Continuous wave synthetic low-coherence wind sensing Lidar: motionless measurement system with subsequent numerical range scanning

Investigating Suppression of Cloud Return with a Novel Optical Configuration of a Doppler Lidar

The full-width at half-maximum or probe length of the Lorentzian weighting function of continuous-wave Doppler lidars increases quadratically with the focus distance, which results in a deterioration in the spatial resolution of measurements. What is worse, a Doppler lidar is susceptible to moving objects that are far away from the intended measurement point. Therefore, we suggest a novel configuration to mitigate these problems by deploying two co-planar quarter-wave plates with orthogonal fast axes in the conventional continuous-wave lidar system, without any change to the other optical or electronic components. If the vertically polarized laser beam that we emit goes out and its backscattered beam returns back through the same quarter-wave plate, the returned beam will become horizontally polarized. The horizontally polarized backscattered beam cannot beat with the vertically polarized local oscillator to generate a Doppler signal. However, the polarization of the returned beam will remain unchanged if the emitted beam travels out through one plate and returns through the other. In this way, the influence of a moving backscattering particle far away from the focus point can be reduced. Both theoretical and experimental results show that, in a proper configuration, the probe length of the continuous-wave lidar can be reduced by 10%, compared with that of the conventional lidar. In addition, the fat tails of the Lorentzian weighting function can be suppressed by up to 80% to reduce the return from a cloud, albeit with a large reduction (perhaps 90%) in the signal power. This investigation provides a potential method to increase the spatial resolution of Doppler wind lidars and suppress the low-hanging cloud return.

Wind ranging and velocimetry with low peak power and long-duration modulated laser

A novel approach to simultaneous wind ranging and velocimetry using low peak power, long-duration modulated laser pulse transmissions is proposed. Received signals backscattered by aerosol particles are processed by a multi-reference matched-filter (MRMF) which performs matched filter processing between the received signal and several reference signals in parallel and outputs a range-velocity profile of received power. Ranging and velocimetry are performed simultaneously by estimating received power, radial velocity, and velocity dispersion from a velocity profile at an arbitrary range in the range-velocity profile. The accuracies of the three estimates improve in proportion to the square root of pulse duration; that is, a 100 times longer pulse is equivalent to a 10-dB amplification.

Influence of laser coherence on reference-matched laser Doppler velocimetry

The probe length is investigated under the influence of the coherence length of Gaussian and Lorentzian spectra for the case that the focal point and the point of highest interference are matched in a strongly focused laser Doppler velocimetry setup (LDV). Isosurfaces of a -3  dB drop of the intensity maximum are estimated and suggested as an alternative, comprehensible way to define probe volumes. In the end, the equations are applied for an exemplary lidar setup to show the reduction of requirements for the coherence length of the laser source in comparison to unmatched cases.