Final PhD Seminar

Wave Front Sensing and Correction Using Spatial Modulation and Digitally Enhanced Heterodyne Interferometry

Mr Tarquin Ralph
Department of Quantum Science

Gravitational wave detectors use laser interferometry to measure the extremely small changes in arm length but can be limited by the quality of the laser injected into the instrument. This is particularly true in space based interferometers which lack the room to include the mode cleaning cavities that the Laser Interferometer Gravitational-Wave Observatory (LIGO) is able to employ. While theoretically Gaussian in profile, real beams are imperfect because of spatially distributed shot noise, microscopic flaws in optics, atmospheric turbulence and vibrations.

Digitally Enhanced Heterodyne interferometry, used in conjunction with spatial light modulation, can be used to map and, if necessary, correct for phase and amplitude variations across a laser wavefront. In a first experiment we measure and map the phase across a beam and demonstrate that phase offsets can be corrected or deliberately introduced using a spatial light modulator.

In a second experiment we measure the spatial distribution of power across a laser wavefront transmitted through a cavity and use this to generate a control signal to maximise this power by correcting for beam imperfections. It also allows us to preferentially select and transmit higher order laser modes.

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