Lidar for Atmospheric Remote sensing Philippe Keckhut et Andrea Pazmiño LATMOS, Institut Pierre Simon Laplace, CNRS-UPMC-UVSQ, Paris, France Historical.

Lidar for Atmospheric Remote sensing

Philippe Keckhut et Andrea PazmiñoLATMOS, Institut Pierre Simon Laplace, CNRS-UPMC-UVSQ, Paris, France

Historical Overview Lidar BasicsThe Lidar Equation Lidar systems Summary

Outline

Historical Overview (1)

1930 Synge proposed a method to determine the atmospheric density with an antiaircraft searchlight and a telescope (bistatic configuration)

1936 First reported results of density profiles: Duclaux (3.4 km), Hulbert (28 km)

1938 First reported use of a monostatic configuration for cloud base height, using a pulsed light source (Bureau)

1953 First retrieval of temperature profiles from density profiles (Elterman)

Emitted beam

Detector field of view

Monostatic co-axial

BistaticMonostatic bi-axial

~km

Transmitter & receiver collocated

(pulsed light source range of scattering)

Receiver’s FOV scanned along the transmitted beam

(geometry range of scattering)

Historical Overview (2)

1956 Friedland et al. reported the first pulsed monostatic system for atmospheric density measurements

Early 1960s Invention of laser powerful new light source for lidar systems

1962 First use of laser in a lidar system (Smullins & Fiocco)

1977 First ozone measurements by lidar (Mégie et al.)

Gérard Mégie

Historical Overview (3)

Networks of ground-based lidar systems as NDSC, EARLINET, etc

Lidars on aircraft

Space-based lidar (ALISSA, LITE, … CALIPSO program)

Present

Lidar Basics (1)

LIDAR: LIght Detection And Ranging

Active remote sensing technique for measuring atmospheric parameters (T, , wind and different constituents: H2O, O3, …, clouds, aerosols)

Same principle as radar but 0.1 < < 10 m

Principle: emission of a light beam that interacts with the medium & detection of radiation backscattered towards the instrument

Interactions with the Atmosphere:elastic (Rayleigh, Mie, Resonance scattering)

inelastic (Raman scattering, Fluorescence)

Absorption

Lidar Basics (2)

Some Optical Interactions of Relevance to Laser Environmental Sensing

Rayleigh Scattering Mie Scattering

Absorption

Elastic Interactions Inelastic Interactions

+ = Differential Absorption & Scattering

• Interaction with the quantized vibrational

& rotational energy levels of the molecule

• >> d Rayleigh = C/4

• ~ d Mie = C/a

• ~ d Mie = C/a

Raman Scattering

• ~ d Mie = C/a

Virtual Level

Vibrationally Excited Level

Lidar Basics (3)

Block diagram of a generic lidar system

Laser Beam expander (optional)

Transmitter

Backscattered light

Emitted light

Light collectin

g telescop

e

Optical filtering

Receiver

Optical to electrical transducer

Electrical recording system

Detector & Recording

Synchronization control

Lidar Basics (4)

Ranging of pulsed monostatic lidar

Each light pulse fired

Complete altitude scattering profile

Time

Altitude

Aerosol layer 2

Aerosol layer 1

Z1

Z2

Laser beam

Scattered light

T1=2.Z1/C T2=2.Z2/C

Signal

Time

Rayleigh scattering

Mie scattering

Noise level

Emission impulsion

tup tdown

tup + tdown =

z = ct/2 1 s 150 m

The lidar equation (1)

)r,(TN Lat

Number of photons detected by a lidar system

oo instrumental parameters

oo geophysical variables

If Ne is the total number of photons emitted by the laser at L

( )Ltet tNN λ=transmission coefficient of optics (0-1)

Total number of photons transmitted into the atmosphere

The number of photons available to be scattered at the distance r

optical transmission of the atmosphere at L along the laser path to the range r

The number of photons backscattered, per unit solid angle due to scattering of type i, from the range interval R1 to R2

dr)r,()r.(TN Li

R

RLat

2

1

β∫ π

backsatter coefficient for scattering of the type i and L

The lidar equation (2)

The number of photons N(s,r) after the detection

Number of photons incident in the collecting optic of the lidar due to scattering of the type i

dr)r,()r,(T)r,(T)r(r

1AN L

iR

RLasa2t

2

1

β∫ ξπ

area of the collecting opticwavelength of the scattered light

overlap factor

decreasing illuminance of the telescope by the scattered light

dr)r,()r,(T)r,(T)r(r

1)(Q)(AtN)r,(N L

iR

RLasa2ssrt

2

1

β∫ ξ=π

transmission coefficient of the reception optics at s

quantum efficiency of the detector at s

The lidar equation (3)

Then, the lidar equation …

In many cases, approximations allow simplification of lidar equation … oo L = s Ta(L) =

Ta(s)oo integral range cte, during acquisition (t = 2 (R2-R1)/c)oo ξ(r) 1

)r,(T)r,(r

r)(Q)(ATN)r,(N L

2L

i2LLrt a

β

=π

instrumental dependency

atmospheric dependency

)]r,(exp[)r,(T LLa τ−=where

∫ ∑ +α+αr

0 kkLkLpLm dr)r(n)r,()r,()r,(

optical depth

cross section of constituent k at

L

concentration of constituent k

pmππ

β+β=βπcontribution of molecules &

particlesextinction coefficient of molecules & particles

Rayleigh-Mie Aerosol Lidar (1)

• Application of inversion method to the lidar equation (Klett) αp(,z), βp(,z) (hypothesis on βp(,z)/αp(,z) )

• Polarization technique: measure the polarization ratio indication of aerosols shape (liquid or solid)

• Multi-wavelength lidar Spectral dependence of aerosol optical thickness (AOT)

Transmitter

Detector

(polarization technique)

Receiverτ−β= 22 eKNrcontribution of aerosols

X Measurements of aerosols & clouds in the troposphere & lower stratosphere

Aerosol and molecular scattering (2)

• Both molecular and aerosol contribution are present

• Aerosols are identified through their vertical shape

• Aerosol analysis consists in estimating– Molecular contribution– Aerosol attenuation

Rayleigh-Mie Aerosol Lidar (3)

Time [UTC] Time [UTC]

ln(Nr2) flag

Temporal evolution of lidar signal at 532 nm (linear polarization component) corrected in distance [ln(Nr2)] for April 1st 2003, (left panel). Classification of the atmospheric layers: noise (flag 0), zone with molecules (flag 1), ABL (flag 2), zone with particles (flag 3 & 4), and indefiended zones (flag > 4)

• Transmitter: Nd:YAG at 532 nm (second harmonic) & linear polarization + expander• Receiver: 2 telescopes (0.1-7 km & 2-15 km)• Detection: 532 nm linear & cross polarization components par PMT, 1064 nm par avalanche photodiodes • Vertical resolution: 15 m & temporal resolution: 30’• Classification of the atmospheric structure from backscattering lidars signals corrected from noise & total overlapping: (Identification of atmospheric boundary layer (ABL), the zones with particles (aerosols & clouds) & finally the zones with molecules)

8 9 10 11 12 13 14 15 8 9 10 11 12 13 14 15

15000

10000

5000

0

Alti

tude

[m

]

15000

10000

5000

0A

ltitu

de [

m]

Measurements in the troposphere (Pietras et al., 2004)

Médiane du nuage filtré

Minimum de la fonction de coût

No = 7.71 cm-3

rm = 0.29 µm

σ = 1.45

Multiwavelenght lidar (4)

Size distribution -> Size distribution -> Aerosol surface and volumeAerosol surface and volume

Temperature measurements (5)

• Required pure molecular scattering

• Density and pressure are relative measurements

• Temperature is absolute

280260240220200180Temperature (K)

90

85

80

75

70

65

60

55

50

45

40

35

30

Altitude (km)

OHP 5 Dec 1991 17:38-03:21

€

ϕ (z) = f (N(z)

dP(z) = −gϕ (z)dz

T(z) =MP(z)

Rϕ (z)

T(z) =M

R

gϕ (κ )Δz0

z

∑ϕ (z)

=Mg

R

N(κ )Δz0

z

∑N(z)

Rayleigh Lidar (6)

(a) (background corrected) raw lidar backscatter profiles with the Rayleigh/Mie/Raman (RMR) and Potassium (K) lidar at Kühlungsborn, Germany on 23 February 2003. (b) Temperatures profiles retrieved from (a)

• Transmitter: Nd:YAG at 532 nm (second harmonic) & 355 nm (third harmonic) for T measurements• 532 nm high Rayleigh signal : 4 telescopes of 50 cm diameter 40-90 km (blocking chopper at 40 km)• 532 nm low Rayleigh signal : 1 telescope of 50 cm diameter 20-50 km (blocking chopper at 20 km)• 1 h integration time• Vertical resolution of 1 km & a heigh-variable smooth filter (0.6-3 km width)• Statistical T error < 10 %

Temperature measurements (Alpers et al., 2004)

To discriminate species: Raman scattering (7)

• Raman consists in a spectral shift of the returned wavelength

• Raman shift is characterized by the molecules considered

• Only attenuation of the bean is required

• Technique useful for pollution

Raman Lidar (8)

Spectral shift of the returned wavelength (Raman= L ) Raman shift is characterized by the considered molecules (unique spectral signature) The vibrational Raman lines are generally selected for detection concentrations High-quality of narrow-band interference filters High-blocking filter for elastic backscatter of molecules & aerosols Small cross-section of Raman scattering molecules with a relatively high abundance (H2O,

N2, O2)

H2O Raman Lidar: q(z) H2O mixing ratio is specified as:

( ) =⎟⎟

⎠

⎞

⎜⎜

⎝

⎛××=

22

2

2

2N

N

OH

N

OH rM

M

)z(n

)z(nzq

kMass H2O / Dry air mass

( )( ) k

K

K

z,T

z,T

N

N

OH

N

OH

N

Rama

Rama

N

OH

2

2

2

2

O2H

2N

2

2

σ

σ

λ

λ

Atmospheric transmission at

Ram

Differential Raman backscattering cross

sections for water vapor & nitrogen

Calibration constant

Lidar Raman signal for

nitrogen & water vapor

X Measurements of temperature using Raman scattering from N2

X Cloud & aerosols can also be studied by this technique

H2O at 660 nmN2 at607 nm

Beam spliter

Filters for Rayleigh rejection

At 532 nm

2 channels: H2O and N2

Sky background calibration during daytime (SZA=60°)

H2O Raman: Calibration (9)

Raman Lidar (10)

29/10/2002

(a) Comparisons with collocated radiosonde. Data are summed over 20 minutes. (b) Height time series for the water vapor mixing ratio for night 29 October 2002. Profiles are summed over 5 minutes. (right column).

• Vertical resolution is variable from 50 to 500 m in order to maintain a good signal to noise ratio• Good agreement between lidar and RS water vapor mixing ratio below 5 km• Same water vapor structures are seen by the two instruments.• Slight overestimation of lidar profile after 4 km due to an undetermined instrumental bias. Relative precision of lidar < 5% (up to 2 km) & 10% (up to 4 km) requirements for boundary layer applications.

Water vapor measurements in the lower troposphere (Tarniewicz et al., 2003)

a) b)

Lidar Retrieval for O3 Measurements (11)

DIfferential Absorption Lidar technique for stratospheric ozone measurements

Measurements of the stratospheric ozone vertical distribution

Two laser wavelengths (on, off) characterized by a different ozone absorption cross section (UV spectral range, great ozone absorption)

Self calibrating technique, no instrumental constants

O3 number density

differential O3 absorption cross-

section

number of detected photons

at i

background radiation at i

correction term

)z(n)z,(N)z,(N

)z,(N)z,(NLn

dz

d

)z(2

1n

33

3 Oonbgon

offbgoff

OO +⎟

⎟⎠

⎞⎜⎜⎝

⎛

−

−⋅

⋅=

Rayleigh & Mie differential scattering

Rayleigh & Mie differential extinction

Absorption by others constituents (SO2, NO2))z,()z,( offOonO 33

−

OHP stratospheric ozone DIAL system (12)

Multiple-fiber collector concept

Abs. radiation : XeCl Lambda Physics EMG 200 Excimer laser (308 nm)

Ref. line: 3rd harmonic Continuum Nd:Yag (355 nm)

4 Collecting mirrors: 0.53 m, F 1.5 m, Ap. F/3

Moveable fiber mounts for the alignment of the XeCl laser radiation

Beam expanders

optical fibersMechanical chopper

spectrometer

Example of ozone profile (13)

• Ozone measurements performed during the night

• Temporal resolution 3 – 4 hours

• Require clear skies

Courtesy S. Godin-Beekmann

Wind measurements (14)

• Wind is based on the Doppler shift of the return signal

• Dynamic of the signal : 5-6 orders of magnitudes

• Emission-reception geometry– Parallax– defocalisation

• Noise and signal-induced-noise

Limitations (1)

Lidar subsystems (2)

Transmitter sub-system

Strategy of measurement choice of the laser source

laser beam expander

Common examples:

Gas laser (ex: excimer laser)

optical medium: gas of molecules only stables in an excited state. Electrical discharge

Solid-state laser (ex: Nd:YAG)

Impurity ions (Nd3+) in a glassy material (YAG). Optically pumped by a flash lamp stimulated emission (1.06 m)

Altitude range and concentration to be detected

Specific Wavelengths (absorption)

Energy & repetition rate

Concentration and spectral characteristics of other gases

Reliability, ease of operation in monitoring applications

Operating costs

Lidar subsystems (3)Receiver sub-system

Collection of scattered laser light back from the atmosphere and focuses it to a smaller spot

ϕ ~ 10 cm, lenses or mirrors (close range)

~ few meters, mirrors (middle & upper atmosphere)

4 fibersgrating

387 nm

308 nmhigh & low energy

355 nmhigh & low energy

347 nm 332 nm

spectral filtering schemes: centered in a specific

wavelength (dichroic, gratings, mirrors, narrowband

interference filters < 1 nm)

separation based on polarization (aerosols)

protection of detector (fast mechanical shutter,

electrical gating)

Processing of scattered laser light chopper

Lidar subsystems (4)Signal Detection & Acquisition sub-system

Conversion of light into an electrical signal & recording in electronic device

Photomultipliers Tubes (PMTs) are generally used in incoherent lidar systems

(direct detection)

To the electronic device

Output of PMT: current pulses produced by photons ++

thermal emission of electrons (dark current)

2 Techniques: - - Photon counting Mode (individual pulses)

- - Analog Mode (multitude of pulses)

Selecting the PMT: - PMT structure optical measurement conditions

- Photocathode Quantum Efficiency high QE in the wavelength range

- Gain > 106

- Dark count lower detection limit

- Response time maximum count rate, time resolution

Hamamatsu

Lidar subsystems (5)Signal Detection & Acquisition sub-system

Photo Counting

Analog Detection• Pulse pair resolution of the detector 10 to 100 MHz

• Fast analog-to-digital converter

x Generally for tropospheric measurements

Coherent Detection• Mixing of backscattered laser light with light from local oscillator on a photomixer radio frequency (RF) signal

• Frequency of RF signal Doppler shift of the scattered laser light wind velocity

x Frequency stability & short laser pulse length is required

• Preamplifier amplification + pulse shape (ringing)

• Main amplifier (if it is necessary)

• High speed comparator (discriminator) remove

a substantial number of the dark current

• Pulse shape & counter

x Generally for low signals detection Typical Photon Counting System

Hamamatsu

Photon counting (6)

• Measurement = Histogram• D

• Improvements = increase the number of collected photons

– Size of the telescope– Laser power– Vertical resolution– Temporal resolution

PHD

HV

Photon

Cooling system

Discriminator level

Counter

€

= 1NbPhotons

t

t