Optical Fibre Sensing Networks
J. L. Santos
a,b
, O. Frazão
 b,a
, J. M. Baptista
c,b
, P. A. S. Jorge
 b
, I. Dias
 b
, F. M. Araújo
 b
, L.A. Ferreira
 b
 
a
D
epartamento de Física da Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto
,
Portugal
 
 b
INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
c
Departamento de Matemática e Engenharias, Universidade da Madeira, Campus da Penteada, 9000-390 Funchal, Portugal
 Abstract
—This presentation gives a review of the main concepts behind fibre optic sensing networks. Some important application areas are mentioned, such as the monitoring of oil fields, of large Civil Engineering structures, as well as of natural environments. Then, it will be addressed the potential relevance of this technology in domains of increasingly social importance, such as Health and Elderly Care, particularly in the development of Personal Health Systems.
 Keywords optical fibre sensors, multi-point and distributed  measurement, sensing networks.
I.
 
I
 NTRODUCTION
 Optical fibre sensing is nowadays an established technology that provides sensing solutions with optical performance for almost all kinds of applications and environments. Fibre sensors can be designed so that the measurand interacts with one or several optical parameters of the guided light (intensity,  phase, polarization and wavelength). The crucial feature of this technology is the dual functionality of the optical fibre, which is a sensing structure in view of the measurand induced changes of the properties of the light that propagates in the fibre, but also a communication channel, meaning there is no need of an additional telemetry path, contrary to what happens in all other sensing technologies. Additionally, fibre optic sensors offer numerous operational benefits: they are electromagnetically passive, so it is possible to operate in high and variable electric field environments (like those typical of the electric power industry) and where there is explosion risk; they are chemically and biologically inert since the basic transduction material (silica) is resistant to most chemical and  biological agents; their packaging can be physically small and lightweight. Considering the intrinsic low optical attenuation of the fibre (around 0.2 dB/km at 1550 nm), the sensors can easily be placed kilometres away from the monitoring station, and it is also possible to perform multiplexed measurements using large arrays of remote point (or distributed) sensors, operated from a single optical source and detection unit, with no active optoelectronic components located in the measurement area, thereby retaining electromagnetic  passiveness and environmental resistance [1,2]. In this presentation we start to present the fundamentals of multi-point and distributed fibre optic sensing. Examples of their application in the context of monitoring of oil fields, of Civil Engineering structures and of large natural environments are provided. The final part of the text addresses the relevance of this technology in the context of increasingly important social problems, such as Health and Elderly Care. II.
 
S
ENSOR
M
ULTIPLEXING
 
i) Multi-Point Sensing
Multi-point sensing means that the measurement is performed in discrete points that can be located along a large area covered by an optical fibre network. Therefore, we are dealing with multiplexed point sensors. In general, multiplexing involves the concepts of network topology, sensor addressing, and sensor interrogation [3]. The first one deals with the way the sensors are arranged into a network, which may have consequences in terms of power budget and sensor crosstalk. The second one involves the study of processes and techniques that permit to address from the emission and detection block a  particular sensor, and typically all of them are related with time, wavelength, coherence, frequency or spatial addressing. Finally, sensor interrogation means how to read the status of a specific sensor and, therefore, to obtain information about the measurand when it is addressed by the system. In general, these concepts are frequently not independent, which means the choice of one implies the selection of the other two. For example, if the sensors are of the reflective type, this requires also the utilization of a reflective network topology. Figure 1 shows a possible architecture for multiplexing point sensors. The network has a reflective ladder topology and, therefore, the sensors operate in reflection. They can be based on any type of optical modulation (intensity, phase,  polarization or wavelength) and can be distributed over a large area that can include hazard environments. If required, optical amplification can be provided along the fibre network, which  permits to have the sensors deployed at very large distances [4]. Fibre optic multipoint sensing with sensors operating on varied physical principles has been applied in many contexts, with a growing visibility comparatively to other sensing technologies. Historically, sensing arrays for underwater acoustic detection were of high importance in view of military
978-1-4244-5357-3/09/$26.00©2009IEEE 290
 
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m
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m
1
i
2
i
im
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n
2
n
nm
 Agressive Environment 
12
out 
im
out 
nm
out 
Circulator 
Fibre Coupler  Fibre Coupler Fibre Coupler 
Optical Fibre
S Sensor 
 
Figure 1. A possible architecture for multiplexing fibre optic sensors.
 applications [5]. Interferometric sensors were used in these arrays and sophisticated addressing/interrogation techniques were developed which permitted to achieve unprecedented detection performances [6]. With the military pressure release as a consequence of the end of the Cold War, this technology was adapted to other applications, most notably the monitoring of the ocean continental platforms and the elaboration of seismic maps of the sea floor [7]. Nowadays, these seismic maps are essential for the permanent monitoring of oil and gas reservoirs, which is achieved analysing the propagation of seismic waves induced by controlled explosions. They are obtained with interferometric fibre optic sensing systems that may require over 30 000 sensors. This scale requires arrays with many sensing fibres and state-of-the art technology allows to have more that 250 sensors supported by a single fibre pair using a specific combination of time and wavelength multiplexing [8]. An illustrative view of this application is shown in Figure 2.
Figure 2. Illustrative view of sea ground seismic monitoring with an optical fibre sensing network for monitoring of oil and gas reservoirs
[9].
 
An important class of fibre sensors are the fibre Bragg gratings (FBG). These are simple, versatile and small intrinsic sensing elements that can be written in silica fibres. Consequently, they have all the advantages normally attributed to fibre sensors. In addition, due to the fact that the measurand information is encoded in the resonant wavelength of the structure, which is an absolute parameter, these devices are inherently self-referenced and can be easily multiplexed, which is particularly important in the context of multi-point sensing. All these characteristics triggered a research burst by the mid-nineties, addressing diversified topics like the fundamentals of UV induced refractive index modulation of the fibre core, interrogation of these wavelength encoded devices, and the development of new sensing head concepts integrating FBGs, including their multiplexing, and applications. Figure 3 shows the structure and spectral characteristics of these devices. The fibre gratings are fabricated UV imprinting an axial periodic pattern in the fibre optic core of higher and lower refractive index modulation (in the sections of UV irradiation, the fibre glass is modified and, under certain circumstances, the refractive index also changes - a process called photosensitivity). When broadband light (with a large width spectrum) propagates down the fibre and reaches the FBG position, at each refractive index interface there will be a tiny reflection (Fresnel reflection), generating very small amplitude reflected waves (the amplitude is small because the amplitude of the refractive index modulation is also small – typically around
3
110
×
). For the large majority of the incident wavelengths (such as
 
1
 in Figure 3) these reflected waves are out of phase and, therefore, when they add up the result is destructive interference and no light is reflected, i.e., all light is transmitted (it is assumed that the number of small reflected waves is large; typically, the refractive index modulation period is around 0.5
m, so for a grating length of 10 mm, this means there are ~20 000 reflected waves, which fulfils the required condition). However, there is a small wavelength window where the reflected waves interfere in  phase (around the wavelength
 
2
 in Figure 3). In these cases the interference is constructive and a strong reflection occurs. The central wavelength of this spectral window satisfies the so called Bragg condition,
2
 B
 n
λ 
 = Λ
, where
n
 is the effective refractive index of the fibre core guided mode and
Λ
 is the  period of the refractive index modulation (Figure 3). The reflected wavelengths are in a spectral window with a width of ~0.1-0.2 nm around
.
 B
λ 
 All other wavelengths are transmitted, i.e., they behave as if the refractive index modulation of the fibre core were not present.
Figure 3. Fibre Bragg grating spectral signature (left) and physical  principle (right).
The Bragg condition indicates the crucial feature of these structures: changes in the period,
Λ
, or in the refractive index,
n
, srcinate a shift in
 B
λ 
, i.e., a small variation occurs in the “colour” of the reflected light. The measurement of this variation gives indications on the action that introduced
FiberBragg GrattingOpticalFibreTransmittedSignalReflectedSignalInputSignal
Λ
FiberBragg GrattingOpticalFibreT
 
ransmittedSignalReflectedSignalInputSignal
Λ
1
λ 
2
λ 
FiberBragg GrattingOpticalFibreT
 
ransmittedSignalReflectedSignalInputSignal
Λ
FiberBragg GrattingOpticalFibreT
 
ransmittedSignalReflectedSignalInputSignal
Λ
1
λ 
2
λ 
2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 291
 
changes in (
,
n
 Λ
). Because the wavelength (“colour”) is an absolute parameter, this process is insensitive to variations that may occur in other light parameters along the optical system (such as intensity, phase and polarization). This feature, and the possibility of having a large number of FBG structures along the same fibre, each one with its own
,
 B
λ 
 brought a qualitative advance in the optical fibre sensing domain [1]. FBG interrogation is the designation commonly used to refer ways to convert the Bragg wavelength value (and variations) into an electrical signal with adequate characteristics to obtain the information about the measurand. The general principle of FBG interrogation is shown in Figure 4. The optical source can be a broadband source (LED, SLD, ASE, Supercontinuum), in which case it operates passively and normally there is no control from the processing unit. This is not the case when spectrally narrow illumination is used, most of the cases from laser sources, in which the wavelength modulation of the emitted light can be a component of the FBG interrogation technique [10].
( )
out B
V
 λ 
= Ψ
FBG
 B
δλ 
(
 Measurand 
 
Figure 4. General layout for interrogation of Bragg grating sensors.
FBGs are intrinsically sensitive to temperature and strain applied to the fibre where they are inscribed, as well as to the refractive index of the external medium when the fibre is etched to a point then the evanescent field of the guided mode in the grating region extends to that medium. Based on these  basic mechanisms, it is possible to build up sensing heads supported by FBGs susceptible to detect a large spectrum of  physical, chemical and biochemical parameters. If, in addition, it is realized the intrinsic amenability of these structures to wavelength multiplexing, it is natural the statement that these structures can be considered almost ideal sensing elements,  being therefore their development a technological  breakthrough (certainly not only in the fibre sensing domain). Many sensing applications have been supported by the FBG technology, particularly when there is a need to monitor large structures. It is what happens in the field of Civil Engineering,  probably the first application area where FBG-based fibre sensing was introduced. Indeed, Bragg gratings can directly replace electric strain gauges surpassing the limitations of these devices. In Portugal, this technology was developed in the nineties, and the first field trials occurred at the beginning of this decade. Its maturity allowed its application in the real time monitoring of large structures, the first one being in 2004 in the context of the repair and reinforcement of bridge Luis I in Porto (Figure 5). More than 120 sensing points where addressed with FBGs located in specially designed fibre optic cables with several kilometers length. This sensing infrastructure, that keeps working properly up to now, was developed by
FibreSensing
, a local fibre optic sensing company that moves in the worldwide sensing market.
Figure 5. Bridge Luis I in Porto where in Portugal was installed the first professional fibre optic FBG-based monitoring system.
Environmental monitoring is also an area where fibre optic sensing systems based on FBGs have increasingly been applied. Just to point out an example with direct involvement of the authors of this work, it is mentioned here a project directed to the study of the dynamics of
 Ria de Aveiro
, an important Portuguese coastal lagoon. One of the required  parameters to be monitored was the distribution of temperature of water along the 12 km extension of the lagoon, from the connection to the sea up to the location where fresh water is delivered to the system by the feeding river. An optical cable incorporating fibre Bragg gratings was developed and installed in 2002. The optical cable had several fibres with the total length of the cable, but with gratings written in specific regions in order to be able to determine the water temperature along the full extension of the cable. Redundant fibres with sensors were also included in the cable. The gratings in each fibre were sequentially interrogated using an optical switch, in a layout shown in Figure 6.
Figure 6. Structure of the FBG-based sensing system for measurement of water temperature in the Portuguese lagoon
 Ria de  Aveiro.
 
J.
PC
GPIB
 . . .
 . . . .
 UNIDADE DEINTERROGAÇÃOINTERUPTOR ÓPTICO
Fibra 1Fibra 2Fibra N
Fibre OpticSensor
 
Illumination andInterrogation UnitFiber NFiber 2Fiber 1
PC
Optical Switch
 
 PC
GPIB
 . . .
 . . . .
 UNIDADE DEINTERROGAÇÃOINTERUPTOR ÓPTICO
Fibra 1Fibra 2Fibra N
Fibre OpticSensor
 
Illumination andInterrogation UnitFiber NFiber 2Fiber 1
PC
Optical Switch
2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 292
 
The interrogation equipment, installed near the sea, is continuously acquiring data from the FBGs, delivering it to the remote sites of the institutions involved in the project for further processing and analysis. Figure 7 shows a geographic map of the optical fibre cable installation, a photo of its deployment, a scheme of the support that raises the optical cable in each sensing position to avoid the mud of the lagoon floor, and an example of the data stream relative to water temperature measured by three FBGs of the cable [11].
606570758085901820222426
L
= 0 m (sensor 1 -
λ
=1526 nm)
 
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= 450 m (sensor 19 -
λ
=1552 nm)
 
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= 850 m (sensor 25 -
λ
=1556 nm)
 
   T  e  m  p  e  r  a   t  u  r  e   (   º   C   )
Days
 (d)
Figure 7. Map of the location of the optical fibre sensing cable in
 Ria de Aveiro
 Lagoon (a), diagram of the support to raise the cable in the sensing regions (b), photo of the installation phase (c), and daily variation of water temperature registered along a period of one month  by some of the FBGs sensors (L = 850 m is closer to the sea).
Besides structural monitoring in Civil Engineering applications and in monitoring of natural environmental, FBGs are increasingly been used in many other domains. It is the case of aeronautics, in land and ocean transportation, in oil and gas exploration and production (the sensing layout shown in Figure 2 could be based on fibre Bragg gratings), and many others. One of these involves security and intrude detection. In this field, Figure 8 illustrates a possible application where optical fibres with FBGs run along a fence. With adequate FBG arrangement and signal processing, the strain change in a specific location of the fence can be associated with touch, vibration or disconnection, triggering the adequate countermeasures. This is a typical application where fibre optic based distributed measurement is particularly well suited, but new developments in the interrogation of fibre Bragg gratings permit to read with high resolution a large number (hundreds) of closely distributed, equal-wavelength, low-reflectivity FBGs, permitting a very effective solution  based on this technology [12].
Figure 8. Illustrative view of an intrusion detection system based on fibre Bragg gratings (adapted from
[13]).
 
ii) Distributed Sensing
The ability to carry out distributed sensing over long distances is one of the most important and distinguishing aspects unique to fibre optic sensing technology. The term “distributed” refers to the ability to simultaneously detect scale and location of a measurand anywhere along a continuous length of sensing Fibre. This differs from the concept of multi-point sensing  presented in previous section, where the measurement is done at specific locations with point sensors, such as fibre Bragg gratings. Figure 9 illustrates both concepts. The basis of distributed sensing is the scattering of light that propagates in the fibre core, particularly the back-scattering to permit the  propagation of the scattered light back to the detection unit. The scattering can be elastic, in which case the frequency of the scattered light coincides with that of the incident radiation (Rayleigh scattering) or inelastic, where there is a shift of the frequency of the scattered light (Brillouin and Raman scattering).
   A   t   l  a  n   t   i  c   O  c  e  a  n
BarraS.JacintoCosta NovaVagueiraVista AlegreVarelaTorreiraLaranjo
Vouga River
AVEIRO8 45'
o
40'40 50'
o
45'40'35'
- Transverse electric cables- Longitudinal optical cable
(a) (b) (c)
Intrusion Detection Based on StrainMeasurement with FBGs
 
Intrusion Detection Based on StrainMeasurement with FBGs
2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 293
 
 
Figure 9. Sensing with optical fibres.
The Rayleigh scattering appears from the interaction of the light with refractive index fluctuations in the fibre core that appear in spatial scales much shorter than the light wavelength. The Brillouin scattering is generated by interaction of the light with acoustic modes in the medium, which are induced by the light propagation. Raman scattering is generated by the interaction of the propagating light with molecular vibrations in the medium. Figure 10 shows the spectral characteristics of the several types of scattered light and the type of variation that is used for sensing [1].
Figure 10. Spectral characteristics of scattered light in optical fibres (adapted from Hiroshi Nasure, Mie University, Japan).
Optical Time Domain Reflectometry (OTDR) based on the monitoring of the Rayleigh backscatter (and of discrete reflection points) is widely used for distributed fibre (network) monitoring based on the observation of the loss profile along the fibres. To achieve spatial resolution, the light injected into the system is short pulsed and the detection of the backscatter light with a certain time delay relatively to the emission identifies the region where the scatter srcinated. The intensity of the scattered light relatively to adjacent regions permits the measurement of the loss in that region. This technology has  progressed substantially in the last 30 years and it is possible nowadays to have access to equipments at an affordable price that provides a loss distribution map along the fibre with centimetre spatial resolution [14]. The Rayleigh scattering based OTDR concept rarely has been used for sensing. Actually, it is an interesting approach to read multiplexed intensity-based fibre optic sensors [15], but for some reason this technique has not been explored widely. Recently, optical backscatter reflectometry (OBR) based on frequency domain reflectometry (OFDR) has opened Rayleigh scattering techniques to applications in sensing [16]. The OBR technique measures distributed Rayleigh backscatter with very high spatial resolution along a single mode optical fibre and records a
 finger print 
 scatter pattern as function of length. Although the pattern is random and statistically distributed, it is a stable and invariant property of a fibre segment. External  perturbations (strain and temperature) on a fibre result in a shift or change in
 periodicity
 of this
 finger print 
 and, using suitable algorithms, the magnitude and location of  perturbations can be recovered with (sub)mm spatial resolution, permitting temperature and strain sensitivitivies in the order of a fraction of 1ºC and few microstrains, respectively, in fibre lengths up to 70 m. This technology fills a gap in sensitivity and spatial resolution between FBG based sensing, which is well suited for distributed sensing over short lengths or at discrete points, and Brillouin scatter technology, which is ideally suited to sensing over very long lengths at a more reduced spatial resolution [17, 18]. Optical Fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements [19]. The Brillouin interaction causes the coupling between optical and acoustic waves when a resonance condition is fulfilled. The resulting back-scattered light shows a frequency shift relatively to the incident light of
 11 GHz, which is strain and temperature dependent, so determining this Brillouin shift directly provides a measure of temperature or strain. This frequency shift is an intrinsic property of the material that may observed in any silica fibre. This is very attractive since the bare fibre itself acts as sensing element without any special processing or preparation. Standard optical cables can be used, resulting in a low-cost sensing element that may be left in the structure. Since the optical effect only depends on the fibre material, it is absolutely stable in time and independent of the instrument. Different measurements performed over a long-term period are thus fully comparable. The basic layout for Brillouin sensing is shown in Figure 11. Pulsed light with enough power to stimulate the Brillouin  phenomena is injected into the fibre. The time width of the  pulse defines the spatial resolution that can be achieved with the system. It is not feasible to reduce too much this width  because that will limit the power into the system, which should imply large integration times to obtain a reasonable signal-to-noise ratio. The scattered light is mixed with the injected light, which permits to determine the beat frequency,
Single-Point SensorMulti-Point (Quasi-Distributed) Sensing Configuration
Sensing Element 
Optical Fibre
Multiple Sensing Points 
Distributed Sensing Configuration
Continuous Sensing Element 
0
ν 
Single-Point SensorMulti-Point (Quasi-Distributed) Sensing Configuration
Sensing Element 
Optical Fibre
Multiple Sensing Points 
Distributed Sensing Configuration
Continuous Sensing Element 
0
ν 
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Optical Frequency
0
ν 
Optical Fibre
0
ν 
Input LightRayleigh Scattering(Intensity Variation)Frequency Shifts
11 GHz
3
1 THz
BrillouinScattering(Frequency Shift Variation)Raman Scattering(Intensity Variation)
 
   I  n   t  e  n  s   i   t  y  o   f   S  c  a   t   t  e  r  e   d   L   i  g   h   t
Optical Frequency
0
ν 
Optical Fibre
0
ν 
Input LightRayleigh Scattering(Intensity Variation)Frequency Shifts
11 GHz
3
1 THz
BrillouinScattering(Frequency Shift Variation)Raman Scattering(Intensity Variation)
2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 294
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