Biomedical Engineering Reference
In-Depth Information
other. When the difference is 180°, the condition is reversed; hence, the dif-
ference output is zero for the undisturbed state and goes positive or negative
depending on which fiber is longer. Since the wavelengths in use are on the
order of 1 μm (0.000039 in.), the system is in effect an ultrasensitive strain
gage. Any measure and that can be converted to a fiber strain can be mea-
sured with comparable sensitivity. If one arm is spatially displaced from the
other, the system becomes a gradiometer sensitive to the spatial derivative of
the measure and field.
Pressure variations can be converted to axial fiber strains by wrapping
the fiber around a pressure expandable mandrel or by jacketing the fiber
with an elastic plastic or rubber material. Figure 4.26 shows the latter. Here
an increase in ambient pressure causes the jacket to elongate in one direc-
tion. This strain is transferred to the fiber, increasing its optical phase length.
The ultimate resolution of the fiber-optic interferometer is determined by the
smallest detectable phase difference between the light waves emerging from
the separate paths. The literature reports a noise-limited maximum phase
resolution of 1 μrad under a standardized set of reference conditions that
control other variables affecting sensitivity. These conditions are as follows:
• A signal bandwidth of 1 Hz.
• A difference in the mechanical length of the two fibers of 2 mm or
less.
• Two photodetectors are used with a difference amplifier as shown
in Figure 4.25.
• The signal frequency is 2 kHz or is an envelope on a 2 kHz carrier.
A few words about these conditions are in order. The 1 Hz bandwidth is a
convenient reference, and the SNR or limiting sensitivity of any other signal
bandwidth can be scaled directly. A difference in the mechanical lengths of
the fibers will make the system output a function of the laser wavelength.
Optical
input
ℓ
Jacket material
Physical field
“
P
”
To be measured
P
a
FIGURE 4.26
Acoustic pressure transduction mechanism.
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