An efficient and robust reconstruction method for optical tomography with the time-domain radiative transfer equation

An efficient and robust method based on the complex-variable-differentiation method (CVDM) is proposed to reconstruct the distribution of optical parameters in two-dimensional participating media. An upwind-difference discrete-ordinate formulation of the time-domain radiative transfer equation is well established and used as forward model. The regularization term using generalized Gaussian Markov random field model is added in the objective function to overcome the ill-posed nature of the radiative inverse problem. The multi-start conjugate gradient method was utilized to accelerate the convergence speed of the inverse procedure. To obtain an accurate result and avoid the cumbersome formula of adjoint differentiation model, the CVDM was employed to calculate the gradient of objective function with respect to the optical parameters. All the simulation results show that the CVDM is efficient and robust for the reconstruction of optical parameters.     

Detection and characterization of breast tumours by time-domain scanning optical mammography

The paper gives a short overview of various methods of optical mammography, emphasizing scanning time-domain mammography. The results of a clinical study on time-domain optical mammography are reviewed, comprising 154 patients carrying a total of 102 carcinomas validated by histology. A visibility score attributed to each carcinoma as qualitative measure of tumour detectability indicates acceptable sensitivity but poor specificity for discrimination between malignant and benign lesions. Likewise, a multi-variate statistical analysis yields sensitivity and specificity between 80% and 85% for tumour detection and discrimination with respect to normal (healthy) breast tissue, but values less than 70% for discrimination between malignant and benign breast lesions, being too low to be of clinical relevance. For 87 of the 88 tumours detected retrospectively in both projection optical mammograms, optical properties and tissue parameters were derived based on the diffraction of photon density waves by a spherical inhomogeneity as forward model. Following injection of a bolus of indocyanine green as non-targeted absorbing contrast agent, dynamic contrast-enhanced time-domain optical mammography was carried out on a small number of patients, but no differences in wash-out kinetics of indocyanine green between tumours and healthy breast tissue were observed.     

An efficient method for model refinement in diffuse optical tomography

Diffuse optical tomography (DOT) is a non-linear, ill-posed, boundary value and optimization problem which necessitates regularization. Also, Bayesian methods are suitable owing to measurements data are sparse and correlated. In such problems which are solved with iterative methods, for stabilization and better convergence, the solution space must be small. These constraints subject to extensive and overdetermined system of equations which model retrieving criteria specially total least squares (TLS) must to refine model error. Using TLS is limited to linear systems which is not achievable when applying traditional Bayesian methods. This paper presents an efficient method for model refinement using regularized total least squares (RTLS) for treating on linearized DOT problem, having maximum a posteriori (MAP) estimator and Tikhonov regulator. This is done with combination Bayesian and regularization tools as preconditioner matrices, applying them to equations and then using RTLS to the resulting linear equations. The preconditioning matrixes are guided by patient specific information as well as a priori knowledge gained from the training set. Simulation results illustrate that proposed method improves the image reconstruction performance and localize the abnormally well.     

An efficient locally one-dimensional finite-difference time-domain method based on the conformal scheme

An efficient conformal locally one-dimensional finite-difference time-domain(LOD-CFDTD) method is presented for solving two-dimensional(2D) electromagnetic(EM) scattering problems. The formulation for the 2D transverse-electric(TE) case is presented and its stability property and numerical dispersion relationship are theoretically investigated. It is shown that the introduction of irregular grids will not damage the numerical stability. Instead of the staircasing approximation, the conformal scheme is only employed to model the curve boundaries, whereas the standard Yee grids are used for the remaining regions. As the irregular grids account for a very small percentage of the total space grids, the conformal scheme has little effect on the numerical dispersion. Moreover, the proposed method, which requires fewer arithmetic operations than the alternating-direction-implicit(ADI) CFDTD method, leads to a further reduction of the CPU time. With the total-field/scattered-field(TF/SF) boundary and the perfectly matched layer(PML), the radar cross section(RCS) of two2 D structures is calculated. The numerical examples verify the accuracy and efficiency of the proposed method.     

An efficient lattice Boltzmann method for fluorescent diffuse optical tomography on GPUs

Optical Review - Fluorescent diffuse optical tomography (FDOT) is an emerging imaging modality, with great prospects in areas such as biology and medicine. However, current FDOT encounters...     

声学中用于时域有限差分法的一种高效吸收边界

本文提出一种用于声场时域有限差分法(FDTD)的高效吸收边界条件。通过伪逆矩阵对截断边界附近网格线之间的转移矩阵作最小二乘估计,所得到的吸收边界条件性能明显优于多种常用方法。新方法的数值计算效率高,算法稳定。对点源辐射,尖劈衍射和圆柱体散射问题的FDTD数值计算结果显示了这种方法的优异性能。     

用推广的Jacobian椭圆函数方法解离散的mKdV格子

本文用推广的Jacobian椭圆函数方法求解了离散的mKdV格子，得到了Jacobian椭圆函数双周期解，当模取1时，可得到钟型孤波解和冲击型孤波解。     

Auxiliary differential equation: efficient implementation in the finite-difference time-domain method

An efficient algorithm for modeling dispersive media in finite-difference time-domain methods is presented. It is based on the auxiliary differential equation method for treatment of Lorentz media with an arbitrary number of relaxations. The algorithm shows excellent accuracy of second order in time and space and is efficient in both memory requirements and computational effort.     

An efficient algorithm used for full-field optical coherence tomography

An efficient algorithm for computing optical coherence tomography is presented, which is based on the discrete differences of a time/depth sequence of interferograms. The existing multiple-step phase-shift algorithms, including those solved from Carré and Hariharan equations as well as circular-step algorithm proposed by Dubois et al., are analyzed and compared with the proposed algorithm. The analytical and experimental results show the computational efficiency of the new algorithm outperforms others. The simulations also demonstrate that the algorithm based on derivatives of four phase-shifted images is less sensitive to the phase-shift noise comparing to traditional 4-step phase-shift-based algorithm.     

Effective wavelength scaling for optical antennas

In antenna theory, antenna parameters are directly related to the wavelength lambda of incident radiation, but this scaling fails at optical frequencies where metals behave as strongly coupled plasmas. In this Letter we show that antenna designs can be transferred to the optical frequency regime by replacing lambda by a linearly scaled effective wavelength lambda(eff)=n(1)+n(2)lambda/lambda(p), with lambda(p) being the plasma wavelength and n(1), n(2) being coefficients that depend on geometry and material properties. It is assumed that the antenna is made of linear segments with radii R < lambda. Optical antennas hold great promise for increasing the efficiency of photovoltaics, light-emitting devices, and optical sensors.     

Effective optical response of silicon to sunlight in the finite-difference time-domain method

The frequency dependent dielectric permittivity of dispersive materials is commonly modeled as a rational polynomial based on multiple Debye, Drude, or Lorentz terms in the finite-difference time-domain (FDTD) method. We identify a simple effective model in which dielectric polarization depends both on the electric field and its first time derivative. This enables nearly exact FDTD simulation of light propagation and absorption in silicon in the spectral range of 300-1000 nm. Numerical precision of our model is demonstrated for Mie scattering from a silicon sphere and solar absorption in a silicon nanowire photonic crystal.     

An optical method for wavelength fine-selection in optical spectrum analysers

This letter describes a novel optical method for wavelength fine-selection in the optical spectrum analysers(OSAs) for dense wavelength division multiplexing (DWDM) applications. The proposed new method employs a 'refractive optical lever'system consisting of a rotating optical wedge prism.A new OSA system based on Littman-type monochromator is proposed and the wavelength selection accuracy and resolution of OSA that has included such an optical lever system have been improved by a factor of 20 to100 depending on the wedge angle and offset orientation angle of the optical wedge prism. This proposed 'refractive optical lever' may also simplify the rotation mechanism of the mirror in the commercially available OSAs.     

An optical method for simulating nonuniform systems

Model experiments of a new type in the physics of nonuniform systems are proposed. The method is based on the production of a randomly nonuniform distribution of charge carriers in a uniform semiconductor by means of photoexcitation with a nonuniform radiation flux. The method makes it possible to vary easily the character of the nonuniformities over wide limits. It has been used to investigate the effective transverse conductivity of nonuniform p-Si plates in a magnetic field (H). An anomalous transverse conductivity, previously predicted in a number of theoretical works, has been observed. As the electric field (E) increases, the anomalous conductivity decreases as a result of smoothing of the nonuniformities. The nonuniformities have virtually no effect on the conductivity in an open Hall circuit regime. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 207–211 (25 January 1997)     

An optical method for measuring nanoparticle size

We describe a method for determining the size of nanoparticles and their optical constants that is based on analysis of the spectrum of extinction of light by these particles. We show that the results of our mathematical modeling agree well with the experimental data.     

Grating-assisted polarization optical time-domain reflectometry for distributed fiber-optic sensing

We report a novel type of polarization optical time-domain reflectometry (POTDR) for fully distributed fiber-optic sensing, in which the reflected optical signal is from a series of fiber Bragg gratings that are uniformly distributed along the fiber. Compared with a conventional POTDR that uses the Rayleigh backscattering, this grating-assisted POTDR can have a much better signal-to-noise ratio and consequently a better measurement resolution and a larger measurement range of the fiber birefringence. Experimental results have shown that the measurement resolution of the grating-assisted POTDR is almost an order of magnitude better than that of a conventional POTDR.     

Overview on finite-element time-domain approaches for optical propagation analysis

Several finite-element based time-domain approaches for the analysis of photonic devices are discussed in details. Narrowband and different kinds of wideband formulations are derived highlighting strengths and weaknesses related to the initial analytical equation and to the numerical implementation through the finite element method. A formulation which directly implements the unknown field without separating the envelope and the fast varying carrier is also presented. Performances and limits of the various algorithms are discussed by analyzing the simple and clear case of time and space evolution of gaussian beams propagating in a homogeneous medium. Comparison of the modules and arguments of the spectral response is also provided.     

Vector Brillouin optical time-domain analyzer for high-order acoustic modes

Thanks to a double-frequency phase modulation scheme, we report a vector Brillouin optical time-domain analyzer (BOTDA). This BOTDA has a high immunity level to noise, and it features a phase spectrogram capability. It is well suited for complex situations involving several acoustic resonances, such as high-order longitudinal modes. It has notably been used to characterize a dispersion-shifted fiber, allowing us to report spectrograms with multiple acoustic resonances. A very high 57 dB dynamic range is also reported for 100-ns-long pulses simultaneously with a 16 cm numerical resolution.     

An analytical derivation of the optimum source patterns for the pseudospectral time-domain method

In the computational electromagnetics and acoustics, spatially smoothed sources are often utilized to alleviate the aliasing errors in the pseudospectral time-domain (PSTD) algorithms. In our work, an analytical derivation of the optimum source patterns is presented according to the accurately derived expressions of the dominant source-introduced aliasing errors according to the circular discrete convolution and Tailor series expansion method. We quantitatively demonstrate, for the first time in literature, that the aliasing errors can be optimally suppressed and rapidly reduced to the negligible levels by these optimum patterns and with the increment of source cells. We also provide the different implementation schemes of the optimal patterns both for the soft and hard source cases. The numerical calculation and 1D PSTD transient simulations are conducted to verify the excellent performance of these optimum sources.