透過在公用二元相位偏移調變(public binary phase-shift keying, BPSK)通道中傳輸隱匿的光分碼多重擷取(optical code-division multiple-access, OCDMA)信號來實現光隱寫術(optical steganography)系統。我們結合頻譜幅度編碼（spectral amplitude coding, SAC）和極化分集(polarization diversity)的概念來設計我們所提出的編解碼器。極化分光器（polarization beam splitters, PBS）和陣列波導光柵（arrayed waveguide grating, AWG）則是被用來建構採用非相干光源的頻域極化編碼（spectral-polarization coding, SPC）系統。希望在系統中發送隱藏信息的用戶的簽章碼(Signature code)是通過選擇Walsh-Hadamard矩陣中任一行來獲得的Walsh-Hadamard code。
光學隱寫術的目的在於用戶可以發送隱匿的私有信號藏在公用通道之中傳輸，而不被他人注意，以增加通信系統的保密性。可以通過啁啾光纖布拉格光柵（chirped fiber-Bragg gratings, CFBGs）來將訊號在時域上隱藏在公用通道之中，而頻域上則是藉由頻域極化編碼的技術來達成。在此篇論文中我們將分別討論隱匿傳輸於有線以及自由空間傳輸(free space optical, FSO)的公用頻道中的情況
接收端可以通過光分碼多重擷取解碼器中的相關性檢測和平衡檢測法來提出秘密信息，同時來自公共通道的其他信號則是會根據傳輸方式以特定的方法消除掉。在公共通道的接收端中，在二元相位偏移調變的解調過程消除了隱匿信號，使得公用通道的訊號不會受隱匿信號的影響。研究的結果顯示在隱匿訊號、公用訊號以及雜訊分別為-5, 0和-3dBm 的情況下，隱匿訊號可以安全地隱藏在公用通道之中，我們提出的光隱寫系統可以提供高保密性。此外在傳輸功率為20dBm的情況下，光纖傳輸最遠為75公里而自由空間傳輸則是380公尺。

An optical steganography scheme is proposed to a public binary phase-shift keying (BPSK) channel by transmitting stealth optical code-division multiple-access (OCDMA) signal. We combine the concepts of spectral amplitude coding (SAC) and polarization diversity to design the proposed coder/decoder (codecs). Polarization beam splitters (PBSs) and arrayed waveguide gratings (AWGs) are used to construct the spectral-polarization coding (SPC) system with incoherent optical source. The signature code of the user who wants to transmit the stealth information in the system is Walsh-Hadamard code. It is obtained by selecting the row of the Walsh-Hadamard matrix.
The purpose of optical steganography is that user can transmit private signal which is hidden in the existing public channel without attentions by others and increase the secrecy of the communication system. The stealth signal can be hidden under the public channel in temporal domain and spectral domain by chirped fiber-Bragg gratings (CFBGs) and SPC techniques. Steganography transmissions with optical fiber and free space optical (FSO) links are discussed in this thesis.
At the receiver side, the secret information is extracted by correlation-detection and balanced-subtraction in the OCDMA decoder. At the same time, the other signals from public channel is also reduced with specific methods according to the way of transmission. At the demodulator of public channel, the procedure of BPSK demodulation eliminates the stealth signal so that the public signal wouldn’t affect by stealth signal. The results of this study show that the stealth signal is safely hidden in public channel and highly security is available in our proposed optical steganography system when the average power of the stealth signal, public signal and public noise are -5 dBm, 0 dBm and -3 dBm, respectively. In addition, the transmission distances with fiber and FSO links are 75 km and 380 m when the transmitter power is 20 dBm.

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