微透鏡組陣列應用於高穿透深度之平行取樣光學同調斷層掃描

Abstract

光學同調斷層掃描技術(Optical Coherence Tomography，OCT)，為近十幾年來在國內外被極力發展出的一種新穎的影像斷層掃描技術，其利用低同調高頻寬的雷射光源射入待測物內，然後收集射入待測物內產生反射與反向散射之訊號，利用麥克森干涉儀來解析訊號，其解析度可以達到數微米等級的縱向解析度；在本論文中，將首次利用微機電系統(Micro-Electro-Mechanical System，MEMS)中的微透鏡組陣列(Compound Microlens Array)，使用在光學同調斷層掃描技術中，利用微光學元件之優勢，同時完成平行取樣與增加光源使用效率之目的，目前已經成功的將250微米見方內的光束，成功的縮小成平行射出的陣列光源，且在透鏡組陣列後方10公分仍有60微米的平行光束陣列，預期利用此種特性應用在穿透深度方面有很高價值。 近年來光學同調斷層掃描技術，隨著光電感測器技術的進步，在掃描訊號的擷取方面，漸漸以二維陣列式面型的感測器代替傳統利用單點式感測器來擷取訊號，此法可以大大提升單點式光學同調斷層掃描系統的掃描速度，不再受限於機械式元件的掃描速度，以整個平面平行取樣之概念，來代替傳統從點到線到面的方式，此法通常叫做平行取樣之光學同調斷層掃描技術或是全區域光學同調斷層掃描技術(Parallel OCT or Full-field OCT)，本論文就是建立在此種技術下，希望能夠提高系統光源的使用效率。 應用於本研究利用灰階光罩技術所製作出之微透鏡陣列，其設計填充係數達到100%，驗證其在陣列聚焦的光點以達到30~40微米，遠勝於傳統技術製作之90微米；而利用此種技術所製作出來的表面粗糙度，也在數十奈米等級，足以符合光學元件之要求。 而在微透鏡陣列的製作方面，將利用微機電技術中的灰階光罩微影製程(Gray-Scale Microlithography)，不同於一般傳統二元式光罩微影技術，可以利用灰階光罩上黑白濃度造成不同透光率的灰階值，利用一次微影就可以製作出三維的透鏡結構，再經由熱壓成型製作單凸透鏡陣列，或是利用晶圓濕蝕刻110的切面，精準製作雙面結構之雙凹透鏡陣列，利用聚二甲基矽氧烷（polydimethylsiloxane，PDMS）來翻模，成功結合高分子光學塑膠材料在微成型技術的使用平台，使本技術除了操作在微米等級的微光學元件，更具有高附加價值與批次量產的優點。

Over the past 14 years a technique called optical coherence tomography (OCT) has been developed for noninvasive cross-section imaging in clinical diagnosis system. OCT uses low-coherence interferometry to produce to en-face and three dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. In this thesis, we know that parallel OCT in sample arm is demonstrated illuminating with a uniform extended beam and imaged on a two-dimensional array of photodetectors, so it can’t use focal lens in front of the sample. Recently, more and more innovative optical components be manufactured by the fabrication of MEMS (Microelectromechanical System), such as optical-communication system. The objective of this thesis is demonstrated using a 20x20 microlens arrays (in 5 millimeter square) in parallel OCT that compound microlens arrays could enhance both the incident light and collected back scattering efficiency in sample arm. However, there is a trade-off between lateral resolution and focusing depth when conventional optical elements (spherical lenses, mirrors, etc.) are used, because a beam cannot be produced that has simultaneously a long focal length and a narrow lateral width. Whereas high-lateral-resolution imaging requires small spot size by a large numerical aperture, a long focal depth requires a small numerical aperture. When We demonstrated to transform a parallel light beam to an array of narrow parallel light beam by a combination of two different microlens arrays. The size of each narrow parallel light beam is about 60 micrometers, and the penetration depth is about 10 centimeters from 250 micrometers microlens array. The roughness of microlens arrays is about 10-40 nanometers that be useful to avert diffraction. In the recent years, a parallel detection scheme with a CCD camera has gradually substituted for scanning system of single-point detector. This technique, which employs a two-dimensional array of photodetectors to receive the signal from XY-plane, avoids recording point by point with a fast two-dimensional mechanical scanning system. We called it “parallel OCT or full-field OCT”. Basing on the optical theory, gray-scale lithography, precision electroforming, and optical plastic material molding technique, several kinds of micro optical component could be developed by the novel Optical-MEMS technology. The 100% fill factor of compound microlens array is designed to enhance the incident light efficiency and increase the penetration depth in OCT system. Moreover, the low cost, simple process and mass production would be its advantages to impact the commercial market.