Enhanced and controllable reflected group delay based on Tamm surface plasmons with Dirac semimetals

The reflected group delay from a multilayer structure comprising a one-dimensional photonic crystal coated with a bulk Dirac semimetal(BDS)separated by a spacer layer is investigated theoretically.It is shown that the group delay of the reflected beam in this structure can be significantly negatively enhanced and switched from negative to positive.The enhanced group delay originates from the steep phase change caused by the excitation of the optical Tamm state at the interface between the BDS and spacer layer.Moreover,positive and negative group delays can be actively tuned through the Fermi energy and the relaxation time of the BDS.We believe that this enhanced and tunable delay scheme has important research significance for the fabrication of optical delay devices.

Enhancement of the group delay in quadratic coupling optomechanical systems subjected to an external force

We theoretically study the effect of the quadratic coupling strength on optomechanical systems subjected to a continuous external force. Quadratic coupling strength originates from strong coupling between the optical and the mechanical degrees of freedom. We show that the quadratic coupling strength reduces the amplitude of the dispersion spectra at the resonance in both blue-and red-sideband regimes. However, it increases(decreases) the amplitude of the absorption spectrum in the blue-(red-)sideband regime. Furthermore, in both sideband regimes, the effective detuning between the pump and the cavity deviates with the quadratic coupling strength. Thereby, appropriate selection of the quadratic coupling strength results in an important magnification(in absolute value) of the group delay for both slow and fast light exiting from the optomechanical cavity.

基于group delay函数的分布式阵列三角定位算法

为了提高信源三角定位的定位精度和噪声鲁棒性,采用群时延(group delay)函数和分布式阵列,提出一种高精度的三角定位算法。通过group delay函数的负差分形式与传统MUSIC空间谱的乘积消除无关伪峰,得到高精度DOA估计,然后根据三角定位方程和阵列模型得到精确信源位置估计,再利用分布式阵列的阵列孔径大、对内部子阵位置变化不敏感等特性,提高三角定位算法对噪声的鲁棒性。实验结果表明,该算法是一种精度高、鲁棒性强的信源三角定位算法。

Time-domain nature of group delay

The characteristic of group delay is analyzed based on an electronic circuit, and its time-domain nature is studied with time-domain simulation and experiment. The time-domain simulations and experimental results show that group delay is the delay of the energy center of the amplitude-modulated pulse, rather than the propagation delay of the electromagnetic field. As group velocity originates from the definition of group delay and group delay is different from the propagation delay, the superluminaiity or negativity of group velocity does not mean the supeduminal or negative propagation of the electromagnetic field.

Optical soliton with group delay in microring coupled-resonator optical waveguides

This paper derives the dispersion relation of microring coupled-resonator optical waveguides （CROWs） without any approximation by using the transfer matrix method. Based on the established dispersion relation of CROWs it obtains the slow group velocity and dispersion coefficient. It finds that the effect Of dispersion on optical pulses can be adjusted to balance the effect of nonlinearity by changing coupling coefficient or loss, so optical soliton with group delay can be obtained in microring CROWs. The optical soliton with group delay is of great significance for applications of microring CROWs in delay lines and optical buffers of future all-optical communication systems.

Optical time-domain differentiation based on intensive differential group delay

An optical time-domain differentiation scheme is proposed and demonstrated based on the intensive differential group delay in a high birefringence fibre waveguide. Results show that the differentiation waveforms agree well with the mathematically calculated derivatives. Both error and efficiency will increase when the birefringence fibre becomes longer, and the error rises up more quickly while the efficiency approaches to a maximum of ~0.25. By using a 1-m birefringence fibre a lower error of ~0.26% is obtained with an efficiency of 1% for the first-order differentiation of 10-ps Gaussian optical pulses, and the high-order optical differentiation up to 4th order is achieved with an error less than 3%. Due to its compact structure being easy to integrate and cascade into photonic circuits, our scheme has great potential for ultrafast signal processing.

Renormalization group methods for a Mathieu equation with delayed feedback

This paper presents the application of the renormalization group （RG） methods to the delayed differential equation. By analyzing the Mathieu equation with time delay feedback, we get the amplitude and phase equations, and then obtain the approximate solutions by solving the corresponding RG equations. It shows that the approximate solutions obtained from the RG method are superior to those from the conventionally perturbation methods.

A large bandwidth photonic delay line using passive cascaded silicon-on-insulator microring resonators

This paper investigated the design and the characterization of a photonic delay line based on passive cascaded silicon-on-insulator （SOI） microrings. We considered the compromise of group delay, bandwidth and insertion loss. A 3-stage double channel side-coupled integrated spaced sequence of resonator （SCISSOR） device was optimized by shifting the resonance of each microring and fabricated with electron beam lithography and dry etching. The group delay was measured to be 17 ps for non-return-to-zero signals at different bit rates and the bandwidth of 78 GHz was achieved. The experiment result agreed well with our simulation.

基于MEMS开关的可调实时延时器设计

为了有效解决相控阵雷达/天线瞬时带宽小、孔径效应严重、功耗大等问题,将射频MEMS开关引入实时可调延时器结构中,设计了一种基于射频MEMS开关的实时可调延时器。通过MEMS双掷开关选择具有不同电长度的延时路径,在DC~20 GHz内实现了5位的射频信号延时。利用HFSS三维电磁仿真软件对延时单元的几何参数进行仿真优化,得到5个可切换延时状态的延时量,分别为64.76 ps,101.46 ps,137.97 ps,174.61 ps,210.98 ps,延时步进为36.5 ps,整体面积约为5 mm^(2)。与其他实时可调延时器相比,该实时可调延时器具有多可控位数、大延时带宽积、高集成度等优点。

群组层次分析法在海底管道工艺设计中的应用研究

海底管道工艺设计是海底管道总体设计中的重要组成部分,工艺设计方案的优劣将对海底管道设计、施工安全、维护产生重大影响。由于决定工艺方案的因素涉及到技术、经济、环境等诸多领域,问题较复杂,到目前还没有形成一套量化优选工艺方案的科学模式。文章提出了一种基于工程实践及群组层次分析法(GD-AHP)的海底管道工艺方案半定量评价模型,建立了相应的评价指标体系,并应用专家群组决策理论进行科学优选。将该模型应用于实际工程项目论证,结果表明本评估体系合理有效。

动态群时延的自适应补偿算法

为了补偿数据传输中的动态群时延,提出了一种简单有效的基带自适应的补偿算法。该算法基于一种并行全数字接收机结构,采用梯度下降算法迭代的最小化恒模算法的代价函数,完成对群时延补偿器系数的更新。仿真结果表明,该算法可以对群时延进行有效的自适应补偿,将关闭的眼图成功的打开。

颜色辨识状态下ERP时频特征与认知能力的相关性研究

研究不同程度认知能力人群的事件相关电位(ERP)的特征,从中提取出能有效识别血管性痴呆(VD)的电生理指标。选择正常青年组、VD组以及与VD同一年龄段的正常老年组作为被试人群,采用视觉通道诱发的oddball实验模式,对ERP信号运用平滑伪魏格纳分布(SPWVD)进行时频分析。通过不同被试人群的群延迟(GD)和瞬时频率(IF)的对比发现,大脑额、中央和顶区ERP信号的群延迟可反映3组不同被试之间认知能力的差异,为VD患者的早期诊断识别提供一个新的途径。

一种通信卫星在轨测试中群时延特性测量方法及工程实现

讨论了群时延的概念及群时延的测试原理和方法,并在通信卫星在轨测试的工程实现中,针对卫星转发器频段多及Ka频段转发器频率较高的特点,重点描述了综合群时延测试的方案设计及实现。经工程验证,该方法提高了在轨测试的效率和群时延测试精度。

Investigation of Helical Pulse Forming Line

To investigate the feasibility for a helical line to be used as a pulse forming line （PFL）, the transmission characteristics of the helical transmission line is studied both theoretically and experimentally. The results indicate that it is feasible to employ a helical line as a long-pulse PFL, and the influence of its dispersion is negligible. Compared with a conventional coaxial PFL, the helical PFL with the same size can produce a longer pulse.

Group delay and phase delay in GNSS systems

GNSS signals have previously been modulated using binary phase shift keying but this modulation scheme is being replaced by binary offset carrier(BOC)modulation.Research has considered how the BOC signals might be affected differently when passed through a surface acoustic wave(SAW)filter.The concern has been that because of the split spectrum nature of the BOC signals,the upper and lower side-lobes will be delayed significantly differently.This was suggested because SAW filters have nonlinear phase characteristics and therefore different frequencies are delayed differently.It was suggested that this difference in delay will result in greater distortion of the correlation triangle.A delay magnification effect was also mentioned when analyzing the delay of a BOC signal.It was not understood why the theoretical delay calculations did not match up with the actual results in both hardware and simulation.This paper clarifies some of the confusion and explains why the“delay magnification”applies to phase delay but not group delay.This paper also takes a look at how the code phase delay can vary with frequency and correlator spacing as a result of the SAW filter properties.

The generation of group delay ripple of chirped fiber gratings

The analysis of the group delay ripple (GDR) generated by chirped fiber gratings is developed based on the Fabry-Perot (F-P) resonator. It shows clearly how the GDR is generated and why the periods of GDR varies along the chirped fiber gratings. It could also explain why apodization could suppress the GDR and how apodization affects the chirp of the grating. The theory could be used to devise the apodization of fiber gratings.

Novel uncertainty relations associated with fractional Fourier transform

In this paper the relations between two spreads, between two group delays, and between one spread and one group delay in fractional Fourier transform （FRFT） domains, are presented and three theorems on the uncertainty principle in FRFT domains are also developed. Theorem 1 gives the bounds of two spreads in two FRFT domains. Theorem 2 shows the uncertainty relation between two group delays in two FRFT domains. Theorem 3 presents the crossed uncertainty relation between one group delay and one spread in two FRFT domains. The novelty of their results lies in connecting the products of different physical measures and giving their physical interpretations. The existing uncertainty principle in the FRFT domain is only a special ease of theorem 1, and the conventional uncertainty principle in time-frequency domains is a special case of their results. Therefore, three theorems develop the relations of two spreads in time-frequency domains into the relations between two spreads, between two group delays, and between one spread and one group delay in FRFT domains.

Frequency correlation and group time delay of ambient noise and ship radiated-noise in the sea

The characteristics of frequency correlation and group time delay of ambient noise and ship radiated-noise in the sea are studied. The theoretical and experimental results show that the frequency correlation of ship radiated-noise is much greater than that of ambient noise,and the frequency correlation of ship radiated-noise at long distance has obvious group time delay

Prediction of Group Delay Distribution Around Receiving Point Using Modified IRI Model and IGRF Model

The international reference ionosphere （IRI） model is generally accepted standard ionosphere model. It describes the ionosphere environment in quiet state and predicts the ionosphere parameters within a certain precision. In this paper, we have made a breakthrough in the application of the IRI model by modifying the model for regions of China. The main objectives of this modification are to construct the ionosphere parameters foF2 and M （3000） F2 by using the Chinese reference ionosphere （CRI） coefficients, appropriately increase hmE and hmF2 height, reduce the thickness of F layer, validate the parameter by the measured values, and solve the electron concentration distribution with quasi-parabolic segment （QPS）. In this paper, 3D ray tracing algorithm is constructed based on the modified IRI model and international geomagnetic reference field （IGRF） model. In short-wave propagation, it can be used to predict the electromagnetic parameters of the receiving point, such as the receiving area, maximum useable frequency （MUF） and the distribution of the group delay etc., which can help to determine the suitability of the communication. As an example, we estimate the group delay distributions around Changchun in the detection from Qingdao to Changchun using the modified IRI model and IGRF model, and provide technical support for the short-wave communication between the two cities.

Definition of Very Long Baseline Interferometry (VLBI) Group Delay and Relativistic Correction Model With Picosecond Precision

In this paper, the incorrect definition of VLBI coordinate time delay in solar system barycenter frame (B-frame) is clarified, thus correcting the error of sign of the individual term in VLBI relativistic time delay model of transformation published earlier. It is proved that the terms, i. e. the tropospheric refraction delay, the axis offset refraction delay, and the gravitational delay caused by the major planets which are respectively multiplied by the velocity and the acceleration vector of the geocenter, should be taken into account in the model mentioned above under the precision of 1 ps. Finally, the VLBI relativistic time delay model with precision of better than 1 ps is obtained.