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
Responsivity R(λ) and Sensitivity Q(λ):
R( λ ) =
X(λ)
Φ(λ )
Q(λ) =
dX( 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
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
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
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
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
• 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
CEM 835 page 4-8
Total rate and current are
r ap = η⋅ r cp
= η⋅ Φ ph ⋅K(λ )
{
photon
flux (s -1 )
i ap = η⋅ e ⋅ r cp
= η⋅ e ⋅ Φ ph ⋅ hυ ⋅ R(
λ)
2
3
1
424
3 1
−1
Φ = radient 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
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 = g n is 10 6-108
• charge pulse at anode is few ns wide
CEM 835 page 4-10
Can measure average current traveling between the cathode and
anode
i ap = m ⋅η⋅Φ⋅ R( λ)
Φ = Φ ph ⋅ h ⋅υ
= m ⋅e ⋅η⋅Φ ph ⋅K(λ )
• R(λ) or K(λ) are functions of photocathode material
• very high sensitivity (10-105 A·W-1)
• Alternatively, the rate of charge pulses can be counted, a
technique called photon counting. In this case, rate is
r ap = η⋅Φ ph ⋅ K(λ)
=
η⋅Φ⋅ K(λ)
h ⋅υ
• same equation as for a phototube but each pulse contains m
electrons not just single electron
- detection limit lower - PMT's best in low incident flux
conditions
- but max iap ~ 1 µA so max Φ is also lower
• Dark currents in PMT's result from similar processes to phototube
- thermionic emission associated with the photocathode can be
significant (multiplied by dynodes) 10-11-10-7 A
- cooling the PMT (0 to -60 °C) helps.
CEM 835 page 4-11
Photodiode:
• contains a reverse-bias semiconductor pn junction
- p-type semiconductor has excess holes (eg B-doped Si)
- n-type semiconductor has excess electrons (eg P-doped Si)
• under reverse bias, depletion layer formed (resistivity of depletion
layer is very high)
CEM 835 page 4-12
• under irradiation, electron-hole pairs created that move under bias
(holes → p-type, electrons → n-type)
- momentary current is produced - ns or sub-ns
• spectral response of a typical photodiode depends on band-gap of
semiconductors used (typically near IR into near UV)
• R(λ) less than PMT (no internal gain) but Q(λ) constant over 6-7
orders of magnitude
• poor sensitivity (10-2-1 A·W-1)
• can be made very small, ideal for use in multichannel devices
Multichannel detectors:
Photographic film:
Photodiode arrays:
• based on pn-photodiodes constructed by semiconductor chip
techniques
• regions of p-type Si deposited onto n-type Si crystal
• distance between elements is typically 25 or 50 µm (up to 4096
elements per array)
• usually operated in the- depletion layer is formed around each p-type islands
• upon irradiation (integration time) bias is turned off and electrons
and holes are created in depletion regions
- holes migrate to the p-type islands and accumulate (max ~106)
CEM 835 page 4-13
CEM 835 page 4-14
• during read-out period, each p-type region is interrogated
(1) capacitor connected to each island is charged by 5 V
supply
(2) each island is connected in turn to it's capacitor and charge
flows out of the capacitor to neutralize the positive charge
accumulated on each island
(3) current needed to return the capacitor to 5 V value is
measured (total time ~25 µs)
(4) once 5 V reverse-bias has been reestablished, holes are
neutralized and system is ready for integration step
• thermal excitation of electron-hole pairs creates difficulties with
long integration times - array is often cooled
• dynamic range 2-4 orders of magnitude
- at detection limit thermal excitation dominates
- at LOL end, charge storage becomes non-linear
• sensitivity can be increased by coupling diode array with
microchannel plate (MCP)
CEM 835 page 4-15
MCP:
CEM 835 page 4-16
Charge transfer devices:
• photodiode arrays are inferior to PMT's in respect to sensitivity,
dynamic range and signal-to-noise ratio
• charge transfer devices approach the performance of PMT's
• each pixel is metal oxide semiconductor
• negative bias is applied to each electrode, a potential well collects
photogenerated holes
- more than 106 holes bleed onto adjacent pixels
• charge accumulated during the integration time can be integrated
in two ways
- charge-injection device (CID)
- charge-coupled device (CCD)
CEM 835 page 4-17
Charge-injection device (CID) (two electrode per pixel):
(1) during integration, one electrode (B) more negative than the
other (A) - all photogenerated holes are accumulated under B
(2) voltage applied to A is removed and the surface charge
measured on A
(3) potential on electrode B is switched to a positive potential,
causing the holes to migrate to electrode A
(4) charge under A is remeasured and the signal is the difference
between the two measurements
(5) positive voltage applied to electrode A to repel accumulated
holes and return system to initial state
CEM 835 page 4-18
Charge-coupled devices (CCD): (three electrodes per pixel)
• substrate is p-type Si so electrons (not holes) are accumulated
• each pixel contains three electrodes
• following the integration period, a three-phase voltage transfers
electrons in a step-wise manner along row
Normalized Voltage
1.0
0.5
0.0
1
2
3
-0.5
-1.0
150
200
250
300
350
Time (a.u.)
• readout process is destructive
CEM 835 page 4-19
400
450
CEM 835 page 4-20