Auxiliary Resonant Scanner to Increase the Scanning Capability for Coarse Integral Holographic Displays
Schematic of the scanned holographic display system. (a) The two-stage scanning structure, which has one resonant scanner and one 2-axis galvanometer scanner. (b) The sub-vertical dither scan pattern. By taking advantage of the high scanning ability of the auxiliary scanner, the overall visual extent of the system is able to be expanded by multiple times.
The light propagation reconstruction feature supported by holography can provide all necessary 3D visual information, in theory. Therefore, holographic display has been considered as the promising candidate of the futuristic 3D image display for decades since the image hologram was realized.
However, there are some constraints limiting the applications of 3D holographic display technique. One of the major challenges to realizing a holographic display is to provide sufficient distribution capabilities for the high amount of optical information from spatial light modulators (SLMs). Nowadays a single SLM can achieve the rate of more than 10 Gpixels/s, nevertheless, all this information needs to be evenly distributed in space to tile up the targeted visual scope (image size and viewing angle). Scanning this information by a galvanometer (rotating mirror), however, is limited by the mechanical limitation, and this method doesn't support to distribute the information at the rate of 10 Gpixels/s or more.
To deal with this difficulty, the research team led by Prof. Daping Chu has proposed a scanned holographic display system which takes advantage of a high-speed resonant scanner to augment a galvanometer and hence improves the opto-mechanical information distribution capabilities, thereby potentially achieving the increased image sizes and the enlarged viewing angles. This work is published in Chinese Optics Letters, Volume 15, No. 4, 2017 (Jhen-Si Chen, et al., Auxiliary Resonant Scanner to Increase the Scanning Capability for Coarse Integral Holographic Displays).
In the proposed scheme, a resonant scanner is combined with a 2-axis galvanometer, to introduce another sub-dimensional scanning. It takes advantage of the high speed of the resonant scanner (because of the small mirror) to scan information into a larger but still small area before the galvanometer further distributes this information to a larger area.
This two-stage scanning structure fully utilizes the scanning capability of the two kinds of scanners and shows its potential to support distributing information rate of 50 Gpixels/s. The present work will play an essential role in the investigation of the coarse integral holographic video displays for achieving the target of a full-bandwidth, large image size and field of view (FOV).
Further work will focus on developing scalable, tile-able, and integrated coarse integral holographic video display systems by using the auxiliary resonant scanner, and approaching the challenges of applying these techniques to different 3D holographic displays systems.
双镜合璧，提升全息显示扫描性能
图片说明：两级扫描方案实现信息高效分配：将一个共振扫描振镜和一个两轴电流计扫描振镜集成在一起，利用前者的高扫描速率和后者的大扫描镜面，可以有效实现信息的分配。（a）两级扫描结构示意图；（b）亚维度扫描原理图（亚垂直颤振扫描线）。
理论上讲，全息显示技术中光传播重构特征能够提供构建三维图像所需的全部三维视觉信息。因此，自全息图实现以来，这一技术一直被广泛看好，它被许多人视为未来三维图像显示的不二之选。
然而，全息显示技术实现应用也面临着一些需要克服的障碍。主要挑战之一便是如何分配来自空间光调制器的大量光信息。当前单个空间光调制器能达到高于10 G 像素每秒的信息率，但所有这些信息需要在空间上均匀地分布，以便根据图像尺寸和视场角拼接成目标视觉范围。通过电流计扫描振镜来传递这些信息受到其自身机械性能的限制，无法满足每秒分配10 G像素或更高速率的信息量的要求。
为了克服这个困难，剑桥大学初大平教授课题组提出了一种扫描式全息显示系统的方案。该方案利用一种共振扫描振镜，与电流计扫描振镜实现互补，从而提高了信息分配能力，满足了大图像尺寸和广视场角的需求。相关研究成果发表在Chinese Optics Letters 2017年第4期上（Jhen-Si Chen, et al., Auxiliary Resonant Scanner to Increase the Scanning Capability for Coarse Integral Holographic Displays）。
该研究工作以疏积分全息显示为例，提出一种共振扫描振镜与两轴电流计扫描振镜相结合，引入另一亚维度扫描的方法。由于该共振扫描振镜的扫描镜尺寸相对较小，可以工作在高速扫描模式，把输入光信息扫描成面积更大但对电流计扫描振镜而言仍然较小的一个区域，以便进一步分配这些信息到一个更大的区域。
利用这种两级扫描结构，可以充分发挥两种扫描振镜各自的优势——共振扫描振镜的高扫描速率和电流计扫描振镜的大扫描镜面，进而有效实现信息的分配。实验结果表明这种方法具有高达每秒分配50 G 像素信息量的潜力。这项工作对于以大图像尺寸、广视场角和全带宽为需求进行疏积分全息显示的研究有着极为重要的价值。
后续工作主要是对提出的技术方案的性能进行深入的分析，研究在长时间工作的情况下，该系统传输性能的稳定性。
该课题组后续将开展基于辅助共振扫描器的可扩展、可片铺和多系统集成的疏积分三维全息显示的研究工作，以及将辅助共振扫描器具体应用到不同的三维全息显示系统中去。