Automatic Interferometric Alignment of a Free-Space Optical Coherence Tomography System

127 Pages·2011·9.94 MB·English

by  Andrew Cenko

Automatic Interferometric
Alignment of a Free-Space Optical
Coherence Tomography System
by
Andrew Cenko
A thesis
presented to the University of Waterloo
in fulfillment of the
thesis requirement for the degree of
Doctor of Philosophy
in
Systems Design Engineering
Waterloo, Ontario, Canada, 2011
(cid:13)c Andrew Cenko 2011
I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis,
including any required final revisions, as accepted by my examiners.
I understand that my thesis may be made electronically available to the public.
ii
Abstract
Optical Coherence Tomography (OCT) is a relatively new interferometric technology
that allows for high-resolution and non-destructive tomographic imaging. One of its pri-
mary current uses is for in vivo and ex vivo examination of medical samples. It is used for
non-destructive examination of ocular disease, dermatological examination, blood vessel
imaging, and many other applications. Some primary advantages of OCT imaging include
rapid imaging of biological tissue with minimal sample preparation, 3D high-resolution
imaging with depth penetrations of several millimeters, and the capability to obtain re-
sults in real time, allowing for fast and minimally invasive identification of many diseases.
Current commercial OCT systems rely heavily on optical fiber-based designs. They
depend on the robustness of the fiber to maintain system performance in variable environ-
mental conditions but sacrifice the performance and flexibility of free-space optical designs.
We discuss the design and implementation of a free-space OCT interferometer that can
automatically maintain its alignment, allowing for the use of a free-space optical design
outside of tightly controlled laboratory environments.
In addition, we describe how similar enhancements can be made to other optical inter-
ferometric systems. By extending these techniques, we can provide similar improvements
to many related fields, such as interferometric metrology and Fourier Transform Spec-
troscopy. Improvements in these technologies can help bring powerful interferometric tools
to a wider audience.
iii
Acknowledgments
It is my pleasure here to thank all the people that made this thesis possible.
I owe my deepest gratitude to Arsen Hajian, who has been a constant supporter and
guide for my work and, without whom, I would not even have begun this endeavour.
I would also like the thank the rest of my thesis committee, namely Paul Calamai,
Maud Gorbet, Glenn Heppler, Ed Vrscay, and James Fraser. Their comments, questions,
and suggestions have greatly enhanced the quality of this thesis and forced me to think in
directions I would not otherwise have considered.
It is an honor for me to thank my fellow graduate student and partner in crime, Jeff
Meade. Without the many late night lab sessions, the immediately following early morning
discussions, and the (often fruitless) food hunting expeditions, this work would have been
infeasible and much more boring.
Iamalsogratefultomyotherfellowgraduatestudents,TomHaylock,FarnoudKazemzadeh,
Simon So, and Linda Vu, who both aided me in completing my research and provided an
escape when I could no longer bear to look at my own.
I am also indebted to the aid and support provided by my collaborators at Tornado
Medical systems, especially Andrew Berkeley, Peter Christensen, Annie Deng, Jan Hen-
drickse, Stefan Larson, Fred Sweeney, Paul van der Vecht, and Jonathan Vermette.
Additional thanks are due to the machinists at the UW Engineering Machine Shop and
Mike Hintermeister, who helped turn system drawings into the components necessary to
complete this work.
I also extend my gratitude to the Thunder Bay Regional Research Institute, a research
partner of the Thunder Bay Regional Health Sciences Centre, for their financial support
of this project. Further support was also provided by Tornado Medical Systems and Arjae
Spectral Enterprises.
Finally, and most importantly, I wish to thank my parents, Alex and Martha Cenko,
my brother, Lesyk Cenko, and all the rest of my extended family for their continued love
and support. Without them, I would never have managed to reach this point.
iv
Dedication
This thesis is dedicated to my mother, who has shown me the value of perseverance,
hard work, and self-esteem. It is also dedicated to my father, who has always pushed me
to do more than I believed possible.
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Table of Contents
List of Tables x
List of Figures xi
List of Symbols xv
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Free-Space Optics Advantages . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Free-Space Optics Limitations . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 Mitigating Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 State-of-the-Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Background 5
2.1 Interferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 What is a Wave? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 How Do Waves Interfere? . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.3 What is an Interferometer? . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.4 Interferometric Tolerances . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.5 Interferometric Devices . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.6 Dispersed Interferometry . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Optical Coherence Tomography . . . . . . . . . . . . . . . . . . . . . . . . 11
vi
2.2.1 OCT Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.2 Uses of OCT Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.3 Limiting Factors of OCT Performance . . . . . . . . . . . . . . . . 14
3 Base OCT System 18
3.1 OCT System Design Choices . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2 Baseline OCT System Layout . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3 Baseline OCT System Results . . . . . . . . . . . . . . . . . . . . . . . . . 22
4 Alignment Enhancements 28
4.1 Expected Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.1.1 Alignment Effects on System Performance . . . . . . . . . . . . . . 28
4.1.2 Expected Tolerance Requirements . . . . . . . . . . . . . . . . . . . 29
4.1.3 Expected Alignment Timescales . . . . . . . . . . . . . . . . . . . . 30
4.1.4 Mitigating factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.1.5 User-level Requirements . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2 Alignment System Degree of Freedom Reduction . . . . . . . . . . . . . . . 32
4.3 Alignment System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.4 Additional Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.4.1 Beam Splitter Choice . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.4.2 Beam Reducer vs Pupil Projection Imager . . . . . . . . . . . . . . 41
4.5 System Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.5.1 Auto-alignment Components . . . . . . . . . . . . . . . . . . . . . . 43
4.5.2 Alignment Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.5.3 Extra Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.6 Software Control of Alignment Hardware . . . . . . . . . . . . . . . . . . . 48
4.6.1 Alignment Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.6.2 Alignment Cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.6.3 Servo Blockers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.6.4 Linear Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
vii
4.7 Software Offset Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.7.1 Tilt Offset Measurement . . . . . . . . . . . . . . . . . . . . . . . . 54
4.7.2 Shear Offset Measurement . . . . . . . . . . . . . . . . . . . . . . . 55
4.8 Computer Controlled Alignment . . . . . . . . . . . . . . . . . . . . . . . . 55
4.9 Optimizing Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.10 Automatic Alignment Interface . . . . . . . . . . . . . . . . . . . . . . . . 58
4.11 Automated Alignment Correction . . . . . . . . . . . . . . . . . . . . . . . 58
4.12 Alignment Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.13 Temperature Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5 Extension to Other Systems 65
5.1 Degree of Freedom Identification and Reduction . . . . . . . . . . . . . . . 65
5.1.1 Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.1.2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.2 Simplified Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6 Conclusions 74
6.1 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.3 Final Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
APPENDICES 77
A Code 78
A.1 Measurement Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
A.1.1 Centroiding Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
A.1.2 Save Current Alignment Code . . . . . . . . . . . . . . . . . . . . . 82
A.1.3 Tilt Alignment Offset . . . . . . . . . . . . . . . . . . . . . . . . . . 84
A.1.4 Shear Alignment Offset . . . . . . . . . . . . . . . . . . . . . . . . . 85
A.2 Correction Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
A.2.1 Motor Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
viii
A.2.2 Alignment Calibration Code . . . . . . . . . . . . . . . . . . . . . . 92
A.2.3 Pixel Offset to Motor Command Conversion . . . . . . . . . . . . . 94
A.3 Reduction Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
A.3.1 Initial Processing Code . . . . . . . . . . . . . . . . . . . . . . . . . 97
A.3.2 Image Generation Code . . . . . . . . . . . . . . . . . . . . . . . . 100
A.3.3 Dispersion Compensation Code . . . . . . . . . . . . . . . . . . . . 102
References 106
ix
List of Tables
3.1 The imaging parameters of our OCT system. For tissue, we assume an index
of refraction n = 1.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.1 The effect of degrees of freedom on our various optical components. The
degrees of freedom are referenced to the centers of our optical components. 33
x