This thesis deals with the subject of fibre optic magnetic field sensors utilizing iron garnet materials. Such materials exhibit a large Faraday rotation which make them advantageous for application in compact mag­netic field sensors. After an introduction, in which fibre optic sensors and optical methods to measure electric current are reviewed, the original research work is summarized. A system for the measurement of the magneto-optic properties of trans­parent materials is described. Measurement results, showing the influence of temperature, magnetic field direction and sample treatment on the magneto-optical proper­ties of YIG-crystals, are presented. The proper­ties of thin magneto-optical waveguiding films have also been studied using different light coupling methods. Measurement results obtained for holo­graphic grating, prism and edge (end-fire) light coupling to different substituted YIG films are presented. It is shown that the launching method may affect the properties to be measured. The design and performance of several versions of extrinsic guided wave fibre optic magnetic field sensors are then reported. The sensors employ substi­tuted YIG (Yttrium Iron Garnet, Y3Fe5O12) thin film waveguides as sensing elements. Polari­zation maintaining fibres were used as feed and return to provide two signal channels. The signals were combined in a balanced measure­ment system, providing insensitivity to both fluctuations in optical power and loss. Sensors have been made both with separate fibres to guide the light to and from the sensing element and with a single fibre for both functions. The two fibre version, although less ”elegant”, is found to have a better performance. This version also makes it possible to determine both the magnitude and sign of the magnetic field. Measurement results indicate a usable measurement range of at least several mT with a noise equivalent magnetic field level of less than 8 nT/root(Hz). The design and performance of multimode fibre optic magnetic field sensors utilizing the Faraday effect in an epitaxially grown thick (YbTbBi)IG film is also described. This type of sensor is found to be linear over a range from 27 mT to less than 270 nT. Sensor prototypes suitable for current monitoring in high voltage transmission lines have also been developed.

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Fibre Optic Magnetic Field Sensors Utilizing Iron Garnet Materials

Dissertation Title : Development of Magnetic Field Sensors Using Bismuth – Substituted Garnets Thin Films with In-Plane Magnetization Iulian Nistor, Ph.D., 2006 Dissertation Directed By: Professor of Engineering Isaak D. Mayergoyz, Electrical and Computer Engineering Department, and Dr. Charles Krafft, Laboratory for Physical Sciences In the dissertation, the use of magnetic single crystal Bismuth-substituted iron garnet thin-films with giant magneto-optical effect as optical sensors for measuring high frequency (up to 1GHz), low intensity magnetic fields is discussed. The advantages of these sensors are high intrinsic sensitivity and the possibility of tailoring the field range of the sensor. The design of these sensors is based on single crystal epitaxial garnet thin films grown by liquid phase epitaxy method from a flux melt. The garnet thin films are fully characterized using various magnetic and optical techniques. The melt compositions that allow for the growth of films with in-plane magnetization, giant Faraday rotation per unit length, large negative uniaxial anisotropies and small cubic anisotropy are identified. The design of garnet based magnetic field sensors is performed and a noise equivalent magnetic field of Hz T μ 5 . 1 and a signal-tonoise ratio of 68dB are demonstrated. DEVELOPMENT OF MAGNETIC FIELD SENSORS USING BISMUTH – SUBSTITUTED GARNETS THIN FILMS WITH IN-PLANE MAGNETIZATION

Development of magnetic field sensors using Bismuth-substituted garnets thin films with in-plane magnetization

Fibre optics is beginning to find use for sensing purposes. Fibre optic sensors have many interesting features, e.g., their immunity to interference from electromagnetic fields. The paper briefly d ...

/pdf/a-fibre-optic-displacement-sensor-4dsqgdkt9b.pdf

A fibre optic displacement sensor

Four examples of specific fiber optic sensor system designs, each of which demonstrates a different optical modulation format, are described. The birefrigent temperature transducer illustrates direct digital signal modulation. The temperature/pressure dependent semiconductor filter illustrates high-pass optical wavelength signal encoding. The coupled polarized-mode transducer shows how a solid-state sensor can produce narrow-bandpass optical wavelength signal encoding. The luminescent temperature sensor illustrates a way to construct a solid state sensor in order to produce pulse width modulation of an optical signal.

Fiber optic sensors for military, industrial and commercial applications

A fiber-optic sensor concept screening study was performed. Twenty sensor subsystems were identified and evaluated. Two concepts selected for further study were the Fabry-Perot fiber-optic temperature sensor and the pulse-width-modulated phosphorescent temperature sensor. Various designs suitable for a Fabry-Perot temperature sensor to be used as a remote fiber-optic transducer were investigated. As a result, a particular design was selected and constructed. Tests on this device show that spectral peaks are produced from visible white light, and the change in wavelength of the spectral peaks produced by a change in temperature is consistent with theory and is 36 nm/C for the first order peak. A literature search to determine a suitable phosphor for implementing the pulse-width-modulated fiber optic temperature sensor was conducted. This search indicated that such a device could be made to function for temperatures up to approximately 200 C. Materials like ZnCdS and ZnSe activated with copper will be particularly applicable to temperature sensing in the cryogenic to room temperature region. While this sensing concept is probably not applicable to jet engines, the simplicity and potential reliability make the concept highly desirable for other applications.

Analysis and preliminary design of optical sensors for propulsion control. [temperature sensors]

/pdf/analysis-and-preliminary-design-of-optical-sensors-for-2suw205dca.pdf

W. H. Quick

Papers

Fiber optic sensors for military, industrial and commercial applications

Analysis and preliminary design of optical sensors for propulsion control. [temperature sensors]

Analysis and preliminary design of optical sensors for propulsion control