臺灣博碩士論文加值系統

當光穿過紊亂介質,與介質接觸時,產生的繞射與散射將嚴重影響觀測的影像,但若利用非線性光學中的相位共軛特性: 當光的相位改變,與原入射波為共軛之關係,光將自行回溯至波源。數位光學共軛鏡(Digital Optical Phase Conjugation)結合了電光調製器(Electro Optical modulator)及空間光調製器(Spatial light modulator),經由此儀器我們可以成功的產生相位共軛的光波,藉以達到光學回溯的效果。

本論文使用有限時域差分法(Finite-Difference Time-Domain technique), 並搭配此方法的額外技術, 針對數位光學相位共軛鏡進行一連串數值模擬與研究。我們在空間中擺放物質, 觀測電磁波經光學相位共軛鏡讀取資訊後之光回溯場型, 並更改模擬區域的寬度, 觀察光學相位共軛鏡與入射波在入射波波源處的能量場型, 企圖藉由此類數值模擬來了解數位光學相位共軛鏡之基本性質。此外, 我們更改變數位光學相位共軛鏡接收向前傳播散射波(Forward scattering wave) 之頻率以及共軛鏡的解析度, 試圖找出最有效率的組合, 並進一步利用此一組合, 模擬數位光學共軛鏡在穿透不同濃度的紊亂介質(turbid medium)時之光學回溯效果。

Due to the complex structure of biological tissues, wave propagateing through turbid medium generates interference and diffraction, generally the image is scattered and blurred. However, in nonlinear optics, with the technique of optical phase conjugation (OPC), for wave propagating through OPC, the wave refocuses to initial emanating point. Digital optical phase conjugation (DOPC) combines electro optical modulator (EOM) and spatial light modulator (SLM), which records forward scattering wave information and digitally reverses the phase information.

In this thesis, we apply the Finite-Difference Time-Domain (FDTD) technique, with additional techniques, to simulate DOPC refocusing phenomenon. Not only we simulate the basic property of DOPC, like placing an object in the simulation area to observe DOPC refocusing phenomenon, altering the width of simulation area to compare the profile of DOPC wave and the profile of incident wave; we also alter the DOPC recording time interval and spatial resolution to find the most efficient wave for DOPC refocusing. Moreover, we make use of these results to simulate DOPC refocusing through turbid medium.

1 Preface 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Research Background and Literature Review . . . . . . 2
2 Theory 5
2.1 Optical Phase Conjugation (OPC). . . . . . . . . . . . 5
2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Reciprocity theorem and scattering matrix . . . . . 8
2.2 Digital Optical Phase Conjugation (DOPC) . . . . . . 13
2.2.1 Introduction . . . . . . . . . . . . . . . . . . . 13
2.2.2 Poisson summation formula . . . . . . . . . . . . . 14
3 Finite-Difference Time-DomainMethod 18
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . 18
3.1.1 Maxwell equations . . . . . . . . . . . . . . . . . 19
3.1.2 Yee algorithm and Taylor expansion . . . . . . . . 22
3.2 Special technique in FDTD method . . . . . . . . . . 26
3.2.1 PerfectlyMatched Layer Absorbing Boundary Condition 26
3.2.2 Total-Field/ Scattered-Field (TF/ SF) technique . . 31
3.2.3 Near-to-Far-Field transformation (NTFF) technique . 34
4 DOPC simulation analyses 40
4.1 DOPC simulation—DOPC property analysis . . . . . . . 40
4.1.1 DOPC refocusing of a plane wave . . . . . . . . . . 41
4.1.2 DOPC refocusing of a diffracted wave . . . . . . . 44
4.1.3 Explanation of DOPC wave refocusing through narrow simulation width . . . . . . . . . . . . . . . . . . . . 48
4.2 Analysis of DOPC optimal selection . . . . . . . . . 52
4.2.1 Analysis of DOPC recording time interval . . . . . 52
4.2.2 Analysis of DOPC refocusing ability in various resolutions . . . . . . 56
4.3 Analysis of DOPC refocusing through turbid medium . . 58
4.3.1 DOPC refocusing through various number of scatterers . . . . . . . . 59
4.3.2 DOPC refocusing via different level precision . . . 64
5 Conclusion and future prospect 67
5.1 Conclusion . . . . . . . . . . . . . . . . . . . . . 67
5.2 Future prospect . . . . . . . . . . . . . . . . . . . 69
Reference . . . . . . . . . . . . . . . . . . . . . . . . 71

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