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Optical fibre long-period grating sensors: characteristics and application

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Optical fibre long-period grating sensors: characteristics and application
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  I NSTITUTE OF  P HYSICS  P UBLISHING  M EASUREMENT  S CIENCE AND  T ECHNOLOGY Meas. Sci. Technol.  14  (2003) R49–R61 PII: S0957-0233(03)55184-0 REVIEW ARTICLE Optical fibre long-period grating sensors:characteristics and application Stephen W James and Ralph P Tatam OpticalSensors Group, Centre for Photonicsand Optical Engineering,Schoolof Engineering,Cranfield University, Cranfield, Bedford MK43 OAL, UKE-mail: s.w.james@cranfield.ac.uk  Received 24 October 2002, accepted for publication 13 November 2002Published 26 March 2003Online at stacks.iop.org/MST/14/R49 Abstract Recent research on fibre optic long-period gratings (LPGs) is reviewed withemphasis placed upon the characteristics of LPGs that make them attractivefor applications in sensing strain, temperature, bend radius and externalindex of refraction. The prospect of the development of multi-parametersensors, capable of simultaneously monitoring a number of thesemeasurands will be discussed. Keywords:  fibre optics, fibre sensors, fibre gratings, long period gratings,strain, temperature, bend sensor, refractive index(Some figures in this article are in colour only in the electronic version) 1. Introduction The development of fibre gratings has had a significantimpact on research and development in telecommunicationsand fibre optic sensing. Fibre gratings are intrinsic devicesthat allow control over the properties of light propagatingwithin the fibre—they are used as spectral filters, asdispersion compensating components and in wavelengthdivision multiplexing systems. The sensitivity of theirproperties to perturbation of the fibre by the surroundingenvironmental conditions has led to extensive study of theiruse as fibre sensor elements.Fibre gratings consist of a periodic perturbation of theproperties of the optical fibre, generally of the refractive indexof the core, and fall into two general classifications basedupon the period of the grating. Short-period fibre gratings,or fibre Bragg gratings (FBGs), have a sub-micron period andact to couple light from the forward-propagating mode of theopticalfibretoabackward, counterpropagatingmode[1]. Thiscoupling occurs at a specific wavelength, defined by the Braggconditionforthefibregrating, withtheFBGactingasanarrow-band reflection filter and as a narrow-band channel-droppingfilterwhen operated in transmission. The Bragg wavelength isgoverned by the period of the FBG and the effective indexof the propagating mode, with the result that a change ineither of these parameters, induced for example by a changein temperature or strain, changes the wavelength, forming thebasis of the many reported FBG sensing schemes. Typically,FBGs deployed as sensors have lengths of the order of 5 mm,with sensitivities to temperature and strain of 13 pm K − 1 and1 pm  µε − 1 respectively, for FBGs operating at 1300 nm [2].The long-period grating (LPG) has a period typically inthe range 100  µ m to 1 mm, as illustrated in figure 1. TheLPG promotes coupling between the propagating core modeand co-propagating cladding modes. The high attenuationof the cladding modes results in the transmission spectrumof the fibre containing a series of attenuation bands centredat discrete wavelengths, each attenuation band correspondingto the coupling to a different cladding mode. Examples of the transmission spectra of LPGs are shown in figure 2. Theexact form of the spectrum, and the centre wavelengths of theattenuation bands, are sensitive to the period of the LPG, thelength of the LPG (typically of the order of 30 mm) and tothe local environment: temperature, strain, bend radius and tothe refractive index of the medium surrounding the fibre [3].Changes inthese parameters canmodify the period ofthe LPGand/orthe differential refractive indexofthe core andcladdingmodes. This then modifies the phase matching conditions for 0957-0233/03/050049+13$30.00 © 2003 IOP Publishing Ltd Printed in the UK  R49  Review Article Figure 1.  Schematic of an LPG. coupling to the cladding modes, and results in a change in thecentral wavelengths of the attenuation bands.The sensitivity to a particular measurand is dependentupon the composition of the fibre and upon the order of thecladding mode to which the guided optical power is coupled,and is thus different for each attenuation band. This rangeof responses makes them particularly attractive for sensorapplications, with the prospect for multi-parameter sensingusing a single sensor element [3].This review will discuss the properties of LPGs and themethods employed in their fabrication. This will be followedby an examination of the theoretical background of LPGs, adiscussion of their sensitivities to a range of measurands witha review of their implementation as sensor elements and adiscussion of current trends in LPG sensor research. 2. LPG fabrication The fabrication of LPGs relies upon the introduction of aperiodic modulation of the optical properties of the fibre. Thismay be achieved by permanent modification of the refractiveindexofthecoreoftheopticalfibreorbyphysicaldeformationof the fibre.The modulation of the core refractive index hasbeen achieved by ultraviolet (UV) irradiation [4–7], ionimplantation [8], irradiation by femtosecond pulses in theinfrared [9], irradiation by CO 2  lasers [10, 11], diffusionof dopants into the core [12, 13], relaxation of mechanicalstress[14] and electrical discharges [15, 16]. The deformationof the fibre has been achieved mechanically [17, 18], bytapering the fibre [19] orbydeformation ofthe core [20, 21] orcladding [22]. LPGs have been fabricated in photonic crystalfibrebyperiodicallycollapsingtheholesofthefibreusingCO 2 laser heat treatment [23].The UV-induced index modulation is typically achievedin Ge-doped silica fibres using wavelengths between 193 and266 nm [24]. This is the most widely utilized method for thefabrication ofLPGs. The refractive index change is associatedwiththeformationofGe-relatedglassdefects. Fibreswithhighphotosensitivity have been developed by co-doping the corewithboron andgermanium [25]andbyhydrogen loading [26].The refractive index modulation may be built up on a point bypoint basis—a very flexible technique—or the entire lengthof the LPG may be formed simultaneously by exposure of the fibre though an amplitude mask [27], via a patternedmirror [28] or using a microlens array [29]—facilitating rapid and reproducible LPG fabrication.A typical LPG fabrication configuration, using UVirradiation through an amplitude mask, is shown in figure 3.The output from a UV laser source is used to illuminate the 800 850 900 950 1000 1050 1100102030405060708090    T  r  a  n  s  m   i  s  s   i  o  n   % Wavelength (nm) 100 (a)(b) Figure 2.  Transmission spectrum of (a) an LPG of length 40 mmand period 400  µ m, fabricated in B–Ge co-doped photosensitiveoptical fibre with a cut-off wavelength of 644 nm (after [55]).(b) An LPG fabricated in Corning SMF-28 with period 320  µ m(after [44]). UVcylindricallensamplitudemask spectrometerV grooveoptical fibrelaserV groovebroadbandoptical source Figure 3.  LPG fabricationusing a UV laser. optical fibre through an amplitude mask of appropriate period,which may be fabricated in chrome-plated silica [27] or froma metal foil, for example by milling a copper foil using a Cuvapour laser [30]. The cylindrical lens produces a line focusoriented parallel to the axis of the fibre.R50  Review Article The use of UV exposure is well established, due to itswidespread use in the fabrication of FBGs [31]. Its use hasimplications for the spectral characteristics and stability of the LPG spectrum. UV exposure of optical fibres is knownto induce birefringence, which can produce a polarizationsplitting in the attenuation bands for LPGs fabricated in non-polarization-maintaining fibres [32]. The refractive indexchange is known to contain an unstable component, whichdecays in time causing a significant change in the centralwavelengths of the attenuation bands and in the couplingstrength [27]. This unstable component may be removed bythermal annealing, but needs to be taken into account whendesigning an LPG for a particular application. The use of hydrogen loading of fibres to enhance their photosensitivitycan result in further changes to the central wavelength andpeak loss of the attenuation bands occurring after fabrication,as the hydrogen diffuses out of the fibre [6]. LPGs fabricatedin hydrogen-loaded fibre by irradiation at 193 nm have beenobserved to exhibit a growth in the peak loss of up to 14 dBand an increase in central wavelength of 40 nm over the first20 h following UV exposure, followed by a slow reduction of thepeak lossanddecrease incentral wavelength of50nmoverthe following 450 h [6]. The effect is attributed to depletionof hydrogen in the exposed regions of the core. The diffusionof hydrogen from unexposed to exposed regions then acts toincrease the amplitude of the refractive index modulation, asthe refractive index increases further in the exposed regions(increasing hydrogen concentration) and decreases in theunexposed region (decreasing hydrogen concentration). Thusthe strength of the LPG grows. This is followed by a slow out-diffusion whereby the refractive index of the core decreases,causing a decrease in wavelength. Similar effects have beenobserved for LPGs fabricated in hydrogen-loaded fibres using248 nm irradiation [33]. These effects limit the ability tofabricate LPGs with precisely defined characteristics withinhydrogen-loaded fibre. It has been shown that pre-exposureof a hydrogen-loaded fibre to a uniform UV beam locks inthe photosensitivity enhancement provided by the hydrogenloadingprocedure[34]. TheUVpre-exposureofthehydrogen-loaded fibre increases the number of defect sites believed tobe responsible for photosensitivity, and these remain afterthe hydrogen has diffused out of the fibre. If an LPG isfabricated in pre-exposed fibre after the hydrogen has diffusedout of the fibre, typically 2 weeks after removal from thehydrogen atmosphere at room temperature, the fibre maintainsthe photosensitivity, but does not suffer from the changesin attenuation band central wavelength and extinction [34].Sucheffects have been observed using continuous-wave (CW)pre-exposure at 244 nm [34] and using pulsed irradiation at157 nm [35] and 193 nm.Post-fabrication tuning of the characteristics of an LPG’stransmission spectrum is possible by reducing the fibrediameter by etching. The reduction in cladding diameterchanges the effective index of the cladding modes, resultingin an increase in the central wavelengths of the attenuationbands. The etching also changes the electric field profileof the cladding modes, resulting in a change in the overlapintegral and a concomitant change in the coupling efficiencyand minimum transmission of the attenuation bands [36, 37].Other UV wavelengths are being investigated for LPGformation, including 157 nm from F 2  lasers which offers the Fixed Point Stretching andtwisting pointCladdingCore Figure 4.  Corrugated fibre LPG. ability to fabricate LPGs in low-GeO 2  fibre with no hydrogenpre-treatment [38]—fibres in which LPGs would not formwhen exposed to other UV wavelengths. In addition, LPGformation in hydrogen-loaded fibres using this wavelengthrequired a fluence 250 times lower than that required froma laser operating at 248 nm [5]. The frequency-doubled(266 nm) [39] and -tripled (355 nm) [7] outputs of Nd:YAG lasers have also been used to fabricate LPGs, as havefemtosecond pulses from an Nd:glass laser [40].LPGs have been fabricated using irradiation withfemtosecond pulses in the near infrared (800 nm) [9]. Theirradiation is believed to cause a densification of the glass,and the resultant index change, and LPG spectrum, is stable attemperatures up to 500 ◦ C. CO 2  laser irradiation at 10 . 6  µ mhas been shown to produce LPGs with high temperaturestability and polarization insensitivity [41], and with spectralcharacteristics that are unchanged even after annealing at1200 ◦ C [10]. The CO 2  laser exposure of the fibre wassrcinally thought to result in the densification of the glass,and/or the relaxation of tensile stresses built into the claddingof fibres such as Corning SMF28 during fabrication [14].However, there is evidence to suggest that the change inrefractiveindexisaresultofbreakage ofSi–O–Gechains[11].CO 2  laser irradiation has allowed LPGs to be written in bothfibres that have been hydrogen loaded and in fibres with nohydrogen pre-treatment. The LPGs fabricated in hydrogen-loaded fibre were associated with few extraneous spectralfeatures, as a result of the lower fluence required for theirfabrication [11].Electric arc fabrication of an LPG relies upon acombination of up to four effects to generate the periodicmodulation of the fibre properties. The mechanisms exploitedinclude the induction of microbends into the fibre [42],the periodic tapering of the fibre [19], the diffusion of dopants [12, 13] and the relaxation of internal stresses [15]. Such LPGs have been shown to operate at temperaturesof up to 800 ◦ C without permanent modification of theirproperties [43], and, if annealed appropriately, they mayoperate at temperatures up to 1190 ◦ C [15]. Typically, theelectrodes of a fusion-splicing machine are used, exposinga region of fibre with a length of the order of 100  µ m tothe arc, limiting the minimum period of LPG that may befabricated. Chemical etching of the cladding of a fibre toproduce a corrugated structure has been shown to allow thegeneration of an LPG [22]. The corrugated fibre is shown infigure4. Whenatensileloadisappliedtothefibre, theperiodicvariation in the diameter of the fibre results in a periodic strainvariation across the corrugated structure, with a concomitantperiodic refractive index induced via the photoelastic effect.R51  Review Article Thus the coupling strength increases with applied load, with asmall change in wavelength of the attenuation bands [22].Generally, LPGs have been fabricated to operate attelecommunications wavelengths (1300and1550nm), andthespectrumismonitored bycoupling lightfrom superfluorescentfibre sources or superluminescent diodes in the fibre andrecording the transmission using an optical spectrum analyser.There have been reports of the use of fibres with lower cut-off wavelengths, 650 nm, allowing the operation of the LPGwithin the response of silicon detectors, facilitating the use of low-cost CCD spectrometers [39]. 3. LPG theory The fibre itself consists of two waveguide structures—onebeing the high-index core surrounded by the lower-indexcladding, the other being the cladding, surrounded by air.Phase matching between the mode propagating in the core of thefibre and aforward-propagating cladding mode isachievedat the wavelength,  λ , where the expression λ = [ n ef f   (λ) − n iclad  (λ) ]   (1)is satisfied [44], where  n ef f  (λ)  is the effective refractive indexof the propagating core mode at wavelength  λ ,  n iclad  (λ)  is therefractive index of the  i th cladding mode and    is the periodof the LPG.The minimum transmission of the attenuation bands isgoverned by the expression [1] T  i  = 1 − sin 2 (κ i  L )  (2)where  L  is the length of the LPG and  κ i  is the couplingcoefficient for the  i th cladding mode, which is determined bythe overlap integral of the core and cladding mode and by theamplitude of the periodic modulation of the mode propagationconstants.Since the cladding generally has a large radius, itsupportsa large number of cladding modes. Theoretical analysis hasshownthatefficientcouplingispossibleonlybetweencoreandcladding modes that have a large overlap integral, i.e. modesthat have similar electric field profiles [45]. Thus coupling isobserved between the core and circularly symmetric claddingmodes of odd order. This is because the electric field profileof the even-order modes is such that the field amplitude is lowwithin the core, whereas the electric field profiles of the oddmodes have a peak located within the core [45].The modelling of LPGs requires the calculation of therefractive indices of the core and cladding modes to allow thecentral wavelengths of the attenuation bands to be determined,and the calculation of the mode electric field profiles tofacilitate determination of the coupling strength and the formof the spectrum. The refractive index of the propagating coremode of the fibre is generally determined using the weaklyguidedfieldapproximationofGloge[46]. Asimplethree-layerslab waveguide model to determine the cladding mode indiceshas been presented, allowing an approximate determination of the coupling wavelengths [27]. However, it has been shownthat the effect of the core upon the effective indices and modeprofiles of the cladding mode cannot be ignored for accuratesimulation of LPG characteristics [45]. 100200300400500600700800900123456789    L   P   G  p  e  r   i  o   d   (     m   ) wavelength (   m) 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 1.154050607080901001101201301.11.0510.950.90.850.80.750.7 1.15 18192021222324252627    L   P   G  p  e  r   i  o   d   (     m   ) wavelength (   m) (a)(b) Figure 5.  Plot of resonant wavelength as a function of LPG periodfor coupling between the guided core mode and cladding modes of order (a) 1–9 and (b) 18–29, calculated using the technique detailedin the text and assuming the fibre’s cut-off wavelength is 650 nm. Figure5showsthedependenceofthecouplingwavelengthfor cladding modes 1–9 (a) and 18–27 (b) upon the period of the LPG. These graphs were calculated following the methodpresented in[47]. The fibre modelled has acut-off wavelengthof 675 nm. The graphs indicate that coupling to lower-ordermodes is achieved using longer periods, while shorter periodsfacilitate coupling to the higher-order modes. In addition,figure 5(b) shows that, for higher-order cladding modes,coupling to one cladding mode can occur at two wavelengths,producing two attenuation bands [48]. It should be noted thatsmall changes in the coupling conditions could result in largechanges in the separation of the two attenuation bands. ForLPGs fabricated such that their period coincides with the peak of the curve (for example, mode 18), it has been observed thatchanges in the coupling conditions result in a change in thecoupling efficiency, but not in the wavelength [49]. 4. LPG sensors 4.1. Temperature sensitivity As discussed in the introduction, the sensitivity of LPGs toenvironmental parameters is influenced by the period of theLPG [3], by the order of the cladding mode to which couplingtakesplace [3] andbythe composition oftheoptical fibre [50].This combination of influences allows the fabrication of LPGsthat have a range of responses to a particular measurand—asingle LPG may have attenuation bands that have a positivesensitivity to a measurand, others that are insensitive to themeasurand and others with a negative sensitivity.R52  Review Article Thispropertyhasbeenwidelyexploitedforcontrollingthetemperature sensitivity of LPGs. For many telecommunica-tionsapplications spectralstabilityisofprime importance, andthe ability tofabricate anLPG withan inherently temperature-insensitive attenuation band is an attractive feature. This isalso attractive for forming temperature-insensitive strain sen-sors. On the other hand, for forming temperature sensors, orthermallytunedfilters,ahightemperature sensitivity(ofeithersign) is required.The origin of the temperature sensitivity may beunderstood by differentiating equation (1) [44].d λ d T  = d λ d (δ n ef f   )  d n ef f  d T  − d n cl d T   +   d λ d  1  L d  L d T  (3)where  λ  is the central wavelength of the attenuation band, T   isthetemperature,  n ef f   istheeffectiverefractiveindexofthecore mode,  n cl  is the effective refractive index of the claddingmode,  δ n ef f   = ( n ef f   − n cl ) ,  L  is the length of the LPG and   is the period of the LPG.The first term on the right-hand side of equation (3) isthe material contribution, and is related to the change in thedifferential refractive index of the core and cladding arisingfrom the thermo-optic effect. This contribution is dependentupon the composition of the fibre and is strongly dependentupon the order of the cladding mode. For coupling to low-order cladding modes (accessed using longer periods,   > 100  µ m), the material effect dominates. For coupling tohigher-order cladding modes (accessed using shorter periods,  <  100 µ m),thematerialeffectforstandardgermanosilicatefibrescanbenegligible[44]. Thesecondtermisthewaveguidecontribution as it results from changes in the LPG’s period.The magnitude and sign of the term depend upon the orderof the cladding mode. For coupling to low-order claddingmodes d λ/ d   is positive, while for the higher-order claddingmodes this term is negative, as can be seen from the graphs infigures 5(a) and (b). Thus, by an appropriate choice of LPGperiod it is possible to balance the two contributions to thetemperature sensitivity to produce a temperature-independentattenuation band and also to produce attenuation bands withtemperature sensitivities (positive or negative) appropriate tospecific applications. Altering the fibre composition, suchthat the thermo-optic coefficient of the core is either largeror smaller than that of the cladding, can also be used to obtaina required temperature sensitivity [51]. In addition to thechange in central wavelength, a change in the extinction of the attenuation band may be observed for LPGs with strength κ  L  > π  [51]. At room temperature, the temperature responseof the wavelengths of the attenuation bands’ is linear, asillustrated in figure 6 [3]. However, it has been shown thatthe response becomes non-linear at cryogenic temperatures,below 77 K [52].LPGs fabricated in standard telecommunications opticalfibreexhibittemperaturesensitivitiesintherange3nm / 100 ◦ Cto10 nm / 100 ◦ C[3]. Thisisanorder ofmagnitude largerthanthe sensitivity of FBG sensors. Temperature-compensatedLPGshavebeendemonstrated usingperiods < 100 µ m. Inthisregime, the coupling is to the higher-order modes, which havea lower material contribution to the temperature sensitivity.For example, an LPG of period 40  µ m was found to have an Figure 6.  (a) Shift in the wavelength of an attenuationband of anLPG with temperature. The LPG was fabricated with period of 280  µ m in Corning SMF-28 fibre. The spectra correspondtotemperaturesof 22.7, 49.1, 74.0, 100.9, 127.3 and 149 . 7 ◦ C.(From [3].) (b) Shift in the wavelengthsof four attenuation bands,A–D, as a function of temperature for the LPG detailed infigure 6(a). The dashed line is the shift for an FBG fabricated at1550 nm for comparison. (From [3].) attenuation bandwithsensitivityof1 . 8pm  ◦ C − 1 [44], anorderof magnitude smaller than that of a FBG.Forthefabricationofhigh-resolutiontemperaturesensors,or to create widely tuneable filters, a number of techniquesfor further enhancing the sensitivity have been reported,including the use of fibres of different composition anddifferent geometries and the use of polymer coatings.By careful choice of the order of the cladding mode andoperatingwavelength,LPGsfabricatedinphotosensitiveB–Geco-doped optical fibres have been shown to offer sensitivitiesof up to 275 nm / 100 ◦ C [53]. Other techniques for enhancingthe temperature sensitivity have been reported based uponsurrounding the fibre by a material of large thermo-opticcoefficient, resulting in the LPG responding to both changesintemperature andtothe temperature-induced refractive indexchange ofthe surrounding medium [54–56]. These techniqueswill be discussed in more detail in section 4.3.In addition to using parameters in equation (3) to designa temperature-insensitive LPG [44], there has been a reportof the use of athermal packaging. The LPG was bonded toa substrate whose thermal expansion coefficient induced astrainthat produced awavelength shift of the opposite sensetothat induced by the change in temperature. This temperatureR53
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