| 摘要: | 本實驗係利用反射式液晶空間光調制器(Reflective Liquid Crystal Spatial Light Modulator, 簡稱RLC-SLM)產生不同樣式之Fresnel Zone Plates (簡稱FZP)，透過調制灰階、聚焦位置及對應波長等參數進行白光色彩分離暨色序式顯示的加法混色調制效果，實現即時性波長及焦距可調之反射式Fresnel光學聚焦透鏡。
首先，我們藉由Matlab進行輸入之FZP圖型資訊繪製與檢視，透過灰階調制選定適宜的參數範圍。利用RLC-SLM輸出FZP為二元相位式FZP，相較於傳統製程的FZP必須犧牲奇數圈或偶數圈的繞射效率，能有更高的光利用效率及繞射效率。當以RLC-SLM呈現之FZP其奇偶圈相位差為π時，元件之第一階繞射效率經誤差校正後約為36～38%，其結果趨近於二元相位式之FZP繞射效率理論最大值40.5%。其次，基於FZP具有高階聚焦點的現象，我們成功地透過程式碼塗改某些不必要圈數的存在，進而製造出該FZP原不存在的聚焦點。
最後，我們利用FZP將白光主要波長分離成對應顏色的聚焦點，證明FZP圖型能夠針對選定波長進行聚焦，而其他波長則呈現非完全聚焦或甚至散焦的成像結果。此外，亦利用網頁編碼器Adobe Brackets以高於人眼分辨率之影格率(frame rate)快速切換對應光源波長峰值之不同FZP形成加法混色效果，透過量測白光光源與聚焦點的光譜交互確認相符，即輸入具有多個波長峰值之光源時，焦距上會呈現對應不同顏色調制結果。換言之，可利用該方法透過FZP及加法混色法達成白光聚焦之反射式Fresnel光學聚焦透鏡。;We have successfully demonstrated electrically switchable wavelength and focal length tunable reflective Fresnel lenses using liquid crystal spatial light modulators (SLM) based on a liquid crystal on silicon (LCoS) device. The SLM device can display various Fresnel zone plates (FZP) with different grayscales, focal lengths, wavelengths, and others. On the basis of the real time switches of FZP with suitable parameters, the white light focusing based on color sequential and additive color techniques can be realized.
First, the corresponding FZPs, plotted by the software of Matlab, for different focal lengths and wavelengths with appropriate ranges of parameters were elucidated. By using such an LCoS device, the phase difference between these two adjacent ring areas of each FZP can be electrically tuned to be about π to approach the maximum focusing efficiency. After analyzing the focusing efficiency of light utilization, the experimentally obtained maximum diffraction efficiencies reached about 36-38%, close to the theoretical limit value of ~40.5%. Second, regarding the FZPs having characteristics of several orders of focal points, we successfully removed the unnecessary rings of FZPs to achieve a new focal point, which was absent in the original FZP. Finally, we also used FZPs to separate the white light, the mixing light of three wavelengths of 632.8 nm, 532 nm, and 450 nm, to the desired color light at their focal points. This experiment proved that the FZP patterns are able to focus the light with the desired wavelength, while others are out of focus or even defocusing. Furthermore, the switching of different FZPs, which correspond to the wavelengths of 632.8 nm, 532 nm, and 450 nm, with its frame time shorter than the limitation of humans’ eyes was adopted to mix these three lights according to the additive color technique. Experimentally, the spectra of focusing lights at their focal points were consistent with the theoretical analyses. The color of the combined lights at the focal points were contributed from the main wavelengths of the incident light source. In other words, we can obtain a white focal point by using various reflective Fresnel lenses. |
