Unbalanced nulling interferometer and precise wavefront control

A nulling interferometer was proposed to achieve direct detection of extra-solar planets (exoplanets) by suppressing light from the central star using a pair of telescopes. Recently, the stellar coronagraph method has shown rapid progress, which uses an extended concept of the nulling interferometer within single telescope optics. A dynamic range larger than 1 × 10⁹ for the detection of Earth-like exoplanets can be attained by reducing diffraction patterns using the nulling coronagraph, and by suppressing speckle noise using an adaptive optics with an accuracy of λ/10000 rms. An unbalanced nulling interferometer (UNI), which is used as fore-optics, improves the wavefront sensing sensitivity and compensation level of the adaptive optics by a factor of 10. Consequently, the dynamic range of the coronagraph can also be improved by two orders of magnitude. The UNI is composed of a modified coronagraph or a traditional interferometer and magnifies the aberrations of incoming wavefronts.     

Automatic alignment of a rigid spacer cavity

A bench-top auto-alignment system employing an invariant Gouy phase telescope is described in this paper. Our system uses external galvanometer-actuated mirrors for the tilt and offset control of a laser beam onto a rigid spacer cavity. Wavefront sensing diagnostics which accurately decouple tilt and offset er-rors are employed, and corresponding orthogonal corrections are enabled with the aid of electronic nulling. The closed loop system achieves an alignment fluctua-tion suppression of 50.4 dB for angular tilt and 58.3 dB for lateral offset at 1 Hz, with a unity gain bandwidth in excess of 115 Hz.     

Development of spatial coherence from an extended source in successive rotational shearing interferometers for achromatic stellar coronagraphy

We propose a new scheme of an achromatic nulling interferometer-coronagraph to gain a higher contrast in the star-planet model. Two successive rotational shearing interferometers improve the interference contrast limited by insufficient spatial coherence of a physically extended source, a star. The theory and simulations have been confirmed by an experiment. The theoretical value of the coronagraphic contrast has been improved from 10⁴ to 10¹⁰ for an angular size of the extended source of 10^?2λ/D, where λ is the wave-length and D is the telescope’s aperture diameter.     

大剪切实时干涉系统的火焰条纹分析

使用平行平晶做分光元件建立了剪切量随平行平晶偏转角可调的横向剪切实时干涉系统,该系统在最佳偏转角下所得最大剪切量与平行平晶的厚度成正比。实验表明,当系统的横向剪切量大于被测火焰畸变波面宽度的一半时,剪切域中的非核心剪切区将出现半幅以上的双曝光条纹,以之进行干涉测温处理可以避开传统剪切干涉测量技术所必需的剪切相位还原算法。这意味着实时双曝光条纹可以简单地通过大剪切量横向剪切干涉系统得到。     

双光栅白光衍射干涉的消色效应

本文提出和研究了白光点源照明下的双光栅衍射干涉效应,报道了白光照明下剪切干涉的“消色效应”,给出了完整的菲涅耳线性标量衍射原理的数学推导和几何光学的解释.     

Ghost telescope and ghost Fourier telescope with thermal light

As important observation tools, telescopes are very useful in remote observations. We report a proof-of-principle experimental demonstration of ghost telescope scheme and show that, by measuring the intensity correlation of two light fields and only changing the position of the detector in the reference path, ghost telescope and ghost Fourier telescope can be obtained even if a single-pixel detector is fixed in Fresnel region of the object. Differences between conventional telescope and ghost telescope are also discussed.     

Virtual shearing interferometry by digital holography

A novel method of virtual shearing interferometry (VSI) is proposed. In this method, the shearogram is obtained by interference of a real object wave-front and a virtual object wave-front. The former is optically recorded and then digitally reconstructed; and the latter is introduced digitally by repositioning or reforming the former. The obvious advantages of VSI over conventional shearing interferometry (SI) are its versatility, accuracy, and simplicity. Only one real field is necessary to produce shearogram; there is no need of any real shearing device or even the phase unwrapping computation; and the digital shear can take any possible form according to different purposes. Both the optical experiments and computer simulations with lateral shearing, 180° rotational shearing and double lateral shearing for evaluation of lens aberrations in the general case including spherical aberration, coma, astigmatism, defocus, and tilts based on phase-shifting interferometry are given to verify the effectiveness of this method.     

Direct observation of double diffraction patterns

The double diffraction light intensity of two identical gratings placed parallel in the Fresnel region is sensitive to the lateral displacement ΔX, and is also affected by the air gap R between the gratings. In this note, the dependence of the double diffraction light intensity on ΔX and R for each order beam has been obtained directly by observing the intensity pattern on a screen.     

菲涅耳望远镜合成孔径激光成像雷达实验室验证

对菲涅耳望远镜合成孔径激光成像雷达进行了实验室尺度条件下的原理验证实验。实验中利用不同曲率半径、垂直正交偏振的两个球面波通过二维(2D)扫描方式照明远距离处的目标,接收望远镜接收到的目标回波经过偏振分光镜分成两束作为信号光和本振光进入2×4 90°桥接器,桥接器输出的四路光信号被两个平衡探测器接收,平衡探测器输出电信号经模数转换后经过复数化、两维相位二次项匹配滤波算法处理后可以重构出目标图像。对4.3m处点目标和2D面目标进行了成像实验,取得了具有良好成像分辨率和对比度且带有散斑效应的预期成像结果,证明了该合成成像激光雷达概念的正确性。     

共路外差法分析牛顿望远镜偏振特性

应用共路外差干涉法分析了牛顿望远镜的偏振特性。根据菲涅耳定律求出了入射光s-偏光和p-偏光入射到望远镜各点的反射率公式。给出了共路外差干涉法测量牛顿望远镜偏振特性的实验装置原理图。采用632.8nm的外差光源,分析了牛顿望远镜对s-偏光和p-偏光反射系数、相位差以及对入射光偏振度的影响,根据入射角度的不同绘制了相应的变化曲线。结果表明：镀有铝膜的牛顿望远镜对入射光偏振特性影响较小,s-偏光和p-偏光反射系数相差不到0.01,偏振度变化不超过0.07,适用于激光遥感偏振成像的接收系统。     

衍射相关和Lau干涉

本文用离轴傅里叶变换系统的模型和菲涅耳衍射理论分析了衍射受限下的离焦相关器的系统特性,证明是第一模板和第二模板衍射像之间的强度相关积分。推导了衍射相关器作为测量相位物体的干涉仪的一般理论,这时要求编码板函数能衍射自成像并且相关函数为周期性函数。讨论了平行光栅作为编码板时的干涉仪的横向剪切干涉特性,给出了实验结果。本文还证明了衍射相关器是推广到任何平面物体在任何观察距离上的Lau效应装置。衍射相关干涉仪产生Lau干涉条纹,并且是直接应用Lau效应的干涉仪。 关键词：     

横向剪切可变的双焦距波带板干涉仪

本文提出了一种应用双焦距波带板做为横向剪切元件的干涉仪,适用于检测会聚波面像差。它结构简单,不仅具有Ronchi光栅检测的优点,而且条纹对比度好,可根据需要改变剪切量,不需要更换波带板。这种干涉仪可由在波带板平面内移动波带板,获得待测波面任意方向的剪切干涉图。双焦距波带板可用全息方法很容易制作。文中给出了实验结果。     

激光雷达偏振成像遥感的望远镜系统偏振分析

激光雷达偏振成像主要是利用不同目标散射光偏振度的差异来实现对目标成像的。由于大部分光学系统都存在消偏振效应,因此系统自身的消偏振问题在成像过程中必须考虑。根据近轴理论和消像差的要求设计出卡塞格伦望远镜的几何结构,应用琼斯理论、光波的相干矩阵和菲涅耳反射理论对空气和卡塞格伦望远镜的交界面进行了消偏振分析,并使用Matlab软件仿真了卡塞格伦望远镜在镀金属反射膜前后,反射光偏振度在望远镜径向直径上各点的变化曲线。根据仿真结果可以看出偏振成像系统中使用的卡塞格伦望远镜在镀铝金属反射膜时其消偏振效应在成像过程中可以忽略。     

合成孔径激光成像雷达(Ⅰ):离焦和相位偏置望远镜接收天线

把合成孔径激光成像雷达的目标衍射区分为三个区域,提出采用离焦或者附加空间相位调制板的光学接收望远镜补偿回波像差.当目标处于菲涅耳衍射区时可采用离焦或偏置望远镜补偿回波二次项离焦像差并产生用于孔径合成的二次项相位历程;目标处于夫琅和费衍射区时可以采用离焦或偏置望远镜补偿回波二次项离焦像差但不产生相位历程;目标处于瑞利-索末菲衍射区域时不可能补偿回波高阶像差.     

Nulling Phenomenon of the New Radio Pulsar J0810+37 at a Frequency of 111 MHz

Pulsar J0810+37 with a period of 1.2483 s is detected at a frequency of 111 MHz during the pulsar search at the Big Scanning Antenna (BSA) radio telescope of the Lebedev Physical Institute [1]. In this paper, we present the results of the detailed study of radio emission from J0810+37 which exhibits a rare nulling effect of different durations in a very wide time interval with an average nulling fraction in “switch-on” days 〈NF〉 = 38%; considering “switch-off” days, 〈NF〉 = 74%, since radio emission from this source is interrupted on average by 2–3 days, and then it again revives on average for 1–2 days. The “switch-off” periods reach 7 days.     

A new method to analyse pulsar nulling phenomenon

Pulsar nulling is a phenomenon of sudden cessation of pulse emission for a number of periods.The nulling fraction was often used to characterize the phenomenon.We propose a new method to analyse pulsar nulling phenomenon,by involving two key parameters,the nulling degree,χ,which is defined as the angle in a rectangular coordinates for the numbers of nulling periods and bursting periods,and the nulling scale,N,which is defined as the effective length of the consecutive nulling periods and bursting periods.The nulling degreeχcan be calculated by tanχ=Nnulling/Nburstingand the mean is related to the nulling fraction,while the nulling scale,N,is also a newly defined fundamental parameter which indicates how often the nulling occurs.We determined the distributions ofχand N for 10 pulsars by using the data in literature.We found that the nulling degreeχindicates the relative length of nulling to that of bursting,and the nulling scale N is found to be related to the derivative of rotation frequency and hence the loss rate of rotational energy of pulsars.Their deviations reflect the randomness of the nulling process.     

Fringes demodulation in time-averaged digital shearography using genetic algorithms

This paper presents a phase recovery method, based on genetic algorithms, in time-average shearography. It is proved that a single Bessel fringe pattern obtained under a subtraction operation could be enough to calculate the phase. A merit function is solved iteratively using genetic operator like selection, reproduction and mutation. Experimental results are presented in this paper using a simple shearing system based on a Fresnel biprism.     

Phase-shifting lateral shearing interferometer with two pairs of wedge plates

We present a compact and robust phase-shifting lateral shearing interferometer that produces shearing fringes in orthogonal directions without any mechanical rotation or precise alignment. It consists of two pairs of wedge plates, a beam splitter, and a single CCD camera. Both phase-shifting and tilt for lateral shearing are achieved with two pairs of wedge plates, which can reduce systematic errors caused by external vibration and atmospheric disturbance.     

Studies on aperture synthesis in digital Fresnel holography

Aperture synthesis can improve image resolution in digital holography by increasing the numerical aperture of the system. In this paper, we show that both the lateral resolution and image field of view can be enhanced at the same time using a more general Fresnel holography setup and hologram stitching. The impact of aperture synthesis on the lateral resolution is investigated both theoretically and experimentally. In the experiment, the synthesis is executed by moving the compact digital holographic system in two directions. Nine holograms are recorded and stitched into one hologram. The reconstruction results show that expanding aperture improves the lateral resolution. The lensless Fresnel holography used in this paper is demonstrated to have the ability to provide a larger numerical aperture and can compress the object spectrum in recording process.     

A successive scattering methodology with application to two cylinders in the Fresnel region of each other

We present an efficient approach to compute the second-order scattering of an electromagnetic wave by two discrete scatterers in proximity to each other. Such a two-body system represents the simplest canonical arrangement to address near-field volume scattering phenomena in microwave remote sensing models of vegetation. Using an analytical wave-based approach, a successive scattering methodology is employed to derive the first interaction term in multiple scattering by two arbitrary scatterers in terms of their transition operators. The general formulation is applied to find the second-order bistatic scattering amplitude for a pair of finite length thin cylinders at arbitrary interaction distances using the exact Green's function. To improve computational efficiency, the solution is then specialized to the Fresnel region. These second-order bistatic scattering amplitude results are in agreement with the exact Green's function model when the scatterers are in the Fresnel region of each other. Additionally, it is demonstrated that using the far field approximation in the Fresnel region can yield significant deviations from the exact results. The Fresnel model, unlike the far field approximation, accurately predicts the scattering amplitude peak values and null locations, and is suited to fast solutions in realistic canopy simulations.