雷射投影系統中以純相位式空間光調變器消除雷射光斑雜訊之研究

Study of Reducing Laser Speckle Noise by Utilizing a Phase-Only Spatial Light Modulator in Laser Projection System

張晏碩(Yan-Shuo Chang)

指導教授 ： 林晃巖

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.01312

摘要

在雷射投影的光學成像系統中由於雷射的高度同調性，使得在螢幕端會產生建設性及破壞性干涉的亮暗交雜的光強分佈，這就是“光斑 (speckle)現象”，然而光斑現象被認為是在影響影像品質的雜訊，因此減弱或消除光斑的影響便成了在雷射投影光學系統上的主要目標之一，在本論文中我們改良已經發表的常見的雷射投影系統，可達到簡化系統架構、抑制光斑現象與提高影像品質，並且在整個系統中加入特殊設計的數位全像片(computer generated hologram, CGH)，使得本質的光斑(intrinsic speckle)被抑制，我們並以此系統探討降低光斑的方法及將其量測出來。
本論文中我們將介紹雷射投影顯示的概述，之後介紹光斑的現象、原理以及整理抑制光斑的方法，其分為極化分集、波長分集、角度分集與其他元件設計四大類別，在我們所研究的抑制光斑方法中以光斑對比 (speckle contrast, SC) 作為我們判別光斑嚴重程度的重要參數和相關係數 (correlation coefficient) 來輔助優化數位全像片的設計。
在我們的系統中，以反射式液晶空間光調變器取代常見抑制光斑的方法中的移(轉)動式擴散片。藉由使用遞迴傅立葉演算法 (iterative Fourier transform algorithm, IFTA) 設計出數位全像片，其可以將雷射光束整形 (beam shaping) 成方形，使雷射光能完整照明在幻燈片上以增進投影效率，同時也利用相位調變的技術來取代移(轉)動的擴散片，如此一來可以避免機械式的轉動或者移動，而且也能以相位調變來降低光斑對比，也透過分析數位全像片間的相關係數了解到光斑對比值在實驗與理論上差距的主因。然而以這個系統為基準下再探討光束整型後的光，利用相位補償的方式設計另一種數位全像片我們稱為，相位補償數位全像片，使光斑對比可以藉由設計相位接近全建設性干涉而達到光斑對比下降而且均勻度上昇。
在本論文中也以組合式全彩的方式分析在我們的系統中使用全彩光源，經過我們的抑制光斑的設計後，分析光斑對比去討論。未來可以使用實際全彩光源來探討分析此雷射投影系統的光斑抑制效果，也可以將數位全像片連續播放與設計相位補償數位全像片做結合來探討光斑抑制情形。

關鍵字

雷射投影顯示系統 ； 雷射投影機 ； 光斑消除 ； 光斑抑制

並列摘要

A speckle distribution is a mixed bright and dark intensity distribution with some constructive and destructive interference which is caused by the high coherence of laser light. It is of practical significance because it introduces noise in Laser Projection Display Systems (LPDS), which damages the image quality. Therefore, speckle suppression is one of the main goals in LDPS. In this dissertation, the general LPDS is optimized to simplify the system, suppress the speckle on the illumination part of the LDPS and improve the image quality by more uniform illumination. A Computer Generated Hologram (CGH) specially designed to reduce the intrinsic speckle from the uniformly illuminated part of the LPDS is added to the system. We discuss the speckle suppression method we design and measure it in the optimized LPDS.
In this dissertation, we will introduce the laser projection display, the speckle phenomenon, the speckle principle, and speckle suppression methods. We separate speckle suppression methods into four parts: polarization diversity, wavelength diversity, angular diversity, and other element designs. The Speckle Contrast (SC) and Correlation Coefficient (CC) are important parameters that can determine the degree of speckle. We design our CGH by analyzing the SC and CC.
In our LPDS, we utilize the reflective spatial light modulator (SLM) to replace the moving diffuser, which is a common speckle suppression method. We use the Iterative Fourier Transform Algorithm (IFTA) to design a CGH which can shape the beam for improving the projection efficiency and modulate the phase to replace the moving diffuser. In this way we can avoid mechanical rotation or vibration and reduce the SC by phase modulation on the illumination part of the LPDS. Moreover, we realize that the main reason for the difference in SC between experiment and theory after analyzing the CC between the front and back CGHs is sequential time. Based on this system, we designed another CGH by phase compensation that we call “compensation CGH”. It can approach full constructive interference to suppress the speckle and improve the uniformity on the illuminated part of the LPDS at the same time.
In this dissertation, we discuss the SC to use the combined analysis of a three color laser light source for simulating the white light source in our LPDS. We can use the real white light laser to carry out experiments for realizing the variation of the SC. Moreover, we can combine the CGHs in sequential time and the compensation CGH to discuss the variation of the speckle phenomenon.

並列關鍵字

laser projection display system ； laser projector ； speckle reduction ； speckle suppression

參考文獻

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