Influence Of Turbulence On Propagationproperties Of Gaussian Laser Beams

Abstract:Laser array beams have already been attracted much attention for which played an important role in obtaining higher quality and high power laser system. Studying the propagation of laser beams in non-Kolmogorov turbulence has an important significance in theory and practical application because the atmospheric turbulence may exhibit non-homogeneity or anisotropy. On the other hand, with the development of underwater optical communications, imaging, sensor and laser radar application, the further study of propagation characteristics of laser beams through water medium is especially important. The main works in this thesis are summarized as follows:(1) The expressions for the Rayleigh range zR, the turbulence distance zT and the far-field angle θ of Gaussian array beams propagating through non-Kolmogorov turbulence are derived. The influence of generalized exponent factor a of the atmospheric power spectrum and the type of beam combinations on the spreading of Gaussian array beams is studied. It is shown that for both coherent and incoherent combinations, the dependence of zR, zT and θ on a is not monotonic. When α=3.108, zR and zT reach their minimums, and θ reaches its maximum.It means that the spreading is largest, and the spreading is most affected by turbulence when a=3.108. For the incoherent combination the spreading is larger than that for the coherent combination, but for the incoherent combination the spreading is less affected by turbulence than that for the coherent combination. It is mentioned that, for the small free-space diffraction we have zT<ZR, i.e., the spreading is affected by turbulence within the Rayleigh range; for the large free-space diffraction we have ZT> zR, i.e., the spreading is less affected by turbulence within the Rayleigh range.(2) It is found that curves of the mean-squared beam width versus the propagation distance may have a cross point in free space, and analytical expressions of the position of the cross point are derived. The position of the cross point for the coherent combination is further away from the source plane than that for the incoherent combination. However, in non-Kolmogorov turbulence the cross point disappears, and the Gaussian array beams will have the same directionality in terms of the angular spread. Furthermore, the short propagation distance is needed to reach the same directionality when the generalized exponent is equal to3.108. In particular, it is shown that in turbulence the condition obtained in previous studies is not necessary for laser beams having the same directionality, which is explained physically. On the other hand, the relative average intensity distributions at the position of the same mean-squared beam width are also studied. It is mentioned that, in free space the relative average intensity distributions are not the same at the position zc, in turbulence the relative average intensity distributions are the same at the position where Gaussian array beams have the same mean-squared beam width.(3) Based on the power spectrum of oceanic turbulence proposed by Nikishov et al, the influence of oceanic turbulence on propagation of Gaussian array beams is studied in detail by using the mean-squared beam width w’, the Rayleigh range z’R, the turbulence distance z’T, the power in the bucket PIB and Strehl Ratio SR. For both the coherent and the incoherent combinations, w’increases, but Z’R,Z’T, PIB and SR decrease, as τ(ratio of temperature to salinity contribution to the refractive index spectrum) and XT(rate of dissipation of mean-squared temperature) increase and ε(rate of dissipation of kinetic energy per unit mass of fluid) decreases. For the coherent combination w’,z’T, SR are smaller than those for the incoherent combination, but z’R and PIB are larger than those for the incoherent combination. We have deeply researched on the oceanic turbulence in theoretical way. We can find the active oceanic regions which less effects the propagation of laser beams have weak turbulence with turbulence parameters of temperature-induced variation, smaller χT and larger ε. And the strongly stratified water has strong turbulence which more effects the propagation of laser beams.