Showing papers on "Bessel beam published in 2008"

High quality quasi-Bessel beam generated by round-tip axicon

Abstract: We study theoretically and experimentally the spatial intensity distribution of the zero-order Bessel beam formed by the axicon which possess a rounded tip. Such a tip generates a refracted beam that interferes with the quasi-Bessel beam created behind the axicon. In turn an undesired intensity modulation occurs that significantly disturbs the unique properties of the quasi-Bessel beam--namely the constant shape of the lateral intensity distribution and the slow variation of the on-axis beam intensity along the beam propagation. We show how the spatial filtration of the beam in the Fourier plane improves this spatial beam distribution and removes the undesired modulation. We use an efficient numerical method based on Hankel transformations to simulate the propagation of the beam behind the axicon and filter. We experimentally measure the intensity distribution of the beam in many lateral planes and subsequently reconstruct the spatial intensity distribution of the beam. Computed and measured beam distributions are compared and the obtained agreement is very good.

Bessel beam spectral-domain high-resolution optical coherence tomography with micro-optic axicon providing extended focusing range

Abstract: Endoscopic imaging in tubular structures, such as the tracheobronchial tree, could benefit from imaging optics with an extended depth of focus (DOF) to accommodate the varying sizes of tubular structures across patients and along the tree within the same patient. Yet the extended DOF needs to be accomplished without sacrificing resolution while maintaining sufficient sensitivity and speed of imaging. In this Letter, we report on the measured resolution and sensitivity achieved with a custom-made micro-optic axicon lens designed to theoretically achieve an 8 mm DOF. A measured invariant resolution of ~8 μm is demonstrated across a 4 mm measured DOF using the micro-optic axicon while achieving an invariant sensitivity of ~80 dB with a 25 mW input power. Double-pass Bessel beam spectral-domain optical coherence tomography with an axicon micro-optic lens (i.e., <1 mm in diameter) is, for the first time to our knowledge, demonstrated in a biological sample demonstrating invariant resolution and signal-to-noise ratio across a 4 mm measured DOF, which is compared to Gaussian beam imaging.

Filamentation in Kerr media from pulsed Bessel beams

Abstract: In contrast with filamentation of ultrashort laser pulses with standard Gaussian beams in Kerr media, three different types of Bessel filaments are obtained in air or in water by focusing ultrashort laser pulses with an axicon. We thoroughly investigate the different regimes and show that the beam reshapes as a nonlinear Bessel beam which establishes a conical energy flux from the low intensity tails toward the high intensity peak. This flux efficiently sustains a high contrast long-distance propagation and easily generates a continuous plasma channel in air.

Nondiffracting vortex beams with continuous orbital angular momentum order dependence

Abstract: We introduce a new class of nondiffracting fractional vortex beams that connect Bessel beams of successive order in a smooth transition. The new fractional-order beams preserve the same nondiffracting feature of Bessel beams of integer order, and their orbital angular momentum per photon can take any value in a continuous range. The propagation of the more physically realizable fractional-order Bessel–Gauss beams, i.e. fractional Bessel beams apodized by a Gaussian transmittance, through general ABCD optical systems is studied in detail and is complemented by the experimental generation of several instances of fractional beams which in turn confirms our theoretical predictions.

Exact and geometrical optics energy trajectories in twisted beams

Abstract: Energy trajectories, that is, integral curves of the Poynting (current) vector, are calculated for scalar Bessel and Laguerre–Gauss beams carrying orbital angular momentum. The trajectories for the exact waves are helices, winding on cylinders for Bessel beams and hyperboloidal surfaces for Laguerre–Gauss beams. In the geometrical optics approximations, the trajectories for both types of beam are overlapping families of straight skew rays lying on hyperboloidal surfaces; the envelopes of the hyperboloids are the caustics: a cylinder for Bessel beams and two hyperboloids for Laguerre–Gauss beams.

Scattering of a Bessel beam by a sphere: II. Helicoidal case and spherical shell example.

Abstract: In prior work [P. L. Marston, "Scattering of a Bessel beam by a sphere," J. Acoust. Soc. Am. 121, 753-758 (2007)] the partial wave series for the scattering by a sphere centered on a zero-order Bessel beam was derived. The present work extends the analysis of the far-field scattering to the case of a Bessel beam having an angular dependence on phase. The beam considered is an example of a helicoidal beam where "helicoidal" refers to a type of beam that possesses an axial null and has an azimuthal phase gradient. This type of beam is sometimes also referred to as an acoustic vortex. The beam considered here has a phase ramp equal to the azimuthal angle. In agreement with symmetry arguments given previously, the backward scattering and forward scattering vanish for all frequencies. Some of the resulting modifications of the scattering are illustrated for a rigid sphere and an evacuated steel shell in water. For some directions and choices for the frequency, the calculated scattering by the shell increases when shifting to a helicoidal beam illumination.

Tunable generation of Bessel beams with a fluidic axicon.

Abstract: This paper describes a tunable fluidic conical lens, or axicon, for the generation and dynamic reconfiguration of Bessel beams. When illuminated with a Gaussian laser beam, our fluidic axicon generates a diverging beam with an annular cross section. By varying the refractive index of the solution that fills our device, we can vary easily the spatial properties of the resulting Bessel beam.

Phase matching with pulsed Bessel beams for high-order harmonic generation

Abstract: We propose a different approach to obtain phase-matched generation of high-order harmonics based on the use of pulsed Bessel beams as pump pulses. By means of the 'coherence map' technique, we show that it is possible to maximize the generation of a chosen harmonic of interest by properly adjusting the phase front tilt of the pulsed Bessel beam to compensate the mismatch arising from material and plasma dispersion and atomic phase.

Theory of the acoustic radiation force exerted on a sphere by standing and quasistanding zero-order Bessel beam tweezers of variable half-cone angles

Abstract: A rigorous theory is developed to predict the radiation force (RF) exerted on a sphere immersed in an ideal fluid by a standing or quasistanding zero-order Bessel beam of different half-cone angles. A standing or a quasistanding acoustic field is the result of counter propagating 2 equal or unequal amplitude zero-order Bessel beams, respectively, along the same axis. Each Bessel beam is characterized by its halfcone angle betalscr;lscr=1, 2 of its plane wave components, such that betalscr=0 represents a plane wave. Analytical expressions of RF are derived for a homogeneous viscoelastic sphere chosen as an example. RF calculations for a polyethylene sphere immersed in water are performed. Particularly, the half-cone angle dependency on the RF is analyzed for standing and quasistanding waves. Changing the half-cone angle is equivalent to changing the beamwidth. Potential applications include particle manipulation in microfluidic lab-on-chips as well as in reduced gravity environments.

Fast focus field calculations

Abstract: We present a method for fast calculation of the electromagnetic field near the focus of an objective with a high numerical aperture (NA). Instead of direct integration, the vectorial Debye diffraction integral is evaluated with the fast Fourier transform for calculating the electromagnetic field in the entire focal region. We generalize this concept with the chirp z transform for obtaining a flexible sampling grid and an additional gain in computation speed. Under the conditions for the validity of the Debye integral representation, our method yields the amplitude, phase and polarization of the focus field for an arbitrary paraxial input field in the aperture of the objective. Our fast calculation method is particularly useful for engineering the point-spread function or for fast image deconvolution. We present several case studies by calculating the focus fields of high NA oil immersion objectives for various amplitude, polarization and phase distributions of the input field. In addition, the calculation of an extended polychromatic focus field generated by a Bessel beam is presented. This extended focus field is of particular interest for Fourier domain optical coherence tomography because it preserves a lateral resolution of a few micrometers over an axial distance in the millimeter range.

Kerr-induced spontaneous Bessel beam formation in the regime of strong two-photon absorption.

Abstract: We study the effect of Two-Photon Absorption (TPA) nonlinear losses on Gaussian pulses, with power that exceeds the critical power for self-focusing, propagating in bulk kerr media. Experiments performed in fused silica and silicon highlight a spontaneous reshaping of the input pulse into a pulsed Bessel beam. A filament is formed in which sub-diffractive propagation is sustained by the Bessel-nature of the pulse.

Generation of Bessel Beams at mm- and Sub mm-wavelengths by Binary Optical Elements

Abstract: In this paper, binary optical elements (BOE’s) are designed for generating Bessel beams at mm- and sub mm- wavelengths. The design tool is to combine a genetic algorithm (GA) for global optimization with a two-dimension finite-difference time-domain (2-D FDTD) method for rigorous electromagnetic computation. The design process for converting a normally incident Gaussian beam into a Bessel beam is described in detail. Numerical results demonstrate that the designed BOE’s can not only successfully produce arbitrary order Bessel beams, but also have higher diffraction efficiencies when compared with amplitude holograms.

Total internal reflection of vector Bessel beams: Imbert–Fedorov shift and intensity transformation

Abstract: Total internal reflection of vector Bessel beams is studied. Transmitted beams are described by the evanescent fields, the energy fluxes of which show the shift of the reflected beam. As is well known, the Imbert–Fedorov shift is the lateral displacement of the plane wave leading the reflected beam out from the plane of incidence. Using an analogy with plane waves, the Imbert–Fedorov shift is introduced for the Bessel beams. This shift results in the intensity redistribution of the reflected beam compared with the incident one. The conditions of the Bessel beam intensity transformation from a squared Bessel beam function of the order m−1 (Jm−12-profile) to a squared Bessel beam function of the order m+1 (Jm+12-profile) are derived.

Diffraction-free beams with elliptic Bessel envelope in periodic media

Abstract: We report on discrete, nondiffracting, paraxial beams with a Bessel spatial envelope in 1D periodic structures of dielectric media. Anisotropy of the envelope profile is demonstrated to behave in the same manner as extraordinary waves in uniaxial crystals.

Properties of Approximate Bessel Beams at Millimeter Wavelengths Generated by Fractal Conical Lens

Abstract: An axicon, which images a point source into a line along the optic axis, is used widely to generate an approximation to a Bessel beam. More recently many novel axicons, such as Fresnel axicons, Fractal axicons and fractal conical lenses (FCLs), have been proposed. Understanding the properties of Bessel beams generated by these axicons is very helpful to research their applications. However, in optical region, all of them are calculated approximately by the scalar theory. To accurately analyze FCLs when illuminated by a plane wave at millimeter wavelengths, the rigorous electromagnetic analysis method, which combines a two-dimension finite-difference time-domain (2-D FDTD) method and Stratton-Chu formulas, is adopted in our paper. By using this method, the properties of approximate Bessel beams generated by FCLs are analyzed and the conclusions are given.

Modification of the structure of Bessel beams under oblique incidence

Abstract: The stability of a Bessel beam generated via an annular slit on the focal plane of a convergent lens as a function of the angle of incidence of a plane wave into the slit was measured. Although the paraxial approximation, along with an infinite lens assumption, predicts that the only change in the Bessel beam should be a displacement, experimental results showed that other slight changes were present. It was shown numerically that the way to take into account those changes is to abandon the quadratic phase approximation for the lens. Good agreement was then found between the experimental data and the numerical simulation.

Annular nanoantenna on fibre micro-axicon

Abstract: In this paper, we propose to extend the concept of loop antenna to the optical domain. The aim is to develop a new generation of optical nanocollectors that are sensitive to specific electric or magnetic vectorial field components. For validating our approach, a preliminary one-micron-diameter gold nanoring is micromachined on the apex of a cone lens obtained from a tapered optical fibre. It is shown that such a nano-object behaves as a nano-antenna able to detect the longitudinal electric field from a Bessel beam in radial polarization and the longitudinal magnetic component from a Bessel beam in azimuthal polarization. In the latter case, the annular nano-antenna exhibits the properties of an optical inductance.

Fabrication of photonic devices directly written in glass using ultrafast Bessel beams

Abstract: Optical waveguides have been inscribed in fused silica by focusing femtosecond laser pulses with an axicon The axicon is a conical lens that allows obtaining an optical beam with a transverse intensity profile that follows a zero-order Bessel function This profile is invariant along a certain distance (>1 cm) The advantage of using axicon is that the beam is focused along a narrow focal line of a few micron width Therefore the inscription of waveguides can be done without moving the glass sample The waveguides so fabricated exhibit low losses and no detectable birefringence due their excellent circular symmetry By translating the glass sample during the inscription process, we have induced a refractive index change along a thin plane in order to fabricate planar waveguides

Propagation of obstructed bessel and bessel-gauss beams

Abstract: The investigation into Bessel beams has been a topic of immense research during the past 20 years, due to the interesting properties they display. Bessel beams not only exhibit diffraction free propagation, but also reconstruction of the amplitude and phase of the beam after encountering an obstruction. Although this self reconstruction property has been previously modelled by numerous groups, the techniques involve rigorous, time-consuming computations. In this work we present an efficient method to accurately calculate the reconstruction of a Bessel beam after an arbitrary obstruction. Our method considers the well-known conical wave features of Bessel beams and looks at the projection of the obstruction in space as a result of the travelling conical waves that produce the Bessel beams.

First experimental demonstration of a Fresnel axicon

Abstract: Nondiffracting or Bessel beams find applications in diverse fields like optical tweezers/spanners, microscopy, super-resolution and optical coherence tomography. An axicon, energywise, is the most efficient method for generating a diffraction-free beam. Yet one of the impediments to wide use of these optical elements is the cost related to the way they are manufactured. Recently we proposed a novel optical element - Fresnel axicon (Opt. Lett. 31, 1890, 2006) - which enables to overcome this problem while providing advantages such as compactness and very low bulk absorption. Prototypes of the Fresnel axicon were manufactured. In the present work we report the first experimental results characterising the optical properties of this novel device, which bridges the gap between diffractive axicons and their refractive counterparts.

Generation of Bessel Beams at mm- and submm-Wave Bands Using Binary Optical Elements

Abstract: We apply binary optical elements (BOE's), designed by using a genetic algorithm (GA) for global optimization and a two-dimension finite-difference time-domain (2-D FDTD) method for rigorous electromagnetic computation, to generate Bessel beams at mm and submm-wave bands. Numerical results demonstrate that the designed BOE's can not only successfully produce arbitrary order Bessel beams, but also have higher diffraction efficiencies when compared with amplitude holograms.

Nonlinear unbalanced Bessel beams in the collapse of Gaussian beams arrested by nonlinear losses

Abstract: Collapse of a Gaussian beam in self-focusing Kerr media arrested by nonlinear losses may lead to the spontaneous formation of a quasi-stationary nonlinear unbalanced Bessel beam with finite energy, which can propagate without significant distortion over tens of diffraction lengths, and without peak intensity attenuation while the beam power is drastically diminishing.

Characteristics of a laser beam spot focused by a binary diffractive axicon

Abstract: Bessel beams formed by an axicon are nondiffracting beams. Such beams are used in the field of laser machining because they have several desirable characteristics, including a micron-sized focal spot and a deep focal depth. Conical lenses are generally employed for producing Bessel beams. A binary diffractive axicon was made by electron beam fabrication; the first-order diffracted beam from this axicon has a similar optical field to that of a refracted beam from a conical lens. The characteristics of a laser beam spot focused by the axicon were investigated. We demonstrated theoretically and experimentally that the peak intensity on the optical axis oscillated with a larger amplitude as the spot size decreased when the beam spot was observed in the vicinity of the axicon. This phenomenon was well explained by taking higher-order diffraction into account. We also investigated the wavelength dependence of the spot size focused by the axicon, and found that the spot size was constant.

Free-space delay lines and resonances with ultraslow pulsed Bessel beams.

Abstract: We investigate the ultraslow motion of polychromatic Bessel beams in unbounded, nondispersive media. Control over the group velocity is exercised by means of the angular dispersion of pulsed Bessel beams of invariant transverse spatial frequency, which spontaneously emerge from near-field generators. Temporal dynamics in transients and resonances over homogeneous delay lines (dielectric slabs) are also examined.