Biomedical Engineering Reference
In-Depth Information
Fixed
mirror
Movable mirror
Laser
Transducer
Beamsplitter
Laser
Beamsplitter
Optical
detector
(a)
Fixed mirror
(c)
Detector
Laser
Detector
Transducer
Optical
detector
Laser
Fixed mirror
Moveable mirror
(b)
(d)
Transducer
FIGURE 4.4
(a) Michelson interferometer, (b) Mach-Zehnder interferometer, (c) SAGNAC interferometer,
and (d) Fabry-Perot interferometer. (After Krohn, D.A.,
Fiber Optic Sensors: Fundamentals and
Applications
, Instrument Society of America, Research Triangle Park, NC, 1988.)
detector. The sensing path is from the laser, through the beam splitter, off
the transducer/mirror, and off the beam splitter to the detector. The Mach-
Zehnder configuration includes a fixed and moving mirror/transducer, with
clockwise and counterclockwise propagation paths. In the SAGNAC inter-
ferometer, the clockwise and counterclockwise paths produce a phase dif-
ference upon rotation of the entire system.
Finally, in the Fabry-Perot interferometer, the transducer/mirror is par-
tially transmitting, so that a change in the total path or length or total num-
ber of reflections at the second mirror produces a change in the total light
intensity at the detector.
The four common fiber interferometer configurations are illustrated in
Figure 4.5. Here, the free-space optical paths are replaced by fiber-optic
waveguides, the mirrors are multilayer dielectric coatings, the couplers are
fused biconical taper couplers or integrated optical couplers, and the lasers
and detectors are coupled to the input and output fibers, respectively.
4.4 Fiber-OpticMagnetometer
The fiber-optic magnetometer is representative of the class of interferomet-
ric sensors. A magnetometer design is discussed to illustrate many of the
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