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Developing a Fiber Optic Magnetic Field Sensor: Fiber Bragg Gratings and the Magnetorefractive Effect

  • Author(s): Strutner, Scott Strutner
  • Advisor(s): Carman, Gregory
  • et al.

Optical fiber Bragg gratings (FBGs) have been shown to provide sensing of strain and temperature at 100's of points along a fiber's path. This work extended that to 1000's of points, and added discrete sensing of magnetic field along the fiber. The first was achieved via an extension of the optical frequency domain reflectometery (OFDR) de-multiplexing method. This was done by treating FBG segments as a series of concatenated FBGs instead of one discrete FBG. Such treatment allows for the resolution of even a micro-crack's strain fields in a pressurized composite overwrapped pressure vessel (COPV), which are on the 0.5 mm scale. The second advancement of FBGs, magnetic sensing, was added via an assembly to transduce magnetic attraction into fiber strain. This assembly allowed multiplexed magnetically sensitive FBG sensors for the first time. Stringing pieces of magnetic material onto a fiber and gluing that fiber onto a structure is an easy method and largely uses off-the-shelf components. This will prove simple to expand and utilize to solve actual engineering problems. To further improve, by removing the strain transduction and thus reduce vibrational sensitivity, a new magneto-optical material is needed. An investigation into the magnetorefractive effect (MRE), and characterization of the shift of index of refraction in the material La1-xSrxMnO3 (LSMO) with magnetic field and temperature was performed. The index of refraction of the LSMO was found to shift with magnetic field. This was leveraged to create a magnetometer interferometer. That device did not function though, as the absorption of the magneto-optic LSMO used prevented light's transmission through that active material. This highlights the complex, i.e. non-real, index of refraction of LSMO an aspect which would warrant further investigation.

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