IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 11, NOVEMBER

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1 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 11, NOVEMBER Secure Chaotic Transmission on a Free-Space Optics Data Link Valerio Annovazzi-Lodi, Senior Member, IEEE, Giuseppe Aromataris, Mauro Benedetti, Member, IEEE, and Sabina Merlo, Senior Member, IEEE Abstract In this paper, we numerically demonstrate secure data transmission, using synchronized twin semiconductor lasers working in the chaotic regime, which represent the transmitter and receiver of a cryptographic scheme, compatible with free-space optics technology for line-of-sight communication links. Chaotic dynamics and synchronization are obtained by current injection into the laser pair of a common, chaotic driving-signal. Results of simulations are reported for the configuration in which the chaotic driving-current is obtained by photodetection of the emission of a third laser (driver), chaotic by delayed optical feedback in a short cavity scheme, selected with different parameters with respect to the laser pair. The emissions of the synchronized, matched lasers are highly correlated, whereas their correlation with the driver is low. The digital message modulates the pumping current of the transmitter. Message recovery is performed by subtracting the chaos, locally generated by the synchronized receiver laser, from the signal obtained by photodetection (at the receiver side) of the chaos-masked message transmitted in free space. Simulations have been performed with the Lang-Kobayashi model, keeping into account both attenuation of the optical signal in a line-of-sight configuration, and noise. Security has been investigated and demonstrated by considering the effect, on synchronization and message recovery, of the parameter mismatch between transmitter and receiver. Index Terms Chaos, cryptography, communication systems. I. INTRODUCTION OPTICAL chaotic cryptography [1], [2] is a hardware technique for secure transmission, which makes use of a pair of lasers routed to chaos. A standard DFB telecommunication laser operating in a chaotic regime, for example by back-reflection from a remote mirror, exhibits a widened spectrum, typically in the order of GHz; its emission in the time domain is amplitude modulated, showing a non periodic and very complex, apparently random, behavior, which, however, can be described on the basis of a deterministic model. In the cryptographic schemes, one of the sources is used for the transmission, i.e., to codify the message with chaos; Manuscript received November 12, 2007; revised April 04, This work was supported in part by the Italian Ministry of University and Research (MUR), under a PRIN-COFIN 2005 contract, and in part by EU Project PICASSO IST V. Annovazzi-Lodi and G. Aromataris are with the Dipartimento di Elettronica, Università degli Studi di Pavia, Pavia, Italy ( valerio. annovazzi@unipv.it; giuseppe.aromataris@unipv.it). M. Benedetti and S. Merlo are with the Dipartimento di Elettronica, Università degli Studi di Pavia, Pavia, Italy ( mauro.benedetti@unipv.it; sabina.merlo@unipv.it). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JQE the other one is used at the receiver for message extraction. In the basic scheme, chaos is simply superposed over the message to strongly reduce its signal-to-noise (S/N) ratio, thus implementing the so-called chaos masking [3]. Extraction of the hidden message from chaos is based on the synchronization of transmitter and receiver, i.e., on the generation of the same chaotic waveform at both ends of the channel. Synchronization can be obtained by optical injection of a fraction of the transmitter laser output into the receiver laser, which, under suitable conditions, replicates the chaotic regime of the transmitter but does not replicate the message. Message extraction is simply performed by making the difference between the signal coming from the transmitter, and the chaotic signal replicated at the receiver. However, it is very difficult, for an eavesdropper, to extract the message, because effective synchronization relies on the use of twin lasers, i.e., two lasers with very similar parameters (typically, the two devices must be not only of the same model, but also selected in close proximity from the same wafer). After initial investigations on basic principles, more recently work has been focused towards the application of all-optical chaotic cryptography to real networks. Digital transmission on a metropolitan network [4] has been performed. Analog transmission of radio and video signals [5], [6] on optical fibers has been also reported. Several basic functional blocks have been already studied and experimentally demonstrated, such as the chaotic signal repeater [7], modules for point multipoint connections [8], for two channel transmission [9], for wavelength multiplexing [10] and for wavelength conversion [11]. In addition to fiberoptic networks, transmission links based on free-space optics (FSO) technology, that exploit a modulated laser beam traveling in open space through the atmosphere, have been envisaged and designed. Point-to-point connections, between two locations on the line-of-sight, are commercially available [12]. Free-space optics links (FSOL) represent an interesting alternative to fiber optics links for small/medium private networks because their installation and maintenance is less expensive and because they are license free. Point-to-point optical interconnections may also work by diffuse radiation, exploiting reflection and diffusion of the walls and the ceiling of a room [13]. Another important application of free-space optics technology is represented by optical transmission links between satellites. Information security remains, however, a major issue in free-space optical networks. The absence of protected propagation increases the risk of eavesdropping and makes this kind of systems intrinsically non-secure. The schemes of optical chaotic cryptography already studied for fiber transmission /$ IEEE

2 1090 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 11, NOVEMBER 2008 Fig. 1. Optical configuration for secure data transmission with semiconductor lasers as Tx and Rx, which are routed into the chaotic regime and synchronized by means of current injection of a common, chaotic driving-signal. can be proposed, in principle, also for this application. However, the different characteristics of the transmitted signal suggest to consider dedicated schemes. More specifically, a major improvement, in terms of cost and practical feasibility, would be offered by schemes allowing electrical, instead of optical, signal amplification. A possible solution is proposed and analyzed in this paper. In the scheme illustrated in detail in Section II, optical injection is replaced by current injection. Two semiconductor lasers (transmitter and receiver) are routed into a synchronized chaotic regime by means of injection into the pump of a common, chaotic driving-signal. The reduced bandwidth requirements, with respect to fiber transmission, make this approach attractive, since low-cost Monolithic Microwave Integrated Circuits (MMIC) can be used for signal amplification. In Section II, we also briefly compare our system with other optoelectronic schemes already present in the literature. In Section III, we report the equations which describe the operation of the selected scheme, based on the Lang-Kobayashi model. Langevin and photodetection noise terms as well as transmission losses are considered. Results of the numerical simulations for a baseband digital transmission at 1 Gb/s are reported in Section IV, whereas the results relative to a 100 Mb/s signal transmitted on a 4.65 GHz carrier are illustrated in Section V. II. FREE-SPACE OPTICS CONFIGURATION The selected configuration for secure chaotic transmission on a free-space optics data link is illustrated in Fig. 1. As in previously investigated schemes for optical chaos cryptography, we assume to use a pair of twin semiconductor lasers, which are subject to optical feedback from external reflectors. These lasers represent the transmitter (Tx) and receiver (Rx) of the communication link. As shown in Fig. 1, a common, chaotic driving-signal is superposed to the pumping currents of the laser pair. We performed numerical simulations for the case in which this common, chaotic driving-current is obtained by photodetecting the intensity emission of a third laser (driver, Drv in Fig. 1), chaotic by delayed optical feedback, selected with different parameters with respect to the matched laser pair. Under suitable conditions, this common chaotic input forces the Tx and Rx lasers to generate highly correlated chaotic waveforms (i.e., to synchronize to each other); their output waveforms are however different from that generated by the Drv. The message to be transmitted modulates the pumping current of the transmitter as in standard Chaos Shift Keying (CSK) [1] [3] for secure data transmission. Message recovery can be attained by subtracting the chaos locally generated by the synchronized receiver, detected by photodiode PD4, from the chaos-masked message (message + masking chaos from the Tx) detected by photodiode PD3 after propagation in free space. Two more photodiodes, PD1 and PD2 are used for current conversion of the optical chaotic signal, generated by the driver laser. In principle, they would not be required if the common chaotic driving were electrically generated and distributed by a RF link. Security of this cryptographic configuration is supported by specific requirements on Rx/Tx matching for ensuring good synchronization, as it will be shown in the following. Synchronization of semiconductor lasers induced by means of a common chaotic signal was recently demonstrated numerically and experimentally in [14], for the distribution of secret communication keys: in this set-up, optical injection from the chaotic driver laser, into the two response lasers, was required for inducing the chaotic regime in these (otherwise unperturbed) lasers and for pursuing synchronization. Electro-optical injection in chaos cryptography was proposed by other authors for data transmission along a fiberoptic link. In [15], Larger et al. generated chaos by exploiting the nonlinearity of a Mach-Zehnder device in an optoelectronic feedback loop. Chaotic communications using semiconductor lasers with op-

3 ANNOVAZZI-LODI et al.: SECURE CHAOTIC TRANSMISSION ON A FREE-SPACE OPTICS DATA LINK 1091 TABLE I PARAMETERS toelectronic delayed feedback and optoelectronic injection between transmitter and receiver were reported in [16] [18]. The chaotic carrier was generated by a semiconductor laser with optoelectronic feedback, and chaotic communication was realized by synchronizing the receiver laser with the transmitter laser by means of electrical injection between these two lasers. In these schemes, feedback/coupling delay times and strengths were required to be carefully adjusted and controlled. On the other hand, the transmission scheme that we present in this paper is a modification of the standard all-optical scheme [1] [3]. This variant is intended for free space links, where, due to reduced bandwidth requirements, propagation losses can be conveniently compensated for by using low-cost electrical RF amplifiers. Differently from [15] [18], optoelectronic feedback is not used in the transmitter or in the receiver. In our architecture, a third laser is required for common chaotic driving, but no optoelectronic feedback loop needs to be adjusted. Optoelectronic coupling exists between driver and transmitter, and between driver and receiver: in principle, this solution would also allow for secure multi-point transmission using a single driver. In these equations, is the slowly varying, complex electric field (normalized, in [m ]) of the driver laser, is the feedback parameter, the carrier density, the constant pumping current, the electron charge, the Planck s constant, the vacuum impedance with vacuum permittivity and speed of light in vacuum. Definitions and values of the other parameters are reported in Table I. Equation (4) indicates how to obtain the true electric field (in [V/m]) from the normalized field, for future comparisons with experimental data. is the spontaneous emission term, and are the Langevin noise terms [20], given by (3) (4) III. NUMERICAL MODELING The well known Lang-Kobayashi model [19] for a singlemode semiconductor laser subject to delayed optical feedback can be easily modified to describe the configuration of Fig. 1. First of all, we can write the following set of equations for the driver laser where are zero-mean, unit-variance Gaussian distributions and is the time resolution in the modeling of white noise. A set of equations can be written for the transmitter laser (1) (2) (5)

4 1092 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 11, NOVEMBER 2008 (6) (7) (8) and another set for the receiver laser (9) (10) (11) (12) In these equations, and are the slowly varying, complex electric fields (normalized, in [m ]) of transmitter and receiver lasers, respectively, is the feedback parameter, and are the carrier densities, and the spontaneous emission terms and the Langevin noise terms, having the same form as for the Driver. The other parameters are specified in Table I. Whereas the pumping current for the driver laser is constant, the pumping currents of transmitter and receiver, and in (8) and (12), contain also time-varying terms. In particular, they include the terms and which are, respectively, the signals obtained after amplification and filtering of the output currents and from photodiodes PD1 and PD2 (see Fig. 1), given by (13) (14) These terms contain the common, chaotic driving-signal,. The dc component of the photodetected currents has been subtracted, since we assume that the dc working condition of Tx e Rx lasers is set by their dc pumping currents ( and ). In (13) and (14), and take account of the propagation losses between driver and transmitter/receiver, indicates the time of flight between driver and transmitter, indicates the time of flight between driver and receiver. For simplicity, we have taken. The symbol means absolute value, or module of the complex field, and means the time average. and keep into account the Johnson noise current of the 50 termination resistance and the shot noise current due to direct detection. The transmitter current, given by (8), contains also the component due to the Fig. 2. Numerical RF chaotic power spectra (2.5 MHz resolution bandwidth) of the driver laser, of the transmitter laser (red trace online) and of the receiver laser (blue trace online). For better visualization, we have shifted upwards (+20 db) the traces relative to driver and receiver. The difference signal (green trace online) is also reported. message. In the next Section, we report the numerical results obtained with this model for an example of baseband digital transmission. IV. BASEBAND TRANSMISSION A. Synchronization The parameters of the laser and of the external cavities were initially taken identical for both Tx and Rx, whereas some parameters were different for the driver, as reported in Table I. First of all, we tested the synchronization of transmitter and receiver, without message transmission. We assumed ma, ma. At this current, the driver laser is chaotic by optical feedback. Fig. 2 illustrates the RF chaotic power spectrum obtained by photodetection, amplification, and low-pass filtering ( GHz for all detected currents) of the driver laser output at the Tx site after propagation. A similar spectrum would be obtained from PD2 at the Rx site. We considered attenuation due to free-space transmission for the chaotic driving by taking : since losses were compensated by electrical amplification, the only effect of attenuation is a reduction of the S/N ratio. Fig. 2 contains also the RF chaotic power spectra obtained, by photodetecting, respectively, the transmitter laser output after propagation and the receiver output, both at the Rx site. These lasers are routed into the chaotic regime and synchronized by current injection of the common chaotic signal. In this graph, for a better visualization, the traces relative to the receiver and to the driver were shifted upwards by 20 db: transmitter and receiver spectra would be otherwise superposed, since Tx and Rx were synchronized by current injection. A factor 10 attenuation suffered in transmission between Tx and Rx was completely compensated for by amplification. On the other hand, no losses were considered at the Rx site for the receiver output detected by PD4. The trace relative to the difference signal between the transmitter and the receiver is also reported showing a chaos cancellation of approximately 15 db on the bandwidth of interest. As already specified for photodiodes PD1 and PD2, in the calculations we took into account the Johnson noise of the 50- termination resistance

5 ANNOVAZZI-LODI et al.: SECURE CHAOTIC TRANSMISSION ON A FREE-SPACE OPTICS DATA LINK 1093 Fig. 3. Simulations of chaotic waveforms I (t);i (t);i (t) (see also Fig. 1) as a function of time. Fig. 5. Simulated current waveforms for a baseband link: (a) original message I (t); (b) system output with Drv OFF; (c) attenuated, photodetected and amplified transmitter output I (t) with the message masked by chaos (red online); (d) extracted message I (t) after subtraction at the receiver side in the case of perfect Tx/Rx matching (green online); (e) unsuccessful message extraction in case of photon lifetime mismatch 1 = 5% (blue online); (f) unsuccessful message extraction in case of feedback parameter mismatch 1K =4%(magenta online). verified that it is impossible for the eavesdropper to extract the message by subtracting the Drv output from the Tx output. The maxima occur at since we neglected any time delay. Fig. 4. Cross-correlation as a function of time delay. (a) Tx and Rx. (b) Driver and Tx. Same results as in (b) are obtained for driver and Rx. Insets: crosscorrelation plots. and the shot noise for both photodiodes PD3 and PD4. In Fig. 3, we show typical chaotic waveforms as functions of time of the photodetected currents relative to Drv, relative to Tx, and relative to Rx, in the same conditions as specified for Fig. 2. In Fig. 4, we show the plot of the cross-correlation function between the various detected outputs: a peak higher than 98% is obtained between transmitter and receiver, in agreement with the results shown in [14], whereas lower correlation is observed between driver and transmitter or between driver and receiver. Cross-correlation plots are also reported in the insets, which clearly show that Tx and Rx were successfully synchronized, but they were not synchronized to the driver laser, which supports the security of this scheme. Indeed, it has been numerically B. Message Recovery To investigate the possibility of using this configuration for secure data transmission, we numerically studied message recovery at the system output. A pseudo-random NRZ digital message at 1 Gb/s was used as modulating current, with a peak-to-peak value of 30 A. In Fig. 5, we present numerical results relative to temporal waveforms for comparison among different signals. Trace a is the modulating current with the original message, whereas trace b is the system output without chaotic encryption (Drv OFF), which describes the channel with all the noise sources. Trace c is the current output when the message is completely hidden by chaos: this would be the signal tapped by the eavesdropper. Trace d is the extracted message after subtraction at the receiver, in the case of optimized synchronization between Tx and Rx (with perfectly matched parameters, and equal cavity lengths). C. Effect of the Laser Parameters The quality of the extracted message, even in presence of attenuations and photodetection noise, is quite good in matched conditions. However, it is important to investigate the effect on message extraction of parameter mismatch between Tx and Rx. We found that the scheme is sensitive to mismatch of most internal and external parameters, as well as to the external cavity length difference. Typically, a parameter difference of a few percent between transmitter and receiver prevents the message from being extracted with an acceptable S/N ratio. For example, the effects of a mismatch of photon lifetime % (trace e) and of the feedback parameter % (trace f) are shown in Fig. 5. For both cases of parameter mismatch, the original message cannot be recognized in the extracted signal. The security of the proposed scheme is thus comparable to that of standard chaotic cryptosystems for fiber transmission.

6 1094 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 11, NOVEMBER 2008 Fig. 6. Numerical RF spectra (1.5 MHz resolution bandwidth) for AM transmission of a message at 100 Mb/s on a 4.65 GHz carrier: (a) original message; (b) transmitted signal, in which the message is hidden by chaos (red online); (c) recovered message in the case of perfect parameter matching (green online). Traces a and b were moved upwards by 20 and 60 db, respectively, for better readability. V. CARRIER TRANSMISSION In addition to base-band transmission, the use of a RF carrier modulated by the message can be considered. Besides allowing the use of different channels over the same optical wavelength, this approach provides a method to optimize performances by positioning the message at the frequency where chaos is larger, or where synchronization is better. In fact, the use of a carrier or Manchester coding has been already proposed [21], [22] for optical chaotic transmission in fiber. Carrier transmission has been studied numerically by assuming the same parameter set as in Section IV, except for ma and. The carrier amplitude, superimposed to the pump current, was 50 A, and its frequency was 4.65 GHz. The message was a pseudo-random NRZ digital signal at 100 Mb/s modulating the carrier in amplitude (AM) with 100% modulation depth. We tested the system for different channel attenuations, finding an acceptable S/N ratio of the recovered message down to. In Figs. 6 and 7, typical system performances are shown for and for attenuation between Tx and Rx of a factor 50. In the simulations, all photodiodes were followed by a band-pass filtering stage with a 400 MHz bandwidth around the carrier frequency. In Fig. 6 the RF spectrum of the original message (trace a) is compared to the spectrum of the extracted message (trace c) showing only a low distortion of the modulation side-bands. Instead, in the transmitted message (trace b) only the carrier is partially visible, while the side-bands, containing the information, are hidden by chaos to the eavesdropper. For better readability, traces a and b in Fig. 6 were shifted upwards by 20 and 60 db, respectively. In Fig. 7, signals are shown in the time domain: trace a is the original message; trace b is the demodulated message without chaotic encryption (Drv OFF). Trace c is the demodulated output when the message is hidden by chaos (eavesdropper site). Trace d is the extracted message demodulated after subtraction at the receiver, in the case of optimized synchronization. From these simulations, we can conclude that transmission Fig. 7. Simulation in the time domain of AM transmission of a 100 Mb/s message on a 4.65 GHz carrier: (a) original modulating message; (b) demodulated message without chaos (Drv OFF); (c) demodulated output with message hidden by chaos (red online); (d) recovered demodulated message in the case of perfect matching (green online). around a carrier can be also proposed for a free-space optics link, which significantly widens the applicability range of our cryptographic scheme. VI. CONCLUSION In this paper, we have analyzed a new scheme for chaotic cryptography of an optical signal transmitted in a free-space optics link. We have numerically demonstrated both effective message masking and message recovery in a point-to-point connection on the line-of-sight, both in baseband and with a carrier. The effect of parameter mismatch has been also considered, for security evaluation. Further work is required to extend our scheme to operation in the diffused regime, where the effects of attenuation and dispersion represent a strong limitation [13] even with standard links. For what concerns attenuation, this is expected to be 1 2 orders of magnitudes higher than operating with a collimated beam. To compensate such losses, a strong electrical amplification is required, and the resulting degradation of S/N ratio must be taken into account. However, the major limitation would probably come from dispersion due to the presence of multiple paths, with a corresponding distribution of flight time, resulting in a distortion of the step response and a reduction of the bandwidth. In the case of chaotic cryptography, these limitations may worsen the quality of synchronization between transmitter and receiver, because they modify the injection process on which this synchronization is based. 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Shore, The relative intensity noise of a semiconductor laser subject to strong coherent optical feedback, J. Opt. B-Quantum Semiclassical Opt., vol. 6, no. 8, pp. S775 S779, [21] A. Bogris, K. E. Chlouverakis, A. Argyris, and D. Syvridis, Enhancement of the encryption efficiency of chaotic communications based on all-optical feedback chaos generation by means of subcarriers modulation, in Proc. CLEO, Jun. 2007, Paper JS13_1. [22] L. Ursini, M. Santagiustina, and V. A. Lodi, Enhancing chaotic communication performances by manchester coding, IEEE Photon. Technol. Lett., vol. 20, no. 6, pp , Mar Valerio Annovazzi-Lodi (M 89 SM 99) was born in Novara, Italy, on November 7, He received the degree in electronic engineering from the University of Pavia, Pavia, Italy, in Since then he has been working at the Department of Electronics of the University of Pavia in the fields of electronics and electro-optics. His main research interests include injection phenomena and chaos in oscillators and lasers, cryptography, optical sensors, passive fiber components for telecommunications and sensing, optical amplifiers, transmission via diffused infrared radiation, micromechanical systems. In 1983, he became a Staff Researcher of the Department of Electronics of the University of Pavia, in 1992 an Associate Professor and in 2001 a Full Professor of the same institution. He is the author of more than 100 papers and holds four patents. Dr. Annovazzi-Lodi is a member of AEIT and a Senior Member of IEEE- LEOS. Giuseppe Aromataris was born in Siderno, Italy, in He received the degree in physics in 2006 from the University of Milan, Italy, with a thesis on covariant quantum measurement of phase on coherent and squeezed states. He is currently working toward the Ph.D. degree in electronics engineering with the Optoelectronics Group of the University of Pavia, Italy. His research interests include non-linear dynamics on optically injected semiconductor lasers, with regard in particular to numerical analysis on optical chaos synchronization and cryptographic communications systems. Mauro Benedetti (M 05) was born in Pontevico, Italy, in He received the degree in micro-electronics engineering in 2002 and the Ph.D. degree in electronics engineering in 2006 from the University of Pavia, Pavia, Italy. He is currently working as a post-doctoral researcher with the Optoelectronics Group of the University of Pavia. His main research interests include nonlinear dynamics in optically injected semiconductor lasers, with regard in particular to optical chaos synchronization and cryptography in fiber-optic communications, characterization of MEMS/MOEMS devices and Photonic Crystals. Dr. Benedetti is a member of the IEEE-LEOS and of the SICC (Italian Society for Chaos and Complexity). Sabina Merlo (M 2001 SM 2005) was born in Pavia, Italy, in She received the degree in electronic engineering from the University of Pavia, Pavia, Italy, in 1987, the M.S.E. degree in bioengineering from the University of Washington, Seattle, and the Ph.D. degree in electronic engineering from the University of Pavia, Pavia, Italy, in In 1993 she became Assistant Professor and in 2001 she became Associate Professor with the Department of Electronics of the University of Pavia. Her main research interests include MEMS, MOEMS, chaos in lasers, laser interferometry, fiber-optic passive components and sensors. She holds three patents and is the author of more then 60 papers. Dr. Merlo She was the recipient of a Rotary Foundation Graduate Scholarship for studying at the University of Washington, Seattle. he is an Associate Editor of the IEEE/ASME Journal of Microelectromechanical Systems. She is a member of AEIT, and Senior Member of IEEE-LEOS.