本篇論文比較在不經過光纖傳輸的情況下，接收端分別使用直接偵測、同調偵測以及自我同調偵測的系統表現。這裡透過數值以及理論分析的方式分別討論各架構的系統表現。不論從理論的分析或者是數值分析的結果，我們都可以發現欲達到相同的系統表現，自我同調偵測系統所需的光信號對雜訊的功率比會比同調偵測系統多上一倍。另外，濾波器頻寬對各系統表現的影響在本論文也有詳細的討論，特別是當接收端使用同調偵測與自我同調偵測的情況，這兩者會有相當類似的結果。
接著，我們在原先的自我同調偵測架構中再加入一個頻譜匹配的濾波器使系統更好，更接近同調偵測系統的表現。在這裡我們也考慮雷射線寬的限制，並且利用不同頻寬以及階數不同的高斯濾波器做為光載波濾波器，使系統能有最好的表現。從模擬的結果可以發現，階數較高且頻寬較窄的濾波器能去除大多數的ASE雜訊，卻也對訊號造成嚴重的相位失真，使系統表現明顯地變差。最後，我們發現當雷射線寬為1MHz時，頻寬為50MHz的一階高斯濾波器會使系統有最好的表現。最後，我們討論頻譜使用效率的問題。改變載波和訊號間的間距，觀察其相對應的系統表現。我們發現使用平衡接收機的架構並不會因為載波和訊號間的間距不同而有不同的系統表現。

Based on the proposed self-coherent detection, several comparisons among coherent detection, direct detection and self-coherent detection is being made in this work. The system performance of self-coherent detection which adopts balanced receiver has a 3-dB penalty to the system performance of coherent detection. The difference comes from the existence of carrier in transmitted signal in self-coherent detection when the carrier to signal power ratio is set to one. The effects of different filter bandwidths will be discussed, especially for coherent detection and self-coherent detection. Some theoretical analysis will be made to explain the resemblance between them.
After then, the spectrally matched optical filters are used to refine the system performance and approach coherent detection as possible. The limitation of laser linewidth is taken into consideration here. To optimize the system performance, optical carrier filters of different bandwidths and different order will be uesd. We find that higher order filters may cause serious phase distortion when the optical filter bandwidth is too small, while lower order optical filters cannot stop the ASE noise from passing through with the OFDM signal. When the laser linewidth equals 1MHz, to get the optimized system performance, 50MHz 1st order Gaussian filter is used as the carrier filter. Then, the issue of spectral efficiency is being discussed. At the beginning, the noise mechanism of self-coherent detection which adopts either single-end receiver or the balanced receiver is studied. Besides, some numerical simulations are made. The result derived from the simulations is consistent with the theoretical result. The system performance of self-coherent detection is invariant when balanced receiver is adopted while it changes significantly when single-end receiver is adopted.

Reference
[1] Richard Van Nee and Ramhee Prasad, OFDM For Wireless Multimedia Communication, (Artech House, Boston/London, 1999)
[2] H. Schulze and C. Lueders, Theory and Applications of OFDM and CDMA, (John Wiley & Sons, Ltd, 2005)
[3] K-Po Ho, “Phase-Modulated Optical Communication Systems”, (Springer Science and Business Media, 2005)
[4] 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
[5] A. J. Lowery and J. Armstrong, “Orthogonal-frequency-division multiplexing for dispersion compensation of long-haul optical systems” Opt. Express, 14, pp. 2079-2084, 2006
[6] A. J. Lowery and J. Armstrong, “Adaptation of Orthogonal Frequency Division Multiplexing to Compensate Impairments in Optical Transmission Systems”, ECOC Tutorial: Berlin 2007
[7] A. J. Lowery, “Amplified-spontaneous noise limit of optical OFDM lightwave systems”, Opt. Express 16, 860-865 (2008)
[8] 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, pp. 131-138, January 2007
[9] B. J. C. Schmidt, A. J. Lowery, and J. Armstrong, “Experimental Demonstrations of Electronic Dispersion Compensation for Long-Haul Transmission Using Direct-Detection Optical OFDM”, Journal of Lightwave Technology, vol. 26, NO. 1, January 1, 2008
69
[10] W. Shieh, H. Bao, and Y. Tang “Coherent optical OFDM: theory and design”, Opt. Express 16, pp. 841-859, 2008
[11] W. Shieh and C. Athaudage, “Coherent Optical Orthogonal Frequency Division Multiplexing,” IEE Electron. Lett., vol. 42, No. 10, May, 2006
[12] I. B. Djordjevic and B. Vasic, “Orthogonal Frequency Division Multiplexing for high-speed optical transmission,” Opt. Express, vol. 14, pp. 3767-3775, 2006.
[13] D. Chanda, A. Sesay and B. Davies, “Performance of clipped OFDM signals in fiber” Proc. IEEE Canadian Conference on Electrical and Computer Engineering, pp. 2401-2404, 2004
[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] L. Xu, J. Hu, D. Qain, and T. Wang, “Coherent Optical OFDM Systems Using Self Optical Carrier Extraction”, OMU4, OFC/NFOEC 2008
[16] A. J. Lowery, “Improving Sensitivity and Spectral Efficiency in Direct-Detection Optical OFDM System”, OMM4, OFC/NFOEC 2008
[17] R. Hui, B. Zhu, R. Huang, C. T. Allen, K. R. Demarest, and D. Richards, “Subcarrier Multiplexing for High Speed Optical Transmission”, Journal of Lightwave Technology, vol. 20, NO. 3, March 2002
[18] Wei-Ren Peng, Kai-Ming Feng, and Alan E. Willner, “Ultimate Sensitivity for Optically- Preamplified Direct-Detected OFDM Systems Using Spectrally Matched Optical Filters,” IEEE Photonics Technology Letter, vol. 21, pp. 1764-1766, Dec. 2009.