Message encoding and decoding using chaotic external-cavity diode ...

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 1, JANUARY 2000

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Message Encoding and Decoding Using Chaotic External-Cavity Diode Lasers S. Sivaprakasam and K. A. Shore

Abstract—Synchronization of chaotic external-cavity diode lasers has been studied in a master–slave configuration. A message is encoded into the chaotic master laser by amplitude modulation and transmitted to the slave laser. A scheme for decoding the message at the slave is demonstrated. Index Terms—Chaos, diode lasers, encryption, synchronization.

I. INTRODUCTION

A

A DIODE laser can be routed to chaos with appropriate optical feedback [1] or by light injection from another source [2]. Practical applications for chaotic dynamics have been proposed in a number of disciplines, especially in communications. Following the publication of a chaos control technique by Ott et al. [3], a number of successful applications of this method and its variants for controlling chaotic dynamics, including lasers, have been reported. In particular, Roy et al.have resolved the problem of multimode instability in the Nd : YAG laser with intracavity frequency doubling in the KTP crystal [4]; Glorieux and co-workers have stabilized a chaotic output of the self-pulsing fiber [5] and modulated CO2 lasers [6]. Attempts have been made to control chaotic dynamics in external-cavity laser diodes [7]. In particular, use was made of occasionally proportional impulsive feedback [8]–[10]. Further interest in controlling semiconductor laser dynamics has been stimulated primarily by the possibilities for achieving secure communication which exploits the properties of chaotic dynamical systems [11]. The underlying concept for this work is that the transmitted message should be encoded within the noise like output of a chaotic transmitter. Extraction of the message requires a receiver in which the same chaos is generated as in the transmitter, which can be achieved by synchronization of the chaos of the transmitter and the receiver. Synchronization of chaos was initially understood to be the perfect coincidence of the chaotic dynamics of two coupled chaotic systems. This kind of synchronized chaos has been observed experimentally in the Nd : YAG laser [12], the CO2 laser [13] and the NH3 laser [14]. Various synchronization algorithms have been proposed in the past [15] with quantitative measures of Lyapunov exponents [16], [17] and with chaotic shift keying [18]. A recent theoretical study by Spencer et al. [19] led to the modeling of optical

Manuscript received April 30, 1999; revised September 10, 1999. The work presented here was supported by the U.K.-Engineering and Physical Science Research Council under Grant GR/K78799. The authors are with the School of Electronic Engineering and Computer Systems, University of Wales at Bangor, Bangor LL57 1UT, Wales, U.K. Publisher Item Identifier S 0018-9197(00)00316-X.

synchronization of chaotic external cavity VCSEL’s and its dependence on the coupling coefficient between the master and slave lasers. Secure communication systems based on chaos in erbium-doped fiber lasers were proposed and studied with message masking and chaos shift keying [20]. Work has been done on digital communication with synchronized chaotic single-mode Nd : YAG lasers [21]. Useful progress has been made toward experimental encryption and decryption by use of a novel form of wavelength chaos [22]. A successful demonstration of chaotic transmission of a message has been reported recently [23] using a fiber laser. These advancements provided a powerful stimulus for further development in this area and led to the demonstration of optical synchronization of chaotic external-cavity Fabry–Perot (FP) laser diodes [24]. In this paper, we report: 1) an experimental study of the dependence of synchronization on the coupling coefficient between the chaotic master and slave external-cavity diode lasers and 2) a scheme implemented experimentally, similar to that proposed in [23], for encoding and decoding a message. II. EXPERIMENTAL SETUP The experimental arrangement is shown schematically in Fig. 1. We have used two single-mode FP lasers emitting at 850 nm, each having a linewidth of 200 MHz (Access Pacific, Model No .APL 830-40.) for our experiments. The side-mode suppression ratio (SMSR) is −20 dB. These lasers are driven by ultralow noise current sources (ILX-Lightwave, LDX-3620) and are temperature-controlled by thermo-electric controllers to a precision of 0.01 K (ILX-Lightwave-LDT-5412). The laser output is collimated using an antireflection (AR)-coated laser diode objective (Newport–FLA11). Both lasers are subjected to an optical feedback from external mirrors (M1 and M2) and the feedback strength is controlled using continuously variable neutral density filters (NDF1 and NDF2). The cavity length is 25 cm in both cases. The optical isolators (OFR-IO-5-NIR-HP) ensure the lasers are free from back reflection, and the typical isolation is −41 dB. The isolator (OI1) ensures that the master is isolated from the slave. The coupling attenuator (CA) enables the percentage of master power fed into the slave laser to be controlled. PD1 and PD2 are two identical fast photodetectors (EG&G-FFD040B) with a response time of 2.5 ns. The output of the master laser is coupled to the photodetector (PD1) by beamsplitters BS1 and BS2. Beamsplitter BS3 acts as the coupling element between the master and slave. Beamsplitter BS4 couples the slave output to photodetector PD2. Photodetector outputs are stored in a digital storage oscilloscope (Fluke Combiscope PM3394B, 200 MHz) and then acquired by a PC.

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 1, JANUARY 2000

Fig. 1. Schematic diagram of the experimental arrangement. BS1–BS4: beamsplitters; PD1, PD2: photodetectors; OI1, OI2: optical isolators; M1, M2: mirrors; NDF’s: neutral density filters; CA: coupling attenuator; CRO: digital oscilloscope.

The output of a frequency generator is used as a message to be encoded. III. EXPERIMENTAL RESULTS A. Synchronization The master laser and slave lasers are driven into chaos by application of appropriate feedback from the external-cavity mirrors. The optical feedback for the master and slave leads to a reduction in their threshold currents of 7.0% and 9.7%, respecvalues are 57.00 tively. The free-running threshold current and 48.50 mA, respectively, for the master and slave lasers. The temperature and current of the master (29.93 C, 1.10 ) and slave (26.66 C, 1.03 ) are adjusted so as to ensure that they operate at the same wavelength. A small percentage (coupling fraction) of the master optical output is fed to the slave, which leads to synchronization of the slave to the master. The traces are stored in oscilloscope memory at a given instant of time and then retrieved to a PC. Fig. 2 shows the time evolution of the master and slave lasers for various coupling coefficients between the master and slave. The coupling coefficient is expressed as the percentage ratio of master laser power injected into the slave cavity to the slave

Fig. 2. Time-series plots of the master laser (top traces) and the slave lasers (bottom traces). The coupling fractions are (a) 1.8%, (b) 7.0%, (c) 21.5%, (d) 34.0%, and (e) 44.0%.

laser output power. The coupling attenuators used to achieve these coupling coefficients have densities of 1.0, 0.5, 0.3, 0.1 and 0.0 (no attenuator), respectively, for percentage coupling coefficients of 1.8, 7.0, 21.5, 34.0 and 44.0. The output power of the lasers are expressed as the relative photodiode output voltage as measured by the oscilloscope and the master laser output is shifted for clarity. It is appreciated that the frequency spectrum for chaotic dynamics in semiconductor lasers typically extends over tens of gigahertz in bandwidth [19]. Due to the limitation imposed by the available oscilloscope, the work reported here considers a part of those chaotic dynamics within a 200-MHz bandwidth of the central frequency. It is noted that the chaotic spectrum is relatively flat around the central frequency and, thus, the present measurements are expected to give a good representation of the dynamical behavior. It would, of course, be of interest to extend the frequency range that is sampled. Synchronization can be further understood by plotting the synchronization plots in which the slave laser output is plotted against master laser output. Fig. 3 shows these synchronization plots for various coupling coefficients. These plots show improvement of synchronization from Fig. 3(a)–(c) and deterioration until Fig. 3(e). This can be quantitatively expressed in terms of the standard deviation obtained by fitting the synchronization plots to a straight line using the least-squares method. Fig. 4 shows a slow decrease in standard deviation initially and then

SIVAPRAKASAM AND SHORE: MESSAGE ENCODING AND DECODING USING CHAOTIC EXTERNAL-CAVITY DIODE LASERS

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Fig. 4. Standard deviation versus coupling coefficient.

Fig. 3. Synchronization plots for coupling fractions. (a) 1.8%. (b) 7.0%. (c) 21.5%. (d) 34.0%. (e) 44.0%.

a sharp increase in its value. The coupling coefficient window is sufficiently wide (from 10% to 35%) to accommodate quite large parameter variations to permit practical use of this effect. This behavior of the standard deviation is in agreement with recent theoretical work [19]. The synchronization is found to persist for many hours continuously, after which the thermal effects due to room-temperature fluctuations lead to its destruction. Temperature stabilization will thus be a prerequisite for practical applications. B. Encoding and Decoding Since synchronization has been demonstrated to be robust over quite large time scales and with a large coupling coefficient window, encoding a message into the chaotic laser for transmission and decoding is relatively easy. The transmitter consists of the chaotic master laser and signal generator from which the message is derived. The receiver setup consists of the slave laser and two photodetectors (PD1 and PD2) to monitor the input to and output from the slave laser. A 1-mV 9.5-kHz square wave from the signal generator is used as and is added to the chaotic master laser the message by direct amplitude modulation. This encoded output goes through the coupling laser output attenuator having an optical density of 0.5 to the slave laser. The receiver input is coupled to photodetector PD1 and the receiver output is coupled to photodetector PD2. The slave

laser setup is as identical to the master laser setup as possible, especially in terms of cavity length, feedback strength, and operating current. A similar technique to that followed by Van Wiggeren and Roy [23] is used to decode the message. is recorded from The transmitter output the output of photodetector PD1 and is shown in Fig. 5(a). is recorded from the output of The receiver output the photodetector PD2 and is shown is Fig. 5(a). The power spectra of the transmitter output and the receiver output are shown in Fig. 5(b) and (c), respectively. The arrow indicates the message component. The intensities at photodetectors PD1 and , respectively. and PD2 are thus The message and the considered dynamics of the diode lasers are on the same time scales, which allows us to recover the message by taking a simple difference in the output intensities, without any filtering. In the process, the master and slave lasers stay synchronized and the synchronization plot for this case is shown in Fig. 6. The difference in the photodetector intensities is shown in Fig. 7, which is a good recovery of the original message. An attempt was made to extract a message transmitted between unsynchronized chaotic lasers. It proved impossible to decode the message in this case, thus establishing that an essential requirement for decoding is the achievement of chaos synchronization. Theoretical studies [19] indicate that synchronization can be obtained over gigahertz bandwidths and, hence, the message extraction at megahertz to gigahertz modulation rates would appear to be feasible. IV. CONCLUSIONS It has been shown experimentally that two chaotic externalcavity semiconductor diode lasers could be synchronized for many hours continuously and the synchronization quality, as expressed in terms of standard deviation, is in agreement with recent theoretical predictions. The degree of synchronization is controlled by the coupling coefficient between the master and slave lasers, and the best synchronization is observed for coupling coefficients between 10% and 35%. A simple message is encoded to the chaotic master laser and transmitted to the chaotic synchronized slave laser. The transmitted message and the receiver output are recorded by two identical photodetectors. The message is decoded by taking the difference between the photodiode intensities. The recovered

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 1, JANUARY 2000

Fig. 5. (a) The chaotic transmitter output with message and the receiver output. (b) Power spectrum of the transmitter output. (c) Power spectrum of the receiver output.

the chaos [25]. The provision of a clear demonstration of the security of the chaotic transmission system will, however, require further experimental and theoretical work. ACKNOWLEDGMENT The authors gratefully acknowledge discussions with P. S. Spencer. REFERENCES

Fig. 6. Synchronization plot between the transmitter output and the receiver output.

Fig. 7. Decoded message corresponding to an encoded square-wave of frequency 9.5 kHz.

message is a good representation of the message. In recent work, we have shown that the message can be effectively masked in

[1] J. Mork, B. Tromborg, and J. Mark, “Chaos in semiconductor lasers with optical feedback: Theory and experiment,” IEEE J. Quantum Electron., vol. 28, pp. 93–107, Jan. 1992. [2] V. Annovazzi-Lodi, S. Donati, and M. Manna, “Chaos and locking in a semiconductor laser due to external injection,” IEEE J. Quantum Electron., vol. 30, pp. 1537–1541, July 1994. [3] E. Ott, C. Greboggi, and J. A. Yorke, “Controlling chaos,” Phys. Rev. Lett., vol. 64, pp. 1196–1199, 1990. [4] R. Roy, T. W. Murphy, T. D. Maier, Z. Gills, and E. R. Hunt, “Dynamical control of chaotic laser: Experimental stabilization of a globally coupled system,” Phys. Rev. Lett., vol. 68, pp. 1259–1261, 1992. [5] S. Bielawski, M. Bouazaoui, D. Derosier, and P. Gloireux, “Stabilization and characterization of unstable steady states in laser,” Phys. Rev A, vol. 47, pp. 3276–3279, 1993. [6] , “Controlling unstable periodic orbits by delayed continuous feedback,” Phys. Rev. E, vol. 49, pp. 971–973, 1994. [7] N. Kikuchi, Y. Liu, and J. Ohtsubo, “Chaos control and noise suppression in external-cavity semiconductor lasers,” IEEE J. Quantum Electron., vol. 33, pp. 56–65, Jan. 1997. [8] G. R. Gray, A. T. Ryan, G. P. Agrawal, and E. C. Gage, “Optical-feedback induced chaos and its control in semiconductor lasers,” Chaos in Optics, vol. 2039, pp. 45–47, 1993. [9] Y. Liu and J. Ohtsubo, “Experimental control of chaos in a laser diode interferometer with delayed feedback,” Opt. Lett., vol. 19, pp. 448–450, 1994. [10] A. V. Naumenko, N. A. Loiko, S. I. Turovets, P. S. Spencer, and K. A. Shore, “Controlling dynamics in external-cavity laser diodes with electronic implusive delayed feedback,” J. Opt. Soc. Amer. B., vol. 15, pp. 551–561, Feb. 1998. [11] S. Hayes, C. Greboggi, and E. Ott, “Communicating with chaos,” Phys. Rev. Lett., vol. 70, pp. 3031–3034, 1993. [12] R. Roy and K. S. Thornburg Jr., “Experimental synchronization of chaotic lasers,” Phys. Rev., Lett., vol. 72, pp. 2009–2012, Mar. 1994. [13] T. Sugawara, M. Tachikawa, T. Tsukamoto, and T. Shimisu, “Observation of synchronization in laser chaos,” Phys. Rev. Lett., vol. 72, pp. 3502–3506, May 1994.

SIVAPRAKASAM AND SHORE: MESSAGE ENCODING AND DECODING USING CHAOTIC EXTERNAL-CAVITY DIODE LASERS

[14] D. Y. Tang, R. Dykstra, M. W. Hamilton, and N. R. Heckenberg, “Observation of generalized synchronization of chaos in a driven chaotic system,” Phys. Rev. E, vol. 57, pp. 5247–5251, May 1998. [15] K. A. Shore and D. T. Wright, “Improved synchronization algorithm for chaotic communications system,” Electron. Lett., vol. 30, pp. 1203–1204, July 1994. [16] L. M. Pecora and T. L. Carroll, “Synchronization in chaotic systems,” Phys. Rev. Lett., vol. 64, pp. 821–824, Feb. 1990. [17] H. G. Winful and L. Rahman, “Synchronised chaos and spatiotemporal chaos in arrays of coupled lasers,” Phys, Rev. Lett., vol. 65, pp. 1575–1578, Sept. 1990. [18] V. Annovazzi-Lodi, S. Donati, and A. Scire, “Synchronization of chaotic injected-laser systems and its application to optical cryptography,” IEEE J. Quantum Electron., vol. 32, pp. 953–959, June 1996. [19] P. S. Spencer, C. R. Mirasso, P. Colet, and K. A. Shore, “Modeling of optical synchronization of chaotic external-cavity VCSELs,” IEEE J. Quantum Electron., vol. 34, pp. 1673–1679, Sept. 1998. [20] L. G. Luo and P. L. Chu, “Optical secure communications with chaotic erbium-doped fiber lasers,” J. Opt. Soc. Am. B, vol. 15, pp. 2524–2530, Oct. 1998. [21] P. Colet and R. Roy, “Digital communication with synchronised chaotic lasers,” Opt. Lett., vol. 19, pp. 2056–2058, Dec. 1994.

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[22] J. P. Goedgebuer, L. Larger, and H. Porte, “Optical cryptosystem based on synchronisation of hyperchaos generated by a delayed feedback tunable laser diode,” Phys. Rev. Lett., vol. 80, pp. 2249–2252, 1998. [23] G. D. VanWiggeren and R. Roy, “Communication with chaotic lasers,” Science, vol. 279, pp. 1198–1200, Feb. 1998. [24] S. Sivaprakasam and K. A. Shore, “Demonstration of optical synchronization of chaotic external-cavity laser diodes,” Opt. Lett., vol. 24, pp. 466–468, Apr. 1999. [25] , “Signal masking for chaotic optical communication using external-cavity diode lasers,” Opt. Lett., vol. 24, pp. 1200–1202, Sept. 1999.

S. Sivaprakasam, photograph and biography not available at the time of publication.

K. A. Shore, photograph and biography not available at the time of publication.