In optical experiments, replacing the usual constant intensity light source with one that has its intensity modulated at a fixed frequency gives a number of advantages, namely moving the signal of interest away from low frequency noise, allowing the signal to be detected even in the presence of background illumination, as well as being able to gain more information about the experiment by measuring any change in the phase of the modulation. A number of different types of integrating pixels have been simulated in Matlab with regard to their suitability for use as a modulated light detector, along with different methods for reconstructing the signal. The quadrature method of reconstruction was chosen as the best technique, where four samples are taken per modulated time period then all four samples are used in the I and Q channels of the demodulator. A modulated light detector has been implemented using a custom integrated circuit in a standard 0:35 micro m CMOS process, linked to a field programmable gate array. The custom circuits consists of a photodiode configured as an integrating type pixel, where the output of the pixel is connected to a comparator so that when coupled with a counter, the pixel can measure the amount of time taken for the photodiode to reach a particular voltage. A camera with 128 x 128 pixels has been implemented and characterised using a modulated laser as the input, with the results obtained compared to the Matlab simulations carried out. Images have been taken with the camera that show the camera is able to detect modulated light signals at a modulation depth of 1.39% and modulation frequency of 420 Hz. The camera has also been operated successfully for a modulation frequency of 2.5 kHz.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:514695 |
| Date | January 2008 |
| Creators | Light, Roger |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/10503/ |
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