Composites have added new dimensions to the design and fabrication of various structures. These structures are usually used to withstand hefty loads, so their integrity must be guaranteed. Due to optical sensors' advantages and FBGs' unique position among optical fibre sensors, we propose in this paper the integration of FBG sensors for the monitorisation of the curing temperature and strain of multidirectional carbon-reinforced polymer cured by microwave radiation heating method and for strain monitoring of a fibre reinforced polymer pultrusion beam.
We propose a novel cryogenic liquid level sensor based on a long-period grating inscribed in one of the external cores of a multicore optical fibre. The proposed device has a small grating period to improve the sensitivity to external refractive index changes while reducing the temperature sensitivity. We compare the results obtained with an LPG inscribed in a single-core fibre, demonstrating that the optical spectrum of our device is not divided into two different resonances and shifts smoothly towards shorter wavelengths as the relative liquid level increases. The sensitivity of the proposed level sensor when measuring liquid nitrogen is -0.772 nm/cm.
Shape sensing has become an area of great interest for many medical applications, such as epidural administration, colonoscopy, biopsies, and cardiac procedures, where real-time data of a dynamic object is required and visual contact is absent. Fiber Optic Shape Sensors (FOSS) consist of optical multi-fiber cables or Multicore Fibers (MCF) with embedded strain sensors, which can reconstruct the sensor shape from its multidimensional bending. Regrettably, the accuracy of three-dimensional shape sensing is remarkably restricted because of twisting, which makes impossible to correctly detect the bending direction. This paper reports an experimental study aimed at investigating the accuracy of optical shape sensors based on spun multicore fibers in sensing twisting, employing one of the most used multicore fiber geometry for sensing applications, the seven-core fiber. Firstly, a theoretical approach to model the mechanical behavior of multicore fiber was developed. Secondly, a pre-twisted fiber optic shape sensor was fabricated in the Institute for Telecommunications and Multimedia Applications (iTEAM), by inscribing four Fiber Bragg Gratings (FBG) in a Spun Multicore Fiber (diameter of 125.1 μm) with a pre-twisting of 64.9 rotation/meter, manufactured and provided by FIBERCORE. To conclude, a series of experiments were performed to corroborate the theoretical approach and evaluate the sensor performance. The proposed Spun-MCF-based Shape Sensor was able to sense twisting with a sensitivity of 0.23 pm/° and accuracy of 4.81° within a wide dynamic range of ± 270°, maintaining a perfectly elastic behavior at high level of twisting deformation
Distributed sensing based on Optical Backscattering Reflectometry (OBR) is a promising solution for medical applications, such as thermal ablation. OBR working principle is based on the change in pattern of small reflections from non-homogeneities inside the fiber as a response to applied strain and temperature. The advantage of OBR is its high resolution and sensitivity. However, since the scattering level is the same for standard single-mode fibers, OBR is unable to discriminate the pattern of different fibers when they are connected in parallel to a single channel. As a result, OBR detection is limited to a single sensing fiber and does not allow multiplexing. This work proposed a method to overcome the aforementioned limitation by the use of high-scattering fibers. Such fibers are achieved by doping their core with magnesium oxide particles, which size varies from 20 to 100 nm. The backscattering level of nanoparticles doped fiber (NPDF) is 50 dB higher than of the standard fibers. Four NPDFs have been spliced to the standard single-mode pigtails with different length to achieve four sensing regions located at different distances from pigtails. The NPDF sensors have been connected to the OBR equipment by the use of optical couplers and the backscattered pattern has shown four high-scattered sensing regions separated by low-scattering regions from standard fibers. The proposed setup has been applied in the thermal ablation experiments and the 2-dimensional map of temperature change with time has been obtained.
Fiber Bragg gratings (FBGs) in a poled silicate fiber are used to detect external voltage applied to the fiber’s internal electrodes. This work shows a basic proof-of-concept of a single-ended, fiber-based voltage sensor that can be used to measure periodic high-voltage signals. The setup can be extended to a multiplexed e-field interrogation system and used in the electric power industry for remote sensing of transmission lines and power plants.
Fiber optic shape sensing is innovative technology, which enables distributed structural health monitoring, providing real-time feedback on shape and position, based on smart sensors arrays, which consists of optical fiber bundles or multicore fibers with embedded strain sensors. This paper describes a numerical analysis carried out to identify the effects of uncertainty in strain measurement and core position on the accuracy of fiber optic shape sensors, taking into consideration one of the most utilized geometry for fiber optic sensing applications, the seven-core Multicore Fiber, and distinct values of core spacings (distance between outer cores and sensor axis). The Monte Carlo Method was employed to simulate the real measurement process and one million of iterations were performed for each simulation with the aim of defining the laws of uncertainty propagation. The results of this study demonstrate the influence of core position uncertainty, strain measurement resolution and core spacing on optical shape sensors accuracy and can support the design of new sensors, predicting the achievable performance.
A temperature-insensitive 2-D inclinometer by incorporating two fiber Bragg gratings (FBGs) with a homogeneous pendulum transducer is proposed and experimentally demonstrated. The two FBGs are located in two orthogonal planes of the pendulum and only half of each FBG is glued on the surface of the columnar rod. Due to the strain difference between glued- and non-glued part of each FBG, the reflection spectrum of each FBG splits into two peaks. The direction and magnitude of inclination can be readily determined by measuring the wavelength separations of the split peak in two FBGs, whilst the temperature-induced cross-sensitivity is eliminated. A sensitivity of 0.047 nm/° with an accuracy of 0.23° is achieved within the tilt angle range of -20°–+20°.
An interrogation technique based on phase modulation to intensity modulation conversion due to FBG filtering is presented. A 10 GHz tone is used to phase modulate an optical carrier located at the Bragg wavelength of a given FBG. The modulation index is set to a small value to keep Bessel identities close to 0 in order to avoid higher harmonics. Changes of the Bragg wavelength cause a power change in the photodetected 10 GHz tone. A remarkable linear sensitivity of 1 dB/pm for a shift up to 10 pm of the Bragg wavelength is demonstrated through experimental measurements. The range with linear sensitivity can be enlarged sweeping the source wavelength. This proves that the presented interrogation technique is able to interrogate FBGs with a resolution far below 1 pm and no need of extra postprocessing.
An interrogation technique of cascaded FBGs sensors based on a Microwave Photonics filtering technique under coherent regime is presented. The sensing information of a 5m fiber coil with 500 weak FBGs with a similar central wavelength is retrieved. The principle of operation is based on the calculation of the impulsive response by recording the electrical frequency response of the system. Hot spot detection and temperature measurement are demonstrated through experimental measurements with a temperature sensitivity of 0.6dB/ºC. The resolution in the measurement is related to the bandwidth of the electrical measurement and we have obtained a resolution of 20 cm for an electrical bandwidth of 1 GHz and a SNR bigger than 16dB.
Multicore optical fiber can be used to implement multidimensional optical fiber sensors including curvature sensors. In this paper, a selective core inscription technique is used in order to inscribe a single long period grating in each of the outer cores of the optical fiber. A set of three different long period gratings is inscribed for implementing the curvature sensor. The ability of the sensor for measuring the magnitude and the direction of curvature is demonstrated. The optical fiber sensor is characterized experimentally for curvature magnitudes from 0 m-1 to 1.77 m-1 and curvature directions from 0° to 360°.The maximum curvature sensitivity of the developed sensor is -4.85 nm/m-1.
The Karhunen-Loeve transform is applied to the coarsely sampled impulse response generated by an FBG cascade in order to calculate the temperature change suffered by the FBGs. Thanks to a dispersive media, the wavelength change performed by the temperature change produces a delay shift in the sample generated by an FBG, delay shift which is recorded in the eigenvalues calculated by the KLT routine, letting to measure the temperature variation. Although the FBGs samples are represented only by four points, a continuous temperature measurement can be performed thanks to the KLT algorithm. This means a three order reduction in the number of points giving this method a low computational complexity. Simulations are performed to validate the interrogation technique and estimate performance and an experimental example is provided to demonstrate real operation.
The use of multicore optical fibres (MCF) in optical sensing applications has gained increasing interest over the past
years due to the benefits directly brought from their inherent spatial diversity. This property allows measuring either
multiple physical magnitudes at the same time or the same magnitude with slight differences in order to compensate the
cross-sensitivities. We have inscribed Regenerated Fibre Bragg Gratings (RFBGs) in MCFs with the aim of
implementing temperature sensors with an enhanced accuracy and for a very wide temperature range (up to 1000°C). The
sensors have been made in 4-core and 7-core commercially available homogeneous MCFs. The fabrication process has
been designed to create different temperature sensitivities among the identical cores of the MCF. We have obtained
significant wavelength-shift differences up to 1.2 nm at 765°C, what has been used to at least double the temperature
accuracy.
An optical fiber sensor composed of six standard FBGs in cascade is interrogated by use of a technique based on wavelength to delay mapping. A microwave-modulated optical signal enters the FBG cascade and, after being sliced and reflected, experiences dispersion in standard single-mode fiber. The Bragg wavelengths of the FBGs are mapped into the delays that determine the peaks in the system’s electrical impulse response. The Bragg wavelength shifts are calculated from the difference of the delays undergone by FBGs samples. A resolution of 9.2 pm in Bragg wavelength shift is demonstrated.
A system to interrogate photonic sensors based on long weak fiber Bragg gratings (FBGs) is presented and experimentally demonstrated, dedicated to measure the precise location of several spot events. The principle of operation is based on a technique used to analyze microwave photonics (MWP) filters. The long weak FBGs are used as quasi-distributed sensors. Several events can be detected along the FBG device with a spatial accuracy under 1 mm using a modulator and a photo-detector (PD) with a modest bandwidth of less than 500 MHz. The simple proposed scheme is intrinsically robust against environmental changes and easy to reconfigure.
A technique to enhance the response of Brillouin distributed sensors is proposed and experimentally validated. The method consists in creating a multi-frequency pump pulse interacting with a multi-frequency continuous-wave probe. The power of each pulse at a distinct frequency is lower than the threshold for nonlinear effects, while the sensor response remains given by the total power of all pulses. Distinct frequency pulses are delayed to avoid temporal overlapping and cross-interaction; this requires to smartly reconstruct the traces before photo-detection. The method is validated in a 50 km-long sensor using 3 frequencies, demonstrating a signal-to-noise ratio enhancement of 4.8 dB.
Multicore optical fibre can be used to implement multidimensional optical fibre sensors. In this paper, an array of four multiplexed FBGs is inscribed in a multicore optical fibre in order to obtain a multipoint curvature sensor. An improved FBG inscription technique is used in order to mitigate the several issues that arise during the inscription of FBGs in this kind of optical fibres. The optical fibre sensor is described and theoretically analysed in order to obtain the magnitude and direction of the curvature and also the strain produced by external forces.
A novel approach for fibre distributed sensing is proposed, conceived to match as closely as possible to an ideally responding distributed sensor. It is demonstrated that it can be actually realized using fibre Bragg gratings of extremely low index contrast and continuously printed over the entire fibre length. The concept is experimentally validated over a restricted distance range that proves the huge potentialities of the technique in terms of response and precision.
The use of microwave radiation for curing carbon-fiber reinforced polymer materials (CFRP) can solve the nonhomogeneous heating problems when using conventional techniques based on the use of catalysts and can reduce the processing times. Optical fiber sensors have well-known advantages for Fiber Reinforced Composites (FRC) monitoring. In this paper fiber Bragg gratings (FBGs) are used for online monitoring of the residual stress and distortions produced during the microwave curing process. The CFRP samples are composed by layers of unidirectional carbon fibers and epoxy resin. The results show a very different behavior between the direction of carbon fibers and the perpendicular direction. Results are compared with the conventional processing technique.
We present the fabrication and characterization of regenerated fiber Bragg gratings (RFBG) in high-birefringence optical fiber for high-temperature sensing. The RFBG presents two Bragg resonances corresponding to the slow and fast axis of the optical fiber. The distance between these two Bragg resonances can be used to reduce the cost of the interrogation unit. Two optical filters are used to convert the wavelength shift due to temperature into optical power changes. The use of modal interferometers for the optical filters simplifies the interrogation setup since they can be easily tuned by strain or temperature.
A technique for liquid-level sensors based on a long fiber Bragg grating (FBG) is presented and experimentally demonstrated. The measurement system is based on the measurement of the central frequency distribution of the FBG based on time-frequency domain analysis. A short optical pulse is injected into a 10-cm long FBG mounted in a container. The back-reflected pulse is scanned by means of an oscilloscope. When part of the grating is immersed in a liquid having temperature higher than the surrounding ambient, the structure of the uniform grating is distorted and its time-frequency response changes. A spatial resolution of 2 mm, given by the input pulse duration, and a 10-cm long measurement range are achieved. Liquid-temperature sensing has also been implemented by scanning the spectral response of the FBG by means of a CW laser and an OSA.
We present the results of a real fire test using optical fiber sensors embedded in concrete samples. The temperature curve used in this experiment is described in the Spanish/European standard UNE-EN 1363-1 temperature profile for normalized concrete resistance to real fire tests, reaching temperatures of more than 1000°C inside the fire chamber and up to 600°C inside the concrete samples. Three types of optical sensors have been embedded in concrete: 1. standard fiber Bragg gratings inscribed in photosensitive germanium-boron co-doped fiber, 2. regenerated fiber Bragg grating (RFGB) inscribed in germanium doped fiber, and 3. RFBG inscribed in germanium-boron co-doped fiber.
Optical fiber sensors based on Fiber Bragg Gratings (FBG) have been embedded in concrete samples for temperature
measurement. Three different types of gratings have been used in this experiment: FBGs inscribed in photosensitive
germanium-boron codoped fiber and Regenerated Fiber Bragg Gratings (RFBG) inscribed in germanium doped and in
germanium-boron codoped fiber. The concrete samples were placed inside a fire chamber where the temperature was
increased above 1000ºC as described in the Spanish/European standard UNE-EN 1363-1 temperature profile for concrete
resistance to real fire. The temperature was monitored in real time. We have compared the performance of the optical
sensors and electrical thermocouples. The RFBGs have shown a very good performance while the FBGs are able to
monitor high-temperatures until their disappearance.
In this paper we have studied the performance of a high-temperature packaged sensor based on Regenerated
Fiber Bragg Gratings (RFBGs). The packaging consists of a two-bore ceramic tube and a 1.5 mm-thick metal
casing. The optical response of packaged and unpackaged RFBG sensors for temperature measurements up to
1100°C have been evaluated and compared to each other. Especial attention has been placed on possible
residual hysteresis in the thermo-optical response after several temperature cycling tests. The response and
recovery times of the packaged sensor has been also measured. The former has been found to be comparable
to that of commercial thermometers based on conventional thermocouples.
Slow light systems are particularly attractive for analog signal processing, since their inherent limitation to a delay-bandwidth
product of 1 is less critical for analog systems such as those used in microwave photonics. We present here
the implementation of two basic functions - phase shifting and true time delaying - fully optically controlled using
stimulated Brillouin scattering in optical fibers. The combination of these two functions makes possible the
implementation of true time delays without limitation on the microwave carrier frequency using the separate carrier
tuning technique. This is illustrated by the implementation of the delaying system for the realization of a microwave
tunable notch filter.
We have successfully created Chemical Composition Gratings (CCGs) into two different types of optical fiber: standard
telecommunications Germanium doped fibers and photosensitive Germanium/Boron co-doped fibers. We have
performed temperature cycles for analyzing the sensing properties and degradation or hysteresis with respect to the CCG
sensors. The results show that CCG sensors based on Germanium/Boron co-doped photosensitive fiber have an almost
linear response and negligible hysteresis effects, with a response of almost 100°C/s.
We present in this paper the results of monitoring the construction process of a steel incrementally launched bridge
located at the Kadagua Valley in Bilbao (Spain) with FBG sensors. The installation of FBG strain and temperature
sensors was done in order to obtain deformation and temperature variations during the launching operation. The
deflection recovery process was also monitored. The setup carried out in the sensors installation process consists of five
optical channels (one for each cross section monitored) and a multiplexed structure of nine strain sensor in each optical
channel. Temperature sensors were also installed in order to measure temperature variation of the steel structure but also
for thermal compensation for the FBG strain sensors. The installation of the optical sensors is explained in detail
including cleaning, bonding and connection of the almost fifty sensors installed in this structure. We also are going to
explain the behaviour of the steel structure by presenting several figures showing the strain values for each sensor taken
in real time during the launching of the bridge.
We have successfully created Chemical Composition Gratings (CCGs) into two different types of fiber: standard
telecommunications Germanium doped fibers and photosensitive Germanium/Boron co-doped fibers. We will present
results on the regeneration process, the sensing properties and also the high temperature decay and maximum operative
temperature for the CCGs created in both types of fiber.
We report the multiplexing of photonic crystal fiber interferometric sensors that exhibit sinusoidal interference pattern.
The frequency division multiplexing technique combined with a simple fast Fourier transform demodulation method was
used to multiplex several devices in a single channel. To avoid crosstalk between the sensors, we calculate the optimal
relationship between their periods. The technique is validated experimentally by multiplexing two-mode PCF
interferometers but it can be adapted to multiplex any other optical fiber sensors that exhibit sinusoidal patterns. The
technique and results reported here may allow the development of PCFI-based sensor networks.
We are going to present and demonstrated the feasibility of a novel sensor interrogation scheme that combines the
advantages of the distributed and point sensor systems using a robust interrogation technique. The experimental results show
the high linearity and stability of the measurements. Also, our proposed scheme uses a tunable source with optical elements
that have very few losses. Thus, the number of sensors that can be allocated in one fiber is very high.
We show the basic mechanism of optical pH sensors using hydrogels and FBGs. We show the experimental results
obtained and the importance of a precise formulation of the hydrogel an appropriated design of our transducer.
A new amplified CWDM (Coarse Wavelength Division Multiplexing) self-referencing sensor network using phase-shifted
fiber Bragg gratings (PS-FBGs) is experimentally demonstrated in this work. The network uses the PS-FBGs to
address intensity sensors in a transmissive configuration, obtaining simultaneously in reflection a wavelength encoded
reference signal. In order to enable the remote operation of the sensors, we have introduced optical amplification at the
interrogation header of the network, using highly doped erbium fiber.
The application of transversal strain on an optical fiber leads to an asymmetry of the fiber section that locally induces
fiber birefringence. In this paper, we show that it is possible to use FBG as a probe to obtain the amount of this induced
birefringence. We describe two techniques able to get the birefringence value. They are both based on differential group
delay (DGD) measurement. The first technique makes use of the comparison between the measurement and the
simulation of the wavelength dependency of grating DGD. Considering that all grating parameters are known, we adjust
the birefringence value to obtain the best fit between experiment and simulation. For the second technique, we first
demonstrate that the wavelength dependencies of chromatic dispersion (CD) and DGD parameters differ in their
amplitude by a quantity proportional to the birefringence. We exploit this relationship to get the birefringence value by
separately measuring the DGD and CD spectral evolutions. The birefringence value is adjusted to obtain the best fit
between the two measurements. These two techniques are discussed and experimental results are finally reported.
V. Micó, M. L. Lozano, J. J. Esteve-Taboada, J. A. Carrión, T. Molina-Jiménez, S. Simón, E. Pérez, J. M. Lloris, M. Cruz-Yusta, M. J. López-Tendero, C. Silvestre, F. López, D. Barrera-Villar, R. García-Olcina, S. Sales
There are a lot of factors that call into question the integrity, safety and reliability of concrete macrostructures such as
bridges, buildings, tunnels and dikes. Examples of such factors are humidity variations, in-excess load supported during
years, vibrations and pH variations, which can damage the concrete structure after extended periods. In order to test the
real state of such structures, we present the design and development of fiber optic based sensors that permit the
measurement of loads and tensions applied to the structure, just as both the humidity and pH of the concrete at the
measurement point. The load of the structure can be measured by means of fiber Bragg grating techniques, which
involves wavelength multiplexing and optical spectrum analysis, and the humidity and pH measurements are achieved
by incorporating different types of hydrogels to the nearness of the fiber Bragg grating. The change in humidity and pH
produces volume changes in these hydrogels that modify the spectral response provided by the fiber Bragg grating. Thus,
it is possible to place multiple sensors along the macrostructure to visualize the on-line status during its life time.
Birefringence in fiber Bragg gratings (FBGs) can result from two distinct effects that combine with the intrinsic fiber
birefringence: the birefringence induced by the UV photo-writing and the birefringence due to a transversal load. In both
cases, it leads to polarization dependent loss (PDL) and differential group delay (DGD) inside gratings. Although these
parameters are not desired for optical telecommunications applications, we demonstrate here that they can be
advantageously used to obtain temperature-insensitive transverse strain measurements with uniform FBGs written into
standard single mode fiber, which is not possible through conventional amplitude spectral measurements. Simulated
results obtained by means of the coupled mode theory and the Jones formalism as well as experimental results measured
by means of a tunable laser source and a polarimeter are presented.
Birefringence in fiber Bragg gratings can result from two distinct effects that combine with the intrinsic fiber birefringence: the birefringence induced by the UV photo-writing and the birefringence due to a transversal load. In both cases, it leads to polarization dependent loss and differential group delay inside gratings. This paper aims to provide a characterization, both theoretically and experimentally, of the polarization dependent loss and the differential group delay generated by uniform Bragg gratings written into single mode optical fibers. We demonstrate that the measured polarization properties can be accurately reconstructed by means of the coupled mode theory and the Jones formalism. We also demonstrate that the PDL and DGD evolutions contain information about birefringence and can thus be used for transverse force sensing purposes. Experimental results obtained on fiber Bragg gratings transversally loaded by an external force confirm the simulated evolutions.
A new interrogation system for a large number of strain sensors is presented. In the residential building sensor market some facts have to be dealt with: low cost, networks with a large number of sensors, reliability, passive devices, amongst others. We have accomplished all of these with the new interrogation system we are going to present.
We propose a new structure for sensing variations in the refraction index independent of fluctuations in temperature. The scope of applications is very wide and can be used to detect electromagnetic, biochemical and even radiation changes. Also, it can be applied to the telecommunications field.
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