Effect Of Twist Phase And Correlation Structure On A Laser Beam In Turbulent Atmosphere

Interest in high-data-rate free-space optical(FSO) laser communication systems has grown significantly in recent years because of the advantages offered by FSO systems over radio frequency systems such as high transfer rate, large bandwidth and high security, etc. These advantages are simple consequences of the high frequency associated with optical waves. However, there are some drawbacks in FSO laser communication systems. When a laser beam propagates through the atmosphere, it can be affected by suspended particles, aerosols and atmospheric turbulence. Basically, the turbulent effect of the atmosphere is due to random changes in refractive index and it can severely influence the beam quality and therefore the efficiency of the technology. For many years, researchers have been studying how to reduce the effects of atmospheric turbulence on the propagation of a laser beam, and put forward a series of methods, such as adaptive optics system modification; modulating the phase, polarization and coherence of light source, etc. This paper based on the modulation of light source, studied the propagation properties of a partially coherent beam with a twist phase and a kind of beam with a special correlation structure in turbulence.Firstly, we investigate the statistical properties of the beam width, radius of curvature, transverse coherence length and twist factor of the twist Gaussian-Schell model(TGSM) beam in non-kolmogorov turbulence. The effect of the fractal constant α of the atmospheric power spectrum and the strength of the turbulence on the statistical properties of the TGSM beam are investigated numerically in detail. It is found that the TGSM beam with larger initial twist factor is less affect by the turbulence induced degeneration than the TGSM beam with smaller initial twist factor under the same condition.Secondly, we investigate the propagation properties of a special correlated radially polarized(SCRP) beam in atmospheric turbulence. Analytical formulas for the average intensity distribution(AID), degree of polarization(DOP) and degree of coherence(DOC) are derived by adopting a beam coherence-polarization(BCP) matrix. The evolutions of the AID, DOP and DOC of the SCRP beam in turbulent atmosphere are illustrated through numerical examples in detail, and the results are compared to those of a partially coherent radially polarized(PCRP) beam under equivalent condition. It reveals that the propagation properties of the SCRP beam is much different from those of the PCRP beam, and closed related to the strength of the turbulence and the parameters of beam.