show attenuation bands that are in approximately the same spectral location,though the grating periods differ by a factor of two. Secondly, it was found that for the LPG with a period of 400?m hadtwo spectrally overlapping attenuation bands corresponding to the two orthogonal polarisation states of the LPG, (circledin figure 2), this spectral property may be used to allow polarisation insensitive interrogation.SPECTRAL BENDING CHARACTERISTICSThe LPGs were clamped mid-way between two towers, one of which was mounted on a translation stage thatwas moved inwards to induced a bend in the optical fibre. Additionally, rotation of the LPG sensor was performed on thisrig by subjecting the sensor to a known curvature then rotating the sensor around its clamped axis with the flat sidealways pointing upwards, defining as a 0? reference the original orientation (fibre hanging downward, flat-side up), seefigure 3. The broadband light source was connected to a polariser, which in turn was connected to a polarisationcontroller; the light from this arrangement illuminated the LPG and observations were made using an OSA. Thepolarisation controller was used to maximise the light in one polarisation state, so one state could be studied at a time.Figure 3. Schematic of the bending/rotational test rig.The sensor's curvature, R, is given by REF4 ? ?222LddR??? (1)where L is the half distance between the edges of the two towers and d is the bending displacement at the centre of theLPG. The experimental results are shown for an LPG with a period of 250?m; the spectral behaviour of this LPG beingtypical of all the LPGs written in the E-core fibre. The curvatures imposed on the LPG in the 0o position (in figure 3) areshown, the other positions were also investigated but only small variations were found. Both polarisation states of theLPG were investigated and the results are shown in figures 4 (slow axis only) and 5.LPG deviceMovement2Ld(-ve)Translationstage0o90o180o270oRotation-19-18-17-16-15-14-13-12-11-10-91500 1550 1600 1650Wavelength nmOptical power dBm(a)-26-25-24-23-22-21-20-19-181500 1550 1600 1650Wavelength nmOptical power dBm(b)712     Proc. of SPIE Vol. 5855Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 08 Nov 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-useFigure 4. The transmission spectra of an LPG attenuation bands associated with slow axis (period 250?m, length 4cm) with increasingcurvature from 0m-1 to 2m-1Inspecting figure 4, as the LPG is subjected to increasing curvature “the normal” attenuation bands experience a bluewavelength shift but there is also the generation of “bend-induced” attenuation bands which have a red wavelength shift.Comparing these results to standard step-index (SI) fibre, two differences were observed. Firstly for the SI fibresignificant induced birefringence occurs at a curvature of about 2m-1, beyond which splitting of the attenuation bands areobserved; in the case of the LPGs written in the E-core fibre the bend-induced attenuation band started to be evident at acurvature of  only about 0.1m-1. This is likely due to the intrinsic birefringence of the E-core fibre, which doesn’t exist inthe SI fibre. Secondly, the evolution with respect to curvature differs from SI fibre, here the bend-induced attenuationband appears spectrally separated from the normal attenuation bands and not from the splitting of the normal attenuationbands. The spectral sensitivities of the LPG attenuation band at 1601nm (figure 4) in both polarisation states as a functionof curvature are shown in figure 5.Figure 5. The spectral curvature sensitivity of both polarisation states of the LPG (period 250?m, length 4cm) written in elliptical corestep-index fibre. (a) the slow axis and (b) the fast axis.  The spectral sensitivity for fast axis with respect to curvature is d?/dR=+6.2 ? 0.7 nm m for the bend-inducedattenuation band and d?/dR=-9.4 ? 0.5 nm m for the normal attenuation band. The spectral sensitivity for slow axis withrespect to curvature is d?/dR=+4.2 ? 0.8 nm m for the bend-induced attenuation band and d?/dR=-8.2 ? 0.6 nm m for thenormal attenuation band.  Using the assumption of linearity between wavelength shift and curvature, the resolution ofslow axis was ? 0.06 m-1 and fast axis was ? 0.05 m-1 (residual analysis). The spectral sensitivities are expected to bedifferent due to the fact that the attenuation bands are associated fast axis possesses an electric field that extends furtherinto the cladding than the slow axis and thus has a higher sensitivity.SPECTRAL TEMPERATURE CHARACTERISTICSThe sensitivity to temperature was investigated by placing the sensor on the top of an insulated Peltier cooler. Thetemperature was varied and, after allowing time for thermal stabilisation, the spectral locations of the central wavelengthsof the attenuation bands for each orthogonal polarisation state were monitored using an OSA. The experiment wasconducted with the fibre in the cooler in two configurations (i) straight and (ii) subject to a curvature of 0.2m-1. Thespectral sensitivity of both orthogonal polarisation states are shown in figure 6.-15-10-505100 0.5 1 1.5Curvature m-1wavelength nm(b)-10-8-6-4-202460 0.3 0.6 0.9 1.2 1.5Curvature m-1Wavelenght nm(a)-13-8-327121722271530 1550 1570 1590 1610 1630 1650 1670 1690Wavelength nmOptical power dBmIncreasing CurvatureProc. of SPIE Vol. 5855     713Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 08 Nov 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-useFigure 6. The spectral temperature sensitivity, in both polarisation states, of the LPG (period 250?m, length 4cm) in elliptical corestep-index. (a) the slow axis (at a wavelength of 1601nm) along with (c) the bend-induced attenuation band. (b) the fast axis (at awavelength of 1620nm)  along with (d) the bend-induced attenuation band.  All the observed attenuation bands had red wavelength shifts with respect to temperature. The spectral sensitivity for fastaxis is d?/dT=+0.048 ? 0.008 nm oC-1 for the bend-induced attenuation band and d?/dT =+0.059 ? 0.005 nm oC-1 for thenormal attenuation band.  The spectral sensitivity for slow axis is d?/dT =+0.035 ? 0.008 nm oC-1 for the bend-inducedattenuation band and d?/dT =+0.051 ? 0.008 nm oC-1 for the normal attenuation band.  Leading the resolution of slowaxis was ? 1.8 oC and fast axis was ? 0.9 oC (residual analysis). Using this LPG to discriminate between curvature andtemperature, for fast axis obtain a sensitivity matrix with a condition number of 174, while slow axis yields a conditionnumber of 200; these figures are comparable to those obtained with other discriminatory sensors.CONCLUSIONThe attenuation bands of an LPG written into elliptical core step-index fibre were found to be sensitive to thepolarisation of the interrogating light with a spectral separation of around 20nm between the two orthogonal polarisationstates. The spectral sensitivity of both orthogonal polarisation states was measured with respect to curvature andtemperature. The bending of the LPG devices produced attenuation bands, which had blue wavelength shifts with sensitivitiesup to –9.4 nm m and red wavelength shifts with sensitivities up to +6.2nm m. With light in the fast axis, the use ofneighbouring normal and bend-induced attenuation bands were used to demonstrate a discriminatory sensor fortemperature and curvature, giving an overall curvature resolution of ?0.05m-1 over the range of 0m-1 to 2.0m-1 and anoverall temperature resolution of ?1.4 oC from 20 oC to 90 oC for this sensor.ACKNOWLEDGMENTThe authors would like to thank Fiberlogix Limited, in particular Saeed Rehman, for supplying the E-core fibre and itsspecification. The work was funded by the UK Engineering and Physical Sciences Research Council.REFERENCES1 V. Bhatia, "Applications of long-period gratings to single and multi-parameter sensing", Opt. Express., 1999, vol. 4, no. 11, pp.457-466.2 H.J. Patrick, et al, "Analysis of the response of long period fibre gratings to external index of refraction", J. Lightwave Technol.,vol.16, no.9, pp.1606-1612, 1998.3 H.J. Patrick, “Self-aligning, bipolar bend transducer based on long period grating written in eccentric core fibre”, Electron. Letts.,vol. 36, no. 21, p. 1763-1764, 2000.4 W. Du, et al, “Long-period fibre grating bending sensors in laminated composite structures,” in Sensory Phenomena andMeasurement Instrumentation for Smart structures and materials. Proc. SPIE 3330, pp.284-292, 1998. 5 A.S.Kurkov, et al “Long-period fibre grating as a wavelength selective polarisation element”, Electron. Letts., Vol. 33 No. 7, 19976 S. Ramachandran, et al, “High extinction, broadband polarisers using long-period fibre gratings in few-mode fibres”, Electron.Letts., Vol. 38 No. 22, 20027 H. Patrick et al, “Hybrid fibre Bragg grating/long period fibre grating sensor for strain/temperature discrimination”, Photon. Technol.Lett., vol.8, no.9, pp. 1223-1225. 8 M.G. Xu, et al, “Discrimination between strain and temperature effects using dual-wavelength fibre grating sensors”, Electron. Lett.,1994, vol.30, no. 13, pp. 1085-1087.00.511.522.533.544.515 35 55 75Temperature oCWavelength shift nm(b)(d)00.511.522.533.5415 35 55 75Temperature oCWavelength shift nm(a)(c)714     Proc. of SPIE Vol. 5855Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 08 Nov 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

The bending and temperature characteristics of long period gratings written in elliptical core step-index fibre

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The bending and temperature characteristics of long period gratings written in elliptical core step-index fibre

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