以空間光調變器取代動態擴散片於二階同調成像系統之應用與研究

Study and Application of Spatial Light Modulator Used as the Dynamic Diffuser in Second-order Coherent Imaging System

劉忠義(Chung-Yi Liu)

指導教授 ： 徐巍峰

國立臺北科技大學/電資學院/光電工程系研究所/碩士(2012年)

https://doi.org/10.6841/NTUT.2012.00221

摘要

使用同調光源(例如雷射)於光學成像系統時，可以投影出三維的立體影像或者是高解析的二維影像，然而，在同調的光學成像系統中無可避免地會出現光斑(Speckles)，使得影像中的微小特徵被光斑雜訊干擾而無法擷取，導致影像品質的下降，一般解決的方法是使用移動的擴散片(Diffuser)來消除光斑，但此舉免不了也會降低影像的解析度。
近年來，以相位式空間光調變器(Spatial light modulator, SLM)來取代擴散片成為一熱門之研究課題，希望能達到以快速顯示隨機相位陣列的方法來取代擴散片的機械式移動，進而達到減低光斑的影響。但是目前的空間光調變器的解析度不足，像素比較大，導致模擬擴散片的效果有限。在論文中，我們採用一個二階同調光學成像系統來探討此一問題，藉著改變第一階光學成像架構的像距來改變光斑大小(Speckle size)，也改變了光斑大小對放置在成像面上SLM像素大小的比值，因而在第二階的成像面上可以得知SLM上的隨機相位陣列對光斑現象的影響，進而發展有效的光斑消除方法。
實驗結果顯示，當第一階成像的光斑大小(繞射極限)大於空間光調變器的像素大小時，在第二階成像中的光斑的確被有效的降低。使用兩張光斑圖案疊加的光斑對比值(Speckle contrast)平均降幅約為0.71，這結果符合理論的預估。

關鍵字

光斑 ； 光斑對比值 ； 光斑大小 ； 空間光調變器

並列摘要

Using a coherent light source such as laser in the optical imaging system, one can project three-dimensional stereoscopic images or high-resolution two-dimensional images. However, the coherent optical imaging system inevitably appear speckles, which make the small features in image unable to be retrieved, and thus reduce the image quality. The general solution is to use a moving diffuser to suppress the speckles.
Recently, using phase-mode spatial light modulator (SLM) to substitute the moving diffuser has been widely studied and successfully implemented. Taking advantage of fast display of multiple random phase patterns on the SLM, the mechanical moving device and the diffuser are replaced and the speckles are reduced. However, the poor resolution and large pixel size of the currently-available spatial light modulation are insufficient to fulfill the requirement of this application. It is the goal of the thesis to investigate the condition that a spatial light modulator can replace the diffuser and gain a significant result in reduction of speckles. In the thesis, we used a coherent optical imaging system with two stages of single-lens imaging systems to achieve the goal. By changing the magnification of the first-stage optical imaging system, the speckle size was changed and the ratio of speckle size to SLM's pixel size in the imaging plane was changed as well. The influence of the random phase arrays displayed in the SLM on speckle phenomenon in the second-stage imaging plane was obtained. Based on the results of the thesis, we can develop a method of reduction of speckles.
The experiment results showed that when the speckle size (diffractive limit) of the first-stage imaging was two times larger than the pixel size of the spatial light modulator, the speckles of the second-stage imaging were effectively reduced. With superposition of two speckle patterns, the average of reduction of the speckle contrast was reduced to 0.71, which confirmed the theoretical estimation in most studies.

並列關鍵字

Speckles ； Speckle contrast ； Speckle size ； Spatial light modulator

參考文獻

[13] 周民俊，被動式雷射光斑之模擬和量測分析，碩士論文，國立臺北科技大學光電工程所，台北，2011。

[14] 施嘉博，LC-R2500液晶空間光調變器之測定研究，碩士論文，國立臺北科技大學光電工程所，台北，2008。

[1] J. I. Trisnadi, "Speckle contrast reduction in laser projection displays," Proc. SPIE, vol. 4657, 2002, pp. 131-137.

[2] M. L. Jakobsen, H. T. Yura, and S. G. Hanson, "Spatial filtering velocimetry of objective speckles for measuring out-of-plane motion," Applied Optics, vol. 51, no. 9, 2012, pp. 1396-1406.

[3] C. Zhou, J. Wang, C. Wang, and W. Yu, "Rapid processing for statistical properties of laser speckle with thermal spraying surface," Proc. SPIE, vol. 5642, 2005, pp. 592-598.