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水导激光技术的核心是激光能够在水束中传输,前提条件是激光焦斑直径小于水束直径和激光能够在水束中发生全反射作用。
激光在喷嘴孔入口面与水束进行耦合对准,如图 1所示。
为避免激光烧蚀喷嘴,影响实际加工质量,激光光斑尺寸与水束直径在喷嘴孔处关系必须满足以下公式:
$ 2 w <d_{\mathrm{w}}=\alpha d_{\mathrm{n}} $
(1) 式中,w为激光焦点半径,dw为水束直径,dn为喷嘴孔直径,α为收缩系数。
由于水束的缩流效应[17],实际水束直径小于喷嘴孔直径。对喷嘴处进行流场仿真分析(如图 2所示),得出收缩系数αs=dw/dn≈0.83。在水束下方放置一反射镜,再利用工业相机——电荷耦合器件(charge-coupled device,CCD)观察到喷嘴孔入水口面处,如图 3所示。喷嘴孔中亮斑即为实际水束直径,可得收缩系数αe≈0.83。所得参数与参考文献中接近[18-19],下文中收缩系数α默认为0.83。
激光能够在水束中发生全反射作用是激光能够在水束中传输的重要条件。水束为圆柱形水射流,激光在水中的折射率保持不变,所以水束可视为多模阶跃折射率光纤。基于光在光纤中的传输特性及光线理论分析,激光在水束光纤中的传输可以分为子午光线和斜光线传输。子午光线的传输路径必经过光纤中心轴,与此相反,与光纤中心轴不相交的光线为斜光线[20]。水束光纤中子午光线和斜光线占比不同也会对激光功率密度分布情况产生影响,如图 4所示。
激光在喷嘴出射的水束与空气界面形成全反射,根据斯奈尔折射定律,由图 4a中几何关系可以得到子午光线在水束中发生全反射时的临界角θw与光线进入水束时的最大入射角θa:
$ \sin \varphi_{\mathrm{w}}=\cos \theta_{\mathrm{w}}=\frac{n_{\mathrm{a}}}{n_{\mathrm{w}}} $
(2) $ \theta_{\mathrm{w}}=\arcsin \left[\sqrt{1-\left(\frac{n_{\mathrm{a}}}{n_{\mathrm{w}}}\right)^2}\right] $
(3) $ \theta_{\mathrm{a}}=\arcsin \left[\sqrt{\left(\frac{n_{\mathrm{w}}}{n_{\mathrm{a}}}\right)^2-1}\right] $
(4) 式中,na和nw分别为激光在空气与水中的折射率,φw是水束中子午光线发生全反射时临界角的余角,也即子午光线与法线的夹角。
斜光线与光纤中心轴不相交,由图 4b中几何关系得斜光线在水束中发生全反射时的临界角θw′与斜光线进入水束时的最大入射角θa′:
$ \sin \theta_{\mathrm{w}}{ }^{\prime} \cos \gamma_{\mathrm{w}}{ }^{\prime}=\sqrt{1-\left(\frac{n_{\mathrm{a}}}{n_{\mathrm{w}}}\right)^2} $
(5) $ \theta_{\mathrm{w}}{ }^{\prime}=\arcsin \left[\frac{1}{\cos \gamma_{\mathrm{w}}{ }^{\prime}} \sqrt{1-\left(\frac{n_{\mathrm{a}}}{n_{\mathrm{w}}}\right)^2}\right] $
(6) $ \theta_{\mathrm{a}}{ }^{\prime}=\arcsin \left[\frac{1}{\cos \gamma_{\mathrm{w}}{ }^{\prime}} \sqrt{\left(\frac{n_{\mathrm{w}}}{n_{\mathrm{a}}}\right)^2-1}\right] $
(7) 式中,γw′是水束中斜光线在横截面处的投影与法线的夹角。
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激光在水束中传输时,由于水对激光的吸收、散射等作用存在,激光能量随着水束长度的变化而存在一定的衰减。当激光传输距离较短时,激光在水中的衰减规律符合比尔-朗伯定律(Beer-Lambert law):
$ P=P_0 \exp (-\beta L) $
(8) 式中,P0和P分别是传输距离为0和L时的激光功率(W);β是包括吸收和散射在内的衰减系数(m-1)。
由(8)式可知,激光在水束中的传输效率与衰减系数和传输距离有关,通过对比不同水束长度下激光功率变化设计实验。
设计如图 7所示实验系统以及图 8所示检测系统示意图,搭建后的实验系统整体实物图如图 9所示。利用CCD工业相机辅助激光与水束耦合对准后,在水束下方放置一挡水板,在水束中耦合传输后的激光束穿过挡水板到达下方的激光接收靶面上,利用XYZ移动平台将激光与接收靶面中心对准,并通过z轴调整水束长度,检测水束下方激光功率。激光功率检测时不添加衰减片。
实验中采用的是分离式镜片安装,环境中灰尘颗粒与镜片安装调整等存在误差,导致激光与水束耦合对准前激光功率与激光器显示功率存在一定差异,这里分别对激光器出射激光功率、激光经镜片传输后聚焦镜下方、耦合对准后水束中的激光功率进行多次测量取平均值,分析对比凸透镜聚焦后的激光束在不同水束长度下传输效率。
水束中激光功率与水束传输激光的长度有关,随着水束传输长度的增大,激光功率逐渐减小;此外,在激光传输过程中环境、镜片、保护玻璃以及用来做挡水板的亚克力玻璃等都会对激光存在一定的衰减作用。如图 9所示搭建实验平台,采用珠海市粤茂激光型号为YMS-20F光纤激光划片机、北京研邦科技有限公司的VLP-2000-50W型号功率计,通过调整下方XYZ移动平台中z轴来检测不同水束长度下激光功率。利用4mm厚亚克力玻璃进行1064nm激光衰减效率实验,得出亚克力玻璃的透过率Ta=92%,而蓝宝石玻璃保护窗口对1064nm激光透过率为Ts=86%。从表 1也可以看出,从激光器出射的激光在镜片间传输过程中存在一定损耗,实际到达耦合对准前的激光功率P1只有激光器出射激光功率P0的91.4%,Pz为激光与水束耦合后不同水束长度段的激光功率。通过对激光在与水束耦合前后功率计算,可得在水束长度lz分别为20mm,30mm,40mm和50mm时,激光在耦合前后的功率传输效率ηz=Pz/(P1Ta),分别为63.6%,55.9%,48.2%,39.5%。通过对比不同水束长度下的激光功率传输效率,可以看出水束长度越短,激光传输效率越高,因此为提高耦合效率,在实际加工时避免激光能量损失过大,在水束稳定长度范围内,加工工件表面与耦合装置下方相距应不宜太长。
Table 1. Laser power before and after laser coupling with water-jet
outgoing laser power P0/W laser power before coupling P1/W laser power of different water-jet lengths Pz/W 20mm 30mm 40mm 50mm 1 0.920 0.538 0.466 0.408 0.337 2 1.836 1.074 0.950 0.800 0.668 3 2.716 1.586 1.405 1.233 0.984 efficiency ηz/% 63.6 55.9 48.2 39.5 -
针对第2节中对不同段水束长度时的激光功率密度分布仿真分析,采用图 8所示的激光功率密度分布检测系统结构,采用德国Cinogy公司的CinCam CMOS 1024相机型号的光束分析仪,考虑光束分析仪探测面的损伤阈值,在挡水板下方放置光密度值为1.0的衰减片,衰减片只影响不同激光功率下的激光强度,不改变水束中激光功率密度分布情况。测定了不同功率(0.2W,0.5W,1.0W)、不同压力(0MPa,1MPa,2MPa)以及不同水束长度(20mm,30mm,40mm,50mm)下的激光功率密度分布情况。
图 10是不同功率下水束中激光散斑分布情况。通过对比激光功率0.2W,0.5W,1.0W时的水束长度20mm处的激光功率密度分布情况,可以看出,随着激光功率的增大,激光光斑分布区域也逐渐增大,输入激光功率的增大必然会导致水束中输出激光功率的增大,衰减片对激光边缘能量的衰减作用也会相应减弱,从而导致激光光斑整体形状尺寸逐渐变大。对于图 11中不同压力条件下,激光散斑的尺寸大小、亮度以及分布情况都存在一定的变化,特别是散斑从0MPa时的相对集中分布到2MPa时的散斑分布更均匀化。当水压变化时,一定范围内水速的增加有利于水束的稳定,从而激光在水束中全反射效率也更高,激光的传输也更为稳定。在激光功率0.5W,压力1MPa条件下,如图 12所示,随着水束耦合传输长度的增大,水束中激光散斑的尺寸大小、亮度逐渐变小、变暗,这一部分原因是由于激光在水束中全反射的次数的差异,更大一部分原因是水束对激光的衰减作用,从第3.1节也可以看出,随着水束传输长度的增大,激光功率逐渐减小。由于激光传输聚焦过程中的环境洁净度、镜片调整以及水束脉动等导致检测结果存在些许偏差,但变化趋势是符合仿真分析结果的。
Figure 10. Distribution diagram of laser power density in water-jet with diffe-rent power when water pressure is 1MPa and coupling transmission length is 20mm
Figure 11. Distribution diagram of laser power density in water-jet with diffe-rent power when laser power is 0.5W and coupling transmission length is 40mm
Figure 12. Distribution diagram of laser power density in water-jet with diffe-rent power when laser power is 0.5W and water pressure is 1MPa
通过对图 12中激光功率0.5W、水压1MPa和水束传输长度20mm时激光功率密度分布情况进行截面分析,取x轴截面, 如图 13所示。可以看出,激光在水束中传输后仍具有一定的高斯分布特性。
水导激光技术中水-光耦合传能规律研究
Study on energy transmission law of water-laser coupling in water-jet guided laser technology
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摘要: 水-光耦合传输效率是实现水导激光可加工性的前提与效率保证。为了研究水导激光中水-光耦合传能规律, 得到较高的水束中激光功率传输效率和均匀的激光功率密度分布, 采用光线追迹原理及物理光学传播方法, 仿真分析了1064nm激光束聚焦后的光束特性及水-光耦合后水束中激光光斑分布形态, 并对不同水束长度下激光功率传输效率, 以及不同功率、压力和水束长度下激光功率密度分布情况进行了系统的实验检测分析。结果表明, 随着水束长度的减小, 1064nm激光在水束中功率传输效率越高, 在水束长度为20mm时, 激光功率传输效率可达63.6%;激光功率的变化对水束中激光功率密度分布影响最大; 当激光功率不变时, 在水束稳定长度范围内水压的增大有利于水束中激光功率密度均匀化分布, 而耦合水束长度的减小可以提高激光传输效率。研究结果为提高水导激光中能量利用率有一定的指导意义。Abstract: Water-laser coupling transmission efficiency is the premise and efficiency guarantee of water-jet guided laser machinability. In order to study the law of water-laser coupling energy transmission in water-jet guided laser, and to obtain high laser power transmission efficiency and uniform laser power density distribution in water beam, by using ray tracing theory and physical optics propagation method, the simulation analysis of the focused beam characteristics at 1064nm and the speckle beam pattern after coupling were carried out. And the laser power transmission efficiency under different water beam length, the distribution of laser power density under different power, pressure and water beam length were tested and analyzed systematically. The results show that the power transmission efficiency of 1064nm laser increases with the decrease of the water beam length, and the power transmission efficiency can reach 63.6% when the water beam length is 20mm. The variation of laser power has the greatest influence on the distribution of laser power density in water beam. When the laser power is constant, the increase of water pressure is beneficial to the uniform distribution of laser power density in the stable length of water beam, and the decrease of coupled water beam length can improve laser transmission efficiency. The research results provide some guidance for improving the energy utilization rate of water guided laser.
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Table 1. Laser power before and after laser coupling with water-jet
outgoing laser power P0/W laser power before coupling P1/W laser power of different water-jet lengths Pz/W 20mm 30mm 40mm 50mm 1 0.920 0.538 0.466 0.408 0.337 2 1.836 1.074 0.950 0.800 0.668 3 2.716 1.586 1.405 1.233 0.984 efficiency ηz/% 63.6 55.9 48.2 39.5 -
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