Biomedical Engineering Reference
In-Depth Information
thermal or noise factors play a significant role on fiber sensor performance,
these compensation schemes can be easily employed.
A final consideration in the choice and operation of the laser diode is opti-
cal feedback. Various optical discontinuities in the fiver sensor transmission
system can reflect light back to the source and deteriorate its operating char-
acteristics. If such optical feedback becomes excessive, performance of the
sensor system may be impaired. In this case, an optical isolator would be
required between the laser pigtail fiber and the system fiber connector. The
isolator rotates the plane of polarization of the feedback light by π/2 so that
it can be filtered by a polarizer.
4.9 LaserFrequencyStabilityConsiderations
The major difficulty in implementing the interferometric fiber sensor involves
the requirement for a coherent and stable light source. Accuracy of sensing is
affected directly by the laser source and any mode jumping, phase noise, or
mode partition noise. Laser structures with the most stable operations pos-
sible are chosen for the fiber sensor system. For applications in which size
and power consumption are not limited, a gas laser should be considered
since the gain curve of the total linewidth is typically on the order of 1 GHz
[11]. With the gas laser, it is relatively easy to define the wavelength to parts
per million; in some cases, the HeNe laser can be locked to the center-gain
curve with stability on the order of parts in 100 million [12]. This parameter
is very important because changes in the operating frequency result in time
delays due to fiber dispersion, even in the low-dispersion operating regions
of the single-mode fiber.
Along with the effect of dispersion and frequency instability, the perfor-
mance of the sensing system can be affected by the presence of phase noise.
Phase noise occurs due to imperfections in the laser cavity, laser drive circuit
noise, and resonance within the laser structure itself [11]. Some control of
excess phase noise is possible through implementation of a feedback control
system in the laser drive. Tight control of the laser drive current is necessary
because the laser resonance is a function of the drive current. Typically, a
change in drive current of 1 mA will result in a change of several gigahertz
in lasing frequency. Other enhancements such as modification to the laser
emission linewidth through the use of an external cavity and the use of ther-
moelectric cooling of the laser body provide only minimum improvements.
Laser source stability improvements are necessary for full implementation
of the interferometric fiber sensor. Further developments are also necessary
before laser manufacturers offer the optimized laser structured in a wide vari-
ety of operating frequencies; therefore, external stabilization techniques must
be employed as an interim solution to the stabilization problem. One such
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