Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 151

leg. The detector output is used to modulate the reference leg to maintain a
constant-phase relationship to the sensing leg. In this approach, the level of
feedback is a measure of the sensing leg perturbation.
Such a constant-phase modulation and detection scheme is commonly
employed because, in many cases, the output sensitivity is more sensitive
when the system is biased at quadrature. This sensitivity factor is illustrated
in Figure 4.3. With an additional 90° phase difference between the two sen-
sor legs, the phase variation to be detected produces a larger detector output
variation than without bias. Movements as small as 10 −13 m can be detected
with the bulk and fiber configurations of these sensors [4].
Perturbing fields detected in this manner can include the following:
acoustic, electric, magnetic, acceleration, trace, vapor, pressure, temperature,
rotation, and electric current. Common applications for such sensors are
hydrophones, magnetometers, gyroscopes, and accelerometers. In the dis-
cussion to follow, the fiber magnetometer is discussed as representative of
the class of interferometric sensors.
Before discussing more details of the sensor design, it is instructive to look
at the major classes of interferometers in their bulk configurations and then
to visualize these sensors in a fiber configuration [5]. Figure 4.4 illustrates
the four most common bulk interferometer configurations: the Michelson,
Mach-Zehnder, SAGNAC, and Fabry-Perot.
In the Michelson configuration, the reference path is from the laser, off
the beam splitter, to the fixed mirror, and through the beam splitter to the

Current out

Phase variation

-20 0 20 40 60 80 100 120 140

Bias angle (°)

Between the outputs of the two branches
of the interferometer

FIGURE 4.3
Sensitivity at 0° and 90° bias.