频率扫描干涉中基于DFT的相位测量

频率扫描干涉可以实现绝对距离测量,其中相位测量至关重要,其精度直接影响干涉测量的最终精度。从信号频域角度出发,提出了一种利用参考信号和测量信号离散傅里叶变换的相位测量方法。在对参考信号和测量信号采样前,分别对它们进行负向过零检测,当信号负向过零时才开始采样。对信号离散傅里叶变换后进行频谱分析,指出在信号采样前进行负向过零检测,可以消除信号频率值的相对偏差。根据理论分析,在LabVIEW中进行了建模仿真。结果表明:该方法具有精度高和抗噪声能力强等优点,在测量距离为10 m时,相位测量平均值与理论值相差2π,相对不确定度仅为0.105%。     

Strongly driven frequency-sweeping events in plasmas

A generic model of a kinetic plasma formed from a source and sink is presented which without instability would form a strongly unstable state due to a single mode. Instead, the resulting wave-particle resonant interaction maintains the distribution near a marginally stable state through the continual production of fast frequency-sweeping modes that sweep unidirectionally (upward in our case) throughout the energy-inverted region of the distribution function. The energy of these modes can be channeled to the background plasma through wave dissipation and, in our particular example, one quarter of the injected energy is available to be channeled.     

An effective method in calibrating nonlinear errors for phase-shifting interferometry

In phase measurement or digital holography for phase-shifting interferometry, the key role is the variation of reference light wave and recover algorithm based on interferograms and reference phase, so the calculation result is directly affected by phase-shift accuracy. However, because of the errors of nonlinear and other random factors, it is difficult to control the actual phase-shifting amount accurately. In this paper, we aim to propose an efficient method for phase-shifting interferometry which does not require accurate initial estimation of phase-shift amounts, only a few pixels with several randomly shifted interferograms are sufficient for accurate extraction of phase information. This method has reduced the dependence of reference phase, and can obtain phase-shifting amount directly without using complex revised algorithm for correcting phase-shifting nonlinear errors.     

高精度干涉检验移相算法对振动误差的免疫能力

对移相干涉测量中影响移相稳定性,从而产生测量误差的小幅度机械振动进行了研究,建立了振动误差模型,仿真分析了Wyant84b三步算法、Hariharan87五步算法、七步算法和十三步算法4种不同移相算法对振动的免疫能力。模拟结果显示,十三步算法对振动的免疫能力最好,与已发表文献成果吻合,说明本文的模拟具有较高的可信度,在一定程度上能够对影响干涉仪精度的振动误差进行合理预测,并能够起到评价和筛选移相算法的作用。     

Two-dimensional measurement of laser-induced target movement by face-on interferometry

Initial movement of laser irradiated foil targets has been measured by face-on interferometry with two-dimensional resolution. This interferometric method provides direct and precise information about the initial movement of the rear surface of the target compared with conventional diagnostics. Target movement of 20–500 nm from the original position has been observed.     

Phase-shift algorithm for white-light interferometry insensitive to linear errors in phase shift

White-light interference has changes in fringe contrast. When phase-shift techniques are applied to white-light interference, the phase-shift algorithm which can extract the phase accurately under the contrast changes is required. There is often another requirement that the phase shift between frames should not be restricted to π/2. Computer simulations show that the well-known algorithms have non-negligible errors under both requirements. To find an algorithm which will satisfy the requirements, I extract individual terms (I _j ∓ I _k ) in an algorithmic equation by considering symmetry of light intensity against phase, where I _j is light intensity just after the j-th phase shift. Using computer simulations, I search for appropriate coefficients by which the terms are multiplied in the equation, finally finding an algorithm which satisfies both the requirements with the phase shift used.     

Carrier squeezing interferometry: suppressing phase errors from the inaccurate phase shift

The carrier squeezing interferometry algorithm is proposed to retrieve the phase from interferograms with phase shift errors. A linear carrier is introduced in the interferograms, and the image data is rearranged by the squeezing interferometry technology. In the spectrum of the rearranged image, the error lobe and the phase lobe are separated so the error-free phase can be retrieved by filtering. The simulated interferograms with phase shift errors are computed, and the precisions are better than 8.4×10(-4)λ. Its validation is verified by experiments, where a mean precision of 0.0040λ is obtained.     

Observation of nonlinear frequency-sweeping suppression with rf diffusion

Nonlinear frequency sweeping of unstable waves in a laboratory plasma is suppressed upon application of rf fields. Frequency sweeping is driven by a population of energetic electrons trapped in a magnetic dipole field that excite drift-resonant potential fluctuations and create coherent structures in phase space. Self-consistent numerical simulation reproduces the suppression and suggests an explanation due to rf scattering of energetic electrons that destroys the phase-space structures.     

Correction of wave-front retrieval errors caused by the imperfect collimation of reference beam in phase-shifting interferometry

In phase-shifting interferometry (PSI) the standard wave retrieval formulae are usually derived based on the condition of a plane reference beam normal to the recording plane. In practice, however, the reference wave may be a spherical wave of large radius or even have a slight inclination at the same time due to the imperfect collimation and coaxality of the optical system, and this fact will introduce phase distortion for the reconstructed object wave-front. A simple digital processing algorithm without any additional measurements is proposed to determine the unknown parameters of the spherical reference wave and then correct the object wave errors reconstructed with standard wave retrieval formulae. The effectiveness of this method is verified by a series of computer simulations, and its limitation is also discussed.     

相移干涉术及广义相移数字全息干涉术

在基础光学框架内说明了传统的相移干涉术及近年来发展的广义相移数字全息干涉术的基本原理;前者要求特殊值等步长相移,而后者则可以采用任意未知相移值.着重介绍了广义相移干涉术中的物波恢复算法,以及基于衍射场统计特性的未知相移提取算法.从一个侧面沟通了基础光学与近代光学之间的关系.     

Optical coherence pulsed interferometry: shaping probe pulses in time-domain interferometry

The autocorrelation of a modulated coherent light source is used as a probe pulse in a time-domain interferometry scheme. With respect to conventional techniques, higher flexibility in selecting the shape of the probe pulse can be achieved by simply acting on the modulation parameters. The complex amplitude of short pulses propagating through a generic optical device can be directly measured, with no need for fast sampling and time synchronization. The potentialities of the technique are shown by reporting measurements of amplitude distortion, group delay, and frequency chirp of optical pulses transmitted through integrated ring resonators.     

Dephasing in neutron interferometry

Wigner functions are used to describe various dephasing effects in neutron interferometry which are caused by thickness variations and density fluctuations of the phase shifter and by variations of the beam parameters. It will be shown that separated coherent Schrödinger-cat-like states, which exist when large phase shifts are applied, are extremely fragile and sensitive to any kind of imperfections. The related dephasing factor depends quadratically on the spatial separation of the coherent states which permits the definition of an upper limit of feasible coherent packet separation. The results show that dephasing is an unavoidable effect caused by intrinsic fluctuations inherent to any physical system.Dedicated to H. Walther on the occasion of his 60th birthday     

Programmable interferometry

The advent of automatic data-processing for interferometry greatly reduced the complexity of interferometric testing, thereby significantly increasing its usage both within and outside the optics community. A great many of these new applications require special data-processing and output data not normally used for testing lenses. To address these new measurement problems, many data-analysis programs were written, ranging from advanced analysis of wavefronts (i.e., Zernike polynominals, point-spread function, modulation-transfer function) to analysis of mechanical surfaces, such as Winchester-disk read/write heads. Some of these programs were very specific to a particular application, and some tried to be general and as such became cumbersome. Even with much of this software available for sale to the general public, most applications other than simple surface and lens measurements usually cannot be solved directly by using the available programs. In an attempt to solve this problem, we have developed software for interferometry that allows users to easily develop their own measurement routines. The solution was to take a version of the BASIC programming language and add the commands necessary to do interferometry. This software is resident in a processor that can easily be adapted to a large number of interferometry applications. By using this processor and its associated software with an appropriate interferometer, it is possible for the user to tailor the measurement to a particular application. This can be very useful in an optical-production shop, where each different testing application can have its own program. If the program is written properly, the operator will not have to set up any default conditions or format the output; the operation of the program can be reduced to the pushing of a single button, and the output will be formatted properly for that particular test. Examples of this system in actual optical shop-testing situations will be discussed.     

Speckle interferometry

Illumination of a rough surface by a coherent monochromatic wave creates a grainy structure in space termed a speckle pattern. It was considered a special kind of noise and was the bane of holographers. However, its information-carrying property was soon discovered and the phenomenon was used for metrological applications. The realization that a speckle pattern carried information led to a new measurement technique known as speckle interferometry (SI). Although the speckle phenomenon in itself is a consequence of interference among numerous randomly dephased waves, a reference wave is required in SI. Further, it employs an imaging geometry. Initially SI was performed mostly by using silver emulsions as the recording media. The double-exposure specklegram was filtered to extract the desired information. Since SI can be configured so as to be sensitive to the in-plane displacement component, the out-of-plane displacement component or their derivatives, the interferograms corresponding to these were extracted from the specklegram for further analysis. Since the speckle size can be controlled by the F number of the imaging lens, it was soon realized that SI could be performed with electronic detection, thereby increasing its accuracy and speed of measurement. Furthermore, a phase-shifting technique can also be incorporated. This technique came to be known as electronic speckle pattern interferometry (ESPI). It employed the same experimental configurations as SI. ESPI found many industrial applications as it supplements holographic interferometry. We present three examples covering diverse areas. In one application it has been used to measure residual stress in a blank recordable compact disk. In another application, microscopic ESPI has been used to study the influence of relative humidity on paint-coated figurines and also the effect of a conservation agent applied on top of this. The final application is to find the defects in pipes. These diverse applications demonstrate the power of this technique.     

Kaon interferometry

2-kaon and 2-pion correlation functions for an expanding thermalized source are compared. In the Yano-Koonin-Podgoretskii parametrization of the correlation function, the HBT radius parameters are shown to obeyM _∟-scaling in the absence of collective transverse flow. This scaling is broken by transverse flow. An accurate comparison of pion and kaon correlations can thus resolve issue whether the observedM _∟-dependence of the transverse radius parameter is due to transverse collective flow or other transverse gradients. Effects from resonance decays are shortly discussed.     

Spiral interferometry

We present a surprising modification of optical interferometry. A so-called spiral phase element in the beam path of a standard microscope results in an interferogram of phase samples, for which the interference fringes have the shape of spirals instead of closed contour lines as in traditional interferograms. This configuration overrides the basic problem of interferometry, i.e., that elevations and depressions cannot be distinguished. Therefore a complete sample profile can be reconstructed from a single exposure, promising, e.g., high-speed metrology with a single laser pulse. The method is easy to implement, it does not require a spatially separated reference beam, and it is optimally stable against environmental noise.     

Recent developments in laser-diode interferometry

Laser-diode (LD) interferometry based on heterodyne techniques is described. The developments covered include: sinusoidal phase- modulating, distance-measuring feedback and phase-shifting interferometry. The wavelength is controlled by the laser injection current and is continuously or stepwise changed to introduce a time-varying phase difference between the two beams of an interferometer with unbalanced optical path lengths. A feedback interferometer is described with electronics to stabilize the phase shift and to lock the interferometer on a preset phase condition by controlling the injection current of the LD. An automated phase-measuring interferometric system is developed in which the laser current is changed to synchronize intensity data acquisition with the clocks of a charge-coupled device. A typical experimental result shows measurements of the profile of a diamond- turned Al surface.     

Regularization methods in radio interferometry

The problem of image retrieval in radio interferometry is a classical example of an ill-posed problem. In this paper, we consider a new approach to the radio-interferometric imaging based on regularization, which is computationally more flexible. A new type of a weighting function suitable for managing the interferometer antenna pattern is obtained. The regularization procedure was implemented in the VLBI-image processing software developed at the Astrospace Center of the Lebedev Physical Institute (ASC LPI). It is shown that the joint use of the developed regularization methods with traditional deconvolution algorithms such as CLEAN and self-calibration improve the quality of images of cosmic radio-emission sources. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 453–460, June 2007.