Suppression of Time Delay Signature based on Brillouin ... - IEEE Xplore

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JPHOT.2017.2690680, IEEE Photonics Journal

Suppression of Time Delay Signature based on Brillouin Backscattering of Chaotic Laser Jianzhong Zhang1,2,, Changkun Feng1,2, Mingjiang Zhang1,2, Member, IEEE, Yi Liu1,2, and Yongning Zhang1,2 1

Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China

2

Institute of Optoelectronic Engineering, College of Physics & Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China

This work is supported by the National Natural Science Foundation of China (NSFC) under Grant 61377089 and 61527819, by Shanxi Province Natural Science Foundation under Grant 2015011049, by Research Project by Shanxi Province Youth Science and Technology Foundation under Grant 201601D021069, by Research Project Supported by Shanxi Scholarship Council of China under Grant 2016-036. Corresponding author: J. Z. Zhang (e-mail: [email protected]) and M. J. Zhang (e-mail: [email protected]).

Abstract: A new method by utilizing Brillouin backscattering of optical fiber to suppress the time delay signature (TDS) of an external cavity semiconductor laser (ECSL) is proposed and experimentally demonstrated. When the average power of the injected chaotic laser is within the range of 400 to 1500 mW, the TDS can be significantly suppressed with the G.655 fiber of the 0.5, 2, 4.5 or 6 km length. The G.652 fiber has the same TDS suppression effect with the G.655 fiber. The TDS suppression effect is relatively stable among a number of measurements ranging from 1 to 50. Moreover, the TDS suppression has nothing to do with the length of the chaotic series by employing the Brillouin backscattering method.

Index Terms: Brillouin backscattering; external cavity semiconductor laser; time delay signature; chaotic laser

1. Introduction Chaotic light, as a novel laser output form, has been extensively applied to all kinds of fields, such as, high speed secure communications [1], fast physical random number generation [2], [3], high-precision ranging radar [4], optical time domain reflectometer [5] and distributed optical fiber sensor [6]. An external cavity semiconductor laser (ECSL) is usually utilized to generate chaotic light because of its simple structure, stable performance and compatibility with optical fiber communication systems. However, chaotic light signals from the ECSL contain the time delay signature (TDS) induced by the optical round trip in the external cavity feedback, which limits the application of chaotic laser to some extent [3]. Therefore, the elimination of the TDS is vital to the application of chaotic laser.

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At present, many methods have been proposed and employed to suppress the TDS of the chaotic laser. On the one hand, from the standpoint of the chaotic laser source itself, the TDS can be suppressed by optimizing the feedback strength and setting the moderate injection current in the single feedback system [7]-[9] which is the simplest feedback configuration. Modified feedback schemes, including double optical feedbacks [10], [11], modulated multiple feedbacks [12], fiber Bragg grating feedback [13], [14], polarization rotated feedback [15], and filtered feedback [16], are also proposed to suppress the TDS under the appropriate feedback strength. However, the suppression mechanism of these schemes regardless of the simplest or relatively complicated feedback is unclear by simply controlling the laser parameters. Whether the TDS can be truly eliminated or not remains up in the air [17]. Besides, cascade-coupled configurations with two [18] or three [19], [20] semiconductor lasers are adopted to conceal the TDS. However, the overall scheme is extremely complicated and more parameters have to be simultaneously optimized [14]. On the other hand, in view of the application of chaotic laser, other approaches have also been proposed to eliminate the TDS. For high speed random bit generation, the periodicity induced by the TDS is eliminated by an exclusive-OR operation on two random sequences [2] or the difference between consecutive sampled 8-bit values [3] extracted from the chaotic fluctuations. However, the adoption of high-performance logic components gives rise to some bad problems, such as high-cost and electronic bottleneck. For chaotic secure communications, the possible clues caused by the TDS are concealed by integrating a pseudorandom binary sequence (as a digital key) in a chaotic delay system [21]. But, this scheme is only numerically verified and has still a long distance to the practical application. Actually, the TDS can be eliminated by the subsequent operation on the direct output signals from the ECSL. For example, the optical delayed self-interference [22] or electrical heterodyning [23] of the chaotic signal is employed to suppress the TDS. Here, a novel operation, i.e., the Brillouin backscattering on the chaotic laser is proposed to suppress the TDS. In this paper, the chaotic TDS suppression of the ECSL is experimentally demonstrated by utilizing Brillouin backscattering of optical fiber and the influence of such factors as the fiber length, the injection power, the fiber type and number of measurements on the suppression effect is further analyzed.

2. Experimental setup Fig.1 shows the experimental setup of the chaotic TDS suppression by adopting Brillouin backscattering of optical fiber. A distributed feedback laser diode (DFB-LD) with a  21 m fiber ring feedback external cavity was referred to as the chaotic light source. The amount and polarization state of the feedback light in the external cavity are adjusted by a variable optical attenuator (VOA) and a polarization controller (PC), respectively. The chaotic output signal through an optical isolator (OI) was amplified by a high power Er-doped fiber amplifier (EDFA). Here, the utilization of the OI ensured that the unwanted optical feedback was prevented into the DFB laser diode. The amplified chaotic light was injected into the standard single mode fiber (SMF) via an optical circulator (OC). At the end of the fiber, the refractive index matching liquid was utilized to prevent the Fresnel reflection. The chaotic Stokes wave was filtered by an optical filter from the backscattered chaotic light, and further divided into two beams by an



optical coupler with the coupling ratio of 50:50. One of both beams was recorded by an optical spectrum analyzer (OSA, APEX AP2041B, 5 MHz resolution). The other was converted into electrical signals by a broadband photodetector (PD), and then measured by a real-time oscilloscope (OSC, LeCroyLabMaster 10 Zi, 36 GHz bandwidth and 80 GS/s sampling rate).

Fig. 1 Experimental setup of the TDS suppression based on Brillouin backscattering of chaotic laser. DFB-LD: distributed feedback laser diode, OC: optical circulator, PC: polarization controller, VOA: variable optical attenuator, 50:50: optical coupler, ISO: isolator, EDFA: erbium-doped optical fiber amplifier, SMF: single mode fiber, PD: photodetector, OSA: optical spectrum analyzer, OSC: oscilloscope.

3. Experimental results The DFB semiconductor laser was biased at 33 mA (1.5 times threshold), and its wavelength was stabilized at 1554.1 nm. The original chaotic output from the ECSL was obtained with - 2.3 dB optical feedback. As plotted in Fig. 2, the dark yellow line denotes the corresponding optical spectrum with the - 3 dB line width of 1.4 GHz or the - 10 dB line width of 6.2 GHz. After the amplified chaotic laser was injected into the different single mode fibers, the chaotic light was scattered back to the injection end via fiber Rayleigh and Brillouin scattering effect. The optical spectra of the backscattered chaotic light through the 6 km and 0.5 km feedback fibers are shown as the red and magenta lines in Fig. 2, respectively. We can see that the optical spectrum of the backscattered chaotic light is hardly influenced by the length of feedback fiber. The optical spectra of the Stokes waves through the 6 km and 0.5 km feedback fibers which were filtered by a tunable optical filter of 12 GHz are denoted as the blue and navy lines in Fig. 2, respectively.



Fig. 2 Optical spectra of the different chaotic laser signals. The dark yellow line denotes the output of the ECSL. The red and magenta lines denote the backscattered chaotic light through the 6 km and 0.5 km feedback fibers, respectively. The blue and navy lines denote the filtered Stokes waves through the 6 km and 0.5 km feedback fibers, respectively.

The suppression effect of the chaotic TDS for the Brillouin backscattering method was analyzed by comparing the self-correlation function (SF) and mutual information (MI) of chaotic signals before and after Brillouin backscattering. Here, the SF and MI are introduced to identify the suppression of the chaotic TDS. For an ECSL system, the SF can be defined as: C ( t ) 

P (t  t )  P (t )  P (t )  P (t )  P (t )  P (t ) 

2

P (t  t )  P (t ) 

2

(1)

Where Ｐ(t)=|E(t)|2 represents the chaotic time series, and Δt is the time delay. The TDS can be retrieved from the peak location of the SF curve. The SF is usually used to represent the similarity between a signal and its time delay one. The MI is a measure of the mutual dependence between the two variables in information theory. The MI betweenＰ(t) andＰ(t+Δt) can be described by:

M ( t ) 

 (P (t ), P (t  t ))  (P (t ), P (t  t ))log  P (t ),P (t t )  (P (t )) (P (t  t ))

(2)

Where the φ(P(t), P(t+Δt)) denotes the joint distribution probability density; φ(P(t)) and φ(P(t+Δt)) refer to marginal distribution probability density. The peak position of the MI curve can also characterize the TDS. In experiment, the time series of the chaotic laser directly generated by the ECSL is shown in Fig. 3(a1). After the chaotic light with the power of 500 mW via the amplification of the EDFA was injected into the 6 km single-mode optical fiber (G.652), the Brillouin backscattering of the



chaotic light took place along the fiber. The time series of the filtered Stokes wave is displayed in Fig. 3(b1). The Stokes wave obtained by Brillouin backscattering remains to be chaotic. By calculation, the corresponding largest Lyapunov exponent is 0.2167, while the number for the ECSL itself is 0.2213. Fig. 3(a2) and Fig. 3(b2) show the corresponding SF curves of the time series from Fig. 3(a1) and Fig. 3(b1), respectively. As plotted in Fig. 3(a2), we can see that the feedback time delay clearly manifests itself as strong self-correlation peaks, at  = 105 ns and 2  = 210 ns, with the correlation coefficients of C = 0.37 and C = 0.087, respectively, when the chaotic laser is under the direct generation of the ECSL. The delay time is related to the feedback external cavity length of the ECSL according to the equation of  = Lfn/c, where Lf is the feedback external cavity length and is set to 2.1 m as indicated before, and c = 3 × 108 m/s is light speed in a vacuum and n = 1.5 the fiber refractive index. Therefore, the delay time is 105 ns. Obviously, as the external cavity length of the ECSL varies, so does the delay time. However, by adopting Brillouin backscattering, Fig. 3(b2) displays the suppression of the self-correlation peaks, where the correlation peak at  = 105 ns is significantly decreased to 0.026, and the correlation peak at 2  = 210 ns almost completely disappears. Fig. 3(a3) and Fig. 3(b3) further give the corresponding MI curves of the time series from Fig. 3(a1) and Fig. 3(b1), respectively. We can find that the MI peak at the feedback time delay  = 105 ns is decreased from M = 0.169 to M = 0.0017. So, the TDS of the ECSL through Brillouin backscattering has been successfully suppressed regardless of the utilization of the SF or MI evaluation means. The chaotic TDS suppression could be due to the following reason. Stimulated Brillouin backscattering itself is a nonlinear process. The input chaotic pump light interferes with the backscattered Stokes light, and then an acoustic wave is generated through the effect of electrostriction. The generating acoustic wave acts as a moving Brillouin dynamic grating, which can scatter chaotic light in the backward direction. Due to the distribution of a great quantity of the special gratings along the fiber, many scattering points are formed to randomly backscatter the chaotic light, which make the backscattering light have no regular transmission routes. In the Brillouin backscattering process of the chaotic laser, the weak period of chaotic light signals corresponding to the time delay is disrupted. Therefore, the TDS can be effectively suppressed.



Fig. 3 Time series, self-correlation traces and mutual information traces of the chaotic light, shown as (a1) and (b1), (a2) and (b2), (a3) and (b3), respectively. Left column: the directly generated chaotic light from the ECSL, Right column: the filtered Stokes wave by the Brillouin backscattering.

To visually verify the suppression effect of the chaotic TDS by utilizing Brillouin backscattering, we integrated SF curves of the chaotic laser to form a two-dimensional map for the injection power from 400 to 1500 mW, as shown in Fig. 4. The map in Fig. 4(a) corresponds to the case of the output chaos from the ECSL via the amplification of the EDFA. Fig. 4(b), 4(c) and 4(d) denote the cases of the chaotic Stokes waves through Brillouin backscattering of G.655 fiber with different lengths, 0.5, 4.5 and 6 km, respectively. We can see that in the whole range of the injection power from 400 to 1500 mW, the TDS at  =  105 ns can be significantly suppressed regardless of the fibers with the 0.5, 4.5 or 6 km length, although the TDS of the ECSL output is obvious under the arbitrary injection power.



Fig. 4 Maps of the integrated SF curves of chaotic laser in the range of injection power, 400-1500mW. (a) before Brillouin backscattering, (b), (c), and (d) after Brillouin backscattering of G.655 fibers with the 0.5, 4.5, and 6 km length, respectively

Besides, we further analyzed the suppression effect of the chaotic TDS based on Brillouin backscattering for different types of optical fiber, namely G.652 and G.655. The optical effective areas Aeff of G.652 and G.655 are 80 μm2 and 50 μm2, respectively, which can get rise to the various Brillouin backscattering efficiencies [24]. Fig. 5(a) and 5(b) show the chaotic TDS suppression by employing optical fibers of G.652 and G.655, respectively. In Fig. 5(a) and 5(b), the blue and purple lines separately denote the correlation coefficient at  = 105 ns as a function of the injection power before and after Brillouin backscattering of the 0.5 km long fiber. We can see that in the whole power range of 400 -1500mW, the correlation coefficients are nearly 0.38 at the fiber injection end, and decrease to almost 0.03 through Brillouin backscattering of G.652 fiber of 0.5 km length. With regard to Brillouin backscattering of 0.5 km long G.655 fiber, the identical suppression effect is also obtained. The black and red lines in Fig. 5(a) and 5(b) refer to the correlation coefficient at  = 105 ns versus the injection power before and after Brillouin backscattering of the 6 km long fiber, respectively. By comparing the two cases before and after Brillouin backscattering, we can get the same TDS suppression effect regardless of G.652 or G.655 fiber, although they have the different specs. Error bars in Fig. 5 further indicate the standard deviation from 50 measurements for each input power with and without the Brillouin backscattering of the fibers, which can fully demonstrate the stability of the suppression effect by using the Brillouin backscattering method. Here, two kinds of fiber, G.652 and G.655, are still employed with the fiber length of 0.5 km and



6 km, respectively. We take the injection power of 600 mW and the 6 km long G.652 fiber as an example. In the range of 50 measurements, the correlation coefficient is nearly kept at 0.38 at the fiber injection end, and declines to 0.012 after Brillouin backscattering. The same results can be achieved for other injection powers, and other types and lengths of fiber. Therefore, this means that the Brillouin backscattering method has a stable suppression effect of the chaotic TDS.

Fig. 5 The suppression effect of chaotic TDS is shown by utilizing two kinds of fiber, G.652 (left) and G.655 (right). The blue and purple lines denote the correlation coefficient at  = 105 ns as a function of injection power from 400 to 1500 mW before and after Brillouin backscattering of the 0.5 km fiber, respectively. The black and red lines refer to the correlation coefficient at  = 105 ns versus injection power before and after Brillouin backscattering of the 6 km fiber, respectively. Error bars are the standard deviation from 50 measurements.

4. Discussions As is indicated previously, the TDS of the ECSL is usually characterized by two parameters. One is the delay value which is decided only by the external cavity length of the ECSL. The other is the delay information strength which is directly described by the correlation coefficient of the SF. As is intuitively understood, the delay information strength is related to the length of the chaotic series, and the longer the series length is, the stronger the delay information strength becomes. This is possibly because that the weak period signal caused by the external feedback delay is not only contained in the chaotic series, itself but also participates in the generation of the chaotic series. Thus, the longer chaotic time series can contain the much more weak-periodicity signals corresponding to the external cavity mode. However, through Brillouin backscattering of optical fiber, the delay information strength is independent of the chaotic series length. Fig. 6 further shows the correlation coefficient at  = 105 ns as a function of the chaotic series length under the different injection powers and fiber lengths. From Fig. 6(a), it can be seen that when the injection powers are 200, 400, 600 and 800 mW, respectively, the correlation coefficient through Brillouin backscattering of the 2-km G.655 fiber is hardly influenced by the chaotic series length ranging from 600 to 11000 ns. When the fiber lengths are separately 0.5, 2, 4.5 and 6 km, the correlation coefficient is almost invariable with the increase of the chaotic series length from 600 to 11000 ns. Therefore, the chaotic TDS



suppression has nothing to do with the length of the chaotic series by employing the Brillouin backscattering method.

Fig. 6 The correlation coefficient at  = 105 ns as a function of the chaotic series length, (a) under different injection powers of 200, 400, 600 and 800 mW, respectively, (b) under different fiber lengths of 0.5, 2, 4.5 and 6 km, respectively.

Compared with the optical delayed self-interference [22] and electrical heterodyning [23] by utilizing the electronic components to achieve the TDS suppression, the Brillouin backscattering method employs the standard single mode fiber to suppress the TDS, and the operation on the chaotic laser fully is in the optical domain. In the process of the TDS suppression by using the optical delayed self-interference and electrical heterodyning, the utilization of the electronic component can cut off the radio frequency (RF) spectrum of the chaotic signal, which makes the high-frequency signals loss. However, the Brillouin backscattering method can avoid the above difficulty.

5. Conclusions In conclusion, the TDS suppression of the ECSL is experimentally demonstrated by utilizing Brillouin backscattering of optical fiber. The influence of some factors, such as the fiber length, the injection power, the fiber type and number of measurements, on the suppression effect is analyzed. The relationship between the chaotic TDS suppression and the length of the chaotic series is further discussed. The experimental results indicate that the Brillouin backscattering method can effectively suppress the TDS of the ECSL. This suppression method will be helpful to provide high quality chaotic signals in some applications including high speed secure communications, fast physical random number generation, high-precision ranging radar, optical time domain reflectometer and distributed optical fiber sensor.

Acknowledgements The authors wish to thank the anonymous reviewers for their valuable suggestions.

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