FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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value of A~$AP below that of fused silica, as was shown
in Fig. 5.3. Theoretical results are shown in Fig. 5.6
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100 2(Y2 300 400 500 600
HYTREL THICKNESS (pm)
Fig. 5.4 The components of the per-unit phase change
of a lightwave in a Hytrelc-jacketed fiber
per pascal as a function of jacket thickness.
After Lagakos and Bucaro, Appl. Opt. ~, 2717 (1981).
In order to calculate the pressure sensitivity
of one meter of Hytrelc-jacketed fiber, consider the
experimental results that were given in Fig. 5.3. The
measured value of A /@jAP, corresponding to a l-mm plastic
jacket is 10-~0 pascal-l. Solving fOr A$/~ and
taking L = 100 cm, n = 1.5, k = 7.4 x 104/cm (for A =
0.85 microns) yields aA$/$of 10-6 radians for 10Q3
pascals. Thus, 103 micropascals produces a one microradian
phase shift. The pressure sensitivity per meter
for this case is 60 dB re 1 micropascal. Increasing
the length of optical fiber to 10 and 100 meters increases
the pressure sensitivity to 40 dB re 1 micropascal
and 20 dB re 1 micropascal respectively. These
results are only 5 dB greater than the quantum limited
theoretical results shown in Fig. 5.5. Included for
3–0.5 —
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CALCIUM ALUMINATE GuSS
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0 20 40 60 SO 100 120 140
COATING THICKNESS (pm)
Fig. 5.6 Acoustic response (phase shift of a lightwave)
in an optical fiber as a function of
coating thickness.
After Lagakos et al., Opt. Lett. ~, 460 (1982).
(See Ref. 6 in Subsection 5.1.7) for jackets of nickel,
calcium aluminate glass, and aluminum. A nickel jacket
as thin as 10 microns should reduce the acoustic sensitivity
of silica fiber to zero; however, the thickness
is critical. On the other hand, a 90-micron jacket of
aluminum is required for zero acoustic sensitivity but
the thickness is much less critical. The variation of
‘L,z~ ‘n,z~ cn,r, and A$/@P versus the thickness of
the the aluminum jacket are shown in Fig. 5.7 (See Ref.
5 in Subsection 5.1.7).
4,
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3 5 10 2 5 100 2 5 1,000 2 5 Io,orm
FREQUENCY (H,]
Fig. 5.5
Variation of acoustic energy spectrum level
as a function of frequency for two coated
fibers along with other noise levels in a
sea subsurface environment.
comparison in Fig. 5.5, are various ambient noises,
such as shipping, weather and seismic noiaes. Also
shown is the equivalent noise pressure of the U.S.
Navy’s H56 hydrophore.
o 20 40 60 80 100 120 140
ALUMINUM THICKNESS (pm)
Fig. 5.7 Pressure components per unit sensitivity
per unit pressure as a function of aluminum
jacket thickness on an optical fiber.
After Lagakoa and Bucaro, Appl. Opt. — 20, 2719 (1981).
5.1.2.2
Pressure Gradient Sensors
The effect of jacketing optical fiber with
aluminum (measured experimentally) is to reduce the
mined by
The direction of a sound source can be deterusing
either an array of omnidirectional sen-
5-3