High-speed free-space optical communication systems using fiber-optic components can greatly improve the stability of the system and simplify the structure. However, propagation through atmospheric turbulence degrades the spatial coherence of the signal beam and limits the single-mode fiber (SMF) coupling efficiency. In this paper, we analyze the influence of the atmospheric turbulence on the SMF coupling efficiency over various turbulences. The results show that the SMF coupling efficiency drops from 81% without phase distortion to 10% when phase root mean square value equals $0.3\lambda$. The simulations of SMF coupling with adaptive optics (AO) indicate that it is inevitable to compensate the high-order aberrations for SMF coupling over relatively strong turbulence. The SMF coupling efficiency experiments, using an AO system with a 137-element deformable mirror and a Hartmann–Shack wavefront sensor, obtain average coupling efficiency increasing from 1.3% in open loop to 46.1% in closed loop under a relatively strong turbulence, $D/r_0=15.1$.

The phase diversity (PD) algorithm is an image-based co-phasing error detection method for segmented mirror synthetic aperture optical systems. Particle swarm optimization (PSO) is suitable for PD for its fast convergence speed and simple structure. However, with the increase of the numbers of subapertures, optimization of cost function formed by the PD algorithm becomes a high-dimension non-linear optimization problem, which would lead PSO to result in a premature solution. Regarding the problem above, this paper presented a modified PSO in the PD algorithm to co-phase the segmented primary mirror, which overcame drawbacks of the traditional PSO, such as premature convergence and deactivation of particles, and enhanced the dig ability of the algorithm. Vast simulation results show that the modified PSO in the PD algorithm can effectively restore the segmented primary mirror, reducing the peak-to-valley (PV) value to 0.0012 λ and the root mean square error to 0.0007 λ , and raising the Strehl ratio to over 0.999. A comparison was also conducted to show that the modified PSO has advantages over two existing algorithms in higher accuracy and faster convergence speed.

A dispersed fringe sensor (DFS) is a promising approach for sensing the large-scale piston error for segmented mirror systems. In this Letter, a novel method, to the best of our knowledge, for piston sensing based on a digital dispersed fringe (DDF), which is generated by wavelength tuning, is introduced. By continuously changing the wavelength of light passing through the sensor, a collection of far-field spot arrays can be acquired, which are used to construct the DDF with steps of accumulation and stacking in sequence for each inter-segment region. A correlation-based method is applied to extract the encoded piston information from the DDF. The method greatly eases the demanding hardware requirement in traditional DFS and shows high efficiency for the reason that it could simultaneously measure all of the relative piston settings; in addition, it exhibits strong robustness to noise.

In this paper, a broadband phasing algorithm is combined with a two-wavelength phasing algorithm to detect the piston error of a segmented mirror with the advantages of long range, high precision, and fast detection. Moreover, an active optics co-phasing experimental system of the segmented mirror is built to verify the algorithm’s effectiveness. The segmented mirror consists of four hexagonal segments, with flat-to-flat lengths of 100 mm and radii of curvature of 2000 mm. First, a Shack–Hartmann sensor and piezoelectric actuators are used to finely co-focus the segmented mirror. Then, the broadband phasing algorithm is used to reduce the piston error of the segmented mirror to several micrometers. Finally, the two-wavelength phasing algorithm is used to reduce the piston error of the segmented mirror to zero. The experimental results show that the measurement accuracy is better than 26 nm, and the adjustment accuracy is approximately 55 nm, which demonstrates that the combined algorithm is valuable for segmented-mirror co-phasing measurement and adjustment.

Free space optical communication (FSOC) systems operating in the space–atmosphere channel are susceptible to severe turbulence-induced scintillation, particularly in up-link configurations where the adaptive optics (AO) pre-correction becomes ineffective due to anisoplanatic constraints. This study presents a novel scintillation suppression strategy utilizing self-focusing optical pin-like beams (OPBs) with tailored phase modulation, combining theoretical derivation and numerical simulation. It is found that increasing the shape factor γ and modulation depth C elevates the average received power and reduces the scintillation index at the focal point. Meanwhile, quantitative evaluation of the five OPB configurations shows that the parameter set γ = 1.4 and C = 7 × 10⁻⁵ gives a peak scintillation suppression efficiency. It shows that turbulence induced scintillation is suppressed by 44% with the turbulence intensity D/r₀ = 10, demonstrating exceptional effectiveness in up-link transmission. The findings demonstrate that OPB with optimized γ and C establish an approach for uplink FSOC, which is achieved through suppressed scintillation and stabilized power reception.