FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

Since neutral bouyancy is required, m/AS = maaa per unit
volume = 1 and Eq. (5.13) becomes:
tin = (2T/k~)pA (5.14)
Thus, once the sound frequency (wavelength, 1s) and
pressure level are chosen, the acceleration can be
determined. For f = 100 Hz (As = 1460 cm) and PA = 50
dB re 1 micropascal, corresponding to the 118-met r
curve that waa shown in Fig.
= 1.67 -3
X 10 g
(1.6 gal). This requires an5;1;r~%Ny sensitive accelerometer.
The two-fiber accelerometer described
below exhibits the required sensitivity.
5.1.3 Fiberoptic Magnetic Sensors
Yariv and Winsor (See Ref. 7) suggested that
an optical fiber could be used to measure the change
in length of a magnetostrictive material subjected to
a magnetic field. The resulting optical phase change
is linearly related to the magnetic field. Jarzynski,
et.al. (See Ref. 8 in Subsection 5.1.7) developed expressions
for the strain induced in a magnetostrictively-jacketed
fiber subjected to a weak axial magnetic
field. These expressions were obtained as a function
of jacket thickness for a variety of magnetostrictive
materials as shown in Fig. 5.11. The magnetooptic
J
10 100 1,000 10,000
FREQUENCY (HZ)
Fig. 5.12 The magnetooptic coupling coefficient versus
frequency for an optical fiberwound
nickel toroid with walls 0.038 cm thick.
After J. Cole et al., Opt. Lett. ~, 216 (1981).
\ /-WINDING
.
10.0
t
4.5% CO.95.5% Ni
-TOROIDAL
WINDING
8.0 -
70
-1
*j
aXO
6.0 -
4.0 -
2.0 -
2V-PERMENDUR
HOUSING
Fig. 5.13 Optical fiber wound on a magnetostrictive
nickel toroid (wall thickneas 0.038 cm) for
use in meaauring the magnetooptic coupling
coefficient.
After J. Cole et al., Opt. Lett. ~, 216 (1981).
I , , # 1 # 1 , 1
0 5 IO 15 20 25 30 35 40
METAL JACKET THICKNESS (pm)
Fig. 5.11 Magnetic sensitivity of magnetostrictive
metal-jacketed optical fiber as a function
of jacket thickness for various magnetostrictive
metals.
Adapted from J. Jarzynski et al., Appl. Opt. ~, 3746
(1980).
coupling coefficient as shown in Fig. 5.12 was measured
for nickel by Cole, et.al. (See Ref. 9 in Subsection
5.1.7) using a nickel cylinder around which the optical
fiber in one arm of the interferometer was wound as
shown in Fig. 5.13). The relevant theory has been compared
with magnetooptical experimental data taken at
low frequency (< 1 kHz) and msgnetomechanical data
taken at frequencies greater than several tens of kilohertz.
Thia study demonstrated that the piezomagnetic
atrain coefficient remains constant from low frequency
up to the frequency at which eddy currents become important.
Therefore, the low frequency measurement reaults
may be uaed to design magnetic sensors that will
operate at high frequencies, i.e., to the eddy current
limit.
Magnetoatrictive materials have been used extensively
as acoustic transducers for the production or
detection of sound. In the preaent application these
materiala are used to detect magnetic fields by measuring
the reaulting atrain produced. The most atraight
forward and sensitive technique for such measurements
involvea uaing an optical fiber in one arm of a Mach-
Zehnder interferometer. The fiber is either jacketed
with the magnetostrictive material or wound around a
magnetostrictive mandrel. The resulting change in optical
path is due to changes in both the refractive
index and the length of the optical fiber core. This
leads to a phase ahift A+ given by Eq. (5.5) in Subsection
5.1.2.1. An expression for S11 can be obtained
from the effective piezomagnetic strain constant dT defined
by the expression:
dT = 4n(3S11/aH)T (5.15)
where T is the streaa.
yields:
Integrating this expression
Sll = (1/41r) a ‘b dTdH (5.16)
where the limits a and b are Ho-Hi/2 and Ho+H1/2, respectively,
H. is the dc bias field choaen to ~ximize
dT, and HI is a small excursion about that point. For
5-5