In this contribution, the method of Code-division Multiplexing (CDM) is investigated for its dynamic measurement capabilities. In earlier publications, this technique has already been shown to be capable of measuring thousands of Draw Tower Grating® (DTGs®) in a single fiber, where many FBGs with identical wavelengths are used. The basics of CDM are explained. In addition, the ability to do dynamic measurements is investigated theoretically and experimentally and the results are presented. Good correspondence between theory and experiment could be found. Possible system improvements are proposed to find a suitable compromise between detection accuracy and system speed for massive optical sensor networks.
In this paper, we report on an offshore field validation of a FBG based optical fiber sensor for simultaneous monitoring of hydrostatic pressure and temperature. The sensor consists of a femtosecond laser induced grating written in a Butterfly microstructured fiber. The sensor has an extremely low cross-sensitivity between temperature and pressure which makes it ideal for monitoring large transients in pressure or temperature, like is the case in wireline intervention. Pressure and temperature readings from the FBG based optical fiber sensor are compared with the readings from a battery powered electrical quartz gauge during an offshore wireline intervention job in an oil well. Good agreement was found between both measurements
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.
A fibre optic sensor design is proposed for simultaneously measuring the 3D stress (or strain) components and temperature inside thermo hardened composite materials. The sensor is based on two fibre Bragg gratings written in polarisation maintaining fibre. Based on calculations of the condition number, it will be shown that reasonable accuracies are to be expected. First tests on the bare sensors and on the sensors embedded in composite material, which confirm the expected behaviour, will be presented.
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.
Optical sensors based on Fiber Bragg Gratings (FBGs) are used in several applications and industries. Several inscription techniques and type of fibers can be used. However, depending on the writing process, type of fiber used and the packaging of the sensor a Polarization Dependent Frequency Shift (PDFS) can often be observed with polarized tunable laser based optical interrogators. Here we study the PDFS of the FBG peak for the different FBG types. A PDFS of <1pm up to >20pm was observed across the FBGs. To mitigate and reduce this effect we propose a polarization mitigation technique which relies on a synchronous polarization switch to reduce the effect typically by a factor greater than 4. In other scenarios the sensor itself is designed to be birefringent (Bi-FBG) to allow pressure and/or simultaneous temperature and strain measurements. Using the same polarization switch we demonstrate how we can interrogate the Bi-FBGs with high accuracy to enable high performance of such sensors to be achievable.
The ability to decouple strain from temperature is being explored using Bragg gratings written in highly birefringent fiber in combination with a high accuracy interrogator. Both the birefringent preform as well as the interrogator have been optimized in order to reach maximum measurement accuracy. The results from calibration measurements will be presented together with the estimated stability.
We review the state-of-the-art in photonic crystal fiber (PCF) and microstructured polymer optical fiber (mPOF) based
mechanical sensing. We first introduce how the unique properties of PCF can benefit Bragg grating based temperature
insensitive pressure and transverse load sensing. Then we describe how the latest developments in mPOF Bragg grating
technology can enhance optical fiber pressure sensing. Finally we explain how the integration of specialty fiber sensor
technology with bio-compatible polymer based micro-technology provides great opportunities for fiber sensors in the
field of healthcare.
KEYWORDS: Fiber Bragg gratings, Sensors, Modulation, Signal processing, Filtering (signal processing), Digital signal processing, Distortion, Optical filters, Linear filtering, Analog electronics
In this paper, we outline the functionality of a new Fiber Bragg Grating (FBG) interrogator that has been developed
based on requirements from the Flight Test Group of a major European aircraft manufacturer and give some
performance figures regarding the dynamic measurement capabilities of the system. The interrogator is designed for
sensing the wavelength of short apodized gratings at high sampling rates and with strict requirements on the signal
quality in the frequency domain. In particular, the specifications on aliasing and phase distortion will be discussed, and
an explanation on how the system can and does meet these specs over different sampling frequencies is given.
A new method is proposed for mounting a Fiber Bragg Grating to a substrate material and in this way forming an optical
strain gauge. The strain gauges were optimized for usage in aeronautical applications and hence with stringent
requirements on their performance. The installation procedure is easy, fast and reliable and happens by means of a
specially designed mounting tool called a sensor pad. It is used in combination with a UV-curable adhesive. The
performance of these sensors was investigated in an extended research program and the results of this program will be
presented. It will be shown that the proposed optical strain gauges exhibit superior performance compared to their
electrical counterparts and hence that this is a very promising method for strain sensing.
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