Doppler velocimetry lidar with amplified reference beam storage loop
Shen, J.-L. (2003). Doppler velocimetry lidar with amplified reference beam storage loop (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13925
Permanent Research Commons link: https://hdl.handle.net/10289/13925
Wind shear and clear air turbulence are phenomena in which wind speeds can change rapidly presenting a threat to aircraft and their passengers. Lidar is a proven technique for wind speed measurement but remains a research tool only and is not yet commercially viable as an early warning system. This is mainly due to the expense and bulk of the single frequency injection locked lasers required for long-range coherent measurements. A different design philosophy for a pulsed Doppler lidar system has recently been presented in the literature, where the reference beam pulse is stored in a fibre optic storage loop. Each successive reference pulse has travelled a distance equivalent to the measurement beam’s path length. This approach reduces the coherence requirements and allows the use of cheaper, smaller, and lower coherence sources. However, a major problem of this system is that the energy of the stored pulses decays exponentially. This thesis introduces a new and improved reference pulse storage loop, which includes an erbium doped fibre amplifier to compensate for the decay of the reference signal. Details of the design are presented. The original storage loop concept is transferred to the fibre optics telecommunication wavelength range of 1.55μm. The noise and gain of the system is characterized. Special attention is paid to avoiding laser oscillation while maintaining sufficient gain. The components, like isolators, filters, and polarisation controllers, required to achieve satisfactory operation are presented. Electronic and software techniques based on phase lock loop and wavelet transformation are included in the overall system design to reduce noise. The experiments prove that the amplitude of 16 stored pulses remains constant to within 10% fluctuation. To prove the validity of the concept the velocity of a rotating mirror is measured at various measurement distances of 0 to 583 m that coincide with the 16 reference pulses. The experiments show successful optical mixing between the stored amplified reference and the delayed measurement beams. In the range 1 to 10 m/s the velocity can be determined to better than ±0.14 m/s (standard deviation). Software simulations of the system indicate that with improved equipment measurements should be possible for up to 70 stored pulses, which correspond to a measurement distance of 2,550 m. The results obtained illustrate that the amplified reference beam storage loop is a viable concept for Doppler lidar velocimetry.
The University of Waikato
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