我们开发了一种芯片大小的光源,它可以通过控制光的空间相位来发射任意光束形状。我们将此器件命名为 iPMSEL®(可积相位调制表面发射激光器,美国商标注册号 5410112)。
最近,LiDAR 技术在碰撞预防、测绘和 3D 测量等多个领域引起了广泛关注。LiDAR 需要可以扫描光束形状的光源。为此,合适的光源应满足几个条件:小尺寸、没有机械移动结构、以电子方式控制光束形状等。光源的尺寸和重量对于无人机尤其重要,因为无人机的重量和可用空间受到严重限制(请参见图 1)。小型光束扫描光源在其他领域也有应用前景,如胶囊内窥镜(图 2)和自然 3D 显示(图 3)。
图 1:iPMSEL 用途 1(无人机用 LiDAR)
图 1:iPMSEL 用途 1(无人机用 LiDAR)
图 3:iPMSEL 用途 3(3D 显示)
我们成功演示了一种静态 iPMSEL 器件,它可以发射任意静态光束形状。如图 4 所示,它可以从针尖大小的芯片发射指定的任意光束形状。尽管光束形状是静态的,但该技术的潜在用途包括 3D 测量和指示,例如抬头显示器。作为 3D 测量的一个示例,我们可以通过将多点、条纹和网格等光束形状投影到物体上,然后拍照,并根据投影的光束形状的失真来计算位置,从而测量 3D 形状(请参见图 5 左)。如果将光源小型化为针尖大小的芯片,则还可以实现便携式 3D 测量系统(请参见图 5 右)。我们还展示了超过 10,000 个点的大规模多点光束形状,这对于 3D 测量很有用(请参见图 6)。手机的面部识别和 3D 建模对 3D 测量系统有很大需求。因此,我们认为静态 iPMSEL 可用于这些用途。
除了开发可投影或切换静态光束形状的静态 iPMSEL 和阵列 iPMSEL 器件,我们还在开发可以从芯片动态发射和控制任意光束形状的动态 iPMSEL 器件。
图 4:静态 iPMSEL 的远场图案
图 5:3D 测量的原理
图 6:静态 iPMSEL 的大规模多点光束形状
静态 iPMSEL 与传统技术(DOE 和 VCSEL 的组合)的比较概念图如图所示。7.
由于静态 iPMSEL 的尺寸要小一个数量级,因此实现高两个数量级的密集集成是可行的,这使得我们在同一区域内可以切换的图案数量比传统技术多两个数量级。请注意,由于尺寸较小,光束发散度会增加。同时,静态 iPMSEL 中不显示垂直点噪声光束,我们认为它适合密集集成。
图 7:2D 图案光源概念图
*DOE:衍射光学元件,VCSEL:垂直腔体表面发射激光器
■自然 3D 显示
3D 物体发出的光线可以通过像素阵列再现,从而可以控制任意光束形状。这种类型的机理广泛用于“光场显示”中。为此,动态 iPMSEL 将来可以用作 3D 显示器的像素。当自然 3D 显示器开发成功并普及后,将使相距遥远的家庭成员和朋友之间可以进行如同面对面的沟通,从而成为不可或缺的新型沟通工具。
■LiDAR
最近,LiDAR 技术被应用于汽车碰撞预防、测绘和 3D 测量。普通 LiDAR 系统配备了机械光束扫描装置,使系统尺寸较大。开发芯片大小的没有机械移动结构的电子光束扫描系统将使系统的尺寸和重量大幅降低。例如,这种光源将是总重量和可用空间受限的无人机中的关键器件。我们相信 iPMSEL 将非常适合此类用途。
■胶囊内窥镜中的激光手术刀
涉及内窥镜的医疗检查可能特别痛苦。然而,胶囊内窥镜可以缓解诊断检查期间的插入疼痛。事实上,如果胶囊内窥镜配备可以控制任意光束形状的激光手术刀,将可以在体内同时诊断和治疗受影响的部位。我们认为 iPMSEL 将来可用于此目的。
■3D 测量
可通过以下简单过程测量 3D 形状。将多点、条纹和网格等光束形状投影到物体上并拍照,然后根据光束形状的失真来计算 3D 形状。到目前为止,我们已成功地从可以发射任意静态光束形状的静态 iPMSEL 器件发射了这样的光束形状。
■指示
静态 iPMSEL 器件可以发射字符甚至灰度光束形状,如图 4 所示。此外,可以从芯片级器件切换光束形状,如阵列 iPMSEL 的视频所示。因此,我们认为它们可以应用于光束形状数量有限的小型抬头显示器,或用于指示各种标记的芯片级指示装置。
在本节中,我们将介绍静态 iPMSEL 的结构和工作原理。该器件的概念图如图 8 所示。与普通半导体激光器类似,静态 iPMSEL 由熔覆层(限制它们之间的载体(电子和孔))和活性层(通过基板上的辐射再结合产生光)组成。与普通激光二极管相比,在活性层附近形成了新型的相位调制层,其中排列了亚微米级的孔。那么,该层的作用是什么? 如图 8 中的相位调制层的顶视图 SEM 图像所示,一些孔初看之下是周期性排列的。然而,这些孔会略微偏移正方形网格的周期格点。实际上,孔的重心在以正方形网格的格点 O 为中心的圆上旋转偏移。根据目标光束形状,利用计算机生成的全息图技术来确定每个孔的位置偏移。
图 8:iPMSEL 的结构
图 9:iPMSEL 中的面内共振原理
当电流注入到器件中时,由于辐射再结合,光波在活性层产生,并向不同方向发射。这些光波的一部分被耦合到活性层附近的相位调制层。
那么,相位调制层会发生什么? 这里,为了简单起见,我们考虑一个周期性孔阵列(2D 光子晶体),因为孔的偏移较小,可以忽略不计。当光波从左侧入射到周期性孔时,如图 9 所示,光波在每个孔处球状散射。请注意,图 9 中每个孔的散射波在四个方向(即上、下、左和右)上产生相长干涉,因为孔的周期对应于波长,这导致每个孔的散射波在四个方向上发生 2π 相移。这在物理学中称为“衍射”。当这四个衍射光波传播时,它们也衍射到相同的四个方向,相互耦合,并扩散到整个面内区域。请注意,反向传播波的耦合会产生驻波,驻波不向任何方向传播,而在一个固定位置振荡。因此,在相位调制层中会形成 2D 驻波。由于驻波的存在,相位调制层充当了激光器的谐振腔。(“2D 驻波的形成”。)
如图 10(a) 所示,当孔的周期等于光的波长时,四个面内衍射波也在垂直方向上衍射,并且平面波在垂直方向上发射。由于平面波是从扩展到数百微米的广阔区域发射的,因此它会产生一个光束发散角小于 1º 的窄光斑光束。
接下来,我们将直观地解释孔与周期性排列偏移的相位调制层中会发生什么。由于孔的位置偏移较小,因此在改变输出光束形状的同时,仍会形成 2D 驻波。图 10(b) 显示了从相位调制层衍射的输出光束形状的概念图。由于孔位置与周期位置偏移,垂直衍射波在局部继续/延迟,因此波前在空间上被调制(“光束形状的空间调制”)。基于我们在全息术方面的成熟技能,我们可以很容易地为任意目标光束形状设计“空间相位调制”。总之,相位调制层形成的“2D 驻波”充当了激光器的谐振腔,同时对输出光束形状进行“空间相位调制”。因此,静态 iPMSEL 器件可以从针尖大小的芯片发射任意光束形状。
图 10 iPMSEL 中的相位调制原理
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