Sensors and Photo-Acoustics

Question 1

What is a PHOTODETECTOR?

Answer 1

A device that converts light energy into an electric current or voltage

Question 2

What is a CHARGE COUPLED DEVICE (CCD)?

Answer 2

A set of light-sensing elements arranged in a 2D array on a thin silicone substrate

Question 3

What are the 4 STAGES of a CCD?

Answer 3

1) Charge Generation
2) Charge Storage
3) Charge Transfer
4) Charge Measurement

Question 4

What happens in the ‘CHARGE GENERATION’ stage of a CCD?

Answer 4

• Light sensing photodiodes respond to photons by absorbing their energy
• Creates an electron deficient site (hole) within the silicone crystal lattice
• An electron-hole pair is made for each absorbed photon

Question 5

What happens in the ‘CHARGE STORAGE’ stage of a CCD?

Answer 5

• The semiconductor properties of silicone allows the CCD to hold photon-induced charge carriers
• The stored charge is linearly proportional to the light flux up to the Full-Well Capacity (FWC)
• FWC mostly related to physical size of each pixel and determines the maximum sensitivity contrast

Question 6

What happens in the ‘CHARGE TRANSFER’ stage of a CCD?

Answer 6

• Charged stored over time (integration/exposure time) is measured
• Charge transferred in sequence to a single measuring node
• A single readout node reads the entire array
• Charge transferred one at a time to serial register, feeds individual values to an on-chip amplifier

Question 7

What happens in the ‘CHARGE MEASUREMENT’ stage of a CCD?

Answer 7

• Each pixel has a readout in the form of intensity for their corresponding location
• Signal passed through an analogue-to-digital converter (ADC) converting a binary signal

Question 8

What is the RESOLUTION of a CCD measured in?

Answer 8

Dots Per Inch (DPI)

Question 9

What are the 4 STAGES of the PHOTO-ACOUSTIC EFFECT?

Answer 9

1) Incident Laser Pulses
2) Pulses Absorbed by Target
3) Rapid Thermo-Elastic Expansion and Contraction
4) Broadband Ultrasonic Emissions from Tissues

Question 10

What happens in the ‘INCIDENT LASER PULSE’ stage of the PHOTO-ACOUSTIC EFFECT?

Answer 10

• Pulsed to ensure sinusoidal wave
• High power - wavelength matches absorption spectra of target

Question 11

What happens in the ‘PULSES ABSORBED by TARGET’ stage of the PHOTO-ACOUSTIC EFFECT?

Answer 11

• Localized temperature increases as energy is absorbed

Question 12

What happens in the ‘RAPID THERMO-ELASTIC EXPANSION’ and CONTRACTION stage of the PHOTO-ACOUSTIC EFFECT?

Answer 12

• Photo-acoustics measures the optical absorption contrast
• Ultrasonic pressure distribution

Question 13

What happens in the ‘BROADBAND ULTRASONIC EMISSIONS from TISSUES’ stage of the PHOTO-ACOUSTIC EFFECT?

Answer 13

• Frequency determined by object size
• Requires specialized detectors to handle the range