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Coherence Measurement Instrument (CMI) demonstrator
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Resumo(s)
This work was inspired by a previous PhD thesis that focused on studying how source correlations affect the normalized far-field spectrum of light, a phenomenon widely known as the Wolf effect, within the astronomical context. Part of that project involved the development of a solar coherence instrument (SCI), designed to measure the spatial coherence of individual solar granules present in the photosphere of the Sun. Since the validation of the instrument involved assessing each subsystem individually, the main objective of this work was to design, assemble and characterize a prototype of a coherence measurement instrument (CMI) demonstrator that integrated all the relevant subsystems. Similar to the SCI, the developed instrument was designed to perform two main tasks, each associated with a dedicated subsystem. The first subsystem involved selective imaging of the structured source, where a first SLM modulated the incoming light polarization to reflect only a desired region of the source. The second subsystem focused on spatial coherence measurements, achieved by displaying different masks on a second SLM and analytically manipulating the resulting diffraction pattern to estimate the magnitude of the complex degree of spatial coherence (µ). These tasks prompted the development of additional software capable of independently managing communication with each SLM, acquiring images of different interferograms, and processing them to finally retrieve the complex coherence factor. The assembly of the CMI proved to be a challenging and time-consuming process. The need to introduce several mirrors to shorten long optical paths within the relatively small optical table, combined with the implementation of a telescope in the second subsystem to enhance SLM-2 illumination, significantly increased the system’s vulnerability. These critical issues were overcome by independently aligning the telescope and then reassembling the entire instrument on a larger optical table. The final configuration of the CMI demonstrated the instrument’s feasibility. By combining the information obtained from the Fourier spectra of the different interferograms (originated by masks with up to 6 apertures) with the intensity profile in the system’s pupil plane, we successfully retrieved the magnitude of the complex degree of spatial coherence.
Descrição
Tese de Mestrado, Engenharia Física, 2025, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Coherence Spatial light modulators Diffraction