Techniques for low coherence pulsed Doppler lidar
Wind shear and clear air turbulence are atmospheric phenomena that pose a hazard to aircraft and their passengers. In the past, these phenomena have caused injuries and the occasional death. The ability of a lidar system to make wind speed measurements and detect wind shear and turbulence has been known for some time. However, lidar systems are still not commercially viable as early warning systems in aircraft and at airports. The main reason is the expense and bulk of the single frequency injection locked lasers currently used. These are required to ensure coherence is maintained between the reference and measurement beams over long pathlength differences. In order to reduce the cost and size of lidar systems, we have developed new lidar design philosophies based on using components that are cheaper and smaller. Two different techniques are presented in this thesis, one to eliminate the requirement for a highly coherent laser source, and the other provides a method to generate a frequency offset that is an alternative to the currently favoured acousto-optic modulator. The first approach presented is a design in which a sample of the transmitted pulse is stored in a fibre optic ring resonator of pathlength longer than the pulse. Each time the pulse circulates the storage loop a fraction is split off and used as the reference beam. Because this reference beam has travelled an equivalent distance to the measurements beam, the coherence requirements are relaxed significantly allowing the use of a much cheaper and smaller low coherence source. In the second method presented, a frequency offset is generated without the use of the currently favoured and relatively expensive acousto-optic modulator. This method utilises a quadrature beam combiner and radio frequency quadrature modulator techniques. Configured as a single sideband (SSB) modulator, two balanced mixers produce double sideband signals in which one sideband is π out-of-phase. When combined, one sideband cancels leaving a single sideband output containing the baseband Doppler signal shifted up the RF spectrum by a predetermined offset. This is implemented in both electronic and semi-optical configurations. Concept validation experiments have been performed for both techniques. Using the reference beam storage loop, the velocity of a rotating mirror was measured at several simulated ranges. Successful optical mixing between the stored reference and the delayed measurement beams has been demonstrated. Utilising the SSB offset generation method, the velocity of a piezo mounted mirror has also been successfully measured, after an offset was applied with the new techniques. The results obtained show that both concepts are viable, and that these techniques could be applied in a new lidar design philosophy resulting in significant benefits with respect to complexity, cost, size, weight, and power consumption.
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