高亮度绿光半导体激光器光纤耦合系统设计与仿真 
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Objective In the field of ultrashort pulse laser, green laser diodes (LDs) as the pump source of Ti: sapphire laser have received increasing attention. It has been reported that increasing the power of the green laser-pumping source can further improve the output power of Ti: sapphire lasers. However, compared with gallium arsenide-based materials in the near-infrared band, gallium nitride-based materials with a wider bandgap can radiate green light and are prone to produce defects in the growth process, which reduce the radiation efficiency and output power. The maximum power of a single LD commercially available is 1.5 W. Thus, it is necessary to use a beam combining technology to improve the output power. Presently, the incoherent beam combining methods, such as spatial or polarized combination, are mostly used to improve the output power of LD modules. However, it is found that the combined beam, in most cases, is a rectangular beam array. When such a beam is focused by a coupling lens, the angular filling factor (AFF) is low (AFF is defined in Section 2.1), resulting in wasting part of the angle space. If the beam filling can be performed in this region, the brightness of the fiber output can be improved further. The brightness of the pump source is also the key factor to increase the output power of the titanium sapphire laser. This study proposes a method for increasing the AFF to fully use the angle space by combining beams in a closest-packed structure. Based on the proposed method, a high-brightness fiber-coupled green LD module is demonstrated using ZEMAX.
Methods To solve the problem of low AFF and the inability of the numerical aperture of a fiber to be fully used when the beam in a rectangular array is coupled into the fiber, we propose to arrange the beams according to a closest-packed structure. Based on this arrangement, we design a high-brightness fiber-coupled module of green LDs. First, we divide 14 1.5-W green LDs into two groups of 7 each and placed them on the six vertices and centers of the regular hexagon to form a dense structure. Then, the beams emitted from green LDs are collimated, respectively, by the fast and slow axis collimators to form a series of parallel beams. Afterward, the beams at the vertices are moved toward the center beam through a custom-designed beam reducer. Thus, the dark area between the beams can be eliminated while the beam divergence angle is constant. Second, beams emitted from one of the two groups pass through a half-wave plate, and the polarization direction is rotated 90°, then two groups of beams with perpendicular polarization direction are combined into a beam through a polarization beam combiner. Finally, the beam is focused through an aspheric lens into a fiber with a core diameter of 105 μm and a Na of 0.15.
Results and Discussions After collimation, the output beam of LD is approximately circular symmetry (Fig. 3). The divergence angles in the fast and slow axes are 0.262 and 0.750 mrad, respectively (Table 1), which can meet the design requirements. Then, the combined beam passes through the beam reducer and eliminates the dark area between the beams. The combined beam diameter in the fast and slow axis is 6.88 and 7.83 mm, respectively (Fig. 7). When the beam is coupled into an optical fiber, the AFF is calculated to be 63.80%, which is higher than that of the rectangular beam (Eq (3)). The simulation results show that the power coupled out of the fiber is 19.13 W—corresponding to a brightness of 3.125 MW·cm -2·Sr -1—and the fiber-coupling efficiency is 93.75%. Notably, to describe the concept of the AFF clearly, we deliberately collimate the output of the LD into an approximate circular spot in the near field, and it result in an asymmetry of the spot focused on the facet of the fiber in Fig. 9. The asymmetry can be improved in practical applications by combining multiple beams into a circular beam, replacing the subspot in Fig. 7.
Conclusions In summary, we propose a design of an optical fiber coupling module based on the closest-packed structure of combined beam array. It can fully use the numerical aperture of fiber and improve beam brightness. The simulation shows that 14 1.5-W LDs can be coupled into an optical fiber with a numerical aperture of 0.15 and core diameter of 105 μm, and a total power of 19.13 W can be obtained with an output brightness of 3.125 MW·cm -2·Sr -1—corresponding to a fiber-coupling efficiency of 93.75%. In the future, experiments should be performed to provide a better pumping source for Ti: sapphire solid-state lasers.
1 引言
绿光半导体激光器(LD)具有体积小、稳定性高和波长可调制等优点,已被广泛应用于激光存储、激光医疗和激光显示等领域[1-5]。近年来,绿光LD作为钛宝石激光器的泵源之一,在超短脉冲激光领域越来越引起人们的关注。2017年,中国科学院西安光学精密机械研究所王向林等[6]使用了两个功率为1.45 W的绿光LD泵浦钛蓝宝石晶体,获得输出能量为232 mJ的脉冲激光,并指出增大绿光泵浦源的功率可进一步提高输出功率。然而,相比于近红外波段的砷化镓基材料,宽禁带氮化镓基材料在生长过程中极易产生位错和沟槽等缺陷,这些缺陷会引起非辐射复合,进而导致LD的发光效率和输出功率降低[7-8]。处于业内领先水平的日本制造商日亚化学生产的绿光LD,其单管功率只能达到1.5 W[9],因此必须采用光束合束的技术来进一步提高输出功率。2020年,中国科学院半导体研究所Miao等[10]将多单管合束功率为21.68 W的绿光LD模块(耦合光纤芯径为200 μm,数值孔径(NA)为0.22)作为Ti∶S泵浦源,获得的输出功率为1.36 W,该研究指出不仅需要高功率的绿光LD泵源提高输出功率,高亮度的绿光LD泵源(可提升聚焦光束的瑞利长度)也是提升钛蓝宝石固体激光器输出功率的关键因素之一。目前,文献[ 11-14]采用了空间或偏振等非相干合束的方法来提高绿光LD模块的输出功率,但相关报道中的组合光束一般呈矩形阵列排布,光束通过耦合透镜聚焦到光纤的角度填充系数小,但这会浪费了部分角度空间。如果能够在该区域进行光束填充,需要充分利用光纤的NA以进一步提升输出激光的亮度。
本文报道一种基于正六边形平面密堆积分布的光束阵列的非相干合束方法,设计一个平行平板光束的缩束装置。该装置可在不改变光束发射角和不引入额外像差的条件下缩小光斑阵列的直径,消除相邻发光单元之间的暗区。经过理论计算以及光学仿真验证,该模块可以将14只功率为1.5 W且单偏振态的绿光单管耦合进芯径为105 μm和NA为0.15的多模光纤中,光纤输出功率为19.13 W,对应亮度为3.125 MW·cm-2·Sr-1,可为实现钛宝石激光器的高亮度和高功率绿光LD泵源提供一条可能的途径。
2 LD光纤耦合的基本参数
2.1 光纤输出亮度
当描述LD光纤耦合的过程时,采用光束参数积(BPP)来描述LD的光束质量更为方便[15],BPP可以定义为
式中:ω为光束的束腰半径;θ为远场发散的半角。由(1)式可知,BPP只与ω和θ相关,与波长λ无关。在数学关系上,BPP是光束衍射倍率因子M2的λ/π倍[16],因此BPP还可用来衡量不同波长激光束的聚焦效果。对于一定M2的激光束,波长越短则聚焦效果越好。
激光源的亮度(B)是激光在应用过程中的一个重要参数,可以同时反映激光的输出功率和光束质量。B与输出功率P和BPP之间的关系可表示为
在相同的功率下,B与BPP的平方成反比,BPP值越小,光束质量越好,亮度也越高。对于光纤输出的光束而言,耦合光纤的芯径和数值孔径越小,则输出光束的亮度越高。理论上,在相同的功率下,对于芯径为105 μm和NA为0.15的光纤,其输出亮度是芯径为200 μm和NA为0.22的光纤的7.8倍。
2.2 光束进入光纤的角度填充系数
正六边形密堆积分布和矩形分布的光束组合阵列[12]在光纤耦合透镜像方主平面上的光斑分布情况如
式中:Ωlaser为光束阵列中每一个子光束对应的立体角;Ωcircle为阵列光束外扩圆的接收立体角;Slaser为子光束在耦合透镜像方主平面上的光斑面积;Scircle为阵列光束外扩圆在透镜像方主平面上对应的范围面积;F为耦合透镜的焦距。由(3)式计算可知,正六边形密堆积分布的光束组合阵列的角度填充系数为77.78%,矩形分布的光束组合阵列的角度填充系数为63.67%。需要说明的是,
图 1. 组合光束阵列在像方主平面上的光斑分布。(a)密堆积阵列光斑分布;(b)矩形阵列光斑分布
Fig. 1. Spot distribution of composite beam array on main plane of image side. (a) Spot distribution of closest-packed array; (b) rectangular array spot distribution
3 光学设计
3.1 光纤耦合模块结构布局
根据2.2节的结论,设计的绿光光纤耦合模块如
3.2 快慢轴准直
光纤耦合模块中使用的绿光LD单管为TO(Transistor Outline)封装的边发射LD,发射波长为520 nm的多模偏振激光束,发光面的尺寸为1 μm×15 μm,距离TO管壳窗口1 mm。LD发光面的短边尺度极小,发射的光束接近衍射极限,发散角大(约为46°),称为快轴;与之垂直的发光面宽度大,其光束质量约为衍射极限的4倍,光束的发散角小(约为11°),称为慢轴。由于快、慢轴方向的光束质量差异较大,为了保证光束传输过程的光斑可控,需分别对LD的快、慢轴方向的光束进行准直。准直过程中,快轴方向选用F=2.27 mm和厚度为1 mm的非球面柱透镜(圆锥系数为-0.797,物方截距为1.612 mm,其大于TO管壳窗口距离),非球面柱透镜的准直可以有效减小大发散角光束的球差;慢轴方向选用F=10 mm,厚度为2 mm的球面柱透镜。当使用ZEMAX软件设置光源时,快轴方向设为高斯光束,慢轴方向设为超高斯因子H为10的光束[17]。准直后的光斑近似圆形,如
图 3. 光束准直图。(a)快轴准直的光线追迹;(b)慢轴准直的光线追迹;(c)LD准直后的输出光斑
Fig. 3. Light beam collimation diagram. (a) Ray tracing for fast axis collimation; (b) ray tracing for slow axis collimation; (c) output spot of LD after collimation
表 1. 准直后单管的光束参数
Table 1. Beam parameters of single tube after collimation
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3.3 光斑阵列缩束
因TO封装的绿光LD的固有尺寸会导致发光芯片之间存在10 mm的距离,不同单管输出的光束之间存在大片暗区,因此需要对光束进行缩束,使光斑紧密集中在轴心光束的周围。若使用传统的望远镜系统对光束进行缩束,仅能压缩光束尺寸而不能有效消除暗区,因此不能增大耦合聚焦的光束角度填充系数。依据传输过程中BPP值不变的原理,光束经过望远镜系统缩束后发散角将变大,同时望远镜系统的透镜会引入球差等额外像差,这会进一步破坏光束质量,不利于提高光纤耦合效率。
为了解决上述问题,设计了阵列光束缩束装置,结构如
六边形顶点上的6只LD单管输出的准直光束经过缩束装置前、后两个平行平面折射后,完成径向平移,光束边缘与六边形中心轴上的LD单管输出光束的边缘相切。装置设计的α=45°,L=31.62 mm。当入射波长为520 nm绿光时,K9玻璃的n=1.5197,D=7.50 mm。缩束装置在不改变单个LD输出光束参数的条件下完成了阵列光束的径向平移,极大地减小了光束之间的暗区。光束经过该缩束装置后,合束光束的发散角不变,但直径缩小,组合光束的BPP值降低5.63 mm·mrad,这有利于耦合进数值孔径更小的光纤,从而提高模块的输出亮度。
图 7. 单组模块的光斑图。(a)单组模块缩束前输出光斑;(b)单组模块缩束后输出光斑
Fig. 7. Light spot pattern of single module. (a) Output light spot before bunch reduction of single group module; (b) output light spot after bunch reduction of single group module
3.4 光纤耦合
光纤耦合需要满足如下条件[18]
式中:Dlaser为聚焦后光斑直径大小;θlaser为光束聚焦后远场发散半角;Dfiber为光纤芯径,值为105 μm;θfiber为NA=0.15的光纤对应的最大入射角,值为150.57 mrad;θ'为快慢轴准直后光束发散半角中的最大值,值为0.750 mrad。光纤耦合的必要条件是聚焦后的光斑应小于芯径,且入射角小于光纤最大接受角。由(5)~(8)式可知,耦合聚焦镜的焦距必须满足
若组合光束未经过缩束装置直接耦合,则单管间的固有尺寸会导致组束光斑之间有大量空隙。快轴方向上的ωfast=19.84 mm,慢轴方向上的ωslow=22.97 mm。将缩束前的光斑数值代入(9)式,F值既要大于101.17 mm又要小于69.95 mm,显然无解。由此可知,组合光束在未经过缩束装置径向缩束的情况下无法耦合进芯径为105 μm和NA为0.15的光纤中。
将缩束后的光斑数值代入(9)式,得到耦合聚集镜的F的取值范围为(34.35,69.95)。由此可知,耦合聚集镜的F值应该大于34.35 mm且小于69.95 mm,缩束后的耦合聚集镜的F有解。
考虑到缩小模块体积,仿真选择F=35 mm的耦合聚焦镜。为了减小聚焦光斑的弥散尺寸,耦合聚焦镜采用消除球差的非球面镜进行设计。该非球面镜后表面为球面,前表面采用的面型为高阶偶次非球面,表达式为
式中:z为面矢高;r为径向坐标;c为曲率;K为圆锥系数;k为大于0的整数;a为各阶非球面系数。该非球面聚焦镜的表面设计参数如
表 2. 聚焦透镜的设计参数
Table 2. Design parameters of focusing lens
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使用ZEMAX软件对整个系统的光线追迹进行仿真,结果如
图 9. 光纤耦合效果图。(a)光纤端面的聚焦光斑;(b)光纤输出的光斑
Fig. 9. Fiber coupling effect diagram. (a) Focusing spot on fiber facet; (b) spot of optical fiber output
需要说明,为了突出光束阵列与光纤耦合的AFF这一概念,令LD快、慢轴准直后光斑之间的距离在近场中较接近,这样光束阵列在快、慢轴方向的发散角差异大,从而导致聚焦在光纤端面上的光斑对称性差,占据光纤端面的尺寸小,如
4 结论
针对矩形阵列光束聚焦到光纤端面的角度填充系数低,以及未能充分利用光纤数值孔径的缺点,提出按平面密堆积排布的光束阵列的光纤耦合方式以最大化角度填充系数,从而提升光纤耦合输出的亮度。基于该排布方式设计无像差平行平板缩束装置,相比于传统望远镜缩束系统,该装置可保持光束发散角不变,消除光束暗区,改善组合光束的光束质量。ZEMAX仿真结果表明,采用该缩束装置可将14支功率为1.5 W的绿光单管耦合进NA为0.15和芯径为105 μm的光纤中,从而获得功率为19.13 W的半导体绿光,输出亮度为3.125 MW·cm-2·Sr-1,对应的光纤耦合效率为93.75%。下一步可以开展实验研究,为LD泵浦钛蓝宝石固体激光器提供更优质的泵浦源。本文中的密堆积及缩束方法不限于绿光LD,可推广到任意波长的LD光纤耦合设计,从而提高光纤耦合的角度填充系数,进而提升光纤耦合的输出功率和亮度。
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Article Outline
丁兵, 赵鹏飞, 段程芮, 娄博杰, 林学春. 高亮度绿光半导体激光器光纤耦合系统设计与仿真[J]. 中国激光, 2021, 48(5): 0501016. Bing Ding, Pengfei Zhao, Chengrui Duan, Bojie Lou, Xuechun Lin. Design and Simulation of High Brightness Fiber Coupling System of Green Laser Diodes[J]. Chinese Journal of Lasers, 2021, 48(5): 0501016.
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