Handbook of Measurement in Science and Engineering, Volume 3
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Myer Kutz holds engineering degrees from RPI and MIT. He was Vice President and General Manager of Wiley's STM division and has consulted and authored for most of the major professional and technical publishing houses. He is the author of 7 books and the editor of more than 20 handbooks.
Content
VOLUME 3
List of Contributors xxi
PREFACE xxv
Part VII Physics and Electrical Engineering 1943
54 Laser Measurement Techniques 1945 Cecil S. Joseph, Gargi Sharma, Thomas M. Goyette, and Robert H. Giles
54.1 Introduction, 1945
54.1.1 History and Development of the MASER, 1945
54.1.2 Basic Laser Physics, 1946
54.1.3 Laser Beam Characteristics, 1951
54.1.4 Example: CO2 Laser Pumped Far-Infrared Gas Laser Systems, 1956
54.1.5 Heterodyned Detection, 1959
54.1.6 Transformation of Multimode Laser Beams from THz Quantum Cascade Lasers, 1962
54.1.7 Suggested Reading, 1965
54.2 Laser Measurements: Laser-Based Inverse Synthetic Aperture Radar Systems, 1965
54.2.1 ISAR Theory, 1966
54.2.2 DFT in Radar Imaging, 1967
54.2.3 Signal Processing Considerations: Sampling Theory, 1970
54.2.4 Measurement Calibration, 1971
54.2.5 Example Terahertz Compact Radar Range, 1972
54.2.6 Suggested Reading, 1974
54.3 Laser Imaging Techniques, 1974
54.3.1 Imaging System Measurement Parameters, 1975
54.3.2 Terahertz Polarized Reflection Imaging of Nonmelanoma Skin Cancers, 1981
54.3.3 Confocal Imaging, 1985
54.3.4 Optical Coherence Tomography, 1987
54.3.5 Femtosecond Laser Imaging, 1990
54.3.6 Laser Raman Spectroscopy, 1996
54.3.7 Suggested Reading, 1997
References, 1997
55 Magnetic Force Images Using Capacitive Coupling Effect 2001 Byung I. Kim
55.1 Introduction, 2001
55.2 Experiment, 2004
55.2.1 Principle, 2004
55.2.2 Instrumentation, 2004
55.2.3 Approach, 2005
55.3 Results and Discussion, 2006
55.3.1 Separation of Topographic Features from Magnetic Force Images Using Capacitive Coupling Effect, 2007
55.3.2 Effects of Long-Range Tip-Sample Interaction on Magnetic Force Imaging: A Comparative Study Between Bimorph-Driven System and Electrostatic Force Modulation, 2012
55.4 Conclusion, 2020
References, 2021
56 Scanning Tunneling Microscopy 2025 Kwok-Wai Ng
56.1 Introduction, 2025
56.2 Theory of Operation, 2026
56.3 Measurement of the Tunnel Current, 2030
56.4 The Scanner, 2032
56.5 Operating Mode, 2035
56.6 Coarse Approach Mechanism, 2036
56.7 Summary, 2041
References, 2042
57 Measurement of Light and Color 2043 John D. Bullough
57.1 Introduction, 2043
57.2 Lighting Terminology, 2043
57.2.1 Fundamental Light and Color Terms, 2043
57.2.2 Terms Describing the Amount and Distribution of Light, 2047
57.2.3 Terms Describing Lighting Technologies and Performance, 2048
57.2.4 Common Quantities Used in Lighting Specification, 2052
57.3 Basic Principles of Photometry and Colorimetry, 2056
57.3.1 Photometry, 2056
57.3.2 Colorimetry, 2063
57.4 Instrumentation, 2072
57.4.1 Illuminance Meters, 2072
57.4.2 Luminance Meters, 2072
57.4.3 Spectroradiometers, 2074
References, 2074
58 The Detection and Measurement of Ionizing Radiation 2075 Clair J. Sullivan
58.1 Introduction, 2075
58.2 Common Interactions of Ionizing Radiation, 2076
58.2.1 Radiation Interactions, 2076
58.3 The Measurement of Charge, 2077
58.3.1 Counting Statistics, 2078
58.3.2 The Two Measurement Modalities, 2080
58.4 Major Types of Detectors, 2081
58.4.1 Gas Detectors, 2081
58.4.2 Ionization Chambers, 2086
58.4.3 Proportional Counters, 2090
58.4.4 GM Detectors, 2092
58.4.5 Scintillators, 2092
58.4.6 Readout of Scintillation Light, 2094
58.4.7 Semiconductors, 2096
58.5 Neutron Detection, 2100
58.5.1 Thermal Neutron Detection, 2102
58.5.2 Fast Neutron Detection, 2104
58.6 Concluding Remarks, 2106
References, 2106
59 Measuring Time and Comparing Clocks 2109 Judah Levine
59.1 Introduction, 2109
59.2 A Generic Clock, 2109
59.3 Characterizing the Stability of Clocks and Oscillators, 2110
59.3.1 Worst-Case Analysis, 2111
59.3.2 Statistical Analysis and the Allan Variance, 2113
59.3.3 Limitations of the Statistics, 2116
59.4 Characteristics of Different Types of Oscillators, 2117
59.5 Comparing Clocks and Oscillators, 2119
59.6 Noise Models, 2121
59.6.1 White Phase Noise, 2121
59.6.2 White Frequency Noise, 2122
59.6.3 Long-Period Effects: Frequency Aging, 2123
59.6.4 Flicker Noise, 2124
59.7 Measuring Tools and Methods, 2126
59.8 Measurement Strategies, 2129
59.9 The Kalman Estimator, 2133
59.10 Transmitting Time and Frequency Information, 2135
59.10.1 Modeling the Delay, 2136
59.10.2 The Common-View Method, 2137
59.10.3 The "Melting-Pot" Version of Common View, 2138
59.10.4 Two-Way Methods, 2139
59.10.5 The Two-Color Method, 2139
59.11 Examples of the Measurement Strategies, 2141
59.11.1 The Navigation Satellites of the GPS, 2141
59.11.2 The One-Way Method of Time Transfer: Modeling the Delay, 2144
59.11.3 The Common-View Method, 2145
59.11.4 Two-Way Time Protocols, 2147
59.12 The Polling Interval: How Often Should I Calibrate a Clock?, 2152
59.13 Error Detection, 2155
59.14 Cost-Benefit Analysis, 2156
59.15 The National Time Scale, 2157
59.16 Traceability, 2158
59.17 Summary, 2159
59.18 Bibliography, 2160
References, 2160
60 Laboratory-Based Gravity Measurement 2163 Charles D. Hoyle, Jr.
60.1 Introduction, 2163
60.2 Motivation for Laboratory-Scale Tests of Gravitational Physics, 2164
60.3 Parameterization, 2165
60.4 Current Status of Laboratory-Scale Gravitational Measurements, 2166
60.4.1 Tests of the ISL, 2166
60.4.2 WEP Tests, 2167
60.4.3 Measurements of G, 2167
60.5 Torsion Pendulum Experiments, 2167
60.5.1 General Principles and Sensitivity, 2168
60.5.2 Fundamental Limitations, 2168
60.5.3 ISL Experiments, 2171
60.5.4 Future ISL Tests, 2172
60.5.5 WEP Tests, 2176
60.5.6 Measurements of G, 2176
60.6 Microoscillators and Submicron Tests of Gravity, 2177
60.6.1 Microcantilevers, 2177
60.6.2 Very Short-Range ISL Tests, 2177
60.7 Atomic and Nuclear Physics Techniques, 2178
Acknowledgements, 2178
References, 2178
61 Cryogenic Measurements 2181 Ray Radebaugh
61.1 Introduction, 2181
61.2 Temperature, 2182
61.2.1 ITS-90 Temperature Scale and Primary Standards, 2182
61.2.2 Commercial Thermometers, 2183
61.2.3 Thermometer Use and Comparisons, 2193
61.2.4 Dynamic Temperature Measurements, 2199
61.3 Strain, 2201
61.3.1 Metal Alloy Strain Gages, 2202
61.3.2 Temperature Effects, 2203
61.3.3 Magnetic Field Effects, 2204
61.3.4 Measurement System, 2205
61.3.5 Dynamic Measurements, 2205
61.4 Pressure, 2205
61.4.1 Capacitance Pressure Sensors, 2206
61.4.2 Variable Reluctance Pressure Sensors, 2206
61.4.3 Piezoresistive Pressure Sensors, 2208
61.4.4 Piezoelectric Pressure Sensors, 2210
61.5 Flow, 2211
61.5.1 Positive Displacement Flowmeter (Volume Flow), 2212
61.5.2 Angular Momentum Flowmeter (Mass Flow), 2212
61.5.3 Turbine Flowmeter (Volume Flow), 2213
61.5.4 Differential Pressure Flowmeter, 2213
61.5.5 Thermal or Calorimetric (Mass Flow), 2216
61.5.6 Hot-Wire Anemometer (Mass Flow), 2217
61.6 Liquid Level, 2218
61.7 Magnetic Field, 2219
61.8 Conclusions, 2220
References, 2220
62 Temperature-Dependent Fluorescence Measurements 2225 James E. Parks, Michael R. Cates, Stephen W. Allison, David L. Beshears, M. Al Akerman, and Matthew B. Scudiere
62.1 Introduction, 2225
62.2 Advantages of Phosphor Thermometry, 2227
62.3 Theory and Background, 2227
62.4 Laboratory Calibration of Tp Systems, 2235
62.5 History of Phosphor Thermometry, 2238
62.6 Representative Measurement Applications, 2239
62.6.1 Permanent Magnet Rotor Measurement, 2239
62.6.2 Turbine Engine Component Measurement, 2240
62.7 Two-Dimensional and Time-Dependent Temperature Measurement, 2241
62.8 Conclusion, 2243
References, 2243
63 Voltage and Current Transducers for Power Systems 2245 Carlo Muscas and Nicola Locci
63.1 Introduction, 2245
63.2 Characterization of Voltage and Current Transducers, 2247
63.3 Instrument Transformers, 2248
63.3.1 Theoretical Fundamentals and Characteristics, 2248
63.3.2 Instrument Transformers for Protective Purposes, 2252
63.3.3 Instrument Transformers under Nonsinusoidal Conditions, 2253
63.3.4 Capacitive Voltage Transformer, 2254
63.4 Transducers Based on Passive Components, 2255
63.4.1 Shunts, 2255
63.4.2 Voltage Dividers, 2256
63.4.3 Isolation Amplifiers, 2257
63.5 Hall-Effect and Zero-Flux Transducers, 2258
63.5.1 The Hall Effect, 2258
63.5.2 Open-Loop Hall-Effect Transducers, 2259
63.5.3 Closed-Loop Hall-Effect Transducers, 2259
63.5.4 Zero-Flux Transducers, 2262
63.6 Air-Core Current Transducers: Rogowski Coils, 2262
63.7 Optical Current and Voltage Transducers, 2267
63.7.1 Optical Current Transducers, 2268
63.7.2 Optical Voltage Transducer, 2271
63.7.3 Applications of OCTs and OVTs, 2272
References and Further Reading, 2273
64 Electric Power and Energy Measurement 2275 Alessandro Ferrero and Marco Faifer
64.1 Introduction, 2275
64.2 Power and Energy in Electric Circuits, 2276
64.2.1 DC Circuits, 2276
64.2.2 AC Circuits, 2277
64.3 Measurement Methods, 2282
64.3.1 DC Conditions, 2282
64.3.2 AC Conditions, 2285
64.4 Wattmeters, 2288
64.4.1 Architecture, 2288
64.4.2 Signal Processing, 2289
64.5 Transducers, 2290
64.5.1 Current Transformers, 2291
64.5.2 Hall-Effect Sensors, 2296
64.5.3 Rogowski Coils, 2297
64.5.4 Voltage Transformers, 2299
64.5.5 Electronic Transformers, 2302
64.6 Power Quality Measurements, 2303
References, 2305
Part Viii CHEMISTRY 2307
65 An Overview of Chemometrics for the Engineering and Measurement Sciences 2309 Brad Swarbrick and Frank Westad
65.1 Introduction: The Past and Present of Chemometrics, 2309
65.2 Representative Data, 2311
65.2.1 A Suggested Workflow for Developing Chemometric Models, 2313
65.2.2 Accuracy and Precision, 2313
65.2.3 Summary of Representative Data Principles, 2316
65.3 Exploratory Data Analysis, 2317
65.3.1 Univariate and Multivariate Analysis, 2317
65.3.2 Cluster Analysis, 2318
65.3.3 Principal Component Analysis, 2323
65.4 Multivariate Regression, 2352
65.4.1 General Principles of Univariate and Multivariate Regression, 2352
65.4.2 Multiple Linear Regression, 2354
65.4.3 Principal Component Regression, 2355
65.4.4 Partial Least Squares Regression, 2356
65.5 Multivariate Classification, 2369
65.5.1 Linear Discriminant Analysis, 2370
65.5.2 Soft Independent Modeling of Class Analogy, 2372
65.5.3 Partial Least Squares Discriminant Analysis, 2381
65.5.4 Support Vector Machine Classification, 2383
65.6 Techniques for Validating Chemometric Models, 2385
65.6.1 Test Set Validation, 2386
65.6.2 Cross Validation, 2388
65.7 An Introduction to Mspc, 2389
65.7.1 Multivariate Projection, 2389
65.7.2 Hotelling's T2 Control Chart, 2390
65.7.3 Q-Residuals, 2391
65.7.4 Influence Plot, 2391
65.7.5 Continuous versus Batch Monitoring, 2392
65.7.6 Implementing MSPC in Practice, 2394
65.8 Terminology, 2397
65.9 Chapter Summary, 2401
References, 2404
66 Liquid Chromatography 2409 Zhao Li, Sandya Beeram, Cong Bi, Ellis Kaufmann, Ryan Matsuda, Maria Podariu, Elliott Rodriguez, Xiwei Zheng, and David S. Hage
66.1 Introduction, 2409
66.2 Support Materials in Lc, 2412
66.3 Role of the Mobile Phase in Lc, 2413
66.4 Adsorption Chromatography, 2414
66.5 Partition Chromatography, 2415
66.6 Ion-Exchange Chromatography, 2417
66.7 Size-Exclusion Chromatography, 2419
66.8 Affinity Chromatography, 2421
66.9 Detectors for Liquid Chromatography, 2423
66.10 Other Components of Lc Systems, 2426
Acknowledgements, 2427
References, 2427
67 Mass Spectroscopy Measurements of Nitrotyrosine-Containing Proteins 2431 Xianquan Zhan and Dominic M. Desiderio
67.1 Introduction, 2431
67.1.1 Formation, Chemical Properties, and Related Nomenclature of Tyrosine Nitration, 2431
67.1.2 Biological Roles of Tyrosine Nitration in a Protein, 2432
67.1.3 Challenge and Strategies to Identify a Nitroprotein with Mass Spectrometry, 2432
67.1.4 Biological Significance Measurement of Nitroproteins, 2434
67.2 Mass Spectrometric Characteristics of Nitropeptides, 2434
67.2.1 MALDI-MS Spectral Characteristics of a Nitropeptide, 2434
67.2.2 ESI-MS Spectral Characteristics of a Nitropeptide, 2437
67.2.3 Optimum Collision Energy for Ion Fragmentation and Detection Sensitivity for a Nitropeptide, 2438
67.2.4 MS/MS Spectral Characteristics of a Nitropeptide under Different Ion-Fragmentation Models, 2440
67.3 Ms Measurement of in vitro Synthetic Nitroproteins, 2443
67.3.1 Importance of Measurement of In Vitro Synthetic Nitroproteins, 2443
67.3.2 Commonly Used In Vitro Nitroproteins and Their Preparation, 2443
67.3.3 Methods Used to Measure in Vitro Synthetic Nitroproteins, 2444
67.4 Ms Measurement of In Vivo Nitroproteins, 2446
67.4.1 Importance of Isolation and Enrichment of In Vivo Nitroprotein/Nitropeptide Prior to MS Analysis, 2446
67.4.2 Methods Used to Isolate and Enrich In Vivo Nitroproteins/Nitropeptides, 2446
67.5 Ms Measurement of In Vivo Nitroproteins in Different Pathological Conditions, 2449
67.6 Biological Function Measurement of Nitroproteins, 2456
67.6.1 Literature Data-Based Rationalization of Biological Functions, 2457
67.6.2 Protein Domain and Motif Analyses, 2459
67.6.3 Systems Pathway Analysis, 2459
67.6.4 Structural Biology Analysis, 2460
67.7 Pitfalls of Nitroprotein Measurement, 2462
67.8 Conclusions, 2463
Nomenclature, 2464
Acknowledgments, 2465
References, 2465
68 Fluorescence Spectroscopy 2475 Yevgen Povrozin and Beniamino Barbieri
68.1 Observables Measured in Fluorescence, 2476
68.2 The Perrin-Jablon¿ski Diagram, 2476
68.3 Instrumentation, 2479
68.3.1 Light Source, 2480
68.3.2 Monochromator, 2480
68.3.3 Light Detectors, 2481
68.3.4 Instrumentation for Steady-State Fluorescence: Analog and Photon Counting, 2483
68.3.5 The Measurement of Decay Times: Frequency-Domain and Time-Domain Techniques, 2484
68.4 Fluorophores, 2486
68.5 Measurements, 2487
68.5.1 Excitation Spectrum, 2487
68.5.2 Emission Spectrum, 2488
68.5.3 Decay Times of Fluorescence, 2490
68.5.4 Quantum Yield, 2492
68.5.5 Anisotropy and Polarization, 2492
68.6 Conclusions, 2498
References, 2498
Further Reading, 2498
69 X-Ray Absorption Spectroscopy 2499 Grant Bunker
69.1 Introduction, 2499
69.2 Basic Physics of X-Rays, 2499
69.2.1 Units, 2500
69.2.2 X-Ray Photons and Their Properties, 2500
69.2.3 X-Ray Scattering and Diffraction, 2501
69.2.4 X-Ray Absorption, 2502
69.2.5 Cross Sections and Absorption Edges, 2503
69.3 Experimental Requirements, 2505
69.4 Measurement Modes, 2507
69.5 Sources, 2507
69.5.1 Laboratory Sources, 2507
69.5.2 Synchrotron Radiation Sources, 2508
69.5.3 Bend Magnet Radiation, 2509
69.5.4 Insertion Devices: Wigglers and Undulators, 2509
69.6 Beamlines, 2512
69.6.1 Instrument Control and Scanning Modes, 2512
69.6.2 Double-Crystal Monochromators, 2513
69.6.3 Focusing Conditions, 2514
69.6.4 X-Ray Lenses and Mirrors, 2515
69.6.5 Harmonics, 2516
69.7 Detectors, 2518
69.7.1 Ionization Chambers and PIN Diodes, 2519
69.7.2 Solid-State Detectors, SDDs, and APDs, 2520
69.8 Sample Preparation and Detection Modes, 2521
69.8.1 Transmission Mode, 2521
69.8.2 Fluorescence Mode, 2521
69.8.3 HALO, 2522
69.8.4 Sample Geometry and Background Rejection, 2523
69.8.5 Oriented Samples, 2525
69.9 Absolute Measurements, 2526
References, 2526
70 Nuclear Magnetic Resonance (NMR) Spectroscopy 2529 Kenneth R. Metz
70.1 Introduction, 2529
70.2 Historical Review, 2530
70.3 Basic Principles of Spin Magnetization, 2531
70.4 Exciting the NMR Signal, 2534
70.5 Detecting the NMR Signal, 2538
70.6 Computing the NMR Spectrum, 2540
70.7 NMR Instrumentation, 2542
70.8 The Basic Pulsed FTNMR Experiment, 2550
70.9 Characteristics of NMR Spectra, 2551
70.9.1 The Chemical Shift, 2552
70.9.2 Spin-Spin Coupling, 2557
70.10 NMR Relaxation Effects, 2563
70.10.1 Spin-Lattice Relaxation, 2563
70.10.2 Spin-Spin Relaxation, 2565
70.10.3 Quantitative Analysis by NMR, 2568
70.11 Dynamic Phenomena in NMR, 2568
70.12 Multidimensional NMR, 2573
70.13 Conclusion, 2580
References, 2580
71 Near-Infrared Spectroscopy and Its Role in Scientific and Engineering Applications 2583 Brad Swarbrick
71.1 Introduction to Near-Infrared Spectroscopy and Historical Perspectives, 2583
71.1.1 A Brief Overview of Near-Infrared Spectroscopy and Its Usage, 2583
71.1.2 A Short History of NIR, 2585
71.2 The Theory behind Nir Spectroscopy, 2588
71.2.1 IR Radiation, 2588
71.2.2 The Mechanism of Interaction of NIR Radiation with Matter, 2588
71.2.3 Absorbance Spectra, 2591
71.3 Instrumentation for Nir Spectroscopy, 2595
71.3.1 General Configuration of Instrumentation, 2595
71.3.2 Filter-Based Instruments, 2597
71.3.3 Holographic Grating-Based Instruments, 2598
71.3.4 Stationary Spectrographic Instruments, 2600
71.3.5 Fourier Transform Instruments, 2601
71.3.6 Acoustooptical Tunable Filter Instruments, 2603
71.3.7 Microelectromechanical Spectrometers, 2604
71.3.8 Linear Variable Filter Instruments, 2605
71.3.9 A Brief Overview of Detectors Used for NIR Spectroscopy, 2606
71.3.10 Summary, 2608
71.4 Modes of Spectral Collection and Sample Preparation in Nir Spectroscopy, 2609
71.4.1 Transmission Mode, 2609
71.4.2 Diffuse Reflectance, 2611
71.4.3 Sample Preparation, 2613
71.4.4 Fiber Optic Probes, 2617
71.4.5 Summary of Sampling Methods, 2619
71.5 Preprocessing of Nir Spectra for Chemometric Analysis, 2620
71.5.1 Preprocessing of NIR Spectra, 2621
71.5.2 Minimizing Additive Effects, 2621
71.5.3 Minimizing Multiplicative Effects, 2627
71.5.4 Preprocessing Summary, 2633
71.6 A Brief Overview of Applications of Nir Spectroscopy, 2633
71.6.1 Agricultural Applications, 2634
71.6.2 Pharmaceutical/Biopharmaceutical Applications, 2636
71.6.3 Applications in the Petrochemical and Refining Sectors, 2644
71.6.4 Applications in the Food and Beverage Industries, 2646
71.7 Summary and Future Perspectives, 2647
71.8 Terminology, 2648
References, 2652
72 Nanomaterials Properties 2657 Paul J. Simmonds
72.1 Introduction, 2657
72.2 The Rise of Nanomaterials, 2660
72.3 Nanomaterial Properties Resulting from High Surface-Area-to-Volume Ratio, 2661
72.3.1 The Importance of Surfaces in Nanomaterials, 2661
72.3.2 Electrostatic and Van der Waals Forces, 2662
72.3.3 Color, 2663
72.3.4 Melting Point, 2663
72.3.5 Magnetism, 2664
72.3.6 Hydrophobicity and Surface Energetics, 2664
72.3.7 Nanofluidics, 2666
72.3.8 Nanoporosity, 2668
72.3.9 Nanomembranes, 2669
72.3.10 Nanocatalysis, 2670
72.3.11 Further Increasing the SAV Ratio, 2671
72.3.12 Nanopillars, 2672
72.3.13 Nanomaterial Functionalization, 2673
72.3.14 Other Applications for High SAV Ratio Nanomaterials, 2674
72.4 Nanomaterial Properties Resulting from Quantum Confinement, 2674
72.4.1 Quantum Well Nanostructures, 2677
72.4.2 Quantum Wire Nanostructures, 2682
72.4.3 Quantum Dot Nanostructures, 2691
72.5 Conclusions, 2695
References, 2695
73 Chemical Sensing 2707 W. Rudolf Seitz
73.1 Introduction, 2707
73.2 Electrical Methods, 2709
73.2.1 Potentiometry, 2709
73.2.2 Voltammetry, 2713
73.2.3 Chemiresistors, 2715
73.2.4 Field Effect Transistors, 2716
73.3 Optical Methods, 2717
73.3.1 In situ Optical Measurements, 2717
73.3.2 Raman Spectroscopy, 2719
73.3.3 Indicator-Based Optical Sensors, 2721
73.4 Mass Sensors, 2722
73.5 Sensor Arrays (Electronic Nose), 2724
References, 2724
Index 2727
54
LASER MEASUREMENT TECHNIQUES
Cecil S. Joseph1, Gargi Sharma1, Thomas M. Goyette2, and Robert H. Giles1,2
1 Biomedical Terahertz Technology Center, Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, MA, USA
2 Submillimeter-Wave Technology Laboratory, Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, MA, USA
54.1 INTRODUCTION
54.1.1 History and Development of the MASER
Lasers have enabled us to investigate the structure of atoms and molecules as well as accurately measure fundamental constants and observe natural phenomenology over a broad range of frequencies. But the applications of laser measurement technologies have advanced rapidly, for the instrumentation is little more than half a century in the making, and many laser-based measurement systems are now commercially available.
Albert Einstein first suggested the concept of stimulated emitted radiation in 1916. Indicating that a photon could cause the energy transition of an atom from an upper level to a lower level, Einstein proposed that the atom would emit a photon with the same energy as the photon initiating the energy transition. It was not until 1928 that Ladenburg observed stimulated emission.
As a faculty member at Columbia University, Charles H. Townes applied the concept to stimulating molecules in a resonant cavity to generate microwave radiation. His insight enabled postdoctoral fellow Herbert Zeiger and graduate student James P. Gordon to build a working maser by 1953. In 1954, Prokhorov and Basov of Moscow's Lebedev Physical Institute published the complete details for establishing stimulated microwave emission. Prokhorov, Basov, and Townes shared the 1964 Nobel Prize for their research.
54.1.1.1 From Maser to Laser: Extending the Operable Region
The science of stimulated emissions at wavelengths shorter than the microwave regime is very different. Though many physicists were now in pursuit of the technology, it was Charles Townes and Bell Lab's researcher Arthur L. Schawlow who first published the requirements for generating visible radiation in 1959. Detailing the parameters such as cavity structures, spontaneous emission ratios, and transition energy levels for generating visible radiation, Townes and Schawlow filed a patent for their development of "optical masers." However, Gordon Gould's graduate work at Columbia University predated Townes and Schawlow's patent and publication so after extended litigation Gould was awarded patents on the optical pumping techniques and his cavity designs using Brewster windows and a number of applications of what he referred to as "lasers."
While most researchers were building gas lasers at this point, Theodore H. Maiman was investigating the energy levels of ruby crystals. By the mid-1960s he demonstrated the first solid-state laser using a rod of synthetic ruby, thereby expanding the possibilities of light sources and the type of devices available. Through continued innovations by the research community, the performance characteristics of lasers were optimized, and these devices became the primary source for a rapidly growing market of measurement technologies.
54.1.2 Basic Laser Physics
Though there exist a large variety of laser devices that cover the electromagnetic spectrum from the microwave to the ultraviolet (higher frequency, i.e., X-ray lasers are also under development), the core elements of any laser device are:
- A laser medium
- A pump process
- Optical feedback elements
Different approaches to these basic elements and varied combinations thereof lead to the output frequency range of the device. Put simply, the pump process injects energy into the laser media and via optical feedback techniques, and part of this energy is recovered from the media in the form of coherent, stimulated photons, which make up a laser beam. Here we discuss the basic requirements of these elements and how they combine to form a laser.
54.1.2.1 Stimulated Emission and Atomic Rate Equations
A laser medium is made up of a collection of atoms/molecules in a gas, liquid, or solid phase. When the medium absorbs energy (e.g., by heating), some of the atoms transition to higher energy levels in the quantum mechanical energy structure. A fraction of these atoms spontaneously lose energy by emitting a photon and transition back to a lower energy state. In general, the spontaneous decay rate of any state is proportional to the number of atoms in that state. So if Ni(t) is the instantaneous population of an energy level, Ei, the spontaneous decay rate is given by , where ?i is the spontaneous decay rate of the energy level Ei with and ti being the lifetime of the state. (Note that both radiative and nonradiative transitions are allowed and implicitly included. Emitted photons are radiative transitions, while dissipation of energy via lattice phonons is nonradiative transitions.)
It is also possible to stimulate absorption from a lower energy level to a higher energy level by providing a photon that corresponds to the energy difference between the levels (i.e., , where h is the Planck constant and ? is the photon frequency). This stimulates both absorption and emission at the applied signal frequency. The primary difference between stimulated transitions and spontaneous transitions is that stimulated transitions are caused by an applied signal and as such the photons emitted are coherent with that signal. Spontaneous transitions, on the other hand, are radiated out by atoms driven independently of each other and as such are incoherent.
Consider a two-energy-level system, where level Ei has a population Ni and Ej has a population Nj. Moreover, let , corresponding to an energy difference, ?Eji. The spontaneous decay rate of level Ej to Ei is given by ?ji. If we now apply a signal to this system that corresponds to the energy difference, that is, , then we stimulate absorption from Ei to Ej, and we stimulate emission from Ej to Ei. If n(t) is the photon density of the incident signal, then the change in population of the higher energy state is given by
that is,
where K is a constant that measures the strength of the stimulated response.
Thus the rate at which atoms make stimulated transitions is proportional to the population difference of the two energy levels and the applied signal intensity. Each absorbed photon attenuates the applied signal, and each emitted photon amplifies it.
Thus the change in photon density of the applied signal due to stimulated transitions can be represented as , where ?Nij is the population difference between the lower and upper energy state. When there are more atoms in the lower energy state, , and the applied signal is attenuated. If, however, , the applied signal is amplified; this condition that requires a larger number of atoms in the higher energy state for signal amplification is one of the basic requirements of a working laser and is referred to as population inversion.
Before discussing the means to achieving population inversion in laser media, it is important to consider the difference between spontaneously emitted photons and stimulated photons. Stimulated transitions are caused by an applied signal and as such are coherent with the applied signal, unlike spontaneous emission, which is incoherent.
54.1.2.2 Population Inversion and Laser Amplification
Population inversion is an essential condition for laser amplification, that is, there must be a larger number of atoms in the upper excited state than the lower in order for the applied signal to be amplified. According to the Boltzmann principle, the relative populations of any two energy levels are given by where and . Therefore in order to create population inversion, we need to "pump" atoms into the upper excited state since the population difference is always attenuating at equilibrium for a two-level system. Typically other upper energy levels are used to "feed" the upper lasing level, thereby creating inversion.
As an example, consider the lasing levels and population inversion in a ruby laser. Ruby is essentially sapphire (Al2O3) doped with chromium. The Cr3+ ions replace a fraction of the Al3+ ions in the sapphire lattice. These Cr3+ ions in the lattice have energy levels in the red at approximately 694 nm. A representative energy-level diagram is shown in Figure 54.1.
FIGURE 54.1 Energy-level transitions in ruby.
In a typical ruby laser, atoms are optically pumped using a xenon arc lamp from the ground state to higher energy levels. Atoms in these levels relax rapidly down to highly metastable "R" levels via nonradiative transitions. With sufficient pumping, it is possible to create population inversion between the metastable (long lifetime) R levels and the ground state. This transition can then be amplified.
The reason inversion occurs without violating the Boltzmann principle is that through sufficient pumping more than half of the atoms transfer from the ground state to the higher energy states. These states have very short lifetimes...
