Electrical characteristics of the waveguide grating system

Electromagnetic wave propagation is conside-red through a waveguide system consisting of arbitrary number of short-period gratings (i.e. whose period is much shorter than the wave-length). We assume that gratings are formed by ideal conductors having arbitrary cross-section. In order to "stitch" together the fields on gratings, equivalent boundary conditions are used.We have analyzed and studied experimentally the electrical characteristics of an adjustable frequency filter. The filter consists of three short-period gratings formed by conductors with circular cross-section.Experimental results are in good agreement with theory.     

Multitap microwave photonic filters with programmable phase response via optical frequency comb shaping

We present a programmable multitap microwave photonic filter with an arbitrary phase response operating over a broad bandwidth. Complex coefficient taps are achieved by optical line-by-line pulse shaping on a 10 GHz flat optical frequency comb using a novel interferometric scheme. Through high-speed real-time measurements, we demonstrate programmable chirp control of a waveform via phase filtering. This achievement enables us to compress broadband microwave signals to their corresponding bandwidth-limited pulse duration.     

Flat-top ultra-wideband photonic filters based on mutual coherence function synthesis

A novel all-incoherent optical circuit that allows for band-pass microwave-photonic filter design is presented and verified through numerical simulation. In contrast to conventional spectrum-sliced optical architectures that operate on the basis of a finite number of discrete taps, our proposal is based on arbitrary shaping of the spectrum of the broadband optical source in a conventional frequency encoder. This fact dramatically increases the free spectral range of the filter with respect to the conventional discrete-time optical processing. The filter transfer function is given by the mutual coherence function of the filtered source which allows, through an inverse problem, sculpting the RF filter response. The effect of higher-order dispersion terms is also included by means of the optical coherence theory. Finally, some strategies are provided in order to alleviate the baseband resonance constraint. Numerical results are also included.     

VHF波段超窄带超导滤波器的设计及其时域调谐

低频超窄带滤波器要求谐振器间的耦合极弱,设计受到薄膜面积限制,其性能对基片介电常数的均匀性极为敏感且受制作和封装精度的限制.这些因素将导致的滤波器中心频率偏移和带内性能恶化.对此,时域调谐提供了很好的解决途径.我们采用嵌套双螺旋型谐振器,在37mm×12mm的MgO基片上设计制作了4节超导滤波器,中心频率为166.9 MHz,相对带宽仅为0.29%.由于基片厚度或介电常数的偏差及不均匀性会导致滤波器中各谐振的谐振频率偏移,使通带性能受到很大影响.我们提出了将机械调谐与时域分析相结合的方法,通过机械调谐纠正各谐振器的谐振频率,改善滤波器性能.同时为了解决多信道滤波器系统中,各滤波器工作于同一温度下,系统频率一致性问题,通过时域调谐获得频率可调范围信息.对上述0.5 MHz带宽的VHF波段滤波器应用时域调谐方法,得到的可调范围为0.7 MHz,测试结果表明该滤波器具有优异性能,带内插损小于0.4 dB,反射损耗达到14.8 dB,带外抑制大于-70 dB.     

Tunable radio-frequency photonic filter based on an actively mode-locked fiber laser

We propose the use of an actively mode-locked fiber laser as a multitap optical source for a microwave photonic filter. The fiber laser provides multiple optical taps with an optical frequency separation equal to the external driving radio-frequency signal of the laser that governs its repetition rate. All the optical taps show equal polarization and an overall Gaussian apodization, which reduces the sidelobes. We demonstrate continuous tunability of the filter by changing the external driving radio-frequency signal of the laser, which shows good fine tunability in the operating range of the laser from 5 to 10 GHz.     

An Approach of Developing High Performance Millimeter-wave Frequency Synthesizer

In this paper, an approach of developing high performance millimeter-wave frequency synthesizer is proposed, which is significantly simpler than the conventional cases. The synthesizer is driven by one triple tuned typed synthesizer, which adjusts the output frequency of DDS and frequency division ratios of variable frequency divider to suppress the spurious level. With the proposed method, a microwave phase locked loop (PLL) PE3236 and a millimeter-wave multiplier HMC283 are also used. Moreover, the PLL is implemented with the form of charge pump followed by a passive three-order low-pass filter which can further suppress the phase noise. Finally, a low spurious level and high frequency resolution millimeter-wave frequency synthesizer without degradation of frequency switching speed is developed. Experimental results show that this method can achieve the performances of low spurious level, low phase noise, and high frequency resolution.     

多通道CCD摄像机的数据采集和图像显示

针对多通道CCD摄像机 ,提出了一种多线程数据实时存储的方法。利用多线程提高硬盘读写速度 ,实现高速、大容量CCD摄像机数据的硬盘实时存储 ,并利用定时中断将多通道信号定时分块存储入硬盘 ,存储的CCD摄像机数据被转换为图像文件加以显示。系统的CCD摄像机通道达到 5个 ,每个通道的数据率达到 10MBit/s。通过实验 ,验证了该方法的正确性和有效性     

A tunable single-passband microwave photonic filter with positive and negative taps using a fiber Mach-Zehnder interferometer and phase modulation

In this paper, a tunable single-passband microwave photonic filter with positive and negative taps based on phase modulation and a fiber Mach-Zehnder interferometer (FMZI)) is presented, theoretically analyzed and experimentally demonstrated. The FMZI as the slicing filter results in a single-bandpass characteristic for the presented filter, and by using phase modulation, a number of negative taps are generated, thus making the baseband resonance suppressed, which is desirable for microwave photonic filter's application. The wavelength spacing of the FMZI can be tuned by employing an optical variable delay line, making the present filter continuously tunable. A tunable single-passband microwave photonic filter without baseband resonance is achieved in the experiment. The present filter shows an extinction ratio of up to ∼20 dB and good tuning linearity.     

空间外差光谱仪中窄带滤光片的光谱特性研究

高光谱空间外差干涉光谱仪的光谱响应由窄带滤光片的光谱特性决定,由于窄带滤光片的制备水平与理论值存在差距,使得发生光谱展宽或波长漂移等现象,在空间外差光谱仪中形成高频和低频光谱混叠从而导致复原光谱失真。在分析滤光片对仪器光谱响应性能影响的基础上,通过在空间外差光谱仪核心干涉组件光栅的胶合过程中,根据窄带滤光片实测波长透过率曲线,利用可调谐激光器动态监测调整光谱仪基频波长的方法改善对其造成的光谱混叠。结果表明,通过实测滤光片特性调整光栅偏转角度来改变基频波长,可最大程度地有效利用仪器所能覆盖的光谱范围,根据干涉图的复原光谱信号可知试验装置的有效光谱范围从758~770.9 nm增加至756~770.9 nm。     

SHF频段超宽带高温超导滤波器的研制

针对超宽带通信系统的发展趋势,采用1/4波长短截线结构设计了一个工作于超高频(SHF)频段的超宽带高温超导滤波器,滤波器的中心频率为4.2 GHz,相对带宽为50%,讨论了1/4波长短截线结构滤波器的设计原理,给出了所设计超宽带滤波器的理论曲线、模拟仿真结果。实际制作的超宽带高温超导滤波器的测量曲线与模拟仿真曲线基本吻合,得到了满意的结果。     

有孔矩形腔屏蔽效能的传输线法分析

首先介绍了用传输线法（transmission line method,TLM）分析有孔矩形腔屏蔽效能的基本原理,然后将基本公式作进一步扩展,使其能计算圆孔、多孔洞以及在任意极化方向时的情形。仿真结果表明：当频率低于主谐振频率时,离孔缝越近,耦合进的电磁能量越大;当处于谐振频率时,屏蔽腔与孔形成共振,屏蔽效能很低甚至为负,而且腔体内任何空间都如此;屏蔽效能随极化角度的递增而递减,低频段的屏蔽比高频段要好;对于相同面积的孔洞,单孔洞的屏蔽效能比多孔洞的屏蔽效能要差,孔洞越多,屏蔽效果越好,而圆形孔（等同于方形孔）的屏蔽效果最好。     

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.     

Mechanically tunable metamaterials terahertz dual-band bandstop filter

We experimentally demonstrate a mechanically tunable metamaterials terahertz(THz) dual-band bandstop filter. The unit cell of the filter contains an inner aluminum circle and an outside aluminum Ohm-ring on high resistance silicon substrate. The performance of the filter is simulated by finite-integration-time-domain(FITD) method. The sample is fabricated using a surface micromachining process and experimentally demonstrated using a THz time-domain-spectroscopy(TDS) system. The results show that, when the incident THz wave is polarized in y-axis, the filter has two intensive absorption peaks locating at 0.71 THz and 1.13 THz, respectively. The position of the high-frequency absorption peak and the amplitude of the low-frequency absorption peak can be simultaneously tuned by rotating the sample along its normal axis.The tunability of the high-frequency absorption peak is due to the shift of resonance frequency of two electrical dipoles,and that of the low-frequency absorption peak results from the effect of rotationally induced transparent. This tunable filter is very useful for switch, manipulation, and frequency selective detection of THz beam.     

Microwave pulse phase encoding using a photonic microwave delay-line filter

A novel technique to perform microwave pulse phase encoding using an incoherent photonic microwave delay-line filter is proposed and experimentally demonstrated. Being different from a regular microwave delay-line filter, in which the time-delay differences are identical between any adjacent taps, the proposed filter has nonidentical time-delay differences. A phase-encoded microwave pulse with the required code pattern is generated by properly adjusting the time-delay differences. The chip number of a generated phase code is determined by the number of the filter taps, and the phase shift of each chip is determined by the corresponding time-delay difference. The proposed technique is verified by experiments. The generation of binary and quaternary phase-coded pulses is experimentally demonstrated.     

一种改进的微弱NQR信号检测算法

核四极矩共振（NQR）是一种固态射频谱分析技术,可用于检测高危险爆炸物. 然而NQR信号本身非常弱,并且易受线圈的热噪声和外部射频干扰的影响,低信噪比限制了NQR的实际应用. 该文提出一种改进的微弱NQR信号检测算法. 首先利用Hankel矩阵方式下奇异值分解的方法,有效地抑制射频干扰和噪声,并将NQR信号分离出来. 然后提出了一种基于MUSIC谱估计的非线性最小二乘检测器,它既保证了高的频率分辨率,又大大降低了运算量. 仿真数据和实测数据结果表明该算法的有效性.     

Image denoising using trivariate prior model in nonsubsampled dual-tree complex contourlet transform domain and non-local means filter in spatial domain

Image denoising is always the basic problem of image processing, and the main challenge is how to effectively remove the noise and preserve the detailed information. This paper presents a new image denoising algorithm based on the combination of trivariate prior model in nonsubsampled dual-tree complex contourlet transformlet transform (NSDTCT) domain and non-local means filter (NLMF) in spatial domain. Firstly, NSDTCT is constructed by combining the dual-tree complex wavelet transform (DTCWT) and nonsubsampled directional filter banks (NSDFB). The noisy image is decomposed by using NSDTCT. Secondly, based on the correlation between the interscale and intrascale dependencies of NSDTCT coefficients, the distribution of the high frequency coefficients is modeled with the trivariate non-Gaussian distribution model. A nonlinear trivariate shrinkage function is derived in the framework of Bayesian theory, and then the denoised coefficients are obtained and inverse NSDTCT is performed to get the initial denoised image. Finally, NLMF is used to smooth the initial denoised image. Simulation experiment shows that our algorithm can obtain better performances than those outstanding denoising algorithms in terms of peak signal-to-noise ratio (PSNR), mean structural similarity (MSSIM) as well as visual quality.     

Scattering Characteristics of Dielectric Periodic Structure Composed of Left-handed Materials with Arbitrary Oblique Incidence

Scattering characteristics of monolayer and multilayer dielectric periodic structure composed of left-handed materials (LH-DPS) with plane wave arbitrary oblique incidence are carefully analyzed using a method which combines multimode network theory with the rigorous mode matching method. Our analysis results reveal that the arbitrary oblique incident angles and relative position between different LH-DPS have great effects to the scattering characteristics of LH-DPS which different from the situation of dielectric periodic structure composed of right-handed materials (RH-DPS). The reasons why the reflection characteristics of the LH-DPS are totally different from those of the RH-DPS with arbitrary oblique incidence are also given. The present quantitive investigation provides guidelines for the design of the monolayer and multilayer dielectric frequency selective surfaces for millimeter wave applications.     

有孔矩形腔屏蔽效能的传输线法分析

 首先介绍了用传输线法（transmission line method,TLM）分析有孔矩形腔屏蔽效能的基本原理，然后将基本公式作进一步扩展，使其能计算圆孔、多孔洞以及在任意极化方向时的情形。仿真结果表明：当频率低于主谐振频率时，离孔缝越近，耦合进的电磁能量越大；当处于谐振频率时，屏蔽腔与孔形成共振，屏蔽效能很低甚至为负，而且腔体内任何空间都如此；屏蔽效能随极化角度的递增而递减，低频段的屏蔽比高频段要好；对于相同面积的孔洞，单孔洞的屏蔽效能比多孔洞的屏蔽效能要差，孔洞越多，屏蔽效果越好，而圆形孔（等同于方形孔）的屏蔽效果最好。     

复杂背景下混合畸变目标识别技术

结合高斯函数和最大平均相关高度算法设计了高斯-最大平均相关高度滤波器,解决了光学相关识别系统对混合畸变目标(同时存在比例畸变和旋转畸变的目标)的识别问题.在设计滤波器时,通过优化平衡高斯-最大平均相关高度滤波器的标准差σ、输出噪声方差系数α、平均相关能量系数β及平均相似测量系数γ来提高高斯-最大平均相关高度滤波器对混合畸变目标的响应能力.最后通过仿真实验可知,在畸变容差范围(比例畸变0.92~1.16倍,旋转畸变-8度~24度)内,Vander Lugt相关器利用单个高斯-最大平均相关高度滤波器可以实现任意状态混合畸变目标的识别.作为实例,用高斯-最大平均相关高度滤波器对复杂背景下的飞机目标进行了识别实验,验证了该方法的可行性.     

Frequency selective surface with better polarization-independency for arbitrary incident angle

This paper investigates the frequency-selective property of a planar layer consisting of period arrays both theoretically and experimentally for different polarizations at arbitrary incident angle. The novel element is designed by loading the rectangular microstrip element with L-shaped conducting patch at its two ends. Based on the spectral-domain method, the frequency response including angle effect and polarization effect of the frequency selective surface (FSS) structure are analysed and the plots of the frequency versus transmission coefficient are obtained. As a result of the numerical analysis, it is shown that if the source polarization is changed, polarization-independence of previous FSS design can be achieved only for normal incidence, which limits most FSS applications. But in our proposed structure, the better polarization-independency for arbitrary incident angle can be achieved. It is observed that the simulated result comes very close to the experimental result.