在全球通訊以及網際網路發達的現在，人們所追求的是更高的傳輸速度與品質，而光纖通訊因具備高頻寬、低損失及傳輸時不受電磁波的影響等優點，再加上光放大器和光調變器發展，光通訊系統技術也隨之快速的進步。光纖通訊的應用範圍相當廣泛，大致上可分為幾個重要領域，分別為電信網路 (Telecom)、數據網路 (Datacom)、有線電視 (CATV)光纖傳輸網路….等等。
相較於傳統的光通訊系統，光學正交分頻多工系統 (optical OFDM system)最大的優勢在於可以在DSP (digital signal process)處理大部分光上面的問題，比如說光訊號傳輸受到色散 (chromatic dispersion)的影響而造成phase delay，可在傳輸端複製一部份的OFDM訊號當作guard interval，避掉訊號之間的影響(intersymbol interference)以及接收時的訊號損失，然而，傳統的光通訊系統需要使用色散補償光纖。以及OFDM的正交特性可以提升頻譜效應，相較於傳統的光通訊系統，越高的傳輸速率就需要更大的頻寬，OFDM可以節省約兩倍的頻寬並且維持高速的位元傳輸速率。
本篇論文主要就是架設出光學正交分頻多工系統的實驗，在接收端使用direct-detection接收，系統複雜度較簡單但是在傳輸端必須傳送一根載波伴隨著OFDM訊號，所以在傳輸端的IFFT內subcarrier作適當的安排[1]，除了傳輸時可以帶一根載波並且訊號可以偏壓在光調變器的線性區，接收時也可以避掉signal-signal beating interference (SSBI)[2]，並且將此系統作CSPR (carrier signal power ratio)的最佳化。就目前direct-detection光學正交分頻多工系統的研究領域中，A. J. Lowery由20 Gb/s的位元傳輸速率[3]到使用self-coherent以及PDM (polarization division multiplexing)的方式大幅提升位元傳輸速率達120 Gb/s [4]。

致謝………………………………………………………………….……I
摘要……………………………………………………………………...II
目錄…………………………………………………………….………III
圖列表…………………………………………………….………….…IV
第一章 正交分頻多工系統簡介……………………………………..…1
第二章　光的正交分頻多工系統原理…………………………………4
2-1 光的正交分頻多工系統的調變…………………..……………….4
2-2 光的正交分頻多工系統的解調………………….………………11
2-3 光學調變器…………….……………………………….………11
2-4 光的正交分頻多工系統架構…………………………….…….…17
第三章　光的正交分頻多工系統實驗重點元件介紹……………..…24
3-1 OFDM ENCODER…………….………………………………...25
3-2 OFDM DECODER…………….………………………………...32
3-3 Single-Side Band Optical Modulator……………………………...37
第四章 光的正交分頻多工系統的實驗工作原理和結果…………....45
4-1 儀器和元件參數的介紹…….…………………………..………...46
4-2 實驗工作流程….…………………………..…………….……...58
4-3 實驗結果….…………………………..……………...….……...65



圖列表
圖1-1 分頻多工訊號頻譜圖..........................................................................1
圖1-2 正交分頻多工訊號頻譜圖...................................................................2
圖2-1 光的正交分頻多工調變器..................................................................4
圖2-2 8-point FFT作radix-2第一層的簡化...................................................5
圖2-3 radix-2 8-point FFT...........................................................................6
圖2-4 不同的zero-padding ratio比較 (a) zero-padding ratio = 4 (b) zero-padding
ratio = 2 …………………………………..………………………..7
圖2-5 插入循環字首的方法……………………..………………………....8
圖2-6 不同頻率的子載波經過光纖傳輸的示意圖…………..….....…………9
圖2-7 光的正交分頻多工解調器………..…...……………………………11
圖2-8 Mach-Zehnder Modulator波導架構圖…..…...……………….……..11
圖2-9 Mach-Zehnder Modulator轉換功率曲線圖…...…………….….……13
圖2-10 Balanced Mach-Zehnder Modulator示意圖..…………….…..………14
圖2-11 不同的bias point相對不同的頻譜圖…………….…..………….….16
圖2-12 直接偵測的OFDM通訊系統………………..…………..…………17
圖2-13 直接偵測系統下訊號經過photodiode的頻譜示意圖………..….……20
圖2-14 避掉多餘ASE和SSBI的received optical spectrum…………..………20
圖2-15 同調偵測的OFDM通訊系統…………..……..……..……………..21
圖2-16 同調偵測接收的示意圖……..……..……..……………………….21
圖2-17 同調偵測和直接偵測系統表現比較..……..……..…………………23
圖3-1 OFDM ENCODER示意圖……..……..……...……………………25
圖3-2 fully-modulation OFDM和gapped-OFDM頻譜示意圖….....……...…26
圖3-3 Optical carrier對OFDM訊號的影響…..……...…………………….27
圖3-4 RTS訊號截取示意圖…...……...………………………………….29
圖3-5 RTS包含OFDM訊號的truncation……...………………………….30
圖3-6 (a) marker相對OFDM訊號開頭的位置.…………….………………30
圖3-6 (b) marker所相對的OFDM訊號放大圖.…………….………………30
圖3-7 (a) 量化bit數較多的truncation………….…………………..………31
圖3-7 (b) 量化bit數較少的truncation……….……………………..………31
圖3-8 OFDM DECODER示意圖…….……………………..……………32
圖3-9 headsearching algorithm示意圖….……………………..……..……33
圖3-10 R過短時Correlation vs truncation point……………………..………34
圖3-11 R過長時Correlation vs truncation point……………………..………34
圖3-12 R適中時Correlation vs truncation point………………..……………34
圖3-13 經過通訊系統的訊號開頭和其他點數的correlation比較圖………….35
圖3-14 訊號頻譜示意圖：(a)AWG傳輸訊號，(b)經過MZM的訊號，
(c)經過photodiode的訊號……………..…………………..………35
圖3-15 系統架構圖……..…………………..………………………...…37
圖3-16 IQ Optical Modulator偏壓量測………..………………………...…38
圖3-17 IQ Optical Modulator相對的transfer function………..……………...38
圖3-18 測量IQ同步的實驗架構圖……..……………................................40
圖3-19 調整IQ同步和光調變器偏壓最佳化的clock光譜圖和OFDM data星座
圖(a)雷射的光譜圖 (b)同步且偏壓在最佳化 (c)非同步但偏壓在最佳化
(d)同步但偏壓非最佳化……..........................................................42
圖3-20 cosin進Mz-a sin進Mz-b所量測的光譜圖.........................................44
圖4-1 光的正交分頻多工實驗架構圖.........................................................45
圖4-2 OFDM duration計算原理說明圖.......................................................46
圖4-3 AWG面板顯示圖 (a)基本的波形 (b)放大後的波形...........................48


圖4-4 OFDM訊號的頻譜圖 (a)OFDM ENCODER出來的訊號頻譜圖 (b)AWG
出來的訊號頻譜圖 (c)Spectrum觀察到I訊號頻譜圖 (d) Spectrum觀察
到Q訊號頻譜圖.............................................................................50
圖4-5 AWG輸出port................................................................................52
圖4-6 沒有LPF的OFDM_I電頻譜圖.........................................................52
圖4-7 有LPF的OFDM_I電頻譜圖............................................................52
圖4-8 IQ Optical Modulator與tunable laser實驗配置示意圖.........................53
圖4-9 IQ Optical Modulator實圖................................................................53
圖4-10 Tunable Laser和PC實圖..................................................................54
圖4-11 Coupler示意圖.........................................................................55
圖4-12 coupler實圖...........................................................................55
圖4-13 Optical tunable filter實圖.................................................................56
圖4-14 Photodiode實圖..............................................................................56
圖4-15 RTS實圖........................................................................................57
圖4-16 實驗工作流程示意圖......................................................................58
圖4-17 EDFA出來觀察到的OFDM訊號......................................................60
圖4-18 OSA觀察到的ASE noise光譜圖......................................................60
圖4-19 濾波器未校準好的訊號...................................................................60
圖4-20 濾波器校準正確的訊號...................................................................60
圖4-21 EDFA出來的OFDM訊號和EDFA noise..........................................62
圖4-22 進入coupler之前OFDM訊號...........................................................62
圖4-23 OSA觀察1550nm的ASE noise........................................................63
圖4-24 進入濾波器前的光譜圖...................................................................63
圖4-25 濾波器後的光譜圖..........................................................................63
圖4-26 photodiode之後的電頻譜圖.............................................................64
圖4-27 光調變器偏壓飄移的接收電頻譜圖..................................................64
圖4-28 OSNR=17dB在decoder解回訊號的星座圖........................................64
圖4-29 CSPR與系統表現相對圖.................................................................65
圖4-30 CSPR = 0dB 系統表現圖.................................................................65

[1] Wei-Ren Peng, “Theoretical and Experimental Investigations of Direct-Detected RF-Tone-Assisted Optical OFDM Systems,” Journal of Lightwave Technology, Vol. 27, NO. 10, MAY 15, 2009
[2] A. J. Lowery, “Amplified-spontaneous noise limit of optical OFDM lightwave systems,” Optics Express 860, Vol. 16, NO. 2, January 21, 2008
[3] A. J. Lowery, “Experimental Demonstrations of 20 Gbit/s Direct-Detection Optical OFDM and 12 Gbit/s with a colorless transmitter,” Optical Fiber Communication Conference (OFC), March 25, 2007
[4] A. J. Lowery, “120 Gbit/s Over 500-km Using Single-Band Polarization-Multiplexed Self-Coherent Optical OFDM,” Journal of Lightwave Technology, Vol. 28, NO. 4, February 15, 2010
[5] R. W. Chang. Synthesis of band-limited orthogonal signals for multi-channel data　transmission【J】.Bell syst.Tech. J.，Dec. 1966，45：1775 – 1796
[6] R. W. Chang. A Theoretical Study of Performance of an Orthogonal Multiplexing　Data Transmission Scheme【J】.IEEE Trans. Commun.，1968，C-16：529-540
[7] S. B. Weinstein and P. M. Ebert. Data transmission by frequency division　multiplexing using the discrete Fourier transform【J】.IEEE Trans. Commun.　Technol.，Oct. 1971，vol. COM- 19：628 – 634
[8] A. J. Lowery and Jean Armstrong, "Orthogonal-frequency-division multiplexing for dispersion compensation of long-haul optical systems," Opt. Express 14, 2079-2084 (2006)
[9] A. J. Lowery and J. Armstrong, “Adapation of Orthogonal Frequency Division Multiplexing to Compensate Impairments in Optical Transmission Systems”, ECOC Tutorial: Berlin 2007
[10] A. J. Lowery, L. B. Du, and J. Armstrong, “Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems”, Journal of Lightwave Technology, vol 25, NO. 1, January 2007
[11] J. W. Cooley and J. W. Tuke, “An Algorithm for Machine Computation of Complex Fourier Series,” Math. Computation, Vol. 19, pp. 297-301, April 1965
[12] W. Shieh and C.Athaudage,“Coherent Optical Orthogonal Frequency Division Multiplexing,”IEE Electron. Lett., vol. 42, No. 10, May, 2006
[13] M. Sieben, J. Conradi, and D. E. Dodds,”Optical Single Sideband Transmission at 10Gb/s Using Only Electrical Dispersion Compensation”, Journal of Lightwave Technology, vol 17, NO.10, October 1999
[14] R. I. Killey, P. M. Watts, V. Mikhailov, M. Glich, and P. Bayvel,”Electronic Dispersion Compensation by Signal Predistortion Using Digital Processing and a Dual-Drive Mach-Zehnder Modulator”, IEEE Photonic technology Letters, vol 17, NO. 3, March 2005
[15] Chun-Ting Lin, “Optical Millimeter-Wave Up-Conversion Employing Frequency Quadrupling Without Optical Filtering,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 8, AUGUST 2009