FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK
FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK
FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK
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Fig. 5.60 lists several sources of error that<br />
must be dealt with in order to achieve the predicted<br />
performance. Perhapa the major source of noise is backscattering<br />
within the fiber (see Ref. 21, Subsection<br />
5.4.20) and at interfaces, particularly in a setup that<br />
employs discrete components. To overcome this problem,<br />
researchers have used broadband lasers (see Ref. 17 and<br />
22, Subjection 5.4.20), frequency jittered lasers (see<br />
Ref. 18, Subsection 5.4.20), phase modulators, (see Ref.<br />
23, Subsection 5.4.20), and even light-emitting diodes<br />
(LED) (see Ref. 24, Subsection 5.4.20). By destroying<br />
the temporal coherence of the light source the detection<br />
system becomes sensitive only to the interference be-<br />
5.4.16 Integrated Fiber “Gyros”<br />
Although the early investigation of fiberoptic<br />
“gyros” employed (see Refs. 19, 30, 32 and 33, Subsection<br />
5.4.20) discrete optical components for convenience,<br />
it is clear that if fiber gyros are to make a<br />
large impact an integrated (see Ref. 34, Subsection<br />
5.4.20) optical system (Fig. 5.61) with a semiconductor<br />
laser/LED as a light source must be uaed. Beamsplitters<br />
can be replaced by either waveguide or fiber 3-dB couplers<br />
(see Ref. 17, Subsection 5.4.20). Nonreciprocal<br />
RAYLEIGH SCAITERING IN FIBER<br />
SCAITERING FROM INTERFACES<br />
POLARIZATION EFFECTS<br />
PRESENCE OF HIGHER ORDER MODES<br />
TEMPERATURE GRADIENTS<br />
NONIDEAL MODULATORS<br />
NONIDEAL POLARIZERS<br />
INTENSITY DEPENDENT NONRECIPROCITY<br />
LIGHT SOURCE PROBLEMS<br />
MEASUREMENT SYSTEM PROBLEMS<br />
STRESS INDUCED EFFECTS<br />
MAGNETIC FIELD EFFECT<br />
Fig. 5.60 Sources of noise and errors that must be<br />
considered in the design of a fiberoptic<br />
rotation-rate sensor.<br />
tween waves that followed identical counter-propagating<br />
paths. Thus, interference due to backacattering will,<br />
in principle, average to zero.<br />
The problem that has received much attention<br />
both theoretically (see Refs. 25 and 26, Subsection<br />
5.4.20) and experimentally (see Ref. 17, Subsection<br />
5.4.20) is the error due to the polarization behavior<br />
of the optical fiber. By uaing polarizers to establish<br />
the axis of polarization (see Ref. 26, Subsection<br />
5.4.20) in the long-fiber interferometer, it has been<br />
possible to reduce polarization-dependent errors. The<br />
use of the now available single-mode polarization-preserving<br />
fibers (see Ref. 27, Subsection 5.4.20) may<br />
turn out to be a convenient solution.<br />
4I<br />
Fig. 5.61<br />
I<br />
Q=POuTpuT<br />
f~<br />
NR MODULATOR<br />
An open-loop integrated fiberoptic rotation-rate<br />
sensor employing a phase transducer.<br />
phase modulators may employ a short length of fiber<br />
wound around a PZT, the Faraday ef feet, an integrated<br />
Bragg cell, or other features.<br />
Integrated polarizers (see Ref. 17, Subsection<br />
5.4. 20) and polarization controllers (see Ref. 17,<br />
Subsection 5.4. 20) are also feasible. In the closed<br />
loop approach shown in Fig. 5.62 the nonreciprocal<br />
phaae transducer could be a Bragg cell, a Faraday effect<br />
device, or other device. In other words various<br />
possibilities exist for constructing a solid-state fiber<br />
gyro. An all-optical-fiber open-loop system has<br />
already been demonstrated with a very promising performance<br />
(see Ref. 17, Subsection 5.4.20).<br />
All the fiber gyros under study so far have<br />
used single mode fibers. Care has to be taken to insure<br />
that higher order transverse modes are highly attenuated.<br />
A number of error sources are related to temperature<br />
gradients, (see Ref. 28, Subsection 5.4.20)<br />
non-ideal polarizers, (see Ref. 29, Subsection 5.4.20)<br />
non-ideal modulators, stress induced effects, external<br />
magnetic field effects (see Ref. 30, Subsection 5.4.20)<br />
and electronics problems in the measurement system.<br />
A very basic source of nonreciprocal phase<br />
shift has been uncovered, namely that due to unequal<br />
intensities propagating along opposite directions in<br />
the fiber (see Ref. 31, Subsection 5.4.20). This is a<br />
nonlinear optical effect based on four-wave mixing that<br />
takes place in a fiber having a third order nonlinear<br />
susceptibility. Such an intensity-induced nonreciprocity<br />
may be reduced by maintaining equal intensities<br />
in the counterpropagating beam.<br />
5-22<br />
Fig. 5.62<br />
I<br />
SERVO<br />
DETECTOR<br />
f~<br />
A closed-loop integrated fiberoptic rotaa<br />
tion-rate sensor employing<br />
phase trans-<br />
ducer.