Development and testing of an Er:Yb:glass coherent laser radar for wind field mapping.

Abstract

Doppler or coherent laser radars (CLR’s) can measure range-resolved velocities of distant hard and diffuse targets. Critical applications include wind shear and wake vortex detection, clear air turbulence warning, wind field mapping, and pollution dispersion monitoring. To monitor these at different geographic locations in the atmosphere in real time requires a system with high temporal resolution. A laser transmitter that provides eye-safe, transform-limited energetic pulses with good beam quality and a sensitive transceiver is suitable for such applications. In this thesis I describe the development of an eye-safe coherent laser radar that has a range resolution of 75 m with single-shot velocity resolution of ∼1.5 ms⁻¹. I also present measurements of atmospheric wind speeds using this laser. The laser source is a travelling-wave oscillator that uses a conduction-cooled, Coplanar Pumped Folded Slab (CPFS) with an Er:Yb:phosphate glass gain medium that is side pumped using fast-axis collimated laser diodes. The laser uses polarisation-controlled outcoupling and is injection-seeded to produce eye-safe, transform-limited long duration Q-switched pulses at a frequency close to that of the master laser. This thesis describes the complete characterisation and development of that laser. It also describes the design and development of the monostatic heterodyne receiver used to detect backscattered returns from targets. Measurements validating the performance of the CLR using stationary and moving hard targets are reported. The thesis also presents initial measurements of atmospheric wind speeds using the CLR. Reproducible range-resolved single-pulse measurements to ≥2 km are reported and compared to results from a boundary layer radar.