Optical Sensor Technology Flashcards
What EM bands are typically involved in optical sensing?
Optical sensors usually detect from UV down to IR
What are the two key detection principles involved? What then are some examples of each type and how do they work?
Quantum sensing and thermal sensing:
- Photodiodes
- Photovoltiac
Produce a descrete signal upon absorption of a photon
- Thermistors (Resistance change)
- Thermocouples/thermopiles (EMF Generation)
- Pyroelectrics (current source change)
Radiation absorption produces a temperature change which produces an electrical signal
Describe the basic element of a micro-bolometer array
- 1mm^3 Ge:Ga thermistor
- Sapphire substrate
- Thermistor mounted using stycast epoxy
- Brass wires to carry signal, soldered using indium
- 120 nm Bi film
Describe the basic design of the photomultiplier tube sensor
- Photocathode and anode which are seperated by several dynodes which multiply the electrons
- two main types: focussed and venetian
- focussed types are fast with short life times and well0defined electron paths
What are the following typical characteristics of a PM tube?
- Spectral range
- cathode material
- anode to cathode voltage
185 - 650 nm
SbCs
1000 V dc
What are the consequences of the intrinsic region in the PIN diode?
- Shorter response time and increased volume in the depletion region
What are the typical silicon diode response for each EM band in terms of AW-1
0.5 AW-1 at near IR, 1 um
0.4 AW-1 for visible, ~500 nm
0.1 AW-1 for UV, about 300 nm
For the three diode bias conditions how does the PIN diode behave?
Zero bias - in photovoltaic mode
reverse bias - linear mode where the photocurrent is proportional to the light intensity
positive bias - reduced response time
What is a dark current?
Dark current is the electric current that flows through photosensitive devices without photons. It is due to the random generation of electrons and holes in the depletion region
What is meant by NETD?
Noise Equivalent Temperature Difference
The temperature difference which gives a signal level which has the same magnitude as the background noise in the detector
How is the voltage of a transconductance amplifier related to the photocurrent? How does the diagram of this tdetector look?
- V=i_photo*R_feedback
- photocurrent input to negative terminal of amp
- feedbackl 10M resistor between negative input terminal and output terminal
- 15 V dc across amp
How does a phototransistor amplifier work?
Photocurrent goes to the base and a collector current with I_c=h_fe * I_photo is output from the collector
What are the three examples of thermal IR detectors?
Pyroelectric: current source with output proportional to the rate of change of temperature
Lead Salt (PbSe, PbS): Photodiodes with superior detectivity over pyroelectrics at 300K . Limited bandwidth
Mercury Cadmium Telluride (MCT, HgCdTe): need to be cooled using liquid nitrogen but offer superior performance
What is meant by the acronym in CCD array?
Charge-coupled discharge array
What are the advantages of using a thinned back-illuminated CCD array?
Good for detection of soft x-rays and UV light
This is because CCDs are sensitive to light from near IR to blue and the electrodes are opaque for shorter wavelengths.
With back illumination the opaque electrodes needn’t be passed and only silicon is passed by the photons.
To allow the photons to reach the active layer the Si is thinned.
Explain how a PMT works
The PMT comprises an evacuated tube with an optically transparent window, containing a thin photocathode and several thin metallic foil photo-anodes, each held at successively greater positive potential with respect to the cathode.
The last anode is a connected to readout circuit.Typically, several hundred volts of potential difference are applied to the tube.
A photon enters the tube and is absorbed in the photocathode liberating an electron, which escapes the photocathode and which is accelerated towards the first anode.
The electron collides with this creating a small shower of electrons, which are further accelerated towards the next anode, where each of these electrons creates its own shower, and so on(i.e. acascade), until perhaps a million electrons finally hit the readout anode –thus the photon creates an easily detectable burst of charge, and multiplies the response of the photocathode by a factor of a million or so
Why must care be taken to not point a PMT at a bright light source such as the sun?
A bright light will cause the photocathode to burn out, destroying the device
What is NEP short for and what does it mean?
Noise equivalent power, the lower the better. The equivalent power to produce the same signal in an ideal noise free detector as that due to the noise in a real detector
What is meant be specific detectivity D* of a detector?
Inverse of NEP and so the bigger the better.
Normalised by dividing by the sqrt of A and B to enable fair comparison between different detector technologies
D* = SQRT(A)SQRT(B)/NEP
[cm Hz^0.5W^-1]
What are the benefits of adding the intrinsic layer to a PN junction to make a PIN diode?
- Larger depletion region thus more chance of photon absorption and consequential generation of electron hole pair in the depletion region
- Reduced capacitance resulting from the increased depletion region size means a faster response time
Why is a Si photodiode not sensitive to long wave IR or short wave UV
Because photons with energy lower than the bandgap energy are not able to generate an electron-hole pair in the photodiode.
For higher energy photons such as short-wave UV, the photon is absorbed too quickly by the photodiode and thus does not get absorbed in the depletion region. The generated electron-hole pair immediately recombine and no photocurrent is generated
What are some advantages of using CCD arrays?
- High sensitivity to light
- detailed resolution
- low noise imaging
- strong quantum efficiency
What are some disadvantages of using CCD arrays?
- expensive
- readout can be slow due to scanning principle
- high energy consumption
Describe how a CCD array operates to produce an electrical signal?
- Every photon incident on the detector of the CCD creates an electron-hole pair in the deplation region of the corresponding electrode
- when the electrode is held at a positive potential, the hole is swept away whilst the e- is attracted to the electrode, therefore charge accumulation at each electrode proportional to the number of absorbed photons occurs
- by manipulating the potential at each electrode the accumulated charges can be swept to an output amplifier. Using clocks, the specific electrode from which the charges came can be identified.
- in a 2d array, charge can be swept along the rows and then along the column to the ouotput amp
