Biomedical Engineering Reference
In-Depth Information
A
(
t
)
V
th
0
to
Time
B
(
t
)
0
to
Time
FIGURE 4.15
Output signals.
Φ
A
L
1
=
L
2
L
1
∆
f
sin
W
0
t
Sine/cosine
demodulator
Sensing fiber
Laser
Φ
A
3 dB
Coupler
3 dB
Coupler
L
2
2
W
0
~
W
0
FIGURE 4.16
Homodyne using phase-generated carrier (carrier generated by frequency-modulation laser).
4.15 EnvironmentalStabilization
Fiber sensors are extremely sensitive mechanical strain detectors, responding
to any strain-producing influence in either fiber path. Optical fibers exhibit a
change in phase length in response to ambient pressure changes. The mag-
nitude of the effect is strongly influenced by the protective jacket material
on the fiber. Rubber or rubberlike elastomeric materials cause the greatest
pressure sensitivity. Coatings are selected to minimize pressure (static and
acoustic) sensitivity and to balance the sensing and reference paths.
Thermal expansion changes the optical length of the fibers. The combined
thermal expansion effects of the fiber, its jacketing, the magnetostrictive or
other transducer material, and the mechanical support material must be con-
sidered. Some control over the balance between the reference and sensing
paths is afforded by the material selected to support the reference fiber.
The laser frequency and intensity are affected to some extent by tem-
perature and power supply variations. Laser frequency changes produce
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