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摘要:
提出了一种基于光纤耦合的光纤激光阵列像差探测方法，介绍了其结构和波前复原过程，采用数值仿真模拟其复原湍流像差的过程，并进行了7单元自适应光纤准直器(AFOC)阵列复原静态像差的实验。仿真结果表明，本文提出的波前传感方法能够有效复原出湍流畸变波前，且对于不同单元数的六边形排布阵列，存在不同的最优复原阶数。阵列填充因子的降低会增大复原残差，填充因子大于0.8可保证复原残差RMS相较于填充因子为1时的增幅不超过10%。实验结果表明，利用填充因子为0.875的7单元AFOC阵列，复原以离焦为主的低阶像差时，初始畸变波前RMS为0.433 μm，复原残差小于0.075 μm。仿真和实验结果验证了本文提出的基于光纤耦合的光纤激光阵列像差探测方法的有效性。该技术有望在激光阵列大气传输及湍流校正等系统中得到进一步应用。
Abstract:A new method of wavefront sensing based on fiber coupling in the fiber laser array has been proposed. The scheme and the recovery process of this sensor are introduced. Numerical simulations of detecting the turbulence-induced aberrations utilizing such method and experiments of recovering static aberrations with 7-element adaptive fiber optics collimator (AFOC) array are presented. Numerical results show that such sensor could effectively recover the wavefront with turbulence-induced aberrations. For hexagonal array with different units, the optimum reconstructed Zernike mode is also different. Smaller array filled factor leads to larger recovery residual error. Compared with array filled factor of 1.0, value of 0.8 is easy to obtain and brings in recovery residual error increment less than 10%. Experimental results reveal that RMS less than 0.075 μm of the recovery residual error is obtained when detecting the static aberration with 7-element AFOC array with filled factor of 0.875. The aberration is with RMS of 0.433 μm and mainly includes Zernike modes of low orders like defocus. Results here validate the effectiveness of the wavefront sensing method proposed here. Such method would get further application in systems like laser array propagating and turbulence aberrations correcting.
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Key words:
- wavefront sensing /
- adaptive optics /
- fiber laser array /
- fiber coupler
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Overview: Coherent beam combining (CBC) of fiber amplifiers with a master-oscillator-power amplifier (MOPA) architecture is a promising way for brightness scaling with excellent beam quality. Fiber laser array, as a typical CBC architecture, has been widely applied in laboratory experiments. Further application aims at eliminating the turbulence-induced dynamic aberrations. The correctable aberrations of the fiber laser array are tip/tilts and pistons distribute on the sub-aperture. Target-in-the-loop (TIL) technique cooperating with optimal method is the only way reported to achieve CBC in atmosphere. Such method suffers from low bandwidth due to large array scale and long air path. Active detecting the atmospheric aberrations becomes necessary. Conventional wavefront sensor, like Hartmann-Shack, needs beam zooming and splitting in the back end of the telescope. Direct spatial beam zooming and splitting in the back end of fiber laser array system is impossible because the system is discrete in space. Meanwhile, setting a splitter large enough to cover the whole array aperture is inconceivable and breaks the compact and flexible character of the fiber laser array architecture. A new method of wavefront sensing based on fiber coupling in the fiber laser array has been proposed. The scheme and the recovery process of this sensor are introduced. Numerical simulations of detecting the turbulence-induced aberrations utilizing such method and experiments of recovering static aberrations with 7-element AFOC array are presented. Numerical results show that such sensor could effectively recover the wavefront with turbulence-induced aberrations. For hexagonal array with different units, the optimum reconstructed Zernike mode is also different. Smaller array filled factor leads to larger recovery residual error. Compared with array filled factor of 1.0, value of 0.8 is easy to obtain and brings in recovery residual error increment less than 10%. Experimental results reveal that RMS less than 0.075 μm of the recovery residual error is obtained when detecting the static aberration with 7-element AFOC array with filled factor of 0.875. The aberration is with RMS of 0.433 μm and mainly includes Zernike modes of low orders like defocus. Results here validate the effectiveness of the wavefront sensing method proposed. Such method would get further application in systems like laser array propagating and turbulence aberrations correcting.
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图 1 基于自适应光纤准直器阵列的波前传感器的结构示意图
Figure 1. Scheme of the wavefront sensor based on adaptive fiber optics collimator array
图 3 不同单元数的正六边形排布的AFOC阵列。(a) 7单元；(b) 19单元；(c) 37单元
Figure 3. Hexagonal AFOC array with different element number. (a) 7-element; (b) 19-element; (c) 37-element
图 4 模拟湍流相位屏及其经AFOC阵列聚焦形成的光斑阵列。(a) D/r0=10的一帧模拟相位屏；(b) AFOC内透镜阵列聚焦形成的光斑阵列
Figure 4. Phase screen and its focal spot array focused by the AFOC array. (a) Simulated turbulence phase screen with D/r0=10; (b) Spot array focused by the lens in the AFOC array
图 5 子孔径波前倾斜量反演结果。(a) X方向倾斜量；(b) Y方向倾斜量
Figure 5. Results of sub-aperture wavefront slope detected. (a) X-direction; (b) Y-direction
图 6 不同阵列单元数和不同复原阶数下的统计平均波前复原残差RMS值
Figure 6. Normalized RMS curve of the recovery residual error as a function of array element number and reconstructed Zernike mode
图 7 不同单元数AFOC阵列复原出的波前。(a)湍流相位屏；(b) 7单元复原结果；(c) 19单元复原结果；(d) 37单元复原结果
Figure 7. Reconstructed wavefront with AFOC array of different element number. (a) Sample phase screen; (b) 7-element; (c) 19-element; (d) 37-element
图 8 不同阵列单元数和不同阵列填充因子下波前复原残差的归一化RMS值曲线。(a) 7单元；(b) 19单元；(c) 37单元
Figure 8. Normalized RMS curve of the recovery residual error as a function of array element number and filled factor. (a) 7-element; (b) 19-element; (c) 37-element
图 9 基于自适应光纤准直器阵列的波前探测实验装置图
Figure 9. Experimental setup of the wavefront sensing based on AFOC array
图 10 根据电压反演得到的单孔径上倾斜量测量结果。(a) X方向；(b) Y方向
Figure 10. Sub-aperture slope detected through the voltages. (a) X-direction; (b) Y-direction
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