Superresolution far-field diffraction spot in the free-space laser communication system due to radially polarized beam

Abstract

In the free-space laser communication, there is a strong need for a technology that can decrease the size of the diffraction spot in the receiver port, because a smaller diffraction spot in the receive port makes the transmit data more secure. In this paper, instead of the usage of the larger size aperture lens in the free-space laser communication system, we introduce a diffractive superresolution technology that changing the received information laser beam into radially polarized beam which is focused on the detector array. In the paper, firstly, the conversion method of the information natural light which the optical antenna received to the radially polarized beam is discussed in detail. Then, in the focal plane, the transverse intensity distribution expression near the focal point for the radially polarized laser beam are presented, and the numerical simulation results of the intensity distributions around the focal point on different numerical apertures (NA) are given. The full width at half-maximum (FWHM) values of the main lobe are considered for the standard of the spot size. Through a comparison of the focal point FWHM values with the natural light and radially polarized beam, we judge the superresolution performance of the receiver optical system with radially polarized beam on different NA of 0.4, 0.6 and 0.75. We find that the method of focusing with radially polarized beam generates a smaller spot size than the Airy spot size when the NA is no less than 0.6; when the NA reach to 0.75, the resolution is 1.5 times than the diffraction limit. But it will decrease the light power in the process of natural light converted to radially polarized beam. When the communication laser is polarized laser, the energy loss can be reduced to around 20%. This technology can be applied when the laser energy is not the main concern in the communication.