quasi-Bessel Beam Research Articles

Efficient subwavelength focusing of light with a long focal depth.

We demonstrate an efficient method for far-field subwavelength focusing by a novel two-component axicon structure. Annular beams generated by a fiber axicon are focused using a micro-cone reflector, creating a quasi-Bessel beam with a high convergence angle of up to 40°. A center focal spot diameter of 0.41λ was achieved at a power efficiency of over 40%, with a focal depth of 9λ and a working distance as long as 35 μm. We further demonstrate that experimental knife-edge measurements mapping the beam focal intensity agree with numerical simulations of the structure. This method shows demonstrable promise in overcoming the optical focusing limit of single-element axicons and great potential for use in high tolerance, high-resolution applications in optical systems.

Laser printed fiber microlens for fiber-diode coupling by direct laser writing.

A printable microlens on fiber end for coupling between a laser diode to a single-mode fiber (SMF) is demonstrated. The fiber microlens fabricated by the novel technique based on laser direct writing using a quasi-Bessel beam has gained high coupling efficiency, long working distance, and sufficient alignment tolerance. A coupling efficiency of 53.5% was measured for SMF coupling at a working distance of 16μm. The tolerances for a 1-dB loss increment for translational displacements and angular deviations between the fiber microlens and the laser diode were 2.5 and 1.2μm, and 2.0 and 5.0 degrees, respectively. The printable fabrication of a microlens on a fiber endface by laser direct writing allows for a batch process to reproduce a multiple microlens with a high consistency in a simple and fast fabrication cycle, with no need of individual fiber loading and unloading.

Novel hydrodynamic instability of the molten Au/Pd alloy film irradiated by tightly focused femtosecond laser pulses

Abstract Features of nanoscale surface hydrodynamic instabilities induced on the thin Au/Pd alloy films by single femtosecond pulses shaped into the tightly focused quasi-Bessel beam by means of a fiber microaxicon were studied. The thickness of the metal film as well as the pulse energy were found to be the key parameters determined the types of the hydrodynamic instability of the molten film, which results in the formation of different frozen surface relief microstructures: nanojets, nanocrowns and hybrid structures. Single nanojets forms on the surface of the 80-nm-thick film at the pulse energies ranging from 6 to 7 nJ, while for thicker films formation of hybrid structure, a nanojet surrounded by the nanocrown, is observed. Single nanocrown can be fabricated at film thicknesses ranging from 120 to 240 nm at near-threshold pulse energies. The number of nanospikes of the nanocrown was found to be linearly dependent on the pulse energy and the inverse film thickness.

Single and Multiple Microparticle Trapping Using Non-Gaussian Beams From Optical Fiber Nanoantennas

Optical trapping of dielectric microparticles is reported using an optical tweezers based on two original chemically etched fiber nanoantenna. The nanoantenna converts Gaussian beam into nondiffracting type quasi-Bessel beam, which is used in trapping microparticles. Stable trapping in three distinct positions is observed for an antenna distance of 32.5 μm and for light powers as low as 1.3 mW. Optical trapping properties are studied by applying Boltzmann statistics to the particle position fluctuations. Harmonic trapping potentials with trap stiffness of 3.5 pN μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are observed. The FDTD simulation results on the antenna optics are also included to understand the trapping mechanism.

Fast and robust piezoelectric axicon mirror.

In this Letter, we demonstrate the first high-speed piezoelectric axicon mirror. We achieve a usable aperture of 10mm up to the maximum radius of the robust, 300μm thick mirror substrate using a floating boundary condition. The highly aspheric, conical shape is programmed into the device by ring-shaped electrodes, for which we have developed an automated optimization strategy for their individual electrode potentials. In addition, we developed a simple control circuit, in which the conical profile can be programmed and adjusted with just one control signal. The device is fabricated by rapid prototyping to avoid cleanroom processing. The tunable mirror features a resonance frequency of 10kHz and a static deflection of 5.8μm at a surface deviation of 63nm, and is thus able to generate a quasi-Bessel beam.

Theoretical analysis of a quasi-Bessel beam for laser ablation

A quasi-Bessel beam (QBB) is suitable for laser ablation because it possesses a micrometer-sized focal spot and long depth of focus simultaneously. In this paper, the characterizations of QBBs formed by the ideal axicon and oblate-tip axicon are described. Strong on-axis intensity oscillations occur due to interference between the QBB and the refracted beam by the oblate tip. Using the axicon for laser ablation was theoretically investigated. Simple analytical formulas can be used to predict the required laser parameters, including the laser pulse energy, the generated fluence distributions, and the beam diameters.

Shaping self-imaging bottle beams with modified quasi-Bessel beams.

Coherent generated self-imaging bottle beams, typically formed by interfering two coherent quasi-Bessel beams, possess a periodic array of intensity maxima and minima along their axial direction. In practice, the overall quality of the self-repeating intensity patterns is prone to unresolved large intensity variations. In this Letter, we increased consistency of intensity of self-imaging bottle beams through a spatial frequency optimization routine. By doing so, we increased the effective length of self-imaging bottle beams by 74%. Further, we showed that this approach is applicable to higher-order self-imaging beams that display complex intensity structures. The enhancement in these modified self-imaging beams could play a significant role in optical trapping, imaging, and lithography.

Tunable MEMS axicon mirror arrays

Highly aspheric reflective micro-optics for the generation of quasi-nondiffracting beams are of great interest for a wide variety of applications. However, up to now it was impossible to fabricate tunable arrays of these elements. In this Letter, we demonstrate the first array of purely reflective tunable microelectromechanical systems (MEMS) microaxicon mirrors with a conical shape and a continuous surface. The actuation is achieved by thermal expansion in a solid state design and the tuning range allows for large conical angles and is able to form concave as well as convex axicons. The deflection of the mirror surface and the propagation of the resulting quasi-Bessel beams have been characterized to prove the functionality of the device.

Foreign Object Detection by Sub-Terahertz Quasi-Bessel Beam Imaging

Food quality monitoring, particularly foreign object detection, has recently become a critical issue for the food industry. In contrast to X-ray imaging, terahertz imaging can provide a safe and ionizing-radiation-free nondestructive inspection method for foreign object sensing. In this work, a quasi-Bessel beam (QBB) known to be nondiffracting was generated by a conical dielectric lens to detect foreign objects in food samples. Using numerical evaluation via the finite-difference time-domain (FDTD) method, the beam profiles of a QBB were evaluated and compared with the results obtained via analytical calculation and experimental characterization (knife edge method, point scanning method). The FDTD method enables a more precise estimation of the beam profile. Foreign objects in food samples, namely crickets, were then detected with the QBB, which had a deep focus and a high spatial resolution at 210 GHz. Transmitted images using a Gaussian beam obtained with a conventional lens were compared in the sub-terahertz frequency experimentally with those using a QBB generated using an axicon.

Cylindrical symmetry breaking leads to multiple filamentation generation when focusing femtosecond lasers with axicons in methanol

We demonstrate that multiple filaments could be generated when focusing femtosecond laser pulses into methanol solution with an axicon. These long multiple filaments are located on the central spot and ring structures of the quasi-Bessel beam created by the axicon. Further numerical simulation reproduces the key features of the experimental observation. The outcome of simulation suggests that the cylindrical symmetry breaking in the initial beam profile could be responsible for the occurrence of multiple filamentation by using an axicon as focusing optics. Since the quasi-Bessel profile is determined by the axicon properties, the axicon has been suggested as a simple optics component to control multiple filaments.

Multiple filamentation generated by focusing femtosecond laser with axicon

Multiple filamentation has been observed when focusing a femtosecond laser pulse into a methanol solution with an axicon. It is found that multiple long filaments are located on the central spot and ring structures of the quasi-Bessel beam created by the axicon. Since the quasi-Bessel profile is determined by the axicon properties, the axicon has been suggested as a simple optics to control multiple filaments.

Kilohertz raman lasers based on bno and kgw crystals for a wide spectral range

Efficient Raman lasers based on Ba(NO3)2 and KGd(WO4)2 crystals which produce nanosecond pulses with repetition rates of 1 and 4 kHz at 26 wavelengths between 280 and 1600 nm are developed and studied. A maximum SRS conversion efficiency of 25% is obtained. It is found that the divergence of the beams generated by the Raman laser can be significantly reduced by using quasi-Bessel pump beams and an unstable telescope cavity.

Quasi Bessel beam by Billet's N-split lens

This study utilizes the focal property of a classical Billet's split lens to create more focal points by splitting the lens. This approach distributes the focal points circularly on the focal plane. This study explores the characteristics of beam propagation and analytically derives the asymptotic characteristics of beam propagation based on the stationary phase approximation and the moment-free Filon-type method. Results show that the unique Billet's N-split lens can generate a quasi Bessel beam if the number of splitting N is large enough, e.g., N ≧ 24. This study also explores the diffraction efficiency of corresponding quasi Bessel beam and the influence of aperture size. The potential advantage of proposed split lens approach is that, unlike the classical means of annular aperture, this simple lens approach allows a much larger throughput in creating the Bessel beam and hence the Bessel beam could have more optical energy.

Formation of expanded smooth quasi-Bessel beam range due to the transformation of the Gaussian beam in absorbing axicon

The quasi-Bessel beam range produced by an absorbing axicon is investigated. It is shown that when an incident Gaussian beam is used, the absorbing axicon stretches (with respect to transparent axicon case) the uniform on-axis light intensity distribution length, as well as expanding the quasi-Bessel beam range. In addition, the peak of axial light distribution is shifted from the axicon tip. As a result any spatial modulation formed due to the bluntness of the tip is reduced close to the on-axis intensity peak, which becomes more smooth and uniform.

Fabrication, characterization and analysis of a microaxicon

A microaxicon is fabricated in the quartz substrate by the combination of grayscale direct laser writing and inductively coupled plasma etching. The atomic force microscopy measurement result indicates each period of the fabricated quartz microaxicon has a fine continuous relief axicon pattern and the maximum height of each period remains quite consistent. The intensity distributions of the beam generated by the microaxicon are theoretically calculated by using the Fresnel diffraction integral and experimentally recorded with a CCD camera at different propagation distances, respectively. The results of theoretical calculation and optical measurement both show the quartz microaxicon can generate a high-quality quasi-Bessel beam and the working distance is tested as at least 10 mm.

Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams

We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.

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

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.

Production of an extended plasma column in vacuumby irradiating a target by a quasi-Bessel beam

A technique of focusing the heating radiation wasinvestigated, which makes it possible to produce an extended(under laboratory conditions, up to 1 m and over) plasmacolumn and enables an easy output of VUV radiation. A planesolid-state target in vacuum was arranged along the caustic ofa conic lens (axicon), which focused the laser beam. An analytic dependence, which describes the spatial intensity distribution of the heating radiation in the case of a nontransparent, partially reflecting target, was derived and experimentally verified. Experiments on the irradiation of an aluminiumtarget in vacuum with a 5-J, 5-ns pulse of a neodymium-glasslaser were performed. A plasma column up to 30 mm in lengthand no greater than 10 μm in diameter was formed. A ratherintense plasma radiation was recorded in the VUV range.

Generation of femtosecond Bessel beams with microaxicon arrays

Multiple quasi-Bessel beams are generated by transmission of sub-30-fs pulses from a Ti:sapphire laser through refractive thin-film microaxicon arrays. Time-integrated intensity distributions at several axial positions and for pulse durations of 26 and 12.5 fs reveal significant changes of contrast, envelope function, and spatial frequency spectrum in comparison with continuous wave data. Evidence is presented that strong space-time coupling results in a time-dependent interference zone.