Biomedical Engineering Reference
In-Depth Information
a magnetoresistive material to the sensing fiber. Allied Corporation's
METGLAS 2605SC is a suitable material whose properties can be tailored to
this application [56].
• Unequal thermal expansion: match thermal coefficients of expan-
sion and subject sensing and reference arms to the same thermal
environment by thermal filtering
• Transmission loss variation: mount fibers to minimize externally
induced stress
• Depolarization, depolarization shifts: use polarization-preserving
fiber and minimize radial stresses, temperature, and environmen-
tally induced stresses
• Laser frequency noise: make reference and sensor fiber lengths
as nearly equal as possible, stabilize laser frequency with current
regulators and temperature controller, and isolate laser from optical
feedback
4.18.5 Light Source
Single-mode fiber pigtailed laser diodes are available at low cost that produce
power levels of milliwatts at the fiber end. The dynamic (slope) resistance is
low at the operating point, and the device is operated at constant current.
Light is proportional to current, but the current is temperature dependent.
Typical lasers operate at currents between 55 and 80 mA and at voltages
between 1.6 and 1.7 V. Of interest is the fact that lasing does not start until the
diode current reaches 55 mA. At this threshold, the input power is approxi-
mately 88 mW. At the 1 mW optical power point, the power input is 102 mW.
Efficiency would be much better if the threshold current could be substantially
reduced. Experimental diodes are reported with threshold currents of 1 mA.
Many diodes have a built-in photodetector that receives a sample of light
from the diode facet opposite the output facet. The photodetector output is
used to drive an external feedback loop that regulates the laser current to
maintain a constant light intensity. Peltier thermoelectric coolers are incor-
porated in some laser diode packages along with thermistors so that closed
loop temperature control can be effected. These coolers require substantial
currents (up to 800 mA). They are probably unnecessary in an underwater
device where excess heat can be dissipated to the water and where tempera-
ture extremes are limited. Underwater temperature changes are expected to
be relatively slow in comparison to sensor response times.
4.18.6 Photodetectors
The photodetector diodes must be matched in spectral response to the laser
wavelength selected and have a low noise output. The response time must
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