lidar r s p c cu-b lecture 16. temperature lidar (5...

LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 2016

Lecture 16. Temperature Lidar (5) Resonance Doppler Techniques

q  Resonance Fluorescence Na Doppler LidarØ  Na Structure and Spectroscopy Ø  Scanning Technique versus Ratio Techniqueq  Principles of Doppler ratio techniquesØ  Two-frequency ratio technique Ø  Three-frequency ratio techniqueØ  Comparison of calibration curvesq  Na Doppler Lidar InstrumentationØ  Summary

1


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. 2


Recall “Doppler Effects in Fe Boltzmann Lidar”

3

NNorm(λ372, z) =NFe(λ372, z)

NR (λ372, zR )Tc2(λ372, z)

=σ eff (λ372,T,σ L )RBλ372nFe(λ372, z)

4πσ R (λ372, )nR (zR )

q  For two channels of Fe Boltzmann lidar,

q  Take the Boltzmann temperature ratio as

€

RT (z) =Nnorm (λ374,z)Nnorm (λ372,z)

=g2g1

RB374RB372

λ374λ372

#

$ %

&

' (

4.0117 σeff (λ374,T,σL374 )σeff (λ372,T,σL372)

exp −ΔE /kBT( )

NNorm(λ374, z) =NFe(λ374, z)

NR (λ374, zR )Tc2(λ374, z)


4πσ R (λ374, )nR (zR )

=NFe(λ374, z)

NR (λ374, zR )Tc2(λ374, z)


4πσ R (λ374, )nR (zR )g2g1exp(−ΔE / kBT )

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.


Doppler Effect in Na D2 Line Resonance Fluorescence

Na D2 absorption linewidth is temperature dependent

(thermal motions of atom assembly)

Na D2 absorption peak frequency is wind dependent

(bulk motions of atom assembly)4

Abs

orpt

ion

Coe

ffici

ent


Na Atomic Energy Levels

5 Textbook Chapter 5 by Chu and Papen


Na Atomic Parameters


An


Doppler-Limited Na Spectroscopy

€

σeff (ν) =12πσe

e2 f4ε0mec

An exp −[νn − ν(1−VR /c)]

2

2σe2

'

( )

*

+ ,

n=1

6∑

€

σabs(ν) =12πσD

e2 f4ε0mec

An exp −[νn − ν(1−VR /c)]

2

2σD2

'

( )

*

+ ,

n=1

6∑

q  Doppler-broadened Na absorption cross-section is approximated as a Gaussian with rms width σD

q  Assume the laser lineshape is a Gaussian with rms width σL q  The effective cross-section is the convolution of the atomic absorption cross-section and the laser lineshape

€

σe = σD2 + σL

2

€

σD =kBTMλ0

2where and

The frequency discriminator/analyzer is in the atmosphere! -- A huge Na vapor cell – the Na absorption lines as a frequency discriminator. 7


Doppler Scanning Technique

€

NNa(λ,z) =PL (λ)Δthc λ

$

% &

'

( ) σeff (λ)nNa(z)Δz( ) A

4πz2$

% &

'

( ) η(λ)Ta

2(λ)Tc2(λ,z)G(z)( )

€

NR (λ,zR ) =PL (λ)Δthc λ

$

% &

'

( ) σR (π ,λ)nR (zR )Δz( ) A

zR2

$

% &

'

( ) η(λ)Ta

2(λ,zR )G(zR )( )

€

σeff (λ,z) =C(z)

Tc2(λ,z)

NNa(λ,z)NR (λ,zR )

€

C(z ) =σR(π ,λ)nR(zR )

nNa(z )4πz 2

zR2where

8

Least-square fitting gives temp[Fricke and von Zahn, JATP, 1985]


Scanning Na Lidar Results

9 [Fricke and von Zahn, JATP, 1985]


Doppler Ratio Technique

10 −2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5x 109

0

0.2

0.4

0.6

0.8

1

1.2 x 10−15

Frequency Offset (Hz)

Effe

ctiv

e C

ross

Sec

tion

(m2 )

−50 m/s0 m/s+50 m/s

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5x 109

0

0.2

0.4

0.6

0.8

1

1.2 x 10−15

Frequency Offset (Hz)

Effe

ctiv

e C

ross

Sec

tion

(m2 )

150 K200 K250 K

fa fc

fa f+f-


2-Frequency Doppler Ratio Technique

€

RT (z) =Nnorm ( fc,z,t1)Nnorm ( fa,z,t2)

=σeff ( fc,z)nNa(z,t1)σeff ( fa,z)nNa(z,t2)

≈σeff ( fc,z)σeff ( fa,z)

€

Nnorm ( f ,z,t) ≡NNa( f ,z,t)

NR ( f ,z,t)Tc2( f ,z)

z2

zR2

€

Nnorm ( f ,z,t) =σeff ( f )nNa(z)σR (π , f )nR (zR )

14π

11

Two frequencies fa and fc are insensitive to radial wind.


3-Frequency Doppler Ratio Technique

€

RT (z) =Nnorm ( f+,z,t1) + Nnorm ( f−,z,t2)

Nnorm ( fa,z,t3)

≈σeff ( f+,z) + σeff ( f−,z)

σeff ( fa,z)

€

RW (z) =Nnorm ( f−,z,t2)Nnorm ( f+,z,t1)

≈σeff ( f−,z)σeff ( f+,z)

12 0

200

400

600

800

1000

1200

1400

1600

0 1000 2000 3000 4000 5000 6000

fa Channelf+ Channelf- Channel

Phot

on C

ount

s

Bin Number

Raw Data Profiles for 3-Frequency Na Doppler Lidar


Calibration Curves for 3-Freq Tech

13

Isoline / Isogram

180 K

-40 m/s

q  For given temperatures and winds, we can compute the Doppler lidar calibration curves from atomic physics and lidar physics.


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 RT and RW 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.

14


Principle of Doppler Ratio Technique q  Lidar equation for resonance fluorescence (Na, K, or Fe)

€

NS (λ ,z) =PL (λ)Δthc λ

$

% &

'

( ) σ eff (λ ,z)nc(z)RB (λ) +σR (π ,λ)nR (z)[ ]Δz A

4πz2$

% &

'

( )

× Ta2(λ)Tc

2(λ ,z)( ) η(λ)G(z)( )+NB

RB = 1 for current Na Doppler lidar since return photons at all wavelengths are received by the broadband receiver, so no fluorescence is filtered off.

€

NNa(λ ,z) =PL (λ)Δthc λ

$

% &

'

( ) σ eff (λ ,z)nc(z)[ ]Δz A

4πz2$

% &

'

( ) Ta

2(λ)Tc2(λ ,z)( ) η(λ)G(z)( )

€

NR (λ ,z) =PL (λ)Δthc λ

$

% &

'

( ) σR (π ,λ)nR (z)[ ]Δz A

z2$

% &

'

( ) Ta

2(λ)Tc2(λ ,z)( ) η(λ)G(z)( )

q  Pure Na signal and pure Rayleigh signal in Na region are

q  So we have

€

NS (λ,z) = NNa (λ,z) + NR (λ,z) + NB 15


Principle of Doppler Ratio Technique q  Lidar equation at pure molecular scattering region (35-55km)

€

NS (λ ,zR ) =PL (λ)Δthc λ

$

% &

'

( ) σR (π ,λ)nR (zR )[ ]Δz A

zR2

$

% & &

'

( ) ) Ta

2(λ ,zR ) η(λ)G(zR )( )+NB

q  Pure Rayleigh signal in molecular scattering region is

q  So we have

€

NS (λ,zR ) = NR (λ,zR ) + NB

€

NR (λ,zR ) =PL (λ)Δthc λ

$

% &

'

( ) σR (π ,λ)nR (zR )[ ]Δz A

zR2

$

% &

'

( ) Ta

2(λ,zR ) η(λ)G(zR )( )

€

NR (λ ,z)NR (λ ,zR )

=σR (π ,λ)nR (z)[ ]Ta2(λ ,z)Tc2(λ ,z)G(z)σR (π ,λ)nR (zR )[ ]Ta2(λ ,zR )G(zR )

zR2

z2=nR (z)nR (zR )

zR2

z2Tc2(λ ,z)

q  The ratio between Rayleigh signals at z and zR is given by

Where nR is the (total) atmospheric number density, usually obtained from atmospheric models like MSIS00. 16


Principle of Doppler Ratio Technique q  From above equations, the pure Na and Rayleigh signals are

€

NNa (λ,z) = NS (λ,z) − NB − NR (λ,z)

€

NR (λ,zR ) = NS (λ,zR ) − NB

q  Normalized Na photon count is defined as

€

NNorm (λ ,z) =NNa(λ ,z)

NR (λ ,zR )Tc2(λ ,z)

z2

zR2

€



=σ eff (λ ,z)nc(z)σR (π ,λ)nR (zR )

14π

q  From physics point of view, the normalized Na count is

q  From actual photon counts, the normalized Na count is

€



z2

zR2 =

NS (λ ,z) −NB −NR (λ ,z)NR (λ ,zR )Tc

2(λ ,z)z2

zR2

=NS (λ ,z) −NBNS (λ ,zR ) −NB

z2

zR2

1Tc2(λ ,z)

−nR (z)nR (zR ) 17


Principle of Doppler Ratio Technique q  From physics, the ratios of RT and RW are then given by

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.

€

RT =NNorm ( f+ ,z) +NNorm ( f− ,z)

NNorm ( fa ,z)=

σ eff ( f+ ,z)nc(z)σR (π , f+)nR (zR )

+σ eff ( f− ,z)nc(z)σR (π , f−)nR (zR )

σ eff ( fa ,z)nc(z)σR (π , fa)nR (zR )

=σ eff ( f+ ,z) +σeff ( f− ,z)

σ eff ( fa ,z)

€

RW =NNorm ( f+ ,z) −NNorm ( f− ,z)

NNorm ( fa ,z)=

σ eff ( f+ ,z)nc(z)σR (π , f+)nR (zR )

−σ eff ( f− ,z)nc(z)σR (π , f−)nR (zR )

σ eff ( fa ,z)nc(z)σR (π , fa)nR (zR )

=σ eff ( f+ ,z) −σeff ( f− ,z)

σ eff ( fa ,z)

18


Principle of Doppler Ratio Technique q  From actual photon counts, the ratios RT and RW are

€


NNorm ( fa ,z)

=

NS ( f+ ,z) −NBNS ( f+ ,zR ) −NB

z2

zR2

1Tc2( f+ ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( ( +

NS ( f− ,z) −NBNS ( f− ,zR ) −NB

z2

zR2

1Tc2( f− ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( (

NS ( fa ,z) −NBNS ( fa ,zR ) −NB

z2

zR2

1Tc2( fa ,z)

−nR (z)nR (zR )

€


NNorm ( fa ,z)

=


z2

zR2

1Tc2( f+ ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( ( −


z2

zR2

1Tc2( f− ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( (


z2

zR2

1Tc2( fa ,z)

−nR (z)nR (zR )

19


How Does Ratio Technique Work? q  From physics, we calculate the ratios of RT and RW as

€

RT =σeff ( f+ ,z) +σ eff ( f− ,z)

σ eff ( fa ,z)

€

RW =σeff ( f+ ,z) −σ eff ( f− ,z)

σ eff ( fa ,z)

q  From actual photon counts, we calculate the ratios as

€


NNorm ( fa ,z)

=


z2

zR2

1Tc2( f+ ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( ( +


z2

zR2

1Tc2( f− ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( (


z2

zR2

1Tc2( fa ,z)

−nR (z)nR (zR )

€


NNorm ( fa ,z)

=


z2

zR2

1Tc2( f+ ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( ( −


z2

zR2

1Tc2( f− ,z)

−nR (z)nR (zR )

#

$ % %

&

' ( (


z2

zR2

1Tc2( fa ,z)

−nR (z)nR (zR ) 20


How Does Ratio Technique Work?

21

Isoline / Isogram

180 K

-40 m/s

q  Compute Doppler calibration curves from physicsq  Look up these two ratios on the calibration curves to infer the corresponding Temperature and Wind from isoline/isogram.


Na Doppler Lidar Data @ SOR

22 0 2 4 6 8 10 12 1475

80

85

90

95

100

105

110

UT Time

Altitu

de (k

m)

SOR 02−18−1999

170

170

170

190

190

190

210

210

210

210

210

230

230

230170

180

190

200

210

220

230

240

160 170 180 190 200 210 220 23075

80

85

90

95

100

105

110

Temperature (K)

Altitu

de (k

m)

SOR 02−18−1999 Night−Mean

75

80

85

90

95

100

105

110

160 180 200 220 240

FB1899.001.TWD @ 2.82 UT

Altit

ude

(km

)

Temperature (K)

75

80

85

90

95

100

105

110

160 180 200 220 240

FB1899.070.TWD @ 5.32 UT

Alti

tude

(km

)

Temperature (K)

75

80

85

90

95

100

105

110

160 180 200 220 240

FB1899.229.TWD @ 10.56 UT

Alti

tude

(km

)

Temperature (K)


Comparison of Calibration Curves q  Different metrics of RW result in different wind sensitivitiesq  The ratio RW= N+/N- has inhomogeneous sensitivity

23


Comparison of Calibration Curves q  The ratio RW= (N+ - N-)/Na has much better uniformity than the simplest ratio

24


Comparison of Calibration Curves q  The ratio RW= ln(N- /N+)/ln(N-×N+/Na

2) has good uniformity

25


Na Doppler Lidar Instrumentation

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!

Transmitter (Radiation Source)

Interaction between radiation and objects

Radiation Propagation Through Medium

Signal Propagation Through Medium

Receiver (Light Collection &

Detection)

Data Acquisition & Control System

Data Processing, Analysis & Interpretation

26


Na Doppler Wind and Temperature Lidar q  Na Doppler lidar is one of the most successful lidars.



STAR Na LIDAR Modernized DAQ, System Control and Receiver

28

BNC

Oscilloscope

Acousto-Optic Modulation

Master Oscillatorλ/2

λ/2

λ/4

CW532nmPumpLaser

10cm AOMAOM20cm 20cmShutter IrisCollimator

Periscope

Opticalisolator

Frequency-DoubledNd:YAGPulsed,Q-SwitchedLaser

Single-ModeRingDyeLaser

PulsedDyeAmplifier

Photodiode

Doppler Free SpectroscopyHotNaVaporCell

Wave-meterSMFiber

NDFilter

ToSky

AOMDriver

RingDyeLaserControlBox

ActuatedSteeringMirror

>LockingFeedback>

ShutterDriver

CMOS

532 nm1064 nm

<CameraControl(USB)>

Receiver Data Acquisition & Control

SHG

Transmitter

TTL

PC

PMT

Newtonian

<MirrorControl(RS-485)>

DiscriminatedPMTSignal

>Q-SwitchTrigger>Analog/Digital/CounterChannel Connections

[Smith et al., ILRC, 2012]


Na Lidar Transmitter

Ring DyeLaser

VerdiLaser

Wavemeter

AOM

Na VaporCell

29


Na Doppler Lidar Receiver

30

CU-Boulder STAR Na Doppler LidarPrimary Focus Telescope

Fiber Coupling

Multimode Optical Fiber

Chopper

Receiver Chain

PMT

Prime-focus telescope’s

primary mirror ê

MM fiber


STAR Na Doppler Lidar DAQ

31

Multimode Optical Fiber

Chopper

Receiver Chain

PMT

Shots/Freq


STAR Na Doppler LIDAR Transmitter

32

BNC

Oscilloscope

Acousto-Optic Modulation

Master Oscillatorλ/2

λ/2

λ/4

CW532nmPumpLaser

10cm AOMAOM20cm 20cmShutter IrisCollimator

Periscope

Opticalisolator

Frequency-DoubledNd:YAGPulsed,Q-SwitchedLaser

Single-ModeRingDyeLaser

PulsedDyeAmplifier

Photodiode

Doppler Free SpectroscopyHotNaVaporCell

Wave-meterSMFiber

NDFilter

ToSky

AOMDriver

RingDyeLaserControlBox

ActuatedSteeringMirror

>LockingFeedback>

ShutterDriver

CMOS

532 nm1064 nm

<CameraControl(USB)>

Receiver Data Acquisition & Control

SHG

Transmitter

TTL

PC

PMT

Newtonian

<MirrorControl(RS-485)>

DiscriminatedPMTSignal

>Q-SwitchTrigger>Analog/Digital/CounterChannel Connections


Master Oscillator and Freq Locking with Doppler-Free Spectroscopy

33 [Smith et al., OE, 2009]

See detailed explanations on Na Doppler-free saturation-fluorescence spectroscopy in

Textbook Chapter 5.2.2.3.2


Doppler-Free Na Spectroscopy

νa νb νc

νc =(νa+νb)/2

See detailed explanation on Na Doppler-free saturation-fluorescence spectroscopy in Textbook Chapter 5.2.2.3.2

34

fa f+f-


Summary (1) q  Resonance fluorescence Doppler lidars apply Doppler technique to infer temperature and wind from the Doppler-broadened and Doppler-shifted 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, RT and RW are sensitive functions of temperature and radial wind, respectively.

35


Summary (2) q  We compute the ratios RT and RW 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

lidar r s p c cu-b lecture 16. temperature lidar (5...

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