Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 200

be adequate for the sensing frequencies desired. A convenient arrangement
incorporates the photodiode and a low noise operational amplifier in the
same case. This reduces noise and lessens the chance of spurious oscilla-
tion or instabilities. These detector amplifiers are available in a variety of
packages. The active area of the detector is much larger than the fiber cross
sectional area, so attaining efficient optical coupling between the fiber and
detector is easily achieved.
4.18.7 Single-Mode Fiber Directional Couplers
These are 3 dB hybrid quadrature power splitters that are optical equivalents
of the microwave waveguide directional coupler. They are formed by reduc-
ing the cladding thickness on two fibers so that the fibers can be placed into
close proximity. A common approach for this is to fixture the fibers so they
can be twisted and drawn axially while heated. Coupled power is monitored
during the process. The resulting fiber structure is quite fragile and must
be encased before removal from the fixture. Typical commercially available
pigtailed couplers are 1.22 in. long × 0.138 in. diameter.
4.18.8 Optical Fibers
Low loss fibers fall into the following categories: multimode, single mode,
single-mode polarization preserving, and single-mode single polariza-
tion. Mulitmode fibers are unsuitable for interferometric phase measuring
systems because various modes propagate at different velocities, tending
to smear any phase information present when the modes are combined at
the detector. Ordinary single-mode fibers support two identical TE 11 (first
order) modes orthogonally polarized to each other. If the fiber geometry and
refractive index distribution are not absolutely circularly symmetric about
the fiber axis, waves of a certain polarization will advance on waves of some
other polarization. A linearly polarized wave at some arbitrary angle can
be resolved into two orthogonal components. If one of these components
advances or is retarded with respect to the other, the polarization will
become circular or elliptical depending upon whether the components are
equal in amplitude.
Small variations in core-cladding geometry, composition, and stress distribu-
tion in an ordinary single-mode fiber will produce this sort of depolarization—
the magnitude of which varies with temperature, fiber bending, and unequal
radial stresses. Ordinary single-mode fibers are used for long distance tele-
communications applications, in which depolarization is not a concern. Single
polarization fibers are designed such that they only trap and propagate light
of a particular polarization. Orthogonally polarized light leaks from the fiber
and is lost.
Polarization-preserving fibers are fabricated to produce a marked differ-
ence in propagation velocity between a particular linear polarization and