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A hybrid high quality factor (Q-factor) microwave photonic filter with a cascaded active filter and a passive filter is presented and experimentally demonstrated. The active infinite impulse response filter is realized by a recirculating delay line loop with a semiconductor optical amplifier, and a much narrower 3 dB bandwidth of response peaks can be achieved. A passive finite impulse response filter is realized by an unbalance Mach-Zehnder interferometer, and it is cascaded to select the desired filter frequencies and to suppress the intermediate peaks. Compared with the purely active filter scheme, the free spectrum range and the Q-factor of the hybrid structure can be doubled. Stable operation and a high Q-factor of 362 are experimentally demonstrated. 相似文献
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We propose and demonstrate single fiber dual-functionality optical tweezers based on a graded-index multimode fiber. By using the multi-angle fiber grinding and polishing technology, we fabricate the multimode fiber tip to be a special tapered shape, contributing to focus the outgoing beam with a large intensity gradient for the first functionality—three-dimensional contactless trapping of a microparticle. By adjusting the radial direction offset between the lead-in single mode fiber and the graded-index multimode fiber, we perform the second functionality—axial shift of the trapped microparticle with respect to the fiber tip without need of moving the fiber probe itself. It is convenient for practical applications. The theoretical and experimental results about the relationship between the radial offset and the equilibrium positions of the microparticle have the good consistency. Tailoring the trap and axial shift of the microparticle based on the graded-index multimode fiber provides convenient avenues for fiber optical tweezers applied in practical researches. 相似文献
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利用单侧回波可估计合成孔径声呐基阵的斜距误差,但无法区分横荡误差和升沉误差。针对此问题,提出了一种双侧回波联合的运动补偿方法。该方法首先根据双侧基阵运动误差的几何关系,建立了双侧基阵的运动误差模型,再结合偏移相位中心算法估计基阵的横荡误差和升沉误差,最后利用所估计的运动误差对不同掠射角上的回波进行运动补偿。仿真结果表明:该方法能精确估计双侧基阵的运动误差,其估计值与实际值的偏差为10-4 m左右,估计结果的标准差接近克拉美罗下界;对回波进行运动补偿后,能获得比基于单侧回波运动补偿方法更好的成像效果。水下球串目标的湖试数据的成像结果显示,与基于单侧回波的运动补偿方法相比,所提方法能更好地抑制图像的散焦现象。 相似文献
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用数值方法研究磁化非均匀等离子体(磁场与密度梯度垂直)中模转换机制对p极化电磁波的吸收效应,重点考虑磁场对吸收率的影响,结果表明:与非磁化等离子体相比,磁场的加入使得吸收率曲线相对入射角的分布发生小的偏移,吸收率为零的位置不是发生在零度入射(即垂直入射)时.而是在一个负的入射角位置,垂直入射时有不为零的吸收率,且正角度范围与负角度范围的峰值吸收率不相等,磁场提高了正入射角范围内的峰值吸收率而降低了负角度范围内的峰值吸收率(或反过来,这取决于磁场方向). 相似文献
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二进制偏移载波 (binary offset carrier,BOC) 调制方式实现了频谱分离和频段共享,被应用于全球导航卫星系统中;但由于BOC信号自相关函数的主峰和副峰幅度差异较小,容易引起捕获和跟踪模糊,导致较大的测距误差;余弦相位BOC(Cosine-phased BOC,CosBOC)信号自相关函数形式相对复杂,给其无模糊处理带来了挑战;基于伪相关函数法(pseudo correlation function, PCF)思想,针对CosBOC信号设计本地参考信号的特殊码片波形,与接收信号相关后经过非线性组合可获得单峰无模糊相关函数,消除了跟踪模糊性;仿真结果表明:参数取值合适时该方法的跟踪和抗多径性能均优于同等条件下的BPSK-like方法。 相似文献
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针对传统剪切光束成像技术的准实时性问题,提出用口字形排布的四束光代替传统L形三束剪切光照射目标,研究了四光束剪切相干成像目标重构算法.只需单次测量就能同时重构出四幅目标图像,减少了用于降低散斑噪声、获取高质量图像所需的测量次数,同时大大减少了多组发射时的光束切换次数,提高了成像效率.在算法实现中,通过最小二乘法恢复出四组波前相位,利用散斑幅值的简单代数运算恢复波前幅值,从而重构出目标图像.仿真结果表明,与传统方法相比,在图像质量相同的前提下,本文方法所需的数据采集时间减少了至少1/2,不但提高了目标重构效率,还可为远程运动目标的成像识别提供更好的手段. 相似文献