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Development of a Novel Multiwavelength, Time Resolved, Near Infrared Spectrometer

Dunne, LT; (2016) Development of a Novel Multiwavelength, Time Resolved, Near Infrared Spectrometer. Doctoral thesis , UCL (University College London). Green open access

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Abstract

During this PhD I have designed, built, and tested a multiwavelength, multi-channel, time resolved, near infrared optical spectrometer. The system uses a supercontinuum laser which is coupled to two acousto-optic tuneable filters (AOTFs) to produce very short pulses (∼4ps) with a narrow spectral bandwidth of 2 - 4 nm in the near infrared region (690 - 900 nm). This narrow band of light is vital in order to accurately resolve chromophore concentrations. The output light from the AOTFs is coupled to biological tissue using two single mode fibre optics. Light escaping the tissue is then collected using up to four fibre bundles, which guide the photons to the system’s detectors via customdesigned connectors. Custom built variable optical attenuators are used to control the amount of light that enters the photomultiplier tubes (PMTs) to prevent over exposure. The PMTs convert the optical signal into an electrical pulse, which is fed through a router to a time correlated single photon counting card (TCSPC). The TCSPC card measures the times of flight of the photons in tissue, which can be used to build up a temporal point spread function (TPSF). This is done at multiple wavelengths using precise timing electronics, with a resolution of 6 ps. This measurement directly allows the mean time of flight to be obtained, and the coefficients of absorption and scattering to be resolved. The novel elements of the system include that it can measure the absorption and scattering of up to 16 wavelengths simultaneously with a total collection time as low as 1 second. The wavelengths are also completely selectable and any combination of 16 in the near infrared region can be used in a single measurement. The system also has custom AOTF crystals that uniquely provide narrowband light. In this thesis I detail the design, development and characterisation of novel hardware and software. The system is then tested and calibrated on tissue-like phantoms to verify performance, before performing several in vivo measurements on the arm and head, including a cuff occlusion and a preliminary visual cortex study.

Type: Thesis (Doctoral)
Title: Development of a Novel Multiwavelength, Time Resolved, Near Infrared Spectrometer
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
URI: http://discovery.ucl.ac.uk/id/eprint/1530790
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