A fiber-optic temperature sensor based on the interference of selective higher-order modes in circular optical fibers is described. The authors demonstrate that by coupling the LP01 mode in a standard single-mode fiber to the LP0m modes in a multimode fiber, and utilizing the interference of the higher-order modes, a fiber-optic temperature sensor which has an extremely simple structure and is suitable for high-temperature measurements can be constructed. The sensing principle, temperature measurement experiments, and results are presented.

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Fiber-optic temperature sensor based on interference of selective higher-order modes

With the oil and gas industry growing rapidly, increasing the yield and profit require advances in technology for cost-effective production in key areas of reservoir exploration and in oil-well production-management. In this paper we review our group’s research into fiber Bragg gratings (FBGs) and their applications in the oil industry, especially in the well-logging field. FBG sensors used for seismic exploration in the oil and gas industry need to be capable of measuring multiple physical parameters such as temperature, pressure, and acoustic waves in a hostile environment. This application requires that the FBG sensors display high sensitivity over the broad vibration frequency range of 5 Hz to 2.5 kHz, which contains the important geological information. We report the incorporation of mechanical transducers in the FBG sensors to enable enhance the sensors’ amplitude and frequency response. Whenever the FBG sensors are working within a well, they must withstand high temperatures and high pressures, up to 175 °C and 40 Mpa or more. We use femtosecond laser side-illumination to ensure that the FBGs themselves have the high temperature resistance up to 1100 °C. Using FBG sensors combined with suitable metal transducers, we have experimentally realized high- temperature and pressure measurements up to 400 °C and 100 Mpa. We introduce a novel technology of ultrasonic imaging of seismic physical models using FBG sensors, which is superior to conventional seismic exploration methods. Compared with piezoelectric transducers, FBG ultrasonic sensors demonstrate superior sensitivity, more compact structure, improved spatial resolution, high stability and immunity to electromagnetic interference (EMI). In the last section, we present a case study of a well-logging field to demonstrate the utility of FBG sensors in the oil and gas industry.

Fiber Bragg Grating Sensors for the Oil Industry.

In-fiber devices enable a vast array of critical photonic functions ranging from signal conditioning (amplification, dispersion control) to network management (add/drop multiplexers, optical monitoring). These devices have become mainstays of fiber-optic communication systems because they provide the advantages of low loss, polarization insensitivity, high reliability, and compatibility with the transmission line. The majority of fiber devices reported to date are obtained by doping, designing, or writing gratings in the core of a single-mode fiber (SMF). Thus, these devices use the fiber only as a platform for propagating light-the device effect itself is due to some extraneously introduced material or structure (dopants for amplification, gratings for phase matching, etc.) There exists another, relatively less explored degree of freedom afforded by fibers-the ability to copropagate more than one mode. Each mode may have a uniquely defined modal dispersion and propagation characteristic. In this paper, we will describe the variety of fiber devices enabled by few-mode fibers-fibers that typically support two to four modes with suitably tailored dispersive properties. We will show that the unique dispersive properties of various modes, in conjunction with the ability to couple between them with gratings, leads to devices that offer novel solutions for dispersion compensation, spectral shaping, and polarization control, to name a few.

Dispersion-tailored few-mode fibers: a versatile platform for in-fiber photonic devices

A sensor for accurate and long-term fluid pressure monitoring in oil bore-holes based on optical fiber Bragg gratings (FBGs) is presented The sensor converts fluid pressure into optical fiber strain by means of a mechanical transducer designed to withstand downhole conditions The sensor response is investigated as a function of pressure and temperature Effects of shock and vibration are also considered The expected stability and lifetime of the polyimide-coated gratings in a downhole environment are estimated by modeling and extended accelerated mechanical and thermal aging tests

Reliability of fiber Bragg grating based sensors for downhole applications

A comprehensive understanding of the development of residual strains in composite processing is essential to manufacture high quality composite parts. In this paper, the use of fibre Bragg grating (FBG) optical sensors was investigated to measure in situ the build-up of the process-induced strains in composite panels manufactured by resin transfer moulding. The FBG sensors, embedded in the composite laminate, successfully measured the evolution of the composite in-plane strains due to the temperature history. The sensors also captured a strain discontinuity during the cool-down related to the debonding of the composite from the mould. Finite element models were then proposed to simulate the strain development measured by the FBG sensors. Three different contact interactions between the tool and the laminate were investigated: no bonding, perfect bonding and frictional contact. The model using the frictional contact interaction described well the evolution of the measured strains.

Investigation of process-induced strains development by fibre Bragg grating sensors in resin transfer moulded composites

The phenomenon of decay in UV-induced Bragg gratings is examined. Thermal stability of gratings with large and small index changes are studied and compared. The role of stabilization in enhancing the thermal stability of gratings is explained. Approaches to analyzing and predicting grating decay are reviewed and compared.

S. Kannan

Papers

Thermal stability analysis of UV-induced fiber Bragg gratings