PowerPoint Presentation
Sensing Systems and Signal Processing
Dr Richard
Copyright By PowCoder代写加微信 powcoder
Sensing Light
Sensing Light
Nature of light
Silicon photodiodes
Avalanche photodiodes
Photomultiplier tubes
https://www.hamamatsu.com/eu/en/product/optical-sensors/photodiodes/index.html
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Sensing Light – Nature of light
Light has a dual nature: it exhibits wave and particle properties
The particle associated with light is called a photon
The energy of a photon depends upon the frequency (or wavelength) of the light
E is the energy of a photon (J)
h is Planck’s constant (6.626 x 10-34 Js)
f is the frequency of the radiation (s-1)
short wavelengths (UV) -> higher energies
long wavelengths (IR) -> lower energies
https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0
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Sensing Light – Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
λ is the wavelength (m)
Example: red HeNe laser, λ = 632.8 nm
What is Energy of this photon?
https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0
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Sensing Light – Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
λ is the wavelength (m)
Example: red HeNe laser, λ = 632.8 nm
f = 2.998×108 / 632.8×10-9 = 4.74 x 1014 ~ 500 THz
E = hf = 6.626×10-34 x 4.74 x 1014 = 3.14 x 10-19 J
https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0
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Sensing Light – Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
λ is the wavelength (m)
Example: laser pointer
λ = 532 nm
How many photons per second is emitted by the laser pointer?
https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0
EEEE3089 2021-2022
Sensing Light – Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
λ is the wavelength (m)
Example: laser pointer
λ = 532 nm
E = hf = hc / λ = 3.73 x 10-19 J
P = 2 x 10-3 Js-1
N = P / E = 5.36 x 1015 photons.s-1
https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0
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Sensing Light – photoconductor
A photoconductor converts energy (and information) from an optical form into an electrical form
Photoconductors can obviously be used as light detectors
An opto-electrical phenomenon where a material’s conductivity increases due to the absorption of em radiation
When light illuminates a semiconductor:
some photons with the right energy are absorbed
electrons from the valence band obtain enough energy to jump to the conduction band
conductivity increases because of the higher number of conduction electrons
An electron requires a minimum energy to jump from the valence to the conduction band
This minimum energy is the energy gap between the valence band and the conduction band
Photons with energies greater than the bandgap can be absorbed
Unfilled Bands
Filled Bands
Valence Band
Conduction Band
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Sensing Light – photodiode
Photodiodes are semiconducting devices that convert light into electrical signals
A photodiode is a reverse-biased p-n junction
positive bias applied to n side of the diode
negative bias applied to p side of the diode
In an (ideal) reverse-biased p-n junction there is no current flow
If an photon with sufficient energy is incident on the junction, it can be absorbed E>Eg
The absorption creates an electron-hole pair as an electron jumps from the valence to the conduction band
The electron and the hole are swept through the junction in opposite directions
This creates a (photo-)current in the photodiode
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Sensing Light – photodiode
Photodiode Characteristics
Different semiconductors are sensitive to different wavelengths of light due to their particular energy bandgap
We shall concentrate on silicon photodiodes
Si bandgap = 1.12 eV
Si long wavelength detection cut-off ~ 1.1 μm
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Sensing Light – Silicon photodiode
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Sensing Light – Quantum Efficiency
The primary factor defining the sensitivity of a photodiode is its quantum efficiency (QE)
QE is defined as the percentage of incident photons generating electron-hole pairs which subsequently contribute to the output signal
QE = Nelectrons /Nphotons
depends on
wavelength
internal electric fields
sensor architecture
Can be as high as 80% for Si PDs in the visible
Silicon PD limits:
Red (long wavelength) limit:
band gap of Si = 1.12 eV corresponds to λ = 1.1 μm
Blue (short wavelength) limit:
not intrinsic, due to surface structure
transparency of electrodes decreases
reflection losses increase
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Sensing Light – Responsivity
The sensitivity of a photodiode can also be expressed in more practical units
The responsivity R(λ), is the amps of photodiode current (Ip) obtained per watt of incident illumination (P)
R(λ) = Ip / P
Thorlabs: https://www.thorlabs.de/NewGroupPage9.cfm?objectgroup_id=2822
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Sensing Light – Responsivity
R(λ) may be derived by multiplying the QE by the electronic charge (e = 1.602 x 10-19 C) and dividing by the photon energy for a particular wavelength (hc / λ)
R(λ) = QE.e/(hc/ λ)
= QE . λ .e/hc
= QE. λ/1.24
R(λ) is in AW-1
λ is in microns for final form
Ip = QE.e. λ .P./(h.c)
= QE.e. .P./(h.f)
= P . R(λ)
P is incident power, Ip is photocurrent
Thorlabs: https://www.thorlabs.de/NewGroupPage9.cfm?objectgroup_id=2822
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