Lecture 16. Temperature Lidar (5) Resonance Doppler Techniques

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1 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Lecture 6. Temperature Lidar (5) Resonance Doppler Techniques q Resonance Fluorescence Na Doppler Lidar Ø Na Structure and Spectroscopy Ø Scanning Technique versus Ratio Technique q Principles of Doppler ratio techniques Ø Two-frequency ratio technique Ø Three-frequency ratio technique Ø Comparison of calibration curves q Na Doppler Lidar Instrumentation Ø Summary

2 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Recall Various Physical Interactions and Techniques are Applied to Doppler Tech q When resonance absorption and fluorescence is available (not quenched), the resonance Doppler lidar can be employed to probe Doppler broadening and Doppler shift. It can be atomic resonance fluorescence or molecular resonance fluorescence in the upper atmosphere. q If molecular absorption is available, then DIAL lidar can be employed to infer the Doppler broadening in molecular absorption for temperature. This is doable in the lower atmosphere. q Rayleigh scattering experiences Doppler broadening, so it can be measured using various techniques (e.g., edge filter, interferometers, etc.) to infer the temperature. q High-spectral-resolution lidar (HSRL) can help remove aerosol contamination to reveal Doppler broadening for temperature. q Vibrational-rotational Raman scattering should experience similar Doppler broadening as Rayleigh scattering, but it has shifted frequencies of return signals, which may help avoid the contamination of aerosol scattering to Rayleigh scattering.

3 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Recall Doppler Effects in Fe Boltzmann Lidar q For two channels of Fe Boltzmann lidar, N Norm (λ 37, z) = N Norm (λ 374, z) = = N Fe (λ 37, z) N R (λ 37, z R )T c (λ 37, z) = σ eff (λ 37,T,σ L )R n Bλ37 Fe(λ 37, z) 4πσ R (λ 37, )n R (z R ) N Fe (λ 374, z) N R (λ 374, z R )T c (λ 374, z) = σ eff (λ 374,T,σ L )R Bλ n 374 Fe (λ 374, z) 4πσ R (λ 374,)n R (z R ) N Fe (λ 374, z) N R (λ 374, z R )T c (λ 374, z) = σ eff (λ 374,T,σ L )R n Bλ374 Fe(λ 37, z) 4πσ R (λ 374,)n R (z R ) q Take the Boltzmann temperature ratio as R T (z) = N norm (λ 374,z) N norm (λ 37,z) = g R B374 # λ 374 % g R B37 $ λ 37 & ( ' g g exp( ΔE / k B T ) 4.07 σ eff (λ 374,T,σ L374 ) σ eff (λ 37,T,σ L37 ) exp ΔE /k BT ( ) Using the Fe atoms in the MLT as an Fe vapor cell, if we scan the laser frequency of a single channel of Fe Boltzmann lidar, we will see a convoluted Doppler broadening of the return Fe signals. This wavelength scan can be used to infer temperature if the laser lineshape is well known, or can be used to infer the laser lineshape if the temperature is known. 3

4 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Doppler Effect in Na D Line Resonance Fluorescence Absorption Coefficient Na D absorption linewidth is temperature dependent (thermal motions of atom assembly) Na D absorption peak frequency is wind dependent (bulk motions of atom assembly) 4

5 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Na Atomic Energy Levels Textbook Chapter 5 by Chu and Papen 5

6 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Na Atomic Parameters A n Textbook Chapter 5 by Chu and Papen 6

7 σ abs (ν) = σ eff (ν) = LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Doppler-Limited Na Spectroscopy q Doppler-broadened Na absorption cross-section is approximated as a Gaussian with rms width σ D πσ D πσ e e f 4ε 0 m e c e f 4ε 0 m e c 6 6 n= n= ' A n exp [ν n ν( V R /c)] * ), ( σ D + q Assume the laser lineshape is a Gaussian with rms width σ L q The effective cross-section is the convolution of the atomic absorption cross-section and the laser lineshape where ' 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! -- A huge Na vapor cell the Na absorption lines as a frequency discriminator. 7

8 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 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] 8

9 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Scanning Na Lidar Results [Fricke and von Zahn, JATP, 985] 9

10 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 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 0

11 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 -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.

12 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 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

13 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Calibration Curves for 3-Freq Tech q 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 3

14 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Main Ideas Behind Ratio Technique q Three unknown parameters (temperature, radial wind, and Na number density) require 3 lidar equations at 3 frequencies as the minimum ð the highest resolution. q 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. q 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. q 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. 4

15 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Principle of Doppler Ratio Technique q 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. q 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 $ N R (λ,z) = P L(λ)Δt ' $ A & )[ σ R (π,λ)n R (z)]δz& % hc λ ( % z ' ) T a (λ)tc (λ,z) ( ( ) η(λ)g(z) ' ) T a (λ)tc (λ,z) ( ( ) ( ) η(λ)g(z) ( ) ' ) ( q So we have N S (λ,z) = N Na (λ,z) + N R (λ,z) + N B 5

16 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Principle of Doppler Ratio Technique q 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 q 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 ( q 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 ) ( ) q 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. 6

17 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Principle of Doppler Ratio Technique q From above equations, the pure Na and Rayleigh signals are N Na (λ,z) = N S (λ,z) N B N R (λ,z) q 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 q 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 q 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 ) 7

18 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Principle of Doppler Ratio Technique q 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. 8

19 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Principle of Doppler Ratio Technique q 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 T c ( fa,z) n R (z) n R (z 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 T c ( fa,z) n R (z) n R (z R ) z R T c ( f,z) n R (z) n R (z R ) & ( ' 9

20 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 How Does Ratio Technique Work? q 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) q 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 T c ( fa,z) n R (z) n R (z 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 ) 0 & ( ' & ( '

21 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 How Does Ratio Technique Work? q Compute Doppler calibration curves from physics q 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

22 0 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Na Doppler Lidar SOR UT 5.3 UT 0.56 UT Altitude (km) Altitude (km) Altitude (km) Temperature (K) Temperature (K) Temperature (K) 0 SOR SOR Night Mean Altitude (km) Altitude (km) UT Time Temperature (K)

23 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Comparison of Calibration Curves q Different metrics of R W result in different wind sensitivities q The ratio R W = N + /N - has inhomogeneous sensitivity 3

24 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Comparison of Calibration Curves q The ratio R W = (N + - N - )/N a has much better uniformity than the simplest ratio 4

25 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Comparison of Calibration Curves q The ratio R W = ln(n - /N + )/ln(n - N + /N a ) has good uniformity 5

26 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 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 q Resonance Doppler lidar has the frequency discriminator in atmosphere - atomic absorption lines! ð Narrowband transmitter, broadband receiver. ð High signal levels and accurate knowledge on the frequency discriminator! 6

27 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Na Doppler Wind and Temperature Lidar q Na Doppler lidar is one of the most successful lidars. Textbook Chapter 5 by Chu and Papen 7

28 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 STAR Na LIDAR Modernized DAQ, System Control and Receiver Receiver PMT Discriminated PMT Signal PC Data Acquisition & Control < Mirror Control (RS -485) > < Camera Control (USB) > Newtonian > Q-Switch Trigger > TTL B N C > Locking Feedback > Analog/Digital/Counter Channel Connections Transmitter SHG 53 nm 064 nm Frequency-Doubled Nd:YAG Pulsed, Q-Switched Laser Shutter Driver Oscilloscope Doppler Free Spectroscopy AOM Driver Photodiode Hot Na Vapor Cell ND Filter SM Fiber Wavemeter λ/ λ/4 Shutter 0 cm AOM 0 cm AOM 0 cm Acousto-Optic Modulation Periscope Pulsed Dye Amplifier λ/ To Sky CMOS Optical isolator Master Oscillator CW 53 nm Pump Laser Collimator Single-Mode Ring Dye Laser Iris Ring Dye Laser Control Box Actuated Steering Mirror [Smith et al., ILRC, 0] 8

29 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Na Lidar Transmitter AOM Na Vapor Cell Verdi Laser Wavemeter Ring Dye Laser 9

30 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Na Doppler Lidar Receiver MM fiber CU-Boulder STAR Na Doppler Lidar Primary Focus Telescope Fiber Coupling Chopper PMT Prime-focus telescope s primary mirror ê Multimode Optical Fiber Receiver Chain 30

31 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 STAR Na Doppler Lidar DAQ Chopper PMT Shots/Freq Multimode Optical Fiber Receiver Chain 3

32 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 STAR Na Doppler LIDAR Transmitter Transmitter Doppler Free Spectroscopy SHG Frequency-Doubled Nd:YAG Pulsed, Q-Switched Laser Oscilloscope Photodiode Hot Na Vapor Cell ND Filter 53 nm 064 nm Shutter Driver AOM Driver SM Fiber Wavemeter λ/ λ/4 Shutter 0 cm AOM 0 cm AOM 0 cm Acousto-Optic Modulation Periscope Pulsed Dye Amplifier λ/ To Sky CMOS Optical isolator Master Oscillator CW 53 nm Pump Laser Collimator Single-Mode Ring Dye Laser Iris Ring Dye Laser Control Box Actuated Steering Mirror 3

33 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 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

34 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 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 )/ 34

35 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Summary () q 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. q 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. q 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). q By taking the ratios among signals at these three frequencies, R T and R W are sensitive functions of temperature and radial wind, respectively. 35

36 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 06 Summary () q 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. q Different metrics exhibit different inhomogeneity, resulting in different crosstalk between T and W errors. q 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. q Na and K Doppler lidars are currently near mature status and making great contributions to MLT science. q 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. 36