Transcript Slajd 1 - Warsaw University of Technology

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~ 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 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
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