Study On Spectral Properties Of Laser Beams Propagating Through Turbulent Atmosphere

In 1986, Professor Wolf, showed that when the source does not satisfy the scaling law, the spectrum of the radiation from the source will experience spectral change. Later it is also found that when partially coherent light whose spectral degree of coherence satisfies the scaling law is diffracted by an aperture, there exists a spectral transition of the light in the diffracted field. On the other hand, turbulence will change properties of Laser beams (including spectral properties). The present thesis is devoted to studying spectral properties of Laser beams propagating through turbulent atmosphere. The main results achieved in the paper can be summarized as follows:1. Based on the extended Huygens-Fresnel principle, the closed-form spectral propagation equation of partially coherent Cosh-Gaussian (ChG) beams propagating through the atmospheric turbulence is derived. The influences of turbulence on spectral properties are studied in detail, and some interesting results are obtained, which are interpreted physically. It is shown that the spectrum of partially coherent ChG beams depends on the source spectrum, the beam coherent parameter, the beam decentered parameter, the structure constant of the refractive index, and the observation point. The turbulence results in a decrease of on-axis spectral shift. There exists an off-axis spectral transition of partially coherent ChG beams when the turbulence is strong enough, and the critical position where the spectral transition occurs and the transition of the relative spectral shift increase with increasing the strength of turbulence. In addition, the critical position is far away from the propagation axis as the beam coherent parameter increases, and the transition of the relative spectral shift increases with increasing the beam decentered parameter.2. The spectral propagation equation of flat-topped beams propagating through atmospheric turbulence is derived by using the quadratic approximation of the Rytov's phase structure function. The spectral properties are studied in detail. It is shown that the on-axis spectrum is blue-shifted, the off-axis spectrum becomes red-shifted, and multi-order spectral transition will appear with the increase of transverse coordinate. Furthermore the spectral shift and the transition of the spectral shift decrease with the increase of turbulence, and the spectral transition gradually disappears when the turbulence is strong enough. Specially, the behavior of the spectral transition of flat-topped beams in turbulence is very different from that of Gauss-Schell-model (GSM) beams, and the difference is discussed.