Page 599 - Introduction to Information Optics

10.2. A Brief Review of Types of Fiber-Optic Sensors 583

10.2.3. PHASE-BASED FIBER-OPTIC SENSORS

As described by Eq. (10.1), the phase of the light field, (f)(r., t) can also be
changed by external perturbations so that the fiber-optic sensor can also be
built based on the phase changes of the light field. The relationship between
the phase change and the optical path change can be written as

271
0(r, f) = — L(r, f), (10.7)
/
where A is the light wavelength and L(r, t) represents the optical path change.
Since the optical wavelength is very small, in the order of microns, a small
change in the optical path may result in a large fluctuation in the phase change.
Thus, in general, the phase-based fiber-optic sensor is more sensitive than the
intensity-based fiber-optic sensor. Note that, since the optical detector cannot
detect the optical phase directly, some types of interferometric techniques are
exploited to implement phase-type fiber-optic sensors, as described in the
following subsections.

10.2.3.1. Fiber-Optic Sensors Based on the Mach-Zehnder Interferometer

Figure 10.8 shows one kind of widely used fiber Mach-Zehnder inter-
ferometer-based fiber-optic sensor [13]. The interferometer consists of two
arms, the sensing arm and the reference arm. The light coming from a coherent
light source, such as from a distributed feedBack (DFB) semiconductor laser,
is launched into the single mode fiber. The light is then split into two beams
of nominal equal intensity by a 50/50 fiber-optic directional coupler, part being
sent through the sensing fiber arm, the remainder through the reference arm.
The output from these two fibers, after passing through the sensing and
reference fiber coils, is recombined by the second fiber-optic directional
coupler. Thus, an interference signal between the two beams is formed and
detected by the photodetector.

Coherent Cou P |er Photodetector
Light source
Reference fiber

Fig. 10.8. Mach-Zehncler interferometer -based fiber-optic sensor.