A method to change the scatterer distribution of a random laser medium using the optical trapping technique is proposed. By focusing trapping beams into small regions of the scattering medium, some scattering particles are concentrated in those regions, causing inhomogeneous distribution of scatterers. We investigate the effect of the scatterer distribution formed by optical trapping on the emission spectrum of a random laser. Experimental results show that the spectral spike intensity of random lasers with trapped particles is higher than that for the random lasers without trapped particles. Furthermore, the spectral spike intensity depends on the power of the trapping spot. The relationship between the number and the intensity of spikes in the emission spectra shows a feature observed in other random lasers with inhomogeneous scatterer distributions.
We experimentally prepared a laser speckle flowgraphy (LSFG) device, and investigated whether it can accurately measure cutaneous blood flow. Cutaneous blood flow was measured in healthy adults using the thermal diffusion method (TDM) and strain-gauge plethysmography (SPG) established as blood flow measurement methods, along with LSFG. The correlation properties between the values measured by LSFG and TDM, and LSFG and SPG, were investigated. Results found that a significant positive correlation is noted between the two conventional methods and LSFG. It is suggested that LSFG is a useful device for cutaneous blood flow evaluation, and that its applications in medical care and esthetic fields are expected.
The reflection and scattering properties of light incident on human skin covered with powder particles have been investigated. A three-layer skin structure with a pigmented area is modeled, and the propagation of light in the skin’s layers and in a layer of particles near the skin’s surface is simulated using the Monte Carlo method. Assuming that only single scattering of light occurs in the powder layer, the simulation results show that the reflection spectra of light from the skin change with the size of powder particles. The color difference between normal and discolored skin is found to decrease considerably when powder particles with a diameter of approximately 0.25 μm are present near the skin’s surface. The effects of the medium surrounding the particles, and the influence of the distribution of particle size (polydispersity), are also examined. It is shown that a surrounding medium with a refractive index close to that of the skin substantially suppresses the extreme spectral changes caused by the powder particles covering the skin surface.
The reflection and scattering properties of light incident on skin covered with powder particles have been investigated. A
three-layer skin structure with a spot is modeled, and the propagation of light in the skin and the scattering of light by
particles on the skin surface are simulated by means of a Monte Carlo method. Under the condition in which only single
scattering of light occurs in the powder layer, the reflection spectra of light from the skin change dramatically with the
size of powder particles. The color difference between normal skin and spots is found to diminish more when powder
particles smaller than the wavelength of light are used. It is shown that particle polydispersity suppresses substantially
the extreme spectral change caused by monodisperse particles with a size comparable to the light wavelength.
An authentication method using dynamic laser speckle has been investigated to identify scattering objects such as papers
or plastic cards. The effects of sampling and quantization of speckle signals on identification performance are examined by
using the equal error rate (EER) as a measure of the accuracy of object identification. It is found that a sampling interval
of more than the correlation length of speckle fluctuations and a quantization of two or three bits give the lowest EER for
data sizes ranging from 100 to 500 bytes.
We study the light scattering properties of random media which have the refractive index distribution of threedimensional
(3D) clipped laser speckle structures. To evaluate the performance of the speckle media as random laser
cavities, we calculated the energy density and the local quality factor inside the media by means of the 3D finite
difference time domain (FDTD) method. It is shown that the random media fabricated with the use of three speckle
waves superposed without interference have the scattering strength as large as that of particulate media used
conventionally for random laser media. To realize the speckle random media, we employed a technique of holographic
lithography and fabricated polymer random media by illuminating photopolymer with one or two speckle waves. The far
field scattering pattern of the sample media was measured to estimate the refractive index distribution of the samples.
The experimental results suggest that a fibrous index structure, which is characteristic of 3D speckle patterns, is formed
inside the sample media.
The correlation properties of fractal speckle distributions arising from superposition of three speckle waves propagating
perpendicularly to one another are studied theoretically. The analytical expressions are derived for the intensity correlation
of the superposed speckle fields, and the theoretical results are verified by means of computer simulations. It is shown that
the spatial anisotropy of speckle patterns exists even when the three speckle waves are interfered with one another. This
spatial anisotropy affects the power law distribution of intensity correlations for fractal speckles, leading to the speckle
pattern which is not self-similar in two or three dimensions. As an application of the superposed speckle field, a method
to fabricate a disordered medium by illuminating photopolymer with multiple laser speckle waves is proposed. Some
preliminary experimental results are shown.
The statistical properties of three-dimensional fractal speckle fields produced by three speckle waves crossed orthogonally are studied theoretically. The analytical expressions are derived for the intensity correlation of the superposed speckle fields, and the theoretical results are verified by means of computer simulations. It is shown that the spatial anisotropy of intensity distributions, which exists in each speckle field, remains even when three speckle waves are interfered with one another. The spatial anisotropy affects the power law distribution of intensity correlations for fractal speckle fields, and leads to speckle patterns that are not self-similar in two or three dimensions. As an application of the superposed speckle field, we propose a method to fabricate a disordered medium by illuminating photopolymer with multiple laser speckle waves.
The statistical properties of three-dimensional laser speckles formed with orthogonally crossed multiple speckle waves are
studied theoretically. The analytical expressions for the first and second order statistics of the crossed speckle fields are
derived, and the theoretical results are verified by means of computer simulations. In the simulations, fractal speckles are
also analyzed to examine the self-similar properties of the resulting field. The correlation properties of clipped speckles
are investigated to explore the possibility of fabricating fractal random media by means of photopolymerization for three-dimensional
patterning of micro structures.
The statistical properties of three-dimensional light scattering fields produced by two or three random optical waves propagating in the directions orthogonal to each other are studied theoretically. As a result of the interference of crossed speckle fields, spatial anisotropy of intensity distributions, which exists in a three-dimensional single speckle field, is almost eliminated. It is found that the resultant field has a random intensity distribution in a macroscopic scale, but it has a periodic spatial structure in a microscopic scale. The possibility of making a new type of random lasers with the use of photopolymers and the crossed scattering fields is shown.
Intensity correlations of speckle patterns formed by multiple scattered light from a powder bed are investigated experimentally. Decorrelation of the speckle patterns is measured by changing the angle and the area of laser beam illumination. The proportional relationship between the transport mean free path and the average particle diameter is obtained when the powder samples of the same type are used in the angular correlation measurements. It is also found that the particle size affects the speckle decorrelation due to the translation of the illuminating area on the powder surface. This result exhibits a contrast to a single scattering diffuse object for which the degree of speckle correlation is independent of the characteristics of the diffuser.
We sometimes want to know some information hidden in highly dense media in which the light suffers a strongly multiple scattering, such as the existence of a tumor in a biological tissue and a state of a material behind a layer of paint. In such situations, optical measurement techniques which have the advantage of a non-contact and non-invasive method have been remarked and studied actively by many scientists.1 In the present paper, we pay attention to speckle-like intensity fluctuations generated by multiply scattered light and to their angular correlation properties, and investigate an effect of objects buried in highly dense media on an angular correlation function of the scattered intensity. The angular correlation function is derived theoretically and numerical simulation results are shown.
The transmission properties of an interference filter with optical gain are studied in which the layers of a gain medium are stacked in a self-similar manner. It is found that the gain variation of transmitted light with layer thickness or input frequency exhibits a fractal structure with sharp gain peaks which are not observed for a multilayer with periodic gain structures.
Effect of objects buried in highly dense media on the properties of angular intensity correlation of the scattered light is investigated by means of a theoretical approach and numerical simulations. In the case of an object located just behind the scattering medium, a Fourier transform relation between the angular correlation function and the intensity distribution in the illumination plane where the object exists is derived theoretically for transmitted light. When the object is buried in the medium, such a boundary condition is not satisfied, and it was found from the simulation results that the effect of the object on the angular correlations can be observed more strongly for the object being somewhat deep rather than near the output surface. For reflected light, on the other hand, the influence of the object on the angular correlation function becomes small because of a scattering component not reaching the object.
Coherent backscattering of light from random media has been studied intensively because of its relation to weak localization of photons [1]. Constructive interference of waves following a certain path and its time-reversed path in the medium yields an enhancement in the backscat- tered intensity. When the term "constructive interference" is used for the explanation of the origin of this enhancement, we implicitly as- sume that the light is perfectly coherent. Most of the studies relating to the coherent backscat- tering have been done under this assumption. In this paper we present an analytical ex- pression for a shape of the backscattering cone formed under illumination of the light with ar- bitrary spatial coherence. In a formulation, the statistical properties of the illuminating light is given by the angular correlation func- tion [2], which characterizes the correlation between two components of the angular spec- trum [3] of a light field. This plane-wave de- composition of the field allows us to make use of the analytical expressions [4] developed pre- viously for the coherent backscattering of scalar waves.
A speckle correlation method is described in which a liquid crystal television (LCTV) is used as a spatial filter to measure the paths of the lateral motion of rough objects. Two kinds of speckle patterns are generated simultaneously, consisting of speckles elongated in orthogonal directions to each other. The two-dimensional motion vector of the object can be measured in real time by correlating these patterns with static speckle patterns that are displayed on the LCTV as the references. Because these reference patterns can be renewed in real time, there is no limit for a measurable range of the present method. However, the spatial resolution and contrast of the LCTV limit the sensitivity and accuracy of the method.
The dynamic properties of doubly-scattered speckles produced at the image plane have been investigated by using a scattering model consisting of a coherent Gaussian beam, a cascade of two moving diffusers and a single imaging lens. It is found that, when the point-spread function and the focal position of the lens satisfy certain conditions, three kinds of velocity information, i.e., the velocity of each diffuser, the average velocity of the diffusers, and the velocity difference between the diffusers, can be obtained by measuring a temporal correlation length of the time-varying speckle intensity. Experimental results confirm the theoretical predictions.
Dynamic speckles always appear when a coherent light illuminates dense scattering
media in motion. These media usually produce multiply-scattered speckles whose statistical
properties are considered to be different from those of singly-scattered norial
speckles. Several theoretical studies13 have been carried out for doubly- or multiplyscattered
speckles. However, most of these analyses deal with speckles in the far
field or image speckles in focus, and few studies have been conducted on the effect of
defocus on the dynamic properties of doubly-scattered speckles.
In the present paper we investigate the dynamic properties of time-varying speckles
formed in an imaging system by the light passing through a sequential pair of two
moving diffusers. The space-time correlation function for the Image speckle patterns
is derived to estisate the influence of the motion of each diffuser on a fluctuating
speed of the speckle intensity. Several interesting features have been deduced.
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