Sources

Usually electrical to optical converters1.Continuum sourcesa. Incandescent sourcesBlackbody sourcesTungsten filament sourcesb. ASE (EDFAs)c. LEDs2. Sources of line spectraa. Discharge lampsb. Arc lamps3. Coherent Sources (Lasers)a. Solid-state lasersb. Gas lasersc. Dye lasersd. 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 material1.Blackbody source2.Nernst glower3.Tungsten filament4.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~ 0.4 -0.5•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 shapedTunable: wavelength emitted depends on lasing medium uv to far ir

Types of Lasers

•Characterized by the active medium1.Solid-state lasers2.Gas lasers3.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 absorption2) 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 theirtemperature (τ~ few seconds)•Very accurate; used in standards labs tocalibrate 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 sensitiveCan detect a few photons per second⇒Intense light (e.g., room light) will damage a PMT due to the high currentsproduced

PMT Characteristics

•Fast response ~ 1 −10 nsdue to spread in arrival time of electronsat the anode•Spectral sensitivity

hν> Φ to eject an electron from thephotocathodeΦ~ 2 eV⇒λ< 620 nmCutoff 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

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 regionTrade-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 thelattice to create new free electrons⇒impact ionization or avalanche multiplication

Avalanche Photodiodes (APDs)

•Response speed is slower due to time requiredin secondary electron generation

Typical Performance Characteristics of p-i-nand APD Photodetectors