Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
Attenuation of laser beam propagation
in the atmosphere
Asist. Teacher Ammar Ayish Habeeb
College of science-University of Diyala
Abstract:
In this search I am calculated theoretical model for attenuation of laser
beam due to the scattering by the atmospheric particle (fog, mist, haze and dust)
as a function of visual range (visibility). the theoretical results show that the
laser wavelength in the Ultraviolet region 0.337 µm have attenuation more than
Visible regions, 0.6328µm and near infrared region 1.06µm, also these result
show that, in fog condition where visibility is less than 0.6km, the atmospheric
attenuation of laser beam is independence of laser wavelength.
1-Introduction
The transmittance of laser beam in atmosphere is very important process
in many military and civilian applications with technique development using
laser beam. During the laser transmission in the atmosphere, the laser beam
suffers attenuation due to both scattering and absorption by all the atmospheric
species present in the path of propagation. The atmospheric path is categorized
to horizontal path (constant pressure) and slant path (changed pressure) [1,3].the
absorption occurs by water vapor, carbon dioxide, ozone. Nitrous oxides, carbon
monoxide, nitrogen and oxygen, while the scattering is produced by gas
molecules, fog, smoke and dust[1,4]. The earth’s atmosphere is surrounded by
number of layers, each layer is characterized by (pressure, density, and
temperature), and these properties of layers are variable with time and location
.The transparency of the atmosphere for laser radiation is one of the most
important parameters in the calculation of the attenuation. This is depending on
the weather and the path length[2,5].
2-Theoretical model
The general process by which radiant flux is attenuated in passing through
the atmosphere is called extinction. The transmittance of a path throught the
atmosphere can be expressed by Lambert law [4,5,7].
TL eL ..............(1)
Where TL Transmittance through path length (L)
S a................(2)
Where Total attenuation coefficient, S =scattering coefficient and a=
absorption coefficient.
0202 /العدد الثالث والستون 02 مجلة كلية التربية األساسية
Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
The type of scattering is determined by the size of the atmospheric particle with
respect to the laser wavelength. The size of the atmospheric particle described
by a dimension less number called size parameter (k) [6].
2r
K ........(3)
Where r = radius of scattering particle and
Laser wavelength λ=
The general relation between wavelength and scattering coefficient is [8,9].
S d / q ..........(4)
Where d = constant parameter and q = a parameter whose value depends on
type of scattering.
There are three type of scattering occurs in the atmosphere, Rayleigh, Mie and
Non-selective or Geometrical scattering. Rayleigh scattering occurs when
wavelength is much larger than the particle size (λ >> r), in this kind of
scattering (q) is equal to 4 such scattering would be present even in completely
clear atmosphere, because the gas molecules themselves would scatter the
radiation. the effect of Rayleigh scattering on the total attenuation is very small,
so it can be neglected[2,4,11].
As the particle size approaches laser wavelength (λ≈r), scattering of radiation
off the larger particles becomes more dominate in the forward direction as
opposed to the backward direction. this type of scattering, where the size
parameter (K) varies between (0.1, 50) such as fog, smoke, haze and dust is
called Mie scattering , where the value of q varies from (0 to 1.6) [3,6] .The
third generalized scattering occurs when the atmospheric particles are much
larger than laser wavelength (r >> λ) size parameter grater than 50, the
scattering is called Geometrical or Nom-selective scattering, the scattering
particles are larger enough so that the angular distribution of scattered radiation
can be described by geometric optics .the Rain drops ,snow, hail, cloud droplets
and heavy fogs are caused Geometrical or Non-selective scattering of laser
wavelength . the scattering is called Non-selective because there is no
dependence of the attenuation coefficient on laser wavelength ,where the value
of (q) equal zero[1].
Calculating 3-
According to general equation (4) an empirical relation often used to
calculate the atmospheric attenuation in term of visibility and wavelength this
relation is [9,11] .
q
3.91 0.55
S .................(5)
V
0202 /العدد الثالث والستون 02 مجلة كلية التربية األساسية
Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
Where λ =wavelength in micrometer (µm),V =visual range in kilometer (Km)
and q = the size distribution of the scattering particles.
q =1.6 for good visibility (V > 50km)…………. (5- a).
q =1.3 for average visibility (6km<V <50km)… (5- b).
q=0.585 for low visibility (V < 6km) q= 3 V ……..(5- c).
The visibility (V) is defined as the path length at which transmission at
0.55µm wavelength (where the sensitivity for the light adapted human eye peak)
is 2%, this is intended to correspond to the distance at which a block object can
just be discerned against the horizon sky, therefore for R=V we have T = 2%
According to equations (4 and 5) the value of (q) is very important because it
determines the wavelength dependence of the attenuation coefficient and the
type of scattering [4,12].
A search in literature agrees with equation (5) but the value of (q) at law
visibility equation (5-c) some in error. in fact there is strong empirical data
which suggests for the value of (q) when the visibility is less than
(6km).Eldridge defined three generalized types of zones of shorter visibility as
range , weather fog for visibility less than ( 0.6 km) , haze for visibilities greater
than( 1km ) and transitional zone called mist for visibilities between (0.6 km and
1km).these zones are based on changes in observed particle size distributions
and changes in the wavelength selectivity of measured attenuation coefficients
which have mentioned previously[2,3,13].
Eldridge indicates that haze is primarily made of microscopic fine dust or
salt or small water dropt on the order of a few tenths of a micron. fog occurs
during very high relative humidity when water droplets of a few microns to a
few tens of microns from over the haze particle. Mist occurs during the
transition from haze to fog as the humidity increases to saturation [2,5].
The unit that is used in this research to measure the
attenuation is Decibel per unit length, from equation (1) the Decibel unit (dB)
is defined as [10] :
TL (dB) (10Log10e)L 4.342L 4.342TL ........(6)
The attenuation by scattering in Decibel per unit Length multiply
equation (5) by no. (4.342).
4-Results and discussion
For different visibility condition derived from the attenuation coefficient
calculated using Equation (5) as in figure (1) it can be noted that the energy
loosing from the laser beam through the atmosphere decreases with increasing
the visibility, where the visibility represents the weather conditions, this means
there is a visual range which is related to weather condition specially related to
radius and distribution of the atmospheric particles the weather condition that is
considered for calculation the atmospheric attenuation is general not limited to a
geographic location and the beam propagate horizontally with one layer in the
0202 /العدد الثالث والستون 00 مجلة كلية التربية األساسية
Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
atmosphere ,where this layer have same optical properties (attenuation and
turbulence) all wave length also have other properties such as pressure
temperature .
It can be concluded from figure (2) that the atmospheric attenuation at a
given visibility value decreases with increasing the wavelength
The energy that is carrying by the wavelength (1.06µm) is essentially less
than the energy that is carrying by the wavelength (0.337µm), therefore the
attenuation for Ultraviolet regions 0.337µm is more than visible regions
0.6328µm and Near infrared region 1.06µm although these wavelengths have
transmittance windows. These results also agree with Plank law.
5-Conclusion
1- From figure (1)and figure(2) it can be seen that, in fog condition where the
visibility is less than 0.6 Km the atmospheric attenuation take the same value
for different laser wavelength because the value of (q) parameter equal zero,
therefore the atmospheric attenuation is independent on wavelength.
2- In general the attenuation of the atmosphere increases when the visibility
decreases and the attenuation of the atmosphere decreases visibility when the
visibility increases.
References
[1] J.W Strohbehn, “Laser Beam Propagation in the Atmosphere”, (Topics in
applied physics, vol. 25), ch. (1,2,5,7), Springer-Verlag, 1978.
[2] V.E. Zuev, “Laser Beams in the Atmosphere”, Consultants Bureau, ch.
(2,3,4,), 1982.
[3] H. Hodara, “Laser Wave Propagation through the Atmospere”, Proceedings
of IEEE, vol. 54, no. 3, 368-375, 1966.
[4] H.A. Mayadeh, “ An Experimental Study of some Effects of Atmospheric
Turbulence on the Propagation of a Laser Beam”, M.Sc. Thesis, University of
Baghdad, 1988.
[5] J.C. Q. Lavergnat, M. Sylvain, “Radio Wave Propagation”, (principles and
techniques, ch .3, John Wiley and sons, 2000.
[6] E.D. Hinkley, “Laser Monitoring of the Atmosphere”(Topics in Applied
physics, Vol 14), ch. (3,4), Springer-Verlag, 1976.
[7] F.x. Kneizys, S.A. Clough, E.P. Shettle, “Atmospheric Attenuation of Laser
Radiation”, Proc. SPIE, vol.410, 1983.
[8] A. Biswas and S. Lee, “Ground-to-Ground Optical Communications
Demonstration”, URL:http://Lasers jpl. nasa .gov./papers/GG/141g. pdf.
0202 /العدد الثالث والستون 00 مجلة كلية التربية األساسية
Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
[9] Kim I.I., B. McArthur and E. Korevaar, "Comparison of laser beam
propagation at 785 nm and 1550 nm in fog and haze for optical
wireless communications," Proc. SPIE, 4214, 26-37, 2001.
[10] Kim I.I, Isaac,J Schuster and E. Korevaar "Wireless optical
transmission of fast Ethernet and ESCO Protocol data using the
Terralink laser communication system" Society of photo-opical
Instrumentation Engineering,1998.
[11] Al Naboulsi M., Sizun H. , F. de Fornel " Propagation of optical and
infrared waves in the atmosphere" Journal SPIE International Society
for Optical Engineering, 2003.
[12] Donati, S "Photo detectors Devices, Circuits, and Application" book ,
2000.
[13] E.D.Hinkely"Laser Monitoring of the Atmosphere" (Topics in
Applied Physics, vol. 14) springer-verlay, 1976.
0202 /العدد الثالث والستون 00 مجلة كلية التربية األساسية
Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
0202 /العدد الثالث والستون 03 مجلة كلية التربية األساسية
Attenuation of laser beam propagation in the atmosphere …………………
Asist. Teacher Ammar Ayish Habeeb
Figure (1) Atmospheric attenuation as a function of the visibility for
different laser wavelength.
Figure (2) Atmospheric attenuation as a function of laser wavelength
for different visibility(V) in kilometer unit (Km) .
0202 /العدد الثالث والستون 03 مجلة كلية التربية األساسية
………………… Attenuation of laser beam propagation in the atmosphere
Asist. Teacher Ammar Ayish Habeeb
اخلالصة:
ل د ب ح نة ددل حل ددة حلج د لقددت د ذددا بد ح حل نددئ ناددمو ج ددمظر حلج د
حلخف دف حت دل ثل حلض مو حلكث ف حلجم جل ح مث حالا طم ة لجا مت حلج
د د ت ح حتط د د حل د ددل حلج دددل كتحلد ددل ل د ددتو حل ة دددم احل ض د د ن لظه حلج دددمظر حلج
ددت د ب ح حل جفاددج ل 0.337µm حل ج ددل حل ة ددل حل حفيددل ذددا حل جطقددل حلف د
د ددح حتطد د د حل حل ج د ددل حل ة د ددل حل حفيد ددل ذد ددا حل جطقد ددل حل ظ د ددل µm حفد ددل جدد
حل جطق ددل ن ددت حلن د د ح حلق ددل 1.06 µmهكد د حلت د د ح حح نمل ددل 0.6328
حلض مو حلكث ف جتح ن ئ ك ح حل ض ن ل حفل دح 0.6kmذدمح حل د ب ح حلجد
ال ي ت ع ى حلط ل ل جا ل الة ف ثم ل ي ت ع ى حل ض ن ل ذقط ه
العدد الثالث والستون0202 / 03 مجلة كلية التربية األساسية
