在繞射投影模組中作光斑現象與相位分佈範圍關係之研究

Abstract

中文摘要 近年來，隨著雷射開始被應用在顯示光源上，「光斑」此一嚴重影響影像品質之現象已逐漸被探討與重視。當高度同調性的輻射源，如雷射光及超聲波，被表面粗糙度為波長數量級的表面所散射或反射時，一個高對比度、細緻的光斑圖案將被觀察者或偵測器所觀測接收。由於光斑現象在影像處理應用上的種種不利因素，多年來世界上許多研究團隊已致力於降低此種強度噪音現象。在國立台灣大學顯示光學實驗室之研究中，我們已針對光斑問題提出了一個雷射光學投影系統的新穎模組，此模組乃基於一種純相位繞射光學元件 (DOE) 設計，藉由調制液晶空間光調變器 (SLM) ，可同時達成雷射光束整形與降低光斑對比。 在本論文中，我們對於光斑現象提出了一個全新的看法，即我們將其光場干涉之本質視為其與光場相位分佈範圍有相關性，並以此概念探討影像品質與透過此觀點更進一步降低光斑現象之可能性。我們對投影顯示科技及光斑現象在雷射投影系統上之影響作全面性的歷史回顧，我們闡述光斑現象之基本性質及其統計之數學表示。我們以統計學方法來推導光斑對比值與相位分佈範圍的函數關係，並在理論證明上發現隨著影像相位分佈範圍降低會得到較低的光斑對比值。根據理論推演的結果，我們設計出兩種依據遞迴傅立葉演算法 (IFTA) 來達到低光斑對比值之實驗架構方法：第一種架構為利用特殊設計的IFTA直接在繞射面上同時產生雷射光束整形與光場相位分佈範圍限制之方法；而另一種架構則是先利用IFTA產生均勻的方形光場分佈，再透過一個純相位元件來達到光場相位分佈範圍限制。我們設計數值模擬實驗模型來分析上述兩種根據IFTA所設計之實驗架構的影像品質。我們試圖以實際的光學系統來驗證數值實驗的結果。因此，我們選擇上述第一種方法以實驗室現有之系統實踐，並比較實際結果與模擬趨勢的差異。我們探討將模擬實驗結果實現到實際光學系統之可行性。

ABSTRACT In the recent years, lasers have been widely used as illumination sources, and a phenomenon called“speckle”that degrades image quality has been observed. When highly coherent radiation, such as laser or ultrasonic wave, is scattered or reflected from a rough surface, compared with wavelength, a high-contrast, fine-scale speckle pattern will be seen by an observer looking at the scattering light from rough surfaces. Because of the disadvantage of speckle in imaging applications, researchers have been engaged in minimizing the speckle noise. In the Display Optics Lab. of National Taiwan university, we have demonstrated a novel laser projection system based on the phase-only diffractive optical element (DOE), spatial light modulator (SLM), to implement beam shaping and suppress speckle noise. In this thesis, we present a neoteric view of speckle phenomenon that treats its optical field interference nature as its phase distribution range dependence of the field, and use this concept to discuss image qualities and possibility to further reduce the speckle effects. In the beginning, we provide an overall review of the projection display technology and laser system with speckle effects. Then, we introduce basic concepts and statistical presentation of speckle phenomena. Moreover, we deduce the speckle contrast value as a function of phase distribution range in Statistical Optics methods, and find out images with lower phase distribution range will have lower speckle contrast value. According to deductive results, we design two kinds of iterative Fourier transform algorithm (IFTA) based methods to achieve lower speckle contrast value. One is using modified IFTAs to implement laser beam shaping and optical field phase distribution range limitation on the diffractive plane. The other is using IFTA to implement an uniform rectangular field and passing by a phase-only element to achieve field phase distribution range limitation. Then, we set up simulation models to analyze image qualities for the two IFTA based methods. In the end, we attempt to use real optical system to implement the simulation results. Therefore, we choose the first method mentioned above to practice in our present systems, and compare the results to the simulation ones. Moreover, we discuss the possibility of applying simulations to the real optical systems.