OPTIMAL BEAM FORMING FOR LASER BEAM PROPAGATION ...

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Free space optical communicationsBecause of the increasing need for bandwidth and high data rates , opticalcommunication and network systems continue to be more and more important.Free space optical communications using laser beams have been developedfor line-of-sight information transmission, such as in satellite communications.Systems with infrared, visible, and ultraviolet carrier frequenciesare all included in optical communications. There are many advantages ofoptical communications compared with an RF system, such as wider bandwidthand higher power density in an extremely narrow beam [27]. But thedetrimental effects of the propagation media form the main drawback of thesystem, because optical wavelengths are commensurate with molecule andparticle sizes. Even in clear sky situations, laser beams are subject to turbulence,scattering and absorption through the atmosphere. In bad weathersuch as fog, rain and snow, the received optical power can be decreased andthe bit error rate increased significantly.Laser radar systemLaser radar, or lidar, is a short wave extension of radar, which uses radiationat wavelengths that are 10, 000 to 100, 000 times shorter than those used byconventional radar. Radiation scattered by a target is collected and processedto yield information about the target and the path to the target. By usinghigher frequency laser beams, higher accuracy and resolving power can beapproached with lidar systems. Phase and amplitude fluctuations of the laser2
eam due to turbulent media can, however, disrupt lidar’s functions such astarget tracking, imaging and ranging.Astronomical imagingAstronomers have known for many years that the Earth’s atmospheric propagationpath causes phenomena such as image jitter and star twinkling. Thephase distortions induced by propagation in turbulence cause degradationproblems in imaging systems such as image blurring and image jitter whichboth contribute to a loss of resolution. This is the main reason that the HubbleSpace Telescope was placed in space. Long exposure images from groundbasedtelescopes are severely blurred due to the much broader point spreadfunction (PSF), a result of the average effects of turbulence. The Earth’sturbulent atmosphere can also degrade radio astronomy systems. Althoughlonger wavelength and different receivers (radio-telescopes) are used in radioastronomy, atmospheric turbulence has similar effects on optical and radioastronomy because both light and radio waves can be corrupted by randommedia.Laser guide starReference beacons are needed in adaptive optics systems because wavefrontcompensation requires measurement of the turbulent propagation path. Twomain requirements for the reference source are that it have enough brightnessand it is located within the isoplanatic patch of the desired view angle [33].Because there are few natural stars or objects that satisfy these requirements,3
Page 5 and 6: AcknowledgementFirst and foremost,
Page 7 and 8: 3.2.1 Paraxial waves and Gaussian b
Page 9 and 10: 6 Summary 996.1 Conclusions . . . .
Page 11 and 12: 4.2 Airy pattern: (a)Normalized Air
Page 13 and 14: 5.9 Monte-Carlo simulation by trans
Page 15 and 16: List of Tables4.1 Physical paramete
Page 17: Chapter 1Introduction1.1 Motivation
Page 21 and 22: the effects of refraction and diffr
Page 23 and 24: strate evident reduction of scintil
Page 25 and 26: are the most important causes of al
Page 27 and 28: mal blooming can be ignored and the
Page 29 and 30: centroid is caused by large eddies
Page 31 and 32: optics systems for laser beam contr
Page 33 and 34: YYouXθvXAZZ = 0Z = dFigure 3.1: Th
Page 35 and 36: When the propagation distance d is
Page 37 and 38: wherew(z) = w 0[1 +( zz 0) 2] 1/2,
Page 39 and 40: Figure 3.2: Hermite Gaussian beams
Page 41 and 42: 3.3 Optical CoherenceThe optical co
Page 43 and 44: where F (⃗r, ν) is the Fourier t
Page 45 and 46: from finite surfaces with small dif
Page 47 and 48: 3.4.1 Kolmogorov turbulence modelKo
Page 49 and 50: structure function of refractive in
Page 51 and 52: The wave number k(⃗r) can be expr
Page 53 and 54: An iterative method can then be tak
Page 55 and 56: We then get( ) aψ s (⃗r) = χ +
Page 57 and 58: It can be seen that the ensemble av
Page 59 and 60: 3.6.1 Maximization of the integrate
Page 61 and 62: Chapter 4Optimal Beam to Maximize t
Page 63 and 64: A with aperture function⎧⎪⎨ 1
Page 65 and 66: 4.1 Optimal Beam in Free SpaceThe m
Page 67 and 68: −0.132.5−0.052u’ (meter)01.51
Page 69 and 70:
plane as∫ z+∆zφ(⃗u) = k n(
Page 71 and 72:
−0.132.5−0.052u’ (meter)00.05
Page 73 and 74:
formh(⃗v, ⃗u) = 1 (jλd exp j 2
Page 75 and 76:
The transverse coherence length has
Page 77 and 78:
models - extended Heygens-Fresnel p
Page 79 and 80:
21.8free spaceturbulence10.91.60.81
Page 81 and 82:
2.5free spaceturbulence10.920.80.71
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Chapter 5Optimal Beam to Minimize t
Page 85 and 86:
and the corresponding received inte
Page 87 and 88:
following two sections, we will dis
Page 89 and 90:
where Γ(⃗u 1 , ⃗u 2 ) = 〈φ(
Page 91 and 92:
intensity can be expressed as∫∫
Page 93 and 94:
5.4 Selection of coherent modesThe
Page 95 and 96:
m = 0m = 1m = 2222000−2−2−2
Page 97 and 98:
Scintillation Index0.140.130.120.11
Page 99 and 100:
p = 0p = 1p = 2420−2420−2420−
Page 101 and 102:
N 1 2 3 4 5 6 7 8α 0 1.0000 0.1177
Page 103 and 104:
120100TheoreticalSimulation1.21Theo
Page 105 and 106:
Scintillation Index0.40.350.30.250.
Page 107 and 108:
Scintillation index of single mode0
Page 109 and 110:
0.90.80.7r 0= 0.05 mr 0= 0.1 mr 0=
Page 111 and 112:
0.6C n2 = 2x10−16C n2 = 5x10−16
Page 113 and 114:
known fact that planets scintillate
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Chapter 6Summary6.1 ConclusionsIn t
Page 117 and 118:
showed that increasing the receiver
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L m p (x) Laguerre polynomialn Inde
Page 121 and 122:
Bibliography[1] L. C. Andrews and R
Page 123 and 124:
[18] D. deWolf. Strong irradiance f
Page 125 and 126:
[36] A. Ishimaru. Flucturations of
Page 127 and 128:
[54] C. Primmerman and D. Fouche. T
Page 129 and 130:
[73] V. I. Tatarski. Waves Propagat
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OPTIMAL BEAM FORMING FOR LASER BEAM PROPAGATION ...

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