Studying propagation of light in random scattering materials is important for both basic and applied research. Such studies often require usage of numerical method for simulating behavior of light beams in random media. However, if such simulations require consideration of coherence properties of light, they may become a complex numerical problems. There are well established methods for simulating multiple scattering of light (e.g. Radiative Transfer Theory and Monte Carlo methods) but they do not treat coherence properties of light directly. Some variations of these methods allows to predict behavior of coherent light but only for an averaged realization of the scattering medium. This limits their application in studying many physical phenomena connected to a specific distribution of scattering particles (e.g. laser speckle). In general, numerical simulation of coherent light propagation in a specific realization of random medium is a time- and memory-consuming problem. The goal of the presented research was to develop new efficient method for solving this problem. The method, presented in our earlier works, is based on solving the Fredholm type integral equation, which describes multiple light scattering process. This equation can be discretized and solved numerically using various algorithms e.g. by direct solving the corresponding linear equations system, as well as by using iterative or Monte Carlo solvers. Here we present recent development of this method including its comparison with well-known analytical results and a finite-difference type simulations. We also present extension of the method for problems of multiple scattering of a polarized light on large spherical particles that joins presented mathematical formalism with Mie theory.
Optical coherence tomography (OCT) is a versatile optical method for cross-sectional and 3D imaging of biological and
non-biological objects. Here we are going to present the application of polarization sensitive spectroscopic OCT system
(PS-SOCT) for quantitative measurements of materials containing nanoparticles. The PS-SOCT combines the polarization
sensitive analysis with time-frequency analysis. In this contribution the benefits of using the combination of timefrequency
and polarization sensitive analysis are being expressed. The usefulness of PS-SOCT for nanoparticles evaluation
is going to be tested on nanocomposite materials with TiO2 nanoparticles. The OCT measurements results have been
compared with SEM examination of the PMMA matrix with nanoparticles. The experiment has proven that by the use of
polarization sensitive and spectroscopic OCT the nanoparticles dispersion and size can be evaluated.
Simulating propagation and scattering of coherent light in turbid media, such as biological tissues, is a complex problem. Numerical methods for solving Helmholtz or wave equation (e.g. finite-difference or finite-element methods) require large amount of computer memory and long computation time. This makes them impractical for simulating laser beam propagation into deep layers of tissue. Other group of methods, based on radiative transfer equation, allows to simulate only propagation of light averaged over the ensemble of turbid medium realizations. This makes them unuseful for simulating phenomena connected to coherence properties of light. We propose a new method for simulating propagation of coherent light (e.g. laser beam) in biological tissue, that we called Coherent-Wave Monte Carlo method. This method is based on direct computation of optical interaction between scatterers inside the random medium, what allows to reduce amount of memory and computation time required for simulation. We present the theoretical basis of the proposed method and its comparison with finite-difference methods for simulating light propagation in scattering media in Rayleigh approximation regime.
The unique features of nanocomposite materials depend on the type and size of nanoparticles, as well as their placement
in the composite matrices. Therefore the nanocomposites manufacturing process requires inline control over certain
parameters of nanoparticles such as dispersion and concentration. Keeping track of nanoparticles parameters inside a
matrix is currently a difficult task due to lack of a fast, reliable and cost effective way of measurement that can be used for
large volume samples. For this purpose the Optical Coherence Tomography (OCT) has been used. OCT is an optical
measurement method, which is a non-destructive and non-invasive technique. It is capable of creating tomographic images
of inner structure by gathering depth related backscattered signal from scattering particles. In addition, it can analyse, in a
single shot, area of the centimetre range with resolution up to single micrometres. Still to increase OCT measurement
capabilities we are using additional system extensions such as Spectroscopic OCT (SOCT). With such addition, we are
able to measure depth related parameters such as scattering spectra and intensity of backscattered signal. Those parameters
allow us to quantitatively estimate nanoparticles concentration. Gaining those, information allows to calculate volume
concentration of nanoparticles. In addition, we analyse metallic oxides nanoparticles. To fully characterize nanoparticles
it is necessary to find and differentiate those that are single particles from agglomerated ones. In this contribution we
present our research results on using the LCI based measurement techniques for evaluation of materials with nanoparticles.
The laboratory system and signal processing algorithms are going to be shown in order to express the usefulness of this
method for inline constant monitoring of the nanocomposite material fabrication.
Multiple scattering of a coherent light plays important role in the optical metrology. Probably the most important phenomenon caused by multiple scattering are the speckle patterns present in every optical imaging method based on coherent or partially coherent light illumination. In many cases the speckle patterns are considered as an undesired noise. However, they were found useful in various subsurface imaging methods such as laser speckle imaging (LSI) or optical coherence tomography (OCT). All features of the speckle patterns and their connection with microstructure of scattering materials was not fully exploited. Further research on this topic may lead to a simple and inexpensive optical diagnostic methods. Theoretical and numerical research could greatly facilitate development of such methods. However, this requires simulations of coherent light propagation through a random scattering media of a length larger than few hundreds of a light wavelength. During such propagation the light can be scattered by many tens of thousands of scattering particles. The numerical methods, such as the radiative transfer theory and the Monte Carlo methods, allows to simulate propagation of the incoherent light in such a media but they do not consider the coherence properties of light. Moreover, they consider only average realization of the scattering medium and therefore does not allow to predict properties of any random fluctuations of the scattered light, such as a speckle patterns. Direct solvers of Maxwell or Helmholtz equations, such as finite-difference or finite-element methods could solve this problem but their computational complexity limits their application to media no longer than about 10 to 20 wavelengths of the light. Here we show an alternative approach based on solving the integral form of the Helmholtz equation written as a so called perturbation expansion. We show the numerical algorithm for solving this equation. The presented algorithm allows to simulate light scattering with accuracy similar to the finite difference methods but with much lower computational complexity. This can make it an useful tool in research on coherent light propagating through the random media.
Simulations of the optical coherence tomography (OCT) systems using the Monte Carlo method is a widely explored research area. However, there are several difficulties that need to be overcome in order to properly model the OCT imaging with the Monte Carlo algorithm. First of all, the temporal and the spatial coherence of the scattered light need to be considered, since OCT is based on the interference phenomenon. For the same reason, the polarization state of the scattered light need to be calculated. Moreover, the OCT systems use light beams that can be described by the Gaussian beam model. However, such beams cannot be directly simulated using the standard Monte Carlo algorithm. Different research groups have developed simulators dealing with some of these problems but the Monte Carlo simulator which considers all of them has not been published yet. Here we present the Monte Carlo program allowing to simulate OCT images of heterogeneous light scattering structures. The presented program considers all of the listed problems and allows to model complex sample geometries with layer boundaries described by a set of polygons.
Spectroscopic optical coherence tomography (SOCT) is an extension of a standard OCT technique, which allows to obtain depth-resolved, spectroscopic information on the examined sample. It can be used as a source of additional contrast in OCT images e.g. by encoding certain features of the light spectrum into the hue of the image pixels. However, SOCT require computation of time-frequency distributions of each OCT A-scan, what is a very time consuming procedure. This is particularly important in a real-time OCT imaging. Here, we present a new approach to SOCT signal processing that allows for nearly tenfold reduction of a required computation time. The presented approach is based on a recursive analysis of OCT scan in time-domain without necessity of computing neither short-time Fourier transform or any other time-frequency distribution.
Bio-ceramics such as hydroxyapatite (HAp) are widely used materials in medical applications, especially as an interface
between implants and living tissues. There are many ways of creating structures from HAp like electrochemical assisted
deposition, biomimetic, electrophoresis, pulsed laser deposition or sol-gel processing. Our research is based on analyzing
the parameters of the sol-gel method for creating thin layers of HAp. In order to achieve this, we propose to use Optical
Coherence Tomography (OCT) for non-destructive and non-invasive evaluation. Our system works in the IR spectrum
range, which is helpful due to the wide range of nanocomposites being opaque in the VIS range. In order to use our method
we need to measure two samples, one which is a reference HAp solution and second: a similar HAp solution with
nanoparticles introduced inside. We use silver nanoparticles below 300 nm. The aim of this research is to analyze the
concentration and dispersion of nanodopants in the bio-ceramic matrix. Furthermore, the quality of the HAp coating and
deposition process repetition have been monitored. For this purpose the polarization sensitive OCT with additional
spectroscopic analysis is being investigated. Despite the other methods, which are suitable for nanocomposite materials
evaluation, the OCT with additional features seems to be one of the few which belong to the NDE/NDT group. Here we
are presenting the OCT system for evaluation of the HAp with nano-particles, as well as HAp manufacturing process.
A brief discussion on the usefulness of OCT for bio-ceramics materials examination is also being presented.
Optical coherence tomography (OCT) is a non-invasive method for 3D and cross-sectional imaging of biological and
non-biological objects. The OCT measurements are provided in non-contact and absolutely safe way for the tested
sample. Nowadays, the OCT is widely applied in medical diagnosis especially in ophthalmology, as well as dermatology,
oncology and many more. Despite of great progress in OCT measurements there are still a vast number of issues like
tissue recognition or imaging contrast enhancement that have not been solved yet. Here we are going to present the
polarization sensitive spectroscopic OCT system (PS-SOCT). The PS-SOCT combines the polarization sensitive analysis
with time-frequency analysis. Unlike standard polarization sensitive OCT the PS-SOCT delivers spectral information
about measured quantities e.g. tested object birefringence changes over the light spectra. This solution overcomes the
limits of polarization sensitive analysis applied in standard PS-OCT. Based on spectral data obtained from PS-SOCT the
exact value of birefringence can be calculated even for the objects that provide higher order of retardation. In this
contribution the benefits of using the combination of time-frequency and polarization sensitive analysis are being
expressed. Moreover, the PS-SOCT system features, as well as OCT measurement examples are presented.
Spectroscopic Optical Coherence Tomography (SOCT) is an extension of Optical Coherence Tomography (OCT). It allows gathering spectroscopic information from individual scattering points inside the sample. It is based on time-frequency analysis of interferometric signals. Such analysis requires calculating hundreds of Fourier transforms while performing a single A-scan. Additionally, further processing of acquired spectroscopic information is needed. This significantly increases the time of required computations. During last years, application of graphical processing units (GPU’s) was proposed to reduce computation time in OCT by using parallel computing algorithms. GPU technology can be also used to speed-up signal processing in SOCT. However, parallel algorithms used in classical OCT need to be revised because of different character of analyzed data. The classical OCT requires processing of long, independent interferometric signals for obtaining subsequent A-scans. The difference with SOCT is that it requires processing of multiple, shorter signals, which differ only in a small part of samples. We have developed new algorithms for parallel signal processing for usage in SOCT, implemented with NVIDIA CUDA (Compute Unified Device Architecture). We present details of the algorithms and performance tests for analyzing data from in-house SD-OCT system. We also give a brief discussion about usefulness of developed algorithm. Presented algorithms might be useful for researchers working on OCT, as they allow to reduce computation time and are step toward real-time signal processing of SOCT data.
Numerical modeling Optical Coherence Tomography (OCT) systems is needed for optical setup optimization, development of new signal processing methods and assessment of impact of different physical phenomena inside the sample on OCT signal. The Monte Carlo method has been often used for modeling Optical Coherence Tomography, as it is a well established tool for simulating light propagation in scattering media. However, in this method light is modeled as a set of energy packets traveling along straight lines. This reduces accuracy of Monte Carlo calculations in case of simulating propagation of dopeds. Since such beams are commonly used in OCT systems, classical Monte Carlo algorithm need to be modified. In presented research, we have developed model of SD-OCT systems using combination of Monte Carlo and analytical methods. Our model includes properties of optical setup of OCT system, which is often omitted in other research. We present applied algorithms and comparison of simulation results with SD-OCT scans of optical phantoms. We have found that our model can be used for determination of level of OCT signal coming from scattering particles inside turbid media placed in different positions relatively to focal point of incident light beam. It may improve accuracy of simulating OCT systems.
Optical coherence tomography (OCT) is one of the most advanced optical measurement techniques for complex structure visualization. The advantages of OCT have been used for surface and subsurface defect detection in composite materials, polymers, ceramics, non-metallic protective coatings, and many more. Our research activity has been focused on timefrequency spectroscopic analysis in OCT. It is based on time resolved spectral analysis of the backscattered optical signal delivered by the OCT. The time-frequency method gives spectral characteristic of optical radiation backscattered or backreflected from the particular points inside the tested device. This provides more information about the sample, which are useful for further analysis. Nowadays, the applications of spectroscopic analysis for composite layers characterization or tissue recognition have been reported. During our studies we have found new applications of spectroscopic analysis. We have used this method for thickness estimation of thin films, which are under the resolution of OCT. Also, we have combined the spectroscopic analysis with polarization sensitive OCT (PS-OCT). This approach enables to obtain a multiorder retardation value directly and may become a breakthrough in PS-OCT measurements of highly birefringent media. In this work, we present the time-frequency spectroscopic algorithms and their applications for OCT. Also, the theoretical simulations and measurement validation of this method are shown.