Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 166

made. This can be done if a third noncoplanar sensor is used or if the angle
can be estimated from the direction of the earth's field. Detailed informa-
tion on R and θ is necessary if, for example, a mine is to be activated at the
most appropriate time.
The analysis above assumes knowledge of the vehicle type and its move-
ment. Only the low-frequency magnitude of the perturbation signal was
considered. To obtain information on vehicle type, the noise spectrum in
the signal can be used and correlated with expected noise for a specific vehi-
cle type. This leads to the appropriate value of the moment, and requires a
transfer of field-time measurements into the frequency domain.

4.13 FiberSensorOperation
The basic fiber sensor output voltage will pass through a series of maxima
and minima as the local magnetic field intensity increases or decreases, pro-
vided the magnetic variations are great enough to change the effective fiber
length by many optical wavelengths. The number of output peaks occurring
in any time interval is a measure of the total change in field strength dur-
ing that interval. The result will, however, be the same for increasing and
decreasing fields, and the negative and positive changes within any mea-
surement interval will add on the basis of absolute magnitude irrespective
of algebraic sign (e.g., suppose that the field went from 0.5 to 1.0 Oe and back
to 0.5 Oe within an observation interval). The net change in field intensity is
zero, but the peak counting technique will indicate a net change of 1.0 Oe.
Furthermore, unavoidable drift in differential optical phase lengths between
the reference sensor fibers will produce spurious maxima and minima that
cannot be distinguished from desired signals.
The drift and ambiguity problems can be overcome by incorporating a
phase-locked feedback system. Such an arrangement is show in Figure 4.11.

Φ A

Sensing fiber High pass

Laser

Φ A

Diff.
amp
Low
pass

3 dB
Coupler

3 dB
Coupler

PZT

Circuit reset

FIGURE 4.11
Active homodyne detection.