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激光源采用18台光纤耦合输出、最大连续输出功率600 W左右的972 nm近红外半导体激光器。传输光纤的纤芯直径为440 μm、数值孔径NA为0.22。光纤输出端采用直径8 mm、带蓝宝石衬底的D80金属连接头连接,以确保其端面在大功率激光输出时的安全性。合束原理如图1(a)所示,18束光纤传输的激光束按照“矩阵方式”排列,每一束由光纤输出的发散激光束对应使用一个相同结构的准直单元,在进行空间合束之前实现准直和相互平行传输。完成准直的18束平行激光束通过共用的合束单元的合束叠加,获得具有单一光束形态且横截面为矩形的合束激光。
图 1 矩形光斑激光非相干空间合束原理。 (a) 18束激光束整体空间位置变化;(b)任意相邻两束激光的光斑空间位置变化
Figure 1. Schematic of laser incoherent space beam combining with a rectangular spot. (a) Changes in the space position of 18 laser beams; (b) Changes in the space position of two adjacent laser spots
通过对远场光斑搭接率的计算,可完成参与合束的准直激光束发散角、相邻光束间距的合理设计。图1(b)展示了相邻两束准直激光束在合束单元第一个光学入射面上的光斑中心间距d和传输一定距离z后的光斑位置的变化关系。假设任意相邻的两束平行准直激光束在合束单元第一个光学入射面上的光束半径分别为r1和r2,r1=r2=r,则根据激光束传输几何关系,任意相邻两束激光束穿过该入射面并传输了一定距离z后的光束半径r1z和r2z可由如下公式表示[10]:
$${x^2} + {y^2}={r_{1{{z}}}}^2$$ (1) $${(x - {d_{\textit{z}}})^2} + {y^2}=r_{2{\textit{z}}}^2$$ (2) $${r_{1{{z}}}}^2=2\tan {{\theta _c}} \times {\textit{z}} + r$$ (3) $${r_{2{{z}}}}^2=2\tan {{\theta _c}} \times \sqrt {d_{{z}}^2 + {{\textit{z}}^2}} + r$$ (4) $${d_{\textit{z}}}=d \times \left| {\frac{{F - {\textit{z}}}}{F}} \right|$$ (5) $${m_{\textit{z}}}=\frac{{r_{1{{z}}}^2 + d_{\textit{z}}^2 - r_{2{\textit{z}}}^2}}{{2d_{\textit{z}}^2}}$$ (6) 式中:d为合束单元第一个光学入射面处任意两束平行准直光束的中心间距,即相邻两个准直单元光轴间距;θc、dz和mz分别为穿过该入射面并传输一定距离z后两束激光束的发散半角、光斑中心间距、光斑边缘临界交叉点在Y轴上的坐标。
由公式(1)~(6)可推导出准直激光束在z位置时重合面积A、光斑搭接率η的计算公式[11]:
$$ \begin{split} A=&2\left[ {\mathop \int \nolimits_{{m_{\textit{z}}}}^{{r_{1{\textit{z}}}}} \sqrt {{r_{1{\textit{z}}}} - {y^2}} {\rm{d}}y + \displaystyle\mathop \int \nolimits_{{d_{\textit{z}}} - {r_{2{\textit{z}}}}}^{{m_{\textit{z}}}} \sqrt {r_{2{\textit{z}}}^2 - {{\left( {y - {d_{\textit{z}}}} \right)}^2}} {\rm{d}}y} \right]=\\ &\frac{{\rm{\pi }}}{2}\left( {r_{1{\textit{z}}}^2 + r_{2{\textit{z}}}^2} \right) + r_{2{\textit{z}}}^2\left[ {\arcsin \left( {\frac{{{m_{\textit{z}}} - {d_{\textit{z}}}}}{{r_{2{\textit{z}}}^2}}} \right) - \arcsin \Bigg(\frac{{{m_{\textit{z}}}}}{{{r_{2{\textit{z}}}}}}\Bigg)} \right] + \\& \left( {{m_{\textit{z}}} - {d_{\textit{z}}}} \right)\sqrt {r_{2{\textit{z}}}^2 - {{\left( {{m_{\textit{z}}} - {d_{\textit{z}}}} \right)}^2}} - {m_{\textit{z}}}\sqrt {r_{2{\textit{z}}}^2 - m_{\textit{z}}^2} \\[-10pt] \end{split}$$ (7) $$\eta =\frac{A}{{\pi r_{2{{z}}}^2}}$$ (8) 根据以上推导公式,准直激光束的发散半角、光斑半径及相邻光束间距对非相干空间合束效果产生直接影响。考虑到激光表面热处理产生的粉尘溅射对合束器安全性的影响,文中将合束单元的焦距F设定为500 mm。根据具体使用需求,将合束激光的焦斑尺寸设定为长30 mm (X轴)×宽10 mm (Y轴)。在传输距离z=F=500 mm处,设定单束激光光斑直径2r2z等于合束焦斑宽度。在傍轴近似下,准直激光束在合束单元第一个光学入射面处的半径r假设为0。根据公式(3)计算出θc=10.0 mrad。设定η>85%对应的传输距离差为合束长度Δz时,利用Matlab软件得到不同d值对应的光斑搭接率η与传输距离z之间的变化规律如图2(a)所示。若使Δz值尽量变长,则d值需要趋近于0。然而,D80光纤连接头的8 mm直径从结构上限制了d值不可能为0。故在确保D80光纤连接头按照矩形阵列分布并考虑合束器结构强度的基础上,d值设定为12 mm。
图 2 光束传播距离与合束激光光斑搭接率之间的变化规律。(a) r=0, θc=10.0 mrad, 不同d值; (b) θc=10.0 mrad, d=12 mm, 不同r值; (c) r=3.5 mm, θc=14.8 mrad, d=12 mm
Figure 2. Variation between the beam propagation distance and the overlapping rate of the combined laser spot. (a) r=0, θc=10.0 mrad, different values of d; (b) θc=10.0 mrad, d=12 mm, different values of r; (c) r=3.5 mm, θc=14.8 mrad, d=12 mm
根据光束衍射极限原理,准直激光束的r值实际不能为0。在F=500 mm、θc=10.0 mrad、d=12 mm的条件下,不同r值对应的光斑搭接率η与传输距离z之间的变化规律如图2(b)所示。r值越大,合束长度Δz越长。由于d值已被确定为12 mm,每套准直单元的光学透镜直径最大值ds将受到d值的限制,ds<12 mm。按照矩形阵列平行排列的18套准直单元由一块金属材料加工获得,同样基于合束器整体结构强度的考虑,ds值被设定为10 mm。文中将准直激光束的直径2r与ds值之间的比值设定为70%,以确保透镜通光面镀制的增透膜在高功率激光长期照射下保持稳定的高透射率,则r值设定为3.5 mm。
根据准直激光束光斑半径r与其发散半角θc之间的变化关系[11]:
$${\theta _c}=\arctan (D/(2f))$$ (9) $$r=D/2 + f \times \arctan {\theta _0}$$ (10) 式中:f为准直单元焦距;D=440 μm为光纤直径;θ0=arcsin0.22为光纤端面处激光束发散半角。
将r=3.5 mm代入公式(9)和(10),可计算出f=14.9 mm,θc则被修正为14.8 mrad。光斑搭接率η与传输距离z之间的变化规律如图2(c)所示。在d=12 mm、r=3.5 mm、θc=14.8 mrad的修正参数下,合束长度Δz的理论值约为215 mm。
将准直激光束的设定参数r=3.5 mm、f=14.9 mm、D=440 μm、θ0=arcsin0.22置入Code V光学设计软件,利用光线追迹法跟踪光学透镜表面发生折射的光线传播路径,模拟计算准直激光束的发散半角θc趋近于14.8 mrad的设定值,从而优化光学透镜的数量、通光面曲率半径、厚度及相邻透镜之间的间距。最终确定每套准直单元均由三个球面透镜(m1、m2、m3)构成,其结构模型及光学参数分别如图3(a)和表1所示。光纤输出端面F、三个球面透镜m1、m2、m3彼此间距分别为5.4 mm、9.4 mm和4.0 mm。
图 3 合束器的结构仿真模型。(a)准直单元-XZ平面;(b)合束单元-XZ平面;(c)合束单元-YZ平面
Figure 3. Structural model of the beam combiner structure. (a) Collimation unit-XZ plane; (b) Combining unit-XZ plane; (c) Combining unit-YZ plane
表 1 透镜参数 (单位:mm)
Table 1. Parameters of the lenses (Unit: mm)
Name Surface type Radius Thickness Diameter X Y m1 In: Sphere −72.1 −72.1 2.0 7.0 Out: Sphere +5.3 +5.3 m2 In: Sphere +20.0 +20.0 1.5 12.0 Out: Sphere −11.8 −11.8 m3 In: Sphere +30.0 +30.0 3.6 12.0 Out: Sphere +15.4 +15.4 M1 In: Sphere +115.35 +115.35 26.0 130.0 Out: Sphere ∞ ∞ M2 In: Sphere +114.02 +114.02 15.0 110.0 Out: Sphere +153.11 +153.11 M3 In: Sphere −138.23 −138.23 10.0 100.0 Out: Sphere +78.40 +78.40 M4 In: Cylinder ∞ +119.48 10.0 90.0 Out: Sphere ∞ ∞ M5 In: Sphere ∞ ∞ 3.0 90.0 Out: Sphere ∞ ∞ 在设计合束单元时,将准直激光束的设定参数r=3.5 mm、θc=14.8 mrad、d=12 mm置入Code V光学设计软件,通过光线追迹获得18束准直平行光在合束单元第一个光学入射面上的整体照射区域尺寸为87.5 mm (X轴)×宽31.0 mm (Y轴)。通过调整合束单元的光学透镜数量、通光面曲率半径、厚度及相邻透镜之间的间距,确保在z=F=500 mm位置处由光线追迹获得的合束焦斑尺寸趋近于30 mm (X轴)×宽10 mm (Y轴),即在合束单元第一个光学入射面与合束焦斑之间,18束平行准直激光束将在X轴方向、Y轴方向分别被实施8.75∶3(X轴)和3.1∶1(Y轴)的不等比例压缩成像。
将Code V软件建立的合束单元结构模型参数置入TracePro光学仿真软件,可仿真模拟合束激光横截面能量分布。在验证合束光斑形貌为矩形的基础上,通过优化设计合束单元结构模型,尽可能增加合束长度。优化设计后的合束单元结构模型如图3(b)和3(c)所示,合束单元由三个球面透镜(M1、M2、M3)、一个柱面透镜M4和一个平面窗口镜M5组成,其中球面透镜组(M1、M2、M3)对18束准直激光束照射区域实施X轴和Y轴同比例压缩成像,M4柱面透镜对18束准直激光束照射区域仅实施Y轴压缩成像。合束单元的最终光学参数如表1所示。准直单元的球面透镜m3、合束单元的光学透镜(M1、M2、M3、M4和M5)的相邻间距分别为6.4 mm、60.5 mm、16.8 mm、15.0 mm和5.0 mm。
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基于准直单元和合束单元的最终结构参数,当18束972 nm激光参与合束、每束激光连续功率为600 W时,沿激光合束方向(Z轴)不同位置处合束激光光斑的能量分布TracePro仿真如图4所示,合束激光光斑尺寸如表2所示。沿Z轴方向,将合束焦斑位置作为0点,图4(e)显示出合束焦斑为单一的矩形光斑形貌,焦斑尺寸接近于30 mm×10 mm的理论设定值。当仿真点与焦斑间距Δl分别为−100 mm、−50 mm、0 mm、+50 mm和+100 mm时,图4(c)~图4(g)分别描述了离焦条件下,矩形合束激光的光斑能量均呈现出类似于单束高斯激光束的聚合分布状态。图4(a)、4(b)、4(h)和4(l)所描述的矩形合束激光的光斑能量分布均出现分离散现象。图4(c)与图4(g)之间的相对距离为200 mm,而图4(b)与图4(c)之间、图4(g)与图4(h)之间相对距离均只有5 mm,可认为合束长度Δz的模拟值为200 mm,即图4(c)与图4(g)仿真点之间的距离。可以看出,合束光斑能量分布会在较短距离内发生剧烈变化,距离焦斑位置越远,合束激光的光斑能量分布越不均匀,合束质量越差。其主要原因在于18束激光束在合束单元的聚焦合束作用下完成了激光空间非相干合束,从合束焦斑的0 mm位置向±100 mm合束位置延伸中,相邻两束激光的光斑搭接率从99%降低至85%,彼此光斑中心间距逐渐增大导致85%的光斑搭接率成为定义合束长度和判断合束光斑能量分布产生聚合的临界值。
图 4 合束激光的光斑能量模拟分布。(a) Δl=−150 mm;(b) Δl=−105 mm;(c) Δl=−100 mm;(d) Δl=−50 mm;(e) Δl=0 mm; (f) Δl=+50 mm; (g) Δl=+100 mm;(h) Δl=+105 mm;(i) Δl=+150 mm
Figure 4. Spot energy simulation distribution of combined laser beam. (a) Δl=−150 mm; (b) Δl=−105 mm; (c) Δl=−100 mm; (d) Δl=−50 mm; (e) Δl=0 mm; (f) Δl=+50 mm; (g) Δl=+100 mm; (h) Δl=+105 mm; (i) Δl=+150 mm
表 2 合束激光的光斑尺寸(单位:mm)
Table 2. Spot size of the combined beam laser (Unit: mm)
Δl Simulation value Measured value Energy distribution −150 57.9×28.5 56.8×27.7 Splitting −105 50.1×22.6 49.0×21.4 Splitting −100 46.5×19.6 47.1×19.2 Aggregation −50 40.2×16.7 38.4×15.0 Aggregation 0 30.9×11.0 31.4×11.4 Aggregation +50 40.4×16.9 39.2×16.3 Aggregation +100 47.1×21.7 46.2×21.3 Aggregation +105 51.8×23.9 51.1×23.1 Splitting +150 58.5×29.1 58.1×28.5 Splitting
High power laser incoherent spatial beam combining with rectangular spot
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摘要: 激光表面热处理技术是进行金属材料表面强化和改性的最有效手段之一。为实现高速、柔性激光表面热处理,按照矩阵平行排列18束光纤输出的972 nm半导体激光束,通过光束准直和空间非相干合束,获得了具有矩形光斑特征的10 kW级合束激光。在理论分析准直激光束的半径、相邻光束间距与合束激光的光斑搭接率之间变化规律、采用Code V光学设计软件建立合束器结构模型及TracePro光学仿真软件模拟合束激光光斑能量分布的基础上,完成了10 kW级18×1矩形光斑激光非相干空间合束器的研制。在200 mm的合束长度内实现了具有单一矩形光斑形貌、最大合束功率10.249 kW、焦斑尺寸31 mm×11 mm、中心波长972.34 nm、谱线宽度2.27 nm的合束激光输出。Abstract: This work lays a foundation for promoting the application of incoherent spatial combining laser in laser surface heat treatment with high speed and flexible processing. 18 semiconductor laser beams at 972 nm output by the fiber were arranged in parallel according to the "matrix". By implementing beam collimation and incoherent spatial beam combination, a 10 kW combined laser beam with rectangular spot characteristics was obtained. The radius of the collimated laser beam, the distance between adjacent laser beams and the overlapping rate of the combined laser were theoretically analyzed, respectively. The structural model of the beam combiner was built using Code V software, and the spot energy distribution of the combined laser was simulated using TracePro software. Based on the above work, a 10 kW 18×1 incoherent spatial laser combiner of outputting a rectangular spot was developed. Within the combined length of 200 mm, the combined laser beam had a single rectangular spot shape. A beam combining power of 10.249 kW was achieved with a focal spot diameter of 31 mm×11 mm, a center wavelength of 972.34 nm and a linewidth of 2.27 nm.
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图 2 光束传播距离与合束激光光斑搭接率之间的变化规律。(a) r=0, θc=10.0 mrad, 不同d值; (b) θc=10.0 mrad, d=12 mm, 不同r值; (c) r=3.5 mm, θc=14.8 mrad, d=12 mm
Figure 2. Variation between the beam propagation distance and the overlapping rate of the combined laser spot. (a) r=0, θc=10.0 mrad, different values of d; (b) θc=10.0 mrad, d=12 mm, different values of r; (c) r=3.5 mm, θc=14.8 mrad, d=12 mm
图 4 合束激光的光斑能量模拟分布。(a) Δl=−150 mm;(b) Δl=−105 mm;(c) Δl=−100 mm;(d) Δl=−50 mm;(e) Δl=0 mm; (f) Δl=+50 mm; (g) Δl=+100 mm;(h) Δl=+105 mm;(i) Δl=+150 mm
Figure 4. Spot energy simulation distribution of combined laser beam. (a) Δl=−150 mm; (b) Δl=−105 mm; (c) Δl=−100 mm; (d) Δl=−50 mm; (e) Δl=0 mm; (f) Δl=+50 mm; (g) Δl=+100 mm; (h) Δl=+105 mm; (i) Δl=+150 mm
图 5 18×1矩形光斑激光空间非相干合束器。(a) 工程立体设计图;(b) 工程设计图Y-Z轴截面;(c) 合束器光纤连接端面照片;(d) 合束器整体照片
Figure 5. Photos of 18×1 laser incoherent space beam combiner. (a) Engineering three-dimensional design drawing; (b) Engineering design drawing Y-Z axis section; (c) Photo of the fiber connection end face of the combiner; (d) Overall photo of the combiner
图 6 合束激光射孔钢板的孔洞形貌图。(a) Δl=−150 mm;(b) Δl=−105 mm;(c) Δl=−100 mm;(d) Δl=−50 mm;(e) Δl=0 mm;(f) Δl=+50 mm;(g) Δl=+100 mm;(h) Δl=+105 mm;(i) Δl=+150 mm
Figure 6. Hole morphology of the combined laser perforated steel plate sample along the combined beam direction. (a) Δl=−150 mm; (b) Δl=−105 mm; (c) Δl=−100 mm; (d) Δl=−50 mm; (e) Δl=0 mm; (f) Δl=+50 mm; (g) Δl=+100 mm; (h) Δl=+105 mm; (i) Δl=+150 mm
图 8 18束激光合束前、后的激光光谱。(a) 18束激光独立光谱叠加;(b) Δl=−100 mm;(c) Δl=−50 mm;(d) Δl=0 mm;(e) Δl=+50 mm;(f) Δl=+100 mm的合束激光光谱
Figure 8. Laser spectrum before and after beam combination. (a) 18 laser beam independent spectrum superposition; Combined beam laser spectroscopy with (b) Δl=−100 mm; (c) Δl=−50 mm; (d) Δl=0 mm; (e) Δl=+50 mm; (f) Δl=+100 mm
表 1 透镜参数 (单位:mm)
Table 1. Parameters of the lenses (Unit: mm)
Name Surface type Radius Thickness Diameter X Y m1 In: Sphere −72.1 −72.1 2.0 7.0 Out: Sphere +5.3 +5.3 m2 In: Sphere +20.0 +20.0 1.5 12.0 Out: Sphere −11.8 −11.8 m3 In: Sphere +30.0 +30.0 3.6 12.0 Out: Sphere +15.4 +15.4 M1 In: Sphere +115.35 +115.35 26.0 130.0 Out: Sphere ∞ ∞ M2 In: Sphere +114.02 +114.02 15.0 110.0 Out: Sphere +153.11 +153.11 M3 In: Sphere −138.23 −138.23 10.0 100.0 Out: Sphere +78.40 +78.40 M4 In: Cylinder ∞ +119.48 10.0 90.0 Out: Sphere ∞ ∞ M5 In: Sphere ∞ ∞ 3.0 90.0 Out: Sphere ∞ ∞ 表 2 合束激光的光斑尺寸(单位:mm)
Table 2. Spot size of the combined beam laser (Unit: mm)
Δl Simulation value Measured value Energy distribution −150 57.9×28.5 56.8×27.7 Splitting −105 50.1×22.6 49.0×21.4 Splitting −100 46.5×19.6 47.1×19.2 Aggregation −50 40.2×16.7 38.4×15.0 Aggregation 0 30.9×11.0 31.4×11.4 Aggregation +50 40.4×16.9 39.2×16.3 Aggregation +100 47.1×21.7 46.2×21.3 Aggregation +105 51.8×23.9 51.1×23.1 Splitting +150 58.5×29.1 58.1×28.5 Splitting -
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