多模光纤反馈半导体激光器产生无时延特征混沌【增强内容出版】
Chaos laser has important applications in the fields of secure communication, key distribution, physical random number generation, and radar. In these applications, the key is a chaotic source, a common choice for which is a semiconductor laser with optical feedback because it is characterized by simple structure, easy integration, and complex dynamics. However, external-cavity resonance between the laser facet and the reflector in the conventional optical feedback structure gives the chaotic signal an obvious time-delay signature (TDS). This feature leaks the key parameter of the external-cavity length of the chaotic light source, which makes the system at a risk of being reconstructed and reduces the security of chaotic secure communication and key distribution. In addition, a TDS also introduces a weak periodicity to the chaotic signal, which limits the randomness of physical random numbers and the anti-jamming performance and resolution of radar. Therefore, the suppression of the TDS is an important prerequisite for the best use of chaos laser. The main methods of suppressing the TDS are increasing the complexity of the feedback cavity, introducing nonlinear feedback, and post-processing the chaotic signal. In this study, a TDS-free chaos laser generation scheme using inter-modal dispersion of a multimode fiber (MMF) is proposed. This work provides a basis for the application of TDS-free chaos laser in the fields of secure communication, key distribution, physical random number generation, and radar detection.
The light output from a semiconductor laser is divided into two paths by an optical coupler. One path is fed back to the laser via the MMF to perturb itself to generate chaos, and the other one is for detection. We utilize a variable optical attenuator and a polarization controller to adjust the strength and polarization state of the feedback light. The feedback strength is defined as the power ratio of the feedback light to the static output of the laser. An erbium-doped fiber amplifier is used to amplify the signal's optical power in the feedback and detection paths. When the bias current and operation temperature are set to 15 mA and 25 ℃, respectively, the static wavelength of the laser is stabilized at 1550.1 nm. Numerically, we employ the VPIphotonics design platform to construct the simulation system mentioned above. In the simulation, the bias current of laser is set to 20 mA, giving rise to a central wavelength of 1552.52 nm. Two typical MMFs with core diameters of 50 μm and 62.5 μm are used to analyze the effects of the core diameter (D), relative offset, and length of the MMF on the chaotic optical mode field. In addition, the evolution of the TDS as a function of the relative offset, feedback strength, and length of the MMF is explored. Note that, to quantitatively characterize the magnitude of the TDS, the autocorrelation function is used.
First, a TDS-free chaotic signal is obtained experimentally using an MMF with a length of 4.4 km and a core diameter of 62.5 μm while the optical feedback strength is fixed at 0.1 (Fig. 2). Next, we theoretically study the influences of the core diameter (Fig. 3) and relative offset (Figs. 4 and 5) of the MMF on the number of modes and the distribution of chaotic optical mode fields. As the core diameter and relative offset increase, the number of modes gradually increases and the mode field distributions become more complex. The effect of fiber length on mode separation is also investigated (Fig. 6). Note that the degree of mode separation (that is, the inter-modal dispersion) becomes larger as the fiber length increases. By comparing the typical chaotic characteristics of a laser subject to single-mode fiber and MMF feedback under the same parameter conditions, we find that the approach with MMF feedback can afford the elimination of the TDS, whereas that with single-mode fiber feedback cannot (Fig. 7). Furthermore, the effects of relative offset (Fig. 8), fiber length (Fig. 9), and feedback strength (Fig. 10) on the TDS are given. When the relative offset is 0 and the feedback strength is 0.1, the critical fiber length required to eliminate the TDS can be as short as 1 km.
In this study, we propose a scheme for TDS-free chaos laser generation using a semiconductor laser with MMF feedback. Chaos laser without the TDS is obtained experimentally. Theoretically, the effects of the core diameter, relative offset, and fiber length of MMFs with D=50 μm and D=62.5 μm on the chaotic optical mode field are analyzed. Furthermore, we explore the TDS evolution as a function of the relative offset, fiber length, and feedback strength. Finally, the parameter conditions required to suppress the TDS of chaotic signals are ascertained: When the relative offset is 0, the fiber length is greater than or equal to 1 km, and the feedback strength is greater than or equal to 0.1, chaotic signals without a TDS can be generated. This study underlies secure communication, key distribution, physical random number generation, and radar detection using TDS-free laser chaos.
1 引言
混沌激光具有大带宽、类噪声、可同步、自相关类δ函数等特性,被广泛应用于保密通信[1-2]、物理随机数产生[3-4]、混沌雷达测距[5]、密钥分发[6-8]等领域。光反馈半导体激光器结构简单、易于集成,且动力学特性复杂,是产生混沌激光的主要方法之一[9]。然而,光反馈结构下激光器端面与反射镜之间存在外腔谐振,使混沌信号存在明显的反馈时延特征(TDS)。该特征泄露了混沌光源外腔长度这一关键参数[10-11],使系统存在被重构的风险[12],降低了混沌保密通信、混沌密钥分发的安全性。此外,时延特征的存在也表明混沌信号存在一定程度的相关性(即弱周期性),限制了物理随机数的随机性以及混沌雷达的抗干扰能力和分辨率。
国内外学者针对混沌信号时延特征的抑制提出了很多有效的方案[13-14]。2005年,Lee等[15]在实验上利用双腔反馈半导体激光器抑制了时延特征,发现增加反馈腔的数目有助于增加时延特征分析难度。2007年,Rontani等[16]理论证明了调整激光器反馈强度和工作电流可使弛豫振荡周期接近外腔延迟时间,时延特征得到抑制,此方案适用于短外腔激光器。2009年,Wu等[17]构建了双光反馈实验系统,进一步证明了非相干光反馈可有效消除混沌时延特征。2012年,Li等[18]通过数值模拟证明了光纤布拉格光栅(FBG)滤波效应可消除时延特征,并在2015年通过数值模拟和实验分析了其抑制机理,即FBG的旁瓣群时延模糊了反馈延迟时间[19]。2013年,Zhong等[20]数值模拟了两个相互耦合的垂直腔面发射激光器(VCSEL)方案,证明通过调整合适的耦合强度和频率失谐,两个激光器可同时获得无时延特征的混沌信号。2014年,Hong等[21]通过实验对比四种方案,证明了将光反馈混沌VCSEL输出的混沌激光单向注入另一自由运行VCSEL,可获得宽带无时延的混沌激光。2015年,Yang等[22]在实验上利用三级级联半导体激光器消除了混沌时延特征;同年,Li等[23]实验研究了激光器线宽增强因子对混沌时延特征的影响,提出高线宽增强因子有助于抑制时延特征。2016年,Zhong等[24]实验研究了FBG反馈VCSEL对混沌时延特征的抑制作用。2017年,Jiang等[25]通过构建时间透镜模块对产生的混沌信号进行后处理,在数值上消除了时延特征。2018年,Li等[26]实验证明,激光器的混沌输出经过长距离单模光纤传输后,时延特征得到抑制,并揭示了光纤的自相位调制等非线性效应是其抑制原因。本课题组也对混沌时延抑制进行了研究,先后提出法布里-珀罗滤波反馈[27]、单模光纤的散射反馈[28]、延迟自干涉[29]、光外差法、啁啾光纤布拉格光栅色散反馈[30]等方案。
2018年,Zhang等[31]理论研究了多模光纤模式空分复用的混沌空间符号变换技术,增强了光纤通信系统的物理安全性。本文提出了一种基于多模光纤(MMF)反馈半导体激光器的无时延特征混沌产生方案:利用MMF的模间色散抑制外腔谐振,进而消除时延特征。实验产生了无时延特征混沌,理论探明了MMF对混沌光模场的影响规律及其抑制时延特征的条件,并与单模光纤(SMF)的反馈结果进行了对比。
2 实验装置及典型结果
实验装置如
图 1. MMF反馈激光器产生无时延特征混沌的实验装置图
Fig. 1. Experimental setup of generating chaos without time-delay signature using MMF feedback laser
在实验中,激光器的阈值电流
为定量表征混沌信号时延特征的大小,采用自相关函数表示信号经过延迟时间
式中:
如
图 2. Kf=0.1时不同光纤反馈混沌信号的典型特性对比。(a)时序;(b)自相关;(c)频谱;(d)局部频谱
Fig. 2. Comparison of typical characteristics of chaotic signals fed back by different fibers when Kf=0.1. (a) Time series; (b) autocorrelation; (c) spectrum; (d) local spectrum
3 数值模拟结果
在实验中难以详尽研究MMF对混沌光场以及时延特征的影响规律。因此,我们采用VPIphotonics设计软件[34]进行数值仿真。激光器模型选用LaserTLM模块,部分参数如
表 1. 激光器内部参数
Table 1. Internal parameters of laser
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3.1 MMF参数对混沌光模场的影响
MMF与SMF最显著的区别在于光纤的芯径,SMF芯径通常为6~9 μm,而MMF芯径可达50 μm以上。本文选取商用SMF(芯径D=6 μm)和MMF(芯径D=50 μm,62.5 μm),研究其在不同参数条件下对混沌信号模式数量及模场分布的影响情况。
图 3. 光纤长度为1 km时光纤芯径对模场光斑及模式数量的影响
Fig. 3. Influence of fiber core diameter on mode field spot and number of modes when fiber length is 1 km
此外,混沌光相对于芯轴的偏移会影响光纤内模式的空间分布。我们利用两种芯径MMF探究上述分布规律。当MMF芯径为50 μm时,混沌光在不同偏移量(Δd)下入射MMF,光纤传输的模式数量及对应的光场分布如
图 4. 光纤长度为1 km时D=50 μm的MMF的纤芯偏移对模场光斑及模式数量的影响
Fig. 4. Effects of core offset of MMF with D=50 μm on mode field spot and number of modes when fiber length is 1 km
当MMF芯径为62.5 μm时,混沌光在不同偏移量下入射MMF,光纤传输的模式数量及对应的光场分布如
图 5. 光纤长度为1 km时D=62.5 μm的MMF的纤芯偏移对模场光斑及模式数量的影响
Fig. 5. Effects of core offset of MMF with D=62.5 μm on mode field spot and number of modes when fiber length is 1 km
最后,我们研究了MMF长度对模式分离程度的影响。如
图 6. 当Δd=0时MMF长度对模式分离程度的影响。(a)(b)L=0.1 m;(c)(d)L=10 m;(e)(f)L=1 km
Fig. 6. Effect of MMF length on mode divergence degree when Δd=0. (a)(b) L=0.1 m; (c)(d) L= 10 m; (e)(f) L=1 km
3.2 MMF对混沌信号时延特征的抑制
基于3.1节中MMF对混沌光模场的影响分析,我们进一步探索了MMF抑制混沌信号时延特征的规律。
图 7. Kf=0.1时SMF与MMF反馈混沌激光器的典型信号特征。(a1)~(a4)SMF;(b1)~(b4)D=50 μm的MMF;(c1)~(c4)D=62.5 μm的MMF
Fig. 7. Typical signal characteristics of chaotic lasers subject to SMF and MMF feedback when Kf=0.1. (a1)‒(a4) SMF; (b1)‒(b4) MMF with D=50 μm; (c1)‒(c4) MMF with D=62.5 μm
首先我们研究了纤芯相对偏移量对时延特征的影响。MMF的长度分别选取0.1、1、10、15 km,反馈强度固定为Kf=0.1。可以发现,当长度为0.1 km时,随着纤芯相对偏移量的增大,两种MMF反馈激光器的混沌时延特征曲线呈现下降的趋势,如
图 8. 纤芯相对偏移量对时延特征的影响。(a)L=0.1 km;(b)L=1 km;(c)L=10 km;(d)L=15 km
Fig. 8. Influence of relative offset of fiber core on TDS. (a) L=0.1 km; (b) L=1 km; (c) L=10 km; (d) L=15 km
图 9. 光纤反馈强度对时延特征的影响。(a) L=0.1 km;(b)L=1 km;(c)L=10 km;(d)L=15 km
Fig. 9. Influence of fiber feedback strength on TDS. (a) L=0.1 km; (b) L=1 km; (c) L=10 km; (d) L=15 km
进一步研究了光纤长度对混沌信号时延特征的影响。如
图 10. 光纤长度对时延特征的影响。(a) Kf=0.05;(b) Kf=0.10;(c) Kf=0.25;(d) Kf=0.39
Fig. 10. Influence of fiber length on TDS. (a) Kf=0.05; (b) Kf=0.10; (c) Kf=0.25; (d) Kf=0.39
4 结论
提出了多模光纤反馈激光器产生无时延特征混沌的方案,在实验上获得了时延特征消除的混沌激光。理论分析了D=50 μm和D=62.5 μm的多模光纤的纤芯直径、相对偏移量及光纤长度对混沌光模场的影响。进一步分析了纤芯偏移、光纤长度和反馈强度对时延特征的抑制规律。最终探明了多模光纤抑制混沌信号时延特征的参数条件:当纤芯偏移量Δd=0、光纤长度L
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