Flame four-dimensional deflection tomography with compressed-sensing-revision reconstruction

Deflection tomography with limited angle projections was investigated to visualize a premixed flame. A projection sampling system for deflection tomography was used to obtain chronological deflectogram arrays at six view angles with only a pair of gratings. A new iterative reconstruction algorithm with deflection angle compressed-sensing revision was developed to improve reconstruction-distribution quality from incomplete projection data. Numerical simulation and error analysis provided a good indication of algorithm precision and convergence. In the experiment, 150 fringes were processed, and temperature distributions in 20 cross-sections were reconstructed from projection data in four instants. Four-dimensional flame structures and temperature distributions in the flame interior were visualized using the visualization toolkit. The experimental reconstruction was then compared with the result obtained from computational fluid dynamic analysis.     

两步广义相移干涉术的光学实验验证

提出了一种基于两步广义相移干涉术的波前再现技术以及基于衍射场统计特性的未知相移值提取算法,并且进行了光学实验验证.该方法中可仅利用两幅干涉图及一次不需精确预设的未知相移,参照物光光强和参考光光强,即可通过计算提取真实的相移量,进而恢复出物波波前复振幅,而无需借助傅里叶谱分析或数字滤波等操作.在光学实验中,对借助于空间光调制器（SLM）的间接输入和三维漫反射物体直接输入两种情况进行了详细地研究,得到的光学实验结果验证了所提方案的有效性和可行性. 关键词： 信息光学 相移干涉术 波前再现 数字图像处理     

Optical coherence tomography

Optical Coherence Tomography (OCT) is a new technology for performing high-resolution cross sectional imaging. OCT is analogous to ultrasound imaging, except that it uses light instead of sound. OCT can provide cross sectional images of tissue structure on the micron scale in situ and in real time. OCT functions as a type of optical biopsy and is a powerful imaging technology for medical diagnostics because unlike conventional histopathology which requires removal of a tissue specimen and processing for microscopic examination, OCT can provide images of tissue in situ and in real time. OCT can be used where standard excisional biopsy is hazardous or impossible, to reduce sampling errors associated with excisional biopsy, and to guide interventional procedures.     

Optical beam deflection using dynamic volume reflection gratings

We propose a scheme for optical beam deflection using dynamic volume reflection gratings. Since the direction change of the diffracted beam with the recording wavelength is much larger for a reflection grating a passive beam-tilting element may not be required.     

Optical encryption using phase-shifting interferometry in a joint transform correlator

We propose an optical encryption technique where the encrypted field and the decrypting key are obtained by three-step phase-shifting interferometry and registered as digital Fresnel holograms in a joint transform correlator architecture. Decryption can be achieved by digital or optical means. The technique allows the complete process to be achieved at high speed and data to be transfered via digital communication channels. Experimental implementation is performed in a system based on a programmable liquid-crystal TV display working in pure phase mode to represent the input data and to introduce the required phase shifts. A CCD is used to register the output data.     

Optical scanner - light deflection using computer-generated diffractive elements

Some ideas are presented about optical deflectors. Using computers to form generalized diffractive elements can be used to advantage. Three unique features can be incorporated in the elements. These are: 1) convenient selection of their geometrical shape, 2) changes in speed and direction of the deflected light beam, and 3) variations in focal power. Some practical realizations are discussed.     

Optical encryption of gray-level image using on-axis and 2-f digital holography with two-step phase-shifting method

In this paper we propose an encryption/decryption technique of gray-level image information using an on-axis 2-f digital holographic optical encrypting system with two-step phase-shifting method. This technique reduces the number of holograms in phase-shifting digital holography and minimizes the setup of the encryption system more than multistep phase-shifting technique. We are able to get the complete decrypted image by controlling the K-ratio which is defined as the reference beam intensity versus the object beam intensity. We remove the DC-term of the phase-shifting digital hologram to reconstruct and decrypt the original image information. Simulation results show that the proposed method can be used for encryption and decryption of a 256 gray-level image. Also, the result shows some errors of the decrypted image according to K-ratio.     

Optical coherence tomography with plasmon resonant nanorods of gold

We explored plasmon resonant nanorods of gold as a contrast agent for optical coherence tomography (OCT). Nanorod suspensions were generated through wet chemical synthesis and characterized with spectrophotometry, transmission electron microscopy, and OCT. Polyacrylamide-based phantoms were generated with appropriate scattering and anisotropy coefficients (30 cm(-1) and 0.89, respectively) to image distribution of the contrast agent in an environment similar to that of tissue. The observed signal was dependent on whether the plasmon resonance peak overlapped the source bandwidth of the OCT, confirming the resonant character of enhancement. Gold nanorods with plasmon resonance wavelengths overlapping the OCT source yielded a signal-to-background ratio of 4.5 dB, relative to the tissue phantom. Strategies for OCT imaging with nanorods are discussed.     

Optical tomography of the laser's Gaussian electric field

A method for the measurement of the spatial distribution of birefringence, caused by intense femtosecond laser pulses in condensed plasma, is presented for the case of a Gaussian laser beam. For that only two light intensities are to be measured on the probe beam in a polariscope.     

Optical deflection and temporal characterization of an ultrafast laser-produced electron beam

The interaction of a laser-produced electron beam with an ultraintense laser pulse in free space is studied. We show that the optical pulse with a(0)=0.5 imparts momentum to the electron beam, causing it to deflect along the laser propagation direction. The observed 3-degree angular deflection is found to be independent of polarization and in good agreement with a theoretical model for the interaction of free electrons with a tightly focused Gaussian pulse, but only when longitudinal fields are taken into account. This technique is used to temporally characterize a subpicosecond laser-wakefield-driven electron bunch. Applications to electron-beam conditioning are also discussed.     

Three-dimensional microscopy with phase-shifting digital holography

We applied phase-shifting digital holography to microscopy by deriving the complex amplitude of light scattered from microscopic three-dimensional objects through a microscope objective by video camera recording, phase-shifting analysis, and computer reconstruction. This method requires no mechanical movement and provides a flexible display and quantitative evaluation of the reconstructed images. A theory of image formation and experimental verification with specimens are described.     

Common-path phase-shifting interferometer with binary grating

An experimental setup for phase extraction of 2D phase distributions is presented. The system uses a common-path interferometer consisting of two windows in the input plane and a translating grating as spatial filter. In the output, interference of the fields associated with replicated images of the input windows is achieved by a proper choice of the windows spacing with respect to the grating period, the focal length of the transforming lens and the wavelength of the coherent illumination employed. Because in this type of grating interferometer a grating is placed as a spatial filter, the phase changes which are needed for phase-shifting interferometry can be easily performed with translations of the grating driven by a linear actuator. Some experimental results are shown.     

Optical Fresnel transformation and quantum tomography

Corresponding to optical Fresnel transformation characteristic of a ray transfer matrix (A,B,C,D),AD-BC=1, there exists Fresnel operator F(A,B,C,D) in quantum optics, we show that under the Fresnel transformation the pure-state position density ∣x〉〈x∣ becomes density operator ∣x〉_s,rs,r〈x∣, which is just the Radon transform of the Wigner operator, i.e.,

     

Optical coherence tomography (Review)

The basic principles and possibilities of optical coherence tomography (OCT) as a method for the investigation of pathologies in medical diagnostics are outlined. We discuss the OCT method limitations and issues related to the need for improving the resolution of optical tomographs. The prospects are considered for developing the OCT methods based on probing with diffraction-free light beams.     

Optical image encryption using chaos-based compressed sensing and phase-shifting interference in fractional wavelet domain

In this paper, a novel optical image encryption system combining compressed sensing with phase-shifting interference in fractional wavelet domain is proposed. To improve the encryption efficiency, the volume data of original image are decreased by compressed sensing. Then the compacted image is encoded through double random phase encoding in asymmetric fractional wavelet domain. In the encryption system, three pseudo-random sequences, generated by three-dimensional chaos map, are used as the measurement matrix of compressed sensing and two random-phase masks in the asymmetric fractional wavelet transform. It not only simplifies the keys to storage and transmission, but also enhances our cryptosystem nonlinearity to resist some common attacks. Further, holograms make our cryptosystem be immune to noises and occlusion attacks, which are obtained by two-step-only quadrature phase-shifting interference. And the compression and encryption can be achieved in the final result simultaneously. Numerical experiments have verified the security and validity of the proposed algorithm.     

Closed-loop phase-shifting interferometry with a laser diode

A closed-loop phase-shifting Fizeau-type interferometer at lambda=633 nm was constructed with the direct frequency modulation of laser diodes. The interferometer is servo controlled fully in the phase domain where optical phases are detected by a two-frequency optical heterodyne method. Stepwise phase shifting was accomplished with good stabilization against external disturbances (vibration, air flow, etc.) and laser frequency fluctuations.     

Optical tomography reconstruction based on gradient tree

The adjoint differentiation scheme was first proposed by Alexander D. Klose for the reconstruction of optical tomography [J Q S R T 2002;72:715-32]. However, the scheme proposed there can only be employed for the computation of derivatives with respect to the optical parameters on the boundary, i.e. those derivatives with respect to the internal parameters cannot be obtained there, and as a result, the correct reconstructed result cannot be obtained with the gradient based optimization algorithms. In our paper, based on the adjoint differentiation scheme, a novel algorithm based on gradient tree is proposed for the computation of the derivatives with respect to both boundary and internal optical parameters. A scheme for approximate gradient calculation is also proposed for speeding up algorithm implementation. Experimental results show that the algorithm proposed performs well and good reconstruction results can be achieved.     

Optical tomography with the discontinuous Galerkin formulation of the radiative transfer equation in frequency domain

Optical tomography is an inverse method of probing semi-transparent media with the help of light sources. The reconstruction of the optical properties usually employs finite volumes or continuous finite elements formulations of light transport as a forward model for the predictions. In a previous study, we have introduced a generalization of the inversion approach with finite elements formulations by using an integral form of the objective function. The novelty is that the surfaces of the detectors are taken into account in the reconstruction and compatibility is obtained for all finite element formulations. This present paper illustrates this new approach by developing a Discontinuous Galerkin formulation as a forward model for an optical tomography application in the frequency domain framework. Numerical tests are performed to gauge the accuracy of the method in recovering optical properties distribution with a gradient-based algorithm where the adjoint method is used to fastly compute the objective function gradient. It is seen that the reconstruction is accurate and can be affected by noise on the measurements as expected. Filtering of the gradient at each iteration of the reconstruction is used to cope with the ill-posed nature of the inverse problem and to improves the quality and accuracy of the reconstruction.     

Continuous wave terahertz phase imaging with three-step phase-shifting

We demonstrated a continuous wave terahertz (THz) phase imaging system which can present the depth information of an object's surface interior accurately and non-invasively. With the three-step phase-shifting method, the phase images of samples which can provide the relative optical depth profile of the sample's surface and interior have been achieved. Two samples with different thickness and materials were measured to demonstrate the phase imaging ability of the system. The results show that this method is effective and the measured values agree well with the actual ones. The longitudinal resolution of the system was also measured. To exhibit the advantages of the three-step phase shifting method, the multi-wavelength imaging method was also employed and the results are compared.