Fiber Based Coherent Lidars for Remote Wind Sensing

Abstract

In this thesis a study of coherent lidars for remote wind sensing is undertaken. Remote wind sensing is important when mast mounted anemometers cannot be used due to economical or practical reasons. One such important field is within the wind energy sector where accurate wind sensing at distances of up to a few hundred meters from the ground is important for site evaluation, turbine power verification and for future applications such as active blade pitch and load control for an optimized output. Coherent lidars can detect very small frequency variations in collected weak light. A viable solution to remotely sense wind is thus to sense the Doppler shift induced on the received light scattered off aerosols suspended in the atmosphere. Fiber based systems are cost effective and robust, and can operate without safety restrictions; they are thus an ideal choice for non-scientific applications. The thesis covers a wide field; the state of the art coherent lidar wind sensors is reviewed, the signal model is expanded upon and results of applied research on commercially available lidars, as well as the development and evaluation of a frequency modulated prototype, are presented. The standard model for coherent lidars sensing dispersed targets was expanded upon by considering the range dependent correlated contribution duration of range gated systems, which significantly affects the narrowband signal to noise description and the weighting of the sample volume. A model of the range dependent collection efficiency obtained by focused fiber based lidars has been developed and verified. These advancements are important for predicting the lidar performance and optimizing the signal processing. A novel frequency modulation concept, which is promising for the remote sensing of wind, was invented in this study. The technique is based on equidistantly frequency stepped pulse trains and a frequency pursuing local oscillator. It operates at high pulse repetition but will despite this avoid the customary disadvantages; range ambiguity and a limitation of the sensing range. This novel concept could provide more accurate systems while avoiding the use of high peak powers which introduces stimulated Brillouin scattering and non-linear effects as well as putting higher demands on other components in the transmitter. A frequency sweeper, appropriate for the invented frequency modulation technique, was constructed and evaluated. This sweeper was subsequently used to modify a continuous wave lidar into a frequency modulated prototype and a proof of principle campaign, demonstrated the merits and limitations of the novel technique. Applied investigations of wind data collected from two different commercial systems revealed that the weighting of the wind distribution spectrum is influenced by single particles with atypically long correlated scatter durations, when sensed by focused continuous wave systems, and by the range dependent collection efficiency, when sensed by focused range gated systems. Resume (Abstract in Danish) I denne Ph.D.-afhandling er kohaerent lidar teknologien for afstandsmaling af vind undersogt. Afstandsmaling af vind er vigtig i de tilfaelde hvor kopanemometre der er monteret i en mast, af praktiske eller okonomiske arsager ikke kan bruges. Praecis vindmaling pa afstande op til nogle hundrede meter fra jordens overflade er saerligt vigtigt inden for vindenergisektoren for eksempel til vurdering af potentielle mollepladser, turbine verificering og for fremtidige anvendelser, sasom aktiv bladvinkelregulering og belastningskontrol for at opna et optimeret udbytte. Kohaerente lidars kan detektere meget sma frekvensvariationer i et ekstremt svagt lyssignal. Det er derfor muligt at afstandsmale vindens hastighed ved at male Doppler-skiftet pafort det indsamlede lys reflekteret fra aerosoler der folger luftstrommen. Fiberbaserede systemer er okonomiske i drift, robuste og kan bruges uden sikkerhedsrestriktioner. De er derfor velegnede til ikke-videnskabelige applikationer. Denne afhandling vil daekke en gennemgang af de nyeste vindmalende kohaerente systemer, udviklingen af signalmodellering og resultaterne fra studier af kommercielle systemer savel som udvikling og evaluering af en frekvens moduleret prototype. Signalmodellen for kohaerente lidars, der maler et distribueret mal, som eksempelvis atmosfaeren, er blevet udviklet. Specielt med hensigt pa den afstandsafhaengige korrelerede periode i refleksionerne der bliver samlet under en malingsperiode. Denne effekt har stor betydning for det smalbandede signal/stoj forhold og for vaegtningen af provevolumen. En model, der beskriver den afstandsafhaengige indsamlingseffektivitet af et fokuseret fiberbaseret lidarsystem, er blevet udviklet og verificeret. Disse fremskridt er vigtige for forudsigelser af en lidars ydeevne og for optimal signalbehandling. En ny frekvensmoduleringsmetode, som er tilpasset for afstandsmaling af vind, blev udviklet i dette studie. Teknikken er baseret pa aekvidistant trinvist frekvensmodulerede pulstog og en frekvens folgende lokal oscillator. Den sender pulser med hoj repetition men undgar trods dette de almindelige ulemper sasom afstandsambiguiteter og begraenset malingsafstand. Denne metode kan give mere nojagtige systemer uden at bruge de hoje spidseffekter, der forarsager stimuleret Brillouin spredning, ikke-lineaere effekter og gor, at der stilles hojere krav om holdbarhed for ovrige komponenter i transmitteren. En frekvens skanner, der er tilpasset den opfundne frekvensmoduleringsteknik, er blevet konstrueret og undersogt. Denne frekvens skanner blev herefter brugt til at modificere en kontinuerligt sendende lidar til en frekvensmoduleret prototype. En malingskampagne for at pavise den nye tekniks fordele og begraensninger blev udfort. Undersogelser af vinddata indsamlet med to forskellige kommercielle systemer er ogsa blevet lavet. Det er pavist, at enkelte partikler med en utypisk lang korreleret spredningsperiode pavirker vindspektra malt med kontinuerligt transmitterede lidars, og at den afstands-afhaengige indsamlingseffektivitet pavirker vindspektra malt med fokuserede pulsede lidars. “I had always considered that if you thought hard and long enough, you could work out anything. Wind was a hard one though. It had always puzzled me until the day I realized that it evidently was caused by all the trees waving about.” Terry Pratchett