FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
The ever-present need for increased communication
aystem capacity and reduced cost per message
unit has spurred the development and installation of
hundreds of operating lightwave communication ayatems
around the world. Compared to wire aystems, optical
fiber transmission syatems operate with less energy per
message unit-mile, lower signal attenuation per unit
distance, higher bandwidth for increased channel capacity,
lower electromagnetic interference, lower crosstalk,
higher resistance to clandestine tapping, lower
shock hazard, amaller size, less weight, reduced consumption
of critical metals, and many others. These
advantages have encouraged improvements in light
sources; optical fibers, cables, and connectors; and
photodetectors. Optical fiber data links are off-theshelf
ready-to-install items. Hundreds of millions of
dollars are being spent annually for improving optical
communication system component.
Capitalizing on the availability of optical
components, there haa been significant progress during
the past few years toward the development of a new
claaa of sensors employing fiberoptic. These sensors
are capable of detecting acoustic fields, linear and
rotational acceleration, electric and magnetic fielda,
and many other physical parameters. In effect, the
senaor modulates some feature of the lightwave in an
optical fiber such as the intenaity OT the phase.
Usually phase modulation must be converted to an intensity
modulation prior to detection. This may be accomplished
by means of an optical interferometer. The resulting
signals (intensity or phase) can be telemetered
to places other than the location of the sensor (transducer,
modulator) by means of a fiberoptic signal transmission
(telemetry) system. The optical signal may be
in analog or discrete form and the system may operate
with or without optical-to-electrical or electrical-tooptical
signal conversion. The fiberoptic aensors described
in this manual may use fiberoptic transmission
systems as well as electrical or electromagnetic transmission
systems. Even for the simplest case, one in
which a visual field or image is to be transmitted in
a coherent-fiber cable, the fiber bundle itself must
serve as the sensor and all the aspects of achieving
lightwave acceptance by optical fibers must be considered.
1.2 PURPOSE
This manual on fiberoptic sensors is designed
to be a stand-alone document intended to serve many purposes.
It provides a basic background for understanding
the concepts that make up the field of fiberoptic,
particularly as they apply to fiberoptic sensors. It
describea the propertied of optical fibers, then fabrication,
and the properties of light sources and detectors
associated with fiberoptic sensora. Specific
emphasis is placed on design considerations for these
major components and for associated connector,
splices, couplers, and cables.
Different schemes may be used for controlling
lightwaves in order to sense a physical parameter. Many
of these control schemes are discussed in this manual,
including interferometry, polarization, and modulation.
Intensity and phase modulation are discussed in terms
of homodyne and heterodyne detection. Integrated optical
circuits are introduced with emphasis on their fabrication
and operating principles.
Many different types of fiberoptic sensors
are described in terms of their design and operation.
Some of these include intensity and phase modulation
sensora, rotation sensors, and accelerometers. Devices
discussed include the fiberoptic sensora (transducers,
modulators) used in hydrophores, magnetometera and geophones.
In most fiberoptic sensor and sensor array
applications there will be a requirement to telemeter
sensed data over the full range of distances. Various
fiberoptic aensor arrays and telemetry schemes are discussed.
Information is given concerning risetime and
power budgeting. Overall design considerations for telemetry
systems are also briefly discussed.
1.3 CONTENTS BY CHAPTER
Following thia brief Introduction, (Chapter
1), Chapter 2 is devoted to the properties of the baaic
component of the fiberoptic sensor: the optical fiber
itself. Electromagnetic wave (lightwave) propagation
in optical fibera in terms of the wave equation; the
coupling of lightwaves in and out of fibers; power loss
by absorption, leakage, and scattering; are all discussed
in some detail. Various properties of fibers, their
basic construction and limitations are discussed, including
basic concepts of total internal reflection,
critical entrance angles, and numerical aperture. The
concepts of mode propagation, refractive index profiles,
and polarization are introduced. Various methods of
fiber fabrication are covered, including several methods
of drawing fibers. Obtaining deaired refractive
indices and the size, strength, and level of purity of
fibers are also described. Finally, in Chapter 2, the
characteristics of various types of light sources are
discusaed in sufficient detail to understand their use
in connection with fiberoptic sensors. The chapter
closes with a discussion of the characteristics and
limitations of photodetectors with apecial emphasis on
their importance and use in connection with the output
signal from a fiberoptic sensor.
Chapter 3 follows with a diacuasion of the
various means for connecting fiberoptic aenaor inputs
to electrical or optical sources and outputs to photo-
1-1