Coherent Light Beams Research Articles

Imaging through a weakly scattering medium by spectral-domain optical coherence microscopy with a digital phase conjugation

A spectral-domain optical coherence microscope (SD-OCM) embedded by a digital phase conjugation is presented for high-resolution imaging of objects behind a weakly scattering medium. For high-resolution SD-OCM, keeping a narrow focusing spot even in a scattering medium is important. To compensate for the degradation of the focused spot due to the scattering, digital phase conjugation with a low-coherent light is introduced by measuring the distorted wavefront of coherent light and making wavelength conversion due to the difference between the coherent and low-coherent light beams. Experiments using a weakly scattering sheet with a scattering angle of 30 degrees showed us to improve the focused spot property and enhance the reflected intensity signal. We also demonstrate the feasibility of the system for the imaging of objects with periodic reflectance structures of 10 µm width.

Non-linear X-ray Science

The advent of the laser in the early 1960s represented a revolution in Optics and Spectroscopy that is still impacting our lives to this day. The availability of coherent and intense beams of laser light enabled the birth of non-linear optics as first demonstrated by the historic experiments in 1961 on second harmonic generation, and the first observation of two-photon absorption. The rapid discovery of diverse non-linear (NL) optical methods had a deep impact in Science that were recognised by Physics (1981, 2018) and Chemistry (1999) Nobel Prizes.

Manufacturing Functional Polymer Surfaces by Direct Laser Interference Patterning (DLIP): A Polymer Science View

Direct laser interference patterning (DLIP) involves the formation of patterns of light intensity using coherent laser light beams that interfere between them. Light on the ultraviolet (<350 nm) and NIR (800–2000 nm) is absorbed in chromophores present in the polymer structure or in loaded absorbing species (dyes, polymers, nanoparticles). The absorbed light induces photothermal/photochemical processes, which alter permanently the topography of the polymer surface. The success of DLIP at different wavelengths is discussed in relation to the optical/thermal properties of the polymers and previous data on laser ablation of polymers. The size of the pattern is related directly to the wavelength of the light and inversely to the sine of the angle between beams and the refractive index of the external medium. In that way, nanometric structures (<100 nm) could be produced. Since the patterning occurs in a single short pulse (<10 ns), large surfaces can be modified. Both bacterial biofilm inhibition and human cell differentiation/orientation have been achieved. Large improvements in technological devices (e.g., thin film solar cells) using DLIP structured surfaces have also been demonstrated. Prospective application of DLIP to common polymers (e.g., Teflon®) and complex polymeric systems (e.g., layer-by-layer multilayers) is discussed on the basis of reported polymer data.

Experimental investigation of degree of cross-polarization for an electromagnetic Gaussian-Schell model beam.

We investigate the properties of the degree of cross-polarization (DOCP) for an electromagnetic Gaussian-Schell model (EMGSM) beam in the radial direction of the cross section of the beam. The coherence and polarization features of a partially coherent light beam are engineered to construct the isotropic and non-isotropic EMGSM beams, and the resulting changes in the DOCP are examined. For experimental realization, a double-slit interferometer is utilized at the output to probe the coherence properties for different polarization components across the beam diameter of an electromagnetic source. Experimental observations infer that variation in DOCP does not become apparent for isotropic coherence widths in the orthogonal polarization directions, whereas the variation can be apparent only once both the coherence widths are distinct. Furthermore, experimentally, a special case is also investigated for which the value of DOCP goes beyond unity. The DOCP finds application in areas such as second-order intensity interference, imaging, and characterization of non-homogeneously polarized beams.

Overview of Optical Technology for Aerosols Characterization

The study of the optical properties of particles in micro and macro environments is a field that collects valuable information on the chemistry and microphysics of aerosols (referring to particles and gases). Constant improvements in instrument and detector designs and mechanisms enable measurements with ever-increasing precision and resolution. Moreover, in recent years low-cost open-source technology has emerged as a massive offer, with very good features, which has allowed low-income laboratories and institutions to carry out their own optical measurements on samples and systems of different natures. This review aims to present the main current advances in optical measurements of air samples using optical properties. Emphasis is placed on low-cost technology and its characterization and applications due to its current boom and mass marketing, with the added value of new scientific articles showing advances in its uses and applications. On the one hand, the theory behind spectral photometric measurements is described, and on the other, the bases for detection from the scattering of a coherent light beam. Finally, the main advances in the literature making use of these phenomena are described, complemented with general results of own measurements. In order to focus the analysis, these aspects are presented based on atmospheric measurements.

Investigation of the anisotropy and scaling of the phase structure function of a spatially coherent light beam propagating through convective air turbulence.

We report on applications of moiré deflectometry in measurements of the anisotropy and scaling of the phase structure function (PSF), obtained after passing a laser beam through an indoor enclosure containing convective air turbulence. We combine the use of two telescopes, with a two-channel wavefront sensor based on moiré deflectometry, to attain high sensitivity and resolution to fluctuations in the wavefront phase, caused by turbulent fluctuations in the enclosure. The measurements of the wavefront PSF along two directions perpendicular to the direction of the light beam propagation at different heater temperatures show that the convective air turbulence is anisotropic turbulence, where the value of the anisotropy increases with increasing temperature gradient. Various models are fitted to the measured PSFs, and we find that the turbulent is also non-Kolmogorov, in which, for the separation distances of two points on the wavefront less than 10 cm, the von Kármán PSF is the best fit to the experimental data. For higher values of separations, the experimental data do not fit with existing models. By fitting the von Kármán PSF on the data, we estimate values of the refractive index structure constant, Cn2, as well as the outer scale of the turbulence. The value of the outer scale decreases with increasing temperature of the heater up to approximately 50°C, where it saturates, while the value of Cn2 monotonically increases. Over the complete range of heater temperatures, from 40°C to 160°C, the Rayleigh number, Ra, for the enclosed air flow varied from 5.80×108<Ra<5.89×109 so that all measurements were conducted in a state of developed convective turbulence.

Optical Deformation of Biological Cells using Dual-Beam Laser Tweezer.

Optical tweezer is a non-contact tool to trap and manipulate microparticles such as biological cells using coherent light beams. In this study, we utilized a dual-beam optical tweezer, created using two counterpropagating and slightly divergent laser beams to trap and deform biological cells. Human embryonic kidney 293 (HEK-293) and breast cancer (SKBR3) cells were used to characterize their membrane elasticity by optically stretching in the dual-beam optical tweezer. It was observed that the extent of deformation in both cell types increases with increasing optical trapping power. The SKBR3 cells exhibited greater percentage deformation than that of HEK-293 cells for a given trapping power. Our results demonstrate that the dual-beam optical tweezer provides measures of cell elasticity that can distinguish between various cell types. The non-contact optical cell stretching can be effectively utilized in disease diagnosis such as cancer based on the cell elasticity measures.

Tunable multiband metamaterial coherent perfect absorber based on graphene and vanadium dioxide

In this paper, we propose a tunable multiband metamaterial coherent perfect absorber, where vanadium dioxide (VO2) square particles are periodically arranged on the two sides of polysilicon–silica–graphene–silica–polysilicon​ structure. The absorption spectrum of our absorber exhibits three peaks with high absorptivity which are originated from the plasmon resonance modes of graphene and VO2 particle. Benefitting from the tunable conductivity of graphene, the adjustment range of the absorption peak based on the first-order plasmon resonance mode of graphene is as much as 7.88μm when the Fermi level of graphene is increased from 0.3 to 1 eV. As VO2 is changed from metal phase to insulating phase, the absorption peak stemming from the plasmon resonance mode of VO2 particle disappears. The modulation depth of absorption peak can achieve up to 98.85% by tuning the relative phase of two coherent light beams. The resonance wavelengths and intensities of absorption peaks are also dependent on the array period, side length of VO2 particle and the thickness of silica layer. In addition, the change of surrounding refractive index causes a shift of the absorption peak corresponding to VO2 particle. Our findings are helpful for the engineering of actively tunable nano plasmonic devices and metamaterials.

Tailoring on-axis spectral density with circularly coherent light beams.

The on-axis cross-spectral density (CSD) of a beam radiated by a stationary source with a circular coherence state and a Gaussian spectral density is obtained in the closed form. It is revealed that the on-axis CSD is expressed via the Laplace transform of the source's degree of coherence or the Hilbert transform of the corresponding pseudo-mode weighting function. Such relations enable efficient tailoring of the on-axis spectral density, as we show with a slew of numerical examples.

Rotating and spiraling spatial dissipative solitons of light and cold atoms

Clouds of cold neutral atoms driven by a coherent light beam in a ring cavity exhibit self-structured states transversely with respect to the beam axis due to optomechanical forces and the back action of the atomic structures on the beam. Below the instability threshold for extended hexagonal structures, localized soliton-like excitations can be stable. These constitute peaks or holes of atom density, depending on the linear susceptibility of the cloud. Complex rotating and spiralling motion of coupled atom-light solitons, and hence atomic transport, can be achieved via phase gradients in the input field profile. We also discuss the stability of rotating soliton chains in view of soliton-soliton interactions. The investigations are performed in a cavity scheme but expected to apply to other longitudinally pumped schemes with diffractive coupling.

Imaging Through Random Scatterer with Spatial Coherence Structure Measurement

Optical coherence is becoming an efficient degree of freedom for light field manipulations and applications. In this work, we show that the image information hidden a distance behind a random scattering medium is encoded in the complex spatial coherence structure of a partially coherent light beam that generates after the random scatterer. We validate in experiment that the image information can be well recovered with the spatial coherence measurement and the aid of the iterative phase retrieval algorithm in the Fresnel domain. We find not only the spatial shape but also the position including the lateral shift and longitudinal distances of the image hidden behind the random scatterer can be reconstructed, which indicates the potential uses in three-dimensional optical imaging through random scattering media.

The Invention of the Laser.

In this chapter, I describe a scientific rivalry at Columbia University's physics department in the days of the 1950s before and when the laser invented, and the race to build a laser eventually won by a scientist in California in 1960. It tracks the arc from Charles Townes' success in amplifying microwaves with a device he called the maser, to graduate student Gordon Gould's realization of how shorter and more powerful light waves could also be amplified. The chapter describes Gould's notebook, written in 1957, diagramming an optically pumped laser. In the same notebook, he coined the word laser, an acronym for light amplification by stimulated emission of radiation, and suggested possible uses for the coherent light beams he imagined. The chapter also covers Gould's wish to patent and profit from his invention, his mistaken belief that he had to build a laser to receive a patent, and the flirtation with radical politics that barred him from the laboratory where he worked once the government classified the laser project that it was funding. Townes and his colleague and brother-in-law Arthur Schawlow, both eventual Physics Nobelists, meanwhile filed an application and received a patent for what they called an optical maser. Theodore Maiman at Hughes Laboratories built the first working laser in 1960, as the chapter describes. It concludes with the results of Gould's long battle to win basic laser patents and his eventual success.

Faraday-imaging-induced squeezing of a double-well Bose-Einstein condensate

We examine how nondestructive measurements generate spin squeezing in an atomic Bose-Einstein condensate confined in a double-well trap. The condensate in each well is monitored using coherent light beams in a Mach-Zehnder configuration that interacts with the atoms through a quantum nondemolition Hamiltonian. We solve the dynamics of the light-atom system using an exact wave-function approach, in the presence of dephasing noise, which allows us to examine arbitrary interaction times and a general initial state. We find that monitoring the condensate at zero detection current and with identical coherent light beams minimizes the backaction of the measurement on the atoms. In the weak atom-light interaction regime, we find the mean spin direction is relatively unaffected, while the variance of the spins is squeezed along the axis coupled to the light. Additionally, squeezing persists in the presence of tunneling and dephasing noise. For long atom-light interaction time, we equally show our methods can be used to prepare non-Gaussian correlated spin states.

Diffuse terahertz spectroscopy in turbid media using a wavelet-based bimodality spectral analysis

Current terahertz (THz) spectroscopy techniques only use the coherent light beam for spectral imaging. In the presence of electromagnetic scattering, however, the scattering-mitigated incoherent beams allow for flexible emitter-detector geometries, which enable applications such as seeing through turbid media. Despite this potential, THz spectroscopy using diffuse waves has not been demonstrated. The main obstacles are the very poor signal to noise ratios of the diffused fields and the resonance-like spectral artifacts due to multiple Mie scattering events that obscure the material absorption signatures. In this work, we demonstrate diffuse THz spectroscopy of a heterogeneous sample through turbid media using a novel technique based on the wavelet multiresolution analysis and the bimodality coefficient spectrum, which we define here for the first time using the skewness and kurtosis of the spectral images. The proposed method yields broadband and simultaneous material characterization at detection angles as high as 90° with respect to the incident beam. We determined the accuracy of the wavelet-based diffuse spectroscopy at oblique detection angles, by evaluating the area under the receiver operating characteristic curves, to be higher than 95%. This technique is agnostic to any a priori information on the spectral signatures of the sample materials or the characteristics of the scattering medium, and can be expanded for other broadband spectroscopic modalities.

Speckle in polarization structured light

We show that the far-field speckle patterns observed on the transmission of polarization structured light beams through a random diffuser screen have distinct spatial textures, which significantly differ from the texture of the fully developed scalar speckle. In particular, we report on speckle observed using the V-point and C-point polarization singular beams for illumination in this work. The V-point and C-point polarization singularities are generated by a combination of different orbital angular momentum (OAM) states in orthogonal polarizations. When passed through a random diffuser screen, the on-axis far-field diffraction intensity is an incoherent summation of speckle intensities due to the individual OAM states. Unlike the typical grainy structure in the scalar speckle, the speckle patterns observed with V-point and C-point illuminating beams are seen to have a noodle-like and sponge-like textures respectively. The speckle from V-point and C-point illuminations is difficult to distinguish using the usual intensity statistics which does not account for the distinct spatial structure of the speckle patterns. The Gray Level Co-occurrence Matrix (GLCM) methodology was therefore used to extract a number of spatial textural features from the speckle patterns observed using scalar and polarization structured illuminations. The principal component analysis (PCA) technique when applied to the texture parameters shows that it is possible to identify the nature of the illuminating beam by simply observing the texture properties of the resultant speckle pattern. When coherent light beams pass through a random medium resulting in a fully developed speckle pattern, the information regarding the profile of the input beam is generally considered to be lost. Our new observations on distinct textural features in speckle from structured light therefore suggest the possibility of information transmission through random media by means of switching the polarization structure of the incident beam.

Stellar interferometry for gravitational waves

We propose a new method to detect gravitational waves, based on spatial coherence interferometry with stellar light, as opposed to the conventional temporal coherence interferometry with laser sources. The proposed method detects gravitational waves by using two coherent beams of light from a single distant star measured at separate space-based detectors with a long baseline. This method can be applied to either the amplitude or intensity interferometry. This experiment allows for the search of gravitational waves in the lower frequency range of 10-6 to 10-4 Hz. In this work, we present the detection sensitivity of the proposed stellar interferometer by taking the detector response and shot and acceleration noises into account. Furthermore, the proposed experimental setup is capable of searching for primordial black holes and studying the size of the target neutron star, which are also discussed in the paper.

Optimizing Ballistic Motion of Partially Coherent Multiple Airy Beams by Quadratic and Linear Phases

AbstractBy applying quadratic and linear phase modulations (QPM and LPM), the propagation properties of the partially coherent dual and quad Airy beams which maintain acceleration in free space are studied. They are generated by superimposing two and four partially coherent Airy beams whose phase is modulated in the source field. It is demonstrated that the trajectories of the partially coherent light beams and the location and number of focal points, along with maximum altitude and range of the trajectories can be easily dominated by adjusting the appropriate values of QPM p and LPM l. Furthermore, for the quad case, the transverse spectral density distribution of lacking side lobes exhibits the structure of the optical frame which may be flipped under certain conditions. Both the peak intensity of the beams and the divergence speed of the optical frame can be managed by choosing the different characteristic transverse width of the beams. These results show an energizing new idea in the manipulation of partially coherent multiple fields of accelerating beams.

Tracking the rotation of a birefringent crystal from speckle correlation

The random intensity distribution observed due to the propagation of a coherent beam of light through a scattering medium is known as a speckle pattern. The interaction of speckles with a birefringent crystal, here, YVO4 results in the splitting of the speckles into e-ray and o-ray speckles. It is observed that the e-ray and o-ray speckles experience an angular shift with the rotation of the crystal and the amount of the shift depends on the rotation angle of the crystal. It is also found that the rate of the shift is different for e-ray and o-ray speckles, and is independent of the topological charge of the beam incident on the scattering medium. The observed rate of shift of the speckles is found to be same as that of the e-ray and o-ray beams. It is experimentally demonstrated that the rotation of a birefringent crystal can be tracked very accurately from the speckle correlation.

Examination of the Implementation and Utilization of Optical Fibers Beams: A Review

For light propagation purposes, the optical fibers that are known as waveguides can be applied. A glass or plastic film called cladding covers the central portion of the optical fiber, and is distinguished by a refractive index that is lower relative to the main refractive index. For the fine confines of the light inside the waveguide, the overall internal reflection phenomena are necessary. It is possible to categorize optical fibers according to shape, number of modes, refractive index profile, dispersion, signal processing power, and polarization. We are concentrating on the first three typical forms of optical fibers in this article. This may be used in fiber beams as a typical application of fibers to generate and intensify a small, powerful beam of coherent and monochromatic light. Optical fiber processing requires three steps, such as the development of performs. The process of adjusted chemical vapor deposition (MCVD) is a recognized technique that can be used to manufacture optical fibers. Optical fiber sensors are well known in optics and photonics for their large variety of applications. Optical biosensors can be developed as a sensing application focused on refractive index changes that are commonly utilized for the identification of biomolecules in their natural forms.

Study and simulations of speckle effects on BRDF measurements at very high angular resolution

Goniospectrophotometer ConDOR designed by CNAM, France, measures BRDF at very high angular resolution (0.014°), comparable to that of human vision. This is achieved using a very collimated light beam and a dedicated Fourier lens. Interestingly, the BRDF of glossy surfaces measured at this resolution exhibits a granular aspect around the specular direction, like a rapid angular fluctuation, which is not detection noise as it is repeatable between successive measurements on the same sample. Based on our experiments using ConDOR as well as numerical simulations, we claim that this granular aspect comes from an optical effect called speckle, which occurs every time a sufficiently coherent light beam — which is inevitably the case when the incident solid angle is very narrow — strikes a rough surface. First elements of confirmation are given in this paper.