-
需要整形的是德国DILAS公司生产的LDM-3V-75A-BDS型半导体激光器。该激光器拥有16个发光单元,距光源1m处光束的光强分布如图 1所示。各发光单元间在慢轴(水平)方向上存在与发光单元宽度相当的无光区,且各单元光束互不平行,相邻两发光单元中心轴线间的夹角为19.2mrad,每一发光单元的中心轴线相对光轴方向的张角依次分别为±9.6mrad,±28.8mrad,±48.0mrad,±67.2mrad,±86.4mrad,±105.6mrad,±124.8mrad,±144.0mrad,各发光单元在快轴(竖直方向)和慢轴(水平方向)的发散角分别为38.4mrad和4.6mrad,像散为0.10537cm。
按照几何光学理论,光强均匀度μ定义为:
$ \mu = \frac{{{I_{\rm{u}}} - ({I_{\max }} - {I_{\min }})}}{{{I_{\rm{u}}}}} $
(1) 式中,Imax为表面最强辐射照度,Imin为表面最弱辐射照度,Iu为表面平均辐射照度。为了使此激光器能够用于无线电力传输,需要对其方向性、均匀性和光斑大小进行整形,具体目标为:激光斑在远场(约2m)形状是边长为10cm的方形,光束发散角控制在10mrad内,均匀度不低于60%,整形系统对光强的损失尽量小。为此要对各发光单元光束分别进行慢轴方向的平行化,实现混束效果,再分别进行快慢轴方向的扩束准直,得到矩形光斑,最后对矩形光斑进行均匀化处理,以得到符合要求的矩形光斑。
-
由于需要整形的半导体激光器直接输出的激光为16条分离光束,且各光束间互不平行,为此首先采用了16个顶角不同的光楔对每束光的传播方向进行调节,使其中心轴线全部平行于光轴方向。调节原理如图 2所示。设光楔顶角为α,当激光束以角度i入射到光楔表面时,根据折射定律,可知其出射角β与α的关系为:
$ \alpha = \frac{\beta }{{n - 1}} $
(2) 式中,α为光楔的顶角,β为光束经过光楔后传播方向变化的角度,n为玻璃的折射率。选择适当的光楔顶角α,可使出射光束沿水平方向;同时,由于各激光束在水平方向均为发散的,故当达到某个距离后各发光单元间的无光区会消失。所以,当光楔放置在特定位置上时,可以将16个发光单元发出的光束调整为沿一束光轴方向的平行光。
在此基础上,对光束慢轴方向进行准直,并且将光束的截面光斑整形。对于低功率半导体激光器,由于其发光单元的填充因子小于0.3,慢轴发散角较小,直接利用在快轴准直透镜后叠加慢轴方向微柱透镜的方式进行光束准直即可。但对于要整形的半导体线阵激光器,为了获得高输出功率,其发光单元的宽度明显增大,填充因子达到0.5[17]。这就意味着入射光束在慢轴方向上的总发散角远比低功率激光器大得多,此时再使用微透镜整形方式将面临许多技术上的困难,为此选择柱透镜对其慢轴进行准直。再考虑到对光斑截面大小及形状的要求,光束也需要扩束。综合考虑,选用了倒置的伽利略望远镜系统[18]。该系统由一个平凹透镜和一个平凸透镜按光学间隔Δ=0的方式组合而成。
垂轴放大率M为:
$ M = - \frac{{{f_2}}}{{{f_1}}} = \frac{{{D_2}}}{{{D_1}}} $
(3) 式中,f2为平凸透镜的焦距,f1为平凹透镜的焦距,D2为出瞳直径,D1为入瞳直径。扩束倍数与两柱透镜的焦距比有关。
而经慢轴准直后,光束在慢轴方向的发散角可以认为只由平凸透镜的焦距f2和平凹透镜的焦距f1决定。故可以通过合理选择两个柱透镜的焦距,使光束的截面光斑形状为10cm×10cm的方形,并且将光束的慢轴发散角准直到实验需求的范围内。
经过上述整形后,激光器发出的16条光束被整形成为一条光束,且截面光斑形状为10cm×10cm的方形,慢轴发散角也已符合要求。但均匀度较差,且快轴方向的发散角不符合要求,因此还要对光束进行均匀化和压缩快轴发散角的整形。常用的快轴准直方式分为宏观透镜准直法和微透镜准直法。而关于光束能量均匀化,描述激光束能量均匀度γ的公式为:
$ \gamma = 1 - \frac{{\sum\limits_i^m {{E_i} - \bar E} }}{{m\bar E}} $
(4) 式中,Ei为采样点数值,E为采样平均值,m为采样点个数。由于经过上述整形后的光斑整体强度呈带状分布,且每个带宽内部不同位置的光强也有差别。所以要得到均匀度好的光斑,不仅要对矩形光斑整体上进行均匀化处理,还要对每个发光单元发出的光束进行再均匀化。对这种形式的光斑,难以采用传统柱透镜均匀化的方法[19]。综合考虑,本系统中采用了平凸棱镜组对光斑进行均匀化,并对快轴进行准直。当入射光经过平凸棱镜组时,由于棱镜单元的光学作用形成彼此独立传播的光通道,每束光通道内光能量的均匀度会优于入射光,经过棱镜后,光通道内的能量叠加于光屏的同一区域,叠加后重新生成的光斑,其能量均匀度会远高于初始入射光斑。最终截面光斑的均匀度与棱镜曲面的弧度值θ、光束入射到棱镜的位置、角度等情况相关;而当光束通过平凸棱镜后,快轴方向的发散角也会被准直。最终快轴方向的发散角可以认为只与激光器本身的性能参量,即棱镜距离光源的位置d以及棱镜曲面的弧度值θ有关。因此不仅棱镜对应面的弧度、棱镜与光源的距离必须合适,还要保证每个棱镜的宽度正好等同于对应发光单元发出的光束在此位置处的宽度,长度略大于光束在此位置处的长度,且每个棱镜中心轴线必须对准相应发光单元的中心轴线位置,使每个发光单元发出的光束刚好完全透过棱镜,才能达到预期的整形效果。
-
确定了整形系统的光学结构后,接下来再对其进行数值仿真及优化。首先在ZEMAX中模拟出16个发光单元的发光情况。在非序列模式下,可以利用多个位于同一平面、彼此平行、间距相等的二极管光源来模拟, 效果如图 3所示。在离输出端500mm处光强分布,模拟激光束与实际激光束数据基本一致。
然后在ZEMAX中输入光楔-棱镜-曲面镜组整形系统的参量,由于激光器发出的光束线阵具有对称性,故设计出8对对称的光楔即可。光楔的玻璃材质均选用常见的BK7玻璃,类型选择为多边形物镜。各项参量见表 1。
Table 1. Parameters of optical wedge
wedge length/
mmheight/
mmthickness/
mmdeflection
angle α/
(°)wedge
angle β/
(°)1 150 9.38 12.68 8.25 15.99 2 150 9.38 12.31 7.15 13.86 3 150 9.38 11.94 6.05 11.72 4 150 9.38 11.58 4.95 9.59 5 150 9.38 11.23 3.85 7.46 6 150 9.38 10.87 2.75 5.33 7 150 9.38 10.52 1.65 3.20 8 150 9.38 10.17 0.55 1.07 平凹、平凸透镜以及棱镜组(棱镜组中每个棱镜的参量均相同)的玻璃材质也选择BK7玻璃,类型均选择为环形透镜。各项参量见表 2。
Table 2. Parameters of flat concave lens, flat convex lens and prism lens
flat
concave
lenslength/
mmheight/
mmcentral
thickness/
mmedge
thickness/
mmfirst
radius/
mmsecond
radius/
mm160 50 5 10 66 infimum flat
concave
lenslength/
mmheight/
mmcentral
thickness/
mmedge
thickness/
mmfirst
radius/
mmsecond
radius/
mm160 160 20 3 infimum 197 prism lens length/
mmheight/
mmthickness/
mmfirst radius/
mmsecond radius/
mm160 9.5 5 545 infimum 整形系统的整体仿真结构如图 4所示。
为了在保证输出光斑形状、均匀性的前提下,最大限度地提升激光传输系统的能量传输效率,还需要对系统进行整体性能优化。先在ZEMAX序列模式里调用afocal image space函数,用以优化子午面和弧矢面内的波像差及光线高度,以保证输出光斑为10cm×10cm的方形;然后在ZEMAX非序列模式里调用NSDD与NSTR函数,以优化光斑均匀度及系统整体传输效率,以保证系统在输出光束符合要求的前提下,拥有最大的能量传输效率。
优化后,出射光斑具体状况如图 5所示。光斑形状为边长10cm的正方形,光束发散角为6mrad,光强均匀度达到了82%, 系统的整体能量传输效率为86%。即使在远场范围(2m附近)匀光效果仍较好,在1.5m~4m范围内光斑大小也基本没有变化。ZEMAX仿真结果表明,该系统可达到预期的整形目标。
高功率半导体激光器光束整形的设计和实现
Design and implementation of beam shaping for high power semiconductor lasers
-
摘要: 为了使线阵半导体激光器光束能更好应用于激光远程无线电力传输,设计了基于光楔-曲面镜-棱镜组的线阵半导体激光束整形系统,采用数值计算方法,取得了系统中各元件的参量及理论整形效果。在此基础上加工出实物元件,搭建整形系统。实验中测得整形后的激光光斑尺寸为9.9cm×9.6cm,能量均匀度为68.9%,系统能量传输效率为71.3%,光束质量可满足接收端的光电池对激光空间均匀性的要求。最后分析了仿真系统与实验系统间产生差异的原因。结果表明,该系统可同时实现激光束阵列快轴和慢轴方向的扩束与准直,并能够调节输出光斑的形状及光强均匀度,且采用光学元件数量较少。光电池组件是激光无线电力传输过程的关键元件,该设计对激光转换效率的研究有较重要的实用价值。
-
关键词:
- 光学设计 /
- 光束整形 /
- 线阵半导体激光器 /
- 光楔-棱镜-曲面镜组
Abstract: In order to make the laser beam of linear-array semiconductor laser be better used in laser remote wireless power transmission, a linear-array semiconductor laser beam shaping system based on the set of optical wedges, curved mirrors and prisms was designed. The parameters of components in the system and the theoretical shaping results were derived by numerical calculation. After then the realistic components were processed and the experimental shaping system was built. The experimental results were that the laser spot size after shaping was 9.9cm×9.6cm, energy uniformity was 68.9%, and energy transfer efficiency was 71.3%. The beam quality could meet the requirement of light cell at receiving end for laser space uniformity. The reason of the difference between the simulated and experimental system was analyzed. The results show that the system can simultaneously realize the expanding and collimation of laser beam array along fast axis and slow axis. The system can also adjust the shape and the uniformity of output light spot with less optical components. Light cell components are the key processes of laser wireless power transmission. The study has great practical value for laser conversion efficiency. -
Table 1. Parameters of optical wedge
wedge length/
mmheight/
mmthickness/
mmdeflection
angle α/
(°)wedge
angle β/
(°)1 150 9.38 12.68 8.25 15.99 2 150 9.38 12.31 7.15 13.86 3 150 9.38 11.94 6.05 11.72 4 150 9.38 11.58 4.95 9.59 5 150 9.38 11.23 3.85 7.46 6 150 9.38 10.87 2.75 5.33 7 150 9.38 10.52 1.65 3.20 8 150 9.38 10.17 0.55 1.07 Table 2. Parameters of flat concave lens, flat convex lens and prism lens
flat
concave
lenslength/
mmheight/
mmcentral
thickness/
mmedge
thickness/
mmfirst
radius/
mmsecond
radius/
mm160 50 5 10 66 infimum flat
concave
lenslength/
mmheight/
mmcentral
thickness/
mmedge
thickness/
mmfirst
radius/
mmsecond
radius/
mm160 160 20 3 infimum 197 prism lens length/
mmheight/
mmthickness/
mmfirst radius/
mmsecond radius/
mm160 9.5 5 545 infimum -
[1] BLACKWELL T. Recent demonstration of laser power beaming at DFRC and MSFC[C]//AIP Conference Proceeding Beamed Energy Propulsion. New York, USA: American Institate of Physics, 2005: 73-85. [2] GEORGE T, TOBIAS K, BEN S. High power semiconductor laser cheer the United State light beam transmission competitions[J]. Laser & Optoelectronics Progress, 2008, 45(10):64-66(in Chinese). [3] HOFFMAN J M. To infinity and beyond!—How to get fuel to future interplanetary vehicles: beam it up to them with lasers[J]. Machine Design, 2007, 79(5): 78-88. [4] LIU X G, HUA W Sh, LIU X. Experimental investigations of laser intensity and temperature dependence of single crystal silicon photovoltaic cell parameters[J]. Chinese Journal of Lasers, 2015, 42(8):0802011(in Chinese). doi: 10.3788/CJL [5] YIN Zh Y, WANG Y F, JIA W W, et al. Performance anglysis of beam integrator system based on microlens array[J]. Chinese Journal of Lasers, 2012, 39(7):29-35(in Chinese). [6] YANG P, YANG Y N. The research of monocrystalline silicon solar cells efficiency under laser[J]. Laser Technology, 2012, 36(5):696-699(in Chinese). [7] BERGER J, WELCH D F, STREIFER W, et al. Fiber-bundle coupled, diode end-pumped Nd:YAG laser [J]. Optical Letters, 1988, 13(4):306-308. doi: 10.1364/OL.13.000306 [8] QIAO L, YANG Y N. The experimental research of laser wireless power transmission efficiency[J]. Laser Technology, 2014, 38(5):590-594(in Chinese). [9] LIU L, GUO Sh F, LU Q Sh, et al. Study of thermal distortion in composite slab amplifiers [J]. Chinese Journal of Lasers, 2010, 37(7):1678-1682(in Chinese). doi: 10.3788/CJL [10] GAO Q H. Research on high power semiconductor laser beam shaping[D]. Xi'an: Xidian University, 2005: 25-34(in Chinese). [11] DU K, BAUMANN M, EHLERS B. Fiber-coupling technique with micro step-mirrors for high-power diode-laser bars [C]// Advanced Solid State Lasers. Miami, USA: Fiber-Optic Components, 1997: 390-393. [12] XING Sh Sh, RAN Y H, JIANG H B, et al. Illumination mode conversion system design based on micromirror array in lithography[J]. Acta Optica Sinica, 2015, 35(11): 94-103(in Chinese). [13] LEI Ch Q, WANG Y F, YIN Zh Y, et al. Homogenization system for diode laser stack beams based on microlens array[J]. Chinese Journal of Lasers, 2015, 42(5): 0502009(in Chinese). doi: 10.3788/CJL [14] CLARKSON W A, HANNA D C. Two-mirror beam-shaping technique for high-power diode bars [J]. Optical Letters, 1996, 21(6):375-377. doi: 10.1364/OL.21.000375 [15] DENG X, LIANG X, CHEN Z, et al. Uniform illumination of large targets using a lens array [J]. Applied Optics, 1986, 25(3):377-381. doi: 10.1364/AO.25.000377 [16] LI Z, LI W, LV H, et al. A novel method for respiration-like clutter cancellation in life detection by dual-frequency IR-UWB radar [J]. Microwave Theory and Techniques, 2013, 61(5):2086-2092. doi: 10.1109/TMTT.2013.2247054 [17] WANG Y F, LEI Ch Q, YIN Zh Y, et al. Collimation of high fill factor diode laser slow axis beam [J]. Laser & Optoelectronics Progress, 2015, 52(6):061402(in Chinese). [18] WANG Zh X, PAN Y M, YIN Sh Y, et al. Laser processing lens of long focal depth and high resolution[J]. Acta Optica Sinica, 2013, 33(2):191-195(in Chinese). [19] DICKEY F M, HOLSWADE S C. Laser beam shaping:theory and technology [M]. New York, USA:Marcel Dekker Press, 2000:273-283.