Photonics 2

Photonics 2

1. Optical Detectors

   • Thermal Detectors
     • Uses a change in temperature to detect light. Optical energy is converted into heat. This measurement is independent of wavelength.
     • Microbolometers
       • Light is transmitted onto a suspended amorphous silicon structure. The light causes the temperature of the structure to change and in turn causes the resistance to change which is measured. Advantages: Inexpensive, reacts to a large range of wavelengths, can be utilised at room temperature. Disadvantages: Material must have a large temperature coefficient, i.e. change in resistance due to temperature and have a small thermal mass. https://www.optotherm.com/microbolometers
       • Waveguide integrated microbolometer
         • To implement requires subwavelength grating couplers to minimise loss of light. For a suitable subwavelength grating coupler requires single mode condition, small leakage of the mode, good mechanical stability and low loss. Grating couplers effectively act as lateral cladding when the period of the grating is significantly smaller than that of the wavelength. This means the average reflectance of the coupler is less than the refractive index of the core. This in turn reduces the likelihood of light reflectance or scattering. https://github.com/Black-K1ng568/Year3-Revision/wiki/Bolometer
         • Fabrication
           • Lithography
           • Silicon Etching
           • BOX Etching with HF
             • Requires Large Holes (>400nm)
             • For Suspension of MMIs >3.5mm lateral BOX etch is required
         • Gold Antenna
           • High Absorption
           • Can't Be Etched by HF
   • Quantum Detectors
     • Photoconductor
       • https://github.com/Black-K1ng568/Year3-Revision/wiki/Photoconductors
     • Photodiode
       • Silicon PN Photodiode
         • PIN Diode
           • Thin p+ region
             • Reduced carrier diffusion processes
           • Intrinsic Region
             • Wider depletion region
               • Smaller Junction Capacitance
                 • Faster Photodiode
                 • Longer Transit Times
         • Responsive for light in wavelengths between 0.3um to 1.1um
         • Metal-semiconductor (Schottky) photodiode
         • Junction Capacitance is somewhat large due to small depletion layer width
           • Small depletion layer width causes inability to absorb longer wavelengths
         • Large Time Constant
       • Photodetectors
         • Graphene Photodetectors
           • Good Performance
           • Complex Fabrication due to layers
         • Silicon Photodectors
           • Silicon mid IR Photodector
             • Absorbs sub-bandgap photons
             • Suitable for standard silicon processing
             • Defect States introduced by low does inert ions
             • Low Yield
         • Ge Photodetectors
           • Evanescent Coupling
             • Grown directly on silicon waveguide
               • Simpler Processing
             • Requires 17um to absorb 95% of energy
           • Butt Coupling
             • Silicon is etched in trench, then Ge is grown on a waveguide
               • More Complicated Processing
             • Requires on 7um to absorb 95% of incident light
           • Popular Approach
           • Large Absorption at 1310nm and smaller at 1550nm
           • CMOS Compatible, but avoided by some foundries due to issues with growing Ge on Si
           • PIN Junction
         • III-V Photodetectors
           • Can Be Bonded On To Silion
           • Polymers Can Be Used To Bond
             • BCB
           • Good Performance
           • Low Yield
         • Others
         • Performance Metrics
           • Electro-optic Bandwidth
           • Eye Diagrams
           • Dark Current
           • Noise
         • Avalanche Photodiode
       • Characterisation Of Photodiodes
         • Quantum Efficiency
         • Responsivity
         • Speed/Bandwidth
           • Diffusion Of Carriers
           • Drift time
           • Capacitance Of Depletion Region

2. Laser and amplifier fundamentals

   • Laser Fundamentals
     • What does LASER stand for? Light Amplification by Stimulated Emission Of Radiation
     • Population Inversion and Threshold Conditions
       • Energy Levels In Matter
         • Energy Level Lifetime
         • 3 Level System
         • 4 Level System
       • Rate Equations
         • Rate Equations With Ampllifcation
           • Rate Equations With Saturation
             • Rate Equations with 3 Level System
             • Rate Equations With 4 Level System
         • Rate Equations Without Amplification
     • Active Materials
       • Optical Gain
       • Basics Of Photon Matter Interactions
       • Gas Lasers
       • Liquid Lasers
       • Solid State Lasers
         • Solid State Materials
           • Energy Level Splitting
           • Boltzmann Pop Distribution
           • Fermi-Dirac Distrbution
     • Gain Linewidth
       • Line Shape
         • Functions Describing Lineshapes
         • Transition Cross sectin
           • Fuchbauer-Landenburg relation
           • Probability Cross Sections
           • Gain Cross Section
         • Inhomogeneous Broadening
         • Homogeneous Broadening
     • Absorption and emission
     • Optical Transmission Through A Medium
       • Absorption Of Radiation
         • Amplification Of Radiation
       • Beer-Lambert Law
     • Einstein Coefficients
       • Boltzmann Distribution
         • Degeneracy
           • Energy Level Population Ratio
       • Einstein Relations
       • In thermal equilibrium rate of upward transitions is equal to rate of downward transitions
   • Optical Amplifiers
     • Semiconductor Amplifiers
       • Semiconductors
         • Intrinsic Semiconductors
         • Extrinsic Semiconductors
           • Carrier Concentration
         • Mechanisms for photon absorption and emission
           • Band to band transitions
           • Impurity to band transitions
           • Free carrier transitions
           • Exitonic transitions
         • Junctions
           • Homojunctions
             • pn homojunction
               • Biased pn homojunction
                 • Quasi-equilibrium Fermi Levels
           • Heterojunctions
             • pn heterojunction
         • Bandgap Wavelength
         • Transitions
           • Spontaneous Transitions
           • Stimulated Transitions
           • Amplifier bandwidth
         • Occupancy Probabilities
           • Absorption Conditions
           • Emission Conditions
         • Small Signal gain/ absorption
           • Gain Requirement
         • Pumping
         • Active Waveguides
       • Typical semiconductor amplifier/laser
     • Rare Earth Doped Amplifiers
       • Dielectrics
         • Host Materials
           • Amorphous Materials
             • Glass
           • Crystals Materials
             • Ceramics
       • Active elements
         • Transition Metals
           • Broadband/ tuneable lasers
         • Rare Earth Ions
           • Dominant atom elements for solid state lasers
           • Rare Earth Energy States
             • Quasi four-level
         • Actinides
           • Radioactive limited use
     • Noise Contributions
       • Gain in a lossy medium
       • Amplifier Noise
     • Gain Saturation
       • Transparency
   • Resonators and reflectors
     • Stability and Modes
       • Longitudinal Modes
       • Transverse Modes
     • Distributed Reflectors
       • Resonator
         • Resonator Loss
         • Resonator Free Spectral Range
         • Resonator Finesse
         • Resonator cavity Q
     • Single-frequency Operation
     • Intensity and Phase Noise

3. Semiconductor sources

4. Optical Communications

   • Fundamentals
   • Challenges

5. Radiative Transistions

   • Coherent Absorption
     • Intrinsic
       • Absorption of a photon leads to promotion of electron from the valence band to the conduction band across the bandgap
     • Extrinsic
       • Metal impurity ions absorb incident photons and are in turn promoted to higher energy levels. Occurs when photon energy is less than the bandgap energy. Electron rises to an intermediate energy level. These are formed due to the aforementioned defects.
       • Point Defects
       • Line Defects
       • Surface Defects
       • Volume Defects
     • Derived From Photon Flux per unit area and transition absorption cross sextion
   • Spontaneous Emission
     • LEDs
     • Uncorrelated Phase
     • Any Polarisation State
     • Incoherent emission
     • Rate of excited state decay is an exponetial
   • Stimulated Emission
     • Mutually Coherent
     • Same Frequency, Phase, Polarisation State and Direction
     • Rate Of Stim Emission Defined by photon flux and transition emission cross section
   • Recombination
     • Direct
     • Indirect
     • Nonradiative

6. Passive Devices

   • Fixed response, no change when electrically biased

7. Active Devices

   • Response changes to externally controlled inputs

8. splitters and Combiners

   • MZI
     • Optical Modulators
       • Direct Modulation
         • On-Off Keying In Lasers
       • External Modulation
       • Direct Vs Indirect Modulation
       • Electro-optic interactions
         • Electro-refraction
           • Change In refractive index due applied voltage. Most suitable with MZI
         • Electro-absorption
           • Change in absorption coefficient due to applied voltage. Uses EAM, electro-absorption modulator
       • Modulation Effects
         • Thermo-optic effect
         • Plasma-dispersion effect
         • Pockels Effect
         • Franz-Keldysh Effect

9. PN Junctions