| Preface | xi |
Chapter 1 | Spectroscopy Instrumentation | 1 |
1.1 | Introduction | 1 |
1.2 | Types of Spectrometers | 1 |
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 | 34 |
3.4.3 | Phase correction | 34 |
| References | 34 |
Chapter 4 | Interferometer Alignment Errors | 37 |
4.1 | Error Characteristics | 39 |
4.2 | Interferogram-Modulation Error | 40 |
4.3 | Interferogram-Sampling Error | 45 |
| References | 46 |
Chapter 5 | Motion Components and Systems | 47 |
5.1 | Actuators | 48 |
5.1.1 | Actuator types | 48 |
5.1.2 | Performance specifications for actuators | 48 |
5.2 | Driver and Amplifier | 50 |
5.3 | Bearings and Coupling Mechanism | 50 |
5.3.1 | Bearing types | 51 |
5.3.2 | Performance specifications for bearings | 51 |
5.3.3 | Discussion on bearings | 53 |
5.4 | Position Sensor | 55 |
5.4.1 | Laser interferometer | 56 |
5.4.2 | Sensor specifications | 58 |
5.5 | Homodyne Interferometer Sources of Error | 59 |
5.5.1 | Source wavelength variations | 59 |
5.5.2 | Refractive index variations | 59 |
5.5.3 | Laser detector errors | 60 |
5.5.4 | Polarization errors | 60 |
5.5.5 | Alignment errors | 61 |
5.6 | Actuator, Sensor, and Mirror Mounting | 61 |
5.6.1 | Actuator location vs. center of mass | 61 |
5.6.2 | Sensor location vs. mirror location | 62 |
5.6.3 | Sensor location vs. actuator location | 62 |
5.7 | Digital Motion Controller | 63 |
| References | 64 |
Chapter 6 | Interferogram Data Sampling | 65 |
6.1 | Step Scan vs. Continuous Scan | 66 |
6.2 | Sampling Period | 67 |
6.2.1 | The absolute minimum: the Nyquist criteria | 67 |
6.2.2 | More than the Nyquist minimum | 68 |
6.2.3 | Anti-alias filter | 69 |
6.3 | Accuracy with Respect to OPD | 71 |
6.3.1 | Effects of sampling errors to the output spectrum | 71 |
6.3.2 | Sources of sampling error | 73 |
6.4 | Repeatable Clock Position | 74 |
6.5 | Hardware Considerations | 74 |
| References | 76 |
Chapter 7 | Data Acquisition | 77 |
7.1 | DAQ Hardware Formats | 77 |
7.2 | Analog Inputs | 77 |
7.2.1 | Number of channels | 78 |
7.2.2 | Sampling rate | 78 |
7.2.3 | Resolution | 78 |
7.2.4 | Input ranges | 78 |
7.2.5 | Accuracy | 79 |
7.3 | Hardware Triggering and Clock Signal | 79 |
7.4 | Effects of ADC Resolution on Spectral SNR | 80 |
Chapter 8 | The Detector | 83 |
8.1 | Noise-Equivalent-Power | 83 |
8.2 | Spectral SNR quantification | 84 |
8.3 | Detector Types | 87 |
8.4 | Selection Guidelines | 88 |
8.4.1 | Spectral range and responsivity | 88 |
8.4.2 | Active area | 88 |
8.4.3 | Noise and detectivity | 89 |
8.4.4 | Linearity | 89 |
8.4.5 | Temporal bandwidth | 90 |
8.4.6 | Summary | 90 |
| References | 91 |
Chapter 9 | Consideration of Optics and Interferometer Alignment | 93 |
9.1 | System Throughput | 93 |
9.2 | Lenses vs. Mirrors | 95 |
9.3 | Interferometer Alignment Procedure | 96 |
Chapter 10 | Signal-to-Noise Ratio Enhancement Techniques | 99 |
10.1 | Identification of Error Sources | 99 |
10.1.1 | Detector noise | 99 |
10.1.2 | Digitization noise | 100 |
10.1.3 | Interferometer alignment error | 101 |
10.1.4 | Sampling error | 101 |
10.1.5 | Light-source variation | 101 |
10.2 | Averaging | 102 |
10.3 | Temporal Modulation and Bandwidth Narrowing | 103 |
10.4 | Spectral-Bandwidth Narrowing | 104 |
10.5 | Spectral Post-Processing | 105 |
10.6 | Double Beaming | 106 |
10.6.1 | Electrical subtraction | 107 |
10.6.2 | Optical subtraction | 107 |
10.7 | Gain ranging | 109 |
10.8 | Dynamic Tilt Compensation | 109 |
| References | 110 |
Appendix A | Simulation of Static-Tilt Error | 111 |
Appendix B | Sampling Circuit Example | 113 |
Appendix C | Simulation of Sampling Error | 115 |
| Index | 117 |