Biomedical Engineering Reference
In-Depth Information
4.18 FiberSensorDesignExample:Fiber-Optic
SonarDomePressureTransducer
4.18.1 Identification and Significance of the Problem
Influence-actuated sensors and mines require means of detecting and classi-
fying influences produced by vessels. Among these are pressure fluctuations
produced by the bow wave, propeller cavitation, and engine noise, and mag-
netic field disturbances produced by the presence of the steel vessel in the
earth's field. These influences exist in a noise field created by wind and tide
driven surface wave, sea creatures, distant sources propagating in density
gradient guides, and other out of range shipping.
Reliable sensing often depends upon correlation of all influences with
individual signals weighted according to their reliability and expected
SNR. More sensitive detectors are needed. Fiber-optic interferometric sen-
sors possess very high sensitivities and dynamic ranges. Techniques exist
for applying them to each of the signal types mentioned above. Acoustic and
seismic sensors can also be made directional so that the bearing angle can
be determined from an array of three sensors. If the sensors are adequately
separated, range can be deduced from the time difference of arrival of the
sound. Spectral characteristics of the various influences can be correlated
with known target properties and used for target classification.
4.18.2 Possible Solution for a Sonar Dome Pressure Transducer
The Mach Zehnder fiber interferometer is the most convenient and most
frequently used fiber sensor configuration. Figure 4.25 illustrates this form
schematically. It consists of a coherent laser source whose output is split
into two equal paths by a 3 dB directional coupler. The two light signals are
transmitted through separate equal length fiber paths and recombined in a
second 3 dB directional coupler. The second coupler outputs are converted
to electric signals by photodetector diodes. If the two fibers are identical in
length and are made from identical materials, the two light signals arriving
at the second coupler will be in the same time phase and will add construc-
tively to produce a maximum output intensity.
If one fiber is one half wavelength longer than the other, the two light
signals will be of opposite time phase and will add destructively to produce
a minimum output intensity. This is true irrespective of the total lengths of
the fibers; hence, an elastic elongation of one fiber by one half wavelength
will produce a maximum output variation. The system can be viewed as a
very sensitive strain gage measuring displacements in terms of the wave-
length of light, with typical phase resolutions on the order of 1.0 μrad in a
1 Hz bandwidth.
 
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