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Spie Press Book

Fourier-Transform Spectroscopy Instrumentation Engineering
Author(s): Vidi Saptari
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Book Description

Many applications today require the Fourier-transform (FT) spectrometer to perform close to its limitations, such as taking many quantitative measurements in the visible and in the near infrared wavelength regions. In such cases, the instrument should not be considered as a perfect "black box." Knowing where the limitations of performance arise and which components must be improved are crucial to obtaining repeatable and accurate results. One of the objectives of this book is to help the user identify the instrument's bottleneck.

Book Details

Date Published: 3 November 2003
Pages: 136
ISBN: 9780819451644
Volume: TT61
Errata

Table of Contents
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Preface / xi
Chapter 1
Spectroscopy Instrumentation / 1
1.1 Introduction / 1
1.2 Types of Spectrometers / 1
1.2.1 Dispersive spectrometers
1.2.1 Dispersive spectrometers / 1
1.2.2 Filter-based spectrometers / 2
1.2.3 Fourier-transform spectrometers / 3
1.3 Advantages of FT Spectrometers / 4
1.3.1 Throughput or Jacquinot advantage / 4
1.3.2 Multiplex or Felgett advantage / 6
1.4 Discussions on FT Spectrometer Advantages / 8
References / 9
Chapter 2
Signal-to-Noise Ratio / 11
2.1 Signal-to-Noise Ratio Defined / 11
2.2 Quantifying Signal-to-Noise Ratio / 12
2.3 Practical Considerations / 15
Chapter 3
Principles of Interferometer Operation / 17
3.1 Overview / 17
3.2 Quantitative Explanation / 19
3.2.1 Light as a wave / 19
3.2.2 Measurable light quantity / 21
3.2.3 Interference and superposition / 22
3.2.4 Polychromatic source / 24
3.2.5 Fourier-transform routine / 25
3.3 Theoretical Resolution / 26
3.3.1 Retardation distance / 26
3.3.2 Divergence angle / 29
3.4 Interferogram Digital Processing / 32
3.4.1 Apodization / 32
3.4.2 Zero filling / 33
3.4.3 Phase correction / 34
References / 34
Chapter 4
Interferometer Alignment Errors / 35
4.1 Error Characteristics / 37
4.2 Interferogram-Modulation Error / 38
4.3 Interferogram-Sampling Error / 43
References / 44
Chapter 5
Motion Components and Systems / 45
5.1 Actuators / 46
5.1.1 Actuator types / 46
5.1.2 Performance specifications for actuators / 46
5.2 Driver and Amplifier / 48
5.3 Bearings and Coupling Mechanism / 48
5.3.1 Bearing types / 49
5.3.2 Performance specifications for bearings / 49
5.3.3 Discussion on bearings / 51
5.4 Position Sensor / 53
5.4.1 Laser interferometer / 53
5.4.2 Sensor specifications / 56
5.5 Homodyne Interferometer Sources of Error / 56
5.5.1 Source wavelength variations / 57
5.5.2 Refractive index variations / 57
5.5.3 Laser detector errors / 57
5.5.4 Polarization errors / 58
5.5.5 Alignment errors / 58
5.6 Actuator, Sensor, and Mirror Mounting / 59
5.6.1 Actuator location vs. center of mass / 59
5.6.2 Sensor location vs. mirror location / 59
5.6.3 Sensor location vs. actuator location / 59
5.7 Digital Motion Controller / 60
References / 61
Chapter 6
Interferogram Data Sampling / 63
6.1 Step Scan vs. Continuous Scan / 63
6.2 Sampling Period / 65
6.2.1 The absolute minimum: the Nyquist criteria / 65
6.2.2 More than the Nyquist minimum / 67
6.2.3 Anti-alias filter / 68
6.3 Accuracy with Respect to OPD / 69
6.3.1 Effects of sampling errors to the output spectrum / 70
6.3.2 Sources of error / 72
6.4 Repeatable Clock Position / 73
6.5 Hardware Methods / 73
References / 74
Chapter 7
Data Acquisition / 75
7.1 DAQ Hardware Formats / 75
7.2 Analog Inputs / 75
7.2.1 Number of channels / 75
7.2.2 Sampling rate / 76
7.2.3 Resolution / 76
7.2.4 Input ranges / 76
7.2.5 Accuracy / 77
7.3 Hardware Triggering and Clock Signal / 77
7.4 Effects of ADC Resolution on Spectral SNR / 78
Chapter 8
The Detector / 81
8.1 Noise-Equivalent-Power / 81
8.2 Spectral SNR quantification / 82
8.3 Detector Types / 85
8.4 Selection Guidelines / 85
8.4.1 Spectral range and responsivity / 86
8.4.2 Active area / 86
8.4.3 Noise and detectivity /86
8.4.4 Linearity / 87
8.4.5 Temporal bandwidth / 88
8.4.6 Summary / 88
References / 87
Chapter 9
Consideration of Optics and Interferometer Alignment / 91
9.1 System Throughput / 91
9.2 Lenses vs. Mirrors / 93
9.3 Interferometer Alignment Procedure / 94
Chapter 10
Signal-to-Noise Ratio Enhancement Techniques / 97
10.1 Identification of Error Sources / 97
10.1.1 Detector noise / 97
10.1.2 Digitization noise / 98
10.1.3 Interferometer alignment error / 98
10.1.4 Sampling error / 99
10.1.5 Light-source variation / 99
10.2 Averaging / 100
10.3 Temporal Modulation and Bandwidth Narrowing / 101
10.4 Spectral-Bandwidth Narrowing / 102
10.5 Spectral Post-Processing / 103
10.6 Double Beaming / 105
10.6.1 Electrical subtraction / 105
10.6.2 Optical subtraction / 106
10.7 Gain ranging / 107
10.8 Dynamic Tilt Compensation / 107
References / 108
Appendix A
Simulation of Static-Tilt Error / 109
Appendix B
Sampling Circuit Example / 111
Appendix C
Simulation of Sampling Error / 113
Index / 115

Preface

This book covers the engineering aspects of Michelson-interferometer-based Fourier-transform (FT) spectrometers. While other Fourier-transform spectroscopy textbooks have generally focused on the theoretical aspects and applications, this book provides insights to the design and instrumentation of FT spectrometers. This book is intended for scientists and engineers who wish to understand the operation of the instrument to the extent where they have the knowledge to improve results through hardware, software and/or procedural modifications. This book is also a proper introductory text for an engineer who plans to design a custom FT spectrometer. Many applications today require the FT spectrometer to perform close to its limitations, such as taking many quantitative measurements in the visible and in the near-infrared wavelength regions. In such cases, the instrument should not be considered as a perfect "black box." Knowing where the limitations of performance arise and which components must be improved are crucial to obtaining repeatable and accurate results. One of the objectives of this book is to help the user identify the instrument's bottleneck. The book begins with an overview of the types of spectrometers and the fundamental advantages of FT spectrometers (Chapter 1). Chapter 2 is devoted to clearly defining signal-to-noise ratio (SNR), a performance figure extensively used throughout this book. In Chapter 3, details of the interferometer operation are discussed. The design and limitations of the subsystems and components that include the motion components, the analog-to-digital (A/D) conversion process, the detection system, and the optical components are covered in Chapters 4 through 9. Chapter 10 introduces various practical techniques to identify the limiting source of error, and techniques to improve the obtainable SNR. I am indebted to Professor Kamal Youcef-Toumi at Massachusetts Institute of Technology (MIT) for giving me the opportunities to be involved in various first-rate R&D projects during my time at MIT, one of which led me to the exciting world of FT spectroscopy. I deeply thank Merry Schnell of SPIE, the main editor of this project, for her thoroughness and professionalism. Bradley M. Stone and David Ball did an excellent job in reviewing the original manuscript. Rick Hermann and Timothy Lamkins of SPIE provided generous assistance from the early stage of the project. All have my deepest appreciation.

Vidi Saptari

September 2003


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