Study On The Propagation Properties Of Polychromatic Partially Coherent Electromagnetic Flat-topped Beams In Atmospheric Turbulence

The research of the theory of the laser beam propagation in the atmospheric turbulence is of great importance in the area of remote sensing, tracking and long-distance communication. A large number of theoretical and experimental studies have shown that atmospheric turbulence will lead to beam expansion and the change of propagation properties. In addition, partially coherent beams propagating through atmospheric turbulence are less sensitive to the effects of turbulence than fully coherent beams. Based on the extended Huygens-Fresnel principle, the degree of polarization, the degree of coherence, the spectrum and the total intensity distribution of polychromatic partially coherent electromagnetic flat-topped beams in turbulence are studied, by using the quadratic approximation of Rytov's phase structure function.1. The analytical expression for the degree of polarization of polychromatic partially coherent electromagnetic flat-topped beams in turbulence is derived. It is shown that the degree of polarization is independent of the bandwidth of original spectrum. After sufficient long propagation, the degree of polarization of polychromatic partially coherent electromagnetic flat-topped beams with different beam orders, widths, correlation lengths tend to different limit values in free space. However, the degree of polarization with different parameters tend to the same value at the source plane due to turbulence, after sufficient long propagation.2. The analytical expression for the degree of coherence of polychromatic partially coherent electromagnetic flat-topped beams in turbulence is derived. It is shown that the degree of coherence is independent of the bandwidth of original spectrum. The oscillatory behavior and phase singularities of the degree of coherence may appear in free space. The turbulence makes the difference of the degree of coherence between polychromatic partially coherent electromagnetic flat-topped beams and EGSM beams decrease. The oscillatory behavior and phase singularities of the degree of coherence of polychromatic partially coherent electromagnetic flat-topped beams may disappear in the turbulence.3. The analytical expression for the spectrum of polychromatic partially coherent electromagnetic flat-topped beams in turbulence is derived. It is shown that the spectrum is dependent of the bandwidth of original spectrum. In free space, 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 decreases and the spectral transition gradually disappears due to the turbulence.4. The total intensity distribution of polychromatic partially coherent electromagnetic flat-topped beams in turbulence is obtained. It is shown that the intensity is dependent of the bandwidth of original spectrum. The intensity reaches peak value at less distance and declines more quickly in turbulence. The beams with larger bandwidth have bigger peak values and will be greater affected by the turbulence. The impact of bandwidth on beam expansion will decrease due to the turbulence.