應用於光學辨識系統之可獨立調變貓眼回射器陣列

Abstract

回射器在自由空間光學辨識系統中扮演很重要角色，因為透過回射器的特性能增加資料傳輸的安全性及辨識準確率，回射器顧名思義能將入射光沿著同方向反射回去。回射器依照結構不同如稜鏡型(microprism)回射器、corner cube回射器(CCR)以及貓眼回射器(cat’s eyes retro-reflector)等，而有不同的光學特性，在外面市售稜鏡型(microprism)回射器膠帶已經非常常見，但是這種回射器膠帶通常尺寸較大，且回射光應用在大範圍如道路指標、汽機車反光板或是交通安全上還堪用，若要作為光學標籤(optical tag)應用於光學辨識系統中，回射光恐怕沒有較高準確率，且回射器還必需結合調變的機制而且元件輕巧、擁有高解析度，因此本文將介紹能大量製作且具有調變功能的貓眼回射器陣列。 貓眼回射器是由前半部聚焦微透鏡和後半部反射凹面鏡所構成的，而前後模組之間的gap將決定光線是否能順利聚焦於後反射鏡上，再產生反射沿著原方向反射；由文獻可發現前半部微透鏡的調變方式有很多種，如以電濕潤(electrowetting)效應來改變透鏡曲率、以液體或液晶材料構成的微透鏡等，都是用來調變微透鏡的焦距或是曲率半徑，然而，大部分的調變機制都著重在單顆微透鏡，這邊將結合經過設計的高分子分散型液晶(Polymer-dispersed liquid crystal)薄膜於貓眼回射器陣列，透過施加電壓於兩片鍍有ITO (Indium tin oxide)的玻璃基板，產生一電場使液晶分子轉向，進而改變PDLC薄膜的狀態，讓回射光能夠自由調控，藉由改變回射器中不同的像素，以及變換施加電壓的頻率，這個輕薄、製程簡易且可獨立調變的光學裝置能扮演辨識系統中的鑰匙。

In free-space optical identification system, the optical MEMS device called retro-reflector to return the light signal to the original light source is needed for the purpose of increasing security and precise identification. The structures of retro-reflector such as microprism, corner cube, and cat’s eyes retro-reflector have different characteristics. Although the microprism type commercial retro-reflective tape has wide range of application, the most used of the tape is as road signage, vehicle visibility and traffic safety. To realize a tunable optical tag, a device has to combines with modulability mechanism; moreover, to be compact and high resolution in identification system. Therefore, we present a novel method to modulate cat’s eyes retro-reflector in this paper. The cat’s eyes retro-reflector, which consists of a front lens and back reflecting concave mirror with the gap appropriate for the incident ray focus on the back mirror and return to the direction of incident source. Many methods to modulate micro lens have been reported including electrowetting effect to control the curvature of the lens, lens constituted by liquid crystal or liquid-filled micro lens. However, most works mentioned above are focus on the modulation of single lens rather than array. Here, we introduce a designed PDLC film to switch each pixel of retro-reflector array while an external voltage is applied to glass which is coating ITO, the liquid crystal rotate along the direction of electrical field to change the state of the film. By switching the pixels of retro-reflector and altering the frequency of applied voltage, the compact, simple fabrication procedure, tunable optical device can be used as optical tag in identification system.