Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 201

one orthogonal to it. The positions of these fast and slow polarizations in the
cylindrical coordinate system of the fiber are called the axes of birefringence.
If a linearly polarized wave is launched with its polarization vector parallel
to one of these axes, it will retain this polarization over long distances. If it
is launched with the polarization vector at 45° to the axes, the polarization
will change cyclically from linear to circular to linear, etc., as it propagates
along the fiber. The distance along the fiber for the polarization to complete
a cycle and return to its original condition is called the fiber beat length. A
condition for the effective use of these fibers is the ability to locate the axes of
birefringence and to match them when joining the fibers for coupling fibers
to sources.
The field amplitudes of the waves emerging from the two fibers of the
interferometer add vectorially in the output couplers, and the photode-
tectors produce an output voltage proportional to the intensity, which is
a constant times the amplitude squared. Some of the difficulties involved
in transmitting light into and out of the fiber and in connecting fibers are
shown in Figure 4.28.
Any phenomenon that causes one amplitude to vary with respect to the
other to affect the sum will produce an output. Ordinarily, the laser diode
light is linearly polarized in a transverse direction. In common single-mode
fibers, there is some variation in refractive index from one transverse plane
to another; the amount changes with bending, radial stress, and tempera-
ture. This unwanted variable birefringence causes random variable polar-
ization shifts. Rotation of the polarization vectors of two waves being added
affects the sum in the same way as time delay phase shifts and produces a
spurious output signal.
The best solution to this problem is to deliberately create a birefringence
that is stable and large enough to “swamp” any random environmentally
induced variations. When polarization-preserving fibers are used, it is
essential to launch the light with its polarization vector parallel to one of the
principal planes of birefringence - either the “fast” or the “slow” plane. It is
also necessary to align these planes when splicing fibers. This considerably
increases the complexity of the splicing operation.
4.18.9 Reference Branch Phase Modulator
The phase-locked tracking loop arrangement requires a means for modulat-
ing the phase delay of the reference branch to balance the transducer delay.
Piezoelectric and magnetic materials are used to strain the fiber. The ref-
erence branch fiber is wound under tension (prestressed) around a piezo-
electric cylinder fitted with electrodes and so poled that its circumference
changes with applied voltage. Minimum cylinder diameter is determined by
the fiber bending radius at which fiber transmission losses increase due to
“leakage” into the cladding due to the increased ray incidence angles at the
core-cladding interface.