Brahim Chebbi

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.

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.

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.

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.

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.

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.

Measuring velocity with Bessel beam fringes

Abstract. We analyze theoretically, numerically, and experimentally the spectral response of scattered light intensity from moving particles crossing the fringes of a Bessel beam. This response could be the basis of a simple technique to measure velocity.

Learning disabilities and visual-motor skills; comparing assessment from a hapto-virtual reality tool and Bender-Gestalt test

Abstract Previous investigations conducted on post-secondary adult students with learning disabilities (LD) suggest that deficits in visual-motor skills contribute to difficulties in written expression which impact academic achievement. Intervention strategies for individuals with LD include assistive computer-based technologies (ATs) to compensate for or maximize performance. However, research fails to assess the impact of ATs on performance, learning, and motivation of students with LD. Also, one of the limitations of ATs is that they cannot be used for assessment and training and there are very few methods to assess or train visual-motor skills in this population. The present study explores the usefulness of a hapto-visual virtual reality motor skills assessment (MSA) device for visual-motor functioning in adults with and without LD. This is a preliminary step of developing an intervention to improve impaired visual-motor skills in adults with LD. A sample of 22 male and female university students with and without LD had their visual-motor skills pretested using a standard paper-and-pencil Bender-Gestalt (BG) test and were compared according to their performance on the MSA tool. We hypothesized that our LD participants’ performance would be significantly lower than our control participants on the VR task in terms of number of errors and speed. Results showed that participants without LD performed better and more rapidly on the VR task than participants with LD. There were no correlations between the BG and MSA performance. We did not find significant differences between the groups on the Bender-Gestalt scores, previous experience with video game, arousal, and mood. Our results suggest that a novel 3D virtual reality tool such as the MSA can potentially discriminate motor function of people with and without LD; however, the difference between both may also be due to a lack of problem-solving ability in LD.

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.

Reducing the divergence of a light sheet below diffraction limit

A quasi-nondiffracting 2D light beam is generated by placing a mask consisting of a pair of double slits on a cylindrical lens.