Chap.1. Introduction to Optical Remote Sensing

Transcription

1 Chap.1. Introduction to Optical Remote Sensing ORS active: LIDAR Francesc Rocadenbosch ETSETB, Dep. TSC, EEF Group Campus Nord, D4-016

2 INTRODUCTION LIDAR (LIgth Detection And Ranging) Strong optical interaction between laser/atmospheric species of interest λ rparticles, λ >> r airborne molecules Interacting mechanisms: scattering by gases ( α g,sca ) and particles ( α p, sca) absorption ( ) KEYS: α g,abs Highly collimated R(spatial resolution) meters t = [secondsminutes] Fig. SOURCE: Measures (1992); R.M. Measures, "Laser Remote Sensing. Fundamentals and Applications". John Wiley & Sons, (Reprint de 1992, Krieger Publishing Company). 2

3 INTRODUCTION MOTIVATION OF LASER PROBING: Features Associated To Optical Wavelengths Strong optical interaction High directivity of radiation θ λ D λ = 532 nm D = 1 cm (Comparison with RADAR) to achieve the same angular resolution at 3 GHz, f = 3 GHz λ = 10 cm D 1800 m! θ 50 µrad Larger (optical) Doppler shifts than at RF wavelengths f d 2v = λ r lidar d radar d f f λ λ radar lidar

4 INTRODUCTION 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 (τ l ), high-energy laser pulses (Ep 1J, τ l 10ns, PRF 10Hz) (1973) Semiconductor laser (GaAs) Laser diode arrays. Trade-off between peak energy (Ep) and PRF E = E p τl = T E p τ l PRF (2002) TLD-technologies and ps-lidar Spectroscopic Lidar (detection of chemical species), 3D mapping 4

5 OPTICAL AND TECHNOLOGICAL CONSIDERATIONS BEER S (or BOUGUER S) LAW Describes intensity of a laser beam propagating in a inhomog. medium I I ( λ) 0 = T [ ] dr R ( λ, R) = exp α( r, λ) where: I 0 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 BANDS α = α + α + α [ km g, sca p, sca Lidars operate in atmospheric transmission windows µm (VIS), µm (NIR), 3-5 µm y 9-13 µm (IR) eye-safe : λ >1.4 µm (100 mw/cm 2, 1J/cm 2 ) Trade-off: Laser and detector availability! Ej. Ruby (0.69 µm), Nd:YAG (1.064 µm), CO 2 (9-10 µm), eye-safe 1.55µm 0 g, abs 1 ] 5

6 OPTICAL AND TECHNOLOGICAL CONSIDERATIONS A) Based on their APPLICATION ELASTIC-BACKSCATTER LIDAR (or backscatter lidar ) measures... the average content of particulate and molecular matter (be them contaminating 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 CONFIGURATION MONO-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. 6

7 BACKSCATTER LIDAR OPERATIONAL PRINCIPLE Same emission and reception wavelengths (λ 0 =λ R ) 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) 7

8 UPC BACKSCATTER LIDAR LASER RECEIVER SYSTEM SPECS Gain medium Nd:YAG Focal length 2 m Configuration Vertical biaxial Energy 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 nw Pulse length 10 ns Net Responsivity V/W Acquisition 20 Msps/12bit PRF 10 Hz Bandwidth 10 MHz Spatial resolution 7.5 m R = 7.5 m, t = 1 min 8

9 DIAL 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. N where: a 2 ( σ σ ) a 1 a R ln P P λ λ ( R) ( R) N a is the molecule concentration, σ a, σ a are the molecule absorption cross-sections at λ, λ and, P λ, P λ are the backscattered return powers at λ, λ, normalised to the transmitted ones. Fig. Contours of NO 2 concentration (ppm) in the vicinity of a chemical plant, as measured by differential absorption lidar. (SOURCE: K. W. ROTHE et al Appl. Phys. 4, 181 (1974)). 9

10 DIAL APLICATIONS 1) 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, SF6 Typ. 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),

11 RAMAN LIDAR OPERATIONAL PRINCIPLE 1) In contrast to elastic systems, the return wavelength, λ R, is shifted from the incident one, λ 0. 2) Wavelength shift, κ, depends on each molecular species. λ0 λ R = 1 κλ 3) Very faint returns. requires photon counting very often, night-time operation 0 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,

12 RAMAN LIDAR APLICATIONS 1) Self-calibrated lidar (N2 shift) Absolute concentration of any atmospheric species can be determined by comparison to the N2-atmospheric return 2) Temperature profiler (±2K) 3) Spectroscopic sensing (COMPARISON WITH DIAL) Fig. SOURCE: Measures (1992). 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, ppm, m). In contrast, measurements are always range resolved (RR) and there is no need to tune the laser in absorption bands. 12

13 DOPPLER LIDAR Uses airborne particles&molecules as tracers along with the Doppler principle to invert the wind radial component (1992) First commercial system. Specs.: m range, 1-m/s resolution, 150-m spatial resolution and 5-min integration time. (Today) Wind sensors: LAWS (NASA) and ALADIN (ESA). 13

14 DOPPLER LIDAR TECHNIQUES 2v Coherent Detection: Optical heterodyne detection fd λ Incoherent Detection: E.g. Uses high-resolution filters (Fabry- Pérot) as frequency (fd)-amplitude transducers (edge-technique). = r 14

15 DOPPLER LIDAR Wind measurement example using a Doppler lidar at Eldorado Canyon during a mesofront invasion. SOURCE: Courtesy of NOAA (National Oceanics and Atmospherics Administration). 15

16 ABSORPTION LIDAR OPERATIONAL PRINC.: Long-path absorption. See also TDLAS. APPLICATIONS Column-content (CC) gas detection Sensitivity defined by [ppm m] 16