Lecture 6: Introduction to Lidar

Types of scattering process •  Rayleigh scattering: laser radiation elastically scattered

from atoms or molecules with no change of frequency

•  Mie scattering: laser radiation elastically scattered from

particulates (aerosols or clouds) of sizes comparable to the wavelengths of radiation with no change of frequency

•  Raman Scattering: laser radiation inelastically scattered from molecules with a frequency shift characteristic of the molecule

•  Resonance scattering: laser radiation matched in frequency

to that of a specific atomic transition is scattered by a large cross section and observed with no change in frequency

•  Fluorescence: laser radiation matched in frequency to a specific electronic transition of an atom or molecule is absorbed with subsequent emission at the lower frequency

•  Absorption: attenuation of laser radiation when the frequency matched to the absorption band of given molecule

•  Differential absorption and scattering: Two laser beams is evaluated from their backscattered signals when the frequency of one beam is closely matched to a molecular transition while other somewhat detuned from the transition.

Backscattering cross section

Na Absorption Cross Section:~ 5x10-16 m2

Rayleigh Scattering Cross Section: 14 orders smaller Rot. Raman Scattering Cross Section: 2.5% of Rayleigh Vib. Raman Scattering Cross Section: 3 orders smaller than Rayleigh Bandwidth: ~ 1 GHz for most processes (1nm = 35 cm-1 = 1060 GHz)

Alti

tude

(km

)

Ray

leig

h L

idar

Flou

resc

ence

L

idar

(Na)

Thermosphere

Mie

Lid

ar DIA

L L

idar

(O

3)

Ram

an L

idar

(H

2O)

Lidar sensing of atmospheric layers

Lidar history

v  lidar principle dates back to pre-laser times In the early 1930s, measure air density profiles in the upper

atmosphere using searchlight beam. In 1938, pulses of light were used for the first time to measure

cloud base heights

v Modern lidar technology started with the invention of the laser in 1960 Mie scattering lidar:

“Observations of the aerosol layer at 20 km by optical radar” by G. Fiocco, G. Grams (Journal of the Atmospheric Sciences 1964)

Raman Scattering lidar:

“Observation of Raman scattering from the atmosphere using a pulsed nitrogen ultraviolet laser” by Donald A. Leonard (Nature 1967)

Resonance Fluorescence Lidar: “Daytime laser radar measurements of the atmospheric sodium layer” by A.J. Gibson, M.C.W. Sandford (Nature 1972) “High-spectral-resolution fluorescence light detection and ranging for mesospheric sodium temperature measurements” by C.Y. She, J.R. Yu, H. Latifi, R.E. Bills (Applied Optics 1992)

Differential Absorption Lidar (DIAL):

“Some observations of the vertical profile of water vapor by means of a laser optical radar” by R.M. Schotland (in Proceedings of the Fourth Symposium on Remote Sensing of Environment 1966)

Rayleigh Lidar:

“Density and temperature profiles obtained by lidar between 35 and 70 km” by Alain Hauchecorne, Marie-Lise Chanin (Geophysical Research Letters 1980)

Doppler wind Lidar:

“Laser-Doppler system for detection of aircraft trailing vortices” by R.M. Huffaker, A.V. Jelalian, J.A.L. Thomson (Proceedings of the IEEE 1970)

Lidar design

In principle, a lidar consists of a transmitter, a receiver, electronic control system. Two basic configuration: bistatic (rarely use) and monostatic (widely use) A monostatic lidar: coaxial and biaxial. Coaxial: laser beam is coincident with the axis of the receiver optics Biaxial: laser beam only enter the field view of the receiver optics (as shown in figure).

To avoid near-field strong backscattering, two ways: electronic gating the photodetector (e.g PMT) or use a fast shutter.

Telescope receiver

Two types of main telescopes: Newtonian (left) and Cassegrainian (right) reflecting telescopes Primary mirror: paraboloid primary mirror, Secondary mirror: a flat mirror (Newtonian), or hyperbolic secondary mirror A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror.

Filters Key parameters:

Central Wavelength (CWL) Bandwidth (refer to FWHM) Blocking Range Optical Density (OD): describes the blocking specification of a filter

CWL: 589.1 ± 0.15nm FWHM: 1.00 ± 0.20nm Peak %T: > 65% Out of band blocking: OD5 from 200-1200nm Size: 1.000" +0/-.010" Clear aperture: 0.90" diameter minimum Angle of incidence: Normal Operating temp: 23°C (ambient) Surface quality: 80/50 per MIL-C-48497A Multiple substrate construction, laminated into an Aluminum ring Parts will be tested in air @ ~23°C with a collimated beam

Example of filter parameters

Photodectector Photomultiplier Tube (PMT): Spectral response, quantum efficiency, frequency response, current gain, and dark current

Sample of lidar return signals

Basic physics of lidar remote sensing

N(λλL)

T(λλL,z) T(λλ,z)

ββ (λλL, λλ, θθ, z)

A/z2

ηη(λλL, λλ)ξξ(z)

Fundamental lidar equation (single pulse)

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

( ),sN zλ Number of photon counts recorded by photon counting board for wavelength λ and altitude z Number of photons transmitted from laser System efficiency and overlap factor Forward transmission of laser photon in the atmosphere Probability of a transmitted photon to be scattered per range bin z and per solid angle Backward transmission Signal photon in the atmosphere Probability of scattered photon to be collected by telescope (solid angle of the receiving telescope) Background noise and PMT dark counts

( )LN λ

( ) ( ),L zη λ λ ξ

( ),LT zλ

( ), , ,L z zβ λ λ θ Δ

( ),T zλ

2

Az

BN

In a case of a pulsed, monostatic lidar:

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

Number of photons transmitted from laser

( ) ( )/L

LL

EN

hcλ

λλ

=

Number of photons transmitted per laser pulse =

Transmitted laser pulse energy

Planck constant ×laser frequency

= Transmitted laser pulse energy

Single laser photon energy

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

Angular scattering probability in range bin Δz - the probability that a transmitted photon is scattered by scatters in range bin Δz into a unit solid angle. Angular scattering probability in range bin Δz = volume scatter coefficient β x scattering range bin Δz

Probability of a transmitted photon to be scattered

( ), , ,L z zβ λ λ θ Δ

Volume scatter coefficient ββ

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

Probability of a scattered photon to be collected by telescope

2

Az

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

Forward and backward transmission

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

System efficiency and overlap factor

( ) ( ),L zη λ λ ξ

ξξ(z)

Simple overlap factor

Assuming no apertures or obstructions, the geometrical factor is equal 1, provided that the laser divergence is less than telescope field of view.

A coaxial lidar system

A biaxial lidar system

Circle of least confusion (aberration)

ΣLC is the best place to observed the image.

Geometrical form factor

Background noise and PMT dark counts

( ) ( ) ( ) ( ) ( ) ( ) ( ) BLLLLs NzAzTzzzTzNzN +••Δ•••= 2,,,,,,, λθλλβλξλληλλ

BN

Mainly due to background noise (e.g., solar scattering), detector (PMT) and circuit shot noise.

quantum noise (shot noise): N