Research On Vector Light Field Regulation And Its Transmission Characteristics In Turbulent Atmosphere

Free Space Optical Communications(FSO)has important application value in the satellite-air-ground-sea communication system,and has received widespread attention because of its high capacity,low crosstalk,and flexible link construction.Vector beams with polarized vortex structure open a new multiplexing dimension for high-capacity optical communications.However,conventional vector beams transmitted in free space are subject to atmospheric turbulence,which can severely degrade and even fragment the beam,resulting in communication interruptions.The vector beam field regulation contributes to optimized link transmission performance of the beam,which is important for improving FSO quality.In this paper,an optical field regulation method is adopted to optimize the transmission performance of the vector beams in turbulent atmosphere.The non-uniform partially coherent single-mode/multi-mode vector beams and vector beam arrays transmission performance are investigated.The research contents are summarized as following:Firstly,the propagation characteristics of a non-uniform partially coherent vector beam are investigated.Appropriate regulation on the degree of coherence for the optical field is an effective way to suppress the interference of the beam by atmospheric turbulence.This article uses the correspondence between the scintillation index and the intensity of light to regulate the non-uniform partially coherent vector beam field.According to the atmospheric transmission theory of vector beam,the transmission performance of non-uniform partially coherent vector beam in atmospheric turbulence is analyzed and compared with that of fully coherent and uniform partially coherent vector beam.Experimental results show that the non-uniform partially coherent beam can achieve low scintillation index and significant self-focusing characteristics,and the maximum received energy can reach about 3 times of the fully coherent vector beam,which helps to improve the signal-to-noise ratio.Moreover,the mode purity of the non-uniform partially coherent vector beam is about 50% higher than that of the uniform coherent vector beam,which indicates that the polarization vortex mode of the non-uniform partially coherent vector beam is less affected by turbulence.Secondly,the propagation performance of the non-uniform partially coherent multiplexed vector beam is investigated.An effective way to expand the capacity of free-space vector optical communication systems is mode-division multiplexing.The overall modulation method and the single-mode modulation method are two approaches to coherent regulation of the non-uniform part of the multiplexed vector beam.The overall modulation means that multiplexing is performed first followed by non-uniform part of coherent regulation,while the single-mode modulation method means that the non-uniform part of coherent regulation is performed first followed by multiplexing.In this thesis,the propagation performances of non-uniform partially coherent multiplexed vector beams using these two modulation methods are investigated.The results show that the non-uniform partially coherent multiplexed vector beam obtained by the single-mode modulation method has focusing characteristics and low flicker index,and the maximum received energy is about twice that of the fully coherent multiplexed vector beam.Its mode crosstalk is low and its purity is about 24% higher than the non-uniform partially coherent multiplexed vector beam purity of the overall modulation method,which can improve the demultiplexing performance of the multiplexed vector beam.Finally,the propagation performance of the radial vector beam arrays is investigated.The beam arrays can increase the power of the far-field beam to enhance the communication quality.In this paper,the radial vector beam arrays are constructed by adopting the idea that radial vortex beam arrays are easy to synthesize into the same type of vortex beam at far field.And the influences of geometric distribution and polarization vortex modes on the far-field vector beam are analyzed.The results show that the radial vector beam arrays have high power and high purity at the appropriate established distance.The maximum received power of vector beam arrays with sub-beam numbers of 6 and 8 at the same receiving aperture(5cm,5.5cm)can reach about 20 times and 28 times of the single vector beam,respectively.Moreover,Increasing the number of sub-beams and decreasing the off-axis distance reduce the establishment distance of high-performance far-field vector beams,while increasing the radius of the sub-beam beam waist increases the establishment distance,which provides a simple and effective regulation scheme for different communication distances.In addition,increasing the polarization topological charge of the sub-beam can reduce the scintillation and increase the intensity of the far-field beam,which improves the performance of space vector optical communication.