Page 590 - Introduction to Information Optics

10.2. A Brief Review of Types of Fiber-Optic Sensors 5 /5
the second fiber region. Thus, evanescent wave coupling occurs. The coupling
coefficient is directly proportional to the separation distance between the two
fibers. When an environmental effect such as a pressure, an acoustic wave, or a
temperature change causes a change in the distance between two fibers, the
result is a change in the coupling coefficient. Thus, the detected light intensity
of the second fiber is also changed. Therefore, by monitoring the intensity
change of the second fiber, the change in the environment can be sensed.

10.2,2, POLARIZATION-BASED FIBER-OPTIC SENSORS

10.2.2.1. Mathematical Description of Polarization
As mentioned previously, the light field is a vector. The direction of the
electric field portion of the light field is defined as the polarization state of the
light field. There are different types of polarization states of the light field,
including linear, elliptical, and circular. For the linear polarization state, the
direction of the electric field always keeps in the same line during light
propagation. For the elliptical polarization state, the direction of the electric
field changes during the light propagation. The end of the electric field vector
forms an elliptical shape. That is why it is called elliptical polarized light.
Mathematically, the possible polarization states can be written as

l
E T = A xx + A ve *y, (10.2)
where E T represents the total electric field vector; A x and A y represent the
components of the electric field in the x and y directions, respectively; 8
represents the relative phase difference between x and y components; and x and
y represent the unit vectors in the x and y directions, respectively. When d = 0,
the polarization state is a linear polarization state. When d = n/2, the polariz-
ation state is a circular polarization state. In general, the polarization state is
an elliptical polarization state, as illustrated in Fig. 10.4.
Since any optical detector can only detect light intensity directly, instead of
using (A x,A y,d) to describe a polarization state, a four-component Stokes
vector representation is often used to describe a polarization state. The four
components of this vector are given by

s 0 = Al + A]
S l = A X — Ay
(10.3)
s 2 — 2A xA ycosfi

s 3 = 2A xA vsinS.