Biomedical Engineering Reference
In-Depth Information
Laser
3
dB coupler
Reference arm
Sensing arm
Output
signal
High pass
filter
Signal
∫
Signal & noise
Low pass
filter
3 dB Coupler
Photodidodes
Amplifier &
summer
FIGURE 4.6
Mach-Zehnder fiber-optic interferometer employing phase-locked homodyne detection.
Figure 4.6 illustrates the configuration consisting of a laser source, 3 dB
beam splitter, two optical fiber paths, another 3 dB beam combiner, and a
photodiode detector. One of the fiber paths is designed as the sensing fiber
and the other as the reference fiber. If the path lengths are identical so that
the two light waves arrive at the combiner exactly in phase, the detector out-
put will be a maximum. If one path length is an odd number of optical half-
wavelengths longer or shorter than the other, the two beams will interfere
destructively at the combiner and the detector output will be minimized.
If one fiber is elongated with respect to the other, the detector output
will go through one complete cycle each time the difference in optical path
lengths increases by one wavelength. Because the optical wavelength is very
small and the fiber can be made very long in terms of wavelength, the device
can become an extremely sensitive detector of any phenomenon producing
a change in fiber length. Many magnetic materials are magnetostrictive,
undergoing dimensional changes upon magnetization. Magnetostrictive
materials have been coupled to the sensing fibers of Mach-Zehnder interfer-
ometers to form sensitive magnetometers [1].
The fiber magnetometer sensing fiber can be coupled to the magnetostric-
tive material by coating the bare fiber with the metal or by bonding the fiber
to bulk shapes of material, as illustrated in Figure 4.7. The bounding method
shown in Figure 4.8 is most commonly employed because it yields the largest
system sensitivity and is most suitable for mass production.
4.5 FiberAcoustic/PressureSensors
Fiber sensor transducers can be designed to sense a wide variety of physi-
cal phenomena. One of the more common examples is the use of an acoustic
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