Bessel-like Beams Research Articles

Asymmetric Shaping for Ultrafast Elliptical Bessel-like Beams

The generation of an elliptical Bessel–Gauss beam has become a topic of interest in ultrafast laser processing of transparent materials because of its nearly non-diffractive elliptical central core. These beams can show potential in generating anisotropic structures down to the nanoscale and in producing asymmetries in the induced fields of thermo-mechanical constraints relevant for material structuring. However, maintaining the central core ellipticity is a challenge that requires further analysis, notably in the propagation behavior of phase anisotropies during the conical interference. This paper presents the controlled generation and propagation of a highly elliptical Bessel–Gauss beam using asymmetric phase-modulation technique. The study involves engineering different asymmetric phase holograms and analyzing their performances in terms of the non-diffractive property and uniformity of the generated beams. We indicate the presence in specific cases of diffraction and its influence on the invariance of the beam shape. The simulation results are in excellent agreement with the experimental results, which verifies the accuracy and reliability of our approach.

Theoretical and experimental research on a spatially modulated femtosecond bessel-like laser for microdrilling in silica glass

Beam shaping has been widely used in laser processing. Compared with Bessel beam, Bessel-like beam has unique advantages of fabricating high-aspect-ratio holes due to its tunable and longer non-diffraction length. However, the theoretical and experimental optimization of Bessel-like beams for laser manufacture has remained to be investigated. In this paper, we established a theoretical model of the intensity distribution and peak energy density of the Bessel-like beams based on a simplified phase of arc axicon. It was found that the longer propagation length and the lower peak energy density of the Bessel-like beams, contributed to the longer ablation length and higher ablation precision in the silica glass. The ablation length of the optimized Bessel-like beams was at least 1.6 times longer than that of the Bessel beam, which furtherly improved the fabrication efficiency of microdrilling.

Extended depth of field of an imaging system with an annular aperture.

A common drawback of high-resolution optical imaging systems is a short depth of field. In this work, we address this problem by considering a 4f-type imaging system with a ring-shaped aperture in the front focal plane of the second lens. The aperture makes the image consist of nearly non-diverging Bessel-like beams and considerably extends the depth of field. We consider both spatially coherent and incoherent systems and show that only incoherent light is able to form sharp and non-distorted images with extraordinarily long depth of field.

Axicon metalens for broadband light harvesting

Abstract In this study, an axicon metalens comprising a large central disc surrounded by nanoposts for energy harvesting in composite metal-oxide semiconductor sensors was designed, fabricated, and experimentally characterized. The main role of the central disc is focusing light; the nanoposts of various diameters deflect light to form a Bessel-like beam. The spatial distribution of the optical transmission was measured using micro-hyperspectral imaging. The axicon metalens concentrates the light to the sensitive area of the sensor and also harvests light from adjacent pixels. After adding an axicon metalens, the normalized peak transmission is up to 250% at λ = 700 nm as compared to a blank TiO2 film. The experimental results had fair agreement with the finite-difference-time-domain simulation. The ultra-broadband energy-harvesting performance of the sensor suggests that it could be applied in surveillance and Internet of Things applications.

Optical transport over millimeter distances of a microscopic particle using a novel all-fiber Bessel-like beam generator

Whilst free space Bessel modes can show particle guidance over extended distances, this has been limited for fiber-based Bessel-like beams which have importance for microfluidic applications. We propose and experimentally demonstrate a novel all-fiber Bessel-like beam generator (BBG) that is shown transport a dielectric particle distance in excess of 2mm. This was achieved by optimizing the multimode interference (MMI) in the BBG structure to create a Bessel-like beam of appropriate propagation invariant length (PIL) and judicious choice of laser wavelength suppress thermal effects. By varying the diameter of the region where the MMI occurred we analyzed its impact on PIL and the transverse intensity profile of the Bessel-like beam. Our study paves the way for the fiber optic applications such as novel beam shaping, optical transport, and optical imaging.

High-speed cleaving of glass and polymers using ultrafast fiber laser

Glass cleaving by Bessel-like beams is one of the leading techniques providing high processing throughput and high edge quality. With the possibility to apply it to different transparent materials, this method has great potential in the high-volume production of flat panel displays, camera windows, medical devices, and others. The principle of glass cleaving is the controlled weakening of a hard transparent material by the induction of microcracks along a tight, elongated focus. These microcracks are oriented in a predefined, constant direction, limiting the flexibility of round shape fabrication. In contrast to dynamic beam manipulation techniques, the regime of direction-independent cutting was achieved through proper temporal pulse shaping. The setup does not consist of any active components in the beam path. The obtained quality and process speed are comparable with other, more complex laser cleaving techniques. Additionally, the successful cutting of polymers is presented, which are difficult to separate by the Bessel beam due to the limited possibility of microcrack generation.

Elastic orbital angular momentum transfer from an elastic pipe to a fluid

Research into the orbital angular momentum carried by helical wave-fronts has been dominated by the fields of electromagnetism and acoustics, owing to its practical utility in sensing, communication, and tweezing. Despite the huge research effort across the wave community, only recently has elastic orbital angular momentum been theoretically shown to exhibit similar properties. Here we experimentally observe the transfer of elastic orbital angular momentum from a hollow elastic pipe to a fluid in which the pipe is partially submerged, in an elastic analogue of Durnin’s slit-ring experiment for optical beams. This transfer is achieved by coupling the dilatational component of guided flexural waves in the pipe with the pressure field in the fluid; the circumferential distribution of the normal stress in the pipe acts as a continuous phased pressure source in the fluid resulting in the generation of Bessel-like acoustic beams. This demonstration has implications for future research into a new regime of orbital angular momentum for elastic waves, as well providing an alternative method to excite acoustic beams that carry orbital angular momentum that could create a paradigm shift for acoustic tweezing.

Generation of combined half-integer Bessel-like beams using synthetic phase holograms.

We discuss the generation of combined half-integer Bessel-like (CHB) beams using synthetic phase holograms (SPHs). We assess the efficiency and accuracy of the SPHs, in the task of generating CHB beams. The proposal is illustrated by the implementation of CHB beams, which are experimentally generated in a setup based on a phase spatial light modulator. Also, we analyze, numerically and experimentally, the propagation of the generated CHB beams. As the main result, the SPHs are able to generate several CHB beams with relatively high accuracy. Additionally, it is obtained that the efficiency values of the SPHs are close to the theoretical predictions.

Spatial Bessel-like beams along arbitrary convex trajectories based on a 3D-printed metasurface.

A 3D-printed all-dielectric metasurface is presented in this Letter which can generate an accelerating beam with a circularly symmetric non-spreading transverse profile that can propagate along arbitrary convex trajectories. The curved trajectory is mapped to the corresponding direct-space spatial phases by the basic cube units with different geometrical heights. The required phase distribution is derived in detail based on the enveloping theory of differential geometry and the Bessel beam generation method. A metasurface with a preset trajectory is simulated and measured to demonstrate the validity of the phase distribution calculated by the proposed theory. The full-wave simulation and measurement results verify that the Bessel-like beam whose intensity follows a curved (off-axis) trajectory can be produced by the proposed metasurface. The generated hybrid beam merges the advantages of non-accelerating and accelerating diffractive-free beams. Therefore, the proposed metasurface has great potential in ultrahigh-speed communication, secure communication, near-field imaging, wireless energy transmission applications, and so on. The all-dielectric characteristic provides the proposed metasurface with the competitive advantages of low cost and easy large-scale processing.

Enhancement of focusing properties of Bessel-like beam by metallic surface nanostructure

The focusing properties of a Bessel-like beam emitted from a circular slit surrounded by a surface groove (CSSG) were explored using experimental and numerical methods. The optical properties of the light emitted from the CSSG can be enhanced by using the scattered light from the surface groove. Furthermore, different modes of cylindrical surface plasmons in the circular slit are induced by different wavelengths of incident light, and beam shapes with a single spot and doughnut are created under illumination in the ultraviolet and visible regimes, respectively. A versatile beam shape is obtained from the CSSG structure.

Generation of diffraction-free petallike beams based on stationary phase principle

Lately, a kind of so-called “combined half-integer Bessel-like beams,” in which their subfamilies include radial carpet, petallike, and ringlike vortex beams, has been generated using a radial periodic grating. Different from classical petallike beams, we introduce and generate a kind of new petallike beams, that is, diffraction-free petallike beams (DFPBs). Their diffraction pattern is independent of the propagation distance. Firstly, the generating mechanism of diffraction-free petallike beams is theoretically investigated based on the stationary phase principle. Then, we theoretically construct a family of patterns of DFPBs. Subsequently, the DFPBs is generated by using an amplitude radial grating with a sinusoidal transmission function and two phase modulation elements — phase plate and axicon. Experimental results well confirm the theoretical predictions, thereby showing that the stationary phase principle is an effective approach to generating various diffraction-free combined half-integer Bessel-like beams based on radial grating, including diffraction-free radial carpet, petallike, and ringlike vortex beams.

Tunable high-order Bessel-like beam generation based on cross-phase modulation.

Nonlinear atomic media are promising substitutes for spatial light modulators (SLMs) owing to the high tunability and fast response. We demonstrate the generation of high-order Bessel-like beam based on cross-phase modulation in 85Rb atoms. The atomic medium, whose refractive index is spatially modulated by the focused Gaussian pump beam, acts as a nonlinear focusing lens for the Laguerre-Gaussian probe beam. As a result, the probe beam carries the nonlinear phase shift and is converted into a Bessel-like mode in far-field diffraction. The superior self-healing ability of the generated high-order Bessel-like beam is verified by inserting an obstruction in the beam path, and its high tunability is investigated in terms of the pump beam power and vapor temperature. Furthermore, this novel beam is used in an obstruction-immune rotation sensor to measure the angular velocity. Nonlinear atomic medium as a novel SLM promises considerable application prospects in modulating the light field structure.

Tunable depth of focus with modified complex amplitude modulation of an optical field.

Bessel beams have nondiffraction and self-healing properties in the propagation direction and are widely used in particle optical manipulation and optical microscopy. Bessel beams can be generated by axicons or spatial light modulators, which can produce a zero-order or high-order Bessel beam with different parameters depending on the specific application. The modulation of Bessel beams achieved in the spatial spectrum domain by optimization algorithms has a low light energy utilization rate due to the small effective modulation region. We propose a Bessel-like beam phase generation algorithm based on an improved iterative optimization algorithm directly in the spatial domain to achieve a tunable modulation of the beam's length and the axial center position. The optimization time is reduced from minutes to seconds relative to the genetic algorithm, providing a new means of modulation for different applications in various fields.

The formation of Bessel light beams at large distances from annular fields

In this work, the process of transformation of an annular beam in a Bessel-like field due to diffraction during propagation in a free space over long distances and due to the focusing effect is investigated. A number of models of annular fields are considered, including an analytical model in the form of a polynomial function in a bounded region of space, as well as an experimentally implemented model based on a scheme with two axicons. A comparison is made of the transverse and longitudinal intensity distributions for these models, and a high degree of stability of the structure of the longitudinal distribution of the axial intensity to a change in the model of the annular field is found. This distribution is characterized by the presence of an intense maximum with an asymmetric profile, the appearance of which is not connected with lens focusing. In the initial region of the pointed maximum, the process of formation of a Bessel beam from an annular beam arises, and a sharp increase in intensity takes place. It is also established that the focusing of an annular field at large distances essentially differs from focusing at short distances. In the case of large distances, the increase of the axial intensity does not take place in the vicinity of the focal plane, but much closer to the transmitter, and here the great increase of intensity caused by direct focusing is not identified. The transverse profile of a Bessel-like beam is calculated at large distances. It is shown that this profile is characterized by a small number of lateral rings, and the axial maximum and the first ring contain more than 90% of the light power. The problem of generation of a model annular field by a Fourier-type resonator with a special transparency mirror is considered.

Integrated all-fiber structures for generating doughnut beam arrays and hollow Bessel-like beams

Two all-fiber structures for generating doughnut beam arrays and hollow Bessel-like beams are demonstrated experimentally based on mode-selective couplers at the visible band. The mode-selective couplers are acted as mode converters to produce doughnut beams, which have the properties of high mode conversion efficiency (94%), low insertion loss (<0.5 dB) and wide bandwidth (∼100 nm). A compact and efficient approach is presented through integrating a tapered multicore fiber with a mode-selective coupler, and a doughnut beam array with nearly equal power distribution is obtained based on the mechanism of coupled-power theory in the tapered multicore fiber. The hollow Bessel-like beams with different cylindrical vector modes and orbital angular momentum are achieved by fabricating a polymer microtip at the facet of the few-mode fiber output port of a mode-selective coupler. The self-healing characteristics of a Bessel-like beam are investigated in detail and the application in an optical tweezer system of a hollow Bessel-like beam exhibits a longer focal depth than that of a Gaussian beam. Both methods used for generating all-fiber-based doughnut beam arrays and hollow Bessel-like beams have the advantages of compactness, flexibility, and easy integration with free-space optical systems. Generated two kinds of beams show huge potentials for the application fields of super-resolution microscopy, laser material process and optical trapping and so on.

>50 MW peak power, high brightness Nd:YAG/Cr4+:YAG microchip laser with unstable resonator.

We demonstrated a flat-convex unstable cavity Nd:YAG/Cr4+:YAG ceramic air-cooled microchip laser (MCL) generating a record 37.6 and 59.2 MW peak power pulses with an energy of 17.0 and 24.1 mJ and a width of 452 and 407 ps at 20 Hz by using a uniform power square and hexagon pump, respectively. For hexagon pump, the near field hexagon donut beam was changed in to a Bessel-like beam in far field, whose beam quality was estimated as 2nd moment M2 of 7.67. The brightness scale of unstable resonator MCL was achieved up to 88.9 TW/(sr·cm2) in contrast with flat-flat cavity MCL. However, the high intense center part of Bessel-like beam increased its brightness effectively more than 8 times, up to 736 TW/(sr·cm2).

Spherically polarized vector Bessel vortex beams

A different type of Bessel-like vortex beams with their intensity spherically modified by a singular polarization structure is introduced. Conventional vector Bessel-Gaussian beams are either linearly or radially (and azimuthally) polarized and their transverse structure is preserved under propagation. In this work, the Bessel-like beams with a Gaussian envelope are spherically polarized: the electric fields are oriented along the radius vector or along with a combination of meridional and azimuthal vectors. For the implementation of such beams, we investigate their vector spatial spectra. The intensity distribution of the beam can be controlled by a proper choice of parameters such as the cone angles of individual plane-wave components and sizes of Gaussian apertures. In the two limiting cases of axicon angles, spherically polarized Bessel beams form either two optical needles or optical-bottle-like intensity patterns.

Underwater Bessel-like beams with enlarged depth of focus based on fiber microaxicon

Underwater Bessel-like beams with enlarged depth of focus based on fiber microaxicon

Improved photopolymerization for fabricating fiber optical tweezers

In recent years, it has been a new topic to fabricate fiber optical tweezers probes by using photopolymers. However, this method requires coupling the laser into the fiber through a spatial optical path with strict laser and experimental environment requirements. It significantly limits the application scope of optical tweezers. This paper proposes and demonstrates an improved photopolymerization method to fabricate fiber optical tweezers based on an all-fiber structure, improve the fabrication process, and solve the above problems. We configure the all-fiber structure by coaxial welding a single-mode fiber and a multimode fiber to excite a Bessel-like beam. The power density distribution of the Bessel-like beam is advantageous for forming the photopolymerization tip that makes the output beam highly convergent. The fabrication method of the proposed fiber optical tweezers is economical and straightforward. Moreover, it provides a new way of improving the photopolymerization process.

Extended focal depth Fourier domain optical coherence microscopy with a Bessel-beam - LP02 mode - from a higher order mode fiber.

We present a robust fiber-based setup for Bessel-like beam extended depth-of-focus Fourier-domain optical coherence microscopy, where the Bessel-like beam is generated in a higher order mode fiber module. In this module a stable guided LP02 core mode is selectively excited by a long period grating written in the higher order mode fiber. Imaging performance of this system in terms of lateral resolution and depth of focus was analyzed using samples of suspended microbeads and compared to the case where illumination is provided by the fundamental LP01 mode of a single mode fiber. Illumination with the LP02 mode allowed for a lateral resolution down to 2.5 µm as compared to 4.5 µm achieved with the LP01 mode of the single mode fiber. A three-fold enhancement of the depth of focus compared to a Gaussian beam with equally tight focus is achieved with the LP02 mode. Analysis of the theoretical lateral point spread functions for the case of LP01 and LP02 illumination agrees well with the experimental data. As the design space of waveguides and long-period gratings allows for further optimization of the beam parameters of the generated Bessel-like beams in an all-fiber module, this approach offers a robust and yet flexible alternative to free-space optics approaches or the use of conical fiber tips.