Phase-matching analysis of four-wave mixing induced by modulation instability in a single-mode fiber

Four-wave mixing induced by modulation instability in a single-mode fiber is analyzed from the phase-matching point of view. For the two-channel transmission, a method is proposed to select the four-wave-mixing-induced sidebands, which is based on the proper use of a continuous-wave and a pulse as light sources. We find that a mass of sidebands are generated in the modulation instability resonance region, and the power of the sideband increases with not only the peak power of the pump pulse but also the continuous-wave power which acts as a seed. The research will provide guidance for fiber communication and sensing systems using wavelength division multiplexing technology.

Numerical Simulation of Water Waves’ Modulational Instability under the Effects of Wind’s Stress and Gravity Force Relaxation

The waves driven by the wind do not move on the water as ordinarily done by sailboats. Indeed, the movement of the waves driven by the wind is more complex than the sailboats’ translation movement that we know. The movement of the wave in our particular case results from the chain-job done by wind’s stress and gravity forces: material is collected upstream (erosion phenomenon) and then deposited on the wave’s summit by the wind. This material deposited on the summit of the wave by the wind is then removed and dispatched on the downstream side of the wave by gravity forces. As always happens in any chain-job: if the wind works faster than gravity forces, great accumulation of material will occur at the summit of the wave that will lead to an increase in its (the wave in this case) height. If conversely the wind works more slowly, a deficit in material delivery will occur and gravity force goes directly to remove material on the wave’s summit and lead to a decrease in its height. In terms of Mechanics, we know that the main obstacle that can seriously disturb the work of the wind is the unavailability of water or so its viscosity. Given the complexity of the process to be studied, it seemed necessary for us to make a use of modulational instability theories such as the standard NLSE in order to better understand the contribution of wind and water viscosity to modulations of driven waves’ amplitudes (or phases): modulations which sometimes can accidentally trigger unpredictable rogue waves.

片上集成FMCW激光雷达研究进展(封面文章·特邀)

随着硅基光电子集成技术快速发展,具有测距测速一体、探测精度高、抗干扰能力强的片上集成调频连续波(Frequency Modulated Continuous Wave,FMCW)激光雷达已成为研究热点,并在智能驾驶、航空航天、目标监测和公共安全等领域释放出越来越大的应用潜力。文中聚焦基于光学相控阵(Optical Phased Array,OPA)的片上集成FMCW激光雷达,介绍了系统工作原理及典型组成,针对国内外研究进展,概述分析了片上集成FMCW激光雷达的发展现状,同时从调频、发射、扫描、探测和处理等角度凝练了主要关键技术,并梳理了片上集成FMCW激光雷达在相关领域的应用前景和典型系统示例。最后,分析并展望了片上集成FMCW激光雷达面临的潜在挑战及发展趋势。

基于光电锁相环的高线性FMCW激光雷达频率调谐技术

搭建了可调谐半导体激光器扫频非线性校正系统,研究了锯齿波调制下半导体激光器的扫频非线性特性,将光电锁相环技术引入调频连续波光源系统中,提出在预失真校正电流基础上采用光电锁相环的方法来克服扫频非线性,以防止大范围扫频非线性频差导致的失锁现象。基于调频连续波激光测距实验,验证光频线性化的效果,并分析了扫频非线性对精确距离测量的影响。实验结果表明,经过非线性校正后,激光器输出光频与理想线性光频差的均方根误差缩小到23.7 MHz(减小为校正前的1/219),残余非线性仅余0.0339%,1-r2的数量级达到10-7,测距实验中得到的目标厚度误差在不同测量距离下在0.05 mm~0.15 mm之间变化,表明非线性校正取得了良好的效果。

76~81 GHz高精度CMOS有源移相器设计

为了实现低成本、高性能的毫米波汽车雷达,基于TSMC 40 nm CMOS工艺,设计了一款高精度低噪声系数的有源矢量合成移相器,其频率范围为76~81 GHz,移相步进为5.625°。采用了一种RC-RL两级多相滤波结构,在产生稳定的四路正交信号的同时,不会增加过多的插入损耗;在多相滤波器之前添加了放大单元电路,提高了整个电路的增益,并且降低了噪声系数;设计了数模转换器(DAC),对矢量合成模块的尾电流进行控制,进而实现高精度的移相。由于相位分辨率由DAC的分辨率所决定,因此该移相器的控制分辨率不会影响其尺寸和插入损耗。为了实现电路的性能,合理地选择了参数模型和拓扑结构,并且优化了版图布局。后仿真结果表明,在76~81 GHz范围内,RMS移相误差<2.1°,平均增益>1.7 dB,RMS增益波动<0.5 dB,噪声系数<9.5 dB,功耗为15.7 mW,1 dB增益压缩点>-7 dBm。该有源移相器具有高精度、低成本、低功耗等特点,与传统有源移相器相比,具有更低的噪声系数与更大的增益。

Efficient Focusing Models for Generation of Freak Waves

Four focusing models for generation of freak waves are presented. An extreme wave focusing model is presented on the basis of the enhanced High-Order Spectral （HOS） method and the importance of the nonlinear wave-wave interaction is evaluated by comparison of the calculated results with experimental and theoretical data. Based on the modification of the Longuet-Higgins model, four wave models for generation of freak waves （a. extreme wave model ＋ random wave model; b. extreme wave model ＋ regular wave model; e. phase interval modulation wave focusing model; d. number modulation wave focusing model with the same phase） are proposed. By use of different energy distribution techniques in the four models, freak wave events are obtained with different Hmax/Hs in finite space and time.

Dynamic Analysis of Turret-Moored FPSO System in Freak Wave

Freak wave is the common wave which has significant wave height and irregular wave shape, and it is easy to damage offshore structure extremely. The FPSOs（Floating Production Storage and Offloading） suffer from the environment loads, including the freak wave. The freak waves were generated based on the improved phase modulation model, and the coupling model of FPSO-SPM（Single Point Mooring） was established by considering internal-turret FPSO and its mooring system. The dynamic response characteristics of both FPSO and SPM affected by the freak wave were analyzed in the time domain. According to the results, the freak waves generated by original phase modulation model mainly affect the 2nd-order wave loads. However, the freak waves which are generated by random frequencies phase modulation model affect both 1st-order and 2nd-order wave loads on FPSO. What is more, compared with the irregular waves, the dynamic responses of mooring system are larger in the freak waves, but its amplitude lags behind the peak of the freak wave.

An efficient focusing model for generation of freak waves

Based on the Longuet-Higgins wave model theory, the previews studies have shown that freak waves can be generated in finite space and time successfully. However, as to generating high nonlinear freak waves, the simulation results will be unrealistic. Therefore, a modified phase modulation method for simulating high nonlinear freak waves was developed. The surface elevations of some wave components at certain time and place are positive by modulating the corresponding random initial phases, then the total surface elevation at the focused point is enhanced and furthermore a freak wave event is generated. The new method can not only make the freak wave occur at certain time and place, but also make the simulated wave surface time series satisfy statistical properties of the realistic sea state and keep identical with the target wave spectrum. This numerical approach is of good precision and high efficiency by the comparisons of the simulated freak waves and the recorded freak waves.

Influence of degenerate four-wave mixing on the performance of supercontinuum-based multiwavelength optical source

The influence of degenerate four-wave mixing （FWM） on the performance of supercontinuum-based multiwavelength optical source has been investigated in detail experimentally and theoretically. Numerical simulation results show that the degenerate FWM effect has a deteriorative influence on the spectral uniformity and the optical signal-to-noise ratio （OSNR） of supercontinuum-based optical source, and by suppressing degenerate FWM effect the performance enhancement of the supercontinuum can be achieved successfully. These results are also confirmed by our experiments. Experimentally, by suppressing degenerate FWM the crosstalk of adjacent channels to the filtered channel can be reduced by as much as 15 dB, and consequently the measured receiver sensitivity at 10 Gbit/s for the filtered optical source is improved from -1.7 to -17.8 dBm.

Optical A/D Quantizer Scheme Based on Parallel Phase Mod ulators

A high-speed and high-resolution optical A/D quantizer is proposed.Its architecture is discussed.Bit circuits are built by using the phase modulators in parallel.Based on the different character of the half-wave voltage for every phase modulator and the polarized bias design of incident light,the RF input signal is coled and transmitted in the form of optical digital signal.According to the principle of the architecture,the high-resolution quantizers with 8-bit and 12-bit,et al.are built,which operate at 100 GS/s.Their quantization noise is invariable almost with bit circuits increasing.The simulation result of 4-bit A/D quantizer is also given.

Evolution of Hyperbolic-Secant Pulses Towards Cross-Phase Modulation Induced Optical Wave Breaking and Soliton or Soliton Trains Generation in Quintic Nonlinear Fibers

The approximate analytical frequency chirps and the critical distances for cross-phase modulation induced optical wave breaking(OWB) of the initial hyperbolic-secant optical pulses propagating in optical fibers with quintic nonlinearity(QN) are presented. The pulse evolutions in terms of the frequency chirps, shapes and spectra are numerically calculated in the normal dispersion regime. The results reveal that, depending on different QN parameters, the traditional OWB or soliton or soliton pulse trains may occur. The approximate analytical critical distances are found to be in good agreement with the numerical ones only for the traditional OWB whereas the approximate analytical frequency chirps accords well with the numerical ones at the initial evolution stages of the pulses.

基于质心寻峰的调频连续波激光干涉位移传感器

调频连续波激光干涉技术在精密测量领域中应用广泛,针对其高精度位移解调问题,本文将质心法应用于其拍频信号的解调中,提出了一种基于质心寻峰法的相位解调算法并进行实验与分析。提出的算法在对截取的拍频信号进行平滑滤波与分峰截幅等处理基础上,通过质心坐标公式得到拍频信号的质心,质心横坐标即为峰值位置,最后通过相位鉴别算法解调出位移。在仿真中设置信噪比(SNR)为15 dB,算法的相位误差为0.016 rad,位移误差为2.04 nm。搭建调频连续波激光干涉位移测量系统进行实验验证,实验结果表明:当固定距离为44 mm时,位移随机误差标准差为2.18 nm。与常规的过零点检测法进行对比分析,结果表明该算法的测量误差降低了49%,分辨率获得了提高,具有广泛的应用前景。

应用于毫米波段的宽带波束调控超表面设计

毫米波段因其较高的数据传输速率和较大的容量而备受关注。为满足现代通信系统对宽带、波束偏转和高辐射效率等天线需求,本文提出了一种工作在88.5~94.6 GHz频带范围内的石墨烯-金属复合结构超表面。该超表面通过调节石墨烯的费米能级,对超表面单元的反射相位进行实时调控编码。调整金属层结构尺寸,可以调制不同编码状态下超表面单元的相位、幅度响应以及编码频段范围。经优化设计,在石墨烯费米能级分别为0 eV和1 eV时,编码超表面单元在88.5~94.6 GHz的宽频带内具有180°±20°的反射相位差,并且反射幅度均大于0.7。利用该单元设计了7×7的超表面,通过合理排布编码单元,可对毫米波段波束进行偏转和多波束切换等动态调控。实验结果显示,单波束最大偏转角度可达105°(-55°~50°),且可实现1~5个波束的切换。该超表面为多功能天线设计提供了新的解决方案,在高速无线局域网、车载雷达、卫星通信等领域具有潜在的应用价值,对毫米波段通信技术的发展具有重要意义。

基于调频连续波相位敏感特性的电缆局部缺陷检测方法

灵敏地检测和定位电缆缺陷对于电缆安全运行具有重要的作用。调频连续波(FMCW)法是一种基于频域反射法的电缆缺陷检测方法,该方法相对于宽频阻抗谱法具有更高的缺陷检测灵敏度及抗干扰能力。该文在现有FMCW方法的基础上,提出利用FMCW的相位敏感特性对电缆缺陷进行检测的方法,提高了现有FMCW方法缺陷检测的灵敏度。首先,结合FMCW相位敏感特性与电缆分布参数模型进行了电缆缺陷检测仿真;然后,在实验室的射频同轴电缆、10 kV交联聚乙烯(XLPE)电缆上模拟不同程度的缺陷并测试,观察缺陷处相位的变化情况,验证了所提方法的有效性。结果表明:相敏-FMCW方法对于射频及10 kV XLPE电缆上模拟的缺陷敏感,不同严重程度的缺陷相位变化量不同,缺陷处对应相位变化的取值区间可为180°。定义缺陷引起的相位、幅值变化量占取值区间的比例为灵敏度,相敏-FMCW方法可以提高现有FMCW方法对该文中所设缺陷检测的灵敏度:对于电缆局部异常温升造成的容性缺陷,当局部温差为37℃时,可将灵敏度由36%提升至71%;对于电阻性、电导性及铜屏蔽破损缺陷,相敏-FMCW方法的灵敏度均优于传统FMCW方法,检测灵敏度提升了7%~62%。

基于相位聚焦的驻波声场悬浮与运输方法研究

高频驻波声场中声波入射波与反射波非线性叠加作用下会产生声辐射力,从而实现微小物体的悬浮.使用幅值调制或频率调制改变悬浮位置存在空间上限制,且运输平稳性较差.在换能器阵列形成声场中,使用相位调制可以实现定点聚焦与物体平稳运输.基于超声驻波悬浮原理,详细分析了单驻波、平面驻波、空间驻波声场声压、相位分布情况,并基于聚焦原理使用相位调制的方法实现对声场的定点聚焦、聚苯乙烯小球悬浮与运输.结果表明:单驻波声场可通过双向相位调制方法实现小球稳定运输;在横向相位差为π的情况下使用PLL多时钟信号可实现多方向、不同速度的同时运输;空间阵列驻波声场通过定点聚焦声压提高了83%,且通过连续改变焦点位置实现了小球直线运输.

石墨烯/锰锌铁氧体/硅薄膜特性及其太赫兹波的磁场调制作用

近年间,基于磁性薄膜的太赫兹调制器件有了初步进展。对磁控溅射锰锌铁氧体薄膜的制备工艺进行了详细研究。在此基础上,制备了石墨烯/锰锌铁氧体/硅(graphene/MnZn ferrite/Si)结构的太赫兹调制器件。其在太赫兹波段的透射可以通过源-栅电压来调制。当栅极电压V_(sg)为-25 V时,垂直膜面磁场从20 Oe增大到200 Oe,复折射率和复介电常数的实部随之增高。由于锰锌铁氧体薄膜的反常塞曼效应和各向异性磁电阻效应引起薄膜整体电导率和折射率的变化,实现了外加磁场对太赫兹波相位的调制。

Effects of group-velocity mismatch and cubic-quintic nonlinearity on cross-phase modulation instability in optical fibers

The synthetic effects of group-velocity mismatch and cubic-quintic nonlinearity on cross-phase modulation induced modulation instability in loss single-mode optical fibers have been numerically investigated. The results show that the quintic nonlinearity plays a role similar to the case of neglecting the group-velocity mismatch in modifying the modulation instability, namely, the positive and negative quintic nonlinearities can still enhance and weaken the modulation instability, respectively. The group-velocity mismatch can considerably change the gain spectrum of modulation instability in terms of its shape, peak value, and position. In the normal dispersion regime, with the increase of the group-velocity mismatch parameter, the gain spectrum widens and then narrows, shifts to higher frequencies, and the peak value gets higher before approaching a saturable value. In the abnormal dispersion regime, two separated spectra may occur when the group-velocity mismatch is taken into account. With the increase of the group-velocity mismatch parameter, the peak value of the gain spectrum gets higher and shorter before tending to a saturable value for the first and second spectral regimes, respectively.

双偏振干涉式光纤陀螺的多相位调制技术

在双偏振干涉式光纤陀螺的发展过程中,光纤环上的应力、扭转等会造成正交偏振态之间的交叉耦合,降低陀螺系统的稳定性。提出了一种基于双偏振干涉式光纤陀螺的六态方波调制技术并进行了理论推导。该技术与传统的方波和四态方波调制相比,降低了偏振交叉耦合误差,提高了信噪比,大大增加了信号解算精度。通过实验对比测试了干涉式光纤陀螺在不同调制技术下的偏置稳定性。实验结果表明六态方波调制技术的偏置稳定性达到了9.85×10-4(°)/h,验证了六态方波提高信号解算精度的效果。

微波耦合四能级原子系统二维电磁感应光栅

提出一种利用微波场在四能级原子系统中实现二维电磁感应光栅的理论研究方案,此原子系统中两个偶极禁戒的亚稳态之间采用微波场来耦合。分析了微波场存在与否对弱探测场夫琅禾费衍射图样的影响。讨论了有微波场存在时,探测场失谐量、控制场强度、相互作用长度等对二维电磁感应光栅衍射效率的影响。结果表明,通过合适地调节系统参数,在所研究的系统中可以实现高衍射效率的二维电磁感应光栅。该方案可应用于全光分束和光开关,对光信息处理及光网络通信研究有一定帮助。

二氧化钒对太赫兹波的调制性能测试

二氧化钒具有显著的可逆绝缘体-金属相变特性,二氧化钒在温度达到68℃时发生一级相变,其电导率、磁导率、可见光反射率等性能都会发生显著的可逆变化,因而可以利用二氧化钒制成智能窗、微驱动器等先进器件。已有研究表明,二氧化钒也能对太赫兹波响应,但是关于二氧化钒对太赫兹波的调制性能还不是很清楚。本文以二氧化钒粉末为研究材料,分别采用了X射线衍射、扫描电子显微镜、X射线能量色散谱对二氧化钒粉末进行了形貌和物相表征,使用差示扫描量热仪对二氧化钒的相变温度进行了测试,使用太赫兹时域光谱仪对二氧化钒的太赫兹调制性能进行了测试,总结了二氧化钒相变过程对太赫兹波调制性能的影响规律。