Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 158

directional and may not produce a uniformly coated fiber. Sputtering depo-
sition rates are slow (less than 1000 Å min −1 ), compared to vapor deposition
(up to 1 μm min −1 ). Magnetostrictive coatings approaching 40 μm are neces-
sary in the magnetometer design. The problem of directionality is eliminated
with electrodeposition or electroless deposition. The former requires a thin
electrical conducting layer deposition on the fiber prior to the process. This
approach has been used in the successful electrodeposition of nickel and iron
alloys on fibers; however, electrodeposition of binary and multicomponent
alloys becomes difficult because of the different deposition potentials of the
individual components. Thus, electroplated alloy compositions do not readily
correlate with solution compositions. These problems can be overcome by a
change of solution type or adjustment of solution composition.
Electroless deposition has an advantage over electrodeposition in that a
thin conducting layer is not necessary beforehand. This process requires a
suitable metal-bearing salt, a reducing agent, and an appropriate catalyst. It
has been used successfully in the deposition of cobalt for metallic film disks.
Problems with alloy compositions are similar if not more severe than with
electrodeposition. For optimum magnetic response, the coated fiber must
be heat treated. Heat treatment relieves deposition strains, homogenizes
the coating, and adjusts the ferromagnetic domain structure. The goal is to
obtain the maximum possible value of the quantity dλ/d H for the coated
fiber. Here dλ is the change in length of the coated fiber (per unit length)
due to magnetostriction, and d H is the change in the field applied along the
length of the fiber. This rate of change of length with field depends in a com-
plex way on both structure-insensitive factors such as magnetic material, the
anisotropy constant, and the level of magnetization, and structure-sensitive
factors, including the ferromagnetic domain configuration and orienta-
tion, grain size and orientation, residual stress, inclusions, and the sample
demagnetization factor. We note here that the maximum dλ/d H value for a
given sample need not necessarily coincide with the maximum d B /d H value,
where B is the magnetic flux density.
In coated fibers, the optimum value of dλ/d H is unlikely to be obtained
because of the limitations imposed on the structure-intensive parameters by
the material systems that can be readily coated from solution and on the
structure-sensitive properties by the limited processing and heat treatment
that can be imposed. For example, the thin layer of copper used to provide
electrical conductivity in the electroplating of iron-nickel alloys will dif-
fuse under heat treatment at 1000°C, resulting in degraded magnetostrictive
properties [9]. A longitudinal annealing field could be applied with a sole-
noid, but the only way to apply a circumferential field would be to pass a DC
current along the wire.
Stresses can also develop in the fiber either from any bending that might be
necessary in the configuration of the magnetic-coated-fiber part of the sensor
or from differential thermal contraction between the coating and the optical
fiber upon cooling from an annealing treatment. The bending stresses should