Haichuan Zhao

Average spreading of a Gaussian beam array in non-Kolmogorov turbulence

By using the rms beam width as the parameter to characterize the spreading property, the average spreading of Gaussian beam array propagates in non-Kolmogorov turbulence is studied. Numerical examples reveal that the beam width depends on the exponent value alpha, and the beam width in non-Kolmogorov turbulence is different from that in Kolmogorov turbulence with alpha=3.667. The beam width also depends on the distance between adjacent beamlets, the inner and outer scale parameters of the non-Kolmogorov turbulence. The beam array spreads more significantly with smaller inner scale and larger outer scale parameters.

Adaptive conversion of multimode beam to near-diffraction-limited flattop beam based on dual-phase-only liquid-crystal spatial light modulators

We propose and demonstrate the adaptive conversion of a multimode beam into a near-diffraction-limited flattop beam 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 multimode beam, and the other 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 fiber multimode beam show that the system is capable of adaptively creating square and rectangle flattop beams with desired parameters. Beam quality can be greatly improved by this system. The power in the main lobe of the far-field spot is about 4 times larger than that of the input multimode beam.

Beam shaping for uplink transmission of a relay mirror system

We establish the model of a two-Cassegrain-telescope system that is commonly used in a relay mirror system. With this model, uplink transmission of the relay mirror system is theoretically analyzed. We determined that uplink transmission with turbulence completely corrected is not an optimal mode. We improve power coupling efficiency of the two-Cassegrain-telescope system by optimizing the optical phase at the launching telescope by use of the stochastic parallel gradient descent algorithm. For a 10 km vertical uplink transmission, power coupling efficiencies of the system are 63.10%, 87.82%, and 97.80% corresponding to an open-loop mode, a closed-loop mode, and a closed-loop with optimization mode, respectively. For a 30 km vertical uplink transmission, power coupling efficiencies of the system are 22.35%, 82.66%, and 91.91%, corresponding to an open-loop mode, a closed-loop mode, and a closed-loop with optimization mode, respectively. The results show that power coupling efficiency of the two-Cassegrain-telescope system is significantly improved.

Adaptive conversion of a high-order mode beam into a near-diffraction-limited beam.

We present a new method for efficiently transforming a high-order mode beam into a nearly Gaussian beam with much higher beam quality. The method is based on modulation of phases of different lobes by stochastic parallel gradient descent algorithm and coherent addition after phase flattening. We demonstrate the method by transforming an LP11 mode into a nearly Gaussian beam. The experimental results reveal that the power in the diffraction-limited bucket in the far field is increased by more than a factor of 1.5.

Coherent beam combining of high-order-mode fiber lasers with active phase control

Coherent beam combining of high-order-mode fiber lasers is an attractive choice for high-power lasers as high-order-mode fiber lasers have a higher power handling capability than fundamental mode lasers. As active phasing provides a robust way for in-phase mode coherent beam combining, coherent beam combining of high-order-mode fibers with active phase control is proposed and a proof-of-concept experiment is performed. In the experiment, LP11 mode lasers are generated by off-center launching of single-mode fiber laser beams into few-mode fibers and active phasing is realized using a stochastic parallel gradient descent algorithm. The results show that the long-exposure fringe contrast is increased from zero to 40% when the system evolves from open-loop to closed-loop. A scalable architecture for coherent beam combining of high-order-mode amplifiers with active phase control in a master oscillator power amplifier configuration (MOPA) is also proposed.

Ef fects of fill factor of fiber combiner on the all-fiber coherent combining

A new approach for an all-fiber coherent beam combining by active phasing a fiber combiner is presented, and effect of fill factor of the fiber combiner on the all-fiber coherent combining is also numerical analyzed. Experimental results show that the power in the bucket (PIB) increases more than a factor of 2.35. The results validate the feasibility of coherent combining using a fiber combiner. The PIBs with different fill factors are calculated and the results show that the coherent combining is reduced with the increasing fill factor.

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.