Page 587 - Introduction to Information Optics

572 10. Sensing with Optics

In recent years, with the development and incorporation of optical fibers,
sensing with optics has been largely focused on optical fiber sensors; this offers
some potential advantages such as immunity to electromagnetic interference, a
nonelectrical method of operation, small size and weight, low power, and
relatively low cost. Therefore, this chapter will focus on fiber-optic sensors. In
particular, since distributed fiber-optic sensors offer the unique advantage of
multiple location measurements with only a single fiber, we will discuss
distributed fiber-optic sensors in a separate section. Note that the distributed
measurement capability of the fiber-optic sensor distinguishes it from other
types of sensors, such as electronic sensors. This is one of the major benefits of
employing fiber-optic sensors.

10.2. A BRIEF REVIEW OF TYPES OF FIBER-OPTIC SENSORS

As mentioned in Sec. 10.1, the key parameters of light fields include
amplitude (or intensity), polarization, frequency, and phase. Thus, we categor-
ize types of fiber sensors based on their sensing parameters. For example, if the
sensing parameter is based on a change in light-field intensity, this type of
sensor will belong to the category of intensity-based fiber-optic sensors.

10.2.1. TNTENSITY-BASED FIBER-OPTIC SENSORS

There are many transduction mechanisms which can result in a change in
light intensity when light passes through an optical fiber, so intensity-based
fiber optic sensors can be used. These mechanisms may include:

• Microbending loss
• Breakage
• Fiber-to-fiber coupling
• Modified cladding
• Reflectance
• Absorption
• Attenuation
• Molecular scattering
• Molecular effects
• Evanescent fields [1]

For the purpose of illustration, several kinds of widely used intensity-type
fiber-optic sensors are briefly summarized in the following subsections.