Lecture 12. Temperature Lidar (2) Resonance Fluorescence Doppler Lidar

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1 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Lecture. Temperature Lidar () Resonance Fluorescence Doppler Lidar Resonance Fluorescence Na Doppler Lidar Na Structure and Spectroscopy Scanning Technique versus Ratio Technique Principles of Doppler ratio techniques Two-frequency ratio technique Three-frequency ratio technique Comparison of calibration curves Na Doppler-Free Spectroscopy Summary

2 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Na Doppler Wind and Temperature Lidar Na Doppler lidar is one of the most successful lidars. Textbook Chapter 5 by Chu and Papen

3 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Na Atomic Energy Levels 3

4 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Na Atomic Parameters 4

5 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Doppler Effect in Na D Line Resonance Fluorescence Na D absorption linewidth is temperature dependent Na D absorption peak freq is wind dependent 5

6 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Doppler-Limited Na Spectroscopy Doppler-broadened Na absorption cross-section is approximated as a Gaussian with rms width σ D e 6 f σ abs (ν) = A n exp [ν n ν( V R /c)] πσ D 4ε 0 m e c n= σ D Assume the laser lineshape is a Gaussian with rms width σ L The effective cross-section is the convolution of the atomic absorption cross-section and the laser lineshape σ eff (ν) = where πσ e e f 4ε 0 m e c 6 n= A n exp [ν n ν( V R /c)] σ e σ e = σ D + σ σ D = L and k B T Mλ 0 The frequency discriminator/analyzer is in the atmosphere! 6

7 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Doppler Scanning Technique N Na (λ,z) = P L(λ)Δt σ eff (λ)n Na (z)δz hc λ ( ) N R (λ,z R ) = P L (λ)δt σ R (π,λ)n R (z R )Δz hc λ σ eff (λ,z) = C(z) T c (λ,z) ( ) N Na (λ,z) N R (λ,z R ) A 4πz A z R η(λ)t a (λ)t c (λ,z)g(z) ( ) η(λ)t a (λ,z R )G(z R ) ( ) where C(z) = σ R(π,λ)n R (z R ) n Na (z) 4πz z R Least-square fitting gives temp [Fricke and von Zahn, JATP, 985] 7

8 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Scanning Na Lidar Results [Fricke and von Zahn, JATP, 985] 8

9 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Doppler Ratio Technique Effective Cross Section (m ). x f - f a f + 50 K 00 K 50 K Effective Cross Section (m ) Frequency Offset (Hz) x 0 9. x f a f c 50 m/s 0 m/s +50 m/s Frequency Offset (Hz) x 0 9 9

10 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 -Frequency Doppler Ratio Technique R T (z) = N norm( f c,z,t ) N norm ( f a,z,t ) = σ eff ( f c,z)n Na (z,t ) σ eff ( f a,z)n Na (z,t ) σ eff ( f c,z) σ eff ( f a,z) N norm ( f,z,t) N Na ( f,z,t) N R ( f,z,t)t c ( f,z) z z R N norm ( f,z,t) = σ eff ( f )n Na (z) σ R (π, f )n R (z R ) 4π Two frequencies f a and f c are insensitive to radial wind. 0

11 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 3-Frequency Doppler Ratio Technique R T (z) = N norm ( f +,z,t ) + N norm ( f,z,t ) N norm ( f a,z,t 3 ) σ eff ( f +,z) + σ eff ( f,z) σ eff ( f a,z) R W (z) = N norm( f,z,t ) N norm ( f +,z,t ) σ eff ( f,z) σ eff ( f +,z) 600 Raw Data Profiles for 3-Frequency Na Doppler Lidar Photon Counts fa Channel f+ Channel f- Channel Bin Number

12 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Calibration Curves for 3-Freq Tech For given temperatures and winds, we can compute the Doppler lidar calibration curves from atomic physics and lidar physics. -40 m/s Isoline / Isogram 80 K

13 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Main Ideas Behind Ratio Technique Three unknown parameters (temperature, radial wind, and Na number density) require 3 lidar equations at 3 frequencies as the minimum the highest resolution. In the ratio technique, Na number density is cancelled out. So we have two ratios R T and R W that are independent of Na density but both dependent on T and W. The idea is to derive temperature and radial wind from these two ratios first, and then derive Na number density using derived temperature and wind at each altitude bin. However, because the Na extinction coefficient is involved, the upper bins are related to lower bins, and extinction coefficient is related to Na density and effective cross-section. The solution is to start from the bottom of the Na layer and then work bin by bin to the layer top. 3

14 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Principle of Doppler Ratio Technique Lidar equation for resonance fluorescence (Na, K, or Fe) N S (λ,z) = P L (λ)δt [ σ eff (λ,z)n c (z)r B (λ) +σ R (π,λ)n R (z)]δz hc λ ( T a (λ)tc (λ,z) )( η(λ)g(z) )+ N B A 4πz R B = for current Na Doppler lidar since return photons at all wavelengths are received by the broadband receiver, so no fluorescence is filtered off. Pure Na signal and pure Rayleigh signal in Na region are N Na (λ,z) = P L(λ)Δt A [ σ eff (λ,z)n c (z)]δz hc λ 4πz T a (λ)tc (λ,z) ( ) η(λ)g(z) ( ) N R (λ,z) = P L(λ)Δt hc λ [ σ R (π,λ)n R (z)]δz A z T a (λ)tc (λ,z) ( ) η(λ)g(z) ( ) So we have N S (λ,z) = N Na (λ,z) + N R (λ,z) + N B 4

15 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Principle of Doppler Ratio Technique Lidar equation at pure molecular scattering region (35-55km) N S (λ,z R ) = P L (λ)δt [ σ R (π,λ)n R (z R )]Δz hc λ A z R T a (λ,z R )( η(λ)g(z R ))+ N B Pure Rayleigh signal in molecular scattering region is N R (λ,z R ) = P L (λ)δt A [ σ R (π,λ)n R (z R )]Δz T a (λ,z R ) η(λ)g(z R ) hc λ z R So we have N S (λ,z R ) = N R (λ,z R ) + N B N R (λ,z) [ N R (λ,z R ) = σ R (π,λ)n R (z) ]T a (λ,z)tc (λ,z)g(z) [ σ R (π,λ)n R (z R )]T a (λ,zr )G(z R ) z R z = n R (z) n R (z R ) ( ) The ratio between Rayleigh signals at z and z R is given by z R z T c (λ,z) Where n R is the (total) atmospheric number density, usually obtained from atmospheric models like MSIS00. 5

16 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Principle of Doppler Ratio Technique From above equations, the pure Na and Rayleigh signals are N Na (λ,z) = N S (λ,z) N B N R (λ,z) Normalized Na photon count is defined as N R (λ,z R ) = N S (λ,z R ) N B N Norm (λ,z) = N Na (λ,z) z N R (λ,z R )T c (λ,z) z R From physics point of view, the normalized Na count is N Norm (λ,z) = N Na (λ,z) = σ eff (λ,z)n c (z) N R (λ,z R )T c (λ,z) σ R (π,λ)n R (z R ) 4π From actual photon counts, the normalized Na count is N Norm (λ,z) = N Na (λ,z) z N R (λ,z R )T c (λ,z) = N S (λ,z) N B N R (λ,z) z R N R (λ,z R )T c (λ,z) = N S (λ,z) N B z N S (λ,z R ) N B z R z z R T c (λ,z) n R (z) n R (z R ) 6

17 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Principle of Doppler Ratio Technique From physics, the ratios of R T and R W are then given by R T = N Norm( f +,z) + N Norm ( f,z) N Norm ( f a,z) σ eff ( f +,z)n c (z) σ = R (π, f + )n R (z R ) + σ eff ( f,z)n c (z) σ R (π, f )n R (z R ) σ eff ( f a,z)n c (z) σ R (π, f a )n R (z R ) = σ eff ( f +,z) +σ eff ( f,z) σ eff ( f a,z) R W = N Norm ( f +,z) N Norm ( f,z) N Norm ( f a,z) σ eff ( f +,z)n c (z) σ = R (π, f + )n R (z R ) σ eff ( f,z)n c (z) σ R (π, f )n R (z R ) σ eff ( f a,z)n c (z) σ R (π, f a )n R (z R ) = σ eff ( f +,z) σ eff ( f,z) σ eff ( f a,z) Here, Rayleigh backscatter cross-section is regarded as the same for three frequencies, since the frequency difference is so small. Na number density is also the same for three frequency channels, and so is the atmosphere number density at Rayleigh normalization altitude. 7

18 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Principle of Doppler Ratio Technique From actual photon counts, the ratios R T and R W are R T = N Norm ( f +,z) + N Norm ( f,z) N Norm ( f a,z) N S ( f +,z) N B z N S ( f +,z R ) N B z = R T c ( f+,z) n R (z) n R (z R ) + N S ( f a,z) N B z N S ( f a,z R ) N B z R N S( f,z) N B z N S ( f,z R ) N B z R T c ( fa,z) n R (z) n R (z R ) T c ( f,z) n R (z) n R (z R ) R W = N Norm ( f +,z) N Norm ( f,z) N Norm ( f a,z) N S ( f +,z) N B z N S ( f +,z R ) N B z = R T c ( f+,z) n R (z) n R (z R ) N S ( f a,z) N B z N S ( f a,z R ) N B z R N S( f,z) N B z N S ( f,z R ) N B z R T c ( fa,z) n R (z) n R (z R ) T c ( f,z) n R (z) n R (z R ) 8

19 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 How Does Ratio Technique Work? From physics, we calculate the ratios of R T and R W as R T = σ eff ( f +,z) +σ eff ( f,z) σ eff ( f a,z) R W = σ eff ( f +,z) σ eff ( f,z) σ eff ( f a,z) From actual photon counts, we calculate the ratios as R T = N Norm ( f +,z) + N Norm ( f,z) N Norm ( f a,z) = N S ( f +,z) N B z N S ( f +,z R ) N B z R R W = N Norm( f +,z) N Norm ( f,z) N Norm ( f a,z) = N S ( f +,z) N B z N S ( f +,z R ) N B z R T c ( f+,z) n R (z) n R (z R ) + N S ( f a,z) N B z N S ( f a,z R ) N B z R T c ( f+,z) n R (z) n R (z R ) N S ( f a,z) N B z N S ( f a,z R ) N B z R N S ( f,z) N B z N S ( f,z R ) N B z R T c ( fa,z) n R (z) n R (z R ) N S ( f,z) N B z N S ( f,z R ) N B z R T c ( f,z) n R (z) n R (z R ) T c ( f,z) n R (z) n R (z R ) T c ( fa,z) n R (z) n R (z R ) 9

20 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 How Does Ratio Technique Work? Compute Doppler calibration curves from physics Look up these two ratios on the calibration curves to infer the corresponding Temperature and Wind from isoline/isogram. -40 m/s Isoline / Isogram 80 K 0

21 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 UIUC Na Doppler Lidar SOR

22 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Comparison of Calibration Curves Different metrics of R W result in different wind sensitivities The ratio R W = N + /N - has inhomogeneous sensitivity

23 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Comparison of Calibration Curves The ratio R W = (N + - N - )/N a has much better uniformity than the simplest ratio 3

24 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Comparison of Calibration Curves The ratio R W = ln(n - /N + )/ln(n - N + /N a ) has good uniformity 4

25 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Na Doppler Lidar Instrumentation Interaction between radiation and objects Radiation Propagation Through Medium Transmitter (Radiation Source) Signal Propagation Through Medium Receiver (Light Collection & Detection) Data Acquisition & Control System Data Processing, Analysis & Interpretation Resonance Doppler lidar has the frequency discriminator in atmosphere - atomic absorption lines! Narrowband transmitter, broadband receiver. High signal levels and accurate knowledge on the frequency discriminator! 5

26 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 STAR Na LIDAR Modernized DAQ, System Control and Receiver [J. A. Smith, W. Fong, W. Huang, and X. Chu, University of Colorado] 6

27 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Na Lidar Transmitter Photo AOM Na Vapor Cell Verdi Laser Wavemeter Ring Dye Laser 7

28 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Na Lidar Transmitter Photo Ring Dye Laser PDA Nd:YAG 8

29 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 STAR Na Doppler LIDAR Transmitter 9

30 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Master Oscillator and Freq Locking with Doppler- Free Spectroscopy [Smith et al., OE, 009] See detailed explanations on Na Doppler-free saturation-fluorescence spectroscopy in Textbook Chapter

31 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Doppler-Free Na Spectroscopy f - f a f + See detailed explanation on Na Doppler-free saturation-fluorescence spectroscopy in Textbook Chapter ν a ν c ν b ν c =(ν a +ν b )/ 3

32 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Summary () Resonance fluorescence Doppler lidars apply Doppler technique to infer temperature and wind from the Doppler-broadened and Dopplershifted atomic absorption spectroscopy. The Doppler-limited atomic absorption spectroscopy is inferred from the returned fluorescence intensity ratios at different frequencies. Both scanning and ratio techniques can work for the Doppler lidar. With scanning technique, the laser will be operated at many different frequencies, and then a least-square fit derives the width of the atomic absorption line, thus deriving the temperature. Its advantage is to provide more than 3 frequency information, so providing checks on more system parameters. But it requires longer integration time. Doppler ratio technique takes advantage of the high temporal resolution feature by limiting the lidar detection to 3 preset frequencies (usually one peak and two wing frequencies) for 3 unknown parameters (T, W, and density). By taking the ratios among signals at these three frequencies, R T and R W are sensitive functions of temperature and radial wind, respectively. 3

33 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Summary () We compute the ratios R T and R W from atomic physics first to form the lidar calibration curves, and then look up the two ratios calculated from actual photon counts on the calibration curves to infer the corresponding temperature T and radial wind W. Different metrics exhibit different inhomogeneity, resulting in different crosstalk between T and W errors. There are several different atomic species (Na, K, Fe, Ca, Ca +, etc.) originating from meteor ablation in the mesosphere and lower thermosphere (MLT) region. They all have the potentials to be tracers for resonance fluorescence Doppler lidars to measure the temperature and wind in the MLT region. Na and K Doppler lidars are currently near mature status and making great contributions to MLT science. Fe Doppler lidar has very high future potential due to the high Fe abundance, advanced alexandrite laser technology, Doppler-free Fe spectroscopy, and bias-free measurements, etc. 33

Lecture 13. Temperature Lidar (2) Resonance Fluorescence Doppler Tech

LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Lecture 13. Temperature Lidar () Resonance Fluorescence Doppler Tech Resonance Fluorescence Na Doppler Lidar Na Structure and Spectroscopy Scanning