PowerPoint Presentation
Sensing Systems and Signal Processing
Dr Richard
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Sensing light – Temporal Bandwidth
Bandwidth is the maximum frequency (or bit rate) that a photodiode can detect.
Limited by two time constants
the transit time τtr
the capacitance time constant of the diode τRC
τtr = width depletion layer / saturation velocity (~105m/s)
This is large for some diode designs and can be the main limiting factor for the speed of device.
what about τRC?
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Sensing light – Temporal Bandwidth
What about τRC?
This depends on the properties of the diode (capacitance C, the junction and series resistance , ) and the load .
The capacitance arises because it looks like two charged plates with a spacer (the depletion region)
The junction resistance is usually very high and is in parallel with the series resistance of the p-type and n-type contacts which is usually low.
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PIN Photodiodes
PIN stands for:
p-type / intrinsic / n-type
or p-type / insulator / n-type
Thicker depletion region
Allows a reduced capacitance, even at low reverse bias
Also improves quantum efficiency
Most commercial photodiodes are “pinned”
GHz bandwidths are possible with small area diodes
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Detection Light – low light levels
Darkfield and fluorescent imaging systems must perform at very low light levels
The simple photodiode has an NEP which is too high for these applications
We need some intrinsic gain to overcome the noise inherent in PDs
Two main technologies exist
avalanche photodiodes (APDs)
photomultiplier tubes (PMTs)
APDs share many characteristics with simple photodiodes
can detect photons with energies higher than the bandgap, so different materials can be used for different spectral ranges
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Detection Light – APD
APDs are operated at high reverse bias (typically > 100 V)
This imparts a lot of energy to the photoelectrons
Impact ionisation, or the avalanche effect, imparts an internal current gain effect, as the initial photoelectron generates additional free electrons through multiple collisions with the silicon lattice
Typical gain is around 100, but some specially tailored devices can be operated at very high reverse-bias (>1 KV) and provide gains of >1000
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Detection Light – APD
Single photon avalanche diode (SPADs) are designed to operate in the reverse breakdown condition – known as Geiger mode
Operation in Geiger mode allows gains of up to 106, so single photons can be detected
SPADs require special quenching circuits to limit the current and reverse the breakdown process
A circuit called a pulse discriminator is used to output a digital pulse each time a photon is detected
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Detection Light – APD
APD avalanche current intensity is linearly related to the optical signal intensity.
However, for a SPAD this is not the case as it operates with a bias voltage above the breakdown voltage which is unstable.
The amount of current doesn’t correlate with input signal intensity, but a single photon generates a large current which can be gated for photon counting.
Linear amplifier
Geiger mode
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Detection Light – APD
Limitations:
The large reverse-bias required by APDs leads to a high dark current, which reduces SNR
The avalanche effect is somewhat random – the actual gain varies from photon to photon
This leads to “excess noise”
Advantages:
relatively cheap
high quantum efficiency
robust – difficult to overload
immune to magnetic fields
http://hamamatsu.magnet.fsu.edu/articles/avalanche.html
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Detection Light – PMT
Photocathode
converts an incident photon into a single electron (photoelectric effect)
Focusing electrodes
guide the electron into the electron multiplying dynodes
electrostatically accelerates the electron
on impact with the dynode, each electron generates several secondary electrons
https://www.hamamatsu.com/preview/en/images/xx_pmt_01/img03.png
collects the electrons and produces an electrical (current) output
PMT characteristics (vs. APD)
Bulky and expensive
Low quantum efficiency
Low dark current
Can be operated in linear mode, or used with a pulse discriminator to count single photons
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Detection Light – PMT
PMT characteristics (vs. APD)
Bulky and expensive
Low quantum efficiency
Low dark current
Can be operated in linear mode, or used with a pulse discriminator to count single photons
https://www.hamamatsu.com/eu/en/product/optical-sensors/pmt/pmt_tube-alone/index.html
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