Theoretical and experimental signal-to-noise ratio assessment in new direction sensing continuous-wave Doppler lidar

A Tegtmeier Pedersen,T Mikkelsen,C F Abari,J Mann

doi:10.1088/1742-6596/524/1/012004

Abstract

A new direction sensing continuous-wave Doppler lidar based on an image-reject homodyne receiver has recently been demonstrated at DTU Wind Energy, Technical University of Denmark. In this contribution we analyse the signal-to-noise ratio resulting from two different data processing methods both leading to the direction sensing capability. It is found that using the auto spectrum of the complex signal to determine the wind speed leads to a signal-to-noise ratio equivalent to that of a standard self-heterodyne receiver. Using the imaginary part of the cross spectrum to estimate the Doppler shift has the benefit of a zero-mean background spectrum, but comes at the expense of a decrease in the signal-to noise ratio by a factor of √2.

Highlights

Coherent Doppler lidars have in recent years started to play an increasingly important role within the wind energy industry and are widely used for especially resource assessment
As can be seen the auto spectrum of the complex signal has a Doppler peak located around −2 MHz which for this specific lidar system is equivalent to a wind speed of approximately −1.5 m/s
There are two ways of processing the signals generated by the lidar; calculate either the auto spectrum of the complex signal or the imaginary part of the cross spectrum

Summary

Introduction

Coherent Doppler lidars have in recent years started to play an increasingly important role within the wind energy industry and are widely used for especially resource assessment. One can work around the latter limitation by shifting the frequency of the reference local oscillator (LO) compared to the transmitted signal, e.g. with the aid of an acoustooptic modulator (AOM). At DTU Wind Energy a different technique to achieve direction sensing has recently been demonstrated with great success [3] This detection scheme is based on an image-reject homodyne receiver, known as coherent in-phase and quadrature (IQ) detection, which in essence works by dividing the received backscattered signal in two and mixing one half with a reference local oscillator signal and the other half with a 90◦ delayed copy of the LO [4, 5]. In this study we analyse the signal-to-noise ratio (SNR) of the IQ detection lidar theoretically and experimentally, and compare with that of a lidar detection system using the standard self-heterodyne technique

Experimental setup

Auto spectrum of individual signals

Auto spectrum of the complex signal

Cross spectrum

SNR from hard target

Discussion and conclusion