Optical fiber Bragg grating sensors (FBGs) were used to measure strain and the temperature field that develop during laser drilling of carbonate rock samples. The shear deformation and high temperature gradient measured are clearly correlated with traces of fractures observed. Beyond the volume directly evaporated by laser exposure, a greater volume around the drilling area was fractured. From the perforation process point of view, it results in an increase of efficiency.
A magnetic field sensor comprised of a high birefringence photonic crystal fiber coated by a Terfenol-D/Epoxy
composite layer is proposed. Magnetic fields induce strains in the magnetostrictive composite that are transferred to the
fiber interfering with light propagation. The sensitivity of the developed sensor with magnetic fields is measured to be 6
pm mT-1.
A simple interrogation system for semi-distributed fiber Bragg grating (FBG) sensor array with a tunable pulsed laser is
demonstrated. The pulsed light source is used to discriminate, in space or in time, the FBG position. The sensor array can
then be constituted with low reflectivity FBG (<5%) and with the same wavelength (the FBGs have to be spatially
separated). This improvement increases the capacity of the tunable pulsed laser system interrogation, which can reach up
to 1000 sensors in one single fiber. The signals are measured with only one photodetection system, which makes
detection less dependent of intensity variations and minimizes external influences on the circuit, such as variations in the
environment temperature. A test approach has been assembled for the interrogation of five sensors in the same fiber,
varying its reflectivity from 0.8% to 1.6% and with the same nominal wavelength.
An analysis of the Bragg wavelength deviation generated by TDM/WDM multiplexing of large number of fiber Bragg grating (FBG) sensors has been proposed. Demodulation technique based on Fixed Spectrum Filters is compared with peak position of full wavelength spectrum. Results indicate that the fixed filter approach is less sensitive to wavelength distortion due to shadow of other sensors operating at the same nominal wavelength, allowing much larger number of FBG sensors. A simulated result show less than 1 pm deviation with up to 100 sensors at same wavelength, with 1% peak reflectivity for each sensor.
This work presents a system for the interrogation of optical fiber Bragg grating sensors based on time domain reflectometry and fixed filters. In this system, filtering is accomplished by fiber Bragg gratings, and a pulsed broadband light source is employed to illuminate the gratings. The spectral informations from the sensors are related to the ratio of two pulse intensities, each from a different filter. The signals are measured with a unique photodetection system, which makes detection independent of intensity variations and minimizes external influences on the circuit, such as variations in the environment temperature. Also, a small number of optical couplers and circulators are needed, so the interrogation system costs per sensor is considerably reduced. A test approach has been assembled for the interrogation of six sensors. Comparisons between experimental results and simulations show a good agreement. Extrapolations indicate that it would be possible to interrogate sensors with spectral variations up to 2 nm, providing uncertainties smaller than 5 pm, which is adequate for multiplexed sensing of temperature in a range of 200°C.
Fiber Bragg grating (FBG) sensors offer many advantages for monitoring strain and temperature, other physical parameters can be measured through the use of mechanical transducers. However, FBGs are sensitive to strain and temperature and, in many cases, it is difficult to discrminate both measurements. To overcome this problem several techniques have been proposed, most of them employing more than one grating. This work demonstrates the possibility to discriminate temperature and pressure measurements using only one FBG sensor and a low cost demodulation technique based on two fixed filters. A pressure transducer has been used to transfer a lateral force to the fiber, proportional to the applied pressure, generating birefringence in the Bragg grating. The system allowed to measure pressure in the range of 0 to 400 psi with uncertainty of 4 psi and, simultaneously, measure temperature in a range of 22°C with uncertainty of 0.1°C.
Corrosion control in pipelines and wells is a critical issue in the oil industry. In this paper, we present an optical fiber sensing technique devoted to monitor one of the parameters involved in corrosion: the environment acidity. In the proposed technique, a transducer mechanically couples a fiber Bragg grating to a pH sensitive hydrogel. The possibility of determining pH values with resolution of 10-2 in a range from 3 to 6 is evaluated and discussed.
Laboratory setups are used to simulate real conditions in which drilling fluid and shales interact during an oil well drilling process. The present work describes the development of fiber optic systems capable of measuring the ionic diffusion in water-based fluids under high pressure. Two alternatives have been tested and calibrations are presented for both. The most successful one was tested in a real experiment in which the concentration of CaCl2 has been continuously measured during five days. Starting from pure water, the final ionic concentration measured by this method was compared with the result from chemical analysis of the fluid with very good agreement.
Two experimental setups, using the fourth harmonic of a Nd:YAG laser (266 nm), for producing Bragg gratings by the external method are described and their results presented and discussed. The two different setups allow the fabrication of gratings of controlled spectral bandwidth (0.3 to 5 nm) with peak position between 1.3 and 1.6 micrometers . The stability of the system and the possibility of writing type IIA gratings are also discussed.
Fiber Bragg Gratings (FBG) have been applied to a wide range of systems both in telecommunication and sensors systems. For sensors the main advantages in using FBG are the ease of multiplexing and the reliability of measurements due to spectral encoding. Although many different ways of analyzing the spectral response of a FBG have been proposed and demonstrated, most of them involve sophisticated electronics and do not take into account practical problems. We describe a simple, robust and low cost technique in which a reference grating is used as a spectral filter for the sensing grating. In our prototype a 1550 nm pig-tailed LED is used as broad band source. As the system is proposed for strain measurements, special attention is paid to field calibration and long term measurements. Two calibration methods are demonstrated. One uses the digitalized reflection spectra of sensor and filter grating and numerically predicts the sensor output. The other method is based on the application of a controlled opposite strain on the filter grating. Both methods were compared with conventional resistance strain gage and the measuring accuracy is estimated. We also point out applications where FBG strain sensor can have significant advantages over conventional electric counterparts.
An optical fiber thermometer for electrical energy applications has been developed. The instrument uses the radiation of leaky modes in a short bend of standard singlemode fiber covered with polymeric resin, to sense temperature variations in the external medium. Suitable precision (<EQ +/- 2 degree(s)C) and good stability in the usable range of 5 degree(s)C - 170 degree(s)C are achieved.
We have observed several differing photorefractive effects on transmission in polarization maintaining optical fibers. Both the strain-induced and shape-induced birefringence fibers can exhibit strong photorefractive behavior, depending on fiber composition, structure and pre- treatment, while pre-treatment alone can influence a single fiber sample to exhibit differing behavior. Subsets of these transmission effects appear to originate from qualitatively differing photorefractive mechanisms which we have not identified. Contrasting behavior is most clearly seen in the characteristics of photo-generated components such as reflection gratings and polarization couplers. High-efficiency polarization couplers, including 100% and overcoupled versions, can be made in some fibers, but can apparently always be erased by heating to 240 degree(s)C. Reflection gratings written in a fresh fiber sample appear to be thermally stable. However, further gratings, written in the same fiber sample after optical erasure of the original grating, are thermally erasable.
We designed a new optical fiber current sensor based on the
Faraday Effect. The electronic detection scheme uses two polarizers in
order to avoid ambiguities due to polarimetric detection. In this way
we may detect current beyond the folding point of the square cosine
law inherent to that sensing technique. The sensor presents excellent
linear response in the measured range, front 0 to 7.5 kA (peak to
peak) , 60 Hz AC current.
A digital electronic system has been designed and implemented to be used in polarimetric optical fiber sensors. The system expands the dynamic range of these sensors by counting peaks of its periodic output. High accuracy can be achieved with relatively simple optics and analog electronics. In ac applications it uses a self-reference that compensates for optical power fluctuations.
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