Study On Important Techniques For Chaotic Optical Communications Through Fiber-optic Channel

With the arrival of the information epoch, high-speed, efficient, and secure transport and communications of information become the core issue. Optical fiber communications have become the major high-speed wire communication bearers for the merits of high transmission bandwidth, low channel attenuation, and anti-interference, etc. However, the physical layer of optical fiber communications with the technique of electric time division multiplexing (TDM) or wavelength division multiplexing (WDM) is transparent without any encryption. Therefore, the information eavesdropping at the physical layer becomes possible.The bandwidth for the chaos generated by semiconductor laser through perturation can be up to several to more than ten GHz, whose correlation dimension is big. What’s more, the bit rate for chaotic optical communications can be up to Gb/s or more. Since the optical fiber communications network has been widely employed, the chaotic optical communications should be compatible with existing optical fiber communications network by multiplexing and dispersion compensation. Hence, it is important not only to achieve the secure transmission of the secure message, but also to improve the spectral efficiency of the optical fiber channel to save the costs of lines and equipments. This important scientific and technical issue becomes urgent for the chaotic optical secure communications.This dissertation is based on the national and international research status of the chaotic optical secure communications. The key issues of the WDM transmission between the conventional fiber-optic channels and the chaotic optical channels, the WDM transmission among the chaotic optical channels, the division multiplexing of optical chaotic channels, and the dispersion compensation are detailed investigated by the theoretical analyses, numerical simulations, and experimental validations. The detailed contributions of this dissertation are listed as follows.(1) Due to the dispersion and nonlinear effects, the chaotic optical signal will be distorted during the propagation through the optical fiber. Therefore, the quality of the chaotic synchronization and communications declines. Consequently, the scheme of single-channel chaotic optical communications utilizing symmetrical dispersion compensation technique is proposed. The message recovery performance with and without dispersion compensation for different bit rates are compared. Moreover, the effect of the dispersion compensation period on the system performance is also investigated. The waveform change for the chaotic optical carrier after the transmission of dispersion-shifted fiber is analyzed and simulated. The results show that the quality of decrypted sequence is significantly improved by allocating dispersion compensation fiber. (2) The optimal methods for the WDM transmission between the conventional fiber-optic channels and the chaotic optical channels are proposed. The message encryption scheme is the chaos shift keying to increase the bit rates. And the symmetrical dispersion compensation is adopted to reduce the effect of optical fiber dispersion on the performance of chaotic optical communications. Moreover, a reasonable adjustment of the optical power is used to reduce the nonlinear effects. A2.5-Gb/s secure message masked by the chaotic optical channel and a10-Gb/s message sequences carried by the conventional fiber-optic channel with the transmission distance up to500km can be realized simultaneously, when the channel spacing is0.8nm.(3) The WDM communications setup with multiple chaotic optical channels is put forward. The effects of the second-order dispersion coefficient and the nonlinear coefficient of fiber, the channel spacing, the bit rates, and the message amplitude on the chaotic synchronization and the multiplexing communications are investigated. The results show that the performance for the chaotic synchronization and communications decrease with the increase of the length of optical fiber link, and otherwise, the effect of the channel spacing on chaotic synchronization and communications is not significant. The1Gb/s,1.25Gb/s, and2.5Gb/s messages are securely transmitted with wavelength division multiplexing by the proposed system.(4) The division multiplexing of optical chaotic channels is proposed to transmit more than one chaotic optical signal with the same wavelength. Two necessary conditions including chaotic synchronization condition and orthogonal condition of chaotic carriers are defined to quantify the performance of division multiplexing for optical chaotic channels. The chaotic characteristics of the output optical signal, including the time series, power spectrum, auto-correlation function and the cross correlation function, are numerically analyzed. The effects of internal and external parameter mismatch and the spontaneous emission noise on the performance of division multiplexing for optical chaotic channels are analyzed in detail. The results show that the parameters for the semiconductor lasers can be continuous values as long as the synchronization conditions and the orthogonal conditions are met, which indicates that the orthogonal space for the chaotic optical division multiplexing is large.(5) The chaotic optical transmitter and receiver for the bit rate of1.25Gbit/s are designed and developed. The chaotic optical communications are experimentally investigated in detail. Firstly, a chaotic optical carrier easy to be compatible with existing optical fiber communications is generated by the optical fiber feedback loop. The chaotic optical synchronization and a1.25Gbit/s secure message transmission are achieved by the experimental setup for back-to-back chaotic optical communications. Then, a1.25Gbit/s secure message transmission up to100km is implemented by the experimental setup for chaotic optical communications. In addition, the WDM between the chaotic optical secure channel masking a1.25-Gb/s message and the conventional fiber-optic channel carrying a1.25-Gb/s message up to100km, are experimentally realized without dispersion compensation during transmission when the channel spacing is3nm. Finally, the WDM between two chaotic optical secure channels masking a1.25-Gb/s message for each channel up to25km can be realized simultaneously without dispersion compensation during transmission.