Fiber Optic Magnetic Field Sensor David Matthew Frailey and Hella M Willis Department of Electrical Engineering and Computer Science Dr. Chiu T. Law Objectives Fabricate an apparatus such that the ends of two optical fibers couple within an optical cavity. Calibrate the optical power transfer while the ends of the fibers separate as Terfenol-D expands due to its magneto-strictive properties in a magnetic field. Apparatus Figure 1: Schematic of the apparatus.  The well-cleaned capillary tube acts as an optical cavity.  The signal is a broadband source centered on 1550 nm. Figure 2: Schematic of Monel Terfenol-D apparatus.  Monel is insensitive to magnetic fields.  In magnetic fields Terfenol-D expands due to its magnetostrictive properties.

Optical Power Transfer Peak Power vs. Distance Optical Power vs. Magnetic Flux Density Figure 4: When mounted to a Monel / Terfenol-D composite and placed in a magnetic field changing in magnitude and direction, Monel is insensitive to the field while Terfenol-D elongates.  These results demonstrate the principle for magnetic field or current detection. Figure 3: The peak power transfer is nearly inversely proportional to the fiber tip separation.

Optical Power Transfer Conclusions & Further Study Optical Power Transfer TIME-VARYING MAGNETIC FIELD OPTICAL POWER vs TIME SINGLE MODE FIBER The use of a capillary glass to guide the wave from a fiber, a range of 600µm was measured with distinguishable optical power. Further work will establish the optimal fiber type: single- or multi-mode. MONEL  /  TERFENOL - D Demonstrated, in principle, fiber optic magnetic field or electric current detection. Demonstrated fiber optic hysteresis detection. Further study will calibrate the optical power transfer while the ends of the fibers separate as Terfenol-D expands in a magnetic field. ALTERNATING MAGNETIC FIELDS Demonstrated detection of oscillating magnetic fields. Further research will be needed to determine how to keep fibers aligned during expansion and compression of Terfenol-D. Figure 5: When mounted to  Terfenol-D and placed in a 6 Hz time varying magnetic field, the sensor detects 12 elongations per second.  The 150 G field-strength plot indicates the sensor has been saturated.  Behavior in a field greater than 160 G, yet to be elucidated as non-linear effects were noted.