Automated phase retrieval of a single-material object using a single out-of-focus image

Phase retrieval is widely used in phase contrast microscopy. Here we present an autofocus algorithm that allows the phase of the exit wave function, from a single-material object, to be reconstructed at medium resolution from a single phase contrast image without any a priori knowledge of the imaging system or object. The algorithm is demonstrated on coherent out-of-focus electron micrographs of 30 nm latex sphere calibration standards, giving <10% RMS error over a large defocus range.     

Phase-resolved optical coherence tomography and optical Doppler tomography for imaging blood flow in human skin with fast scanning speed and high velocity sensitivity

We have developed a novel phase-resolved optical coherence tomography (OCT) and optical Doppler tomography (ODT) system that uses phase information derived from a Hilbert transformation to image blood flow in human skin with fast scanning speed and high velocity sensitivity. Using the phase change between sequential scans to construct flow-velocity imaging, this technique decouples spatial resolution and velocity sensitivity in flow images and increases imaging speed by more than 2 orders of magnitude without compromising spatial resolution or velocity sensitivity. The minimum flow velocity that can be detected with an axial-line scanning speed of 400 Hz and an average phase change over eight sequential scans is as low as 10 microm/s, while a spatial resolution of 10 microm is maintained. Using this technique, we present what are to our knowledge the first phase-resolved OCT/ODT images of blood flow in human skin.     

天文望远镜像差对斑点成像技术的影响

着重研究了采用斑点成像技术处理天文望远镜图像时,光学系统固定像差对图像恢复结果的影响。在详细研究各种恢复天文图像振幅和相位的理论和方法的基础上,建立了一个包括Labeyrie振幅恢复方法和KnoxThompson相位重构方法的恢复扩展目标的斑点成像处理模型,分析了光学系统固定像差对系统传递函数相位分布和目标相位重构的影响：天文目标通过大气成像,固定像差将会被淹没在大气湍流随机起伏中,像差对相位重构没有显著影响。处理图像结果表明,斑点成像技术能同时消除大气湍流和望远镜系统固定像差的影响,得到高分辨力的扩展目标图像。还提出了一种消除光学系统像差的方法。     

多通道超声相控阵驱动控制系统设计

低强度经颅聚焦超声是一种利用脉冲聚焦超声调控脑神经元的治疗技术。为抑制人颅骨的非均质性和个体结构差异影响,须对多阵元相控阵聚焦换能器的各阵元进行参数调控实现经颅精准定位聚焦,各阵元参数调控需通过相位控制和驱动电路系统来实现。该文设计并搭建了一种基于直接数字式频率合成技术的多通道相位、幅值独立可调的相控驱动系统。实测结果表明,可实现正弦波和方波高精度相控输出,输出信号电压峰峰值在0~37.5 V可调,相位分辨率为0.1°,延时误差小于1 ns,可满足多阵元相控阵聚焦换能器驱动及其所需相位分辨率的需要。     

Guided wave arrays for high resolution inspection

The paper describes a general approach for processing data from a guided wave transducer array on a plate-like structure. The raw data set from such an array contains time-domain signals from each transmitter-receiver combination. The technique is based on linear superposition of signals in the frequency domain with some amplitude and phase factors and can be applied to any array geometry and any types of array elements. The problem of finding optimal coefficients, which allow the best resolution to be achieved with the minimum number of array elements, is investigated. It is shown that improvements in resolution are obtained at the expense of sensitivity to noise. A method of quantifying this sensitivity is presented. Results are shown that illustrate the application of the technique to a linear array and an array of circular geometry (containing a single ring of elements). Experimental data obtained from a guided wave array containing electromagnetic acoustic transducer elements for exciting and detecting the S0 Lamb wave mode in a 5-mm-thick aluminum plate are processed with different algorithms and the results are discussed. Generalization of the technique for the case of multimode media is suggested.     

Coherent diffractive imaging using phase front modifications

We introduce a coherent diffractive imaging technique that utilizes multiple exposures with modifications to the phase profile of the transmitted wave front to compensate for the missing phase information. This is a single spot technique sensitive to both the transmission and phase shift through the sample. Along with the details of the method, we present results from the first proof of principle experiment. The experiment was performed with 6.0 keV x rays, in which an estimated spatial resolution of 200 nm was achieved.     

Analysis of coherent gradient sensing (CGS) by fourier optics

A coherent laser beam reflected or transmitted from a deforming plate specimen acquires an optical path difference (phase change) which is related to the deformation and stress field. The optical method of Coherent Gradient Sensing (CGS) uses two parallel grating plates to displace (shear) and recombine the distorted light beam emerging from the specimen. Fringes are produced on the image plane by the interference of the shifted and unshifted beams. The fringe pattern is related to the spatial difference of the phase in the shearing direction. If the shearing distance is small enough, the spatial difference of the phase approximates to its gradient. Each fringe in the image represents a locus of equal gradient component of the phase in the shearing direction. The technique is interpreted by means of wave optics, using the Fraunhofer approximation and the paraxial theory of lenses. The assumptions made in earlier analyses have been removed here. A more precise analysis based on Fourier optics is presented. The simplicity of the optical setup and variable resolution of the technique have led to its frequent use in the area of solid mechanics, including fracture mechanics. Some examples are discussed in the second half of this paper.     

A general phase retrieval algorithm based on a ptychographical iterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

A general phase retrieval algorithm based on ptychographicaliterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

Irregular sampling to reduce the limitations induced by the nonlinearities in wavelength scanning

A new numerical method for reducing the effects of the nonlinearity signal in the distance measurements by the frequency-modulated continuous wave technique is presented. It is shown that the phase shift of the interferogram produced by a light-scanning signal could be efficiently considered as polynomial. In the distance measurement method considered, the information is carried by the first-order polynomial phase signal. Our method eliminates the high orders of the polynomial pulse, which allows an improvement of the resolution. First results are reported in order to illustrate the new technique.     

Halo Reduction Technique in Phase Contrast Microscopy

A new phase contrast microscopy technique has been developed which achieves halo reduction by applying an apodization to the Zernike phase contrast method. In this technique, a new apodized phase plate is placed at the back focal plane of the objective. This plate consists of a quarter wave phase shift ring conjugated of the illuminating aperture, which has a 25#x0025; transmittance, and a pair of adjacent rings having 50#x0025; transmittance. The developed technique yields the highest resolution images without a bluish hue effect ever achieved in phase contrast microscopy.This paper was originally presented at the 2nd International Conference on Optical Design and Fabrication, ODF2000 which was held on November 15-17, 2000 at the International Conference Center, Tokyo, Waseda University, Japan.     

A method for the evaluation of the grating envelope in volume holographic media

A novel method for measuring the three-dimensional spatial distribution of gratings in a volume holographic medium is described. In this method, the diffraction of a uniform probe light wave is measured as a function of the deviation (either in angle or wavelength) from the Bragg condition. The envelope function of the grating is obtained by solving the coupled wave equations inversely. Not only the amplitude but also the phase of the envelope function is obtained. This method allows for the depletion of the probe light wave in the hologram and is applicable to both dielectric (refractive index) and absorption gratings. The resolution and the maximum measurable depth in this method are discussed.     

Shearing interferometer for quantifying the coherence of hard x-ray beams

We report a quantitative measurement of the full transverse coherence function of the 14.4 keV x-ray radiation produced by an undulator at the Swiss Light Source. An x-ray grating interferometer consisting of a beam splitter phase grating and an analyzer amplitude grating has been used to measure the degree of coherence as a function of the beam separation out to 30 microm. Importantly, the technique provides a model-free and spatially resolved measurement of the complex coherence function and is not restricted to high resolution detectors and small fields of view. The spatial characterization of the wave front has important applications in discovering localized defects in beam line optics.     

光束漂移的莫尔条纹检测原理

针对大气湍流光束漂移，设计了一套使用莫尔条纹进行检测的系统，该系统具有检测微小长度和角度的特点，它是将光束漂移的微小角度借助条纹的方式进行放大。分析大气湍流条件下可能获得的最大条纹宽度，然后将条纹宽度和现有较好的CCD分辨率作比较。在CCD分辨率不足的情况下，采用莫尔条纹细分技术，通过适当的电路，将条纹的光信号转换成电信号，使相位变化转换为脉宽的变化，并在形成的方波中内插高频脉冲，通过测量脉冲数目可进一步实现高倍率细分，从而达到较高的分辨率，理论上系统的检测精度可达到0.025μrad。     

Design and Evaluation of Computer Generated Hologram with Binary Subwavelength Structure Designed by Deterministic Process

A binary subwavelength structure for multilevel phase modulation can be designed by our previously proposed deterministic design method without iterative optimization method. To use our design technique in various applications of a computer generated hologram (CGH) like an array illuminator, beam-shaping, signal processing, and so on, an image quality of a reconstructed image from a CGH has become much more important. In this paper, we verify the image quality of a reconstructed image from a CGH designed by our method in terms of the modulation transfer function (MTF) and the spatial resolution. Simulation results show that our technique can theoretically achieve a MTF of more than 99% over a wide range and a spatial resolution of less than 9.66μm.     

Measurement for Scattering Media by Synthesis of Optical Coherence Function with Super-Structure Grating Distributed Bragg Reflector Laser Diode

The application of the technique of synthesis of optical coherence function for detection in scattering media is investigated. By modulating the optical frequency, the technique synthesizes the coherence function into a delta-function-like peak at an arbitrary location, and thus can select interferometrically the information at that location. The location is adjustable by the modulation parameter or additional phase modulation. A multi-section super-structure grating distributed Bragg reflector laser diode (SSG-DBR-LD) of THz-order tunable range is employed to enhance the spatial resolution for suppressing the multiple scattering from locations other than that detected. In a preliminary experimental demonstration, a reflectometry of 550 μm spatial resolution was achieved and was used to detect scattering media.     

Enhanced wavefront reconstruction by random phase modulation with a phase diffuser

A phase retrieval technique for enhanced wavefront reconstruction using random phase modulation and a phase diffuser is proposed. The speckle field generated is sampled at multiple axially displaced planes and the speckle patterns are used in an iterative algorithm based on the optical wave propagation in free space. The presentation of this technique is carried out using two setups. In the first setup, a diffuser plate is placed at the image plane of a metallic test object. The benefit of randomizing the phase of the object wave is the enhanced intensity recording due to high dynamic range of the diffusely scattered beam. The use of demagnification optics will also allow the investigations of relatively large objects. In the second setup, a transparent object is illuminated using a wavefront with random phase and constant amplitude by positioning the phase diffuser close to the object. The benefit of phase-only modulation is the increased resolution of the phase structures of the transparent test objects.     

Particle detection and non-detection in a quantum time of arrival measurement

The standard time-of-arrival distribution cannot reproduce both the temporal and the spatial profile of the modulus squared of the time-evolved wave function for an arbitrary initial state. In particular, the time-of-arrival distribution gives a non-vanishing probability even if the wave function is zero at a given point for all values of time. This poses a problem in the standard formulation of quantum mechanics where one quantizes a classical observable and uses its spectral resolution to calculate the corresponding distribution. In this work, we show that the modulus squared of the time-evolved wave function is in fact contained in one of the degenerate eigenfunctions of the quantized time-of-arrival operator. This generalizes our understanding of quantum arrival phenomenon where particle detection is not a necessary requirement, thereby providing a direct link between time-of-arrival quantization and the outcomes of the two-slit experiment.     

Extended resolution wide-field optical imaging: objective-launched standing-wave total internal reflection fluorescence microscopy

Standing-wave total-internal-reflection fluorescence (SW-TIRF) microscopy uses a super-diffraction-limited standing evanescent wave to extract the high-spatial-frequency content of an object through a diffraction-limited optical imaging system. The effective point-spread function is better than a quarter of the emission wavelength. With a 1.45 numerical aperture objective and 532 nm excitation wavelength, a Rayleigh resolution of approximately 100 nm can be achieved, which is better than twice the resolution of conventional TIRF microscopy. This first experimental realization of SW-TIRF in an objective-launched geometry demonstrates the potential for extended resolution imaging at high speed by using wide-field microscopy.     

Multiresolution Approach to Image Reconstruction with Phase-Diversity Technique

A multiresolution analysis is adopted to improve an image reconstruction with a phase-diversity technique. The phase distribution at low resolution is first estimated and then is used as an initial estimate for phase reconstruction at a higher resolution level. Computer simulations are conducted to confirm the proposed method. The method is applied for image reconstruction of solar granulation.