Biomedical Engineering Reference
In-Depth Information
approach to achieving stable operation of the laser source involves the use
of an external electronic feedback circuit and the fiber-optic ring resonator.
Experimental measurements of the output spectrum produced by single-
mode solid-state semiconductor lasers [13] have resulted in observations of
instabilities, including mode jumping, phase noise, mode partition noise,
line broadening, and frequency shifting. Unstable operation in fiber-optic
interferometric sensing systems has also been observed. Although stabil-
ity problems exist for certain applications, semiconductor lasers have been
demonstrated with excellent performance in communications and coherent
transmission under carefully controlled operating conditions (for a complete
discussion, see Ref. [14]).
In communications, the signal bandwidth can and usually does exceed the
laser linewidth [12]. For example, a communications system using a directly
modulated laser can support a signal bandwidth of 2-3 GHz. This band-
width is much wider than the laser linewidth (a laser linewidth of 10 kHz
was reported through the use of a fiber-optic ring resonator). Because of
this wide bandwidth, some of the problems associated with the frequency
instability are not critical to operation in noncoherent communication sys-
tems. In sensors and interferometers, however, the signal bandwidth may be
very small in comparison to the laser linewidth. For example, if the sensor
is designed to monitor ambient room temperature or perhaps the magnetic
profile of a slowly moving object, the signal sensed may be in the near-dc
range, while the linewidth of the laser used in the sensor itself may be in the
1 GHz range. If the laser linewidth suddenly broadens or changes frequency
during operation, a resulting amplitude or phase fluctuation may be pro-
duced by the sensor, thus degrading performance due to “random noise.”
The need for frequency stability and low noise in the light source for sensor
applications is extremely important and may even impose more stringent
requirements on laser operation than can be presently achieved in many
commercially available devices.
Some laser structures, such as short cavity, external cavity, distributed
feedback (DFB), and rare-earth-doped heterostructure, offer various degrees
of stability; but in general, the ideal stabilized laser diode remains to be
developed. Various stabilization techniques, such as thermoelectric control
of the laser structure, feedback control of the laser injection current, the use
of the external fiber-optic ring resonator, and so on, are designed to improve
laser stability [14].
4.10 CouplersandConnectorsforFiberSensors
The fiber-optic single-mode coupler is an essential component of the fiber sen-
sor system, serving as the power divider and combiner in the interferometer.
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