In this paper, a packaged FBG based optical fiber sensor written by femtosecond laser pulses in highly birefringent micro-structured optical fiber (MS-FBG sensor) is presented and validated for simultaneous pressure and temperature monitoring. The MS-FBG sensor is capable of separating the temperature information from pressure information without the need for an additional transduction mechanism and this with a negligible pressure-temperature cross-sensitivity. However, in order to use the sensor for downhole applications, a ruggedized sensor housing is required that not only offers mechanical protection to the fiber, but also provides pressure transfer from the well fluid to the sensing element without inducing an additional pressure-temperature cross-sensitivity. In this article, the design of the sensor housing is reported as well as the lab-scale validation up to a temperature and pressure of 150 °C and 700 bar, respectively.
The applicability of fiber Bragg gratings written in highly birefringent Butterfly micro-structured optical fibers (MSFBG) for simultaneous High Pressure/High Temperature monitoring without a significant pressure/temperature cross sensitivity have recently been shown. This makes these MS-FBG sensors extremely interesting for downhole monitoring in the Oil and Gas industry. However, an important effect to be taken into account for these applications is the presence of hydrogen, as hydrogen is known to diffuse into the fiber structure and therefore might affect the wavelength responses of the sensor element. In this paper, the effect of hydrogen gas on the MS-FBG sensor readings by monitoring the wavelength changes of the MS-FBG sensor in a hydrogen rich environment have been investigated.
In this experiments, two MS-FBG sensors were placed in a hydrogen test chamber: one with its fiber end sealed for pressure sensing and the other with its fiber end kept open for referencing purposes. It could be demonstrated that both sensors show a similar wavelength shift after some time and that due to the hydrogen diffusion, the pressure in the airholes of the sealed MS-FBG sensor equalizes the hydrogen pressure in the chamber. Furthermore, it could be demonstrated that the refractive index seen by the waveguide of the fiber is also affected. Based on all these observations, the influence of the hydrogen on the temperature and pressure measurement performance of the MS-FBG sensor is estimated, and a mitigation scheme that partially compensates for this influence is discussed.
In this paper, we demonstrate that femtosecond laser pulse written fiber Bragg gratings (FBGs) fabricated in specialty highly birefringent micro-structured optical fiber (MSF) can be used for high pressure and high temperature monitoring in downhole applications. The design of the micro-structure allows encoding the pressure information into the spectral separation between the two Bragg peaks reflected by the obtained MS-FBG. We obtained a differential pressure sensitivity of 3.30 pm/bar over a pressure range from atmospheric up to 1400 bar and at temperatures between 40 °C and 290 °C. Owing to the negligible differential pressure-temperature cross-sensitivity of 6.06E-3 bar/°C, the proposed MSFBG sensor is an ideal candidate for pressure monitoring in the presence of high temperature transients.
In this paper, we describe a theoretical study on the interference created by a phase mask when a femtosecond laser is used. The limitations of the phase mask-to-fiber distance are discussed and the optimal inscription range is established. Femtosecond lasers have the unique feature of short coherence length and thus the diffraction orders do not interfere after a certain distance travelled from the phase mask even if the phase mask has a poor zero order suppression. The equation describing this behaviour is presented and simulations are included for validation. The intensity profile of the overlapping ±1 diffraction orders after the phase mask is also studied for 1st order (1070 nm pitch) and for 2nd order (2140 nm pitch) phase masks.
In this work, we present the results of the tests for NO2 detection using tilted fiber Bragg gratings (TFBGs) inscribed with a pulsed femtosecond laser and coated with the polymer P(S-b-MMA) and the NO2 sensitive molecule LuPc2. Two coating methods were carried out: using a porous and a non-porous polymer matrix. The results show that a previous treatment to create pores in the polymer matrix achieves a faster response but also a lower sensitivity compared to the non-porous matrix.
In this paper, several configurations of optical fiber refractive index sensors based on a hetero-core structure are presented. These sensors are made by splicing short lengths of several specialty fibers with different core and cladding diameters between two standard single-mode fibers. In addition, two fiber bragg gratings are inscribed in single-mode fiber at both sides of the specialty fibers in order to create an in-fiber Fabry-Perot interferometer and the behavior of the transmission spectra is analyzed. The sensitivities and spectral behavior of the sensors are obtained by immersion in solutions of different refractive index.
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.
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 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.
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