Diode laser lidar wind velocity sensor using a liquid-crystal retarder for non-mechanical beam-steering.

Peter John Rodrigo,Theis F Q Iversen,Qi Hu,Christian Pedersen

doi:10.1364/oe.22.026674

Abstract

We extend the functionality of a low-cost CW diode laser coherent lidar from radial wind speed (scalar) sensing to wind velocity (vector) measurements. Both speed and horizontal direction of the wind at ~80 m remote distance are derived from two successive radial speed estimates by alternately steering the lidar probe beam in two different lines-of-sight (LOS) with a 60° angular separation. Dual-LOS beam-steering is implemented optically with no moving parts by means of a controllable liquid-crystal retarder (LCR). The LCR switches the polarization between two orthogonal linear states of the lidar beam so it either transmits through or reflects off a polarization splitter. The room-temperature switching time between the two LOS is measured to be in the order of 100 μs in one switch direction but 16 ms in the opposite transition. Radial wind speed measurement (at 33 Hz rate) while the lidar beam is repeatedly steered from one LOS to the other every half a second is experimentally demonstrated - resulting in 1 Hz rate estimates of wind velocity magnitude and direction at better than 0.1 m/s and 1° resolution, respectively.

Highlights

As light sources of coherent lidar systems for wind sensing, narrow-linewidth semiconductor lasers operating in the eye-safe telecom wavelength regime (λ ~1.55 μm) have been proven as compact and cheaper alternative to fiber lasers [1–3]
We extend the functionality of a low-cost CW diode laser coherent lidar from radial wind speed (scalar) sensing to wind velocity (vector) measurements
The dual-LOS lidar may be used to measure light depolarization by atmospheric particles

Summary

Introduction

As light sources of coherent lidar systems for wind sensing, narrow-linewidth semiconductor (diode) lasers operating in the eye-safe telecom wavelength regime (λ ~1.55 μm) have been proven as compact and cheaper alternative to fiber lasers [1–3]. Previous laboratory investigations into diode laser linewidth requirements in CW coherent lidar have found that measurement at distances beyond the coherence region is still feasible if the laser phase fluctuations are overwhelmed by target speckle fluctuations [7]. The vector measurement principle as well as the design, implementation, and experimental characterization of the enhanced lidar are discussed

Operational principle of the dual-LOS wind lidar

Characterization of the LCR based beam-steering system

Conclusion and outlook