title: Doppler velocimetry lidar with amplified reference beam storage loop

authors: Shen, Jyi-Lai

abstract: 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.

text: Doppler velocimetry lidar with amplified reference beam storage loop 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.

year: 2003

title: Electromagnetic damper control strategies for light weight electric vehicles

authors: Fow, Alista John

subjects: Science & Technology

abstract: An investigation is conducted into the performance of passive, semi-active and active electromagnetic dampers. Theoretical models are constructed of the dampers and these are included in two degree of freedom models of the suspension. The passive and semi-active electromagnetic dampers are significantly heavier than commercial hydraulic dampers. In the case of active electromagnetic damper, the reduction in passenger acceleration is 88 percent when compared to passive damper and 61 percent when compared to a semi-active damper. The power consumption is similar to a magnetorheological semi-active damper.

text: Electromagnetic damper control strategies for light weight electric vehicles An investigation is conducted into the performance of passive, semi-active and active electromagnetic dampers. Theoretical models are constructed of the dampers and these are included in two degree of freedom models of the suspension. The passive and semi-active electromagnetic dampers are significantly heavier than commercial hydraulic dampers. In the case of active electromagnetic damper, the reduction in passenger acceleration is 88 percent when compared to passive damper and 61 percent when compared to a semi-active damper. The power consumption is similar to a magnetorheological semi-active damper.

year: 2016

title: Travel writing and the first Marists in New Zealand

authors: Jennings, William

subjects: travel writing

abstract: French Marist missionaries arrived in New Zealand in 1838 and travelled extensively throughout the country during the next two decades. The letters they sent to France, now published in Girard's Lettres recues d'Ocanie (2009), include accounts of journeys that reveal the Marists' different perspectives of New Zealand's landscape and inhabitants. French romanticism and their religious vows emphasised a spiritual connection with the landscape, in sharp contrast with prevailing utilitarian European views. The Marists' status as Frenchmen in a British colony meant they encountered two alterities: British and Maori. Influenced by the noble savage of the French Romantics and by anti-Protestant bias, the Marists portrayed Maori favourably, reserving their more hostile comments for the British missionaries.

text: Travel writing and the first Marists in New Zealand French Marist missionaries arrived in New Zealand in 1838 and travelled extensively throughout the country during the next two decades. The letters they sent to France, now published in Girard's Lettres recues d'Ocanie (2009), include accounts of journeys that reveal the Marists' different perspectives of New Zealand's landscape and inhabitants. French romanticism and their religious vows emphasised a spiritual connection with the landscape, in sharp contrast with prevailing utilitarian European views. The Marists' status as Frenchmen in a British colony meant they encountered two alterities: British and Maori. Influenced by the noble savage of the French Romantics and by anti-Protestant bias, the Marists portrayed Maori favourably, reserving their more hostile comments for the British missionaries.

year: 2010-12