1. Lidar for Atmospheric Remote sensing
Philippe Keckhut et Andrea Pazmiño
LATMOS, Institut Pierre Simon Laplace, CNRS-UPMC-UVSQ, Paris, France
Historical Overview
Lidar Basics
The Lidar Equation
Lidar systems
Summary
Outline

2. 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
~km
Monostatic co-axial
Monostatic bi-axial
Bistatic
Transmitter & receiver collocated
(pulsed light source  range of scattering)
Receiver’s FOV scanned along the transmitted beam
(geometry  range of scattering)

3. 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

4. Historical Overview (3)
Present
Networks of ground-based lidar systems as NDSC, EARLINET, etc
Lidars on aircraft
Space-based lidar (ALISSA, LITE, … CALIPSO program)

5. LIDAR: LIght Detection And Ranging
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

6. Lidar Basics (2) + = Differential Absorption & Scattering
Some Optical Interactions of Relevance to Laser Environmental Sensing
Elastic Interactions
Inelastic Interactions
Rayleigh Scattering
Mie Scattering
Virtual Level
Vibrationally Excited Level
 ~ d  Mie = C/a
 ~ d  Mie = C/a
Raman Scattering
 >> d  Rayleigh = C/4
 ~ d  Mie = C/a
Interaction with the quantized vibrational
& rotational energy levels of the molecule
Absorption
+ = Differential Absorption & Scattering

7. Block diagram of a generic lidar system
Lidar Basics (3)
Block diagram of a generic lidar system
Emitted light
Backscattered light
Laser
Beam expander (optional)
Transmitter
Light collecting telescope
Optical filtering
Receiver
Synchronization control
Optical to electrical transducer
Electrical recording system
Detector & Recording

8. Complete altitude scattering profile
Lidar Basics (4)
Ranging of pulsed monostatic lidar
Time
Altitude
Aerosol
layer 2
layer 1 Z1 Z2
Laser beam
Scattered
light
T1=2.Z1/C
T2=2.Z2/C
Signal
Rayleigh
scattering
Mie
Noise
level
Emission impulsion
tup
tdown
tup + tdown =
Each light pulse fired 
Complete altitude scattering profile
z = ct/2  1 s 150 m

9. The lidar equation (1)  Number of photons detected by a lidar system
o instrumental parameters
o geophysical variables
 If Ne is the total number of photons emitted by the laser at L
Total number of photons transmitted into the atmosphere
transmission coefficient of optics (0-1)
 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
backsatter coefficient for scattering of the type i and L

10. The lidar equation (2)
 Number of photons incident in the collecting optic of the lidar due to
scattering of the type i
area of the collecting optic
overlap factor
wavelength of the scattered light
decreasing illuminance of the telescope by the scattered light
 The number of photons N(s,r) after the detection
transmission coefficient of the reception optics at s
quantum efficiency of the detector at s

11. The lidar equation (3) o L = s  Ta(L) = Ta(s)
 In many cases, approximations allow simplification of lidar equation …
o L = s  Ta(L) = Ta(s)
o integral range  cte, during acquisition (t = 2 (R2-R1)/c)
o (r)  1
 Then, the lidar equation …
instrumental dependency
atmospheric dependency
where
contribution of molecules & particles
optical depth
extinction coefficient of molecules & particles
cross section of constituent k at L
concentration of constituent k

12. Rayleigh-Mie Aerosol Lidar (1)
Receiver
contribution of aerosols
Transmitter
Application of inversion method to the lidar equation (Klett)  p(,z), p(,z) (hypothesis on p(,z)/p(,z) )
Polarization technique: measure the polarization ratio  indication of aerosols shape (liquid or solid)
Multi-wavelength lidar  Spectral dependence of aerosol optical thickness (AOT)
Detector
(polarization technique)
Measurements of aerosols & clouds in the troposphere & lower stratosphere

13. 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

14. Rayleigh-Mie Aerosol Lidar (3)
Measurements in the troposphere (Pietras et al., 2004)
ln(Nr2)
15000
10000
5000
flag
15000
10000
5000
Altitude [m]
Altitude [m]
Time [UTC]
Time [UTC]
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)

15. Multiwavelenght lidar (4)
Médiane du nuage filtré
Minimum de la fonction de coût
No = 7.71 cm-3
rm = 0.29 µm
σ = 1.45

16. Size distribution ->
Aerosol surface and volume

17. Temperature measurements (5)
Required pure molecular scattering
Density and pressure are relative measurements
Temperature is absolute

18. Rayleigh Lidar (6) Temperature measurements (Alpers et al., 2004)
(a) (background corrected) raw lidar backscatter profiles with the Rayleigh/Mie/Raman (RMR) and Potassium (K) lidar at Kühlungsborn, Germany on 23 February (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  km (blocking chopper at 40 km)
532 nm low Rayleigh signal : 1 telescope of 50 cm diameter  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 %

19. 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

20. 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)
X Measurements of temperature using Raman scattering from N2
X Cloud & aerosols can also be studied by this technique
H2O Raman Lidar: q(z) H2O mixing ratio is specified as:
Atmospheric transmission at Ram
Differential Raman backscattering cross sections for water vapor & nitrogen
Calibration constant
Lidar Raman signal for nitrogen & water vapor k
Mass H2O / Dry air mass

21. H2O Raman: Calibration (9)
H2O at 660 nm
N2 at
607 nm
Filters for
Rayleigh rejection
At 532 nm
Beam spliter
Sky background calibration during daytime (SZA=60°)
2 channels: H2O and N2

22. Raman Lidar (10)
Water vapor measurements in the lower troposphere (Tarniewicz et al., 2003) a)
29/10/2002 b)
(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 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.

23. 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
Rayleigh & Mie differential scattering
Rayleigh & Mie differential extinction
Absorption by others constituents (SO2, NO2)
O3 number density
differential O3 absorption cross-section
number of detected photons at i
background radiation at i
correction term

24. OHP stratospheric ozone DIAL system (12)
Multiple-fiber collector concept
Mechanical chopper
optical fibers
Beam expanders
Moveable fiber mounts for the alignment of the XeCl laser radiation
spectrometer
4 Collecting mirrors:  m, F 1.5 m, Ap. F/3
Ref. line: 3rd harmonic Continuum Nd:Yag (355 nm)
Abs. radiation : XeCl Lambda Physics EMG 200 Excimer laser (308 nm)

25. Example of ozone profile (13)
Courtesy S. Godin-Beekmann
Ozone measurements performed during the night
Temporal resolution 3 – 4 hours
Require clear skies

26. Wind measurements (14)
Wind is based on the Doppler shift of the return signal

27. Limitations (1) Dynamic of the signal : 5-6 orders of magnitudes
Emission-reception geometry
Parallax
defocalisation
Noise and signal-induced-noise

28. Lidar subsystems (2) Transmitter sub-system Strategy of measurement
laser
beam expander
Strategy of measurement
 choice of the laser source
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
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)

29. Lidar subsystems (3) Receiver sub-system
Collection of scattered laser light back from the atmosphere and focuses it to a smaller spot
  ~ 10 cm, lenses or mirrors (close range)
  ~ few meters, mirrors (middle & upper atmosphere)
4 fibers
grating
387 nm
308 nm
high & low energy
355 nm
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

30. 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)
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)
To the electronic device
Hamamatsu
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

31. Lidar subsystems (5) Signal Detection & Acquisition sub-system
Photo Counting
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
Hamamatsu
Typical Photon Counting System
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

32. Photon counting (6) t t Measurement = Histogram D
Improvements = increase the number of collected photons
Size of the telescope
Laser power
Vertical resolution
Temporal resolution t t