在現代通訊網路系統中，光分碼多重存取系統具有很大的安全效益，但在現行文獻中，很少對於實體層安全性作完整並詳細的分析。一般網路系統的安全性分為三大類：保密性、完整性、可用性，光分碼多重存取系統能夠提供系統完整性與可用性的保護，因此光分碼多重存取系統在提升系統安全性方面是非常具有吸引力之技術。
我們以陣列波導光柵為基礎提出一全新的混合式技術以提升網路系統之保密性，利用多組最大長度序列碼建構出一組複合式指配碼，且將其建構於陣列波導光柵上，各組複合式指配碼將被隨機指派給各使用者通訊。我們利用中央控制台傳送變碼指令於各使用者，當使用者接收到指令，其將實行變碼動作，藉此以提升系統之保密性。
在一般通訊網路中，我們最在意的是系統的保密性，因此在此篇論文中，我們主要針對系統的保密性作完整的分析，我們也分析變碼率對於系統保密性之影響；結果顯示，我們所提出之複合式指配碼概念確實可以提高系統之保密性，主要的分析結果我們將在論文裡探討。

Security has often been seen as an important benefit that could be obtained from optical code-division multiple access (OCDMA) system, but has seldom been analyzed in detail. In communication networks, security is traditionally divided into the categories of confidentiality, integrity, and availability. OCDMA system could potentially provide both confidentiality and availability protection. OCDMA has been considered as a good candidate to provide confidentiality.
We propose a new hybrid scheme with arrayed-waveguide gratings (AWGs) to enhance OCDMA network confidentiality. We use several relatively prime-length M-sequence codes to compose composite code sets structuring on several AWGs. The new scheme is proposed to exclusively assign each network user a set of composite signature codes and, upon reconfiguration, randomly select one among them to represent each user’s signature codes. Reconfiguration command controlled by central center used to change the signature codes. By randomly changing the codes assign for each user, the system will be more secure.
We know that confidentiality is probably the most commonly sought form of security in communication networks. In this thesis, we focus on evaluating the degree of confidentiality that may be provided by proposed system. The result shows that the probability of correct detection gets worse when we use the composite codes and the eavesdropper processes code word detection more difficult, thus data confidentiality could be significantly increased. We also analyze the influence of the reconfiguration rate for proposed system.

[1] G. Kramer and G. Pesavento, “Ethernet passive optical network (EPON): Building a next-generation optical access network,” IEEE Commun. Mag., vol. 40, pp. 66-73, Feb 2002.
[2] Prucnal, P.R., Krol, M.F., and Stacy, J.L., “Demonstration of a rapidly tunable optical time division multiple-access coder”, IEEE Photonics Technol. Lett., 1991, 3, (2), p. 170.
[3] M. D. Ma, Y. Q. Zhu, and T. H. Cheng, “A systematic scheme for multiple access in Ethernet passive optical access networks,” IEEE J. Lightwave Technol., vol. 23, pp. 3671-3682, Nov 2005.
[4] P. Prucnal, M. Santoro, and T. Fan, “Spread spectrum fiber optic local area network using optical processing,” IEEE J. Lightwave Technol., vol. LT-4, no. 5, pp.547-554, May 1986.
[5] J.A. Salehi, “Emerging OCDMA communication systems and data networks,” J. Opt. Netw., vol. 6, pp. 1138-1178, Sept 2007.
[6] J.P. Heritage and A.M. Weiner, “Advances in spectral optical code-division multiple-access communications,” IEEE J. Sel. Top. Quant. Electron., vol. 13, pp. 1351-1369, Sep-Oct 2007.
[7] D. Zaccarin, and M. Kavehrad, “An optical CDMA system based on spectral encoding of LED,” IEEE Photon. Technol. Lett., vol. 4, no. 4, pp. 479-482, April 1993.
[8] A. Stok and E.H. Sargent, “The role of optical CDMA in access networks,” IEEE Commun. Mag., vol. 40, no. 9, pp. 83–87, Sept 2002.
[9] W. Ford, Computer Communications Security. Upper Saddle River, NJ: Prentice-Hall, 1994, ch. 2.
[10] M. K. Simon, J. K. Omura, R. A. Scholtz, and B. K. Levitt, Spread Spectrum Communications. Rockville, MD: Computer Science Press, 1985.
[11] J. R. Stern, C. E. Hoppitt, D. B. Payne, M. H. Reeve, and K. A. Oakley, “ TPON-A Passive Optical Network for Telephony,” Fourteenth European Conference on Optical Communication (ECOC'88), vol. 1, pp. 203-206, Brighton, UK, Sep. 1988.
[12] M. Kavehrad and D. Zaccarin, “Optical Code-Division-Multiplexed Systems Based on Spectral Encoding of Noncoherent Sources,” IEEE J. Lightwave Technol., vol. 13, no. 3, pp. 534-545, March 1995.
[13] Y.T. Chang, C.C. Sue, and J.F. Huang, “Robust design for reconfigurable coder/decoders to protect against eavesdropping in spectral amplitude coding optical CDMA networks,” IEEE J. Lightwave Technol., vol. 25, pp. 1931-1948, August 2007.
[14] L. Tancevski, I. Andonovic, and J. Budin, “Secure optical network architectures utilizing wavelength hopping time spreading codes,” IEEE Photon. Technol. Lett., vol. 7, pp. 573-575, May 1995.
[15] B.B. Wu and E.E. Narimanov, “A method for secure communications over a public fiber-optical network,” Optics Express, vol. 14, no. 9, pp.3738-3751, May 2006.
[16] M.P. Fok, K. Kravtsov, Y. Deng, Z. Wang, T. Wang, and P.R. Prucnal, “Securing data networks using optical signal processing,” International Conference on Photonics in Switching, invited S-03-5, August 2008.
[17] J.M. Castro, I.B. Djordjevic, and D.F. Geraghty, “Novel super structured Bragg gratings for optical encryption,” IEEE J. Lightw. Technol., vol. 24, no. 4, pp. 1875–1885, April 2006.
[18] K. Vahala, R. Paiella, and G. Hunziker, “Ultrafast WDM logic,” IEEE J. Sel. Top. Quant. Electron., vol. 3, no. 2, pp. 698 – 701, April 1997.
[19] B.B. Wu and E.E. Narimanov, “Analysis of stealth communications over a public fiber-optical network,” Optics Express, vol. 15, no. 2, pp. 289-301, Jan. 2007.
[20] T.H. Shake, “Security performance of optical CDMA against eavesdropping,” IEEE J. Lightwave Technol., vol. 23, pp. 655-670, Feb 2005.
[21] G.C. Yang and W.C. Kwong, “Two-dimensional spatial signature patterns,” IEEE Trans. Commun., vol. 44, no. 2, pp. 184–191, Feb 1996.
[22] L. Tancevski, I. Andonovic, and J. Budin, “Secure optical network architectures utilizing wavelength- hopping/time-spreading codes,” IEEE Photon. Technol. Lett., vol. 7, no. 5, pp. 573-575, May 1995.
[23] L. Tancevski and I. Andovic, “Hybrid wavelength-hopping/time-spreading schemes for use in massive optical networks with increased security,” IEEE J. Lightwave Technol., vol. 14, no. 12, pp. 2636-2647, Dec 1996.
[24] Z. Wang, J. Chang, and P.R. Prucnal, “Theoretical Analysis and Experimental Investigation on the Confidentiality of 2-D Incoherent Optical CDMA System,” IEEE J. Lightwave Technol., vol. 28, no. 12, pp. 1761-1769, June 2010.
[25] S. Hara, R. Prasad, “Overview of multicarrier CDMA,”IEEE Communications Magazine, Vol.35, pp. 126 –133, Dec 1997.
[26] J. A. Salehi, “Code division multiple-access techniques in optical fiber network-Part I: Fundamental principles,” IEEE Transactions on Communications, vol. 37, no. 8, pp. 824-833, August 1989.
[27] J. A. Salehi and C. A. Brackett, “Code division multiple-access Techniques in optical fiber networks-Part II: Systems performance analysis,” IEEE Transactions on Communications, vol. 37, no. 8, pp. 834-842, August 1989.
[28] Z. Wei, H. M. H. Shalaby, and H. Ghafouri-Shiraz, “Modified quadratic congruence codes for fiber bragg-grating-based spectral-amplitude-coding optical CDMA systems,” J. Lightwave Technol., vol. 19, no. 9, pp. 1274-1281, Sep 2001
[29] E. Marom and O. G. Ramer, “Encoding-decoding optical fiber network,” Electron. Lett., vol. 14, no. 3, pp. 48, 1978.
[30] H. Takahashi, K. Oda, and H. Toba, “Impact of Crosstalk in an Arrayed-Waveguide Multiplexer on N×N Optical Interconnection,” J. Lightwave Technol., vol. 14, no. 6, pp. 1097-1105, June 1996.
[31] L. R. Chen, “Technologies for hybrid wavelength-time optical CDMA transmission”, Conference on Electrical and Computer Engineering, 2001. Canadian, vol. 1, pp. 435-440, 2001.
[32] M. Kavehrad and D. Zaccarin, “Optical code- division-multiplexed systems based on spectral encoding of non-coherent sources,” J. Lightwave Technol., vol. 13, no. 3, pp. 534 -545, March 1995.