In free space optical communication links, due to the fast divergence of orbital angular momentum (OAM) beam, the issue of aperture mismatch is very common. However, in order to realize the aperture adaptation, this problem can be solved by introducing a defocusing double-lens-system at transmitter-side to control the transmitting beam divergence. In this paper, the expression for equivalent radius of Laguerre–Gaussian (LG) beams through defocusing double-lens-systems after a certain propagation distance is derived using Collins' diffraction integral formula. The expression of defocus distance with a determined transformation beam size is also derived. We numerically analyze how the defocus distance changes with OAM modes, transmitting beam size and transmission distance in free space. The calculating results show that the divergence of high-order OAM-beams in free space transmission can be effectively restrained by setting proper defocus distance to realize the aperture adaptation at the receiver. The results also show that defocus distance decreases with topological charge number, transmitting beam size or transmission distance increasing when equivalent focal length is smaller than transmission distance. When equivalent focal length is larger than transmission distance, defocus distance increases with increasing topological charge number or transmitted beam size and decreases with increasing transmission distance, in the meanwhile the smaller the ratio of equivalent radius of OAM beam at the receiver to the transmitted beam size is, the larger the required defocus distance for same OAM modes is. This paper will be beneficial to the parameters choosing for the OAM-FSO communication system.
Atmospheric turbulence limits the performance of orbital angular momentum-based free-space optical communication (FSO-OAM) system. In order to compensate phase distortion induced by atmospheric turbulence, wavefront sensorless adaptive optics (WSAO) has been proposed and studied in recent years. In this paper a new version of SPGD called MZ-SPGD, which combines the Z-SPGD based on the deformable mirror influence function and the M-SPGD based on the Zernike polynomials, is proposed. Numerical simulations show that the hybrid method decreases convergence times markedly but can achieve the same compensated effect compared to Z-SPGD and M-SPGD.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.