Interference of two intersecting Bessel beams and application to laser Doppler velocimetry.

We study the interference of two intersecting similar Bessel beams. The front waves of the two beams interfere to form a distinct pattern of parallel fringes superimposed on the circular concentric fringes of the Bessel beams. We show that the two sets of fringes at the intersection region can be used in a simple laser Doppler velocimetry system to measure two velocity components. Simulations and measurements are presented to characterize the fringes and prove the concept of two-component measurement.

Extension of the span and optimization of the optical "magic carpet": generation of a wide quasi-nondiffracting light sheet.

Light sheet illumination is the basis in developing light sheet microscopy (LSM), a technique with significant advantages compared with other classical techniques. Most proposed optical systems to generate light sheets for LSM use many optical elements, which require extensive adjustments and are costly; moreover, they generate a nonuniform or semiuniform light sheet or have a short depth of field (DOF). A simple scheme using a pair of double slits and a cylindrical lens for generating a quasi-nondiffracting 2D light sheet was reported in Opt. Lett.40, 5121 (2015)OPLEDP0146-959210.1364/OL.40.005121. In the present investigation, we elaborate on the optimization of the mask used. As the separation between the two slits increases, the light sheet becomes thinner and the DOF smaller and vice versa. The slits' width does not affect the light sheet thickness, but it does affect the intensity of the side lobes. For convergence angles of the inner slits from 0.75° to 8°, an optimum ratio of the slits' separation/width of 2.182 is recommended. The obtained light sheet is quasi-diffraction-free, namely, while its DOF is comparable with that of a Gaussian beam, its diffraction broadening is substantially smaller. We also add to the previously developed configuration a Powell lens in order to expand the beam in the spanwise direction while keeping nearly constant intensity in this dimension. We perform scalar diffraction theory calculations and conduct measurements showing the nearly constant intensity in the significantly broadened span of the light sheet. Potential applications for the augmented width include imaging of certain large embryos, laser micromachining, and microparticle image velocimetry.

Toward the optical "magic carpet": reducing the divergence of a light sheet below the diffraction limit.

In 3D, diffraction-free or Bessel beams are well known and have found applications in diverse fields. An analog in 2D, or pseudonondiffracting (PND) beams, is a nontrivial problem, and existing methods suffer from deficiencies. For example, Airy beams are not highly localized, some PND beams have significant side lobes, and a cosine beam has to be truncated by a very narrow aperture thus discarding most of the energy. We show, both theoretically and experimentally, that it is possible to generate a quasi-nondiffracting 2D light beam in a simple and efficient fashion. This is achieved by placing a mask consisting of a pair of double slits on a cylindrical lens. The applications include light sheet microscopy/optical sectioning and particle manipulation.

Development of spot size and lateral intensity distribution generated by exponential, logarithmic, and linear axicons.

Axicons are known to produce a nearly Bessel beam transverse intensity distribution. It has been previously recognized that linear axicons and logarithmic axicons with a predefined distant depth of field (DOF) and no blocking present a development region characterized by an enlarged central spot size. In this paper, we aim to obtain a better insight on the formation of lateral light distribution in this development region. We also examine the spot size and transverse intensity distribution of the recently developed exponential axicon. We present experiments and detailed nonparaxial numerical simulations for a plane wave passing through these optical elements and show that the lateral intensity distribution they generate differs from that of a Bessel beam for a significant part of their DOF. This anomaly/irregularity in the formation of nondiffracting Bessel beams has to be taken into account in applications of these axicons, such as imaging or optical trapping.

Characterization of a refractive linear axicon with distant depth of field and no central blocking.

We report the manufacturing and characterization of a refractive linear axicon producing a linearly increasing axial intensity Bessel-type beam over a predetermined range starting away from the axicon and without central blocking when illuminated by a plane wave. This is in contrast to a classical axicon that generates a diffraction-free beam starting from the axicon tip and extending to a range limited by the input beam aperture. The measured characteristics of the beam produced by the linear axicon, including its intensity distribution and spot size, are in good agreement with the theoretical predictions. Together with logarithmic axicon and exicon, this is another element of the axicon family that can generate a prescribed intensity distribution over a chosen range/depth of field.

Imaging properties of three refractive axicons.

The imaging properties of three types of refractive axicons are examined by using them in an imaging system. A linear axicon, a logarithmic axicon, and a Fresnel axicon are characterized by determining their point spread functions (PSFs) experimentally and by numerical simulation. The PSFs, which vary along the depth of field for the cases considered in the present investigation, are used in digital filters to denoise the images. A comparison of the imaging performance of these three optical elements is presented.

Demonstration of a Fresnel axicon.

We design and manufacture a Fresnel axicon (fraxicon) that generates a quasi-diffraction-free/Bessel beam with a large depth of field. The novel optical element is characterized with both coherent and incoherent light, and its behavior is compared with that of a classical axicon. While the fraxicon exhibits a strong interference pattern in the on-axis intensity distribution, it can be smoothed out when using broadband light, partial spatial coherence light, or by period randomization. As inexpensive, compact/lightweight, and low-absorption elements, fraxicons may find applications in imaging, illumination, and situations where low absorption and dispersion are important.

Characterization of a refractive logarithmic axicon.

We show that it is feasible to design and manufacture a refractive logarithmic axicon that generates a quasi-diffraction-free/Bessel beam with nearly constant beam size and intensity over a predetermined range. The novel optical element was characterized with both coherent and incoherent light, and good correspondence with the predicted behavior of the intensity distribution and spot size was found. The energy flow was also found to be nearly constant over most of the designed range. Logarithmic axicons may find applications in situations where large depth of field and uniform axial intensity/energy distributions are important.

Body Image and Anti-Fat Attitudes: An Experimental Study Using a Haptic Virtual Reality Environment to Replicate Human Touch.

It is well documented that anti-fat attitudes influence the interactions individuals have with overweight people. However, testing attitudes through self-report measures is challenging. In the present study, we explore the use of a haptic virtual reality environment to physically interact with overweight virtual human (VH). We verify the hypothesis that duration and strength of virtual touch vary according to the characteristics of VH in ways similar to those encountered from interaction with real people in anti-fat attitude studies. A group of 61 participants were randomly assigned to one of the experimental conditions involving giving a virtual hug to a female or a male VH of either normal or overweight. We found significant associations between body image satisfaction and anti-fat attitudes and sex differences on these measures. We also found a significant interaction effect of the sex of the participants, sex of the VH, and the body size of the VH. Female participants hugged longer the overweight female VH than overweight male VH. Male participants hugged longer the normal-weight VH than the overweight VH. We conclude that virtual touch is a promising method of measuring attitudes, emotion and social interactions.

The development of a haptic virtual reality environment to study body image and affect.

We report the results of a preliminary study testing the effect of participants' mood rating on visual motor performance using a haptic device to manipulate a cartoonish human body. Our results suggest that moods involving high arousal (e.g. happiness) produce larger movements whereas mood involving low arousal (e.g. sadness) produce slower speed of performance. Our results are used for the development of a new haptic virtual reality application that we briefly present here. This application is intended to create a more interactive and motivational environment to treat body image issues and for emotional communication.