This document provides an overview of laser remote sensing techniques. It begins with definitions of remote sensing and a brief history. It describes how laser remote sensing works using light detection and ranging (LIDAR) and discusses differences between LIDAR and radar. It then covers various laser remote sensing techniques including passive and active sensing, direct and indirect sensing using fibers, and applications like LIDAR for atmospheric sensing, differential absorption LIDAR for chemical detection, Doppler LIDAR for wind and temperature measurements, and fluorescence LIDAR for solid targets. Key enabling factors for laser remote sensing like the properties of lasers and components like detectors are also summarized.

1. B Y : Z . M O H A M M A D P O U R
S P R I N G 2 0 1 5
1
Laser remote sensing

2. Definition
 Science and art of obtaining information
about an object, area or phenomenon through
an analysis of data acquired by a device that
is not in direct contact with the area, object or
phenomenon under investigation.

3. History of Remote Sensing
 McClung & Hellwarth (1962)
 Fiocco & Smullin (1963)
laser echoes from the upper regions of the atmosphere
 Ligda (1963)
 Generation of second harmonic & Invention of
nitrogen and the tunable dye laser
Probe the troposphere
Q-switching
LIDAR stand for light identification, detection, and ranging.
3

4. Radar Lidar
 Radio detection and ranging  Light detection and ranging
Differences
4

5. The entire process of remote sensing
1) Excitation of the analyte within a matrix using a proper optical
frequency
2) Collection of a light signal carrying the analytical chemical
information from this target by a suitable optical system
3) Processing by a detector system not in physical contact with the
object of analysis
5

6.  Passive remote sensing:
employing natural radiation
sources like the sun
 Active remote sensing: utilizing
a laser or lamp as a light source
Spectroscopic remote sensing
6

7. Types of remote sensing
I. Direct remote sensing
both the laser and signal are used along an open ( atmospheric )
path
II. Indirect remote sensing
the laser or the signal is brought through fiber optics or
some other means into contact or near the target
7

8. Direct remote sensing
Lidar in the atmosphere
8

9. Optical
interactions of
relevance to
laser
environmental
sensing
 Fluorescence
 Raman techniques
 Absorption
9

10. Molecular scattering
EΔ
1h
2h
a
b
c
d
1h
1h
a
b
c
d
1h
3h
EΔ
a
b
c
d
elastic Stokes shift anti-Stokes shift
Rayleigh scattering Raman scattering Raman scattering
Ehh Δ12   Ehh Δ12  
Raman spectra preserve
the vibrational energy E!
 Raman techniques:
10

11. Absorption:
 Absorption of radiation
 DIAL (differential absorption Lidar)
 DAS( differential absorption and scattering)
11

12. Laser remote sensor system
12

13. bistatic monostatic
13

14. Why laser?
Monochromatic
Narrow bandwidth
Low degree of divergence
Short duration
High peak power
High repetition rate
14

15. Lasers relevant to remote sensing
 Tunable organic dye laser
 In the IR
 In the UV/VIS range excimer or Nd:YAG-pumped dye
lasers
tunable diode lasers (TDL) based on lead salt semiconductors
&
Molecular gas lasers ( tunable CO and CO2 laser )
15

16. Detectorwavelength selector
 Excellent quantum efficiency
needed
 Spectral & Frequency response
 Current gain
 Low noise needed
 Typical detectors
Photomultiplier
Photodiodes
CCDs
Prism
Difraction gratting
Beam-expanding telescope
Fabry-Perot etalon
Receiver system
16

17. incoherent coherent
Detection systems
17

18. Position and navigation systems
18

19. LIDAR
 Four types
 Range finders: it is the simplest lidars, it measures the distance,
then create the topographic map
 DIAL: Differential Absorption Lidar is used to measure chemical
concentrations (such as ozone, water vapor, pollutants) in the
atmosphere.
 Doppler Lidar: it’s used to measure the velocity of a target
 Fluorescence Lidar: solid targets in the biosphere
19

20. Lidar Range
finder
 The most common form of laser
rangefinder operates on the time of
flight principle by sending a laser pulse
in a narrow beam towards the object
and measuring the time taken by the
pulse to be reflected off the target and
returned to the sender.
𝐷 =
𝑐𝑡
2
𝑡 =
𝜑
𝜔
𝐷 =
1
2
ct =
1
2
𝑐
𝜑
𝜔
=
𝑐
4𝜋𝐹
𝑁 + 𝜋 + ∆𝜑
=
λ
4
(𝑁 + ∆𝑁)
20

21. Application
 Military
 3-D modelling
 Foresting
 Sport
 Industry production processes
21

22. Differential Absorption Lidar
DIAL
 Range-resolved optical
transients due to elastic aerosol
backscattering are recorded for
`on' and `off' laser wavelengths
in alternating laser pulses. The
losses for the `off' wavelength
reflect only 1/ r2-losses of the
divergent laser, respectively the
signal, while the `on'
wavelength is also attenuated
by absorption from gas
molecules. The ratio of the
resulting curves yields the
absorbance as a function of
distance
22

23. Doppler Lidar
 The DL uses a heterodyne detection technique in which the return
signal is mixed with a reference laser beam (i.e., local oscillator) of
known frequency
 Signal processing computer then determines the Doppler frequency
shift from the spectra of the heterodyne signal
 Used to measure temperature and/or wind speed along the beam by
measuring the frequency of the backscattered light
23

25. Fluorescence LIDAR
 Is employed successfully for solid targets in the biosphere
 Pollution monitoring of oil spill, bathymetric
measurements of sea depth, and algal bloom patches
25

26. Lidar overview

27.  Both sensitivity and selectivity are related to the spectral
brightness and tunability of the source and to the
spectral resolution and responsitivity of the detector.
27

28. Indirect remote sensing
28
The development of fiber optical materials for long distance sensing provides an
opportunity to avoid significant limitations in conventional direct laser remote
sensing e.g.
I. The 1/ r2 reduction of radiation power with distance r and by interference
from other airborne species.
II. Reduction of replication and quantitative analysis due to dust, particle,
aerosol and chemical interference in the beam path.
III. Physical limitations responsible for weak analytical signal made make direct
remote sensing to expensive cumbersome instrumental systems

29. 29

30. Principle
30
 Refraction of light at the interface
between two media of different
refractive indices, with n2 > n1.
Since the velocity is lower in the
second medium (v2 < v1), the angle
of refraction θ2 is less than the
angle of incidence θ1; that is, the
ray in the higher-index medium is
closer to the normal.
𝑛1 sin 𝜃1 = 𝑛2 sin 𝜃2

31. Charactristic
 The fiber's flexibility,
long-range transmission,
high bandwidth, small
size, and imaging
capability allow a huge
variety of design options

32. Configurations for indirect fiber optical remote sensing
I. direct laser excitation,
fiber optical guided signal
II. fiber optical guided signal
and laser excitation
III. sensing through
evanescent field

33. 33
 Fiber optic sensors have greater
I. Sensitivity
II. Selectivity
III. Versatility
IV. Remote capability
V. Freedom from interference

34. 34

35. 35
 2-km long very low-loss silica optical-fibre
 InGaAs light emitting diode (LED) with the central
wavelength around 1.64 µm
 A remotely located 50-cm long absorption cell as a gas
sensor head
 Grating monochromator with 300 lines mm-l grating
 Ge detector cooled by a dry ice-methanol mixture.

36. 36
 Block diagram of the experimental setup for remote absorption
measurement of low-level propane gas in the absorption cell
using a 2-km long very low-loss optical-fibre link incorporating
an InGaAs light emitting diode around the 1.68 µm band.

37. 37
 Measured dependence of the absorbance at 1.6837 km on
propane partial pressure in a 1 atm propane-air mixture
contained in a 50-cm long absorption cell, using a 2-km long
very low-loss optical-fibre link. Spectral resolution was 1.0
nm.

38. 38
 All-optical remote sensing system employing long
distance low- loss optical-fibre links in the near-infrared
region will extensively provide a very powerful, reliable
and safe scheme for a large number of
inflammable/explosive gases including LPG, LNG,
natural gas and some city gases, in the industrial and
mining facilities as well as urban and residential areas.

39. 39

40. 40

41. 41

42. 42
Difference of 685 nm LIF images obtained with lenses
(a)small lens: diameter of 20 mm and (b) large lens:
diameter of 42 mm

43. 43
Monthly variation of the LIF image ratio. The
LIF image ratio is the ratio of the intensity of
the 740 nm image to the 685 nm image.
Comparison of ginkgo tree LIF image
ratio, LIF spectra ratio and chlorophyll
concentration for the different months
observed.

44. References:
44
 Y. Saito et al. / Forest Ecology and Management 128 (2000) 129±137
 Trends in analytical chemistry, vol. 17, nos. 8+9, 1998
 Optics and Lasers in Engineering 6 (1985) 119-123
 Miguel A. Pérez, Olaya González and José R. Arias, Optical Fiber
Sensors for Chemical and Biological Measurements, chapter 10 page
265
 Raymond M. Measures, Chemical Analysis, Vol 94,Laser remote
chemical analysis.

45. Thank you
?