Experimental evaluation of an open-loop all-optical chaotic communication system

An experimental investigation of the performance of an open-loop optical chaotic communication system has been carried out under different optical injection conditions. The most popular message encoding techniques have been considered, tested and compared at gigabit rates and for different levels of optical injection to the receiver, by bit-error-rate (BER) measurements. Different low-pass electrical filters have been implemented for the various message bit-rates tested, in order to employ the most appropriate one for message decoding. The best performance of all the examined encryption methods for messages of 1.5 Gb/s bit-rate emerged for the chaos modulation encryption method and was characterized by Q-factor values up to 5.1, after synchronizing in the strong injection regime, while the corresponding BER of the decoded message was as low as 7/spl middot/10/sup -5/. In order to confirm the secure encryption of the message, BER measurements are also performed for the chaotic carrier into which the message is encrypted.     

Spectral synchronization in chaotic optical communication systems

The ability of chaotic carriers to synchronize in optical chaotic communication systems is studied experimentally under a spectral domain analysis. Synchronization of high-frequency components up to tens of gigahertz of the chaotic carriers can thus be evaluated. On the contrary, the traditional method of measuring the synchronization through chaotic carrier time-traces is constrained by the limited bandwidth of the real-time oscilloscopes used for such measurements, excluding the contribution of the high-frequency components of the chaotic carriers to the synchronization characterization of the system.     

Performance characterization of high-bit-rate optical chaotic communication systems in a back-to-back configuration

A comparative study of three data-encoding techniques in optical chaotic communication systems is reported. The chaotic carrier is generated by a semiconductor laser subjected to optical feedback and the data are encoded on it by chaotic modulation (CM), chaotic masking (CMS), or chaotic shift keying (CSK) methods. In all cases, the receiver-which is directly connected to the transmitter-consists of a semiconductor laser similar to that of the transmitter subjected to the same optical feedback. The performance of this back to back configuration is numerically tested by calculating the Q-factor of the eye diagram of the received data for different bit rates from 1 to 20 Gb/s. The CM scheme appears to have the best performance relative to the CMS and CSK scheme, before and after filtering the residual high-frequency oscillations remaining due to nonperfect synchronization between the transmitter and receiver. Moreover, in all encoding methods, a decrease in the Q-factor is observed when the repetition bit-rate of the encoding message increases. In order to achieve as high Q-factor values as possible, a well-synchronized chaotic master-slave system is required.     

Numerical investigation of fiber transmission of a chaotic encrypted message using dispersion compensation schemes

A detailed numerical investigation of the transmission properties of all-optical chaotic communication systems is presented for two data-encoding techniques and for various dispersion compensation maps. A semiconductor laser subjected to optical feedback generates the chaotic carrier, and the data is encoded on it by chaotic modulation (CM) or chaotic-shift-keying (CSK) methods. The complete transmission module consists of different types of fiber, inline amplifiers, and Gaussian optical filters. Different dispersion maps based on either Nonzero dispersion-shifted fibers (NZ-DSFs) or combinations of single-mode fibers (SMF) along with dispersion-compensating fibers (DCF) were considered. The system's performance is numerically tested by calculating the Q factor of the eye diagram of the received data for 1 and 2.4 Gb/s. The influence of the optical power launched into fiber and the transmission distance to the quality of the decoded message has been investigated. The CSK scheme appears to have better performance relative to the CM scheme, while dispersion maps utilizing NZ-DSFs are superior to that employing DCF. In all encoding methods and transmission maps, a decrease in the Q factor is observed when the repetition bit rate of the encoding message and the transmission distance increases.     

Regenerative properties of wavelength converters based on FWM in a semiconductor optical amplifier

A detailed experimental investigation of the regenerative properties of the four-wave mixing (FWM) for the conventional nonreturn-to-zero data format is reported. A key feature in the proposed scheme, and contrary to the conventional approach, is application of the ON-OFF keying modulation on the pump wave. Measurement of the static transfer functions of the FWM process in a semiconductor optical amplifier shows that certain operating regimes exist, mainly related to the power levels "one" and "zero" of the modulated pump, where a significant improvement of the extinction ratio (ER) can be achieved. This conclusion has been confirmed by dynamic measurements at 2.4 Gb/s, where up to 3-dB improvement in the ER has been observed.     

Performance characterization of a closed-loop chaotic communication system including fiber transmission in dispersion shifted fibers

The transmission performance of an all-optical closed-loop chaotic communication system is numerically studied, assuming various span lengths of dispersion shifted fiber between transmitter and receiver. Calculation of the mean synchronization error and Q-factor values obtained from the corresponding eye diagrams has been carried out for two encoding methods (chaotic modulation and chaos shift keying) and two repetition rates (2.4 and 5 Gbps). It is shown that transmission impairments including chromatic dispersion, fiber nonlinearity, fiber losses, and amplified spontaneous emission noise of the inline amplifiers degrade significantly the synchronization quality especially when high-bit-rate message encoding is applied. The influence of key system parameters such as launched optical power, amplifier spacing, positive or negative dispersion, etc., to the transmission performance has been investigated. Acceptable system performance can be achieved for approximately 200 km at 2.4 Gbps.