Spectral Detection of Molecular Scattering with Coherent Heterodyne Lidar

Abstract

Coherent heterodyne lidars are widely used to produce real time wind speed profiles for weather, air quality and aviation safety purposes. The instruments receive the backscattering signal from atmospheric aerosols and the data can be converted into attenuated backscattering profiles coarsely presenting aerosol density along the beam path. However, the laser experiences attenuation as the aerosols scatter and absorb the light and the profiles are typically skewed showing strong signal in short distances and weak signal in long distances making the actual data analysis difficult even with inversion algorithms.

Simultaneous measurement of molecular scattering is typically used to calibrate the laser power along the measurement distance. However, the coherent hetero-dyne technique could not be applied for measuring molecular scattering due to laser power and receiver electronic bandwidth constraints until recently. The heterodyne receiver is essentially a downconversion spectrometer and ideally can record the complete scattering spectrum without needing optical filters, extreme laser stabilisation or wavelength scanning techniques. Successful spectral measurement provides laser power calibration and aerosol analysis capabilities but also a possibility to examine remote temperature profiling.

The research presented in this thesis demonstrates an overhaul to fiber based heterodyne lidar experiment enabling spectral detection of molecular scattering. A novel tapered fiber amplifier is presented for laser peak power scaling up to 2.2 kW while maintaining the beam quality and laser linewidth optimal for coherent lidar. A high speed FPGA preprocessing circuit is applied for the capture and averaging of the spectral data. Finally, atmospheric measurements, signal analysis, data validation and calibration are presented and compared to state of the art Raman lidar Polly Xt.