Light Propagation In Random Media Research Articles

Single-ended characterization of the coherent transfer matrix of coupled multimode transmission channels

Light propagation in random media is a subject of interest to the optics community at large, with applications ranging from imaging to communication and sensing. However, real-time characterization of wavefront distortion in random media remains a major challenge. Compounding the difficulties, for many applications such as imaging (e.g., endoscopy) and focusing through random media, we only have single-ended access. In this work, we propose to represent wavefronts as superpositions of spatial modes. Within this framework, random media can be represented as a coupled multimode transmission channel. Once the distributed coherent transfer matrix of the channel is characterized, wavefront distortions along the path can be obtained. Fortunately, backreflections almost always accompany mode coupling and wavefront distortions. Therefore, we further propose to utilize backreflections to perform single-ended characterization of the coherent transfer matrix. We first develop the general framework for single-ended characterization of the coherent transfer matrix of coupled multimode transmission channels. Then, we apply this framework to the case of a two-mode channel, a single-mode fiber, which supports two randomly coupled polarization modes, to provide a proof-of-concept demonstration. Furthermore, as one of the main applications of coherent channel estimation, a polarization imaging system through single-mode fibers is implemented. We envision that the proposed method can be applied to both guided and free-space channels with a multitude of applications.

Time‐Resolved Nonlinear Diffuse Femtosecond‐Pulse Reflectometry Using Lithium Niobate Nanoparticles with Two Pulses of Different Colors

Time‐resolved nonlinear optical sum‐frequency generation (SFG) of two differently colored, infrared (sub‐)picosecond laser pulses is studied by means of nonlinear diffuse femtosecond‐pulse reflectometry using lithium niobate nanoparticle pellets. The visible SFG emission exhibits an asymmetric pulse shape in the time domain which is explained within the framework of light propagation in random media. The analysis of the spectro‐temporal data set indicates that nanoparticle pellets can be used for an alternative type of an optical correlator, e.g., for the determination of a pulse's chirp parameter. This finding is generalized by means of numerical simulations. As a result, ultrashort pulse shapes can now be predicted inside nanoscaled, densely packed media with a nonlinear optical response on a comprehensive basis, giving rise to a number of prospective fields of applications.

A Hybrid Inversion Scheme Combining Markov Chain Monte Carlo and Iterative Methods for Determining Optical Properties of Random Media

Near-infrared spectroscopy (NIRS) including diffuse optical tomography is an imaging modality which makes use of diffuse light propagation in random media. When optical properties of a random medium are investigated from boundary measurements of reflected or transmitted light, iterative inversion schemes such as the Levenberg–Marquardt algorithm are known to fail when initial guesses are not close enough to the true value of the coefficient to be reconstructed. In this paper, we investigate how this weakness of iterative schemes is overcome using Markov chain Monte Carlo. Using time-resolved measurements performed against a polyurethane-based phantom, we present a case that the Levenberg–Marquardt algorithm fails to work but the proposed hybrid method works well. Then, with a toy model of diffuse optical tomography we illustrate that the Levenberg–Marquardt method fails when it is trapped by a local minimum but the hybrid method can escape from local minima by using the Metropolis–Hastings Markov chain Monte Carlo algorithm until it reaches the valley of the global minimum. The proposed hybrid scheme can be applied to different inverse problems in NIRS which are solved iteratively. We find that for both numerical and phantom experiments, optical properties such as the absorption and reduced scattering coefficients can be retrieved without being trapped by a local minimum when Monte Carlo simulation is run only about 100 steps before switching to an iterative method. The hybrid method is compared with simulated annealing. Although the Metropolis–Hastings MCMC arrives at the steady state at about 10,000 Monte Carlo steps, in the hybrid method the Monte Carlo simulation can be stopped way before the burn-in time.

Spin Hall Effect of Light in a Random Medium.

We show that optical beams propagating in transversally disordered materials exhibit a spin Hall effect and a spin-to-orbital conversion of angular momentum as they deviate from paraxiality. We theoretically describe these phenomena on the basis of the microscopic statistical approach to light propagation in random media, and show that they can be detected via polarimetric measurements under realistic experimental conditions.

Analysis of Light Propagation in Random Media Based on The Dependent Scattering Theory

Analysis of Light Propagation in Random Media Based on The Dependent Scattering Theory

Design and Realization of a Novel Optically Disordered Material: A Demonstration of a Mie Glass

Herein, a diffusive material presenting optical disorder is introduced, which represents an example of a Mie glass. Comprising spherical crystalline TiO2 nanoparticles randomly dispersed in a mesoporous TiO2 matrix, it is proved that the scattering of light in this inhomogeneous solid can be predicted in an unprecedented manner from single‐particle considerations employing Mie theory. To that aim, a study of the dependence of the key parameters employed is performed to describe light propagation in random media, i.e., the scattering mean free path and the transport mean free path, as a function of the size and concentration of the spherical inclusions based on a comparison between experimental results and analytical calculations. It is also demonstrated that Mie glasses enable enhanced fluorescence intensity due to a combined absorptance enhancement of the excitation light combined with an improved outcoupling of the emitted light. The method offers the possibility to perform a deterministic design for the realization of a light diffuser with tailor‐made scattering properties.

Regularization and decimation pseudolikelihood approaches to statistical inference inXYspin models

We implement a pseudolikelyhood approach with l2-regularization as well as the recently introduced pseudolikelihood with decimation procedure to the inverse problem in continuous spin models on arbitrary networks, with arbitrarily disordered couplings. Performances of the approaches are tested against data produced by Monte Carlo numerical simulations and compared also from previously studied fully-connected mean-field-based inference techniques. The results clearly show that the best network reconstruction is obtained through the decimation scheme, that also allows to dwell the inference down to lower temperature regimes. Possible applications to phasor models for light propagation in random media are proposed and discussed.

Asymmetric transmission and optical low-pass filtering in a stack of random media with graded transport mean free path

Light transport and the physical phenomena related to light propagation in random media are very intriguing, they also provide scope for new paradigms of device functionality, most of which remain unexplored. Here we demonstrate, experimentally and by simulation, a novel kind of asymmetric light transmission (diffusion) in a stack of random media (SRM) with graded transport mean free path. The structure is studied in terms of transmission, of photons propagated through and photons generated within the SRM. It is observed that the SRM exhibits asymmetric transmission property with a transmission contrast of 0.25. In addition, it is shown that the SRM works as a perfect optical low-pass filter with a well-defined cutoff wavelength at 580nm. Further, the photons generated within the SRM found to exhibit functionality similar to an optical diode with a transmission contrast of 0.62. The basis of this functionality is explained in terms of wavelength dependent photon randomization and the graded transport mean free path of SRM.

Hybrid model of light propagation in random media based on the time-dependent radiative transfer and diffusion equations

Numerical modeling of light propagation in random media has been an important issue for biomedical imaging, including diffuse optical tomography (DOT). For high resolution DOT, accurate and fast computation of light propagation in biological tissue is desirable. This paper proposes a space–time hybrid model for numerical modeling based on the radiative transfer and diffusion equations (RTE and DE, respectively) in random media under refractive-index mismatching. In the proposed model, the RTE and DE regions are separated into space and time by using a crossover length and the time from the ballistic regime to the diffusive regime, ρDA~10/μt′ and tDA~20/vμt′ where μt′ and v represent a reduced transport coefficient and light velocity, respectively. The present model succeeds in describing light propagation accurately and reduces computational load by a quarter compared with full computation of the RTE.

Diffusion of Polarized Light

We study partially polarized light propagation in random media governed by the theory of radiative transport. In particular, we derive a diffusion approximation when scattering is strong and absorption is weak. This diffusion approximation is substantially easier to solve than the vector radiative transport equation because it requires only the solution of a scalar problem. Included in this analysis is the derivation of a boundary layer solution that corrects the diffusion approximation near the boundaries and provides the means to derive boundary conditions for the diffusion approximation. We give an explicit solution for this boundary layer solution in terms of plane wave solutions that we calculate numerically using the discrete ordinate method. We evaluate the effectiveness of this diffusion approximation through comparison with the numerical solution of the full problem.

Short-range interference effect in the diffusion of light in random media

We study the interference effect of multiply scattered waves in the diffusive light propagation in random media. We take porous glass samples of various pore radii $a$ as an example of random media, and measure the transport mean free path ${l}^{*}$ and the diffusion constant $D$ for various light wavelengths $\ensuremath{\lambda}$ over a wide range of the size parameter $a/\ensuremath{\lambda}=0.05$\char21{}0.85. A crossover is observed from the Rayleigh scattering region to the geometrical optics region. The transport velocity ${v}_{\mathrm{E}}{=3D/l}^{*}$ determined from the data is found to decrease monotonically with $a/\ensuremath{\lambda},$ falling well below the light velocity in glass. We also present a framework of theoretical analysis of wave energy diffusion in a most general setting, taking explicit account of the broadening of light dispersion and paying attention to the requirement of Ward-Takahashi identity. We calculate diffusion parameters as functions of $a/\ensuremath{\lambda}$ for a model of spatially fluctuating dielectric constant, using two types of self-consistent scattering approximation to the self-energy and the scattering kernel. Comparison of the experiment against the calculation with and without interference terms of multiple scattering reveals the importance of short-range interference effects in the diffusive propagation of light.

Trends in optical biomedical imaging

Abstract Recent progress in the development of optical techniques for non-invasive imaging in biology and medicine is reviewed. We start with some fundamental considerations of light propagation in random media. We then discuss imaging using coherent unscattered light and diffuse light. Microscopic techniques based on one- and two-photon fluorescence are described and several types of nonlinear and time-resolved microscopy are explained. We conclude with some application examples and an outlook.