Sanhong Wang

Near-diffraction-limited flattop laser beam adaptively generated by stochastic parallel gradient descent algorithm.

We demonstrate the adaptive generation of a near-diffraction-limited flattop laser beam in the near field based on the stochastic parallel gradient descent algorithm and dual-phase-only liquid crystal spatial light modulators (LC-SLMs). One LC-SLM redistributes the intensity, and the other compensates the wavefront of the output beam. The experimental results show that approximately 69% of the power is enclosed in a region with less than 6% rms intensity variation. The 5mm diameter near-diffraction-limited output beam retains a flattop intensity distribution without significant diffraction peaks for a working distance of more than 30 cm.

Coregistration based on stochastic parallel gradient descent algorithm for SAR interferometry

The coregistration of complex image pairs is a very important step in interferometric synthetic aperture radar (InSAR) data processing. This article proposes a coregistration method based on the stochastic parallel gradient descent (SPGD) algorithm. Stochastic parallel perturbations are imposed on the translation coefficients of the polynomial coregistration model to make the performance evaluation function converge to a global extremum, which allows the translation coefficients to be obtained, and then the coregistration is achieved after resampling. Data processing of images from Kashgar and Mount Etna show that the proposed method is effective and robust. Furthermore, a series of experiments is designed to evaluate the convergence characteristics of the proposed method, which indicates that it has a stable convergence process and good robustness.

Beam cleanup technique based on stochastic parallel gradient descent wavefront control method

The feasibility of realizing beam cleanup of high power lasers using stochastic parallel gradient descent (SPGD) wavefront control method has been demonstrated numerically. The numerical model of an adaptive optics system comprising a 44-element deformable mirror and a far-field system performance metric sensor is first setup which operates with the SPGD wavefront control method. The system is then used to correct for the dynamic aberrations of a laser beam where the phase screens of the beam are constructed from the simulation data of a high power laser system and are introduced into the light wave time sequentially according to the iteration rate of the SPGD wavefront controller. The correction results show that the beam cleanup system investigated here can effectively compensate for the dynamic aberrations of the laser beam involved.

Coherent beam combination of fiber lasers based on the stochastic parallel gradient descent algorithm

Coherent beam combination of fiber laser arrays plays an important role in realizing high power, high radiance fiber laser systems. The stochastic parallel gradient descent (SPGD) algorithm is a newly developed optimization method using the technique of parallel perturbation and stochastic approximation and it is expected that this algorithm can reduce the cost and complexity of a high power fiber laser system when incorporated in its beam combination scheme. In this paper, a numerical simulation model about the fiber laser beam combination system is then established based on beam-quality-metric optimization method. The SPGD algorithm is introduced and used to realize the beam-quality-metric maximization, leading to the maximum output power of the fiber laser system. The results of numerical simulation indicate that the far-field beam intensity optimization method using SPGD algorithm can realize coherent beam combination of fiber laser arrays effectively.

Adaptive conversion of input beam with unsymmetrical intensity distribution to flattop beam based on dual SLMs

We propose and demonstrate the adaptive conversion of input beam with unsymmetrical intensity distribution into a near-diffraction-limited flattop beam with desired parameters in the near field based on a combination of dual phase-only liquid crystal spatial light modulators (LC-SLMs) and the stochastic parallel gradient descent (SPGD) algorithm. One phase-only LC-SLM redistributes the intensity of the input beam to the desired distributions at the other phase-only LCSLM plane, and the other phase-only LC-SLM compensates the wavefront of the output beam. The SPGD algorithm adaptively optimizes the phase distributions of dual phase-only LC-SLMs to reduce the variance between the actual beam shape and the target beam shape. The experimental results on a prefabricated unsymmetrical input beam show that the technique is capable of adaptively creating near-diffraction-limited flattop beams with desired parameters.

Simulation study on fiber-coupling efficiency for quantum communication through atmospheric turbulence

Quantum communication is a high secure and high efficiency communication method. The received laser beam in the quantum communication system must be coupled into the fibers, by which the communication signal can be detected, amplified and processed for the latter devices. However, atmospheric turbulence will degrade the spatial coherence of the laser beam and limits the fiber- coupling efficiency. In the paper, the propagation model of the laser beam through atmospheric turbulence is established and the fiber-coupling efficiency for the laser beam distorted by atmospheric turbulence is evaluated, and the optimal coupling geometry parameter a=1.12 is given, with which the theoretical maximal coupling efficiency is reached. Then the relationship between the atmospheric coherence constant r0 and the fiber-coupling efficiency is analyzed. The simulation also indicates that the fast steering mirror (FSM) can reduce the influence of the atmospheric turbulence and improve the coupling efficiency distinctly.

Parameters optimization of the beam clean-up system based on stochastic parallel gradient descent method

In a high-energy laser, the thermal aberrations degrade the beam quality and reduce the laser’s output power. Adaptive optics (AO) technique based on a stochastic parallel gradient (SPGD) algorithm can be used to compensate for the distortions in real time to clean up the laser beam. Such a beam clean-up system was simulated and experiments were conducted to study the optimization of the parameters of the gain coefficient and the amplitude of the perturbation. The results show that the convergence property of the SPGD algorithm is improved after the parameters being optimized.

Numerical simulation of the high-order Gaussian laser beam cleanup system based on optimization-translation adaptive optics

We present research on high-order Gaussian laser beam cleanup by using adaptive optics (AO) technique. A 64-element adaptive optics beam cleanup simulation system based on Stochastic Parallel-Gradient-Descent (SPGD) Algorithm was set up. The far-filed energy centrality of TEM10 mode was advanced and the evaluation function increased more than a factor of 4.6 when the system evolves from open-loop into close-loop state. The experiments results indicate the feasibility of Adaptive Optics in mode transform system.

Wave-front correction with a multilevel stochastic parallel gradient descent technique

An adaptive optics (AO) system composed of multiple control levels based on the stochastic parallel gradient descent (SPGD) technique is discussed. Results of numerical simulations demonstrate that the convergence rate of 3-level SPGD wave-front correction increased by 23% compared with that of the usual SPGD wave-front correction.