Circular polarization memory effect in low-coherence enhanced backscattering of light

We experimentally study the propagation of circularly polarized light in the subdiffusion regime by exploiting enhanced backscattering [(EBS), also known as coherent backscattering] of light under low spatial coherence illumination. We demonstrate for the first time, to the best of our knowledge, that a circular polarization memory effect exists in EBS over a large range of scatterers' sizes in this regime. We show that low-coherence EBS signals from the helicity preserving and orthogonal helicity channels cross over as the mean free path length of light in media varies, and that the cross point indicates the transition from multiple to double scattering in EBS.     

Coherent backscattering spectroscopy

Coherent backscattering (CBS) of light in random media has been previously investigated by use of coherent light sources. Here we report a novel method of CBS measurement that combines low spatial coherence, broadband illumination, and spectrally resolved detection. We show that low spatial coherence illumination leads to an anomalously broad CBS peak and a dramatic speckle reduction; the latter is further facilitated by low temporal coherence detection. Thus CBS can be observed in biological tissue and other media that previously were beyond the reach of conventional CBS measurements. We also demonstrate, for the first time to our knowledge, spectroscopic analysis of CBS. CBS spectroscopy may find important applications in probing random media such as biological tissue in which depth-selective measurements are crucial.     

Effect of the illumination region of targets on waves scattering in random media with H-polarization

This paper addresses some parameters that have a significant effect on wave scattering in random media. These parameters are: target configuration, including size and curvature; random media strength, represented in the spatial coherence length; and incident wave polarization. Here, I present numerical calculations for the radar cross-section (RCS) of conducting targets and analyze the backscattering enhancement with different configurations. I postulate a concave illumination region and consider targets taking large sizes of about five wavelengths. In this aspect, waves scattering from targets are assumed to propagate in free space and a random medium with H-polarization. This polarization produces what is well known as creeping waves which in turn have an additional effect on the scattering waves that is absent in the case of E-polarization.     

Using dynamic low-coherence interferometry to image Brownian motion within highly scattering media

Dynamic low-coherence interferometry was used to measure Brownian motion of submicrometer particles within highly scattering media. Strong rejection of multiply scattered light was obtained by combination of a coherence gate with a confocal microscope, thus allowing particle characterization methods generally reserved for optically dilute materials to be applied to optically concentrated suspensions. The Brownian diffusion coefficient of highly scattering media was determined with an accuracy better than 5%. Furthermore, we show that spatial variations in the Brownian diffusion coefficient can be imaged with an axial resolution determined by the coherence length of the light source (~30 mum) . The experiments also show broadening of the power spectrum as a function of depth into the sample, most likely as a result of detecting multiply scattered light.     

Spectral Changes of Light and Scattering Phenomena

In the last few years renewed attention has been paid to the study of light scattering from random media. This is due to the recent discovery of a phenomenon that dynamic light scattering from a fluctuating random medium may produce shifts of spectral lines, even when the source and the scattering medium are at rest relative to the observer. It has also been demonstrated that a similar phenomenon may occur in static light scattering from spatially random media without dynamic fluctuations. By the well-known analogy between the processes of scattering and radiation, these phenomena in scattering are found to be closely related to the correlation-induced spectral changes in the coherence theory which are often referred to as the Wolf effect. In this paper some recent developments are reviewed on research regarding the phenomena of changes in the spectrum of light induced by scattering from random media. Emphasis is placed on a number of up-to-date attempts for elucidating the effects of multiple scattering on these phenomena.This review was presented as an invited paper at the Symposium on Spectral Effects in Collective Phenomena organized as a satellite meeting of the Seventh Rochester Conference on Coherence and Quantum Optics, June 7–10 1995 (Rochester, NY).     

Bloch-wave homogenization for spectral asymptotic analysis of the periodic Maxwell operator

The time-reversal effects on super-resolution in random scattering media are analysed using numerical finite-difference time-domain (FDTD) simulations. The analytical solutions and results have been presented previously in the literature, which provide confirmation of spot-size reduction and also explanations of the shower curtain effects and backscattering enhancement. However, the analytical solutions are based on several approximations. Thus, validation of the analytical results against realistic scattering events is necessary. Two-dimensional FDTD Monte Carlo simulations have been employed for this investigation to simulate wave propagation and scattering in a random medium. The scattering environments are created by randomly locating cylindrical rods in the background medium. The simulation process involves a point source emitting a Gaussian pulse wave that propagates through the scattering medium, gets time-reversed, and then back-propagated into the same scattering medium. The focusing behaviours including the location of the focal point and its spot-size as a function of its transverse position are analysed. The shower curtain, particle size, and time domain effects are also investigated. In comparison, the behaviours of focusing derived by numerical results are consistent with those of previously reported analytical results. However, there are some differences, which we speculate to be mainly because of the different phase functions.     

Spatial correlations in the near field of random media

The conventional coherence theory suggests that the fields radiated by statistically homogeneous sources correlate over spatial regions of the order of the wavelength irrespective of the distance from the surface of the source. Contrary to these predictions, we show that the spatial correlations of optical fields in close proximity of highly scattering, randomly inhomogeneous media depend on this distance and, moreover, their extent can be significantly smaller than the wavelength. The contribution of evanescent fields is experimentally demonstrated and the coherence length in the near field is shown to relate to the coherence properties at the surface which are, in turn, determined by the structural characteristics of the random media.     

Radar cross sections of conducting elliptic cylinders embedded in strong continuous random media

The radar cross section (RCS) of a conducting elliptic cylinder in a strong continuous random medium is analysed numerically for E-wave and H-wave incidences, by solving the wave scattering as a boundary value problem. The numerical analysis shows that the spatial coherence of an incident wave on the cylinder has an important effect on the RCS, besides the well known effect of double passage, and that the spatial coherence effect depends on the curvature of the elliptic surface illuminated by an incident wave and the size of the ellipse.     

Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media

An optical Doppler tomography (ODT) system that permits imaging of fluid flow velocity in highly scattering media is described. ODT combines Doppler velocimetry with the high spatial resolution of low-coherence optical interferometry to measure fluid flow velocity at discrete spatial locations. Tomographic imaging of particle flow velocity within a circular conduit submerged 1 mm below the surface in a highly scattering phantom of Intralipid is demonstrated.     

Holographic Image Denoising using Random Laser Illumination

In holographic applications, coherent lasers are indispensable source of illumination. Despite high intensity from coherent light sources, they fail in full-field image projection and illustrate speckle images due to high spatial coherence. This article demonstrates speckle-free high contrast computer-generated holographic image projection upon illumination with a perovskite–polystyrene 10 wt%-based random laser. Solvent-engineered efficient and durable perovskite and perovskite–polystyrene 10 wt%-based random lasers are fabricated. Optical characterizations are elucidated and controlled coherence random lasing operation is achieved under room temperature upon addition of polystyrene concentration 10 wt% on perovskite thin film. The addition of 10 wt% polystyrene concentration results in a low far-field divergence angle of ≈10⁰. The controlled coherence in random lasers is necessary to produce a stable interference pattern and to retain the depth of field in holograms. Additionally, the holographic image projection using random lasers reduces diffraction noise, and exhibits high spatial resolution with full-field imaging. Moreover, this study is clear evidence of an effective strategy to achieve high-performance, indigenous designed-controlled coherence in disordered random lasing media and its application to novel holographic image projection.     

Radar cross sections of conducting elliptic cylinders embedded in strong continuous random media

Abstract

The radar cross section (RCS) of a conducting elliptic cylinder in a strong continuous random medium is analysed numerically for E-wave and H-wave incidences, by solving the wave scattering as a boundary value problem. The numerical analysis shows that the spatial coherence of an incident wave on the cylinder has an important effect on the RCS, besides the well known effect of double passage, and that the spatial coherence effect depends on the curvature of the elliptic surface illuminated by an incident wave and the size of the ellipse.     

Correlation between intensity fluctuations of light scattered from a quasi-homogeneous random media

The correlation between intensity fluctuations of light scattered from a quasi-homogeneous random media was analytically derived. We showed the correlation depends on spatial Fourier transforms of both the intensity and degree of spatial correlation of scattering potentials of the media, while the normalized correlation equals the squared modulus of the degree of spatial coherence of the scattered fields.     

Measurement of optical constants for dense media by low-coherence dynamic light scattering

We perform the optical constants measurements for different absorption dense media by low-coherence dynamic light scattering (DLS) technique. The estimated particle size is used to calculate the scattering coefficient of particles suspended in dense media. The path-length resolved intensity distributions of light backscattered from the absorbing dense media are investigated experimentally by virtue of path-length resolved performance in low-coherence DLS measurements. The absorption coefficient can be obtained by applying the measured path-length resolved intensity distributions to the modified Lambert-Beer law. As a result, we proposed a new low-coherence DLS technique in simultaneous measurement of the scattering and absorption coefficients of absorbing dense media.     

Imaging through scattering media with depth resolution by use of low-coherence gating in spatiotemporal digital holography

We demonstrate optical gating through scattering media based on low-coherence spatiotemporal digital holography. The method combines the advantages of low-coherence gating, both temporal and spatial, with the advantages of methods using heterodyning and phase-sensitive detection. Spatiotemporal data are captured on a CCD detector in a single exposure, without mechanical scans, and processed digitally. Examples of reconstructions and sectioning of objects hidden behind a ground-glass diffuser are shown.     

基于蒙特卡罗模拟的散射介质中后向光散射模型及分析应用

构建了内光源模型探讨散射介质中的光散射现象,利用蒙特卡罗方法研究了逃逸出组织的后向散射光子数随光子在组织内部发生的散射次数的分布关系,探讨了光源照明方式、辐射强度、接收方式、调制等参数的变化对后向散射的影响,结果表明后向散射光子的数量随散射次数的分布并非简单的单调递增或递减,而是一条先增大后减小出现峰值的曲线. 峰值位置、峰值大小及曲线形状与光源、探测方式、组织光学特性参数等有关. 关键词： 医用光学与生物技术 散射介质 后向散射 蒙特卡罗模拟     

Spatial and temporal coherence effects in interference microscopy and full‐field optical coherence tomography

Low‐coherence optical microscopy or optical coherence microscopy uses light with short coherence length. The well‐known case is: “white‐light interferometry”, which became recently more known as: “optical coherence tomography”. However, when lenses and microscope objectives are used to create interferometric images, in what is known classically as “interference microscopy” or today as “full‐field optical coherence tomography” the spatial coherence starts to play a critical role. In this article the coherence effects in low‐coherence optical microscopy are reviewed. As this technology is becoming increasingly publicized due to its importance in three‐dimensional imaging, particularly of scattering biological media and optical metrology, the understanding of the fundamental physics behind it is essential. The interplay between longitudinal spatial coherence and temporal coherence and the effects associated with them are discussed in detail particularly when high numerical apertures are used. An important conclusion of this study is that a high‐contrast, high‐resolution system for imaging of multilayered samples is the one that uses narrowband illumination and high‐NA objectives with an index‐matching fluid. Such a system, when combined with frequency‐domain operation, can reveal nearly real‐time three‐dimensional images, and is thus competitive with confocal microscopy.     

Limits of time-reversal focusing through multiple scattering: long-range correlation

Experimental results of time-reversal focusing in a high-order multiple scattering medium are presented and compared to theoretical predictions based on a statistical model. The medium consists of a random collection of parallel steel rods. An ultrasonic source (3.2 MHz) transmits a pulse that undergoes multiple scattering and is recorded on an array. The time-reversed waves are sent by the array back to the source through the scattering medium. The quality of temporal focusing is very well predicted by a simple statistical model. However, for thicker samples, persistent temporal side-lobes appear. We interpret these side-lobes as a consequence of the growing number of crossing paths in the sample due to high-order multiple scattering. As to spatial focusing, the resolution is practically independent from the array's aperture. With a 16-element array, the resolution was found to be 30 times finer than in a homogeneous medium. Resolutions of the order of the wavelength (0.5 mm) were attained. These results are discussed in relation with the statistical properties of time-reversal mirrors in a random medium.     

Spatial coherence of random laser emission

We experimentally studied the spatial coherence of random laser emission from dye solutions containing nanoparticles. The spatial coherence, measured in a double slit experiment, varied significantly with the density of scatterers and the size and shape of the excitation volume. A qualitative explanation is provided, illustrating the dramatic difference from the spatial coherence of a conventional laser. This work demonstrates that random lasers can be controlled to provide intense, spatially incoherent emission for applications in which spatial cross talk or speckle limit performance.     

Impact of large-angle scattering on diffusively backscattered halos

We discuss the impact of large-angle scattering events in highly forward-scattering media on the spatial distribution of the diffusively reflected light. We show that, even for highly forward-scattering media, the reflected light near the incident beam axis is strongly dependent on the small number of large-angle scattering events. Reliable modeling of near-axis reflection thus requires accurate knowledge of the scattering phase function's behavior at large angles.     

Low-coherence doppler lidar with multiple time coherence of reference and probe waves

The notion of multiple time coherence of optical beams is introduced and mathematically analyzed for the first time. Mathematical simulation is used to demonstrate the possibility of the generation of multiple fiber pulses (MFPs), which exhibit multiple time coherence, and the possibility of their application for the laser Doppler anemometry of spatially separated scattering objects (including the realization of wind coherent Doppler lidars). A method to obtain MFPs of a given duration with the means of fiber-optic commutation is proposed. The method is based on nanosecond lasers with a limited coherence length realized in a set of fiber topologies. The concept of a low-coherence Doppler lidar with the multiple time coherence of the reference and probe waves is formulated. The energy parameters of the lidar are mathematically simulated for one of the topologies of the MFP device.