The Effect of Transverse Load on
Fiber Bragg Grating Measurements
A Thesis
Submitted to the Faculty
of
Drexel University
by
Stephen A. Mastro
in partial fulfillment of the
requirements for the degree
of
Master of Science
in
Materials Engineering
May 2000
ii
Acknowledgements
I gratefully acknowledge Dr. Mahmoud El-Sherif, whose academic, professional, and
personal motivation and support initiated and sustains my continued professional and
academic endeavors. I would also like to thank the faculty and staff of the Materials
Engineering Department of Drexel University for their experience, professionalism, and
high standards, which have supported this work significantly. Thanks are also extended to
Dr. Rachid Gafsi, whose intellectual and hands-on experience contributed much to this
work.
I am grateful to those at the Naval Surface Warfare Center, Carderock Division,
Philadelphia, whose support made this work possible. I extend special thanks to Jack
Overby for invaluable laboratory and theoretical support through the experimental work
of this thesis. Thanks also go to Henry Whitesel, John Sofia, Bill Valentine, Al Ortiz and
Charlie Zimmerman.
Special thanks also go to those in academia that fostered a desire to work in the sciences,
especially Mr. Louis Detofsky, Mrs. Cyndi Nolan and Mr. Gil Fitzgerald.
I would also like to thank my parents, Amedeo and Loretta, for the countless years of
support and encouragement from Bell’s School to Drexel University.
Most special thanks go to my wife, Donna, for her support and encouragement.
iii
Table of Contents
List of Tables................................................................................................................. v
List of Figures .............................................................................................................. vi
Abstract ......................................................................................................................viii
1. Introduction .............................................................................................................. 1
2. Literature Review of Fiber Optic Sensors for Materials Characterization................ 6
2.1 General ........................................................................................................ 6
2.2 Fundamentals of Optics and Material Properties ........................................ 6
2.3 Light Acceptance and Propagation in Optical Fiber ................................... 9
2.4 Optical Fiber Material ............................................................................... 12
2.5 Materials Related Loss Mechanisms......................................................... 14
2.6 Optical Fiber Fabrication .......................................................................... 16
2.7 Fiber Optic Sensors ................................................................................... 22
3. Manufacturing, Operation and Signal Anomalies of Fiber Bragg Gratings .......... 29
3.1 General ...................................................................................................... 29
3.2 Manufacturing ........................................................................................... 31
3.3 Operation................................................................................................... 34
3.4 Observed Signal Anomalies ...................................................................... 37
3.5 Sources Of Bragg Grating Signal Anomalies ........................................... 39
4. Experimental Investigation and Analysis of Fiber Bragg Gratings ....................... 46
4.1 General ...................................................................................................... 46
4.2 Experimental Procedure ............................................................................ 47
iv
4.3 Polarization Effect on FBG Signal............................................................ 48
4.4 Characterization of FBG Using Polarized Light ....................................... 49
4.5 Wavelength Analysis................................................................................. 53
5. Experimental Analysis of Fiber Bragg Grating Response To Transverse Load............................................................................................... 56
5.1 General ...................................................................................................... 56
5.2 Characterization of FBG Under Transverse Load Using Polarized Light 56
5.3 Wavelength Analysis................................................................................. 61
6. Conclusions and Recommendation For Future Work ............................................ 65
6.1 Transverse Load Effects on FBG Structure .............................................. 65
6.2 Recommendation For Future Work........................................................... 69
List of References........................................................................................................ 71
v
List Of Tables
2.1 Characteristics of common optical fiber types……………………………….… 20
2.2 Classification of fiber optic sensors……………………………………………. 23
vi
List of Figures
2.1 Arrangement of typical optical fiber (cross section)............................................... 7
2.2 (a) Refraction of light beam at interface of two homogenous materials................. 8
2.2 (b) Total internal reflection where θi > θcr (θcr is the critical angle) ....................... 8
2.3 Light acceptance at the input of an optical fiber ................................................... 10
2.4 Light loss vs. wavelength (λ) in silica................................................................... 15
2.5 Optical fiber drawing tower .................................................................................. 19
2.6 Index of refraction profile of typical optical fiber................................................. 21
2.7 Intensiometric fiber optic sensor examples........................................................... 24
2.8 Non-intensiometric fiber optic sensor examples................................................... 27
3.1 Internally written Bragg grating............................................................................ 31
3.2 Side-written Bragg grating .................................................................................... 33
3.3 Reflected and transmitted Bragg grating signal with LED source........................ 35
3.4 Bragg wavelength shift with change in Λ ............................................................. 36
3.5 Multiplexing of Bragg grating sensors.................................................................. 37
3.6 Bragg grating signal under transverse strain ......................................................... 38
3.7 Birefringence effect on Bragg grating conditions and reflected signal................. 42
3.8 Transverse loading and effect on index of refraction............................................ 43
4.1 Changing polarization With birefringence............................................................ 48
4.2 Experimental setup for characterization of unloaded FBG................................... 49
4.3 Adjustable bayonet-type optical fiber connector .................................................. 50
4.4 Initial data of polarization baseline study ............................................................. 52
vii
4.5 Intensity vs. analyzer angle for various input polarizations.................................. 53
4.6 Intensity vs. analyzer angle for various input polarizations, overlapped .............. 54
4.7 Intensity vs. wavelength for four input polarizations............................................ 55
5.1 Experimental setup used in transverse load tests .................................................. 57
5.2 The designed mechanical device for transverse loading....................................... 58
5.3 Experimental setup for polarization analysis of FBG under transverse loading... 59
5.4 Polarization analysis data ...................................................................................... 59
5.5 Polarization analysis data, load vs. intensity......................................................... 60
5.6 Experimental setup for wavelength analysis......................................................... 61
5.7 Wavelength analysis data from OSA .................................................................... 63
viii
Abstract
The Effect of Transverse Load on Fiber Bragg Grating Measurements Stephen A. Mastro
Advisor: Mahmoud A. El-Sherif, Ph.D.
The field of communications has been revolutionized by the advent of optical fiber.
Optical fiber now connects most of the world carrying a vast amount of information
through a very limited physical medium. To the materials engineer, optical fiber
technology has stimulated interest in a new type of micro-sensor application, where the
size, weight, and the ability to integrate the sensor into a material structure play a major
role. Areas of interest currently include materials characterization, cure monitoring, and
structural health monitoring. The fiber Bragg grating (FBG) is such a sensor technology,
creating an optical strain gauge within the core of an optical fiber through the use of a
wavelength specific filter. As the FBG experiences induced strain along its major axis,
its light signal indicates the amount of strain with great accuracy and sensitivity. In FBG
applications, where loading may occur in all directions, complex changes take place in
the FBG signal. As these changes impact the usefulness of the FBG as a strain sensor,
this thesis endeavors to study and demonstrate the effect of transverse load on the FBG
structure. A review of the basics of optical fiber fabrication, light transmission, and
optical fiber sensor technology is presented first, followed by a detailed discussion of the
FBG’s manufacturing and operation. The crux of the thesis is presented with the results
of experiments conducted to characterize the FBG signal in both unloaded and
transversely loaded configurations. Polarization and wavelength based experiments and
analyses are conducted to detect changes in the FBG signal. The results of the study are
used to propose cause and effect relationships between transverse loading, its effect on
the material properties of the FBG, and changes in the FBG signal.
1
CHAPTER 1
INTRODUCTION
The communications field has been revolutionized by the advent of optical fiber.
Optical fiber now connects most of the world, carrying a vast amount of information
through a very limited physical medium. Textbooks and sources often cite the same
advantages of optical fiber as a communication medium: high bandwidth, low loss,
electromagnetic interference (EMI) immunity, inert nature, reduced weight etc. In the
late 1970s and early 1980s, it became evident to some researchers that communications
technology was not the only possible beneficiary to optical fiber’s benefits, and the
concept of optical fiber sensors was born.
This new technology has stimulated interest in a new type of micro-sensor
application where the size and weight of the components play a major role. The
development and application of such devices is where the contribution from materials
engineers is evident. Effective use of such technology requires a full understanding of
the material behavior, physical mechanisms, and the design of the proper material system
for sensor applications.
The field of sensors embraces both new and old technology. Cost, performance,
accuracy, robustness, response time and size are major considerations when choosing
sensors for various applications. In many cases, mature technologies, proven and
inexpensive, continue to be used in applications where their capabilities suffice with little
modification. Applications with more demanding in-situ or real-time measurement
2
requirements have emerged with the development of smart structures, advanced medical
and laboratory devices, and other applications [1]. The use of highly advanced fiber optic
sensors has therefore become more commonplace. This is especially true for embedded
fiber optic sensors that require lightweight microstructure and immunity to
electromagnetic fields.
Areas of interest of fiber optic sensors applications include materials
characterization and structural health monitoring. Many different types of fiber optic
sensors have been developed for such applications that operate principally by modulating
the intensity, phase, or polarization of the light passing these sensors. These sensors have
been developed to measure strain, temperature, pressure, proximity, and chemical
presence. One type of sensor used for strain measurement is based on the application of
fiber Bragg grating (FBG) structures. They are known as optical strain gauges. These
optical strain gauges modulate a particular wavelength of the light passing through them,
in a very sensitive manner, as a strain is induced in the optical fiber. It is assumed that
the sensor measurements are related only to axial strain, however looking to the FBG as a
material system raised a number of critical questions based on Poisson’s effect and
transverse loads. The answer to these questions is the core of this thesis work. A study
on the material and structural behavior of FBG sensors under various loading conditions
is presented in this thesis.
In an attempt to provide a suitable background of the FBG under loading, a large
amount of information was gathered on the fabrication and operation of optical fibers.
An emphasis was placed on material properties. These material properties dictate the
fiber’s use as a waveguide, and the altering of these properties is critical to their use as
3
sensors. After examining the broad field of fiber optic sensors, a more detailed look at
the manufacturing and theory of operation of an FBG was completed. In addition, signal
anomalies observed in the literature and in previous experiments by the author were
reviewed. After this review, the study was commenced. First accomplished was the
analysis of unloaded FBGs, after fabrication. Very special attention was paid to
establishing a suitable baseline from which changes in the sensor’s signal under loading
could be observed later, and to attempt to detect any changes in the material properties of
the optical fiber simply through the process of writing the gratings into the fiber core. An
experimental procedure was then developed to observe the FBG signal under transverse
loads, which not only produced the anomalies, but also provided significant insight as to
their causes. After designing such laboratory tests, they were conducted and data were
collected. While a broadening and bifurcation of the FBG signal were expected, other
unexpected data were collected. These unexpected results required studies on induced
changes in the material properties and structure. With the data collected, possible
explanations to the FBG signal anomalies under transverse loads were formulated, and
additional endeavors suggested. To this end, the study on the material and structural
behavior of FBG sensors is presented in the next five chapters.
A literature review is presented in Chapter 2. The basics of optical fiber
fabrication and light transmission are provided, as well as a background on optical fiber
sensors technologies and applications. These fundamentals are crucial to the discussion
of FBG sensors and their behavior. Of specific interest are the material properties of the
silica used in optical fiber. These material properties dictate the fiber’s ability to carry a
light signal, and the altering of these properties allow for novel sensor design.
4
In Chapter 3 the fabrication and operation of the FBG is discussed. Specific
attention is paid to the Bragg condition, and the formation of the Bragg structure in the
optical fiber core. Understanding the FBG necessitates an understanding of the material
structure within the FBG and its relationship to light signals passing through it. Detailed
is the operation of a FBG as a strain sensor and the relationship of the changing material
properties of the FBG and the resulting light signal output. Previously observed FBG
signal anomalies, the impetus of this thesis, are also presented.
The analysis of the material properties of the FBG, and subsequent designing of
appropriate laboratory tests are covered in Chapter 4. An important but often neglected
step in analysis of the FBG is a close look of the material system immediately after
manufacturing, before any loading or embeddment has occurred. It is convenient to
assume that the FBG creates no other effect in the fiber core aside from creating the strain
sensor described above. It is appropriate, however, to take a close look at the optical
fiber’s material properties in an unstrained state to establish an accurate baseline for
loading tests to occur later. At this stage, laboratory tests were conducted, and a strategy
for load testing of the FBG was devised.
Chapter 5 outlines the transverse load testing of the FBG. Laboratory tests
performed in the past by various researchers showed anomalies in the FBG signal when
introduced to transverse loads. In this chapter, laboratory tests are conducted to observe
these effects. The laboratory setup and its relevance to various possible effects on the
data is described. After presenting data from the experiments, attempts are made to relate
the changing material properties of the FBG structure and their manifestation in the light
signal data.
5
The data from the experiments in Chapters 4 and 5 is discussed in Chapter 6. In
addition to drawing conclusions as to the FBG’s changing material properties under
transverse loads, future work in this area is suggested.
6
CHAPTER 2
LITERATURE REVIEW OF FIBER OPTIC SENSORS
FOR MATERIALS CHARACTERIZATION
2.1 General
The technology of transmitting information optically has grown rapidly since the
development of low loss silica fiber in 1970 [2]. Amorphous silica provides an ideal
material to construct an optical waveguide. With proper processing, it is very transparent
with little porosity or scattering centers. An optical fiber can best be defined as a thread
of dielectric material using total internal reflection to transport light enormous distances
and with great accuracy. All silica optical fibers provide extremely low optical signal
loss, and as such provide an ideal medium for sensors and sensor communications. This
chapter presents an overview of the manufacturing and operational characteristics of
optical fiber, and their use as sensors.
2.2 Fundamentals of Optics and Material Properties
In general, an optical fiber comprises two slightly different optical materials. In a
cylindrical arrangement, the center cylinder, called the core, carries the optical
information; the outer layer, the cladding, serves to provide an optical boundary,
reflecting all signals back into core. This reflection is accomplished by the core having a
slightly higher (sometimes as low as 0.1%) index of refraction (n) [3,4]. The core and
cladding share a common boundary. For practical application, other layers of polymer
coatings, braided fiber, and cable jacketing for mechanical strength and protection
7
surround the cladding (Figure 2.1). These additional layers are not meant to interact with
the optical signal at all. For certain applications, polymer fibers can also be used to
transmit light signals, although a high amount of scattering points make them
inappropriate for coherent long distance signal transmission.
Figure 2.1 Arrangement of typical optical fiber (cross section)
To provide for the guidance of light through optical fiber a construction of two
layers of dielectric transparent materials with different indices of refraction (representing
the core and cladding), n1 and n2 is commonly used, where n1>n2 [2]. The refractive
index of a material is the ratio of the speed of light in a vacuum, c, to the speed of light in
the material, v (i.e. the light travels more slowly in materials with larger refractive index):
vcn = (2.1)
Core Cladding
Polymer Coating
Jacketing / Additional Protective Layers
8
Snell’s Law governs the directional change in a beam of light (i.e. refraction of
light) when the beam crosses a boundary with a change in refractive index. This
phenomena, refraction of light, is commonly explained using ray theory. The refraction
angle as depicted in figure 2.2 is given by the relation [5,6]:
ri nn θθ sinsin 21 = (2.2)
where θi is the angle of incidence and θr is the angle of refraction.
(a) (b)
Figure 2.2 (a) Refraction of light beam at interface of two homogenous materials (b) Total internal reflection where θi > θcr (θcr is the critical angle)
n1
n2
Incident Beam
Refracted Beam
n1
n2
θi > θcrReflected Beam
θi
θr
9
As the angle of incidence θi is increased so too will θr increase. The angle θr will
eventually reach 90 degrees, at which point the incident angle θi is known as the critical
angle of incidence, or θcr. Solving Snell’s law for θcr gives the expression:
)(sin1
21
nn
cr−=θ (2.3)
If θi is increased up to a value higher than θcr, then the beam is said to be “totally
reflected,” as indicated in Figure 2.2 (b) [2]. The light suffers no intensity loss through
its reflections but does experience a polarization shift, or change in phase as it travels
through the medium [2]. Total internal reflection is the mechanism of light transmittance
through an optical fiber, with the cladding reflecting all of the light energy back into the
core.
2.3 Light Acceptance and Propagation in Optical Fiber
Snell’s law describes the reflection of light between media and, therefore, within
an optical fiber. Choosing and matching an optical source to the fiber to launch and
transmit a signal requires some additional information. It is important to know the light
collecting ability of the optical fiber’s terminus. In general, three parameters affect the
ability of optical fiber to accept light energy into its core: physical size of the core, and
the core and cladding indices. The maximum angle between the direction of the
incoming light energy and the center core axis of the fiber is called the acceptance angle
and is a function of the core and cladding indices [2]. The term “acceptance angle” is
10
commonly used to describe the fiber’s light gathering capability. The maximum
acceptance angle (α) is limited by the critical angle of incidence at the core-cladding
boundary. Figure 2.3 depicts a ray of light entering the end face of an optical fiber. A
light ray, shown travelling in a refractive medium of n0 then enters the front face of the
optical waveguide at an angle α to the fiber’s major axis. Subsequently the ray strikes
the core-cladding interface at an angle equal to or greater than the critical angle. From
Snell’s law this scenario can be described as
)cos()90sin(sin 110 crcr nnn θθα =−= (2.4)
Since n0 is equal to one for air, which is constant, then the value of (n0 sin α) is constant
for an optical fiber and is known as the numerical aperture (NA) of the waveguide [2]
where
NA1sin −=α (2.5)
Figure 2.3 Light acceptance at the input of an optical fiber
n2
n2
n1
>θcr
α
≤ 90º- θcr
n0 cladding
core
11
and the NA can be written as:
[ ] 2/121 sin1 crnNA θ−= (2.6)
substituting from equation (2.3) for θcr
[ ] 2/122
21 nnNA −= (2.7)
Thus, it is shown that the NA or the light acceptance at the fiber’s end is a function of the
indices of refraction of the core and cladding [7].
A lengthy electromagnetic theory discussion would be necessary to fully describe
guided light waves and their propagation, far from the scope of this thesis; however a few
key features warrant mention. It is necessary to recognize that the electromagnetic field
of the light waves propagating through the optical fiber can be described by Maxwell’s
Equations. Specific solutions to these equations are satisfied through certain parameters
of the fiber boundary conditions, which is a function of the fiber geometry and materials
properties. The solutions allow for discrete waveforms, called modes, to be propagating
through the fiber core [2]. Not surprisingly, the fiber core diameter has a direct
relationship to the number of modes that can propagate [2]. Each solution to the
electromagnetic field equations presents the wave function for a single mode. Optical
fiber that carries many modes is referred to as “multimode,” while that small enough to
carry only one mode is referred to as “single mode.”
Based on Snell’s law, electromagnetic wave theory, and Maxwell’s solutions, it is
clear that the properties of the optical fiber materials are the key parameters for the
12
transmission of optical signals within optical fibers as well as for sensor applications. An
understanding of the materials properties of a waveguide and its interaction with a light
signal is a crucial and often overlooked step in describing fiber optic sensor signals. The
next few sections focus on this point. Later chapters follow through specifically with
sensors.
2.4 Optical Fiber Material
In selecting and manufacturing optical fibers, some basic requirements must be
addressed. First, the material must be transparent at particular wavelengths so that
signals may be transmitted efficiently. The material must also be of a composition such
that thin strands or fibers may be drawn from it that are long, thin, and flexible. Lastly,
materials must be used to construct the core and the cladding that can be tuned or
modified to have slightly different in index of refraction, but are physically and
chemically compatible. The materials that may best address these requirements are glass
and some types of plastics [3]. Noted is the fact that the generic term glass may apply to
many different materials in a non-crystalline state, but in this discussion is
interchangeable with silica glass.
The majority of optical fiber are made of silica glass, SiO2 or a silicate. Glass
fibers can be made with very large cores (up to 100 µm) and reasonable losses or with a
very small core with extremely low losses [3]. Plastic fibers are used in much lower
numbers due to their much higher attenuation. These fibers are used primarily in short
distance applications and in applications where very abusive or harsh conditions exist. In
13
these applications the mechanical strength of plastic fiber gives it an advantage over all
silica fiber [8].
Typical glass fibers are made by mixing and melting metal oxides and silica. The
desired result is a molecular network that is amorphous rather than a structured type
arrangement as found in crystalline material. A crystalline or porous structure would
create reflection and refraction points where the light would be attenuated or dispersed
[9]. As a result of this structure, glass does not have a well-defined melting point. Glass
will remain solid up to several hundred degrees centigrade, at which time it softens and
starts to flow. At this stage it is a viscous liquid. In fact, solid glass can be considered a
supercooled liquid [9]. The raw material for silica is sand. Glass composed of pure silica
is usually referred to as silica glass or fused silica. This particular material has the
desirable properties of being resistant to deformation at temperatures as high as 1000 C°,
having a resistance to breakage due to thermal shock because of its low thermal
expansion, its good chemical durability, and high transparency in the visible and near
infrared (IR) regions [3].
Oxide glasses are the most common type for use in manufacturing optical fibers.
The most common is the aforementioned silica, SiO2. A typical glass of this chemistry
will have an index of refraction of 1.458 at 850 nm [8]. To produce two materials with
slightly different indices of refraction for use as the core and cladding of the fiber,
fluorine or various oxides are added to the silica glass. This process has the effect of
changing the index of refraction or optical density of the material for the desired effect.
These materials added to the silica glass are commonly called dopants. Examples of
common dopants are B2O3, GeO2 (perhaps the most common), and P2O5. Some dopants
14
increase the silica glass’s index of refraction while some lower it. In any case, when
manufacturing optical fiber for light transmission, the core material is made to have a
slightly higher index of refraction than the cladding [10]. A very common example is a
GeO2-SiO2 (signifying a germanium doped silica glass) core, with a SiO2 cladding.
2.5 Materials Related Loss Mechanisms
An important consideration in the use of optical fiber as a light guide is the
mechanisms that cause the loss of signal. While light loss is very low in optical silica
fiber, it still remains a critical factor in designing communication and sensor systems [3].
There are three major material related mechanisms inherent in glass material and in the
fabrication of optical fiber that cause light signal loss. These losses are generally due to
intrinsic material losses, absorption losses due to impurities, and scattering losses [3].
There are also many mechanical mechanisms of light signal loss.
Optical fibers are used to transmit light signals as a transparent window in the
visible and near IR regions. Intrinsic material losses are absorption losses due to the
molecular structure and atoms that comprise the glass material itself. Absorption of a
light signal takes place in the UV region when photons of light have enough energy to
excite the electrons of the glass material from the valence to conduction band. In the case
of pure fused silica the oxygen ions have very tightly bound electrons, and an energy gap
of 8.9 eV. The minimum wavelength required for this electron promotion is 140nm,
which represents the location of the UV absorption peak. The absorption tail then
becomes almost negligible at infrared wavelengths. The IR absorption peak is due to
15
molecular vibrations, which contributes very little loss up to 1500 nm. The absorption
tail becomes significant up to the peak of 8000 nm [3,11].
The region of 800 to 1550 nm is ideal for light transmission from the standpoint
of intrinsic losses, but the silica glass in optical fiber may also contain dopants and
occasionally unwanted transition-metal impurities or hydroxl (OH-) ion [12]. Atoms of
transition metal can change the UV absorption curve and add additional absorption peaks
in the visible range due to differing electron excitation energies. Similarly, the OH- ion
causes significant absorption spikes. Concentration of the impurity ion is directly
proportional to the absorption of light at the characteristic wavelength [3].
In addition to absorption, scattering is a major mechanism of light signal loss in
optical fiber. Scattering loss refers to a condition in which a portion of propagating
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.0
0.1
0.2
0.4
0.6
1.0
2.0
0.8
Loss (dB/km)
Wavelength (µm)
Higher Mode Cutoff Band
OH Band
IR Absorption Edge
Rayleigh Scattering
Limit
Figure 2.4 Light loss vs. wavelength (λ) in silica
16
radiation is deflected out of the fiber’s core, or is scattered back (reflected) in the fiber
core. The dominant mechanism of scattering loss, which sets the lower limit of signal
loss, is Rayleigh scattering [2], a basic phenomenon that results from density and
compositional variation in the fiber material itself. These variations in the glass can
occur during manufacturing, when the material passes through the glass transition point
in becoming an amorphous solid [13]. At this transition point are thermal and
compositional fluctuations that become locked into the lattice at the softening point and
are dependent on the material used. In optical fiber the dopants used to control index of
refraction (which have large index variation with concentration) will increase the
Rayleigh scattering. The scattering losses due to material properties are inherent in glass
materials and cannot be eliminated [3].
2.6 Optical Fiber Fabrication
Two distinct steps are commonly cited in manufacturing optical fibers. First, a
slug or preform is made, and then the preform is melted and drawn to form the optical
fiber. Although a number of variations exist, a vapor deposition process is most
commonly used to build up, on a rotating mandrel, glass to form the preform. During this
process highly pure vapors of metal halides (such as SiCl4 and GeCl4) react with oxygen
to form a powder of SiO2 (or SiO2 + dopant). Typical completed chemical reactions are
[3]:
SiCl4 + O2 � SiO2 + 2Cl2
GeCl4 + O2 � GeO2 + 2Cl2
2POCl3 + 3/2O2 � P2O5 + 3Cl2
2BCl3 + 3/2O2 � B2O3 + 3Cl2
17
Dopant materials GeO2 and P2O5 increase the refractive index of silica, and B2O3 lowers
it. Germanium doped silica and pure silica are very commonly used as material in optical
fiber.
There are two major processes used to create preforms – the modified chemical
vapor deposition method (MCVD), or the inside process, and the so-called outside
process. This preform can vary greatly in size but is typically about 60 to 120 cm long,
and about 10 to 25cm in diameter [8]. The final preform is essentially a cylinder within a
cylinder containing the proper material for the core and cladding. Both use vapor
deposition techniques, and are outlined below.
2.6.1 MCVD Process for preform manufacturing
This manufacturing process is used to form successive glassy layers of doped
silica on the inside of a silica tube. First a tube of silica is externally heated to an
appropriate temperature. Next, one of the oxidation reactions described above is allowed
to occur within the silica tube, with a torch or heat source located near one end of the
tube. Within the tube, the oxidation reaction occurs as the reactants are pumped inside.
The product, a very fine particulate glass referred to as soot sticks to the portion of the
glass tube downstream creating a thin porous glassy layer. Any soot that does not stick to
the tube is ejected from the end. The torch or heat source is slowly moved along the
glass tube, zone-sintering the material that has been deposited to a clear glass layer
without any air bubbles or voids. The reaction may be done to deposit pure silica, or a
combination reaction may be performed to create doped silica. For graded index fiber
18
(fiber in which index refraction of the core varies with radius), each layering pass can
have varied concentrations of dopant. In a process such as this, up to 150 passes may be
used [3]. The final step in the MCVD process is to collapse the tube on itself by passing
the heat source along the tube with sufficient temperature to soften the tube and create
higher surface tension. The tube will then collapse into a solid preform rod.
This process offers many advantages. The reactants come in very pure form and
remain pure through the reaction as they are vaporized. They also have higher vapor
pressures than contaminant transition metal chlorides. As the process takes place in a
rather controlled environment within the glass tube, water infiltration can be kept to a
minimum. The presence of contaminant transition metal chlorides and water in the
optical fiber can cause degradation of fiber properties such as signal loss and brittleness.
2.6.2 Outside process for preform manufacturing
The outside process of creating a preform is similar to the MCVD process, except
the layers of glass are deposited on a “bait rod” rather than inside a tube. SiCl4 and the
dopant chlorides are oxidized in a flame to form hot soot stream. The soot stream is
directed at a rotating bait rod, and shifted back and forth. The hot glass particles stick to
the rod in a partially sintered state. The soot stream is passed over the rod numerous
times to form multiple layers of a porous glass preform. As with the inside process, the
type and amount of dopant halides can be changed to form the fiber core with different
index of refraction, or a graded index fiber by using different amounts of dopant in up to
200 passes of the soot stream [8].
19
When the deposition of soot is completed, the porous preform is removed from
the bait rod. The porous glass preform is then zone sintered to remove bubbles/pores.
This is usually accomplished by putting the preform in a furnace hot zone in a controlled
atmosphere such as helium. The preform is now complete, and ready for drawing. The
final preform in this process leaves a small hole in the center from the bait rod, which
disappears upon drawing.
2.6.3 Fiber drawing
The preform, which now contains a high quality transparent glass, can now be
drawn into optical fiber using an optical fiber drawing system. The apparatus, commonly
and collectively named a “draw tower” is depicted in Figure 2.5 [8]. The entire apparatus
Take Up Drum
Proof Tester
Capstan
Curing Oven
Coating Applicator
Diameter Gauge
Feedback
Loop
Furnace
Precision Preform FeedPreform
Figure 2.5 Optical fiber drawing tower
20
typically stands 15 to 25 feet tall. The preform is precision fed into a furnace, which may
be heated with electrical resistance or combustion gas. Here, the preform is heated
sufficiently to cause softening.
To begin the process of drawing fiber from the preform, a small portion of the
lower face of the preform is hand pulled to the take up drum. The take up drum rotates
and pulls the thin fiber from the preform. The diameter of the fiber is dependent on the
rate of rotation of the take up drum [8]. As the diameter of the fiber is critical to splicing
and cabling, a diameter gauge and feedback loop are used in conjunction with the take up
drum to ensure a constant diameter. To eliminate weakening of the fiber to surface
cracks and abrasions, a first coating is applied immediately after drawing the fiber. This
process is possible as the surface-to-volume ratio increases dramatically after the fiber is
drawn from the preform. Optical fiber can be drawn to various dimensions and index of
refraction profiles depending on the drawing setup and composition of the preform.
Table 2.1 lists various fiber specifications [2].
Table 2.1 Characteristics of common optical fiber types
Fiber Type Core Diameter
(µµµµm)
Cladding Diameter
(µµµµm)
Numerical Aperature
Single Mode 4-8 62.5-125 0.10-0.15
Multimode
step index
62.5-200 100-250 0.1-0.3
Multimode
graded index
50-100 125-150 0.1-0.2
21
Figure 2.6 shows the major fiber types and index of refraction profiles. As noted earlier,
the graded index optical fiber has a core that has a varying index of refraction with
radius. This structure limits dispersion induced in transmitted optical signals by speeding
up rays of light that are taking a longer path through the fiber core near the cladding and
keeps a high level of coherency among a large number of signals [12].
Before the fiber is wound on a spool, it can be run through a proof tester, typically
a pair of pulleys, which applies a fixed amount of strain to verify that the optical fiber is
strong enough to be used in cabling. Standard communications fiber will then have
various coatings, jackets, and sheaths to ensure there is no infiltration of significant
moisture, or scratches, abrasions and the like which can significantly embrittle or weaken
the optical fiber.
n
Multimode Single Mode
n1
n2
STEP INDEX GRADED INDEX
cladding
core
buffer
Figure 2.6 Index of refraction profile of typical optical fiber
22
2.7 Fiber Optic Sensors
Successfully manufactured and used for the transmission of data, the early 1980’s
brought a new application for optical fiber: sensors [1]. The groundwork for the use of
optical fiber for sensors came with observed anomalies in light signals sent through fiber
for the purposes of communication. Small bends, changes in the geometrical and
material condition of the fiber, and special characteristics of the light sources and
detectors used with optical fiber all produced signal changes appropriate for use as a
sensor applications [2,14].
The fiber optic communication system has, as its most basic arrangement, a light
source, an active or electro-optic modulator, the optical fiber, and a detector. This also
would comprise the basis of a fiber optic sensor system, except that a passive modulator
or a sort of measurement device replaces the electro-optic modulator. The light source is
most commonly a light emitting diode (LED) or laser diode [2]. The light proceeds
through the fiber under the principles of total internal reflection as previously discussed.
Then the light is collected with a photodiode, CCD array, or other electronic light
detection device. In the case of measuring strain, temperature, pressure etc., the need
arises for a method of modulating the light with the desired physical parameter. A fiber
optic sensor measures its environment through physical mechanisms that modulate the
light signal. This can be achieved through the fiber itself in the case of an intrinsic
sensor, or outside the fiber as in an extrinsic sensor. Various fiber optic sensors can be
organized into various groups by the method in which they measure physical parameters.
There are two basic ways to classify fiber optic sensors, as seen in Table 2.2. One
is by the sensor configuration, (extrinsic or intrinsic), and the other is by the method of
23
light modulation (intensiometric or other non-intensity type). An intrinsic fiber optic
sensor uses the fiber itself as the sensor, whereas an extrinsic sensor uses some sort of
Table 2.2 Classification of fiber optic sensors
Sensor Classification Intrinsic Extrinsic
Intensiometric Microbend, Distributed Sensors, Rayleigh Scattering
Fluorescence, Proximity, Beam Interrupt, Mode
Coupling
Interferometric/ Other Non-Intensity
Measurement
Internal Fabry-Perot, Polarization
Bragg Grating Mach-Zender Interferometer
External Fabry-Perot, Etalons
sensory element, or transducer to change the light signal. This sensor element may be
located anywhere inside or outside the fiber. Extrinsic sensors are usually located in
specific locations, and as such are classified as localized sensors. Intrinsic sensors are
generally classified as distributed sensors, which means that measurements can be made
anywhere along the fiber axis. This is possible as the fiber itself is the transducer.
Categorizing by light modulation, the sensors can be separated as intensiometric
or interferometric (non-intensity) type modulation. A sensor that operates
intensiometrically changes light intensity with the measurand. Interferometric type
sensors change the phase, or modal properties with the physical parameter being
measured [1]. In many of these categories the signal may be read through transmission
(at the end of the optical fiber), or through a reflected signal near the optical source.
Below, these two classifications are further described.
24
Intensiometric sensors allow the light signal propagating through the optical fiber
to be altered in intensity by the physical parameter being measured. As the grid in Table
2.2 indicates, both extrinsic and intrinsic sensors can be used to change light’s intensity
[2]. In an extrinsic sensor, the light energy can be deflected/modulated between the fiber
end and detector by particulates, smoke, gas, and other matter. In an intrinsic sensor the
light energy can be absorbed into the cladding due to bends or twists in the fiber or
mechanical and/or chemical changes in the fiber can cause changes in the waveguide to
couple, disturb, or eliminate modes of propagation [15]. Intrinsic and extrinsic forms of
intensiometric sensors are highlighted in Figure 2.7. In all three cases the measurement is
Source/ Detector Matter
Detector
Detector
cladding
core
perturbation
Source
Source
Figure 2.7 Intensiometric fiber optic sensor examples (a) Beam reflection (b) Wear damage (c) Light loss to cladding
(a)
(b)
(c)
25
being made by altering the intensity of the light propagating through the fiber. In Fig.
2.7(a) an example of an extrinsic intensiometric sensor is shown. Light launched from
the optical fiber is reflected from the matter just outside the optical fiber’s end face.
Other sensors of this type would have a shutter, or mechanical device interrupt or reflect
the beam depending on the application. In any case, the measurement is intensiometric as
the intensity of the light is measured and extrinsic because the sensing element is a
transducer interacting with the optical fiber. In Figure 2.7 (b) an optical fiber which is
being worn by some hypothetical system is shown. This is an example of an
intensiometric sensor that is intrinsic. The fiber is worn through and the intensity of light
decreases – an intrinsic system as the entire fiber itself, with no external transducer, acts
as the sensor. A more sophisticated version of this concept is shown in Figure 2.7(c).
Light is coupled out of the core into the cladding due to external perturbations. These
perturbations cause a disturbance in the condition of total internal reflection and in the
core/cladding interface. The result is a change in intensity of the propagating light signal.
As with the previous example, this sensor is intrinsic. The effect of the perturbation can
be seen regardless of its location along the fiber. The entire sensor acts as the sensor. In
addition to this effect occurring in standard communications grade fiber, this effect can
be taken advantage of in other ways. For example, replacing the cladding with a special
material which may be sensitive to a certain chemical, or creating a transducer that will
bite into the sides of the fiber creating microbends and light loss into the cladding [16].
Intensiometric sensors offer the advantage of low cost and simplicity of operation but in
many cases lack the sensitivity to make high accuracy measurements.
26
Applications, which require measurements including strain, current, and
pressure, require a more sensitive sensor. Non-intensity-based sensors, like
intensiometric sensors, can be both intrinsic or extrinsic depending on the sensor design.
These sensors modulate the phase of the electromagnetic waves propagating within the
optical waveguide. The sensor output will change in a way independent of light intensity
– a trait very useful in applications where light intensity variations may be common. The
most common types of sensors in this classification are interferometers, phase shift and
Bragg grating sensors.
Figure 2.8 shows some examples of the non-intensity-based sensors. In Figure
2.8(a), a Mach-Zehnder interferometer sensor is depicted. Interferometers in general
measure the interference between to different light paths. This can be accomplished in
two ways. One method is to observe the light signal interference between two optical
fibers one measuring the physical parameter in question, and one control, which is
unperturbed. The two paths differ by virtue of the perturbation in one of the fiber legs,
and the phase shift between the two light signals provides the measurement. Because the
measurements are made with the interference of two light signals with a small
wavelength (typically 850 - 1550nm) the level of sensitivity is extremely high. The light
input in 2.8 (a) is coupled to the optical fiber, and split into a test fiber and a reference
fiber. The test fiber encounters a perturbation from which the reference fiber is free. At
the detector, the perturbed and unperturbed signals interfere with each other and a
measurement is taken. As this perturbation can occur at any point along the fiber’s length
it is an intrinsic sensor [1].
27
Another popular non-intensity sensor technology is the cavity or external
interferometer, also called a phase shift interferometer. Seen in Figure 2.8(b), this device
uses an external cavity with a moveable end face (often made of silicon) to create a
variable sized cavity. As this cavity changes size, an interference pattern is formed at the
detector by successive reflections of light from the end face of the fiber, and the fiber-
cavity interface. The interferometers described above comprise both intrinsic
(interferometer) and extrinsic (cavity) type sensors [1,2]. The other most common non-
intensity types of fiber optic sensor use a grating or wavelength specific filter as a sensor.
One of the most common and most promising fiber optic sensors the Bragg grating, a
Figure 2.8 Non-Intensiometric fiber optic sensor examples (a) Mach-Zehnder (b) Cavity (phase shift) Interferometer (c) Bragg grating
Source/ Detector
Source/ Detector Detector
Source
perturb.
Detector
(a)
(b)
(c)
reference fiber
28
unique grating type sensor, is formed within the optical fiber by altering its material
properties. Changing the fiber core’s material properties to create a sensor classifies it as
intrinsic – the fiber core itself is the sensing element. The Bragg grating, seen in Figure
2.8 (c) is an inter-fiber optical filter that is tuned to a specific wavelength [17]. The filter
is comprised of grating lines that have a slightly different index of refraction than the rest
of the fiber core. Through a separation of grating lines and change of characteristic
wavelength, strain along the fiber’s major axis is measured with extremely high
sensitivity and accuracy [2]. When this type of sensor is subjected to transverse loading,
however, the sensor’s behavior is less predictable.
With the above review of optical fiber and optical fiber sensors as a background,
this thesis endeavors to examine the Bragg grating structure when subjected to loading
not in the axial direction. In proposing the use of Bragg grating sensors to characterize
materials, their response to non-uniaxial strains is critical.
29
CHAPTER 3
MANUFACTURING, OPERATION, AND SIGNAL
ANOMALIES OF FIBER BRAGG GRATINGS
3.1 General
Bragg gratings are named after Sir (William) Lawrence Bragg (1890-1971),
Australian-born British physicist and Nobel Prize winner. The work that brought Bragg
fame was based on the phenomenon of X-ray diffraction in crystals. Bragg discovered
that certain planes in a crystal reflect X rays, in accordance with the normal law of
reflection. The distance between parallel planes of atoms determines the angle at which
reflection can take place for a certain wavelength of the X-rays. This relation is known
as Bragg's law, which is based on the constructive interference between signals reflected
from the crystal planes. In a somewhat similar fashion, a Bragg grating fiber optic sensor
acts as an optical filter tuned for a very specific frequency or wavelength. It acts as a
resonator, whose filter tuning mechanisms are based on a 180º phase shift of incoming
signals. Bragg gratings can be used very effectively to measure induced strain from its
surroundings. Moreover, since they are created within an optical fiber’s core and can be
manufactured repeatedly (and individually coded) within a single optical fiber, they
present a powerful tool in assessing material characteristics in situ, in both embedded and
surface mounted conditions, during fabrication or in use [18-22].
These sensors are of great interest in industrial and military applications for
materials characterization and condition monitoring. The first obvious advantage of this
technology is the ability of the FBG to act as an “absolute” strain gauge. This means that
30
the FBG provides an absolute measure of strain, which can be connected to
optoelectronics at any time for real time measurements. This kind of absolute
measurement, not available with standard foil type strain gauges offers many applications
to materials, mechanical and civil engineers. FBGs offer other advantages such as
remote operation, elegant multiplexing, high sensitivity, and imperviousness to EMI [17].
As stated earlier, a greater understanding of the nuances of performance are needed in
order to take advantage of these capabilities.
A fiber Bragg grating is constructed of a periodic modulation of the index of
refraction of the optical fiber core. This portion of the fiber containing the modulation
acts as a selective filter that passes all but a narrow band of light when illuminated by a
broadband optical source, such as an LED. The remaining narrow band of light is
reflected toward the source. The periodicity of the Bragg structure, or the distance
between lines of the region of modulated index, determines the center of this band,
known as the Bragg wavelength. In most fiber Bragg gratings, the length of the modified
section range between 3mm to 1cm. When the fiber itself is stretched or compressed
along its axis, the modified section is subsequently stretched or compressed, inducing
changes in the periodicity of the structure. As a result, a shift in the Bragg wavelength is
induced. Monitoring the center wavelength of the reflected band is therefore equivalent to
monitoring the induced strain in the fiber. Many sensors can be multiplexed with one
source and detector by manufacturing multiple gratings with unique Bragg wavelengths
on a single optical fiber. This chapter details the manufacturing and operation of the
Bragg grating along with the effect of transverse (off-axis) strain on the Bragg grating
strain measurements. The explanation of these effects is the core of this thesis.
31
3.2 Manufacturing
The manufacturing of Bragg gratings was first made possible in 1978 with the
first observation of photosensitivity in germanosilicate fiber [23-26]. In these
experiments, permanent optically induced changes of refractive index in the core of an
optical fiber were made. This was accomplished by producing a standing wave within
the core from coherent radiation at 514nm reflected from the fiber ends (Figure 3.1).
This standing wave produced a periodic refractive index change (a few percent) in the
core along the fiber’s length. This periodic structure is known as an internally written
Bragg grating [27,28,29]. The photochemical change to the core of the fiber material is
attributed to the breaking of the Ge-O-Si bonds, made possible by doping the silica glass
with germania (on the order of 10 mole %). Specifically, it is thought that the oxygen
defects created in this process cause a change in the electronic structure of the dielectric
Gratings
Period, Λ
Standing Wave
nn+∆n
Coherent Reflecting
Light z x
y
Figure 3.1 Internally written Bragg grating
32
[30,31,32]. Using the internal writing technique limits the periodicity of the grating, Λ,
to the region where photosensitivity occurs, UV to approximately 500nm. Since this
restricts the use of the gratings to those wavelengths, a method was developed to write
the grating externally with an interference pattern of two UV sources (Figure 3.2) [33].
By changing the angle between two sources, θ, the grating can be written with varying
values of Λ (Figure 3.2). The periodicity is related to the wavelength of light and θ by:
θλ
sin2=Λ (3.1)
The result is a grating structure that can be used at wavelengths where optical fiber light
loss is minimized (see Figure 2.4) and where most fiber optic communication hardware
operates. Side-written gratings are typically manufactured after stripping the jacket and
coatings from the core and cladding or written as an integral part of the draw tower
(Figure 2.5) before any additional layers are applied. In addition, a phase mask is often
used in sharpening the periodic structure [34,35]. Some manufacturers use a coherent
light source and a phase mask to create gratings. The mask creates the required
interference pattern. Initial experiments used lasers in the mW range for periods as long
as an hour. Manufacturing techniques have been improved to use powerful UV source
and create grating in as little as a few seconds [36].
33
Figure 3.2 Side-written Bragg grating
The ability to manufacture a Bragg grating is significant. In essence, the filter
created is an intrinsic strain sensor with a discrete gauge length located right in the core
of an optical fiber. This process allows for the core itself, and not an external transducer
to detect strain. Even more significant is that a periodic structure is created through the
changing of material properties. Through the use of coherent radiation, chemical bonds
are selectively broken, electronic configurations are changed, and the index of refraction
of portion of a dielectric waveguide is altered. This is an extremely elegant method of
manufacturing an optical strain gauge or strain sensors.
Ge-doped Core
Cladding
Gratings Period, Λ
Interference Pattern
Coherent UV Beams
PhaseMask
nn+∆n
θ
34
3.3 Operation
Now that a periodic structure of index of refraction has been manufactured into
the core of the optical fiber, the Bragg structure can be employed in sensor applications.
As described in section 2.2, when a light signal encounters a material of higher index of
refraction, some of the light signal is reflected, some is refracted, and some is transmitted.
The Bragg grating allows a portion of a light signal propagating through an optical fiber
to be reflected. Light is reflected in a Bragg grating when the light satisfies the Bragg
condition. This occurs when the wavelength (known as the Bragg wavelength) is
Λ= effb n2λ (3.2)
where neff is the effective index of refraction (an average of the slightly higher index of
the grating and the lower index of the regular fiber core). This condition allows a 180º
phase shift of the light signal at the Bragg wavelength. The amount of light reflected at
that wavelength is dependent on the efficiency, which is based on index of refraction
change or ∆n, of the gratings, and also the total grating length, as each grating line serves
to reflect a portion of the light signal [37,38,39]. This effect is seen in Figure 3.3. Light
from an LED is launched into the fiber, a broadband source whose center wavelength is
close to the Bragg wavelength. The light propagates through the grating, and a portion of
the signal is reflected at the Bragg wavelength. The complimentary portion of the
process shows a small sliver of signal removed from the total transmitted signal. This
clearly shows the Bragg grating to be an effective optical filter. With today’s
manufacturing techniques, 98% and greater reflection is commonly found at the Bragg
35
wavelength. A small percentage broadband absorption and instrumentation tolerances are
also present. The reflected signals have a wavelength spread of up to a few tenths of a
nanometer, or about 0.1nm in most cases.
As defined in Equation 3.2, the Bragg wavelength is directly proportional to the
spacing of the grating lines Λ. Therefore, any change in the grating spacing will cause a
direct change in the Bragg wavelength λb. This is the basis for the use of a Bragg grating
as a strain sensor [40,41,42].
A change in dimension of the fiber core along its major axis causes changes in the
spacing of the grating lines. This change is usually due to thermal expansion, or
mechanically induced strain. As the fiber core is stretched or compressed, there is a
corresponding increase or decrease in the value of Λ, and therefore, a corresponding
change in the value of λb. By tracking the changes of the Bragg wavelength, the
elongation or compression of the fiber core can be measured [39,43]. Under constant
λ λ λλb λb
LED Source Input
Reflected Signal
Transmitted Signal
Input
Reflected Transmitted
Figure 3.3 Reflected and transmitted Bragg grating signal with LED source
36
pressure a direct measurement of strain in the direction of the fiber’s axis is possible by
measuring ∆λb. Since a Bragg grating has a well-defined λb, a number of sensors can be
put on the same fiber core (Figure 3.5). The gratings can therefore be uniquely identified
regardless of the distance between them along an optical fiber. This would allow for a
number of strain measurements along one fiber, with one light source and detector. This
arrangement maximizes the multiplexing potential of Bragg gratings, and keeps system
cost to a minimum. The reflecting wavelength of a Bragg grating will shift linearly with
temperature (expansion and contraction) and with strain. For simplicity, most stresses are
considered just in direction of the fiber’s major axis, as early experiments showed that
response in this direction was by far the most sensitive, and that effects in other directions
were at most two orders of magnitude less.
λ λb
Reflected Signal
(No strain)
λλb
Reflected Signal
(Tensile Strain)
λλb
Reflected Signal
(Compressive Strain)
Tensile
Compressive
Figure 3.4 Bragg wavelength shift with change in Λ
37
3.4 Observed Signal Anomalies
The tracking of strain with the changing of λb as described before, is made simple
through basic assumptions about the measurement system. In looking at the strain
condition purely, it is first necessary to remove temperature effects (thermal expansion
and contraction). Any analysis of complex strain effects on the signal necessitates this
step. The other major assumption made is that axial strain, and perhaps a small Poisson
effect, is the only strain reflected in the shift of λb. In addition, it is assumed that the
fiber core is perfectly cylindrical, and that the material properties in all directions are
homogenous. In some applications, these assumptions are acceptable. With the proposed
use of fiber optic Bragg gratings as an embedded sensor used to characterize materials,
however, it is important to observe the sensor’s response to mechanical perturbations in
all directions.
λ λλb1
λb1 λb2 λb3 λb4
λb2 λb3 λb4
λλb1 λb2 λb3 λb4
LED Source Input
ReflectedSignal
Transmitted Signal
Input
Reflected Transmitted
Fiber Core
Figure 3.5 Multiplexing of Bragg grating sensors
38
While experiencing transverse load, orthogonal to the fiber core’s axis, anomalies
have been observed in the Bragg grating signal, as seen in Figure 3.6. The conceptual
Bragg reflection at the top is depicted in actual signal traces below, before and after
transverse loading. The laboratory experiment design, data acquisition and analysis of
this anomaly is the crux of this study. A portion of the total transmitted signal, as seen at
the top of the figure, is examined with an optical spectrum analyzer. The trace on the left
is the transmitted unperturbed Bragg grating signal, and the trace to the right is the signal
λλb
Figure 3.6 Bragg grating signal under transverse strain
39
with a transverse load applied to the sensor. The Bragg grating signal has broadened,
shifted to a higher wavelength, and has started to form multiple distinct peaks. Poisson
effects can easily explain the general shift to the right: as the fiber is compressed from the
side, it expands in the direction along its major axis, causing the same effect as
elongating the fiber. In addition to shifting, the signal has broadened, and split into two
well-defined components. It is evident that the optical properties of the waveguide have
changed, as have the material properties of the Bragg structure to cause this unusual
signal [44]. It is also evident that additional explanation is needed to properly infer strain
measurement from such a signal.
3.5 Sources Of Bragg Grating Signal Anomalies
The signal anomaly seen in Figure 3.6 was observed in various forms by
introducing the fiber Bragg grating to transverse loading (see Chapter 4). It is obvious
that in this case some sort of bifurcation is taking place, which is splitting the optical
signal in two. Examining the signal on the right in Figure 3.6, it is clear that the signal
shows two predominant waveforms with two predominant wavelengths. In some fashion,
it can be assumed that the material properties of the silica waveguide are being altered as
to allow reflection at more than one dominant wavelength. While the literature and
previous experiments touch on this subject, the studies are not clear.
Common in descriptions of the above effect is the concept of birefringence. By
definition, birefringence is the condition where two orthogonal components of the optical
fiber cross section have different indices of refraction. In practice, this relation to
transverse load is explained next.
40
In looking at the Bragg grating condition (Equation 3.2), a temperature change of
∆T and a strain of ε would result in a wavelength shift of the Bragg condition (∆λb/λb)
given by [41]:
Tpppn
zxyeff
zb
b ∆++++−=∆ )(])([2 121211
2
ζαεεελλ (3.3)
Where α is the thermal expansion coefficient for the fiber, ζ is the thermoptic coefficient
or dn/dT of the doped silica core material, εz is strain in the axial direction, εxy is strain in
the radial direction and p is the photoelastic constant which relates changes in n with
strain [37]. This is sometimes called the strain-optic coefficient, and it relates the
concepts of photoelasticty to the change in Bragg wavelength.
By relating the basic equations of three-dimensional mechanics with equation 3.3,
a number of important equations can be devised to generically describe the mechanics of
the system. Ignoring temperature changes to concentrate on stress related concepts, the
change in Bragg wavelength can be described as [45-48] where F is the load on the fiber
dFF
nFn
d tete cTb
effcTeff
bb ])(2)(2[ 0,0, == ∂Λ∂+
∂∂
Λ=λ (3.4)
Introducing n as a function of direction, both parallel and perpendicular to a side load to
the fiber cross-section, birefringence, B can be defined as:
0,0
0,
parallel
eff
xy
eff
perp
nnn
Bn
nnB
∆−∆+=
−= (3.5)
41
where B0 is any inherent birefringence present before loading, and neff,0 is the initial
effective index of refraction before loading. These equations lead to important opto-
mechanical relations that can provide the change in index of refraction and the resultant
change in Bragg wavelength, due to loading in all three axes. These relations are
described generically as [46,48]:
zyxzyxzyxeffzyxeff Epnfn ,,,,,,0,,, ),,,,()( νσ=∆ (3.6)
zyxzyxzyxeffbzyxb Epnf ,,,,,,0,0,,, ),,,,,()( νσλ Λ=∆ (3.7)
where σ is stress, E is Young’s modulus, and ν is Poisson’s ratio. Skipping the full set of
mechanical equations, the key feature is the ability to relate mechanically induced
changes in index of refraction to changes in the Bragg condition. The ultimate goal is to
mathematically predict the changing Bragg signal under transverse loading, xy
b
dF
dλ. While
presenting a transverse load to the optical fiber the cross section becomes elliptical,
becoming compressed in the direction of force, and being out in tension in the orthogonal
direction [33]. In the compressed direction, the index of refraction will increase and in
the orthogonal direction the index of refraction will decrease. The compressed direction
is commonly called the “slow” axis, referring to the decrease in light speed along the path
with higher n, and vice versa. Under these concepts the bifurcation of the Bragg grating
signal is due to these two indices of refraction (the maximum and minimum found in any
particular situation) that can satisfy the Bragg condition. Experiments conducted under
42
this thesis, showing more than two distinct peaks (see Chapter 4) in a fiber Bragg grating
under purely transverse load, inspired a new look into this phenomena.
There are two common ways to create birefringence in Bragg gratings. One is the
application of transverse load. Another is polarization maintaining optical fiber (useful in
telecommunications applications), which creates a condition of birefringence in the fiber
core during the manufacturing process [49-53]. In both cases the data show two distinct
peaks forming from the single peak predicted by the models, as seen in Figure 3.7.
Where the cross section of the FBG has a distribution of values of index of refraction, the
maximum and minimum values are observed to satisfy the Bragg condition and result in a
double peak signal. In some cases, bending of the fiber, which can cause unequal
grating spacing or other mechanical effects, has been seen to create multiple peaks (2+)
from a single Bragg grating signal [46]. Experiments during this work (see Chapter 4),
however, were able to produce 2+ peaks with purely transverse strain.
λ
λb1 λb2
Figure 3.7 Birefringence effect on Bragg grating conditions and reflected signal
43
To develop a clearer understanding of effects seen when transverse strain is
introduced to a Bragg grating, a relationship must be formed between the Bragg
condition, the material properties and structure of the FBG, and the ability to observe
these effects.
As described above, the Bragg condition, when satisfied, is what causes a
reflection of the light signal at a particular wavelength. This condition ( Λ= effb n2λ ) can
be theoretically satisfied for various values of n and periodicity of the grating Λ. It has
been established that in most cases, any splitting of the Bragg grating signal results in two
peaks – ostensibly one for each birefringent index of refraction found in the core
[54,55,56]. Focus is normally given to the fact the index of refraction change is made in
two extremes, the maximum and the minimum as defines by birefringence, and that there
are usually two orthogonal components of electromagnetic fields propagating through a
single mode fiber core. This idea limits discussion of the presence of more than one
Bragg condition to a split into two. The observation or discussion of multiple peaks (2+)
xz
y
n0
n0
nmin
nmax
n(θ)
Figure 3.8 Transverse loading and effect on index of refraction
Load
44
is usually explained away by considering chirping effects or bending which would cause
Λ to change within a single grating. Theoretically, however, wherever the Bragg
condition is satisfied, through neff or Λ, a reflection will occur.
In describing the theoretical operation of a Bragg grating, a very specific set of
material conditions is assumed within the fiber core. This ideal case description will
provide the basis for analysis. Described is a perfect cylinder of silica, doped with Ge.
There are no residual stresses from manufacturing, and the density, molecular
composition, and index of refraction are all homogenous throughout. After
manufacturing the grating, there would be a periodic slight change in the refractive index
in the z-direction. However, it is assumed that the refractive index is uniform across the
core cross-section at any location in the z-direction. The case in question is transverse
load. In this case, the core cross section would be deformed into an elliptical shape, with
a compressive force on the short axis, and vice versa. While there is indeed a condition
of birefringence, different indices of refraction along two orthogonal components, it is
important to note that there is a gradual change in values of n between the maximum and
the minimum (Figure 3.8). In this case, if linearly polarized light were launched through
the grating along the direction of the birefringent axes, then only an analysis along these
two axes would be necessary – but this is not the case in the general condition, with non-
polarized light used in the experiment. The index of refraction, as a function of
photoelastic effects of the fiber being loaded transversely, causes a continuous change of
values of n to be present throughout the cross section of the Bragg structure. This means,
theoretically, that a number of solutions to the Bragg condition might exist.
45
If, as explained above, there is a continuous change of indices of refraction in the
FBG cross-section, and Poisson effects in the z-direction, it is possible that a number of
different solutions to the Bragg condition would exist, causing numbers of peaks in the
Bragg signal. In most cases these peaks may not be observed. Why? The answer lies in
the level or amplitude of the physical and material parameters discussed above, as well as
the resolution of the devices used to observe the Bragg signal. In the manufacturing of a
Bragg grating, an increase of index of refraction of only a few percent in each line of the
grating is enough to satisfy the Bragg condition at a particular wavelength. When
introducing a light transverse load the index of refraction is changed though photoelastic
effects, and the grating spacing is changed through Poisson’s effect. This initially causes
weak birefringence and a broadening of the reflected spectrum may exist. As the level of
loading increases and the birefringence increases as well two peaks may appear,
depending on the resolution of the spectrum analyzer. As the loading and birefringence
is increased further, two solutions to the Bragg condition become much clearer,
corresponding to the maximum and minimum index of refraction. With additional
loading and commensurate high level birefringence (continued pure transverse loading)
and particularly near the yield point of the fiber core, additional solutions (more than two
peaks) will become evident as the Bragg condition (2neffΛ) can be satisfied for more than
two wavelengths in the broadband light spectrum. The data show (see Chapter 4) that
after the initial bifurcation of the Bragg signal, a sufficient amount of additional loading
will cause additional solutions to become evident, as described above. This is not
thought to be due to chirping, unequal loading, or bending of the fiber, but most possibly
additional solutions to the Bragg condition.
46
CHAPTER 4
EXPERIMENTAL INVESTIGATION AND
ANALYSIS OF FIBER BRAGG GRATINGS
4.1 General
The goal of the laboratory experiments in this thesis is to observe anomalies in the
signal of a Bragg grating strain sensor when subjected to transverse strain as described in
the previous chapter. The proposition has been made that the bifurcation found in Bragg
grating signals under transverse loading is due to birefringence, or the presence of
different indices of refraction within the fiber core due to photoelastic effects. In addition
it has been stated that additional peaks are due to the manifestation of additional solutions
to the Bragg condition becoming evident with sufficient changes in values of n and/or Λ.
Experimentally, these concepts were to be proven, and comparisons made to the
mathematical relations presented in Chapter 3.
A number of separate experiments were required to observe the effect of induced
photoelastic effects in the optical fiber core due to transverse loading on a Bragg grating
signal. Tests were first conducted to validate the experimental setup & evaluate losses
through various connectors and hardware. Next, an experiment was conducted to
establish a baseline for measuring birefringence, or stress induced photoelastic effects by
analyzing an unstressed Bragg grating. Lastly, tests were conducted to analyze the Bragg
grating under transverse loading. The data for all of the aforementioned experiments was
collected, logged and analyzed. Chapter 4 will deal with the evaluation of the unloaded
47
Bragg grating and the establishment of an accurate baseline. The behavior of the Bragg
grating under transverse loading is presented in Chapter 5,.
4.2 Experimental Procedure
Before proceeding with examining the unloaded Bragg grating a method of
testing was devised. The purpose of this first group of tests had two major components.
First, it was necessary to design and construct a test setup, and evaluate any potential
problems, signal losses through connectorization and the like. Second, the process of
evaluating the propagation of the light signal through the unloaded Bragg structure would
be accomplished. This was designed specifically to address assumptions dealing with the
Bragg structure. Specifically these assumptions purport a homogenous nature of the
manufactured Bragg structure. By accurately examining the unloaded Bragg, there
would be a suitable basis to compare transverse loading data.
Previous work in this area assumed a homogenous nature of the Bragg structure.
Thorough study of the Bragg structure requires an analysis of the FBG in its unstressed
state. At this point an experimental procedure was developed that would most effectively
test the FBG. Pursuant to the goals of the thesis, measurements of λb and n throughout
the cross-section of the FBG were needed. The test procedure would have to measure
these two parameters in the most accurate way possible, and in as much of the FBG’s
cross section as possible. Two tests were conceived and are described below. Both
involve using polarized light as a source, but the light signal is analyzed two ways, one
with a polarization analyzer, and one with an optical spectrum analyzer (OSA).
48
4.3 Polarization Effect on FBG Signal
As stated above the polarization effect on FBG signals was chosen as an analysis
method to detect material changes in the core. This analysis method was chosen as the
presence of birefringence has a specific effect on polarized light. If linearly polarized
light is introduced into the FBG, and no birefringence exists, then the light will remain
linearly polarized. If, however, birefringence is present, the state of polarization will
change, as seen in Figure 4.1. The light input is linearly polarized light. As the light
propagates through the fiber, its state of polarization changes. Photoelastic effects
induced from loads on the fiber have created changing values of n within the fiber core.
When the silica is stressed it becomes anisotropic, and the state of polarization changes as
the light refracts through the stressed fiber core. With this in mind, it is possible to use
polarized light to detect any presence of birefringence in the fiber core. Polarized light
launched into the FBG can be analyzed at the output with another polarizer through 360º.
If any birefringence exists in the FBG, the polarization state will change, and the
L Beat Length
φ(z)=0
φ =2π
φ = πφ = 3π/2
φ = π/2
Figure 4.1 Changing polarization with birefringence
49
amplitude and phase of the light at the fiber output will change as well. By using various
linear polarized light input angles, the FBG cross section can be examined.
4.4 Characterization of FBG Using Polarized Light
The first tests conducted were to measure variations in n throughout the fiber
cross-section. To accomplish this measurement a test setup was designed using a
polarized light source. By rotating the direction of polarization of the input light, and
analyzing the output through 360º, the light signal throughout the core could be analyzed.
The fiber Bragg gratings used were standard communications grade single mode optical
fibers with λb ~1300nm. The experimental setup consists of an edge light emitting diode
(ELED) with a center wavelength of 1300nm, an in-line single mode polarizer, a rotating
polarizer plate (analyzer), an optical spectrum analyzer, and related fiber chucks, holders,
connectors and hardware. The overall setup of the unloaded experiment is depicted in
Figure 4.2. The first experiment was conducted to establish a baseline of characteristics
LED
Polarizer (in-line)
Grating
Photodetector/OSA
Polarizer (plate)
Connector Connector
Figure 4.2 Experimental setup for characterization of unloaded FBG
50
of the Bragg grating sensor as obtained from the manufacturer. As stated earlier, it is
assumed that the Bragg grating has no inherent birefringence, or photoelastic induced
variations in index of refraction. Before proceeding with this assumption, the first
experiment was conducted to prove that n is sufficiently homogenous throughout the
fiber core cross section. This was accomplished by using the test setup seen in Figure
4.2. The goal was to launch linearly polarized light through the Bragg grating at various
angles, and by analyzing the result with a rotating plate (free-space) polarizer, observe
any changes in light propagation due to the rotation of the input polarized light.
The setup was assembled one section at a time to measure light loss at each of the
sections. The ELED was rated as delivering 15µW into single mode fiber with an ST
connection. For flexibility in rotation and interchangeability of fibers, a bayonet type
connector (Figure 4.3) was used. Adjusting the end of the fiber through the focus area of
the ELED assembly, which contained a focusing micro-lens, allowed for light power
Figure 4.3 Adjustable bayonet-type optical fiber connector
51
readings up to 19µW. A total transmission loss of about 3dB was measured due to the
experimental setup components. These losses are due to losses in the in-line polarizer,
the splice between the in-line polarizer and the Bragg grating, the Bragg grating itself,
and the free-space loss around the plate polarizer. After taking measures to improve
coupling and reduce losses in the experimental setup, 9.8mW was the highest
transmission power possible.
After assessing losses, the next step was to determine if any birefringence or
directional dependent index of refraction was present in the FBG cross section as
received from the manufacturer. Linearly polarized light was launched into the Bragg
grating through the in-line fiber optic polarizer. This polarization device is a metallic
tube, pigtailed on each end with polarization maintaining (PM) single mode fiber.
Unpolarized light launched through one end will be linearly polarized through the device,
then be maintained as linearly polarized through the PM fiber through the other end. This
linearly polarized light is coupled to the fiber Bragg grating. The Bragg gratings are
located in the center of approximately 1m length of a single mode optical fiber. The light
was then launched from the fiber’s endface through a quarter-wave rotating plate
polarizer (sometimes referred to as an analyzer) to a photodetector, where the light was
collected and intensity measured. The polarizer was rotated through 360° in 30°
intervals.
An example of data collected at certain angles of polarization is shown in Figure
4.4. As the rotating analyzer interacts with the linearly polarized light coming through
the Bragg grating, the classic sine wave pattern is seen. When the plate polarizer is
rotated 90° out of phase with the polarized light coming through the grating, the intensity
52
drops to nearly zero. This means that the component of the electric field in the
perpendicular direction to the polarized light is almost zero. After repeating and
verifying this technique, the procedure was repeated for various polarized input angles
and again analyzed through 360° in 30° intervals. The input polarization changes were
made by rotating the FBG fiber. After collecting this data, it was observed that the peaks
of maximum intensity shifted along the plate polarization axis to correspond with the
rotation of the polarized light input. This is shown in Figure 4.5. Normalizing these
signals and overlapping them could provide a direct comparison of the light propagation
conditions for various input polarization conditions. In addition, any directional
dependence of n, or light characteristics could be detected through observed changes in
the light intensity. As shown in Figure 4.6 the linearly polarized light propagating at five
different input angles all had very similar characteristics after being analyzed. No
Intensity vs. Freespace Polarizer Angle
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0 90 180 270 360
Freespace Polarizer Angle (degrees)
Inte
nsity
(mic
row
atts
)
Figure 4.4 Initial data of polarization baseline study
53
significant change in light intensity is seen in the signals for the five input polarization
conditions.
4.5 Wavelength Analysis
In the second set of experiments, for the characterization of the unloaded Bragg
fiber, polarized input light in the Bragg grating was rotated while observing the signal
output with an optical spectrum analyzer (OSA). This device measures light intensity vs.
wavelength, which is a very clear method to examine the FBG signal (Chapter 3). The
experiment was assembled as seen in Figure 4.1. Instead of using a rotating plate
polarizer, the FBG was connected directly into the OSA. The goal of this experiment
was to examine the changes in the Bragg wavelength for various input polarized light
Intensity vs. Analyzer AngleFor Five Different Angles of Polarized Light Input
0.000.501.001.502.002.503.003.504.004.50
0 90 180 270 360
Analyzer Angle (degrees)
Inte
nsity
(mic
row
atts
)
072144216288
Figure 4.5 Intensity vs. analyzer angle for various input polarizations
Input Plane Of Polarization (degrees)
54
through the OSA, if any. The OSA used has a resolution of 0.1nm, and as such, allows a
rather close examination of the FBG signal. As with the previous experiment, the
polarized light input was rotated at the splice between the in-line polarizer and the FBG.
The data were then collected in the OSA and compared. A representative trial is shown
in Figure 4.7. As the input polarization was rotated, almost no change in the FBG signal
was observed. In this case, broadening or shifting of the FBG signal may have indicated
birefringence or a photoelastic effect from the manufacturing of the FBG.
Following these evaluations, it was determined that there was no (or negligible)
inherent birefringence, or directionally dependent index of refraction to be detected at the
sensor output. These would put the value of these effects well below the level of
Intensity vs. Analyzer AngleFor Five Different Angles
of Polarized Light Input (overlapped)
0.000.501.001.502.002.503.003.504.004.50
Analyzer Angle (degrees)
Inte
nsity
(mic
row
atts
)
072144216288
Figure 4.6 Intensity vs. analyzer angle for various input polarizations, overlapped
Input Plane Of Polarization (degrees)
55
significance related to transverse loading experiments. This provides an accurate basis
for the loading experiments found in Chapter 5.
Figure 4.7 Intensity vs. wavelength for four input polarizations
Input Polarization = 0º Input Polarization = 72º
Input Polarization = 144º Input Polarization = 216º
56
CHAPTER 5
EXPERIMENTAL ANALYSIS OF FIBER BRAGG GRATING
RESPONSE TO TRANSVERSE LOAD
5.1 General
In Chapter 4 it was determined experimentally that any birefringence or
photoelastic effects in the FBG upon manufacturing, and as received was minimal. This
information provides a basis and initial conditions for examining the FBG under
transverse loading. Previously observed anomalies in the FBG structure, as manifested
by a bifurcation in the light signal (in wavelength), initiated this study as to investigating
their cause. In this chapter, results on laboratory testing of FBG structures under
transverse loading are presented and discussed. The goal was to replicate the bifurcation
found in previous experiments (discussed in Chapter 3), and gain greater insight as to
their causes.
To properly evaluate the effect of transverse loads on the FBG structure, a similar
approach was taken in these studies as were in Chapter 4. Both a polarization analysis, as
well as analysis based on the OSA measurements would be conducted. The goal was to
incrementally load the FBG transversely and record the light signal for analysis.
5.2 Characterization of FBG Under Transverse Load Using Polarized Light
A polarization analysis similar to the one used in Chapter 4 was conducted to
examine the light propagation through the FBG during transverse loading. In the
previous baseline study, the goal was to launch polarized light into the fiber at various
57
angles, and examine the FBG output through a rotating plate polarizer. In this
experiment, the more important measurement was observing the change in the FBG
signal under loading. It is valuable to analyze the FBG signal through 360° to observe
light propagation changes in the cross section of the FBG – indicating changes in nΛ as
stated earlier. The general test setup is shown in Figure 5.1. A goal in the experimental
setup was to devise a method to introduce purely transverse loads to the optical fiber. In
order to measure the effects of transverse loading alone a special mechanical device was
designed and fabricated. It consists of two parallel metal blocks fixed to a base and a
cantilever to which weight could be added (Figure 5.2). Spacer fibers were used on
either side of the test fiber to further ensure that the loading to the FBG was purely
transverse. Small weights, providing approximately 0.5N each were added to the
cantilever to provide incremental loading up to 6.5N, for the polarization analysis. The
same mechanical device was used for the detection of the shift in the Bragg wavelength
under transverse loading using the OSA, explained in section 5.3.
Load
LED
Polarizer(in-line)
Grating
Photodetector/OSA
Polarizer (plate-
free rotate)
Connector Connector
Figure 5.1 Experimental setup used in transverse load tests
58
After assembling the polarization analysis setup, seen in Figure 5.3, the data were
taken for the unloaded fiber, incrementally loaded fiber, and then an unloaded fiber again
to verify a return to zero. This is important to identify any possible plastic deformation
or fiber damage that occurred during the loading tests. The data taken was light intensity
vs. angle of the rotating polarizer. This was accomplished by rotating the plate polarizer
through 360° in 30° increments and recording the light intensity as recorded by the
photodetector. This process was repeated for the FBG under each loading condition,
adding one weight at a time. The results are seen in Figure 5.4, which shows a typical set
of data. Throughout these test the incident light at the FBG input is held constant in
polarization and intensity. The direction of the input polarized light was fixed and
parallel to the direction of the applied load. Note the characteristic sine waves as seen in
the experiments in Chapter 4 (Figures 4.4 & 4.5). Also note that with the application of
transverse loading, both the phase and amplitude of the light signal through the FBG
changes. In addition, data were taken where the angle of the analyzer was held constant
and the intensity data were taken with respect to the applied load (Figure 5.5). In both
Load
Spacer Fiber(s)
Test Fiber
Figure 5.2 The designed mechanical device for transverse loading
59
Trial IIISignal Intensity vs. Plate Polarizer Anglefor Various Transverse Load Conditions
0.001.00
2.003.00
4.005.00
6.007.00
8.009.00
10.00
0 90 180 270 360Plate Polarizer Angle (degrees)
Sign
al In
tens
ity (m
icro
wat
ts) No Load
0.48 N1.068 N1.656 N2.244 N2.832 N3.420 N4.008 N4.596 N5.184 N5.772 N6.360 N
cases, the oscillatory nature of the effect transverse load has on the light propagation is
present.
Figure 5.3 Experimental setup for polarization analysis of FBG under transverse loading
Figure 5.4 Polarization analysis data
60
In the unloaded original case, linearly polarized light shows a maximum when it
is parallel and minimum when it is perpendicular with the plate polarizer. With
transverse loading, some of the linearly polarized light is coupled to an orthogonal
component, shifting some of the light power 90º out of phase with the previous
maximum. This can not happen unless the direction of the linearly polarized light has
been changed within the FBG structure as a result of transverse loading. This results in a
“flattened” signal, where the former maximum has decreased in intensity, and the former
minimum (which was near zero) has increased in intensity. The modification of light
amplitude through 360º continues to change with increased transverse loading as intensity
will reach a maximum and then return to the original condition found when the fiber
Load vs. Signal Intensityat Various Plane Polarizer Angles
0.001.002.003.004.005.006.007.008.009.00
10.00
0 1 2 3 4 5 6Load (N)
Sign
al In
tens
ity (m
icro
wat
ts)
28032534612
Plane Polarizer Angle (deg)
Figure 5.5 Polarization analysis data, load vs. intensity
Analyzer Angle
(degrees)
61
experienced no loading. Most importantly, the data above show the state of light
propagation is continuously changing within the cross section of the FBG. The applied
transverse loads control these changes. The correlation between the polarized light input
and the analyzer output are no longer having a fixed relation. Photoelastic effects are
changing the light signal in the core of the FBG, and are therefore necessary for proper
analysis of the sensor output.
5.3 Wavelength Analysis
To observe the effect of transverse load on the Bragg structure from a wavelength
point of view (i.e. Bragg conditions), an experimental setup identical to the one
mentioned in section 5.2 was used, except the rotating polarizer and photodetector were
Figure 5.6 Experimental setup for wavelength analysis
62
replaced with an OSA. The setup can be seen in Figure 5.6. The fiber was loaded, as in
the previous case, using the developed mechanical transverse loading device. Weights
were added to the cantilever to introduce the load to the block that rested on the Bragg
grating, and two spacer fibers. The spacer fibers were used to maintain pure transverse
loading on the FBG under test. In addition, care was taken to ensure that the FBG was
under uniform loading. Weights were carefully added to the transverse loading rig, up to
approximately 45N, and data were collected. Since the interface area between the fibers
and the two parallel metallic plates of the loading device is very small, it was expected
that when 45N was applied the fiber would be under a very high level of stress. Loads in
excess of 50N were found to cause crushing and fiber breakage in some initial trials.
Weights were added then removed to examine if the FBG signal returned to the zero load
condition. This procedure was repeated for four Bragg grating sensors. Representative
results are shown in Figure 5.7. These results display a number of interesting traits,
keeping in mind that the FBG is acting as an optical filter for all wavelengths that can
satisfy the Bragg conditions.
Unpolarized light from the ELED was launched through the FBG to the OSA. In
Figure 5.7(a) there is no transverse loading introduced to the FBG, and a single clear
peak is seen. In Figures 5.7(b) and 5.7(c) the transverse load was increased to 4.75N and
9.25N respectively. The effect of transverse loading is visible at this point. The
transmission peaks have has broadened compared to that seen in Figure 5.7(a), and has
also shifted to a higher center wavelength. As presented in Chapter 3, the transverse
loading will induce tension in the FBG due to Poisson’s effect, i.e. increasing Λ, and
therefore shifting the signal to the right. The signal broadening is due to the induced
63
Figure 5.7 Wavelength analysis data from OSA
(a) (b)
(c) (d)
(e) (f)
(g)
64
changes in the index of refraction and birefringence. As more load was added the signal
broadening increased to reveal two discernable Bragg signals. This is seen in Figures
5.7(d) and 5.7(e) where the load was increased to 13.75N and 18.25N respectively. This
is referred to as bifurcation, and is the sign that birefringence, or two values of neff, exists
in the Bragg grating. With increasing transverse load, the two signal peaks, seen in
Figures 5.7(d) and 5.7(e) separate further, and become very clear. Here, the Bragg
grating is displaying two clear solutions to the Bragg condition. As the applied load
increased through 20N, however, an additional peak appeared at a slightly higher
wavelength. Figure 5.7(f) shows this effect at a transverse load of 27.25N. As with the
appearance of the second peak, the third peak appears and shifts to higher wavelength
with increasing load. As with the previous peaks, it appears to be a a well-defined FBG
signal as it occurs at a specific wavelength. This process can be seen in Figures 5.7(d)
through 5.7(g). At very high loads, some FBGs tested displayed a fourth peak in the
same manner. In four trials, all FBGs displayed three or four peaks. This experimental
data, along with the data in Chapter 4, and the background information in Chapter 3,
provides the necessary information with which to draw conclusions as to the effect of
transverse loading on the FBG signal, presented in Chapter 6.
65
CHAPTER 6
CONCLUSIONS AND RECOMMENDATION FOR FUTURE WORK
6.1 Transverse Load Effects on FBG Measurements
In Chapters 1 and 2, a thorough review of optical fiber, and optical fiber sensors
was presented. In Chapter 3, specific details relating to the structure and operation of the
FBG were presented. It was established that while axial loading will present an expected
response from the FBG, transverse loading creates specific anomalies. Suggestions as to
the cause of these anomalies, usually in the form of broadening or splitting of the FBG
signal, were presented. In Chapters 4 and 5, laboratory experiments were conducted to
evaluate the FBG after manufacturing, and under transverse loading.
The results of Chapter 4 are significant. The homogenous nature of the Bragg
structure as received, at least to the level of accuracy possible with the test equipment,
was confirmed through experiments. Two experimental procedures were used. The first
test used polarized light at various input angles and a rotating plate polarizer (analyzer) at
the output to observe any inherent birefringence. Simply stated, the data from the test
would show inherent photoelastic effects if the data from various input polarization
angles did not match in phase or amplitude. Why? The assumption made in this case is
that the optical fiber, even after the manufacturing of the FBG, is homogenous in the
cross section. This indicates that the fiber core is cylindrical, and that the index of
refraction is constant across the fiber cross section. If this were not the case, as polarized
light was launched into the fiber, the output would show a change in the Bragg
66
wavelength and light intensity as the angle of incidence of the polarized light is changed.
As shown in Chapter 4, this test indicated that there is no birefringence inherent to the
manufacturing of the FBG. This is more clearly seen in the wavelength analysis using
the OSA. The data in this experiment showed the consistency of the FBG signal in Bragg
wavelength for different input polarization conditions. Again, no change was seen in the
FBG signal. It is important to note that while multiple samples were used to confirm the
data, the fibers tested were from one manufacturer (Innovative Fibers, Inc.) and were 9-
micron core single mode fiber. Nonetheless, the industry accepted notion that the
manufacturing of the FBG does not induce birefringence, or a direction dependence of
index of refraction held in these tests.
In Chapter 5, the methodology for mechanical testing of optical fibers under
transverse loading was developed. A special mechanical loading device was designed
and manufacture for applying uniform transverse loads to the optical fiber. The first test
was done using polarized light. The light was launched into the FBG and was analyzed
with a rotating plate polarizer and a photodetector. As transverse load was added to the
FBG changes in the output signal were detected. These changes are due to photoelastic
effects and the Poisson’s effect. The stress field created with the side load resulted in a
change of the material properties of the FBG. Specifically, the index of refraction of the
FBG changes with respect to the cross sectional area and the value of Λ will change. As
this happens, the data shown in Figure 5.4 depicts what happens. Instead of the output
intensity of the analyzer being based on a linearly polarized light, as indicated in the
initial conditions (Chapter 4) the polarization conditions have been changed to circular or
elliptical polarization. This can be explained by the changes in the output signal, where
67
the maximum intensity decreased while the minimum intensity increased, as shown in
Figure 5.4. This means that the cross section of the optical fiber is no longer
homogenous, i.e. the refractive index is not constant across the cross sectional area. As
increasing load causes a change in the material properties of the core the sinusoidal
output changes as well. This is to say that the light coupled away from the original
linearly polarized condition to elliptical or circularly polarized conditions based on the
changes induced in the fiber refractive index across the fiber cross section.
The effect of transverse loading is observed even clearer in the wavelength
analysis using the OSA. As seen in figure 5.7 the application of transverse load causes
the signal to broaden, then split. In the case of very high loads, the signal presents an
additional peak, and in some cases a total of four or five. The key is to determine what
causes these multiple peaks. When transversely loading the fiber, the material properties
of the FBG change. The FBG is manufactured in a silica cylinder, being loaded by two
flat plates. In cross section, the circular shape changes with load into an elliptical shape.
More importantly, this loading causes a photoelastic effect. In the direction of loading
the silica is compressed, in the orthogonal direction stretched. A field of various stresses
exists in the FBG cross section, and as such the index of refraction, which is related to the
density of the material, changes. In addition, the fiber section being loaded experiences
Poisson effects. The fiber expands in the axial direction as it is loaded transversely,
changing the Bragg condition in both neff and Λ. Other factors that can contribute to the
FBGs response are the condition of the interface between the core and cladding and the
state of friction between the FBG and the loading plates. These concepts can be related
to the FBG signals shown in Figure 5.7. The experiments presented acquired data in
68
transmission. The total output signal is shown in Figure 5.7. The difference between the
input and the output signals is due to the Bragg reflection and losses. As the transverse
load was applied to the FBG, the signal began to broaden, then split. The most likely
explanation is that the Bragg solution can be satisfied at more than one wavelength, that
are close to each other with weak interaction. Once a second Bragg solution can be
satisfied at a wavelength far enough from the first one, based on the OSA resolution, two
peaks can be detected. Issues such as grating sharpness, signal noise, and the consistency
of Λ through the grating play a part in the sharpness of the FBG signal. After sufficient
loading, two peaks are clearly seen. At this point it is easy to claim that the elliptical
cross section and related photoelasticity have created two Bragg solutions, located in
orthogonal directions at the maximum and minimum values (solutions) of nΛ. However,
when high levels of transverse loading causes an additional peak, another explanation is
needed. Again, the peak is shown in the transmitted signal – so all possibilities of
reflections as well as losses must be considered. However, it is clear that these changes
in the transmitted signal spectrum are wavelength dependent. This means that it is
related to the Bragg structure as well as the materials properties. The additional peak in
wavelength could be theoretically possible for a number of cases. Light could be coupled
from the core into the cladding due to excessive pinching. The FBG could extend past
the loading blocks, causing only a portion of transverse strain to be couple to the FBG.
The loading on the FBG could be a gradient with uneven surfaces, or localized friction on
the loading blocks. The additional peak could also be an additional satisfactory solution
to the Bragg condition λb=2neffΛ. The data in Chapter 5 indicates that the greatest
possibility lies with an additional solution or solutions to the Bragg condition. The data
69
indicate that the additional peak/loss is wavelength specific. It is difficult to match the
other loss mechanisms with a peak that matches the characteristics of the other Bragg
solutions so well. In addition, the other loss mechanisms would have caused a greater
overall loss of intensity of the signal regardless of the wavelength. The data indicates
that for values of neffΛ that include indices of refraction between the maximum and
minimum, additional Bragg solutions are possible.
6.2 Recommendation For Future Work
This thesis has brought about new approaches in observing transverse loading
response of Bragg grating fiber optic sensors. Specifically, the initial bifurcation of the
signal as a function of transverse load was verified, as was the mathematical relationship
of this effect. Further discussed was the ability to detect multiple solutions to the Bragg
condition, in addition to the initial two as a result of birefringence, when sufficient
transverse strain was applied to the FBG. The experimental work supplements and
proposes some explanations to the theory in this regard. The key relationship in these
phenomena is that of the material properties of the waveguide, namely index of
refraction, and the resulting FBG response.
Additional work in this field is needed. Work endeavoring to further explain the
effect of 3-D loading of a FBG structure would prove very useful. In applying the
concepts proposed in this thesis to applications in which FBGs would be embedded to
provide sensory data for smart structures, the following key tasks could be undertaken:
• Study of interface characteristics of core/cladding/coatings/attachment to host
• Stress strain modeling of host
70
• Stress strain modeling of fiber embedded in host material
• Complete 3-D model of loading and response – experiments (of fiber, fiber + host)
• Study of fiber & host as system (w/above info.)
71
List of References
[1] Eric Udd, An Overview of Fiber Optic Sensors, Rev. Sci. Instrumentation, American Institute of Physics, 66 (8), 1995.
[2] K.T.V. Grattan and B.T. Meggitt, Editors, Optical Fiber Sensor Technology, Chapman & Hall, p.1, 1995.
[3] Allen H. Cherin, An Introduction to Optical Fibers, McGraw-Hill, 1983.
[4] Luc B. Jeunhomme, Single Mode Fiber Optics, Marcel Dekker, 1990.
[5] Joseph Palais, Fiber Optic Communications, Prentice Hall, 1992.
[6] Eugene Hecht, Optics, Addison Wesley, 1987.
[7] Stewart D. Personick, Fiber Optics Technology and Applications, Plenum, 1985.
[8] Gerd Keiser, Optical Fiber Communications, Second Edition, McGraw-Hill, 1991.
[9] Michel Barsoum, Fundamentals of Ceramics, McGraw-Hill, 1997.
[10] William D. Callister, Jr., Materials Science and Engineering, An Introduction, Fourth Edition, John Wiley & Sons, 1997.
[11] Paul Klocek and George H. Sigel, Jr., Infrared Fiber Optics, SPIE Press, 1989
[12] B.E.A. Saleh and M.C. Teich, Fundamentals of Photonics, John Wiley & Sons, 1991
[13] Ephraim Suhir, Mechanical Approach to the Evaluation of the Low Temperature Threshold of Added Transmission Losses in Single Coated Optical Fibers, Journal of Lightwave Technology, Vol. 8, No. 6, June, 1990.
72
[14] John Dakin and Brian Culshaw, Editors, Optical Fiber Sensors: Principles and Components, Artech House, 1988.
[15] M.A. Sherif, J. Radhakrishnan, Advanced Composites with Embedded Fiber Optic Sensors for Smart Applications, Journal of Reinforced Plastics and Composites, Vol. 16, No. 2, January, 1997.
[16] J. Yuan, J. Feng, M. El-Sherif, A.G. MacDiarmid, Development of an On-Fiber Chemical Vapor Sensor, OSA Annual Meeting, Baltimore, MD, Oct. 4-9, 1998.
[17] Stephen Mastro, Fabry-Perot, Bragg Grating, and Optical MEMS Sensors for Naval Shipboard Use, Laser Diode and LED Application III, SPIE – The International Society for Optical Engineering, San Diego, (1997)
[18] A.D. Kersey, T.A. Berkoff, and W.W. Morey, Multiplexed Fiber Bragg Grating Strain Sensor System With a Fiber Fabry-Perot Wavelength Filter, Optics letters, Vol. 18, No. 16, 1993.
[19] Eric Udd, Editor, Fiber Optic Sensors, Critical reviews of Optical Science and Technology, Volume CR44, SPIE Press, 1992.
[20] Gregg Johnson, Sandeep Vohra, and Stephen Mastro, Fiber Optic Strain Sensors for Surface Ship Bending and Vibration Monitoring, Proceedings - ASNE Intelligent Ship Symposium, Philadelphia, PA, 41 (1999).
[21] James S. Sirkis, Embedded Fiber Optic Strain Sensors, Manual on Experimental Methods for Mechanical Testing of Composites, University of Maryland Dept. of mechanical Engineering, May, 1996.
[22] Mahmoud A. El-Sherif, K. Fidanboylu, D. El-Sherif, R. Gafsi, J. Yuan, C. Lee, J. Fairney, A Novel Fiber Optic System for Measuring the Dynamic Structural Behavior of Parachutes, Journal of Intelligent Materials Systems and Structures, accepted for publication in April 2000.
[23] K.O. Hill, Y. Fujii, D.C. Johnson, and B.S. Kawasaki, Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication, Applied Physics Letters, 32 (647), 1978.
73
[24] Francois C. Bilodeau, et al., Photosensitization of Optical Fiber and Silica Waveguides, United States Patent # 5,495,548, February 27, 1996.
[25] Mahmoud El-Sherif, Process for Producing a Grating On An Optical Fiber, United States Patent # 4,842,405 June 27, 1989
[26] Robert M. Atkins and Rolando P. Espindola, Photosensitivity and Grating Writing in Hydrogen Loaded Germanosilicate Core Optical Fibers at 325 and 351 nm, Appl. Phys. Lett. 70, (9), March 3, 1997.
[27] R.J. Campbell et al, Narrow-Band Optical Fibre Grating Sensors, 7th Optical Fibre Sensors Conference, Institution of Radio and Electronic Engineers, Australia, 1990.
[28] Thomas Strasser et al, Ultraviolet Laser Fabrication of Strong, nearly Polarization-Independent Bragg Reflectors in Germanium-Doped Silica Waveguides on Silica Substrates, Appl. Phys. Lett., 65, (26), December 26, 1994.
[29] K.O. Hill et al, Photosensitivity in Optical Fibers, Annual Review of Material Science, 23, 1993.
[30] G. Meltz, W.W. Morey, W.H. Glenn and J.D. Farina, In-Fiber Bragg Grating Sensors, Optical Fiber Sensors, 1988 Technical Digest Series, Vol. 2, Optical Society of America, Washington, D.C., 1988.
[31] Department of Optics and Instrumentation, Technical University Hamburg Harburg Homepage (http://www.om.tu-harburg.de/homepage.asp), Hamburg Germany, July 21, 1999
[32] K.O. Hill, B.Malo, Bragg Gratings Fabricated in Monomode Photosensitive Optical Fiber by UV Exposure Through a Phase Mask, Appl. Phys. Lett., 62, (10), 1993.
[33] W.W. Morey, G. Meltz, and W.H. Glenn, Bragg Grating Temperature and Strain Sensors, Sixth International Conference on Optical Fiber Sensors, Heidelberg, Germany, 1989.
74
[34] Kenneth O. Hill, Fiber Bragg Gratings: Properties and Sensing Applications, Eleventh International Conference on Optical Fiber Sensors, Advanced Sensing Photonics, Japan Society of Applied Physics, 1996.
[35] G. Meltz, W.W. Morey, and W.H. Glenn, Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method, Optics Letters, Vol. 14, No. 15, 1989.
[36] R.B. Wagreich, W.A. Atia, H. Singh and J.S. Sirkis, Gratings Fabricated in Low Birefringent Fibre, Electronics Letters, Vol. 32, No. 13, June 20, 1996.
[37] W.W. Morey, Distributed Fiber Grating Sensors, Seventh Optical Fibre Sensors Conference, The Institution of Radio and Electronics Engineers, Australia, 1990.
[38] Kenneth O. Hill, et al., Optical Fiber Reflective Filter, United States Patent # 4,474,427, October 2, 1984
[39] M. Schmitz, R. Braeuer, O. Bryngdahl, Phase Gratings with Subwavelength Structures, J. Optical Society Am. A12, 2458-2462, 1995.
[40] X.D. Jin and J.S. Sirkis, Simultaneous Measurement of Two Strain Components in Composite Structures Using Embedded Fiber Sensors, OSA Technical Digest Series, Vol. 16, Optical Society of America, Washington, DC, 1997.
[41] Sylvain Magne et al, State-of-strain Evaluation with Fiber Bragg Grating Rosettes: Application to Discrimination Between Strain and Temperature Effects in Fiber Sensors, Applied Optics, Vol. 36, No. 36, December 20, 1997.
[42] S. Tanaka, K. Yoshida, and Y. Ohtsuka, A New Type of Birefingent Fiber Fabricated For Sensor Use, Eleventh International Conference on Optical Fiber Sensors, Advanced Sensing Photonics, Japan Society of Applied Physics, 1996.
[43] Yo-Lung Lo and J.S. Sirkis, Simple method to measure temperature and Axial Strain Simultaneously Using One In-Fiber Bragg Grating Sensor, The International Society for Optical Engineering, Volume 3042, 1997.
[44] K.S. Chiang, Theory of Pressure-Induced Birefringence In a Highly Birefringent Optical Fibre, Seventh Optical Fibre Sensors Conference, The Institution of radio and Electronics Engineers, Australia, 1990.
75
[45] S.Y. Huang, J.N. Blake, and B.Y. Kim, Mode Characteristics of Highly Elliptical Core Two Mode Fibers Under Perturbations, OSA Technical Digest Series, Vol. 2, Optical Society of America, Washington, DC, 1988.
[46] R.B. Wagreich and J.S. Sirkis, Distinguishing Fiber Bragg Grating Strain Effects, OSA Techical Digest Series, Vol. 16, Optical Society of America, Washington, DC, 1997.
[47] M.J. Marrone, C.A. Villarruel, N.J. Frigo and A. Dandridge, Rotation of Polarization Axes in High-Birefringence Fibers, Fourth International Conference on optical Fiber Sensors, Institute of Electronics and Communication Engineers of Japan, 1986.
[48] Rachid Gafsi, Mahmoud El-Sherif, Theoretical Analysis On Spectra of Fiber Bragg Gratings Under Transverse and Static Load, Optical Fiber Technology, Ms. No. OFT99-0015, pending.
[49] Jim Sirkis, Interferometric Optical Fiber Strain Sensor Under Biaxial Loading, Optical Fiber Sensors, 1988 Technical Digest Series, Vol.2, Optical Society of America, Washington DC, 1988.
[50] Leif Bjerkan and Kjetil Johannessen, Measurements of Bragg Grating Birefringence due to Transverse Compressive Forces, 12th International Conference on Optical Fiber Sensors, OSA technical Digest Series, Vol. 16, Optical Society of America, Washington DC, 1997.
[51] Roger Stolen, Polarization Holding Fibers, Conference on Optical Fiber Communication and Third International Conference on Optical Fiber Sensors, Optical Society of America, Washington, DC, 1985.
[52] S.C. Rashleigh, Polarimetric Sensors: Exploiting The Axial Stress In High Birefringent Fibers, First International Conference on Optical Fibre Sensors, The Institution of Electrical Engineers, London, 1983.
[53] Laurence N. Wesson and Simon P. Bush, Keep Your Photons In Line, Photonics Spectra, September 1988.
[54] S. Carrara, B.Y. Kim, and H.J. Shaw, Elastooptic Determination Of Birefringent Axes in Polarization-Holding Optical Fiber, Conference of Optical Fiber
76
Communication and Third International Conference on Optical Fiber Sensors, Optical Society of America, Washington, DC, 1985.
[55] Ephraim Suhir, Bending Performance of Clamped Optical Fibers: Stresses Due To The End Offset, Applied Optics, Vol. 28, No.3, February 1, 1989.
[56] E. Suhir, Spring Constant in the Buckling of Dual Coated Optical Fibers, Journal of Lightwave Technology, Vol. 6, No. 7, July, 1988.
[57] Yoh Imai, Takashi Seino and Yoshihiro Ohtsuka, Temperature Or Strain-Insensitive Sensing Based On Bending-Induced Retardations In A Birefringent Single Mode Fiber, Fourth International Conference on Optical Fiber Sensors, Institute of Electronics and Communication Engineers of Japan, Tokyo, 1986.
[58] Ephraim Suhir, Stresses In A Coated Fiber Stretched On A Capstan, Applied Optics, Vol. 29, No. 18, June 20, 1990.
