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On the possible use of optical fiber Bragg gratings as strain sensors for geodynamical monitoring
Optics and Lasers in Engineering 37 (2002) 115–130
On the possible use of optical fiber Bragg gratings as strain sensors for geodynamical monitoring Pietro Ferraroa,*, Giuseppe De Nataleb a
Istituto Nazionale di Ottica Applicata, Sez. di Napoli, c/o Instituto di Cibernetica del CNR, ‘‘Eduardo Caianiello’’, Compr. ‘‘A. Olivetti’’, Fabbr. 70, Via Campi Flegrei 34, 80078 Pozzuoli (Na), Italy b INGV-Osservatorio Vesuviano, Via Diocleziano, 328, 80124 Naples, Italy Received 9 April 2001; accepted 13 August 2001
Abstract Optical fiber sensors can be used to measure many different parameters including strain, temperature, pressure, displacement, electrical field, refractive index, rotation, position and vibrations. Among a variety of fiber sensors, fiber Bragg gratings (FBG) have numerous advantages over other optical fiber sensors. One of the major advantages of this type of sensors is attributed to wavelength-encoded information given by the Bragg grating. Since the wavelength is an absolute parameter, signal from FBG may be processed such that its information remains immune to power fluctuations along the optical path. This inherent characteristic makes the FBG sensors very attractive for application in harsh environments, ‘‘smart structures’’ and on-site measurements. This paper reviews the achievements about the FBG as a strain and temperature sensor and describes the potential applications of FBG sensors for applications in the field of geophysics and its expected development in the near future. The applications could include: rock deformation, fiber-optic geophone, optical based seismograph, vertical seismic profiling and structural monitoring of civil structures. Different techniques to detect strains and various applications will be reviewed and discussed. The problem of temperature–strain cross sensitivity, that is particularly difficult to eliminate, is addressed and approaches to overcome it are discussed. r 2002 Published by Elsevier Science Ltd. Keywords: Fiber Bragg gratings; Wavelength-encoded information; Strain sensor; Temperature sensor
*Corresponding author. Tel.: +39-081-867-5041; fax: +39-081-804-2519. E-mail address: [email protected] (P. Ferraro). 0143-8166/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 4 1 - 5
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1. Introduction In the last few decades, solid earth sciences, mainly driven by large developments in geophysics, have undergone a progressive transition from a qualitative towards a quantitative, rigorous physical approach. In the meantime, problems like catastrophe hazard assessment and forecasting, involving the most practical aspect of such disciplines, have been recognized as very complex ones, unaffordable by any simple empirical approach. In particular, a modern view of volcano and earthquake hazard assessment, must recognise the intimate link between physical understanding of the basic processes and the monitoring of any changes in the systems which reveal itself to be useful to forecast the system evolution. Stress and strain changes at volcanic and tectonic areas are, by far, recognised as the best indicators of changes in the activity of the system, and its possible evolution towards critical stages. Stress and strain changes in active zones can be both static and dynamic. Static changes involve static deformations produced by volcano and tectonic sources, which are localized close to the sources at maximum distances on the order of some source lengths. Dynamic changes are best represented by seismic waves radiated by earthquake sources, which can be observed at distances of several terrestrial circumferences. Geodesy has been the classical research field for static changes and seismology for dynamic ones. In the recent years, the need for integrating the two disciplines in a unitary view has become more and more evident, also because of the growing of importance, in terms of observations and modelling of the slow processes, involving frequencies, which are at the higher limit for static and lower limit for dynamic changes. For the volcanoes, for instance, the most recent seismological research [1–3] shows that the most interesting range of frequencies to follow magma movements towards the surface is between 0.1 and 0.01 Hz. Such a range is well below the frequencies for usually employed monitoring networks (1– 30 Hz) and at the lower limit for the most recent broad-band instruments (0.01– 30 Hz). In volcano geodesy, in the meantime, the best results for monitoring volcanic sources have been obtained by recently designed borehole strain-meters, which have been able to follow the fast pre-eruptive magma rising of some kilometers in tenths of minutes/hours [4,5]. Recent researches also point out the need for dense arrays of such stress–strain meters, in order to give the best sensitivity to local source activation and the best description of the source geometry and evolution [6]. The future of stress–strain monitoring, hence, appears to be conditioned by the development of very broad-range sensors, able to somewhat unify the classical ranges of seismology and geodesy, which are of low cost and are easy to install in the form of dense arrays. Moreover, they should be directional, i.e. not only measuring volumetric changes, to allow for complete stress–strain tensor retrieving, but should be the best way to follow time-variable source geometry for a moving source. Fiber optic sensors could be, potentially, the solution for the increasing demand in having reliable strain sensors since they allow remote sensing and high networking capability. Moreover, the advancements in the field of opto-electronics, in the recent years, have made new instruments available at even lower costs and with improved characteristics such as durability and compactness allowing the on-field
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application. Among the large number of sensors developed in the recent years for strain monitoring, FBG seems to be the most promising for reliable applications. In this paper, the most important achievements about the FBGs as strain sensors are described. The problem of the strain/temperature cross sensitivity, being one of the key factors to be solved, is analyzed and the possible solutions are discussed. An overview of the existing applications in the field of geophysics of FBGs is given.
2. Principle of operation of Bragg fiber sensors Fiber Bragg gratings (FBG) are obtained by producing a periodic variation in the index of refraction along the short sections in the core of an optical fiber (Fig. 1). The Bragg grating inside the core of an optical fiber is produced illuminating from the side by interference pattern with UV light (holographic method) [7], as shown in Fig. 2. For a given angle 2y between the two interfering beams, the pitch of the periodic modulation of the refractive index is given by luv L¼ : ð1Þ 2 sin ðy=2Þ The symbol L is named Bragg wavelength; luv is the wavelength of the interfering UV waves. If the core of the optical fiber is photosensitive to UV radiation that is able to change the refractive index to the core, then the index of refraction n; in the exposed region, is modified along the axis of the fiber, giving nðzÞ ¼ ncore þ dn 1 þ cos ð2pz=LÞ ; ð2Þ
Fig. 1. Bragg grating into an optical fiber.
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Fig. 2. Holographic setup to write a Bragg grating in an optical fiber.
where ncore is the unexposed core refractive index and dn is its amplitude modulation due to the photo-induced change. Germanium-doped SiO2 fibers are photosensitive to UV radiation so that Bragg gratings can be successful written in. The first report of photosensitivity in optical fiber was due to Hill et al. in 1978 [8], but the first practical demonstrations of grating formation was reported by Meltz et al. [7], ten years later and was based on the transverse method depicted in Fig. 2. After the work of Meltz et al., a considerable amount of research activity began on FBG along two directions: sensing applications [9] and optical communications [10]. Once an FBG has been realized, it basically works as a wavelength selective filter for propagation radiation along the optical fiber. If light from a broadband source is coupled into the optical fiber with a FBG written in it, then a narrow band of light with center wavelength, lB ¼ 2nL; is reflected at the FBG location, while the rest of the spectrum is transmitted as shown in Fig. 3. Strain sensing by means of FBGs is very simple, in principle. In fact, while straining the fiber, the Bragg wavelength lB ¼ 2nL varies as a consequence of both the grating pitch L change (due to the simple elastic elongation), and photo-elastic change in the refractive index. The relative change of L is given by Dlb ¼ ð1 pe Þe; lb
ð3Þ
where e is the longitudinal strain experienced by the optical fiber at FBG location and pe ¼ n2 =2½p12 nðp11 þ p12 Þ is the effective photo-elastic constant of the fiber core material. Typical wavelength shift @1550 nm of lB ¼ 2nL is Dlb =De ¼ 0:001 nm=me:
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Fig. 3. Depiction of the reflected and transmitted spectrum of FBG for a broadband light source.
Fig. 4. Example of an optical fiber with an FBG written in it, embedded in a composite material beam.
In Fig. 4, an example of application of FBG as a strain sensor is shown. The FBG has been embedded into a beam-made carbon fiber with reinforced epoxy resin. The beam is subjected to tensile mechanical load. Fig. 5 gives the graph of surfacemounted electrical strain-gage versus the Bragg wavelength shift, as measured in the embedded fiber optic grating. Fig. 5 demonstrates the very good linearity and sensitivity of the FBG as a strain sensor.
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Fig. 5. Example of wavelength shift of an FBG embedded in a composite material beam subjected to a mechanical load vs. electrical strain-gage.
The Bragg wavelength L exhibits a shift even when the temperature changes; this is due to the summation of two effects, the thermal expansion of core material and the thermo-optic behavior that induces variation in the refractive index, expressed by Dlb ¼ ða þ xÞDT; lb
ð4Þ
where a ¼ DL=L and x ¼ dn=dT represent the thermal-expansion and thermooptical coefficients, respectively. Typical shift for the Bragg wavelength @1550 nm is Dlb =DT ¼ 0:009 nm=o C: By determining the wavelength shift of peak reflectivity, the strain and/or temperature can be evaluated. FBGs present some interesting advantages that make them strong candidates for on-field applications for a variety of uses as strain sensors: * * * *
*
FBG can be used as localized (point-wise) and/or quasi-distributed sensors; good linearity; simple demodulation concept; insensitivity to intensity fluctuation since the information is encoded as wavelength; very high multiplexing capability;
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resistance in hostile environments (immune to e.m. fields and high temperature); embedding capability; and commercially available and low cost.
2.1. Interrogation methods for FBG Different interrogation methods can be used for reading out Bragg wavelength shifts experienced by the FBGs, from which strain is evaluated: (a) direct spectroscopy using optical setup with dispersive optical elements (grating, prism) and photo-detectors (linear array, CCD) of fiber spectrum analyzers; (b) passive optical filtering components (e.g. WDM couplers); (c) tracking tuneable filters (Fabry-Perot, acoustic-tuned optics filter, filter based on a tuneable FBG); and (d) interferometers. The choice of the more suitable interrogation method depends on several factors. In fact, the FBGs can be used to measure static and/or dynamic strain with different ranges, accuracy, sensitivity, number of sensors, complexity and cost of instrumentation. Numerous methods have been proposed and reported in the scientific literature. Interferometric detection of the wavelength shift of FBG sensors yielding very high resolution for dynamic strain has been reported by Kersey et al. [11]. The method is based on a fiber unbalanced Mach-Zehnder interferometer and an pffiffiffiffiffiffi ffi experimental resolution of E0:6n strain= Hz has been reported. Other interrogation methods include schemes that are based on using two cavity lengths in FabryPerot read-out interferometers for producing two-quadrature phase-shifted signals from a FBG [12] or quadrature sampling of Mach-Zehnder interferometer [13]. A de-multiplexing scheme for FBGs sensors, where a received grating is matched to a corresponding sensor has been proposed by Jackson et al. [14]. A demodulation based on an acoustic tunable filter has been proposed by Xu et al. [15]. Demodulation of FBG sensors based on dynamic tuning of multimode laser diode or tuneable laser have also been proposed [16,17]. Active control of the spectral response of a fused biconical wavelength division multiplexer (WDM) has also been reported for strain and temperature measurements by using FBG sensors [18]. Among the possible methods, there is one that uses an acoustic-tuned integrated optics spectrometers (IOS) [19,20]. IOS is made of an optical wave-guide created by diffusing titanium in a lithium niobate crystal in which even a surface acoustic wave (SAW) is made to propagate collinearly with the optical radiation. The SAW produces a longitudinal modulation of the index of refraction along the wave-guide similar to that described by Eq. (2). The optical radiation propagating along the wave-guide will experience Bragg filtering. The period of the SAW can be changed by producing a tuneable non-permanent Bragg grating along the wave-guide. In this way, the IOS can act as a wavelength filter for the radiation reflected for the FBG in allowing the measurement of the wavelength shift and of consequence of strain. In Fig. 6, the experimental configuration adopted with IOS is shown. A broad band optical source like a superluminescent laser diode (SLD) is coupled to a 2 2 fiber coupler. At one end of the 2 2 coupler is a fiber with multiple FBGs written in
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Fig. 6. Interrogation setup of multiple FBGs along a single optical fiber by an acoustic tuned integrated optics filter (AOTF).
Fig. 7. Principle of operation of an AOTF for measuring quasi-static strain.
it, at different central Bragg wavelengths. The reflected signal is analyzed by the IOS. The IOS can be used in three different configurations: as a simple spectrometer; as a wavelength discriminator and finally as a spectral peak tracker. The first configuration is suitable for interrogation of multiple FBGs and for static strain measurement needs. The transmission filter function of the IOS is made to move through its tuning range (see Fig. 7). When the filter function of IOS is on the actual Bragg center wavelength of the FBG, the light intensity as transmitted by the IOS with a photodiode allows measurement of the strain. An alternate configuration to be adopted for the IOS is specially suited for low amplitude but high frequency bandwidth. The IOS filter function can be positioned with linear portion on the central Bragg wavelength (Fig. 8). Dynamic strain makes
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Fig. 8. Principle of operation of an AOTF for measuring low-amplitude dynamic strain.
the reflected spectrum to oscillate around the central wavelength. The transmitted intensity through the IOS gives a periodic intensity signal from which the amplitude and frequency of the dynamic strain can be evaluated. The interrogation method based on IOS has a very important advantage for infield applications in geophysics; in fact, the IOS is very compact (100 33 10 mm3) and stable since it is not made of optical components like classical spectrometers and by consequence, it is very much less affected by the external disturbances. Measurement for dynamic pffiffiffiffiffiffiffi strain with very high frequency and with high resolution (E2n strain= Hz; with bandwidth of 4 KHz) on four FBGs sensors along the same optical fiber has been reported [21]. Finally, one-third of the configuration allows automatic tracking of the reflected peak form an FBG allowing static and static strain measurement. In this configuration, the tuneable filter can be automatically tuned to stay centered even while the sensor is caused to shift by a large static strain. Detection of low level of dynamic strain regardless of static strain and temperature change can be evaluated in this way.
3. Strain–temperature cross sensitivity One of the most important point that must be addressed, for applying FBGs as reliable strain sensors, is the strain–temperature cross sensitivity. In fact, in order to develop a strain sensor capable of measuring static strain, for example in rock deformation, it is necessary to find a practical and effective solution to separate the two contributions for the overall Bragg wavelength shifting due to strain and temperature. For an FBG sensor under operative conditions, the overall shift of central Bragg wavelength is given by the summation of the two effects, strain and temperature, given by Eqs. (3) and (4) as follows: Dlb ¼ ða þ xÞDT þ ð1 pe Þe: lb
ð5Þ
Several solutions have been proposed for separating the two contributions. One method uses a reference grating subjected to the same thermal load but free from
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mechanical load [22]. Another possibility is to use a single grating that is pre-stressed and partially glued to the surface of the component to monitor for the strain and partially free [23]. The portion not glued serves as reference for temperature compensation. The combination of long period FBG and two short FBG sensors can also be applied to discriminate strain over temperature induced shifts [24]. By using a single Bragg grating together with an Erbium-doped fiber amplifier, the temperature can be evaluated by the change of spontaneous emission power effect [25]. Furthermore a single FBG written in an erbium : ytterbium-doped fiber allows synchronous measurement of strain and temperature over ranges of 1100 me and 50– 1801C [26]. Another solution based on the use of induced birefringence caused by the embedding process has been proposed [27]. An optical fiber with FBGs written in is embedded in the composite materials made of carbon fibers and epoxy resin, it has been observed that in the reflected spectrum of an FBG, two peaks appear at two different Bragg wavelengths lb1 and lb2 ; instead of the single peak; the two reflection peaks corresponding to the ordinary and extraordinary index of refraction, respectively (see Fig. 9). The two peaks are generated by the non-isotropic induced stress onto the optical fiber and grated by the shrinkage effect of the epoxy resin after the polymerisation process. The peak separation depends on the induced birefringence and could, in principle, be programmed by the type of embedding lay-out around the optical fiber. Temperature changes induce variation in the birefringence. In this way, the separation between the two peaks varies as a function of temperature. The strain can be measured by evaluating the average shift of the two wavelengths lb1 and lb2 ; while the temperature is measured by evaluating the wavelength separation, Dlb ¼ jlb1 lb2 j: Figs. 10–12 show, respectively, the average wavelength shift for a FBG grating experiencing a birefringence and subjected to a mechanical tensile load, the average wavelength shift as function of temperature and, finally, the peak wavelength spacing as a function of temperature. By measuring the peaks separation and average peaks separation, simultaneously, it is possible to obtain the strain and temperature at the same time. The adoption of interferometric interrogation method with FBG written in bowtie birefringent fiber allows the simultaneous measurement of strain and temperature pffiffiffiffiffiffi ffi pffiffiffiffiffiffiffi with resolution of 72.51C/ Hz: Also, 726me Hz for a fiber with birefringence equals to 5.5 101 [28].
Fig. 9. (a) Reflection spectrum of an FBG before embedding; (b) reflection spectrum after the embedding process in a carbon fiber reinforced palastic material, two peaks appear due to the induced birefringence in the core of the optical fiber.
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Fig. 10. Example of average wavelenght shift vs. electrical strain-gage for an embedded FBG with birefringence subject to mechanical load.
Fig. 11. Example of average wavelength shift vs. temperature for an embedded FBG with birefringence.
4. Exisiting applications of FBGS in geophysics FBGs sensors could be very useful for strain measurement in the field of geophysics. Application could include: rock deformation, fiber-optic geophone,
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Fig. 12. Peaks wavelength spacing vs. temperature.
optical-based seismograph, vertical seismic profiling, and structural monitoring of civil structures. Up-to-now, only in very few cases, FBGs have been applied in those fields of application. In fact, FBG sensors have been designed and used to measure dynamic deformations in rock masses and strain variations in the range of 109 with a bandwidth of 0.1–2 KHz and results have been compared with respect to conventional geophone registrations [29]. Furthermore, special steel rock bolts using FBG sensors have been designed to measure relative strain [30]. Moreover, a report on application tests of novel sensor elements for long-term surveillance of tunnels, based on FBG has been published [31]. A study, presenting a system to measure the ground strain, using FBG sensors, has been reported even if an improvement in the accuracy was recognized to be necessary [32]. The study was developed with the aim to measure the characteristics of the ground behavior during earthquakes. Moreover, very recently, several patents have been issued involving the use of the FBG sensors in geophysics. A system for vertical seismic profiling of the earth borehole has been proposed [33]. Different configurations to design and realize accelerometers with one or more axis have been designed. Along each axis, one FBG sensor provides the measurement of acceleration through the evaluation of dynamic strains [34]. Another important issue for the practical implementation of strain sensing network for geophysics application is the number of sensors that can be usefully interrogated along a single optical fiber. The FBGs offer a very interesting possibility of multiplexing a large number of sensors. Different approaches can be followed: the time domain reflectometry (TDR) and wavelength division (WD) multiplexing. With TDR, it is possible to monitor simultaneously more than one FBG sensor. The optical setup is shown in Fig. 13. Pulsed light from a broadband light source is
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Fig. 13. Example of a TDR multiplexing configuration to interrogate multiple FBG sensors.
Fig. 14. Example of a TDR/WD multiplexing configuration to interrogate multiple FBG sensors.
coupled into the optical fiber where a number of FBGs are written along the fiber equally spaced but with different Bragg wavelengths. The reflected signals are analyzed by the receiving apparatus that can comprise, for example, a wavelength discriminator. Depending on the source bandwidth and the strain, range can be experienced from each FBG sensor it is possible to monitor 10–20 FBG sensors along the same fiber (Fig. 13) [35]. A combination of TDR with WDM could, in principle, allow to monitor more than 100 sensors (Fig. 14) [35]. The high number of sensors that are possible to monitor along the same optical fiber makes the approach
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very attractive for on-field application for sensing, for example, a seismic area with the same apparatus in a large number of points.
5. Conclusions A review has been reported on FBG, principle of operation, its applications and potentialities for application in a new and very important field of seismic and volcano monitoring. A number of key factors should be investigated to implement a real strain sensing network to monitor seismic and volcanic areas: strain/temperature cross sensitivity, design of a reliable FBG sensor/rock interface, design of the best interrogation approach as a function of the required sensitivity and bandwidth, finally the multiplexing approach depending on the needed number of sensors. The main advantages of Braggs-gratings based strain sensors would be the low cost, the easy implementation of large arrays covering a very wide frequency range (from static to thousands of Hz), the directionality, i.e. the capability to reconstruct the complete strain tensor. Presently, such a broad frequency range needs to be covered by many different instruments, going from acoustic sensors and broad-band seismometers to tilt-meters, volumetric strain meters and CGPS. The main disadvantage is represented by the lower sensitivity of FBG in the quasi-static regime (107 strains), when compared with the most sensitive instruments working in this frequency range, namely the volumetric strain meters (1012 strains). Anyway, volumetric strain meters require, to exploit such a high sensitivity, to be carefully installed in deep boreholes (hundreds of meters), practically preventing the possibility to realize dense arrays because of high costs and efforts required. Furthermore, volumetric strain meters only sense the isotropic part of the strain tensor, thus preventing a complete description of the strain and of its evolution in time. The effectiveness of dense arrays of strain-meters, over a broad range of frequencies, to resolve the source geometry and source migration in times has been clearly shown by the most recent research, mainly at volcanic areas [1–6]. The use of Braggs grating based sensors could open up new perspectives on geophysical research in geodynamical active areas, both filling important gaps in some frequency ranges and in unifying a large variety of different sensors in a unique, compact and low cost instrument capable of being easily implemented in dense networks. Besides the theoretical high capabilities of FBG based sensors, enlightened in this paper, it is important to stress that the effective implementation of monitoring optical fiber sensing networks still requires an extensive work aimed to investigate all the possible problems, and to definitively assess the feasibility and the limits of such instruments.
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On the possible use of optical fiber Bragg gratings as strain sensors for geodynamical monitoring Pietro Ferraroa,*, Giuseppe De Nataleb a
Istituto Nazionale di Ottica Applicata, Sez. di Napoli, c/o Instituto di Cibernetica del CNR, ‘‘Eduardo Caianiello’’, Compr. ‘‘A. Olivetti’’, Fabbr. 70, Via Campi Flegrei 34, 80078 Pozzuoli (Na), Italy b INGV-Osservatorio Vesuviano, Via Diocleziano, 328, 80124 Naples, Italy Received 9 April 2001; accepted 13 August 2001
Abstract Optical fiber sensors can be used to measure many different parameters including strain, temperature, pressure, displacement, electrical field, refractive index, rotation, position and vibrations. Among a variety of fiber sensors, fiber Bragg gratings (FBG) have numerous advantages over other optical fiber sensors. One of the major advantages of this type of sensors is attributed to wavelength-encoded information given by the Bragg grating. Since the wavelength is an absolute parameter, signal from FBG may be processed such that its information remains immune to power fluctuations along the optical path. This inherent characteristic makes the FBG sensors very attractive for application in harsh environments, ‘‘smart structures’’ and on-site measurements. This paper reviews the achievements about the FBG as a strain and temperature sensor and describes the potential applications of FBG sensors for applications in the field of geophysics and its expected development in the near future. The applications could include: rock deformation, fiber-optic geophone, optical based seismograph, vertical seismic profiling and structural monitoring of civil structures. Different techniques to detect strains and various applications will be reviewed and discussed. The problem of temperature–strain cross sensitivity, that is particularly difficult to eliminate, is addressed and approaches to overcome it are discussed. r 2002 Published by Elsevier Science Ltd. Keywords: Fiber Bragg gratings; Wavelength-encoded information; Strain sensor; Temperature sensor
*Corresponding author. Tel.: +39-081-867-5041; fax: +39-081-804-2519. E-mail address: [email protected] (P. Ferraro). 0143-8166/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 4 1 - 5
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1. Introduction In the last few decades, solid earth sciences, mainly driven by large developments in geophysics, have undergone a progressive transition from a qualitative towards a quantitative, rigorous physical approach. In the meantime, problems like catastrophe hazard assessment and forecasting, involving the most practical aspect of such disciplines, have been recognized as very complex ones, unaffordable by any simple empirical approach. In particular, a modern view of volcano and earthquake hazard assessment, must recognise the intimate link between physical understanding of the basic processes and the monitoring of any changes in the systems which reveal itself to be useful to forecast the system evolution. Stress and strain changes at volcanic and tectonic areas are, by far, recognised as the best indicators of changes in the activity of the system, and its possible evolution towards critical stages. Stress and strain changes in active zones can be both static and dynamic. Static changes involve static deformations produced by volcano and tectonic sources, which are localized close to the sources at maximum distances on the order of some source lengths. Dynamic changes are best represented by seismic waves radiated by earthquake sources, which can be observed at distances of several terrestrial circumferences. Geodesy has been the classical research field for static changes and seismology for dynamic ones. In the recent years, the need for integrating the two disciplines in a unitary view has become more and more evident, also because of the growing of importance, in terms of observations and modelling of the slow processes, involving frequencies, which are at the higher limit for static and lower limit for dynamic changes. For the volcanoes, for instance, the most recent seismological research [1–3] shows that the most interesting range of frequencies to follow magma movements towards the surface is between 0.1 and 0.01 Hz. Such a range is well below the frequencies for usually employed monitoring networks (1– 30 Hz) and at the lower limit for the most recent broad-band instruments (0.01– 30 Hz). In volcano geodesy, in the meantime, the best results for monitoring volcanic sources have been obtained by recently designed borehole strain-meters, which have been able to follow the fast pre-eruptive magma rising of some kilometers in tenths of minutes/hours [4,5]. Recent researches also point out the need for dense arrays of such stress–strain meters, in order to give the best sensitivity to local source activation and the best description of the source geometry and evolution [6]. The future of stress–strain monitoring, hence, appears to be conditioned by the development of very broad-range sensors, able to somewhat unify the classical ranges of seismology and geodesy, which are of low cost and are easy to install in the form of dense arrays. Moreover, they should be directional, i.e. not only measuring volumetric changes, to allow for complete stress–strain tensor retrieving, but should be the best way to follow time-variable source geometry for a moving source. Fiber optic sensors could be, potentially, the solution for the increasing demand in having reliable strain sensors since they allow remote sensing and high networking capability. Moreover, the advancements in the field of opto-electronics, in the recent years, have made new instruments available at even lower costs and with improved characteristics such as durability and compactness allowing the on-field
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application. Among the large number of sensors developed in the recent years for strain monitoring, FBG seems to be the most promising for reliable applications. In this paper, the most important achievements about the FBGs as strain sensors are described. The problem of the strain/temperature cross sensitivity, being one of the key factors to be solved, is analyzed and the possible solutions are discussed. An overview of the existing applications in the field of geophysics of FBGs is given.
2. Principle of operation of Bragg fiber sensors Fiber Bragg gratings (FBG) are obtained by producing a periodic variation in the index of refraction along the short sections in the core of an optical fiber (Fig. 1). The Bragg grating inside the core of an optical fiber is produced illuminating from the side by interference pattern with UV light (holographic method) [7], as shown in Fig. 2. For a given angle 2y between the two interfering beams, the pitch of the periodic modulation of the refractive index is given by luv L¼ : ð1Þ 2 sin ðy=2Þ The symbol L is named Bragg wavelength; luv is the wavelength of the interfering UV waves. If the core of the optical fiber is photosensitive to UV radiation that is able to change the refractive index to the core, then the index of refraction n; in the exposed region, is modified along the axis of the fiber, giving nðzÞ ¼ ncore þ dn 1 þ cos ð2pz=LÞ ; ð2Þ
Fig. 1. Bragg grating into an optical fiber.
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Fig. 2. Holographic setup to write a Bragg grating in an optical fiber.
where ncore is the unexposed core refractive index and dn is its amplitude modulation due to the photo-induced change. Germanium-doped SiO2 fibers are photosensitive to UV radiation so that Bragg gratings can be successful written in. The first report of photosensitivity in optical fiber was due to Hill et al. in 1978 [8], but the first practical demonstrations of grating formation was reported by Meltz et al. [7], ten years later and was based on the transverse method depicted in Fig. 2. After the work of Meltz et al., a considerable amount of research activity began on FBG along two directions: sensing applications [9] and optical communications [10]. Once an FBG has been realized, it basically works as a wavelength selective filter for propagation radiation along the optical fiber. If light from a broadband source is coupled into the optical fiber with a FBG written in it, then a narrow band of light with center wavelength, lB ¼ 2nL; is reflected at the FBG location, while the rest of the spectrum is transmitted as shown in Fig. 3. Strain sensing by means of FBGs is very simple, in principle. In fact, while straining the fiber, the Bragg wavelength lB ¼ 2nL varies as a consequence of both the grating pitch L change (due to the simple elastic elongation), and photo-elastic change in the refractive index. The relative change of L is given by Dlb ¼ ð1 pe Þe; lb
ð3Þ
where e is the longitudinal strain experienced by the optical fiber at FBG location and pe ¼ n2 =2½p12 nðp11 þ p12 Þ is the effective photo-elastic constant of the fiber core material. Typical wavelength shift @1550 nm of lB ¼ 2nL is Dlb =De ¼ 0:001 nm=me:
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Fig. 3. Depiction of the reflected and transmitted spectrum of FBG for a broadband light source.
Fig. 4. Example of an optical fiber with an FBG written in it, embedded in a composite material beam.
In Fig. 4, an example of application of FBG as a strain sensor is shown. The FBG has been embedded into a beam-made carbon fiber with reinforced epoxy resin. The beam is subjected to tensile mechanical load. Fig. 5 gives the graph of surfacemounted electrical strain-gage versus the Bragg wavelength shift, as measured in the embedded fiber optic grating. Fig. 5 demonstrates the very good linearity and sensitivity of the FBG as a strain sensor.
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Fig. 5. Example of wavelength shift of an FBG embedded in a composite material beam subjected to a mechanical load vs. electrical strain-gage.
The Bragg wavelength L exhibits a shift even when the temperature changes; this is due to the summation of two effects, the thermal expansion of core material and the thermo-optic behavior that induces variation in the refractive index, expressed by Dlb ¼ ða þ xÞDT; lb
ð4Þ
where a ¼ DL=L and x ¼ dn=dT represent the thermal-expansion and thermooptical coefficients, respectively. Typical shift for the Bragg wavelength @1550 nm is Dlb =DT ¼ 0:009 nm=o C: By determining the wavelength shift of peak reflectivity, the strain and/or temperature can be evaluated. FBGs present some interesting advantages that make them strong candidates for on-field applications for a variety of uses as strain sensors: * * * *
*
FBG can be used as localized (point-wise) and/or quasi-distributed sensors; good linearity; simple demodulation concept; insensitivity to intensity fluctuation since the information is encoded as wavelength; very high multiplexing capability;
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resistance in hostile environments (immune to e.m. fields and high temperature); embedding capability; and commercially available and low cost.
2.1. Interrogation methods for FBG Different interrogation methods can be used for reading out Bragg wavelength shifts experienced by the FBGs, from which strain is evaluated: (a) direct spectroscopy using optical setup with dispersive optical elements (grating, prism) and photo-detectors (linear array, CCD) of fiber spectrum analyzers; (b) passive optical filtering components (e.g. WDM couplers); (c) tracking tuneable filters (Fabry-Perot, acoustic-tuned optics filter, filter based on a tuneable FBG); and (d) interferometers. The choice of the more suitable interrogation method depends on several factors. In fact, the FBGs can be used to measure static and/or dynamic strain with different ranges, accuracy, sensitivity, number of sensors, complexity and cost of instrumentation. Numerous methods have been proposed and reported in the scientific literature. Interferometric detection of the wavelength shift of FBG sensors yielding very high resolution for dynamic strain has been reported by Kersey et al. [11]. The method is based on a fiber unbalanced Mach-Zehnder interferometer and an pffiffiffiffiffiffi ffi experimental resolution of E0:6n strain= Hz has been reported. Other interrogation methods include schemes that are based on using two cavity lengths in FabryPerot read-out interferometers for producing two-quadrature phase-shifted signals from a FBG [12] or quadrature sampling of Mach-Zehnder interferometer [13]. A de-multiplexing scheme for FBGs sensors, where a received grating is matched to a corresponding sensor has been proposed by Jackson et al. [14]. A demodulation based on an acoustic tunable filter has been proposed by Xu et al. [15]. Demodulation of FBG sensors based on dynamic tuning of multimode laser diode or tuneable laser have also been proposed [16,17]. Active control of the spectral response of a fused biconical wavelength division multiplexer (WDM) has also been reported for strain and temperature measurements by using FBG sensors [18]. Among the possible methods, there is one that uses an acoustic-tuned integrated optics spectrometers (IOS) [19,20]. IOS is made of an optical wave-guide created by diffusing titanium in a lithium niobate crystal in which even a surface acoustic wave (SAW) is made to propagate collinearly with the optical radiation. The SAW produces a longitudinal modulation of the index of refraction along the wave-guide similar to that described by Eq. (2). The optical radiation propagating along the wave-guide will experience Bragg filtering. The period of the SAW can be changed by producing a tuneable non-permanent Bragg grating along the wave-guide. In this way, the IOS can act as a wavelength filter for the radiation reflected for the FBG in allowing the measurement of the wavelength shift and of consequence of strain. In Fig. 6, the experimental configuration adopted with IOS is shown. A broad band optical source like a superluminescent laser diode (SLD) is coupled to a 2 2 fiber coupler. At one end of the 2 2 coupler is a fiber with multiple FBGs written in
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Fig. 6. Interrogation setup of multiple FBGs along a single optical fiber by an acoustic tuned integrated optics filter (AOTF).
Fig. 7. Principle of operation of an AOTF for measuring quasi-static strain.
it, at different central Bragg wavelengths. The reflected signal is analyzed by the IOS. The IOS can be used in three different configurations: as a simple spectrometer; as a wavelength discriminator and finally as a spectral peak tracker. The first configuration is suitable for interrogation of multiple FBGs and for static strain measurement needs. The transmission filter function of the IOS is made to move through its tuning range (see Fig. 7). When the filter function of IOS is on the actual Bragg center wavelength of the FBG, the light intensity as transmitted by the IOS with a photodiode allows measurement of the strain. An alternate configuration to be adopted for the IOS is specially suited for low amplitude but high frequency bandwidth. The IOS filter function can be positioned with linear portion on the central Bragg wavelength (Fig. 8). Dynamic strain makes
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Fig. 8. Principle of operation of an AOTF for measuring low-amplitude dynamic strain.
the reflected spectrum to oscillate around the central wavelength. The transmitted intensity through the IOS gives a periodic intensity signal from which the amplitude and frequency of the dynamic strain can be evaluated. The interrogation method based on IOS has a very important advantage for infield applications in geophysics; in fact, the IOS is very compact (100 33 10 mm3) and stable since it is not made of optical components like classical spectrometers and by consequence, it is very much less affected by the external disturbances. Measurement for dynamic pffiffiffiffiffiffiffi strain with very high frequency and with high resolution (E2n strain= Hz; with bandwidth of 4 KHz) on four FBGs sensors along the same optical fiber has been reported [21]. Finally, one-third of the configuration allows automatic tracking of the reflected peak form an FBG allowing static and static strain measurement. In this configuration, the tuneable filter can be automatically tuned to stay centered even while the sensor is caused to shift by a large static strain. Detection of low level of dynamic strain regardless of static strain and temperature change can be evaluated in this way.
3. Strain–temperature cross sensitivity One of the most important point that must be addressed, for applying FBGs as reliable strain sensors, is the strain–temperature cross sensitivity. In fact, in order to develop a strain sensor capable of measuring static strain, for example in rock deformation, it is necessary to find a practical and effective solution to separate the two contributions for the overall Bragg wavelength shifting due to strain and temperature. For an FBG sensor under operative conditions, the overall shift of central Bragg wavelength is given by the summation of the two effects, strain and temperature, given by Eqs. (3) and (4) as follows: Dlb ¼ ða þ xÞDT þ ð1 pe Þe: lb
ð5Þ
Several solutions have been proposed for separating the two contributions. One method uses a reference grating subjected to the same thermal load but free from
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mechanical load [22]. Another possibility is to use a single grating that is pre-stressed and partially glued to the surface of the component to monitor for the strain and partially free [23]. The portion not glued serves as reference for temperature compensation. The combination of long period FBG and two short FBG sensors can also be applied to discriminate strain over temperature induced shifts [24]. By using a single Bragg grating together with an Erbium-doped fiber amplifier, the temperature can be evaluated by the change of spontaneous emission power effect [25]. Furthermore a single FBG written in an erbium : ytterbium-doped fiber allows synchronous measurement of strain and temperature over ranges of 1100 me and 50– 1801C [26]. Another solution based on the use of induced birefringence caused by the embedding process has been proposed [27]. An optical fiber with FBGs written in is embedded in the composite materials made of carbon fibers and epoxy resin, it has been observed that in the reflected spectrum of an FBG, two peaks appear at two different Bragg wavelengths lb1 and lb2 ; instead of the single peak; the two reflection peaks corresponding to the ordinary and extraordinary index of refraction, respectively (see Fig. 9). The two peaks are generated by the non-isotropic induced stress onto the optical fiber and grated by the shrinkage effect of the epoxy resin after the polymerisation process. The peak separation depends on the induced birefringence and could, in principle, be programmed by the type of embedding lay-out around the optical fiber. Temperature changes induce variation in the birefringence. In this way, the separation between the two peaks varies as a function of temperature. The strain can be measured by evaluating the average shift of the two wavelengths lb1 and lb2 ; while the temperature is measured by evaluating the wavelength separation, Dlb ¼ jlb1 lb2 j: Figs. 10–12 show, respectively, the average wavelength shift for a FBG grating experiencing a birefringence and subjected to a mechanical tensile load, the average wavelength shift as function of temperature and, finally, the peak wavelength spacing as a function of temperature. By measuring the peaks separation and average peaks separation, simultaneously, it is possible to obtain the strain and temperature at the same time. The adoption of interferometric interrogation method with FBG written in bowtie birefringent fiber allows the simultaneous measurement of strain and temperature pffiffiffiffiffiffi ffi pffiffiffiffiffiffiffi with resolution of 72.51C/ Hz: Also, 726me Hz for a fiber with birefringence equals to 5.5 101 [28].
Fig. 9. (a) Reflection spectrum of an FBG before embedding; (b) reflection spectrum after the embedding process in a carbon fiber reinforced palastic material, two peaks appear due to the induced birefringence in the core of the optical fiber.
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Fig. 10. Example of average wavelenght shift vs. electrical strain-gage for an embedded FBG with birefringence subject to mechanical load.
Fig. 11. Example of average wavelength shift vs. temperature for an embedded FBG with birefringence.
4. Exisiting applications of FBGS in geophysics FBGs sensors could be very useful for strain measurement in the field of geophysics. Application could include: rock deformation, fiber-optic geophone,
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Fig. 12. Peaks wavelength spacing vs. temperature.
optical-based seismograph, vertical seismic profiling, and structural monitoring of civil structures. Up-to-now, only in very few cases, FBGs have been applied in those fields of application. In fact, FBG sensors have been designed and used to measure dynamic deformations in rock masses and strain variations in the range of 109 with a bandwidth of 0.1–2 KHz and results have been compared with respect to conventional geophone registrations [29]. Furthermore, special steel rock bolts using FBG sensors have been designed to measure relative strain [30]. Moreover, a report on application tests of novel sensor elements for long-term surveillance of tunnels, based on FBG has been published [31]. A study, presenting a system to measure the ground strain, using FBG sensors, has been reported even if an improvement in the accuracy was recognized to be necessary [32]. The study was developed with the aim to measure the characteristics of the ground behavior during earthquakes. Moreover, very recently, several patents have been issued involving the use of the FBG sensors in geophysics. A system for vertical seismic profiling of the earth borehole has been proposed [33]. Different configurations to design and realize accelerometers with one or more axis have been designed. Along each axis, one FBG sensor provides the measurement of acceleration through the evaluation of dynamic strains [34]. Another important issue for the practical implementation of strain sensing network for geophysics application is the number of sensors that can be usefully interrogated along a single optical fiber. The FBGs offer a very interesting possibility of multiplexing a large number of sensors. Different approaches can be followed: the time domain reflectometry (TDR) and wavelength division (WD) multiplexing. With TDR, it is possible to monitor simultaneously more than one FBG sensor. The optical setup is shown in Fig. 13. Pulsed light from a broadband light source is
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Fig. 13. Example of a TDR multiplexing configuration to interrogate multiple FBG sensors.
Fig. 14. Example of a TDR/WD multiplexing configuration to interrogate multiple FBG sensors.
coupled into the optical fiber where a number of FBGs are written along the fiber equally spaced but with different Bragg wavelengths. The reflected signals are analyzed by the receiving apparatus that can comprise, for example, a wavelength discriminator. Depending on the source bandwidth and the strain, range can be experienced from each FBG sensor it is possible to monitor 10–20 FBG sensors along the same fiber (Fig. 13) [35]. A combination of TDR with WDM could, in principle, allow to monitor more than 100 sensors (Fig. 14) [35]. The high number of sensors that are possible to monitor along the same optical fiber makes the approach
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very attractive for on-field application for sensing, for example, a seismic area with the same apparatus in a large number of points.
5. Conclusions A review has been reported on FBG, principle of operation, its applications and potentialities for application in a new and very important field of seismic and volcano monitoring. A number of key factors should be investigated to implement a real strain sensing network to monitor seismic and volcanic areas: strain/temperature cross sensitivity, design of a reliable FBG sensor/rock interface, design of the best interrogation approach as a function of the required sensitivity and bandwidth, finally the multiplexing approach depending on the needed number of sensors. The main advantages of Braggs-gratings based strain sensors would be the low cost, the easy implementation of large arrays covering a very wide frequency range (from static to thousands of Hz), the directionality, i.e. the capability to reconstruct the complete strain tensor. Presently, such a broad frequency range needs to be covered by many different instruments, going from acoustic sensors and broad-band seismometers to tilt-meters, volumetric strain meters and CGPS. The main disadvantage is represented by the lower sensitivity of FBG in the quasi-static regime (107 strains), when compared with the most sensitive instruments working in this frequency range, namely the volumetric strain meters (1012 strains). Anyway, volumetric strain meters require, to exploit such a high sensitivity, to be carefully installed in deep boreholes (hundreds of meters), practically preventing the possibility to realize dense arrays because of high costs and efforts required. Furthermore, volumetric strain meters only sense the isotropic part of the strain tensor, thus preventing a complete description of the strain and of its evolution in time. The effectiveness of dense arrays of strain-meters, over a broad range of frequencies, to resolve the source geometry and source migration in times has been clearly shown by the most recent research, mainly at volcanic areas [1–6]. The use of Braggs grating based sensors could open up new perspectives on geophysical research in geodynamical active areas, both filling important gaps in some frequency ranges and in unifying a large variety of different sensors in a unique, compact and low cost instrument capable of being easily implemented in dense networks. Besides the theoretical high capabilities of FBG based sensors, enlightened in this paper, it is important to stress that the effective implementation of monitoring optical fiber sensing networks still requires an extensive work aimed to investigate all the possible problems, and to definitively assess the feasibility and the limits of such instruments.
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