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两路光束合成示意图如图1所示。根据高斯光束在自由空间的传输公式,给出两路光束分别在各自空间坐标系中沿Z轴传输的表达式。其中一路为理想光束,另一路为存在位置偏移及倾斜误差的光束。对于两个坐标系其变换关系为:
式中:
${\theta _x}$ 为光束沿着$x$ 轴($yo{\textit{z}}$ 平面)转过的角度;${\theta _y}$ 为光束沿着$y$ 轴($xo{\textit{z}}$ 平面)转过的角度。因此,可以对两路中心波长不同的激光合成光束质量进行数值仿真。在仿真中,采用光束传输因子(BPF)作为光束质量的评价参数,其计算式为:
式中:
$P$ 为目标处半径为$1.22\lambda L/D$ 大小的桶中功率,L为传输距离,D为光束发射口径的外接圆直径。在仿真中,理想光束中心波长为1080 nm,存在误差的光束中心波长为1070 nm,计算时取二者均值,发射面每个光束直径为${d_0}$ ,具体仿真参数如表1所示。$\lambda $/nm ${\omega _0}$/cm ${d_0}$/cm $L$/m $D$/cm 1075 0.5 2 1000 1.5 Table 1. Simulation parameters
仿真中,使用国产光学系统虚拟仿真平台seelight软件模拟不同位置和倾斜误差情况下的光束传输,计算不同情况下的光束传输因子BPF。
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位置误差是指各输出光束在保持平行的条件下,在输出面其相对位置发生变化,它反映了光轴整体的平移误差。在数值仿真过程中,相对位置的不确定度是通过在原中心位置坐标
$({x_1},{y_1})$ 上分别加上一个随机数组$({\delta _{{x_1}}},{\delta _{{y_1}}})$ 实现的。计算结果如图2所示,仿真中以发射面光束直径为基准,光束坐标偏移成规则变化的情况。可以看到随着位置误差的增大,BPF值越来越小,当位置误差控制在1.95内时,可获得理想光束质量95%的效果,因此,为了获得较高的BPF值,有必要尽可能控制位置误差的偏移。 -
所谓倾斜误差,指各输出光束的光轴并不是完全平行的。实际上,由于实验操作和器件的误差要实现完全的平行也是不可能的。若假设一条理想光轴,那么这种倾斜误差可以看作是各输出光束的光轴与理想光轴之间的夹角。计算结果如图3所示,仿真中以存在光轴倾斜的光束半发散角为基准,光轴倾斜误差成规则变化的情况。可以看到随着倾斜误差的相对增大,BPF值越来越小,尤其是倾斜误差大于0.4时,合成光束质量显著退化。因此,为了获得较高的BPF值,需要精确控制光轴的倾斜。
2.3 kW fiber laser spectral beam combination based on dichromatic mirror
doi: 10.3788/IRLA20200385
- Received Date: 2020-11-05
- Rev Recd Date: 2020-12-11
- Available Online: 2021-02-07
- Publish Date: 2021-02-07
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Key words:
- laser technology /
- fiber laser /
- spectral beam combining /
- dichromatic mirror
Abstract: Spectral beam combining technology based on dichromatic mirror can overcome the limitation of the output power limit of one single-mode fiber laser, which is an effective technical means to obtain laser output with high power and perfect beam quality. Theoretically, the influence of the beam position shift and tilt error on the quality of the combined beam was preliminarily explored. The results show that the beam tilt error has a significant influence on the output characteristics of the combining system. In the experiment, the combining experiment of two narrow linewidth fiber lasers was carried out. Using dichromatic mirrors as the combining element, a high beam quality and common-aperture combining with 2355 W combined output power was achieved, the beam quality factor M2 was 1.9, and the efficiency was greater than 99%, which proves that the dichromatic mirror has high efficiency for both the reflection and transmission cases. The experiment result shows that it is possible to achieve a common aperture laser output with higher power and better beam quality by further increasing the number and power of the channel.