basic electric components

Optical Transducers
Transducer converts non-electrical domain data to electrical domain
data (charge, resistance, voltage, current etc.)
Thermal transducers:
• respond to incident energy rate
• relatively flat spectral response curves (determined by window
and coating)
• generally slow (milliseconds or slower)
• usually single channel
Photon transducers:
• respond to incident photon rate
• highly variable spectral response (determined by photosensitive
material)
• respond quickly (microseconds or faster)
• single or multichannel (1-D or 2-D)
CEM 835 page 4-1

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Responsivity R(λ) and Sensitivity Q(λ):
R( λ ) = X(λ)
Φ
Q(λ) = dX( λ)
(λ )
dΦ(λ)
where
X(λ) is output signal (voltage, current, charge)
Φ(λ) is incident flux (W)
Plot of R(λ) or Q(λ) versus λ is called the spectral response
CEM 835 page 4-2

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Thermal detectors:
Thermocouple:
- based on thermoelectric potential when two dissimilar metals
are in contact
- junction attached to blackened disc of known area but small
heat capacity (0.8-40 µm)
- output is nV-µV range (limited sensitivity)
- Q(λ) constant over modest temperature range (10-10-10-7 W)
- moderate responsivity R(λ) 5-25 V·W-1
- junctions with different sensitivities are available
- response time limited by capacitance of wires to ms
- multiple junction thermocouples called thermopiles
CEM 835 page 4-3

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Thermistor bolometer
• blackened semiconductor with narrow band-gap (0.8-40 µm)
• radiation excites electron-hole pairs which decrease resistance
• decrease in resistance is compared with unirradiated bolometer
• difference is amplified - Q(λ) constant 10-6-10-1 W
• high responsivity R(λ) 1000 V·W-1
• response time 1-10 ms
Pyroelectric detector
• based on a piezoelectric material - eg DTGS
• non-centrosymmetric crystal has permanent dipole moment
across unit cell - acts like a capacitor
• when irradiated crystal expands slightly, capacitance decreases,
current flows
• high responsivity R(λ) up to 104 V·W-1
• Q(λ) constant 10-6-10-1 W
• response time <10 ms
CEM 835 page 4-4

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Photon detectors:
Photon detectors are based on
• photoconductive materials (MCT transducer)
• photovoltaic cells (Si, Se photocell)
• photoemissive materials (PMT's, phototubes)
• semiconductor pn junctions (photodiodes)
CEM 835 page 4-5

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Photoconductive cell:
• semiconductor (CdS, PbS, PbSe, InSb, InAs, HgCdTe, or
PbSnTe) behaves like resistor
• in series with constant voltage source and load resistor
- voltage across load resistor used to measure the resistance of
the semiconductor
• incident radiation causes band-gap excitation and lowers the
resistance of the semiconductor
• most sensitive in near IR (PbS)
• sometimes cooling is necessary to reduce thermal band-gap
excitation
Photovoltaic cells:
• thin layer of crystalline semiconductor (Se, Si, Cu2O, HgCdTe)
sandwiched between two different metal electrodes
• no bias but irradiation causes formation of electron hole-pair
formation
CEM 835 page 4-6

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• electron migrates one way, holes migrate in opposite direction
• if resistance of external circuitry is small, microamps produced
• high sensitivity in near IR to UV (102-106 V·W-1)
- eg Fe-Se-Ag 300-700 nm R(λ) peaking near 550 nm
Phototubes:
• two electrodes enclosed in glass or silica envelope
• bias (70-180 V) is applied between two electrodes
• cathode is a photoemissive material (Cs3Sb, NaO, AgOCs) -
emits photoelectrons
• current collected by anode
- photoemission only if hν > surface workfunction (1-5 eV)
CEM 835 page 4-7

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CEM 835 page 4-8

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Total rate and current are
r
= η⋅
ap
r cp
= η⋅ Φ
⋅
ph
K(λ )
{
photon
flux (s-1 )
i
= η⋅
ap
e ⋅ r cp
= η⋅ e⋅ Φ ⋅
⋅
ph hυ
R( λ )
1 2
4 3
4 1
2
3
Φ
−
= radient
1
A⋅W

flux (J ⋅s-1 )
where
η - anode collection efficiency (0-1) - design and bias dependent
K(λ) - number of electrons produced per incident photon
photocathode quantum yield (0 to ~0.5)
R(λ) - responsivity (A·W-1)
Φph - incident flux (s-1)
e - electron charge (C)
• High sensitivity (10-3-10-1 A·W-1)
• If iap >0.1 mA, secondary electrons and Bremsstrahlung are
ejected from the anode when the photoelectrons are incident
• Dark currents (typically 10-12-10-14 A) caused by
- thermionic emission
- field ionization (high bias)
- ohmic resistance
CEM 835 page 4-9

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Photomultiplier tubes (PMT's):
• similar to phototubes - photoemissive cathode and anode
• multiple secondary electron emissive dynodes (MgO, GaP)
- each dynode is biased ~100 V more positive than previous
to accelerate electrons from dynode to dynode
- gain per dynode, g, is typically 2-5
- total gain m = gn is 106-108
• charge pulse at anode is few ns wide
CEM 835 page 4-10

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