長焦深雙波長繞射光學元件應用在光學讀取頭上之研究

Abstract

近年來，由於半導體技術持續不斷進步與發展，帶動高科技技術的進步，相對地檢測儀器與消費性電子產品大都朝向輕薄短小之積體化、微小化、多功能化發展，如攝影機、光碟機、數位相機等。有些傳統光學元件已經被繞射光學元件（diffractive optical elements/DOEs）所取代，使得光學讀寫頭系統的設計也多朝向微小化與模組化發展。 本論文研究長焦深雙波長繞射光學元件在光學讀取頭上之應用，包括研究各種光學讀寫頭結構之優缺點，以提出新的構想與設計理念，並對其進行基礎理論推導與演算法、模擬分析與計算、元件製作、及檢測實驗。本研究以共軛梯度法來設計雙波長繞射式DVD( digital versatile disks )光學讀寫頭聚焦鏡，並使其具有贗無繞射光束（pseudo-nondiffractiing beam/PNDB）的特性。由於光學讀取頭的焦距很短（在1.8mm∼2.4mm之間），且甚小於近場繞射區域，故在自由空間中，於傍軸近似下Fresnel繞射積分函數之線性變換，其Bessel項之積分函數不能省略。但卻也因此造成在處理過程中產生高複雜度、低收斂性與高計算量的現象。藉由採用梯形輪廓來表現軸向光場強度分布，並引入含有二維權重因子與輻射通量因子之誤差函數來評估所設計之繞射元件之性能。分析的結果顯示整體收斂效率增進8∼9個數量級，而軸向光強度之非均勻度亦減少2個數量級，並成功設計出一種具簡單結構之多功能元件。將物鏡組與準直鏡整合成一片繞射型光學元件，從此省卻諸多元件安裝與光路對準問題。此外，橫向解析度的提高與縱向光強分布的加長，已經由理論與實驗結果證實其可行性。

In recent years, as the semiconductor technologies progress continuously, the high-tech industries have achieved considerable technical and commercial success. Furthermore, the consuming electronic products are also developed along the trends of lightweight, miniaturization, and multi-function. Some of the conventional optical elements have been replaced by diffractive optical elements (DOEs), which make the design of the optical pickup head system toward miniaturizing and modularizing. The application of diffractive optical elements with long focal depth and dual wavelength on optical pickup head is studied in the thesis. The merits and drawbacks of various structures of pickup head are evaluated firstly, then some novel ideas and design concepts are proposed and established by theoretical deduction and algorithm, computer simulation analysis, practical fabrication, and measurement verification. The design of DVD (digital versatile disks) optical pickup head with dual wavelength diffractive focal lens and the pseudo-nondiffracting beam (PNDB) characteristic is carried out by using a conjugate-gradient method. Because the focal length of optical pickup head is very short (within 1.8-2.4 mm), it is extremely smaller than the near field diffractive region. Consequently, the integral function of the Bessel term, belonging to the linear transfer equation of Fresnel diffraction integral function within the paraxial approximation, could not be neglected in the free space. But it resulted in high complexity, lower convergence and heavy computing load during the simulating procedure. The axial intensity distribution can be presented by selecting a trapezoid profile. The performances of designed DOEs are evaluated by using an error function with 2-D weighting factor and radiant flux factor. The analysis results show the whole convergence has increased 8-9 orders and the non-uniformity of axial intensity has decreased 2 orders. A simple structure with multi-function is successfully designed and fabricated in the study. The objective lens and collimator lens have been integrated as the diffractive optical elements. This implies that the problems of assemble and optical alignment of many elements will be solved completely. Moreover, the increased lateral resolution and expand longitudinal axial distribution in this novel diffractive optical elements have been demonstrated by theoretical analysis and experimental results.