Biomedical Engineering Reference
In-Depth Information
differential phase changes and hence noise if the two optical path lengths
are not exactly balanced (i.e., there will be a phase shift proportional to the
frequency difference multiplied by the difference in path lengths). This effect
is near the system noise levels for typical fiber sensors.
Fiber sensor models present to the detector the sum of two sinusoids of
equal and constant amplitude but of variable differential phase so that the
resultant intensity is a measure of the differential phase. Light-intensity
modulation by the laser or fibers produces apparent phase noise and must
be minimized. A feedback-stabilized laser is typically employed. Procedures
used to reduce unwanted phase modulation in the fibers reduce intensity
modulation as well.
Optical standing waves can exist between two reflective mismatches, cre-
ating, in effect, a Fabry-Perot resonator. The transmission of such a resona-
tor depends on its length at any particular frequency. Possible sources of
reflections are separation of fiber ends at connectors and/or misalignment
at connects, couplers, or splices. Temperature variations may change the
separation of reflective mismatches and alter the transmission loss. Careful
attention to the assembly of components is required to minimize transmis-
sion losses, to lower the
Q
of the resonant sections, and to reduce frequency/
thermal transmission variations.
Very slow thermal effects can be tolerated provided they are below the
band of frequencies to be detected. A large thermal time constant is desirable.
Thermal mass will be inherently small in the fiber sensor design. Thermal
response time can be increased by increasing the thermal resistance to sur-
roundings. Low-density plastic foams coated with thin reflective films can
be used for this purpose.
4.16 FiberSensorSystemDesignConsiderations
To achieve optimum fiber sensor performance, commercially available com-
ponents must be analyzed and carefully selected. Fibers, lasers, couplers,
connectors, and detectors should be evaluated for their individual suitabil-
ity to a particular sensor system design. Although the ultimate length of
the sensing and reference arms will dramatically affect the system's optical
power budget, a 30 dB power dissipation between the laser output and the
detector window is a reasonable guideline by which to evaluate the permis-
sible losses through individual components and connectors. As a typical
“worst-case” example of a system power budget, consider the following:
Laser diode pigtail output (min): 1.5 mW
Fiber-coupler connector loss (max): −1.0 dB
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