Sources Usually electrical to optical converters 1.Continuum sources a. Incandescent sources Blackbody sources Tungsten filament sources b. ASE (EDFAs) c. LEDs 2. Sources of line spectra a. Discharge lamps b. Arc lamps 3. Coherent Sources (Lasers) a. Solid-state lasers b. Gas lasers c. Dye lasers d. Semiconductor lasers

Continuum Sources Most continuum sources can be approximated as blackbodies Blackbodies: an object in thermal equilibrium with its surroundings (e.g., a cavity with a small hole)

Emissivity

Incandescent Sources A source that emits light by heating a material 1.Blackbody source 2.Nernst glower 3.Tungsten filament 4.Tungsten arc lamp Nernst glower

Tungsten Lamps A W filament heated by an electrical current and sealed in a glass tube Quartz used for uvemission (cutoff λ ~180 nm vs. 300 nm for glass) Emits from uv to ir Gray body with Є~ Halogen vapour (iodine or bromine) used to regenerate the filament

Amplified Spontaneous Emission (ASE) Sources Erbium-doped fibre amplifier (EDFA) An optical fibre doped with erbium and excited by a pump laser Spontaneous emission in the Er-doped fibre is amplified (amplified spontaneous emission, ASE)

Light Emitting Diodes (LEDs) Wavelength of light emitted depends onbandgapof semiconductor material

Spectral Bandwidth

Sources of Line Spectra Due to electronic transitions between energy levels in gas atoms Well-known transitions wavelength standards

Sources of Line Spectra Discharge & arc lamps: Large voltage applied between electrodes in a gas-filled tube Electrons in gas atoms are excited to higher energy levels, leading to light emission Wavelengths emitted depend on the gas

Lasers Light amplification by stimulated emission of radiation 3 processes involved in the interaction of em radiation with matter: Absorption, Spontaneous Emission, Stimulated Emission. Properties of Laser Light identical energy, direction, phase & polarization Monochromatic: Δλ~ 10^-4nm (laser diode) to 10-10nm (HeNelaser) Coherent: lc~ 15 x 10^6m (HeNelaser) Directional: Δθ~ 10^-3rad(due to diffraction) Intense: few mW(HeNelaser) to 800 W (Nd:YAG) Focused: beam can be focused down to ~ λ, far-field pattern of beam is usually Gaussian shaped Tunable: wavelength emitted depends on lasing medium uv to far ir

Types of Lasers Characterized by the active medium 1.Solid-state lasers 2.Gas lasers 3.Dye lasers Dye Lasers A liquid (usually organic molecules) excited optically Some of the organic molecules used in these lasers are commercial dyes

Femtosecond Lasers, Resonance Frequencies

Generacja drugiej harmonicznej

Detectors Detectors are usually optical to electrical converters Two types: 1)Thermal detectors: Detect light by measuring the heat produced upon absorption 2) Quantum detectors: Detect light by the generation of electron-hole pairs The photon plays a major role in these detectors

Thermal Detectors Detect light by measuring the heat produced upon absorption Types: Thermocouples/thermopiles (voltage-based) Thermistors/bolometers(resistance-based) Pyroelectric(surface charge) Pneumatic (gas pressure) Low sensitivity ( 1 μW) Slow due to time required to change their temperature (τ~ few seconds) Very accurate; used in standards labs to calibrate other detectors & light sources Wavelength insensitive

Jeśli detektor ma czułość 1 mikoWat, to ile fotonów musi jednocześnie dotrzeć do detektora, by wytworzyć sygnał? Przyjąć długość fali l=500 nm.

Quantum Detectors Detect light by the generation of electron-hole (e-h) pairs Very sensitive (~1 pW, 90 dBm) Fast (i.e., high modulation frequency bandwidth) Types: Photon absorption produces e-h pairs that escape from the detector material as free electrons e.g., photomultiplier tubes (PMT) Electrons remain within the material and serve to increase its conductivity e.g., p-i-nphotodiode avalanche photodiodes (APD)

Photocathode Alkali metals usually used due to their low work functions Some photocathode materials

Electron Multiplication Secondary electron emission

PMT

Multiplication Factor PMTsare highly sensitive Can detect a few photons per second Intense light (e.g., room light) will damage a PMT due to the high currents produced

PMT Characteristics Fast response ~ 1 10 ns due to spread in arrival time of electrons at the anode Spectral sensitivity hν> Φ to eject an electron from the photocathode Φ~ 2 eV λ< 620 nm Cutoff wavelength due to glass (~ 300 nm) or quartz (~120 nm) PMTs are only useful in the uv and visible regions

p-n Photodiodes A reverse-biased p-njunction Operates like a surface-emitting LED but in reverse

I-V Curve & Responsivity

Responsivity

Quantum Efficiency

Absorption

p-i-nPhotodiode Design Want x1to be small (minimum absorption through p region) Introduce thin heterostructure Want L to be large (maximize absorption) Introduce thick intrinsic region Want R´ to be small Use anti-reflection coating

p-i-nResponse Speed The speed of a photodiode is determined by the transit time for electrons to cross the intrinsic region We want a thin depletion region Trade-off between sensitivity and speed

Avalanche Photodiodes (APDs) APDshave an internal gain Operate in the breakdown region of the I-V curve

Avalanche Photodiodes (APDs) Electrons are accelerated and collide with the lattice to create new free electrons impact ionization or avalanche multiplication

Avalanche Photodiodes (APDs) Response speed is slower due to time required in secondary electron generation

Typical Performance Characteristics of p-i-nand APD Photodetectors