chap.1. introduction to opticalocw.upc.edu/sites/ocw.upc.edu/files/materials/... · lidar radar...

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NS Chap.1. Introduction to Optical Chap.1. Introduction to Optical

Remote SensingRemote SensingORS active: LIDAR

Francesc Rocadenbosch

ETSETB, Dep. TSC, EEF GroupCampus Nord, D4-016

[email protected]

(C) F. Rocadenbosch 2005-2006

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INTRODUCTIONINTRODUCTION

LIDAR (LIgth Detection And Ranging)Strong optical interaction between laser/atmospheric species of interest

• λ ≈ r particles, λ >> r airborne moleculesInteracting mechanisms:

• scattering by gases ( ) and particles ( )• absorption ( )

KEYS:• Highly collimated →• ∆R(spatial resolution) ≈ meters

• ∆t = [seconds-minutes]

Fig. SOURCE: Measures (1992); R.M. Measures, "Laser Remote Sensing. Fundamentals and Applications". John Wiley & Sons, 1984. (Reprint de 1992, Krieger Publishing Company).

scap ,αscag ,αabsg ,α


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MOTIVATION OF LASER PROBING: Features Associated To Optical Wavelengths

• Strong optical interaction• High directivity of radiation

!1800103 mDcmGHzf ≈⇒=λ⇒=

Dλ

≈θ∆ ⇒⎭⎬⎫

⎩⎨⎧

==λ

⇒cmD

nm1532

µrad50≈θ∆

– (Comparison with RADAR) to achieve the same angular resolution at 3 GHz,

• Larger (optical) Doppler shifts than at RF wavelengths

5102≈

λλ

≈→λ

−=lidar

radarradar

d

lidardr

d ffvf


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HISTORICAL BACKGROUND• (1930) Searchligths• (1960) Laser invention

– Offers: High collimation, purity and spectral coherence (∆λ≈ 0.01 nm)• (1962) Fiocco & Smullin

– bounce a laser beam off the Moon. Study atmospheric turbid layers• (1963) Ligda

– Q-switching: Enables short width (τl), high-energy laser pulses– (Ep ≈ 1J, τl ≈ 10ns, PRF ≈ 10Hz)

• (1973) Semiconductor laser (GaAs)– Laser diode arrays. Trade-off between peak energy (Ep) ↓ and PRF ↑

PRFET

EE lpl

p τ=τ

=

• (2002) TLD-technologies and ps-lidar– Spectroscopic Lidar (detection of chemical species), 3D mapping


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OPTICAL AND TECHNOLOGICAL CONSIDERATIONSOPTICAL AND TECHNOLOGICAL CONSIDERATIONS

BEER’S (or BOUGUER’S) LAWDescribes intensity of a laser beam propagating in a inhomog. medium

( ) ( ) ( )[ ]∫ λα−=λ=λ R drrRT

II

00

,exp,

• where: I0 is the intensity at r=0, I is the intensity at r=R, α is the atmospheric extinction coef., T(λ,R) is the transmissivity in (0,R) and,

SPECTRAL BANDSLidars operate in atmospheric transmission windows

• 0.4-0.7 µm (VIS), 0.7-1.5 µm (NIR), 3-5 µm y 9-13 µm (IR)• “eye-safe”: λ >1.4 µm (100 mW/cm2, 1J/cm2)• Trade-off: Laser and detector availability!

– Ej. Ruby (0.69 µm), Nd:YAG (1.064 µm), CO2 (9-10 µm), “eye-safe” 1.55µm

][ 1,,,

−++= kmabsgscapscag αααα


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OPTICAL AND TECHNOLOGICAL CONSIDERATIONSOPTICAL AND TECHNOLOGICAL CONSIDERATIONS

A) Based on their APPLICATIONELASTIC-BACKSCATTER LIDAR (or “backscatter lidar”) measures...

• the average content of particulate and molecular matter (be themcontaminating or not) in the atmosphere

• winds (cross-correlation techniques) and others (range-finders, CMM, ...)

WIND LIDAR (Doppler lidar)

SPECTROSCOPIC LIDAR → measurement of chemical species

B) Based on their CONFIGURATIONMONO-STATIC LIDAR

• Types: 1) Backscatter, 2) DIAL, 3) Raman, 4) Doppler, 5) Fluorescence, 6) Others

BI-STATIC LIDAR• Types: 1) Long-path absorption

Airborne (helicopter, plane, satellite), mobile (van, truck), or ground-based.


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BACKSCATTER LIDARBACKSCATTER LIDAR

OPERATIONAL PRINCIPLE• Same emission and reception wavelengths (λ0=λR)• Uses elastic Mie scattering (λ ≈ r, aerosols) and Rayleigh scattering

(λ >> r, molecules) to interrogate the intervining atmosphere

ENVIRONMENTAL APPLICATIONS• Pollution monitoring (source strength and location)• Aerosol monitoring: Air Quality regulations, Fires• Feedback to/from Transport models

– to forecast movement of pollutants and related photochemical effects

METEOROLOGICAL APPLIC.• Rain, snow, clouds, ...• Atmospheric attenuation

estimation (dB/km)


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UPC BACKSCATTER LIDARUPC BACKSCATTER LIDAR

∆R = 7.5 m, ∆t = 1 min

LASER RECEIVER SYSTEM SPECSGain medium Nd:YAG Focal length 2 m Configuration Vertical biaxialEnergy 0.5 J/532 nm Aperture ∅ 20 cm System NEP 70 fW·Hz-1/2

Divergence 0.1mrad Detector APD (EGG C30954) Min. Det. Power < 5 nWPulse length 10 ns Net Responsivity 6×101-3×106 V/W Acquisition 20 Msps/12bitPRF 10 Hz Bandwidth 10 MHz Spatial resolution 7.5 m


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DIALDIAL

OPERACIONAL PRINCIPLE• DIAL (Differential Absorption Lidar)• Uses two (or more) tuning wavelengths, one of which is absorbed by the

atmospheric species of interest, and another one that is not.

( )( )( )RPRP

RN

aaa

λ′

λ

σ−σ′≈ ln

21

where:Na is the molecule concentration, are the molecule absorption cross-sections at and, are the backscattered return powers at , normalised to the transmitted ones.

aa σ ′σ ,λ ′λ ,

λ ′λ PP ,λ ′λ ,

Fig. Contours of NO2 concentration (ppm) in the vicinity of a chemical plant, as measured by differential absorption lidar. (SOURCE: K. W. ROTHE et al. 1974. Appl. Phys. 4, 181 (1974)).


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DIALDIAL

APLICATIONS1) Concentration of chemical species in the atmosphere, car exhausts, refineries,...Measurement types:

• range-resolved (RR), and• column-content (CC)• e.g., SO2, NH3, O3, CO, CO2,

HCl, vapor H2O, NO, N2O, SF6Typ. Resolutions: ppb to ppm. Typ. Ranges: a few kms.

2) Temperature and humidity

Fig. SOURCE: Whiteman, D. N.; Melfi, S. H. Cloud liquid water, mean droplet radius and number density measurements using a Raman lidar. J. Geophys. Res. 1999, 104 (D24), 31411-31419


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RAMAN LIDARRAMAN LIDAR

OPERATIONAL PRINCIPLE1) In contrast to elastic systems, the return wavelength, λR, is shifted from the incident one, λ0.2) Wavelength shift, κ, dependson each molecular species.

3) Very faint returns.• requires photon counting• very often, night-time

operation

0

0

1 κλ−λ

=λR

Fig. ADAPTED FROM: Inaba, H. Detection of Atoms and Molecules by Raman Scattering and Resonance Fluorescence. In Laser Monitoring of the Atmosphere, Hinkley, E. D., Ed.; Springer-Verlag: New York, 1976; Chap. 5, 153-236.


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RAMAN LIDARRAMAN LIDAR

APLICATIONS1) Self-calibrated lidar (N2 shift)

• Absolute concentration of anyatmospheric species can be determined by comparison to theN2-atmospheric return

2) Temperature profiler (±2K)

3) Spectroscopic sensing (COMPARISON WITH DIAL) • Low detection sensitivity at long ranges due to the low Raman cross

sections that ...• limit the method to the detection of species present in high concentrations

(e.g. smoke stacks in industrial plants, 100-1000 ppm, 30-100 m).• In contrast, measurements are always range resolved (RR) and there is no

need to tune the laser in absorption bands.

Fig. SOURCE: Measures (1992).


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DOPPLER LIDARDOPPLER LIDAR

Uses airborne particles&molecules as “tracers” along with theDoppler principle to invert the wind radial component

• (1992) First commercial system. Specs.: 30-3000 m range, 1-m/s resolution, 150-m spatial resolution and 5-min integration time.

• (Today) Wind sensors: LAWS (NASA) and ALADIN (ESA).


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TECHNIQUES• Coherent Detection: Optical heterodyne detection• Incoherent Detection: E.g. Uses high-resolution filters (Fabry-

Pérot) as frequency (fd)-amplitude transducers (edge-technique).

λ−= r

dvf 2


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Wind measurement example using a Doppler lidar at Eldorado Canyon during a mesofrontinvasion.

SOURCE: Courtesy of NOAA (National Oceanics and Atmospherics Administration).


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ABSORPTION LIDARABSORPTION LIDAR

OPERATIONAL PRINC.: “Long-path absorption”. See also TDLAS.

APPLICATIONSColumn-content (CC) gas detection

• Sensitivity defined by [ppm·m]

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