• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.247-258
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• 2020

In this paper, based on a stochastic parallel gradient descent (SPGD) algorithm we study phase control of a coherent-beam-combining system under turbulent atmospheric conditions. Based on the statistical theory of atmospheric turbulence, we carry out the analysis of the phase and wavefront distortion of a laser beam propagating through a turbulent atmospheric medium. We also conduct numerical simulations of a coherent-beam-combining system with 7- and 19-channel laser beams distorted by atmospheric turbulence. Through numerical simulations, we characterize the phase-control characteristics and efficiency of the coherent-beam-combining system under various degrees of atmospheric turbulence. It is verified that the SPGD algorithm is capable of realizing 7-channel coherent beam combining with a beam-combining efficiency of more than 90%, even under the turbulent atmospheric conditions up to cn2 of 10-13 m-2/3. In the case of 19-channel coherent beam combining, it is shown that the same turbulent atmospheric conditions result in a drastic reduction of the beam-combining efficiency down to 60%, due to the elevated impact of the corresponding refractive-index inhomogeneity. In addition, by putting together the number of iterations of the SPGD algorithm required for phase locking under atmospheric turbulence and the time intervals of atmospheric phenomena, which typically are of the order of ㎲, it is estimated that hundreds of MHz to a few GHz of computing bandwidth of SPGD-based phase control may be required for a coherent-beam-combining system to confront such turbulent atmospheric conditions. We expect the results of this paper to be useful for quantitatively analyzing and predicting the effects of atmospheric turbulence on the SPGD-based phase-control performance of a coherent-beam-combining system.

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.274-280
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• 2020

In this paper, the effect of atmospheric turbulence is numerically modeled and analyzed via a phase-screen model, in regard to long-range optical energy transfer using coherent beam combination. The coherent-beam-combination system consists of three channel beams pointing at a target at a distance of 1-2 km. The phase and propagation direction of each channel beam are assumed to be corrected in an appropriate manner, and the atmospheric turbulence that occurs while the beam propagates through free space is quantified with a phase-screen model. The phase screen is statistically generated and constructed within the range of fluctuations of the structure constant Cn2 from 10-15 to 10-13 [m-2/3]. Particularly, in this discussion the shape, distortion, and combining efficiency of the 3-channel combined beam are calculated at the target plane by varying the structure constant used in the phase-screen model, and the effect of atmospheric turbulence on beam-combination efficiency is analyzed. Analysis with this numerical model verifies that when coherent beam combination is used for long-range optical energy transfer, the received power at the target can be at least three times the power obtainable by incoherent beam combination, even for maximal atmospheric fluctuation within the given range. This numerical model is expected to be effective for analyzing the effects of various types of atmospheric-turbulence conditions and beam-combination methods when simulating long-range optical energy transfer.

• Korean Journal of Optics and Photonics
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• v.34 no.1
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• pp.1-12
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• 2023

In this study, we propose a hybrid phase-control algorithm for multi-channel coherent beam combining (CBC) system by combining the covariant matrix adaption evolution strategy (CMA-ES) and stochastic parallel gradient descent (SPGD) algorithms and analyze its operational performance. The proposed hybrid CMA-ES/SPGD algorithm is a sequential process which initially runs the CMA-ES algorithm until the combined final output intensity reaches a preset interim value, and then switches to running the SPGD algorithm to the end of the whole process. For ideal 7-channel and 19-channel all-fiber-based CBC systems, we have found that the mean convergence time can be reduced by about 10% in comparison with the case when the SPGD algorithm is implemented alone. Furthermore, we analyzed a more realistic situation in which some additional phase noise was introduced in the same CBC system. As a result, it is shown that the proposed algorithm reduces the mean convergence time by about 17% for a 7-channel CBC system and 16-27% for a 19-channel system compared to the existing SPGD alone algorithm. We expect that for implementing a CBC system in a real outdoor environment where phase noise cannot be ignored, the hybrid CMA-ES/SPGD algorithm proposed in this study will be exploited very usefully.

• Korean Journal of Optics and Photonics
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• v.32 no.1
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• pp.1-8
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• 2021

We have studied a tiled-aperture coherent-beam-combining system based on constructive interference, as a way to overcome the power limitation of a single laser. A 1-watt-level, 3-channel coherent fiber laser and a 3-channel fiber array of triangular tiling with tip-tilt function were developed. A monitoring system, phase controller, and 3-channel phase modulator formed a closed-loop control system, and the SPGD algorithm was applied. Eventually, phase-locking with a rate of 5-67 kHz and peak-intensity efficiency comparable to the ideal case of 53.3% was successfully realized. We were able to develop the essential elements for a tiled-aperture coherent-beam-combining system that had the potential for highest output power without any beam-combining components, and a multichannel coherent-beam-combining system with higher output power and high speed is anticipated in the future.

• Korean Journal of Optics and Photonics
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• v.33 no.6
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• pp.245-251
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• 2022

We have developed a multi-channel fiber laser for tiled laser beam combining and a laser output array system for multi-beam alignment. The fiber laser is a master oscillator power amplifier configuration that has a common seed, a preamplifier, and a 7-channel amplifier. The output power of each channel is more than 10 W. The laser output array system is a packed cylindrical configuration for a high fill-factor, and it has capabilities for collimation and tilt control with built-in PZT. Multi-beam alignment to a target is successfully implemented using PZT controlled with a stochastic parallel gradient descent (SPGD) algorithm.

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.290-294
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• 2020

In this paper, we quantitatively analyze the effect of lens aberration on the degradation of beam-coupling efficiency of a tiled coherent-beam combining system. The Zernike polynomial is used to quantify the aberration of the lens, and Fresnel diffraction is applied to numerically simulate the change in the peak light intensity when combined at a distance. The results of this paper will be useful for quantitative prediction of the beam-combining efficiency that is degraded by aberration of the lens, and it is expected to be helpful for the optimal design of a practical tiled coherent beam-combining system.

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.268-273
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• 2020

We analyze the characteristics of the coherent beam combination of lasers by monitoring the speckle pattern of the beam reflected from a scattering medium. Three collimated laser sources with high coherence are focused on a scattering target using a lens, and we then examine the speckle pattern of the returned beam in the Fourier domain. We observe that the size of the speckle pattern changes, depending on the focused-beam size or degree of spatial overlap of the three beams. Furthermore, through Fourier-domain analysis of the speckle pattern we obtain the monitoring variable to qualify the efficiency of the coherent beam combination.

• Proceedings of the Optical Society of Korea Conference
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• 2003.02a
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• pp.206-207
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• 2003

전자기 유도 투과(electromagnetically induced transparency ； EIT)는 원자의 공명 진동수를 갖는 조사광이 원자 매질을 통과할 때, 강한 결합광에 의한 효과로 매질에 흡수되지 않고 투과하는 양자 간섭 효과로써 원자결맞음 현상의 가장 대표적인 현상 중의 하나이다. EIT 현상은 Boller 등에 의해서 고출력 펄스 레이저를 이용하여 Strontium 증기에서 처음 관측된 이후, 여러 가지 원자와 분자의 증기 셀, 원자 빔, 고체, 냉각된 원자, 그리고 BEC(Bose Einstein condensate)상태에서 다양한 연구들이 이루어지고 있다. (중략)