1. FIBER OPTIC MAGNETIC FIELD SENSOR
Epoxy block
FIBER OPTIC MAGNETIC FIELD SENSOR
BASED ON MAGNETOSTRICTIVE ACTUATION
AN IMPROVED DESIGN
Hella M Willis
Chiu T. Law, Ph.D | Department of Electrical and Computer Engineering
Objectives
Mechanical Actuation
Magnetostrictive
Actuation
Conclusions
A fiber optic magnetic field sensor was developed with a narrow, air-filled channel replacing a mechanical splice capillary tube. The improved design remedies:
Degradation|
Capillary tube optical couplant degrades with time.
Air filled channel has limited range compared to capillary tube.
Misalignment|
Fiber tips readily misalign in a capillary tube; large tube diameter.
Fiber misalignment and nonuniform cleaving of fiber tips possible.
Develop a narrow, air-filled channel instead of a mechanical splice capillary tube to remedy:
Degradation| Capillary tube optical couplant degrades with time
Misalignment| Fiber tips readily misalign in a capillary tube; large tube diameter
Custom Composite| Allow fabrication of custom Terfenol-D / Monel composite surrounding air channel
Mold epoxy block with channel diameter slightly larger than fiber cladding.
Mount apparatus on TerfonolD / Monel composite.
Measure coupled optical optical power to magnetic flux density.
Develop transfer function linking magnetic flux density to optical power. 1
Mold air-filled channel of diameter approximate to that fiber cladding.
Insert fiber with its cladding removed in channel. Break the fiber by scoring the stripped fiber then applying tension.
Measure optical power coupling between the cleaved fiber tips as a function of their separation.
Mount fibers on Monel / Terfenol D composite.
Measure the coupled optical power as the magnetic flux density varies.
Approach
Magnetic Flux Density / Coupled Optical Power Relation
When the magnetic flux density increases, Terfenol D expands, causing the fibers to separate, decreasing the coupled optical power.
The coupled optical power relates directly to the magnetic flux density. As the magnitude of Magnetic Flux Density increases by 1 gauss, the coupled optical power decreases by nW.
IMPROVED DESIGN
INITIAL DESIGN
Figure 1 |Optical power coupling between fiber tips as function of separation inside a capillary tube channel. Fiber tips actuated mechanically.
Figure 5 |Optical power coupling between fiber tips actuated via magnetostriction. As the magnitude of the flux density increases, Terfenol D expands, and the coupled optical power decreases.
Results
Further Research
Broaden sensor effective range |
Measurements range 150 gauss and 6 nW.
Optical couplant filled capillary tube allows for broader operational region.
Address fiber misalignment and non-normal fiber tip faces|
Uniformly severing fiber within channel critical.
Examine sensor behavior in an AC magnetic field.
Figure 3 |Geometry of the improved design: an air filled channel molded from epoxy.
Results
Bibliography
Figure 6| The coupled optical power relates directly to the magnetic flux density magnitude. As the magnitude of magnetic flux density increases by 1 gauss, the coupled optical power decreases by nW; the fiber tip separation increases as the magnitude of the magnetic flux density increases.
G. Engdahl, Handbook of Giant Magnetostriction. London, UK: Academic Press, 2000.
Contact Information
Figure 2 |Dimensions of fiber optic layers.
Hella M Willis |
Chiu T. Law, PhD |
Figure 4 |Coupled optical power as a function of fiber tip separation in air-filled channel. The optical power decreases within 8% of the inverse square of the fiber tip separation.

2. Redesigned Channel Optical Couplant Filled Capillary Tube
Epoxy Molded Air-Filled channel

3. Optical Power Coupling Fiber Separation Magnetic Flux Density