Course 2. Lasers and Other Light Sources

Module 1. Principles of Laser Operation

    1.1 Energy levels
        1.1.1 Examples
            1.1.1.1 Ruby
            1.1.1.2 Nd:YAG
            1.1.1.3 HeNe
        1.1.2 Interaction of light with energy levels
            1.1.2.1 Absorption
            1.1.2.2 Spontaneous emission
            1.1.2.3 Stimulated emission
        1.1.3 Population inversion
            1.1.3.1 Optical pumping
            1.1.3.2 Electron excitation
            1.1.3.3 Resonant transfer of energy
    1.2 Principles of laser operation
        1.2.1 Laser structures
            1.2.1.1 Resonant cavity
            1.2.1.2 Plane and spherical mirrors
            1.2.1.3 Resonant cavity provides feedback
        1.2.2 Gain in laser media
            1.2.2.1 Under conditions of population inversion, there is net gain as light passes thru the laser medium
            1.2.2.2 Sources of loss
            1.2.2.3 Gain must exceed loss
        1.2.3 Buildup of laser operation
            1.2.3.1 Start with one spontaneous photon
            1.2.3.2 Stimulated emission of photons
                    - same frequency as stimulating photon
                    - same direction as stimulating photon
                    - continued stimulated emission process
            1.2.3.3.Energy in population inversion converted to light
            1.2.3.4 Light escapes thru partially reflecting mirror
            1.2.3.5 Threshold for laser operation
    1.3 Characteristics of laser emission
        1.3.1 Longitudinal modes
            1.3.1.1 Origin
            1.3.1.2 Spacing
            1.3.1.3 Number compared to spectral width
        1.3.2 TEM modes
            1.3.2.1 Origin
            1.3.2.2 Spatial profiles
            1.3.2.3 Gaussian TEM00 mode
            1.3.2.4 Beam quality (M2)
        1.3.3 Q-switching
            1.3.3.1 Definition of Q
            1.3.3.2 Basic operation of Q-switches
            1.3.3.3 Properties of Q-switched pulses


Module 2. Visible gas lasers

    2.1 Introduction to visible gas lasers
        2.1.1 Electrical discharges in gases
            2.1.1.1 Current-voltage characteristics
            2.1.1.2 Low voltage-low current regime
            2.1.1.3 Breakdown
            2.1.1.4 Negative resistance regime
        2.1.2 Types of visible gas lasers
            2.1.2.1 HeNe
            2.1.2.2 Argon
            2.1.2.3 HeCd
            2.1.2.4 Krypton
    2.2 The helium-neon laser
        2.2.1 Energy level diagram
            2.2.1.1 Gas mixture
            2.2.1.2 Resonant excitation
            2.2.1.3 Laser transitions
            2.2.1.4 Buffer gas
            2.2.1.5 .Gas pressures
            2.2.1.6 Effect of external magnetic field
        2.2.2 Plasma tube
            2.2.2.1 Capillary tube
            2.2.2.2 Gas reservoir
            2.2.2.3 Electrode structure
            2.2.2.4 Getter
            2.2.2.5 Breakdown characteristics
        2.2.3 Power supply
            2.2.3.1 Step-up transformer
            2.2.3.2 High-voltage exciter
            2.2.3.3 Rectifier/voltage multiplier
            2.2.3.4 Ballast resistance
        2.2.4 Resonant cavity
            2.2.4.1 Mirror transmission vs wavelength
            2.2.4.2 Mirror configuration
        2.2.5 Output characteristics of HeNe laser
            2.2.5.1 Output vs gas pressure
            2.2.5.2 Output vs current
            2.2.5.3 Beam divergence
            2.2.5.4 Other wavelengths than 632.8 nm
        2.2.6 Care, maintenance and operation
            2.2.6.1 Cleaning
            2.2.6.2 Alignment
            2.2.6.3 Operating procedures
    2.3 The argon laser
        2.3.1 Energy level diagram
            2.3.1.1 Levels are those of argon ion
            2.3.1.2 Laser transitions
            2.3.1.3 Excitation processes
            2.3.1.4 Competition between lines
            2.3.1.5 Effect of external magnetic field
        2.3.2 Plasma tube
            2.3.2.1 Segmented bore
            2.3.2.2 Gas bypass channels
            2.3.2.3 Gas ballast
            2.3.2.4 Gas fill valves
            2.3.2.5 Solenoid
        2.3.3 Resonant cavity
            2.3.3.1 Mirror transmission vs wavelength
            2.3.3.2 Mirror configuration
            2.3.3.3 Wavelength selecting prism
        2.3.4 Cooling system
            2.3.4.1 Need for cooling
            2.3.4.2 Design of cooling system
        2.3.5 Operating characteristics
            2.3.5.1 Output vs current
            2.3.5.2 Output vs gas pressure
            2.3.5.3 Output vs magnetic field
            2.3.5.4 Power available in different lines
            2.3.5.5 Beam divergence
            2.3.5.6 Ultraviolet operation
        2.3.6 Care, maintenance and operation
            2.3.6.1 Cleaning
            2.3.6.2 Alignment
            2.3.6.3 Degradation mechanisms
            2.3.6.4 Cooling system maintenance
            2.3.6.5 Installation of wavelength-selecting prism
            2.3.6.6 Operating procedures
    2.4. The HeCd laser
        2.4.1 Energy levels
            2.4.1.1 Uses Cd ions
            2.4.1.2 Similarities to HeNe
        2.4.2 Structure
            2.4.2.1 Cadmium vapor source
            2.4.2.2 Problems arising from presence of Cd vapor
            2.4.2.3 Plasma tube design
        2.4.3 Output characteristics
            2.4.3.1 Wavelengths
            2.4.3.2 Output vs current
    2.5 The krypton laser
        2.5.1 Energy levels
            2.5.1.1 Uses ion energies
            2.5.1.2 Similarities to argon
            2.5.1.3 Additional red lines
        2.5.2 Structure    
            2.5.2.1 Similarities to argon laser
            2.5.2.2 Wavelength selection
        2.4.3 Output characteristics
            2.5.3.1 Wavelengths
            2.5.3.2 Output vs current
            2.5.3.3 Mixed argon-krypton lasers


Module 3. CO2 Lasers

    3.1 Basics of CO2 lasers
        3.1.1 Energy level diagram
            3.1.1.1 Vibrations of the CO2 molecule
            3.1.1.2 Excitation process
            3.1.1.3 Laser transitions
            3.1.1.4 Rotational substructure
            3.1.1.5 Complete CO2 laser spectrum
            3.1.1.6 Usual spectrum for uncontrolled emission
        3.1.2 Gas mixtures
            3.1.2.1 Role of each gas
            3.1.2.2 Typical gas mixtures for laser operation
    3.2 Types of CO2 laser
        3.2.1 Axial flow
            3.2.1.1 Slow flow
            3.2.1.2 Scaling of output with flow rate
            3.2.1.3 Fast flow
            3.2.1.4 Output power available
        3.2.2 Transverse flow
            3.2.2.1 Allows fast flow with less impedance
            3.2.2.3 2Output power available
            3.2.2.3 Transverse excitation
            3.2.2.4 Output available and size for commercial slow flow, fast flow and transverse flow lasers
        3.2.3 Waveguide
            3.2.3.1 Scaling with tube diameter
            3.2.3.2 Structure of waveguide lasers
            3.2.3.3 Output available from waveguide lasers
        3.2.4 TEA
            3.2.4.1 Scaling of pulse energy with pressure
            3.2.4.2 Breakdown of gas mixture at high pressure
            3.2.4.3 Solutions to breakdown problem
            3.2.4.4 Output from TEA lasers
        3.2.5 Gas dynamic lasers
            3.2.5.1 Production of population inversion during gas expansion
            3.2.5.2 Structure of gas dynamic lasers
            3.2.5.3 Output of gas dynamic lasers
    3.3 CO2 laser structure
        3.3.1 Tube structures
            3.3.1.1 Scaling of output versus tube length
            3.3.1.2 Bending of tubes to attain longer length
            3.3.1.3 Electrode structure
            3.3.1.4 Cooling provisions
        3.3.2 Resonant cavity
            3.3.2.1 Mirror substrate materials
            3.3.2.2 Mirror configurations
            3.3.2.3 Wavelength tuning using grating
        3.3.3 Gas flow
            3.3.3.1 Gas supply
            3.3.3.2 Gas handling system
        3.3.4 Power supply
            3.3.4.1 Functions of power supply
            3.3.4.2 Typical power supply circuit
    3.4 Characteristics of CO2 lasers
        3.4.1 Continuous
            3.4.1.1 Effect of tube current
            3.4.1.2 Effect of gas pressure
            3.4.1.3 Effect of gas flow rate
            3.4.1.4 Output power available
            3.4.1.5 Beam divergence
        3.4.2 Pulsed
            3.4.2.1 Electrically pulsed: Pulse energy and pulse shape
            3.4.2.2 TEA: Pulse energy and pulse shape
    3.5 Care, maintenance and operation
        3.5.1 Maintenance
            3.5.1.1 Cleaning of optics
            3.5.1.2 Degradation of optics
            3.5.1.3 Replacement of optics
            3.5.1.4 Cooling system maintenance
            3.5.1.5 Electrode replacement
        3.5.2 Operating procedure    
            3.5.2.1 Cooling system operation
            3.5.2.2 Turn on and off
            3.5.2.3 Operation of gas handling system
            3.5.2.4 Wavelength selection


Module 4. Dye Lasers

    4.1 Energy transfer in dye lasers
        4.1.1 Basic information about organic dyes
            4.1.1.1 Nature of organic dyes
            4.1.1.2 Typical examples of dyes
            4.1.1.3 Optical properties of dyes
        4.1.2 Energy level diagram
            4.1.2.1 Singlet states
            4.1.2.2 .Laser excitation
            4.1.2.3 Tunability of dye lasers
            4.1.2.4 Triplet states
            4.1.2.5 Methods to overcome triplet losses
    4.2 Structure of dye lasers
        4.2.1 Continuous dye lasers
            4.2.1.1 Jet stream
            4.2.1.2 Optical cavity
            4.2.1.3 Tuning elements
            4.2.1.4 Pump sources
            4.2.1.5 Dye handling
            4.2.1.6 .Output characteristics
            4.2.1.7 Tuning range
        4.2.2 Pulsed dye lasers
            4.2.2.1.Pumping methods
            4.2.2.2.Ultrashort pulses
        4.2.3 Ring dye lasers
            4.2.3.1 Principles
            4.2.3.2 Advantages
            4.2.3.3 Characteristics
    4.3 Care, maintenance and operation
        4.3.1 Maintenance
            4.3.1.1 Cleaning of optics
            4.3.1.2 Alignment    
            4.3.1.3 Dye lifetime
            4.3.1.4 Changing of dyes
        4.3.2 Operating procedures        
            4.3.2.1 Turn on and off
            4.3.2.2 Operation of dye handling system
            4.3.2.3 Wavelength tuning


Module 5. Diode Lasers

    5.1 Basics of semiconductor junctions and semiconductor laser operation
        5.1.1 Semiconductor band structure
            5.1.1.1 Origin of band structure
            5.1.1.2 The valence band
            5.1.1.3 The conduction band
            5.1.1.4 The band gap
            5.1.1.5 Holes and electrons
            5.1.1.6 p and n type material
            5.1.1.7 The p-n junction
        5.1.2 Energy levels for laser operation
            5.1.2.1 Current flow through the junction
            5.1.2.2 Production of population inversion
            5.1.2.3 Recombination radiation
    5.2 Structure of diode lasers
        5.2.1 Junction types
            5.2.1.1 Homojunctions
            5.2.1.2 Single heterojunctions
            5.2.1.3 Double heterojunctions
            5.2.1.4 Advantages of the double heterojunction
            5.2.1.5 Deposition techniques to form the junctions
        5.2.2 Confining the beam
            5.2.2.1 The active region
            5.2.2.2 Optical confinement
            5.2.2.3 Stripe geometry
            5.2.2.4 Gain guided and index guided devices
        5.2.3 Diode laser bars
            5.2.3.1 Structure
            5.2.3.2 Allow higher power
        5.2.4 Quantum well devices
            5.2.4.1 Superlattices
            5.4.2.2 Typical quantum well laser structures
    5.3 Semiconductor diode materials
        5.3.1 Aluminum gallium arsenide
            5.3.1.1 Ternary alloys
            5.3.1.2 Index of refraction and band gap vs composition parameter
            5.3.1.3 Wavelength range
        5.3.2 Indium gallium arsenide phosphide
            5.3.2.1 Quaternary alloys
            5.3.2.2 Index of refraction and band gap vs composition parameters
            5.3.2.3 Wavelength range
        5.3.3 Aluminum indium gallium phosphide
            5.3.3.1 Index of refraction and band gap vs composition parameters
            5.3.3.2 Wavelength range
    5.4 Characteristics of diode lasers
        5.4.1 Current-output characteristics
            5.4.1.1 Sub threshold behavior
            5.4.1.2 Threshold
            5.4.1.3 The linear region
            5.4.1.4 Catastrophic optical damage
        5.4.2 Spectrum
            5.4.2.1 Sub threshold spectrum
            5.4.2.2 Above threshold spectrum
            5.4.2.3 Modes
            5.4.2.4 Single mode devices
        5.4.3 Beam divergence
            5.4.3.1 Origin of the fan shaped beam
            5.4.3.2 Typical beam divergence values
            5.4.3.3 Optics to circularize the beam
        5.4.4 Temperature tuning
            5.4.4.1 Magnitude of the temperature shift
            5.4.4.2 Mode hopping
            5.4.4.3 Methods to eliminate mode hopping
        5.4.5 Pulse characteristics
            5.4.5.1 Pulse characteristics
            5.4.5.2 Quasicontinuous operation
            5.4.5.3 Continuous operation
        5.4.6 Output power available
            5.4.6.1 Single junction devices
            5.4.6.2 Laser diode bars
    5.5 Care and operation of diode lasers
        5.5.1 Damage mechanisms
            5.5.1.1 Catastrophic optical damage
            5.5.1.2 Static electricity
        5.5.2 Using diode lasers
            5.5.2.1 Proper handling
            5.5.2.2 Avoiding excessive current
            5.5.2.3 Operating procedures


Module 6. Excimer Lasers

    6.1 Principles of excimer lasers
        6.1.1 Nature of an excimer
            6.1.1.1 Noble gas chemistry
            6.1.1.2 Metastable states
        6.1.2 Energy level diagram
            6.1.2.1 Energy vs separation of atoms
            5.1.2.2 Excitation process
            6.1.2.3 Dissociation
    6.2 Types of excimer lasers
        6.2.1 KrF
            6.2.1.1 Gases used
            6.2.1.2 Wavelength
        6.2.2 .XeF
            6.2.2.1 Gases used
            6.2.2.2 Wavelength
        6.2.3 .XeCl
            6.2.3.1 Gases used
            6.2.3.2 Wavelength
        6.2.4 .ArF
            6.2.4.1 Gases used
            6.2.4.2 Wavelength
    6.3. Structure of excimer lasers
        6.3.1 Gas chamber
            6.3.1.1 Electrode structure
            6.3.1.2 Electric discharge
        6.3.2 Gas mix
            6.3.2.1 The buffer gas
            6.3.2.2 Gas handling
            6.3.2.3 Safety issues
        6.3.3 Optical cavity
            6.3.3.1 Mirrors for the ultraviolet
            6.3.3.2 Mirror configurations
        6.3.3.4 Power supply
            6.3.4.1 Need for a short pulse
            6.3.4.2 Typical circuit
    6.4 Characteristics of excimer lasers
        6.4.1 Pulse shape
            6.4.1.1 Excimers are inherently pulsed
            6.4.1.2 Pulse duration
        6.4.2 Power
            6.4.2.1 Energy per pulse
            6.4.2.2 Variation of energy per pulse with pulse rep rate
        6.4 3 Beam profile
            6.4.3.1 Rectangular beam shape
            6.4.3.2 Presence of dark lines
        6.4.4 Spectrum
            6.4.4.1 Lines present in the output
            6.4.4.2 Control of the lines present
    6.5 Operation and maintenance of excimer lasers
        6.5.1 Degradation of excimer laser output
            6.5.1.1 Decrease of pulse energy with time
            6.5.1.2 Damage from corrosive gases
        8.5.2 Maintenance procedures
            6.5.2.1 Gas replacement
            6.5.2.2 Mirror replacement
            6.5.2.3 Electrode replacement
        6.5.3 Operating procedures
            6.5.3.1 Turn on and off
            6.5.3.2 Control of output power


Module 7. Solid State Lasers

    7.1 The Nd:YAG laser
        7.1.1 Basics of Nd:YAG lasers
            7.1.1.1 Energy level diagram
            7.1.1.2 Excitation process
            7.1.1.3 Laser transitions
        7.1.2 Types of Nd:YAG laser
            7.1.2.1 Continuous
            7.1.2.2 Pulsed
            7.1.2.3 Continuously pumped, repetitively Q-switched
        7.1.3 Nd:YAG laser structure
            7.1.3.1. Rod configurations
            7.1.3.2. Resonant cavity
        7.1.3. Pump sources
            7.3.3.1 Flashlamp
            7.3.3.2 Arc lamp
            7.3.3.3 Laser diode
        7.1 4. Cooling
            7.1.4.1 Cooling requirements
            7.1.4.2 Typical cooling systems
        7 1.5. Power supply
            7.1.5.1 Continuous power supplies
            7.1.5.2 Pulsed power supplies
        7.1.6. Q-switches
            7.1.6.2 Q-switch characteristics
            7.1.6.3 The acousto-optic Q-switch
        7.1.7. Frequency multiplication
            7.1.7.1 Basic principles on nonlinear optics
            7.1.7.2 Frequency doubling, tripling and quadrupling
            7.1.7.3 Nonlinear materials
            7.1.7.4 Structure of a frequency doubles
        7.1.8. Characteristics of Nd:YAG lasers
            7.1.8.1. Continuous (Power, mode structure, beam divergence)
            7.1.8.2 Pulsed (Power, pulse duration, mode structure, beam divergence)
            7.1.8.3. Continuously pumped, repetitively Q-switched (Power, pulse duration, pulse repetition rate, mode structure, beam divergence)
            7.1.8.4 Frequency doubled (Power, mode structure, beam divergence)
            7.1.8.5 Ultraviolet operation with frequency tripling and quadrupling
            7.1.8.6 Diode pumped vs lamp pumped Nd:YAG lasers
        7.1.9. Care, maintenance and operation
            7.1.9.1. Cleaning of optics
            7.1.9.2. Alignment procedures
            7.1.9.3. Cooling system maintenance
            7.1.9.4. Lamp replacement
            7.1.9.5. Operating procedures
    7.2.Nd in other hosts
        7.2.1 Nd:YLF
            7.2.1.1 Wavelength
            7.2.1.2 Comparison to Nd:YAG
        7.2.2 Nd:glass
            7.2.2.1 Wavelength
            7.2.2.2 Low thermal conductivity makes it only a pulsed laser
            7.2.2.3 Medium capable of high energy storage
    7.3 Ruby
        7.3.1 Basics of Nd:YAG lasers
            7.3.1.1 Energy level diagram
            7.3.1.2 Excitation process
            7.3.1.3 The laser transition
            7.3.1.4 Laser transition goes to the ground state
            7.3.1.5 Excitation energy requirements are large
            7.3.1.6 Ruby was the first laser
        7.3.2 Ruby laser structure
            7.3.2.1 Laser rod
            7.3.2.2 Flashlamps
            7.3.3.3 Resonant cavity
            7.3.3.4 Q-switches
        7.3.3 Ruby laser properties
            7.3.3.1 Ruby is only a pulsed laser
            7.3.3.2 Pulse energy: normal pulse and Q-switched
            7.3.3.3 Pulse duration: normal pulse and Q-switched
            7.3.3.4 Beam divergence
    7.4 Other rare earth ions
        7.4.1 Ho:YAG
            7.4.1.1 Wavelength
            7.4.1.2 Properties
        7.4.2 Er:YAG
            7.4.2.1 Wavelength
            7.4.2.2 Properties
    7.5 Vibronic systems
        7.5..1 Nature of a vibronic system
            7.5.1.1 Potential well diagram
            7.5.1.2 Both electronic state and vibrational state change
            7.4.1.3 Offers possibility of tunable solid state lasers
        7.5.2 Alexandrite
            7.5.2.1 Wavelength range
            7.5.2.2 Properties
        7.5.3 Ti:sapphire
            7.5.3.1 Wavelength range
            7.5.3.2 Properties
            7.5.3.3 Frequency doubled operation


Module 8. Modulators, Q-switches, Mode-Locking

    8.1 The electro-optic effect
        8.1.1 Birefringence
            8.1.1.1 Quarter-wave plates
            8.1.1.2 Half-wave plates
        8.1.2 Induced birefringence and the electro-optic effect
            8.1.2.1 Definition
            8.1.2.2 Electro-optic materials
            8.1.2.3 Index change versus voltage
            8.1.2.4 Characteristics of electro-optic devices
    8.2 Use of electro-optic devices as modulators
        8.2.1 Modulator configuration
            8.2.1.1 Longitudinal modulators
            8.2.1.2 Transverse modulators
            8.2.1.3 Crossed field modulators
        8.2.2 Modulator characteristics
            8.2.2.1 Transmission vs voltage
            8.2.2.2 The half wave voltage
            8.2.2.3 Losses
            8.2.2.4 Spectral range
        8.2.3 Procedures
            8.2.3.1 Alignment
            8.2.3.2 Mounting and installation
            8.2.3.3 Operating procedures
    8.3 Applications of electro-optic devices
        8.3.1 Use of electro-optic devices as deflectors
            8.3.1.1 Deflector configuration
            8.3.1.2 Deflector characteristics
            8.3.1.3 Alignment, mounting, installation, and operation of deflectors
        8.3.2 .Use of acousto-optic devices as Q-switches
            8.3.2.1 Q-switch configuration
            8.3.2.2 Alignment, mounting, installation, and operation of Q-switches.
    8.4 The acousto-optic effect
        8.4.1 Basics of acousto-optics
            8.4.1.1 Definition
            8.4.1.2 Acousto-optic materials
            8.4.1.3 Deflection angle
            8.4.1.4 Figure of merit
        8.4.2 Characteristics of acousto-optic devices
            8.4.2.1 Contrast ratio
            8.4.2.2 Speed
            8.4.2.3 Resolvable spots
            8.4.2.4 Efficiency
            8.4.2.5 Structure of devices
            8.4.2.6 Comparison with other devices
    8.5 Applications of acousto-optic devices
        8.5.1 Use as modulators
            8.5.1.1 Modulator configuration
            8.5.1.2 Modulator characteristics
            8.5.1.3 Alignment, mounting, installation, and operation of acousto-optic modulators
        8.5.2 Use of acousto-optic devices as deflectors
            8.5.2.1 Deflector configuration
            8.5.2.2 Deflector characteristics - tradeoff between bandwidth and resolution
            8.5.2.3 Alignment, mounting, installation, and operation of acousto-optic deflectors
        8.5.3 Use of acousto-optic devices as Q-switches
            8.5.3.1 Q-switch configuration
            8.5.3.2 Alignment, mounting, installation, and operation of acousto-optic Q-switches.
    8.6 Mechanical devices
        8.6.1 Types of mechanical devices
            8.6.1.1 Rotating polygons
            8.6.1.2 Rotating prisms
            8.6.1.3 Galvanometers
            8.6.1.4 Piezoelectric devices
        8.6.2 Uses of mechanical devices
            8.6.2.1 Applications as beam scanners
            8.6.2.2 Limitations as modulators and Q-switches
            8.6.2.3 Comparison of mechanical devices, among themselves and with different technologies
    8.7 Mode-locking
         8.7.1 Origin
            8.7.1.1 Requires many longitudinal modes to be present in laser output
            8.7.1.2 Phases lock to produce short high intensity pulses
            8 7.1.3 Relation to linewidth
        8.7.2 Properties of mode-locked pulses
            8.7.2.1 Picosecond duration
            8.7.2.2 Pulse trains
            8.7.2.3 Pulse spacing
            8.7.2.4 Selection of single picosecond pulses
        8.7.3 Devices for mode-locking
            8.7.3.1 Dye cells
            8.7.3.2 Electro-optic devices


Module 9. Incoherent Light Sources

    9.1 Types of light sources
        9.1.1 Spatial criteria
            9.1.1.1 Point sources
            9.1.1.2 Extended sources
        9.1.2 Spectral criteria
            9.1.2.1 Continuum sources
            9.1.2.2 Discrete spectra (e.g. Fraunhofer lines)
    9.2 Incoherent light sources
        9.2.1 Incandescent light sources
            9.2.1.1 Principles of operation
            9.2.1.2 Structure
            9.2.1.3 Tungsten filament lamps
            9.2.1.4 Halogen lamps
            9.2.1.5 Properties
            9.2.1.6 Spectra
        9.2.2 Fluorescent light sources
            9.2.2.1 Principles of operation
            9.2.2.2 Structure
            9.2.2.3 Properties
            9.2.2.4 Spectra
        9.2.3 Arc lamps
            9.2.3.1 Principles of operation
            9.2.3.2 Carbon and mercury arc lamps
            9.2.3.3 Structure
            9.2.3.4 Properties
            9.2.3.5 Spectra
        9.2.4 Gas discharge lamps
            9.2.4.1 Gas discharges in gases
            9.2.4.2 Continuous and pulsed
            9.2.4.3 Osram lamps
            9.2.4.4 Structure
            9.2.4.5 Properties
            9.2.4.6 Spectra
        9.2.5 LEDs
            9.2.5.1 Principles of operation
            9.2.5.2 Structure
            9.2.5.3 Properties
            9.2.5.4 Spectra
        9.2.6 The sun
            9.2.6.1 Spectrum
            9.2.6.2 The solar constant

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