A fibre based quad-cell wavefront sensor for high speed adaptive optics
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The genesis of this thesis came out of the difficulties encountered when trying to apply adaptive optics correction to optical fields that contain scintillation (specklelike amplitude fluctuations). These scintillated fields have non-uniform amplitude distributions and also contain discontinuities in their wavefronts where the amplitude of the field goes to zero. A traditional adaptive optics system that relies on continuous wavefronts and uniform illumination is limited in its ability to correct highly distorted fields. It became clear that adaptive correction using a modular system that does not rely on the continuity of the wavefront or indeed uniformity of the amplitude was better suited to the task. The use of a fibre optics array of sub-apertures to combine regions in close proximity into a mixed signal and to monitor the interference was examined as a more robust means of correcting an aberrated wavefront. Multi-aperture receivers have previously been shown to give an increase in the signal to noise or carrier to noise ratio and perhaps more importantly can also reduce amplitude fluctuations caused by atmospheric aberrations such as speckle. The investigation of a multi-aperture system lead to the development of a fibre based quad-cell wavefront sensor to measure the launching angle to improve the coupling of light to the fibre array. This thesis outlines the development and testing of the fibre quad-cell for use as a cell in a larger array applied to wavefront sensing. The quad-cell is made up of three directional couplers in a cascade arrangement. A directional coupler allows for evanescent coupling of light between two adjacent fibre waveguides. The response of the quad-cell arrangement of fibres was simulated numerically using the beam propagation method and a method for combining the interferometric information from the four output signals was developed. A novel aspect of this sensor compared with other fibre based wavefront sensors is the ability to differentiate between positive and negative gradients. The system was tested for a single bulk fibre optic quad-cell for measuring wavefront gradients. As part of the sensor development an FPGA interface was developed for parallel processing of a quad-cell wavefront sensor and for mirror control. Initial results from the breadboard system are presented and compared with the results of the numerical simulation. The initial results are sufficient to display the proof of concept. However, the use of bulk fibre optics made the sensor very susceptible to thermal noise. The system also proved to be very sensitive to the alignment of the fibres in the quad-cell. It is envisaged that the use of an integrated optics implementation of the sensor would remove much of the thermal noise and also enable the precise orientation and alignment of the fibres in the quad-cell array.