Time Division Multiplexing of a Serial Fibre Optic Bragg Grating Sensor Array

127 Pages·2000·6.26 MB·English

Time Division Multiplexing of a Serial Fibre Optic
Bragg Grating Sensor Array
David J. F. Cooper
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Electrical and Computer Engineering
University of Toronto
O Copyright by David J. F. Cooper 1999
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Time Division Multiplering of a Serial Fibre Optic Bragg Grating Sensor Array
A thesis for the degree of Master of Applied Science, 1999
David J. F. Cooper
Graduate Department of Elecincal and Cornputer Engineering
University of Toronto
A method of multiplexing fibre optic Bragg grating strain sensors is developed. The
proposed design is show to be capable of meeting the requirements demanded by
applications in modem civil engineering constructions. The system is implemented using a
mode-locked erbium doped fibre laser as the interrogation source and an electro-optic
modulator to demodulate the strain encoded sensor information. Both experimental and
theoretical anaiysis is used to examine the system performance.
The requirements of civil engineea for multiplexing fibre optic sensoa were
identified and the proposed system was shown to meet them. Some of the particular
advantages of the system include simple integration with existing single grating sensor
measurernent methods, a sensing range capable of monitoring many structures up to the
failure point, relaxed demands on the manufacture of the grating senson and high sensor
spatial resolution.
1 would iike to thank my thesis supervisor, Professor P. W. E. Smith, for his guidauce
and for his efforts that provided the necessary support for the work in this thesis.
1 would also like to thank Dr. Seldon Benjamin who has since left our research lab to
pursue other oppomuiities. Through discussions with Dr. Benjamin, the original direction of
this thesis was laid out.
1 am grateful for the support of the Canadian Network of Centres of Excellence on
Intelligent Sensing for Innovative Structures (ISIS) for providing funding for the project and
the Natural Sciences and Engineering Research Council of Canada (NSERC) for their
financial support.
Special thanks goes to Trent Coroy for his invaluable knowledge of fibre optic sensor
technology, equiprnent, and for his support.
The support from my colleagues in the Ultrafast Photonics Lab is greatly appreciated.
Mr. Lawrence Chen, Mr. Hany Loka, Dr. Matin, Ms. Li Qian, and Mr. Son Tsou, were
always willing to offer a helping hand and provide advice when needed.
The assistance of Dr. Xija Gu and Dr. Robin Tarn at Photonics Research Ontario at
many stages of this thesis proved to be invaluable.
1 am also thankful to the members of the Fibre Optic Sensor lab at the University of
Toronto Institute for Aerospace Studies (UTIAS) for their kindness and patience. Particular
thanks goes to Prof. R. M. Measures and Prof. R. C. Tennyson.
Finally 1 would like to thank my fnends and family for their encouragement during
the process of this thesis.
Chapter 1 Iiitroduction
1.1 Sensing Requirements for Modem Structures
1.2 Fibre Optic Sensors
1.3 Fibre Optic Bragg Gratings
1.4 Bragg Gratings and Strain Sensing
1.5 Contributions of this Thesis
1.6 Organization of the Thesis
1,7 Re ferences
Chapter 2 Fibre Bragg Gratings as Sensors
2.1 Fundamentals of Fibre Optic Bragg Gratings
2.2 Fiber Optic Bragg Gratings as Strain and Temperature Senson
2.3 Detection of Wavelength Encoded Signals
2.4 Strain/Temperature Discrimination
2.5 Multiplexing Sensors
2.5.1 Spatial Multiplexing
2.5.2 Wavelength Division Multiplexing (WDM)
2.5.3 Tirne Division Multiplexing (TDM)
2.6 References
Chapter 3 Design and Implementation of a TDM Sensor System
3.1 General Requirements for TDM sensor Networks
3.2 System Design
3.2.1 Interference and Cross-talk
3.2.1.1 Spectral Shadowing
3.2.1.2 Multiple Reflections
3.2.1.3 Demultiplexa Cross-tak
3.2.2 Power Budget
3.3 Lmplementation of a Prototype TDM System
3.3.1 Design
3.3.2 Experimental Procedure and Results
3.4 References
Chapter 4 Pertormance Analysis
4.1 Analysis
4.1.1 Detector Performance
4.1.2 Multiplexer Performance
4.1.3 Sensor Performance
4.1.4 Source Performance
4.1.5 Putting it al1 together
4.2 References
Chapter 5 Conclusion and Future Work
5.1 Summary and Conclusions
5.2 Future Work
2.1 : Schematic representation of the index profile forming a fibre optic
Bragg grating.
2.2: Location and direction of the optical waves in equation (2.8).
2.3: (a) Transmission and (b) reflection spectrum of a strong grating
showing coupling to the cladding modes in transmission. From (2.41
2.4: A basic type of interferometric detection using an unbalanced
interferorneter.
2.5: E ffect of applying a rarnped dither signal to one path of the
interferometer when a continuous monochromatic signal is used as input.
2.6: Grating wavelength detection using an Acousto-optic tunable filter
(AOTF). FBG = fibre Bragg grating, VCO = voltage controlled oscillator,
LF = low kquency, ELED = edge emitting light emitting diode. From
[2.26].
2.7: Scanning Fabry-Perot method of measuring wavelength. From 12.271.
2.8: Network topology for spatial multiplexing.
2.9: Optical Spectrum fiom a WDM sensor network. The dotted lines
show the unpertwbed sensor spectrum and the solid lines show the
response to strain or temperature.
3.1 : General block diagram for a time division rnultiplexing sensor system
utilizing fibre Bragg gratings.
3.2: Distortion of optical spectrum caused by spectral shadowing. The
dotted line shows expected spectrum and the solid line shows the distorted
spectnim. The second grating is offset in wavelength by E fiom the fkt
grating.
3.3 : Interference caused by multiple reflections. The reflection of the
optical pulse between gratings in the slot 1 and j r e mw ith the reflection
of the grating in slot k.
3.4: Ratio of power fiom multiple reflected pulses to pnmary reflection.
Cross-tak for different grating reflectivity are shown. This graph
represents the worst-case cross-talk. The cross-tdk in a practical system
will iikely be substantially less[3.2].
3.5: Worst case power penalty versus number of sensors for different
grating reflectivity.
3.6: Prototype system for TDM sensor interrogation.
3.7: Pulse reflection fiom a 5% 0.2 nrn unifonn grating with a 1 ps input
pulse.
3.8: Measured response of multiple quantum well p-i-n. detector. The
output is normalized by a standard photo-detector.
3.9: Setup for measuring the wavelength of incoming light. The box
showing a wavelength sensitive detector is the multiple quantum well
detect or.
3.10: (a) Typical Optical spectnim at the output of the amplifier @) Blow
up of middle section of the spectrum.
3.1 1: Measured temporal response of the pulse From the amplifier using a
fast detector.
3.12: Returned optical spectrum from grating sensors with no
demultiplexing.
3.13 : Pulse output of the shoa pulse generator.
3.14: Output spectrum of TDM system showing successfid seleetion of
each grating.
3.15: Output optical spectrum when an empty time slot is selected by the
demultip lexing unit.
3.16: Strain response of demultiplexing system when a step strain function
is applied to one grating.
4.1 : Structure of an (a) electroabsorption rnodulator [4.3] and (b) digital
optical switch [4.4]
Chapter 1
INTRODUCTION
1.1 SENSING REQUIREMENTS FOR MODERN STRUCTURES ...................................... 1
..................................................................................................
1.2 FIBRE OPTIC SENSORS 2
1.3 FIBRE OPTIC BRAGG GRATINS ................................................................................ 3
i .4 BRAGG GPATINGS AND SIXUN SENSING ........................................................ 9
1.5 CONTRIBUTIONS OF THIS THESIS ............................................................................ 11
1.6 ORGANIZATION OF THE THESIS ...............................................................................1 3
...............................................................................................................
1.7 REFEENCES 14
1.1 Sensing requirements for modern structures
Modem construction techniques increasingly require the use of "Smart Structures and
Materials" [LI] that can sense and in some cases adapt to their environments. The
requirements for these structures are based on a wide range of new constructions in the fields
of civil and mechanical engineering such as buildings, bridges, airports, airplanes, dams.
pipelines, and a host of others. Some structures may be able to detect deterioration caused by
Factors like aging and fatigue, and transmit this information to a central monitoring service
for appropnate corrective action. Other more advanced designs may even be able to
implement self-correcthg maintenance in a continuous closed loop cycle. One cm view the
potential for fbture structures as a biological mode1 where the structure is ouüïtted with a
"nervous system" of sensors throughout, and respouds to stimuli fiom the environment. [1.2]
A major challenge in the realization of Smart Structures is the development of
sensors that can both measure a wide range of parameters and be integrated easily and
cheaply into structural materials. Some of the measurements which are of interest include
vibration frequencies, spatial vibration modes, thermally induced strain and deformations,
loading, impacts, wind monitoring, shear forces, crack formation, darnage assessment, de-
bonding, corrosion, traf'fïc flow, seismic movement, temperature distribution, and chemical
contamination. In order for a sensor to be usefùl it must be sensitive enough to the
measurand of interest, while being able to handle extreme conditions. It must be able to
withstand a possibly harsh installation process, and it should be immune to types of outside
interference such as electromagnetic interference.
1.2 Fibre Optic Senson
One class of sensors that is particularly attractive for use in Smart Structures is the
fibre optic sensor. This class of sensors includes a wide variety of configurations. Sensors
like microbend sensors[l.3], and chemicai sensors [lA]translate measurand signals into a
change of intensity of light propagating in the fibre. Some are based on Iight scattering
processes such as Rayleigh, Raman, and Brillouin scattering. [1.5-1.71 Others are based on
interferornetic measurements of phase or fige visibility. Also, there has been a significant
amount of research in wavelength encoded sensors produced by one dimensional Bragg
gratings fabricated in optical fibres [1.8]. These senson have been used to measure such
parameters as strain, temperature, chemical composition and position. nie main focus of this