FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

in a straight section of the fiber at an angle less
than the critical angle may have their angle of incidence
on the core-cladding interface increased by the
bends and thus be partially transmitted into the cladding.
A more detailed wave theory analysis of periodic
bend-induced coupling indicates that the level of coupling
between modes, including non-attenuating core modes
as well as attenuating core and cladding modes, is
strongest when the difference between the effective
propagation constants of a pair of modes, (Bi - Bj), is
equal to 2 /1..
range 0.5N < F > 1.5 N. In the latter caae, 9“ corresponded
closely to the critical incidence angle, thus
the light waa injected mainly into the highest order
propagating modes and therefore was more eaaily ejected
from the core into the cladding.
loofiT_..-
,_L
1::::-,-
1 I 1 I I I I I I I I I I
T – - * — - 0 - - 1
A number of studies of this phenomenon have
been conducted. For example, the system shown in Fig.
5.27 was used at the Hughes Research Laboratories. As
EO
~ —
L
I
FIBER
DETECTOR MODE DEFORMER
STRIPPER
Fig. 5.27 A microbend intensity-type
sor system developed by the
Laboratories.
fiberoptic sen-
Hughes Research
indicated in the figure, light was injected into a
multimode step-index fiber which was passed through a
deformer element. In this case the intensity of the
core light reaching the end of the fiber was monitored.
By using a helium-neon laser with a well-collimated
beam it was possible to vary the incidence angle of
light into the fiber and thus inject light into a fairly
well defined set of propagating core modes. On sections
of the cladding, just before and just after the
deformer, mode strippers were employed. These elements,
which in their simplest form might consist of black
paint applied to a few centimeters of the outer surface
of the cladding, absorb almost all of the light that
my be propagating in the cladding of the fiber. The
use of cladding mode strippers first insured that only
core light reached the section of fiber in the deformer
and then that any core light ejected into the cladding
by the deformer was absorbed so that it did not reach
the photodetector.
Uaing the system outlined in Fig. 5.27, the
Hughes’ inveatigatora measured the transmitted optical
intensity as a function of the force applied to the
transducer (microbend deformer). This was done for
several different angles of incidence of the input light
and the reaulting data la preaented in Fig. 5.28. Aa
shown in that figure, when the input incidence angle
was set at 0°, i.e., for light injected along the axis
of the fiber, the output intensity decreased by about
twenty percent as the force applied to the deformer increaaed
from O to 2 newtons. On the other hand, for
light incident at 9°, the transmitted intensity decreased
to approximately 40 percent of the input when the
applied force waa again increased to 2 N. In addition,
the slope of the transmission intensity, I, veraus applied
force, F, curve was nearly conatant over the
1
r
r
——— O.ODEG .
20
----- 7.ODEG 0
-“---”- 8.ODEG ❑ 1
} ------- 9.0 DEG ●
4
oo~
0.5 1.0 1.5
FORCE N
Fig. 5.28 The percent transmission of core input
light obtained at the output as a function
of applied force in a microbend intensitytype
fiberoptic sensor.
After J. Fields, et al., J. Acouat. Soc. Am. ~, 816
(1980).
Using the results of this and aimilar experiments,
the Hughes investigatora, in cooperation with
the Physical Acouatica Branch of the U.S. Naval Reaearch
Labora~ory, deaigned and teated a hydrophore employing
such a microbend deformer as the transducer element.
Their first prototype unit ia aketched in Fig. 5.29
DIAPH
/-- FIBERLEADS
/
Fig. 5.29 A microbend intenaity-type fiberoptic sensor
hydrophore developed by the Hughes Research
Laboratories and the U.S. Naval Research
Laboratory.
After Fields and Cole, Appl. Opt. ~, 3265 (1980),
(See Ref. 5 in Subsection 5.2.6). One deformer plate
was rigidly mounted to the cylindrical ahell of the
hydrophore while the other was attached to a thin diapragm.
In addition to the through-put fiber, a second
inactive fiber was included to insure that the deformer
plates remain parallel during operation.
At this point it would be uaeful to review
some of the basic acoustical levels and unita of meaaure.
Referring to Fig. 5.30, one frequently encountered
acoustic reference pressure level encountered in air
acouatics is 0.0002 dynea/cm2. This is the accepted
5-1o