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Sources

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  1. Sources Usually electrical to optical converters 1.Continuum sources a. Incandescentsources Blackbodysources Tungsten filamentsources b. ASE (EDFAs) c. LEDs 2. Sources of line spectra a. Dischargelamps b. Arclamps 3. CoherentSources (Lasers) a. Solid-state lasers b. Gas lasers c. Dyelasers d. Semiconductor lasers

  2. Continuum Sources •Most continuum sources can be approximatedas blackbodies •Blackbodies: an object in thermal equilibriumwith its surroundings (e.g., a cavity with asmall hole)

  3. Emissivity

  4. IncandescentSources •A source that emits light by heating a material 1.Blackbody source 2.Nernst glower 3.Tungsten filament 4.Tungsten arclamp Nernst glower

  5. Tungsten Lamps •A W filament heated by an electrical current andsealed in a glass tube •Quartz used for uvemission (cutoff λ ~180nmvs. 300 nm for glass) •Emitsfromuv to ir •Gray body withЄ~ 0.4 -0.5 •Halogen vapour(iodine or bromine) used to regeneratethefilament

  6. AmplifiedSpontaneousEmission (ASE) Sources •Erbium-dopedfibreamplifier(EDFA) •An optical fibredoped with erbium and excited by a pump laser •Spontaneous emission in the Er-doped fibreis amplified (amplified spontaneous emission, ASE)

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

  8. SpectralBandwidth

  9. Sourcesof Line Spectra •Due to electronic transitions between energy levelsin gas atoms •Well-knowntransitions⇒wavelengthstandards

  10. Sourcesof Line Spectra Discharge & arclamps: •Large voltage applied between electrodes in a gas-filledtube •Electrons in gas atoms are excited to higher energy levels, leading to light emission •Wavelengths emitted depend on the gas

  11. Lasers •Light amplification by stimulated emission of radiation •3 processes involved in the interaction of emradiationwithmatter: Absorption, SpontaneousEmission, StimulatedEmission. Propertiesof 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 mediumuvto far ir

  12. Typesof Lasers •Characterized by the active medium 1.Solid-state lasers 2.Gas lasers 3.Dye lasers DyeLasers •A liquid (usually organic molecules) excitedoptically •Some of the organic molecules used in theselasersare commercial dyes

  13. FemtosecondLasers, ResonanceFrequencies

  14. Generacja drugiej harmonicznej

  15. Detectors Detectors are usually optical to electrical converters Twotypes: 1)Thermal detectors: •Detect light by measuring the heatproduced upon absorption 2) Quantum detectors: •Detect light by the generation of electron-holepairs •The photon plays a major role in thesedetectors

  16. 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) •Lowsensitivity ( 1 μW) •Slow due to time required to change their temperature (τ~ fewseconds) •Very accurate; used in standards labs to calibrate other detectors & light sources •Wavelengthinsensitive

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

  18. 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: •Photonabsorptionproducese-hpairsthat escape from the detector material as freeelectronse.g., photomultipliertubes (PMT) •Electrons remain within the materialand serve to increase its conductivitye.g., p-i-nphotodiodeavalanchephotodiodes (APD)

  19. Photocathode •Alkali metals usually used due to their lowworkfunctions Somephotocathode materials

  20. ElectronMultiplication Secondaryelectronemission

  21. PMT

  22. MultiplicationFactor ⇒PMTsarehighlysensitive Can detect a few photons per second ⇒Intense light (e.g., room light) will damage a PMT due to the high currents produced

  23. PMT Characteristics • •Fast response • ~ 1 −10 ns • due to spread in arrival time of electrons • attheanode • •Spectralsensitivity • hν> Φto eject an electron from the • photocathode • Φ~ 2 eV⇒λ< 620 nm • Cutoff wavelength due to glass • (~ 300 nm) or quartz (~120 nm) • PMTsare only useful in the uv • and visible regions

  24. p-nPhotodiodes •A reverse-biasedp-njunction •Operates like a surface-emitting LED but in reverse

  25. I-V Curve & Responsivity

  26. Responsivity

  27. Responsivity

  28. Quantum Efficiency

  29. Absorption

  30. p-i-nPhotodiode Design •Want x1to be small (minimum absorptionthrough p region) ⇒Introducethinheterostructure •Want L to be large (maximize absorption) ⇒Introducethickintrinsic region •Want R´ to be small ⇒Useanti-reflectioncoating

  31. p-i-nResponseSpeed •The speed of a photodiode is determinedby the transit time for electrons to cross theintrinsic region ⇒We want a thin depletion region Trade-off between sensitivity and speed

  32. AvalanchePhotodiodes (APDs) •APDshave an internalgain •Operate in the breakdown region of the I-V curve

  33. AvalanchePhotodiodes (APDs) Electrons are accelerated and collide with the lattice to create new free electrons ⇒impactionizationoravalanchemultiplication

  34. AvalanchePhotodiodes (APDs) •Response speed is slower due to time required insecondaryelectrongeneration