Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 159

not be unique to the coated fiber provided the sensor configuration of each
unit (coated versus attached fiber) is the same. Similarly, the bending stresses
are only related to the radius of curvature of the bend and will not differ for
the two designs. They are, however, unidirectional and nonuniform, and will
contribute to a birefringence through the piezooptical coefficients.
Differential cooling stresses following heat treatment result in a compres-
sive stress along the fiber as well as a compressive loop stress. Such stresses
result in a change in the optical indices of the fiber and thus a phase change
in the light transmitted compared to a similar length of unstressed fiber. The
alternative approach of fixing the fiber to an already processed and treated
magnetic material deals with the attachment problem. The adhesive acts as
a conduit for the magnetostrictive strain from the magnetic material to the
fiber. As such, its elastic strength and viscoelastic behavior must be suitable
to obtain fiber strain as close as possible to that in the magnetic material. This
is similar to the problem dealt with in the fabrication of strain gauges. The
opportunity to select the optimum magnetic material and to use the optimum
heat treatment makes the last approach discussed preferable in most designs.
The optimum achieved is an appropriate balance between the sensitivity to
field variation (i.e., dλ/d H app ), the bias field applied, and hysteresis effects.
The amorphous alloy METGLAS 2605SC (an Allied Corporation product)
has been shown to exhibit large magnetoelastic coupling. Sensors fabricated
by bonding single-mode unjacketed optical fibers to strips of METGLAS
and to cylinders formed by coiling strips of the material have demonstrated
sensitivities greater than those obtained with the plated fiber sensors. It has
also been reported that annealing the METGLAS in the presence of a mag-
netic field in the direction of the strip width increases the sensitivity and
greatly reduces Barkhausen noise produced by domain wall movement [10].
Depending on the fabrication procedure, unbalanced radial stresses induced
in the fibers may degrade performance due to polarization shifts caused by
strain-induced birefringence. This effect is minimized by sandwiching the
fiber between layers of METGLAS ribbon.
Finally, the material used to bond the fiber to the METGLAS must couple
the magnetostrictive strain to the fiber without introducing spurious signals
due to thermal expansion or ambient pressure changes. It must not relax
excessively with time or temperature. Care must be exercised in the sen-
sor design to avoid resonances and to preserve the frequency response over
about a 5000 Hz bandwidth.
4.8 FiberSensorLaserSelection
Use of a semiconductor laser is essential to the ultimate miniaturization of
the fiber sensor system. The advantage of high efficiency, narrow spectral