Axicon Focusing Research Articles

Interference effects in the formation of the light bullet spectrum under axicon focusing

Specific features of the anti-Stokes band of the light bullet supercontinuum spectrum under filamentation of 1900-nm femtosecond pulses, focused by an axicon in fused silica and calcium fluoride, have been experimentally, numerically, and analytically investigated. It is found that the formation of two or more light bullets in a filament leads to modulation of spectral components’ intensity. The oscillation period of spectral components, defined as the wavelength interval between adjacent intensity minima, decreases with decreasing their wavelength. On a logarithmic scale, the relative change in the oscillation period in the spectrum is proportional to the spectral component wavelength and is independent of the pulse energy and the position of the light bullet formation region in the filament. It is established that the modulation in the spectrum is due to the interference of the broadband supercontinuum emitted by light bullets, and its period decreases with increasing interval between them.

New Approach for Pulsed-Laser Testing That Mimics Heavy-Ion Charge Deposition Profiles

A novel approach for two-photon absorption (TPA) pulsed-laser testing produces extended charge deposition profiles that are analogous to those produced by heavy ions. In this approach, which utilizes an axicon rather than a spherical lens, the conventional Gaussian beam is replaced by a quasi-Bessel beam. The key feature of a quasi-Bessel beam, relevant to pulsed-laser single-event effects (PL-SEE) studies, is that its radial size ( $\sim 1~\mu \text{m}$ ) remains constant over the axial length of the beam (several 100 s of $\mu \text{m}$ ) resulting in a charge-deposition profile that more closely mimics that produced by a heavy ion. This elongated charge distribution directly lends itself to a simple and intuitive description in terms of linear-energy transfer (LET), facilitating laser/ion correlation studies. Experimental results comparing a conventional TPA PL-SEE focusing geometry and the axicon focusing geometry are presented for three different test vehicles: a 45-nm silicon-on-insulator (SOI) nFET, a bulk silicon photodiode, and an LM124 operational amplifier. By using the axicon approach, strong laser/ion correlation is observed in the silicon photodiode for single-event transients (SET) across a wide range of LETs. The correlation is achieved by “tuning” the laser to the desired LET rather than relying on preexisting heavy-ion data, and therefore, this approach holds promise as a predictive tool.

Dispersion of a Femtosecond Wave Packet Focused by an Axicon. A Scaling Parameter

We investigate the propagation of a femtosecond wave packet focused by an axicon in the linear medium. We introduce a scaling parameter for the problem of the Bessel–Gaussian beam formation in the femtosecond light. This parameter is based on a multiplicative representation for spatiotemporal distribution of the light-field complex amplitude. In this approach, the systematic error for estimating the wave-packet energetic parameters is determined. The approximate method presented can be used for analyzing a femtosecond laser-pulse filamentation under axicon focusing.

Plasma channels in a filament of a femtosecond laser pulse focused by an axicon

We report the results of experimental and numerical investigation of the influence of the wavefront curvature of femtosecond light focused by an axicon on the length and position of plasma channels in the filament under conditions of normal and anomalous group velocity dispersion in fused silica. It is shown that a change in the wavefront curvature by a value much greater than the longitudinal dimensions of the filament noticeably changes the geometry of the plasma channel position. The role of axicon focusing for ordering multiple filamentation is studied experimentally.

Optimized Shear Wave Generation Using Hybrid Beamforming Methods

Elasticity imaging is a medical imaging modality that measures tissue elasticity as an aid in the diagnosis of certain diseases. Shear wave-based methods have been developed to perform elasticity measurements in soft tissue. These methods often use the radiation force mechanism of focused ultrasound to induce shear waves in soft tissue such as liver, kidney, breast, thyroid and skeletal muscle. The efficiency of the ultrasound beam in producing broadband extended shear waves in soft tissue is very important to the widespread use of this modality. Hybrid beamforming combines two types of focusing, conventional spherical focusing and axicon focusing, to produce a beam for generating a shear wave that has increased depth-of–field (DOF) so that measurements can be made with a shear wave with a consistent wave front. Spherical focusing is used in many applications to achieve high lateral resolution, but has low DOF. Axicon focusing, with a cone-shaped transducer, can provide good lateral resolution with large DOF. We describe our linear aperture design and beam optimization performed using angular spectrum simulations. We performed a large parametric simulation study in which we varied the focal depth for the spherical focusing portion of the aperture, the numbers of elements devoted to the spherical and axicon focusing portions of the aperture and the opening angle used for axicon focusing. The hybrid beamforming method was experimentally tested in two phantoms, and shear wave speed measurement accuracy and DOF for each hybrid beam were evaluated. We compared our results with those for shear waves generated using only spherical focusing. The results of this study indicate that hybrid beamforming is capable of producing a beam with increased DOF over which accurate shear wave speed measurements can be made for different-size apertures and at different focal depths.

Fabrication of microcapillaries in fused silica using axicon focusing of femtosecond laser radiation and chemical etching

Fabrication of microcapillaries with a diameter of and a length up to in fused silica by axicon focusing of femtosecond laser radiation and subsequent chemical etching in a hydrofluoric acid solution is demonstrated. The etching rate is . It is shown that the microcapillaries have optical waveguiding properties, which testifies to the optical quality of the walls of obtained structures.

Shear wave generation using hybrid beamforming methods

Elasticity imaging is a medical imaging modality that measures tissue elasticity to aid in diagnosis of certain diseases. Shear wave-based methods have been developed to perform elasticity measurements in soft tissue. Hybrid beamforming applies both the conventional spherical and axicon focusing to produce a beam for generating a shear wave with increased depth-of-field so that measurements can be made with a plane-like shear wave. We present our aperture design and beam optimization performed using Field II simulations. We varied the number of elements devoted to spherical and axicon focusing as well as the opening angle used for axicon focusing. We tested hybrid beamforming in three elastic phantoms and an excised kidney. We evaluated the shear wave speed measurements accuracy in the phantoms as well as the depth-of-field for each hybrid beam. We compared our results with those from using beams generated using spherical and axicon focusing. Our results show that hybrid beamforming is capable of producing a long narrow beam that performs well when among the 128 elements of transducer, 48 elements are allocated to each axicon portion and 32 elements for spherical aperture while the angle of the axicon aperture is set less than 20.

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A theoretical model developed for the terahertzradiation generation includes the nonlinear interaction of a femtosecond pulse with an ionized gas and the excitation of transverse oscillations of the plasma column. The results of the theoretical analysis are compared with the experimental data.

Generation of long plasma channels in air by focusing ultrashort laser pulses with an axicon

We show that by focusing ultrashort-pulsed laser beams in air with an axicon, relatively long plasma channels can be generated. The axicon generates Bessel-like beams, where the on-axis intensity stays high over distances much longer compared to focusing with conventional lenses. We developed a scheme to detect the presence of the plasma, based on its screening property. Using this scheme, we detected plasma channels longer than 1 m and 3.5 m generated by 8 mJ and 90 mJ input pulse energies, respectively. Our simulations show that axicon focusing can yield self-guided propagation with or without contribution of plasma, depending on the input pulse power.

Formation of extended plasma channels in a condensed medium upon axicon focusing of a femtosecond laser pulse

The formation of plasma channels of a femtosecond laser pulse in the bulk of fused silica is studied by numerical simulation, and the advantages of using a conical lens (axicon) over conventional parabolic lenses are shown. It is found that the length of the plasma channel formed with the help of an axicon exceeds the length of the channel formed upon lens focusing.

Shock-wave generation upon axicon focusing of femtosecond laser radiation in transparent dielectrics

It is shown experimentally that the axicon focusing of intense femtosecond laser pulses in transparent dielectrics leads to efficient excitation of shock waves. A method is developed for measuring the dynamics of shock waves, which uses a frequency-chirped probe pulse and has high spatial (∼1 μm) and time (∼10 ps) resolutions. The initial stage of the evolution of an intense (up to 10 GPa) shock wave is studied by this method.

Terawatt femtosecond Ti:sapphire laser system

A terawatt femtosecond laser system based on a most promising at present broadband laser medium, Ti-doped sapphire, is created. The system produces 80-fs pulses with energy of 80 mJ and a repetition rate of 10 Hz. The first applications of the laser system demonstrated in the paper are the formation of extended channels with the aspect ratio ~104 upon axicon focusing of laser radiation in transparent dielectrics and transformation of the pulse spectrum during propagation of the pulse in gas-filled low-pressure capillaries.

Experimental investigation of the influence of subterawatt femtosecond laser radiation on transparent insulators at axicon focusing

Experimental investigation of the influence of subterawatt femtosecond laser radiation on transparent insulators at axicon focusing

Eksperimental'noe issledovanie vozdeistviya subteravattnogo femtosekundnogo lazernogo izlucheniya na prozrachnye dielektriki pri aksikonnoi fokusirovke

Экспериментальное исследование воздействия субтераваттного фемтосекундного лазерного излучения на прозрачные диэлектрики при аксиконной фокусировке, Бабин А.А., Киселев А.М., Правденко К.И., Сергеев А.М., Степанов А.Н., Хазанов Е.А.

Modeling of inverse C-caronerenkov laser acceleration with axicon laser-beam focusing.

Acceleration of free electrons by the inverse \ifmmode \check{C}\else \v{C}\fi{}erenkov effect using radially polarized laser light focused through an axicon [J. P. Fontana and R. H. Pantell, J. Appl. Phys. 54, 4285 (1983)] has been studied utilizing a Monte Carlo computer simulation and further theoretical analysis. The model includes effects, such as scattering of the electrons by the gas, and diffraction and interference effects of the axicon laser beam, that were not included in the original analysis of Fontana and Pantell. Its accuracy is validated using available experimental data. The model results show that effective acceleration is possible even with the effects of scattering. Sample results are given. The analysis includes examining the issues of axicon focusing, phase errors, energy gain, phase slippage, focusing of the $e$ beam, and emittance growth.

Focusing of Gaussian and super-Gaussian laser beams by axicons to obtain continuous laser sparks

Calculations are made of the laser radiation intensity in the caustic of an axicon. Calculations are used to develop methods for optimizing the focusing by matching the profiles of axicon focusing elements and the intensity distribution in the initial laser beam cross section. The results are used to determine the dimensions of continuous laser sparks obtained by axicon focusing of laser radiation in a gas.

The improvement and quantitative assessment of B-mode images produced by an annular array/cone hybrid.

Hybrid ultrasound imaging systems, which combine spherical focusing on transmit with axicon focusing on receive, provide excellent resolution over a useful depth of field. This paper presents a new hybrid design with improved sensitivity, in which the axicon focusing is achieved by two conical mirrors and a PZT 5A disk out into 8 sectors. We have investigated two methods of processing the signals from the 8 sectors. In the first, phase insensitive sector addition (PISA), the B-scan is formed from the sum of the 8 demodulated signals. In the second, multiplicative processing (MP), the 8 rf waveforms are multiplied and the resultant is demodulated to form the image. Both techniques result in smoothed speckle but degraded lateral resolution. As well, MP decreases the off-axis sensitivity of the system and artifacts characteristic of axicon focusing. Quantitative assessment of the effects of PISA and MP was performed using a new approach called contrast-to-speckle ratio (CSR). The CSR data, which is a measure of the image contrast of cylindrical voids in a random scattering medium relative to contrast fluctuations due to speckle, shows the superiority of PISA and MP. This conclusion is supported by images of in vitro human breast tissue.

The improvement and quantitative assessment of B-mode images produced by an annular array/cone hybrid

Hybrid ultrasound imaging systems, which combine spherical focusing on transmit with axicon focusing on receive, provide excellent resolution over a useful depth of field. This paper presents a new hybrid design with improved sensitivity, in which the axicon focusing is achieved by two conical mirrors and a PZT 5A disk out into 8 sectors. We have investigated two methods of processing the signals from the 8 sectors. In the first, phase insensitive sector addition (PISA), the B-scan is formed from the sum of the 8 demodulated signals. In the second, multiplicative processing (MP), the 8 rf waveforms are multiplied and the resultant is demodulated to form the image. Both techniques result in smoothed speckle but degraded lateral resolution. As well, MP decreases the off-axis sensitivity of the system and artifacts characteristic of axicon focusing. Quantitative assessment of the effects of PISA and MP was performed using a new approach called contrast-to-speckle ratio (CSR). The CSR data, which is a measure of the image contrast of cylindrical voids in a random scattering medium relative to contrast fluctuations due to speckle, shows the superiority of PISA and MP. This conclusion is supported by images of in vitro human breast tissue.