Research Article
Vol. 54, No. 14 / May 10 2015 / Applied Optics
4509
Experimental demonstration of polarizationdivision multiplexing of chaotic laser secure communications XINYU DOU, HONGXI YIN,* HEHE YUE,
AND
YU JIN
Lab of Optical Communications and Photonic Technology, School of Information and Communication Engineering, Dalian University of Technology, Dalian 116023, China *Corresponding author: [email protected] Received 11 February 2015; revised 15 April 2015; accepted 20 April 2015; posted 21 April 2015 (Doc. ID 234435); published 7 May 2015
Optical polarization-division multiplexing (PDM) can double the capacity of a communication system. In this paper, PDM between a conventional fiber-optic channel and a chaos-encrypted channel, and between two chaosencrypted channels, is proposed and experimentally investigated. The bit rate for each channel is 1.25 Gb∕s, while the transmission in the standard single-mode fiber can be up to 22.54 km. The effect of the mutual power leakages on the receiver quality is experimentally explored, which is induced by the variation in polarization direction during the propagating process. In addition, the effect of optical launched power at the transmitter side on the Q-factor is tested and analyzed. © 2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (140.1540) Chaos; (060.4230) Multiplexing; (260.5430) Polarization. http://dx.doi.org/10.1364/AO.54.004509
1. INTRODUCTION The investigation of chaotic laser secure communications has achieved great progress, due to the superiority of simple realization, real-time encryption, and compatibility with existing optical fiber communication networks and optical wireless communication networks [1–17]. Up to now, many numerical and experimental results have been reported on the subject of single-channel chaos-encrypted transmission [9–17]. For instance, Argyris et al. realized a field experiment of chaotic optical communications using 120 km commercial optical fiber in the metropolitan network of Athens, Greece, with transmission rates of 1 Gbit∕s and bit-error rates (BERs) lower than 10−7 [17]. For modern fiber-optic communication networks, multiplexing is a crucial technique that can increase the transmission capacity of a single physical channel significantly and costeffectively. For this reason, combining the multiplexing technique with chaotic laser communications will realize not only a natural extension of the system capacity but also the security of message transportation utilizing the existing fiber-optic communication network [18–24]. Argyris et al. realized wavelength-division multiplexing (WDM) between a conventional fiber-optic channel carrying a 10 Gbit∕s data sequence and a chaos-encrypted channel masking a 1.25 Gbit∕s message [25]. Chen et al. realized WDM between two chaotic optical secure channels, in which each channel masked a 1.25 Gbit∕s data sequence [26]. If the polarization-division multiplexing (PDM) 1559-128X/15/144509-05$15/0$15.00 © 2015 Optical Society of America
technique is adopted, the transmission capacity will be doubled once again. Jiang et al. numerically investigated the PDM backto-back transmission between two identical mutual-coupling vertical-cavity surface-emitting lasers (VCSELs) driven simultaneously by a third chaotic VCSEL [27]. To the best of our knowledge, experimental results of PDM for chaotic optical secure communications over a long-haul fiber channel have not been reported so far. In this paper, the PDM for a chaotic optical secure communication system employing distributed-feedback (DFB) lasers is established and experimentally investigated. PDM between a conventional fiber-optic channel and a chaos-encrypted channel, and between two chaos-encrypted channels, is realized, in which the bit rate of each channel is 1.25 Gbits∕s. The physical medium is 22.54 km standard single-mode fiber (SSMF). The effect of mutual power leakage between the two polarizationorthogonalized channels on the communication quality is experimentally investigated. In addition, the effect of optical launched power on the Q-factor is also analyzed. 2. SYSTEM ARCHITECTURE The experimental setup of the chaotic laser secure communications based on PDM is depicted in Fig. 1. The conventional fiber-optic channel consists of the DFB laser LDT 1 , the LiNbO3 Mach–Zehnder modulator (MOD1 ), the erbiumdoped optical fiber amplifier (EDFA 1 ), and the variable optical attenuator (VOA 1 ). The CW light from LDT 1 is modulated by
4510
Vol. 54, No. 14 / May 10 2015 / Applied Optics
PRBS3
EDFA 5
PC 7
VOA 9
MOD 3
EDFA 1
PC 2
VOA 1
PC 6
PRBS2
PC 1
ChaosGenerator 2
LD T1
MOD 1
CIR 1 LD T2
1 x
PRBS1
EDFA 2
PC 4
y
VOA 3
PBS
2
3
PC 3
VOA 2
OC 1 Chaos-
MOD 2
Generator 1
VOA 4 CIR 2 LD R
SSMF 22.54 km
1
2
3
PD 1
VOA 5 PC 5 PBS
y
EDFA 3
LPF
PD 2 m2
OC 2 VDL VOA 6
x
Research Article a 3 dB frequency bandwidth of 625 MHz. The receiver of the conventional fiber-optic channel (Rx2) consists of VOA 7 and photodiode PD3 . PRBS2 is identified by energy detection. To achieve PDM between two chaotic channels, the LDT 1 in the conventional channel is substituted by chaos generator 2 (same as chaos generator 1) and Rx2 is replaced by chaotic receiver Rx3 (same as Rx2). The polarization direction of the signals will change during the propagation process in the fiber link. As a result, the state of PC5 must be adjusted to align the polarization direction of the incoming signal to the main axis of the PBS. In this case, the signals of the two channels can be thoroughly divided. Some external interference, e.g., stress or distortion of the fiber, imposed on the fiber channel will bring unpredictable variation to the polarization direction. This will lead to the mutual power leakage between the two channels, which will be recorded at the output ports of the PBS. Therefore, the quality of communication deteriorates.
Chaos-Receiver (Rx1)
EDFA 4
PD 3
m1
VOA 7 Receiver (Rx2)
EDFA 6 Chaos-Receiver (Rx3)
Fig. 1. System architecture of PDM chaotic optical secure communications. LD, laser diode; CIR, circulator; OC, optical coupler; VOA, variable optical attenuator; PC, polarization coupler; MOD, modulator; EDFA, erbium-doped optical fiber amplifier; PBS, polarization beam splitter; PD, photodiode; VDL, variable delay lines; LPF, low-pass filter.
a 1.25 Gbit∕s non-return-to-zero (NRZ) pseudorandom bit sequence (PRBS1) with 215 − 1 length. The polarization direction of the output of VOA 1 is aligned to the x axis by the polarization controller (PC2 ). Concerning the chaos-encrypted channel, the chaotic laser is generated by LDT 2 , the circulator (CIR 1 ), and the optical coupler (OC1 ) through optical feedback [28]. Another 1.25 Gbit∕s NRZ-PRBS (PRBS2) with 215 − 1 length is encrypted through chaos modulation [29]. The polarization direction of the output of VOA 3 is aligned to y axis by PC4 . The two channels are multiplexed into the same fiber link to achieve the PDM. In our experiment, the launched optical power of the two channels is approximately 9 dBm. The central wavelengths of both channels are at 1554.94 nm, while the fiber link is 22.54 km SSMF (G.652) with an attenuation coefficient of 0.2 dB∕km and a dispersion coefficient of 17 ps∕km · nm. At the receiver side, the signals of the two polarization-orthogonalized channels are thoroughly divided by the polarization beam splitter (PBS). In chaotic receiver 1 (Rx1), the chaotic carrier masking the message is divided into two paths by OC2 . One path of the light is injected into LDR to regenerate the chaotic carrier at the receiver side, which is synchronous to the incoming signal. Then, the other path of the incoming chaotic signal is subtracted by the locally generated chaotic carrier. Afterward, the message PRBS1 can be decrypted after a low-pass-filter (LPF). For our experiment, the LPF is a 4th Bessel–Thompson filter with
3. RESULTS AND DISCUSSION The experimental results of the PDM between the conventional channel and the chaotic channel are shown in Fig. 2. Figures 2(a1)–2(a3) are the results of the conventional channel for different leaked powers, while Figs. 2(b1)–2(b3) are those of the chaotic channel under the same conditions. The yellow (top), green (middle), and blue (bottom) time series are the chaotic signal containing the message, the synchronous chaotic carrier emitted from LDR , and the recovered message, respectively. The red time series superimposed on the blue (bottom) waveform is the original message. As can be seen from Figs. 2(a1) and 2(b1), the PDM between the conventional channel and the chaotic channel is realized by the appropriate adjustment of PC5 , since the two polarization-orthogonalized channels are thoroughly divided by the PBS. By comparing the blue and red time series in Fig. 2(b1), the message hidden in the chaotic carrier can be successfully recovered. In this work,
Fig. 2. Effect of power leakage on the communication results of the conventional channel and the chaotic channel. (a1)–(a3) The communication results of the conventional channel when the leaked power is 0, 0.2, and 0.5 mW, respectively; (b1)–(b3) the communication results of the chaotic channel under the same conditions.
Research Article
Vol. 54, No. 14 / May 10 2015 / Applied Optics
the Q-factor is used to evaluate the quality of the secure communications. It is defined as follows [18,19]:
4.5
(1)
in which hP 1 i and hP 0 i represent the average power of the recovered bits 1 and 0, respectively. σ 1 and σ 0 are the corresponding standard deviations. The calculated Q-factors of the conventional channel and the chaotic channel are 4.21 and 2.21, respectively. When the fiber link is affected by some external factors, the communications of both channels are interfered by each other. More noise appears in the conventional channel, while the BERs of the signal for the conventional channel and the recovered message for the chaotic channel are increased, as shown in Figs. 2(a2) and 2(b2). The Q-factors for the two channels decrease to 2.83 and 2.09, respectively. When the leaked power reaches 0.5 mW, both channels suffer severe interference from each other, as shown in Figs. 2(a3) and 2(b3). The conventional fiber-optic channel has noise-like characteristics because of the influx of the chaotic laser. Hence the message cannot be identified by the photodiode. The chaotic optical communication also fails. The Q-factors for both channels fall to 1.41 and 1.39. The eye diagrams of the two channels are shown in Fig. 3 to describe the quality of communications. When the leaked power is increased, the quality of the eye diagrams of the two channels becomes worse. In order to quantify the above results, the Q-factors of both channels are plotted as a function of the leaked power. As shown in Fig. 4, the conventional fiber-optic channel is affected significantly by the power leakage, because the Q-factor descends almost linearly with the increasing leaked power. However, the Q-factor of the chaotic channel degrades more slowly. When the leaked power is 0.5 mW, the Q-factor falls below 1.6, which means that data recovery becomes impossible [21–23]. In addition, according to Ref. [23], the communications may be corrupted by the chromatic dispersion (CD) and the polarization-mode dispersion (PMD) of the fiber link. In Figs. 3 and 4, when the transmission distance is about
Fig. 3. Eye diagrams of the received message of the conventional channel and the recovered message of the chaotic channel. (a1)–(a3) the eye diagrams of the conventional channel, (b1)–(b3) the eye diagrams of the chaotic channel under the same conditions as Fig. 2.
Conventional channel Chaotic channel
4 3.5 Q−factor
hP i − hP 0 i ; Q 1 σ1 σ0
4511
3 2.5 2 1.5 1
0
0.1
0.2 0.3 0.4 Leaked Power [mW]
0.5
Fig. 4. Effect of varying leaked power on the Q-factor of both channels.
20 km, both CD and PMD have little effect on the message recovery. In this condition, the power leakage between the two polarization-orthogonalized channels is the main reason for the deterioration of the communication results. To investigate the PDM between two chaos-encrypted channels, the conventional fiber-optic channel is substituted with another chaos-encrypted channel. The communication results are shown in Fig. 5. Figures 5(a1) and 5(a2) are the results of the x-polarization channel when the leaked power is 0 and 0.05 mW, respectively, while Figs. 5(b1) and 5(b2) are the results of the y-polarization channel under the same conditions. As shown in Figs. 5(a1) and 5(b1), after PC5 is carefully adjusted, both data sequences hidden in the chaotic carrier can be recovered successfully. The calculated Q-factors of the two channels are 2.09 and 2.10, respectively. In contrast to Fig. 4, the PDM between two chaosencrypted channels is very sensitive to power leakage. A slight power leakage leads to a significant deterioration of the quality of communication of both channels, as shown in Figs. 5(a2) and 5(b2). The eye diagrams of the two chaotic channels are shown in Fig. 6 to verify the phenomenon. The Q-factors fall immediately to 1.41 and 1.44, respectively. With increasing leaked power, the effect of the mutual power leakage on the Q-factors is depicted in Fig. 7. Except for being 2.26 at the original point, the Q-factors stabilize around 1.4 at other values
Fig. 5. Communication results of double chaotic channels. (a1) and (a2) The communication results of the x-polarization channel when the leaked power is 0 and 0.05 mW, respectively; (b1) and (b2) the communication results of the y-polarization channel under the same conditions.
4512
Research Article
Vol. 54, No. 14 / May 10 2015 / Applied Optics
2.3 x−pol channel y−pol channel
2.25
Q−factor
2.2 2.15 2.1 2.05
Fig. 6. Eye diagrams of the recovered message of the two chaotic channels. (a1) and (a2) The eye diagrams of the x-polarization channel; (b1) and (b2) the eye diagrams of the y-polarization channel under the same conditions as Fig. 5.
2 0.1
x−pol channel y−pol channel
Q−factor
1.8 1.6 1.4 0
0.1
0.2 0.3 0.4 Leaked Power [mW]
0.5
Fig. 7. Effect of power leakage on the Q-factor of both chaotic channels.
−10
(a)
(b)
0.6
0.7
D E
−30
−30
Power [dBm]
Power [dBm]
−20
A B C
−20
0.4 0.5 Py/Px
nearly flat up to the cutoff frequency of the digital oscilloscope. Thus, the leaked conventional signal affects only the lowfrequency component of the chaotic carrier, as shown by curve C of Fig. 8(a). Accordingly, the chaotic optical communication is not greatly affected. However, the leaked chaotic signal affects nearly the whole spectrum of the other chaotic channel. All the frequency components are elevated, and the spectrum becomes more flat, which is shown as curve E in Fig. 8(b). As a result, the quality of chaotic secure communications degrades rapidly. Therefore, for PDM between two chaos-encrypted channels, external interference should be avoided to ensure the communication quality. Figure 9 depicts the effects of the optical launched power ratios of both chaotic channels on the Q-factors. The power ratio is defined as Py∕Px, where Py and Px represent for the launched optical power of the y- and x-polarization channels, respectively. In this experiment, Py is scanned from 0.15× to 0.7× of Px, while the launched optical power of the x-polarization channel is fixed at 10 dBm. For different power ratios, the Q-factors of both chaotic channels stabilize around 2.1. Thus, for a specific multiplexing distance, the optical launched power of the x- (y-) polarization channel has little effect on the communication quality of the y- (x-) polarization channel. It is
2
1.2
0.3
Fig. 9. Effect of optical launched power on the Q-factor of both chaotic channels.
of the leaked power. This can be explained by the aspect of the frequency domain. For the conventional signal in our experiment, the data is a 1.25-Gbit/s PRBS, and thus the power of the spectrum concentrates mainly between 0 and 1.25 GHz, which is shown as curve A in Fig. 8(a). Compared with A, the spectrum of the chaotic carrier [curve B in Fig. 8(a)] is
2.2
0.2
−40 −50 −60
−40 −50 −60 −70
−70 −80 −80 0
1
2 3 4 Frequency [GHz]
5
−90 0
1
2 3 4 Frequency [GHz]
5
Fig. 8. RF spectrums of the communication signals. (a) Curves A (top, green), B (middle, blue), and C (bottom, red) are the RF spectra of the conventional signal, the chaotic carrier, and the chaotic carrier affected by the leaked conventional signal, respectively. (b) Curves D (top, blue) and E (bottom, red) are the RF spectra of the chaotic carrier and the chaotic carrier affected by the leaked chaotic signal, respectively.
Research Article worth noting that if the WDM technique is introduced to the system described in this work, the capacity of the system can be significantly multiplied. This will be the topic of future research. 4. CONCLUSIONS On the basis of the existing multiplexing technique, PDM can double the transmission capacity of the fiber link. In this paper, PDM between a conventional fiber-optic channel and a chaos-encrypted channel, and between two chaos-encrypted channels, is proposed and experimentally investigated. The bit rate of each channel and the transmission in the SSMF can reach up to 1.25 Gbits∕s and 22.54 km, respectively. The effect of mutual power leakage between the two polarizationorthogonalized channels on the Q-factors is investigated. The experimental results show that the communication quality of the chaotic channel is robust to the optical power leaked from the conventional channel, but is sensitive to that leaked from the chaotic channel. Moreover, whether or not the optical launched power of the two chaotic channels is equal has no effect on the Q-factors of both channels. First HAEPC Science and Technology Project in 2015; National Natural Science Foundation of China (NSFC) (61071123). REFERENCES 1. V. Annovazzi-Lodi, M. Benedetti, S. Merlo, M. Norgia, and B. Provinzano, “Optical chaos masking of video signals,” IEEE Photon. Technol. Lett. 17, 1995–1997 (2005). 2. G. D. VanWiggeren and R. Roy, “Communications with chaotic lasers,” Science 279, 1198–1200 (1998). 3. J. Wu, Z. Wu, Y. Liu, L. Fan, X. Tang, and G. Xia, “Simulation of bidirectional long-distance chaos communication performance in a novel fiber-optic chaos synchronization system,” J. Lightwave Technol. 31, 461–467 (2013). 4. J. Wu, Z. Wu, X. Tang, L. Fan, W. Deng, and G. Xia, “Experimental demonstration of LD-based bidirectional fiber-optic chaos communication,” IEEE Photon. Technol. Lett. 25, 587–590 (2013). 5. S. E. Alavi, I. S. Amiri, S. M. Idrus, A. S. M. Supa’at, J. Ali, and P. P. Yupapin, “All-optical OFDM generation for IEEE802.11a based on soliton carriers using microring resonators,” IEEE Photon. J. 6, 7900109 (2014). 6. I. S. Amiri, S. E. Alavi, S. M. Idrus, A. S. M. Supa’at, J. Ali, and P. P. Yupapin, “W-band OFDM transmission for radio-over-fiber link using solitonic millimeter wave generated by MRR,” IEEE J. Quantum Electron. 50, 622–628 (2014). 7. I. S. Amiri, S. E. Alavi, N. Fisal, A. S. M. Supa’at, and H. Ahmad, “All-optical generation of two IEEE802.11n signals for 2 × 2 MIMORoF via MRR system,” IEEE Photon. J. 6, 7903611 (2014). 8. S. E. Alavi, I. S. Amiri, H. Ahmad, A. S. M. Supa’at, and N. Fisal, “Generation and transmission of 3 × 3 W-band MIMO-OFDM-RoF signals using micro-ring resonators,” Appl. Opt. 53, 8049–8054 (2014). 9. P. Colet and R. Roy, “Digital communication with synchronized chaotic lasers,” Opt. Lett. 19, 2056–2058 (1994). 10. C. R. Mirasso, P. Colet, and P. Garcia-Fernandez, “Synchronization of chaotic semiconductor lasers: application to encoded communications,” IEEE Photon. Technol. Lett. 8, 299–301 (1996).
Vol. 54, No. 14 / May 10 2015 / Applied Optics
4513
11. V. Annovazzi-Lodi, S. Donati, and A. Scire, “Synchronization of chaotic injected laser systems and its application to optical cryptography,” IEEE J. Quantum Electron. 32, 953–959 (1996). 12. J.-P. Goedgebuer, P. Levy, L. Larger, C.-C. Chen, and W. T. Rhodes, “Optical communication with synchronized hyperchaos generated electooptically,” IEEE J. Quantum Electron. 38, 1178–1183 (2002). 13. K. Kusumoto and J. Ohtsubo, “1.5-GHz message transmission based on synchronization of chaos in semiconductor lasers,” Opt. Lett. 27, 989–991 (2002). 14. V. Annovazzi-Lodi, M. Benedetti, S. Merlo, T. Perez, P. Colet, and C. R. Mirasso, “Message encryption by phase modulation of a chaotic optical carrier,” IEEE Photon. Technol. Lett. 19, 76–78 (2007). 15. J. Liu, H. Chen, and S. Tang, “Synchronized chaotic optical communications at high bit rates,” IEEE J. Quantum Electron. 38, 1184–1196 (2002). 16. S. E. Alavi, I. S. Amiri, S. M. Idrus, A. S. M. Supa’at, and J. Ali, “Chaotic signal generation and trapping using an optical transmission link,” Life Sci. J. 10, 186–192 (2013). 17. A. Argyris, D. Synridis, L. Larger, V. Annovazzi-Lodi, P. Colet, I. Fischer, J. Garcia-Ojalvo, C. R. Mirasso, L. Pesquera, and K. A. Shore, “Chaos-based communications at high bit rates using commercial fibre-optic links,” Nature 438, 343–346 (2005). 18. T. Matsuura, A. Uchida, and S. Yoshimori, “Chaotic wavelength division multiplexing for optical communication,” Opt. Lett. 29, 2731–2733 (2004). 19. F. Zhang, P. L. Chu, R. Lai, and G. R. Chen, “Dual-wavelength chaos generation and synchronization in erbium-doped fiber lasers,” IEEE Photon. Technol. Lett. 17, 549–551 (2005). 20. J. Z. Zhang, A. B. Wang, J. F. Wang, and Y. C. Wang, “Wavelength division multiplexing of chaotic secure and fiber-optic communication,” Opt. Express 17, 6357–6367 (2009). 21. Q. C. Zhao and H. X. Yin, “Performance analysis of dense wavelength division multiplexing secure communications with multiple chaotic optical channels,” Opt. Commun. 285, 693–698 (2012). 22. N. Li, W. Pan, L. Yan, B. Luo, M. Xu, Y. Tang, N. Jiang, S. Xiang, and Q. Zhang, “Chaotic optical cryptographic communication using a three-semiconductor-laser scheme,” J. Opt. Soc. Am. B 29, 101–108 (2012). 23. C. Antonelli, A. Mecozzi, M. Santagiustina, and L. Ursini, “Impairments due to polarization-mode dispersion in chaos-encrypted communication systems,” IEEE Photon. Technol. Lett. 21, 1387–1389 (2009). 24. Q. C. Zhao, H. X. Yin, and X. L. Chen, “Long-haul dense wavelength division multiplexing between a chaotic optical secure channel and a conventional fiber-optic channel,” Appl. Opt. 51, 5585–5590 (2012). 25. A. Argyris, E. Grivas, A. Bogris, and D. Syvridis, “Transmission effects in wavelength division multiplexed chaotic optical communication systems,” J. Lightwave Technol. 28, 3107–3114 (2010). 26. X. L. Chen, W. Chang, H. X. Yin, Q. C. Zhao, and H. H. Yue, “Experimental investigation of wavelength division multiplexing secure communications with chaotic optical channel,” in Asia Communications and Photonics Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), paper AW4F.2. 27. N. Jiang, W. Pan, B. Luo, S. Xiang, and L. Yang, “Bidirectional dualchannel communication based on polarization-division-multiplexed chaos synchronization in mutually coupled VCSELs,” IEEE Photon. Technol. Lett. 24, 1094–1096 (2012). 28. J. Mork, B. Tromborg, and J. Mark, “Chaos in semiconductor lasers with optical feedback: theory and experiment,” IEEE J. Quantum Electron. 28, 93–108 (1992). 29. D. Kanakidis, A. Argyris, A. Bogris, and D. Syvridis, “Influence of the decoding process on the performance of chaos encrypted optical communication systems,” J. Lightwave Technol. 24, 335–341 (2006).
Vol. 54, No. 14 / May 10 2015 / Applied Optics
4509
Experimental demonstration of polarizationdivision multiplexing of chaotic laser secure communications XINYU DOU, HONGXI YIN,* HEHE YUE,
AND
YU JIN
Lab of Optical Communications and Photonic Technology, School of Information and Communication Engineering, Dalian University of Technology, Dalian 116023, China *Corresponding author: [email protected] Received 11 February 2015; revised 15 April 2015; accepted 20 April 2015; posted 21 April 2015 (Doc. ID 234435); published 7 May 2015
Optical polarization-division multiplexing (PDM) can double the capacity of a communication system. In this paper, PDM between a conventional fiber-optic channel and a chaos-encrypted channel, and between two chaosencrypted channels, is proposed and experimentally investigated. The bit rate for each channel is 1.25 Gb∕s, while the transmission in the standard single-mode fiber can be up to 22.54 km. The effect of the mutual power leakages on the receiver quality is experimentally explored, which is induced by the variation in polarization direction during the propagating process. In addition, the effect of optical launched power at the transmitter side on the Q-factor is tested and analyzed. © 2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (140.1540) Chaos; (060.4230) Multiplexing; (260.5430) Polarization. http://dx.doi.org/10.1364/AO.54.004509
1. INTRODUCTION The investigation of chaotic laser secure communications has achieved great progress, due to the superiority of simple realization, real-time encryption, and compatibility with existing optical fiber communication networks and optical wireless communication networks [1–17]. Up to now, many numerical and experimental results have been reported on the subject of single-channel chaos-encrypted transmission [9–17]. For instance, Argyris et al. realized a field experiment of chaotic optical communications using 120 km commercial optical fiber in the metropolitan network of Athens, Greece, with transmission rates of 1 Gbit∕s and bit-error rates (BERs) lower than 10−7 [17]. For modern fiber-optic communication networks, multiplexing is a crucial technique that can increase the transmission capacity of a single physical channel significantly and costeffectively. For this reason, combining the multiplexing technique with chaotic laser communications will realize not only a natural extension of the system capacity but also the security of message transportation utilizing the existing fiber-optic communication network [18–24]. Argyris et al. realized wavelength-division multiplexing (WDM) between a conventional fiber-optic channel carrying a 10 Gbit∕s data sequence and a chaos-encrypted channel masking a 1.25 Gbit∕s message [25]. Chen et al. realized WDM between two chaotic optical secure channels, in which each channel masked a 1.25 Gbit∕s data sequence [26]. If the polarization-division multiplexing (PDM) 1559-128X/15/144509-05$15/0$15.00 © 2015 Optical Society of America
technique is adopted, the transmission capacity will be doubled once again. Jiang et al. numerically investigated the PDM backto-back transmission between two identical mutual-coupling vertical-cavity surface-emitting lasers (VCSELs) driven simultaneously by a third chaotic VCSEL [27]. To the best of our knowledge, experimental results of PDM for chaotic optical secure communications over a long-haul fiber channel have not been reported so far. In this paper, the PDM for a chaotic optical secure communication system employing distributed-feedback (DFB) lasers is established and experimentally investigated. PDM between a conventional fiber-optic channel and a chaos-encrypted channel, and between two chaos-encrypted channels, is realized, in which the bit rate of each channel is 1.25 Gbits∕s. The physical medium is 22.54 km standard single-mode fiber (SSMF). The effect of mutual power leakage between the two polarizationorthogonalized channels on the communication quality is experimentally investigated. In addition, the effect of optical launched power on the Q-factor is also analyzed. 2. SYSTEM ARCHITECTURE The experimental setup of the chaotic laser secure communications based on PDM is depicted in Fig. 1. The conventional fiber-optic channel consists of the DFB laser LDT 1 , the LiNbO3 Mach–Zehnder modulator (MOD1 ), the erbiumdoped optical fiber amplifier (EDFA 1 ), and the variable optical attenuator (VOA 1 ). The CW light from LDT 1 is modulated by
4510
Vol. 54, No. 14 / May 10 2015 / Applied Optics
PRBS3
EDFA 5
PC 7
VOA 9
MOD 3
EDFA 1
PC 2
VOA 1
PC 6
PRBS2
PC 1
ChaosGenerator 2
LD T1
MOD 1
CIR 1 LD T2
1 x
PRBS1
EDFA 2
PC 4
y
VOA 3
PBS
2
3
PC 3
VOA 2
OC 1 Chaos-
MOD 2
Generator 1
VOA 4 CIR 2 LD R
SSMF 22.54 km
1
2
3
PD 1
VOA 5 PC 5 PBS
y
EDFA 3
LPF
PD 2 m2
OC 2 VDL VOA 6
x
Research Article a 3 dB frequency bandwidth of 625 MHz. The receiver of the conventional fiber-optic channel (Rx2) consists of VOA 7 and photodiode PD3 . PRBS2 is identified by energy detection. To achieve PDM between two chaotic channels, the LDT 1 in the conventional channel is substituted by chaos generator 2 (same as chaos generator 1) and Rx2 is replaced by chaotic receiver Rx3 (same as Rx2). The polarization direction of the signals will change during the propagation process in the fiber link. As a result, the state of PC5 must be adjusted to align the polarization direction of the incoming signal to the main axis of the PBS. In this case, the signals of the two channels can be thoroughly divided. Some external interference, e.g., stress or distortion of the fiber, imposed on the fiber channel will bring unpredictable variation to the polarization direction. This will lead to the mutual power leakage between the two channels, which will be recorded at the output ports of the PBS. Therefore, the quality of communication deteriorates.
Chaos-Receiver (Rx1)
EDFA 4
PD 3
m1
VOA 7 Receiver (Rx2)
EDFA 6 Chaos-Receiver (Rx3)
Fig. 1. System architecture of PDM chaotic optical secure communications. LD, laser diode; CIR, circulator; OC, optical coupler; VOA, variable optical attenuator; PC, polarization coupler; MOD, modulator; EDFA, erbium-doped optical fiber amplifier; PBS, polarization beam splitter; PD, photodiode; VDL, variable delay lines; LPF, low-pass filter.
a 1.25 Gbit∕s non-return-to-zero (NRZ) pseudorandom bit sequence (PRBS1) with 215 − 1 length. The polarization direction of the output of VOA 1 is aligned to the x axis by the polarization controller (PC2 ). Concerning the chaos-encrypted channel, the chaotic laser is generated by LDT 2 , the circulator (CIR 1 ), and the optical coupler (OC1 ) through optical feedback [28]. Another 1.25 Gbit∕s NRZ-PRBS (PRBS2) with 215 − 1 length is encrypted through chaos modulation [29]. The polarization direction of the output of VOA 3 is aligned to y axis by PC4 . The two channels are multiplexed into the same fiber link to achieve the PDM. In our experiment, the launched optical power of the two channels is approximately 9 dBm. The central wavelengths of both channels are at 1554.94 nm, while the fiber link is 22.54 km SSMF (G.652) with an attenuation coefficient of 0.2 dB∕km and a dispersion coefficient of 17 ps∕km · nm. At the receiver side, the signals of the two polarization-orthogonalized channels are thoroughly divided by the polarization beam splitter (PBS). In chaotic receiver 1 (Rx1), the chaotic carrier masking the message is divided into two paths by OC2 . One path of the light is injected into LDR to regenerate the chaotic carrier at the receiver side, which is synchronous to the incoming signal. Then, the other path of the incoming chaotic signal is subtracted by the locally generated chaotic carrier. Afterward, the message PRBS1 can be decrypted after a low-pass-filter (LPF). For our experiment, the LPF is a 4th Bessel–Thompson filter with
3. RESULTS AND DISCUSSION The experimental results of the PDM between the conventional channel and the chaotic channel are shown in Fig. 2. Figures 2(a1)–2(a3) are the results of the conventional channel for different leaked powers, while Figs. 2(b1)–2(b3) are those of the chaotic channel under the same conditions. The yellow (top), green (middle), and blue (bottom) time series are the chaotic signal containing the message, the synchronous chaotic carrier emitted from LDR , and the recovered message, respectively. The red time series superimposed on the blue (bottom) waveform is the original message. As can be seen from Figs. 2(a1) and 2(b1), the PDM between the conventional channel and the chaotic channel is realized by the appropriate adjustment of PC5 , since the two polarization-orthogonalized channels are thoroughly divided by the PBS. By comparing the blue and red time series in Fig. 2(b1), the message hidden in the chaotic carrier can be successfully recovered. In this work,
Fig. 2. Effect of power leakage on the communication results of the conventional channel and the chaotic channel. (a1)–(a3) The communication results of the conventional channel when the leaked power is 0, 0.2, and 0.5 mW, respectively; (b1)–(b3) the communication results of the chaotic channel under the same conditions.
Research Article
Vol. 54, No. 14 / May 10 2015 / Applied Optics
the Q-factor is used to evaluate the quality of the secure communications. It is defined as follows [18,19]:
4.5
(1)
in which hP 1 i and hP 0 i represent the average power of the recovered bits 1 and 0, respectively. σ 1 and σ 0 are the corresponding standard deviations. The calculated Q-factors of the conventional channel and the chaotic channel are 4.21 and 2.21, respectively. When the fiber link is affected by some external factors, the communications of both channels are interfered by each other. More noise appears in the conventional channel, while the BERs of the signal for the conventional channel and the recovered message for the chaotic channel are increased, as shown in Figs. 2(a2) and 2(b2). The Q-factors for the two channels decrease to 2.83 and 2.09, respectively. When the leaked power reaches 0.5 mW, both channels suffer severe interference from each other, as shown in Figs. 2(a3) and 2(b3). The conventional fiber-optic channel has noise-like characteristics because of the influx of the chaotic laser. Hence the message cannot be identified by the photodiode. The chaotic optical communication also fails. The Q-factors for both channels fall to 1.41 and 1.39. The eye diagrams of the two channels are shown in Fig. 3 to describe the quality of communications. When the leaked power is increased, the quality of the eye diagrams of the two channels becomes worse. In order to quantify the above results, the Q-factors of both channels are plotted as a function of the leaked power. As shown in Fig. 4, the conventional fiber-optic channel is affected significantly by the power leakage, because the Q-factor descends almost linearly with the increasing leaked power. However, the Q-factor of the chaotic channel degrades more slowly. When the leaked power is 0.5 mW, the Q-factor falls below 1.6, which means that data recovery becomes impossible [21–23]. In addition, according to Ref. [23], the communications may be corrupted by the chromatic dispersion (CD) and the polarization-mode dispersion (PMD) of the fiber link. In Figs. 3 and 4, when the transmission distance is about
Fig. 3. Eye diagrams of the received message of the conventional channel and the recovered message of the chaotic channel. (a1)–(a3) the eye diagrams of the conventional channel, (b1)–(b3) the eye diagrams of the chaotic channel under the same conditions as Fig. 2.
Conventional channel Chaotic channel
4 3.5 Q−factor
hP i − hP 0 i ; Q 1 σ1 σ0
4511
3 2.5 2 1.5 1
0
0.1
0.2 0.3 0.4 Leaked Power [mW]
0.5
Fig. 4. Effect of varying leaked power on the Q-factor of both channels.
20 km, both CD and PMD have little effect on the message recovery. In this condition, the power leakage between the two polarization-orthogonalized channels is the main reason for the deterioration of the communication results. To investigate the PDM between two chaos-encrypted channels, the conventional fiber-optic channel is substituted with another chaos-encrypted channel. The communication results are shown in Fig. 5. Figures 5(a1) and 5(a2) are the results of the x-polarization channel when the leaked power is 0 and 0.05 mW, respectively, while Figs. 5(b1) and 5(b2) are the results of the y-polarization channel under the same conditions. As shown in Figs. 5(a1) and 5(b1), after PC5 is carefully adjusted, both data sequences hidden in the chaotic carrier can be recovered successfully. The calculated Q-factors of the two channels are 2.09 and 2.10, respectively. In contrast to Fig. 4, the PDM between two chaosencrypted channels is very sensitive to power leakage. A slight power leakage leads to a significant deterioration of the quality of communication of both channels, as shown in Figs. 5(a2) and 5(b2). The eye diagrams of the two chaotic channels are shown in Fig. 6 to verify the phenomenon. The Q-factors fall immediately to 1.41 and 1.44, respectively. With increasing leaked power, the effect of the mutual power leakage on the Q-factors is depicted in Fig. 7. Except for being 2.26 at the original point, the Q-factors stabilize around 1.4 at other values
Fig. 5. Communication results of double chaotic channels. (a1) and (a2) The communication results of the x-polarization channel when the leaked power is 0 and 0.05 mW, respectively; (b1) and (b2) the communication results of the y-polarization channel under the same conditions.
4512
Research Article
Vol. 54, No. 14 / May 10 2015 / Applied Optics
2.3 x−pol channel y−pol channel
2.25
Q−factor
2.2 2.15 2.1 2.05
Fig. 6. Eye diagrams of the recovered message of the two chaotic channels. (a1) and (a2) The eye diagrams of the x-polarization channel; (b1) and (b2) the eye diagrams of the y-polarization channel under the same conditions as Fig. 5.
2 0.1
x−pol channel y−pol channel
Q−factor
1.8 1.6 1.4 0
0.1
0.2 0.3 0.4 Leaked Power [mW]
0.5
Fig. 7. Effect of power leakage on the Q-factor of both chaotic channels.
−10
(a)
(b)
0.6
0.7
D E
−30
−30
Power [dBm]
Power [dBm]
−20
A B C
−20
0.4 0.5 Py/Px
nearly flat up to the cutoff frequency of the digital oscilloscope. Thus, the leaked conventional signal affects only the lowfrequency component of the chaotic carrier, as shown by curve C of Fig. 8(a). Accordingly, the chaotic optical communication is not greatly affected. However, the leaked chaotic signal affects nearly the whole spectrum of the other chaotic channel. All the frequency components are elevated, and the spectrum becomes more flat, which is shown as curve E in Fig. 8(b). As a result, the quality of chaotic secure communications degrades rapidly. Therefore, for PDM between two chaos-encrypted channels, external interference should be avoided to ensure the communication quality. Figure 9 depicts the effects of the optical launched power ratios of both chaotic channels on the Q-factors. The power ratio is defined as Py∕Px, where Py and Px represent for the launched optical power of the y- and x-polarization channels, respectively. In this experiment, Py is scanned from 0.15× to 0.7× of Px, while the launched optical power of the x-polarization channel is fixed at 10 dBm. For different power ratios, the Q-factors of both chaotic channels stabilize around 2.1. Thus, for a specific multiplexing distance, the optical launched power of the x- (y-) polarization channel has little effect on the communication quality of the y- (x-) polarization channel. It is
2
1.2
0.3
Fig. 9. Effect of optical launched power on the Q-factor of both chaotic channels.
of the leaked power. This can be explained by the aspect of the frequency domain. For the conventional signal in our experiment, the data is a 1.25-Gbit/s PRBS, and thus the power of the spectrum concentrates mainly between 0 and 1.25 GHz, which is shown as curve A in Fig. 8(a). Compared with A, the spectrum of the chaotic carrier [curve B in Fig. 8(a)] is
2.2
0.2
−40 −50 −60
−40 −50 −60 −70
−70 −80 −80 0
1
2 3 4 Frequency [GHz]
5
−90 0
1
2 3 4 Frequency [GHz]
5
Fig. 8. RF spectrums of the communication signals. (a) Curves A (top, green), B (middle, blue), and C (bottom, red) are the RF spectra of the conventional signal, the chaotic carrier, and the chaotic carrier affected by the leaked conventional signal, respectively. (b) Curves D (top, blue) and E (bottom, red) are the RF spectra of the chaotic carrier and the chaotic carrier affected by the leaked chaotic signal, respectively.
Research Article worth noting that if the WDM technique is introduced to the system described in this work, the capacity of the system can be significantly multiplied. This will be the topic of future research. 4. CONCLUSIONS On the basis of the existing multiplexing technique, PDM can double the transmission capacity of the fiber link. In this paper, PDM between a conventional fiber-optic channel and a chaos-encrypted channel, and between two chaos-encrypted channels, is proposed and experimentally investigated. The bit rate of each channel and the transmission in the SSMF can reach up to 1.25 Gbits∕s and 22.54 km, respectively. The effect of mutual power leakage between the two polarizationorthogonalized channels on the Q-factors is investigated. The experimental results show that the communication quality of the chaotic channel is robust to the optical power leaked from the conventional channel, but is sensitive to that leaked from the chaotic channel. Moreover, whether or not the optical launched power of the two chaotic channels is equal has no effect on the Q-factors of both channels. First HAEPC Science and Technology Project in 2015; National Natural Science Foundation of China (NSFC) (61071123). REFERENCES 1. V. Annovazzi-Lodi, M. Benedetti, S. Merlo, M. Norgia, and B. Provinzano, “Optical chaos masking of video signals,” IEEE Photon. Technol. Lett. 17, 1995–1997 (2005). 2. G. D. VanWiggeren and R. Roy, “Communications with chaotic lasers,” Science 279, 1198–1200 (1998). 3. J. Wu, Z. Wu, Y. Liu, L. Fan, X. Tang, and G. Xia, “Simulation of bidirectional long-distance chaos communication performance in a novel fiber-optic chaos synchronization system,” J. Lightwave Technol. 31, 461–467 (2013). 4. J. Wu, Z. Wu, X. Tang, L. Fan, W. Deng, and G. Xia, “Experimental demonstration of LD-based bidirectional fiber-optic chaos communication,” IEEE Photon. Technol. Lett. 25, 587–590 (2013). 5. S. E. Alavi, I. S. Amiri, S. M. Idrus, A. S. M. Supa’at, J. Ali, and P. P. Yupapin, “All-optical OFDM generation for IEEE802.11a based on soliton carriers using microring resonators,” IEEE Photon. J. 6, 7900109 (2014). 6. I. S. Amiri, S. E. Alavi, S. M. Idrus, A. S. M. Supa’at, J. Ali, and P. P. Yupapin, “W-band OFDM transmission for radio-over-fiber link using solitonic millimeter wave generated by MRR,” IEEE J. Quantum Electron. 50, 622–628 (2014). 7. I. S. Amiri, S. E. Alavi, N. Fisal, A. S. M. Supa’at, and H. Ahmad, “All-optical generation of two IEEE802.11n signals for 2 × 2 MIMORoF via MRR system,” IEEE Photon. J. 6, 7903611 (2014). 8. S. E. Alavi, I. S. Amiri, H. Ahmad, A. S. M. Supa’at, and N. Fisal, “Generation and transmission of 3 × 3 W-band MIMO-OFDM-RoF signals using micro-ring resonators,” Appl. Opt. 53, 8049–8054 (2014). 9. P. Colet and R. Roy, “Digital communication with synchronized chaotic lasers,” Opt. Lett. 19, 2056–2058 (1994). 10. C. R. Mirasso, P. Colet, and P. Garcia-Fernandez, “Synchronization of chaotic semiconductor lasers: application to encoded communications,” IEEE Photon. Technol. Lett. 8, 299–301 (1996).
Vol. 54, No. 14 / May 10 2015 / Applied Optics
4513
11. V. Annovazzi-Lodi, S. Donati, and A. Scire, “Synchronization of chaotic injected laser systems and its application to optical cryptography,” IEEE J. Quantum Electron. 32, 953–959 (1996). 12. J.-P. Goedgebuer, P. Levy, L. Larger, C.-C. Chen, and W. T. Rhodes, “Optical communication with synchronized hyperchaos generated electooptically,” IEEE J. Quantum Electron. 38, 1178–1183 (2002). 13. K. Kusumoto and J. Ohtsubo, “1.5-GHz message transmission based on synchronization of chaos in semiconductor lasers,” Opt. Lett. 27, 989–991 (2002). 14. V. Annovazzi-Lodi, M. Benedetti, S. Merlo, T. Perez, P. Colet, and C. R. Mirasso, “Message encryption by phase modulation of a chaotic optical carrier,” IEEE Photon. Technol. Lett. 19, 76–78 (2007). 15. J. Liu, H. Chen, and S. Tang, “Synchronized chaotic optical communications at high bit rates,” IEEE J. Quantum Electron. 38, 1184–1196 (2002). 16. S. E. Alavi, I. S. Amiri, S. M. Idrus, A. S. M. Supa’at, and J. Ali, “Chaotic signal generation and trapping using an optical transmission link,” Life Sci. J. 10, 186–192 (2013). 17. A. Argyris, D. Synridis, L. Larger, V. Annovazzi-Lodi, P. Colet, I. Fischer, J. Garcia-Ojalvo, C. R. Mirasso, L. Pesquera, and K. A. Shore, “Chaos-based communications at high bit rates using commercial fibre-optic links,” Nature 438, 343–346 (2005). 18. T. Matsuura, A. Uchida, and S. Yoshimori, “Chaotic wavelength division multiplexing for optical communication,” Opt. Lett. 29, 2731–2733 (2004). 19. F. Zhang, P. L. Chu, R. Lai, and G. R. Chen, “Dual-wavelength chaos generation and synchronization in erbium-doped fiber lasers,” IEEE Photon. Technol. Lett. 17, 549–551 (2005). 20. J. Z. Zhang, A. B. Wang, J. F. Wang, and Y. C. Wang, “Wavelength division multiplexing of chaotic secure and fiber-optic communication,” Opt. Express 17, 6357–6367 (2009). 21. Q. C. Zhao and H. X. Yin, “Performance analysis of dense wavelength division multiplexing secure communications with multiple chaotic optical channels,” Opt. Commun. 285, 693–698 (2012). 22. N. Li, W. Pan, L. Yan, B. Luo, M. Xu, Y. Tang, N. Jiang, S. Xiang, and Q. Zhang, “Chaotic optical cryptographic communication using a three-semiconductor-laser scheme,” J. Opt. Soc. Am. B 29, 101–108 (2012). 23. C. Antonelli, A. Mecozzi, M. Santagiustina, and L. Ursini, “Impairments due to polarization-mode dispersion in chaos-encrypted communication systems,” IEEE Photon. Technol. Lett. 21, 1387–1389 (2009). 24. Q. C. Zhao, H. X. Yin, and X. L. Chen, “Long-haul dense wavelength division multiplexing between a chaotic optical secure channel and a conventional fiber-optic channel,” Appl. Opt. 51, 5585–5590 (2012). 25. A. Argyris, E. Grivas, A. Bogris, and D. Syvridis, “Transmission effects in wavelength division multiplexed chaotic optical communication systems,” J. Lightwave Technol. 28, 3107–3114 (2010). 26. X. L. Chen, W. Chang, H. X. Yin, Q. C. Zhao, and H. H. Yue, “Experimental investigation of wavelength division multiplexing secure communications with chaotic optical channel,” in Asia Communications and Photonics Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), paper AW4F.2. 27. N. Jiang, W. Pan, B. Luo, S. Xiang, and L. Yang, “Bidirectional dualchannel communication based on polarization-division-multiplexed chaos synchronization in mutually coupled VCSELs,” IEEE Photon. Technol. Lett. 24, 1094–1096 (2012). 28. J. Mork, B. Tromborg, and J. Mark, “Chaos in semiconductor lasers with optical feedback: theory and experiment,” IEEE J. Quantum Electron. 28, 93–108 (1992). 29. D. Kanakidis, A. Argyris, A. Bogris, and D. Syvridis, “Influence of the decoding process on the performance of chaos encrypted optical communication systems,” J. Lightwave Technol. 24, 335–341 (2006).
