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Although in the parallel configuration the PRBS does not completely conceals the delay times T1 and T1+δT1, it can still play a key role in parameter space dimension.

For R^′ 6= R the dynamics of δ1(t) and δ2(t) is given by

These equations indicate that, contrary to the serial configuration, δ2does not always decays to zero independently of any eventual PRBS mismatch. Synchronization degradation therefore takes place both on internal and the transmitted variables.

Furthermore, it is noteworthy that no synchronization is possible between x1(t) and z1(t) when the internal variables do not synchronize, e.g. δ2(t) = 0.

Figure 6.5 shows both the mean square synchronization error (a) and the BER (b) as a function of PRBS mismatch for different values of the internal loop gain G₂. For G₂ = 0, there is no internal variable and therefore synchronization degradation relies on the transmitted variable as found for the serial configuration.

The synchronization error grows faster with the mismatch and just a 1% PRBS mismatch leads to a synchronization error of about 25% similar to that found in the serial configuration. Regarding the BER obtained for G₂ = 0, it grows linearly with the PRBS mismatch and in fact the results are similar to those obtained in the serial configuration for G1 = G2 = 5. When increasing G2, the degradation becomes stronger both in synchronization error and BER. As an illustration, for G2 = 3 the degradation for 0.4% PRBS mismatch is similar to that obtained for 2% of PRBS mismatch when considering G₂ = 0. Also, as for the serial configuration, we have found that for bit rates lower than 1/δT₁, the effect of the PRBS is largely reduced.

Figure 6.5: Influence of the PRBS-mismatch η on (a) Synchronization evaluated through the root-mean square synchronization error σ without the message, and (b) on the BER for a 10Gb/s message. We have considered a PRBS R(t) of length 1024 bits generated at 3 Gb/s, G1 = 5 and G2= 0 (), G2= 2 (△), G²= 3 (•).

6.5 Conclusions

We have studied in this chapter a configuration based on two parallel electro-optic phase-chaos loops. This allows for the generation of two phase-chaos variables, one of which is transmitted to the receiver while the other remains internal. The system also allows for an efficient implementation of a digital key. A suitable receiver is organized in a semi-closed loop configuration since it contains both an open loop for the transmitted variable and a closed one for the internal variable. Starting from single loop system (G2 = 0), we have investigated the conditions for high synchronization quality. Thus, it was found that synchronization takes place even for moderate values of the internal loop gain up to G2 ≈ 3.2. For the values range of G2 for which the system synchronizes, we have found that the system conceals all the internal loop delay times even without any digital key. On the other hand, we have found that the digital key decreases the signature corresponding to the two delay times of the external loop although it does not completely suppress them.

Interestingly, the effect of digital key on synchronization degradation is stronger than for the serial configuration so that its integration efficiently increases the parameter space dimension in chaos-based communications.

Chapter 7

Effect of Fiber Dispersion on

Broadband Chaos Implemented by Electro-Optic Phase Chaos

Systems

7.1 Introduction

Real communication networks will require the use of transmitters and receivers operating in synchronized chaotic regime, even if located far one from another [31].

In general, the signals are transmitted to receiver via either the electrical or the optical channels. The latter is the most used in the current communications because of their very large bandwidth and their low losses. However, besides the problem of signal-to-noise ratio caused by the noise and various mismatches in parameters between the emitter and the receiver, which are general for any communication system, one should additionally overcome the fiber effects. In fact, during its travel through the fiber, the carrier is subjected to attenuation, Kerr nonlinearity and chromatic dispersion. The latter effect is the most damaging in broadband chaos communication because the encrypted signal bandwidth may span over several tens GHz around the nominal frequency of the laser beam. Hence, dispersion shuffles this broadband spectrum and, as a consequence, synchronization noise arises because of imperfect chaos replication at the receiver. This residual cancellation noise is naturally expected to increase with the spectral bandwidth of the signal, with the length of the fiber link, and with the absolute value of the chromatic dispersion.

For optical chaos communication networks, the chaotic carriers used for en-cryption can be generated using a wide variety of architectures ranging from the amplitude/ intensity [67, 74, 142] to phase modulation schemes [75, 76]. In our case, the carriers launched into the fiber are generated from the electro-optic non-linear delay phase chaos generator presented in Sec. 2.3 and the results reported

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in this chapter indicate a better synchronization performance [143] when compared with previous measurements in electro-optical intensity chaos [144]. To the best of our knowledge, very few investigations have been devoted to the topic of fiber propagation effects on the performance of chaos cryptography [42, 129, 130, 145], and they were exclusively based on numerical simulations. Fiber transmission has been considered, nonetheless, in some experimental works [74, 146], but no detailed analysis on the fiber dispersion effects has been reported.

Our aim in this chapter is to address this issue with a joint theoretical and experimental analysis, and with a particular emphasis on the exploration of var-ious dispersion management schemes able to minimize the detrimental effects of chromatic dispersion. The chapter is organized as follows. In Sec. 7.2 we briefly overview the optical fiber effects. In Sec. 7.3 we present the system under study.

The Sec. 7.4 is devoted to the study of cancellation (or synchronization) noise, while Sec. 7.5 is dedicated to the corresponding spectra. The last section summarizes our results and concludes the chapter.