Beam Shaping Method Research Articles

Feasibility of intercostal high intensity focused ultrasound ablation of clinically relevant volumes under the application of beam shaping

Obstruction by the thoracic cage of the high intensity focused ultrasound (HIFU) beam is one of the major challenges in the ablation of abdominal lesions. Several beam shaping methods have been proposed to reduce exposure of the ribs to acoustic energy and to recover focal point intensity. The feasibility of HIFU ablation of a clinically relevant volume applying beam shaping remains however unaddressed. In this study, feasibility was evaluated based on exposure of both the ribs as well as the near-field to acoustic energy. In addition, the volumetric ablation rate was estimated to evaluate whether clinically relevant ablation speeds can be achieved.

Compound focusing mirror and X-ray waveguide optics for coherent imaging and nano-diffraction.

A compound optical system for coherent focusing and imaging at the nanoscale is reported, realised by high-gain fixed-curvature elliptical mirrors in combination with X-ray waveguide optics or different cleaning apertures. The key optical concepts are illustrated, as implemented at the Göttingen Instrument for Nano-Imaging with X-rays (GINIX), installed at the P10 coherence beamline of the PETRA III storage ring at DESY, Hamburg, and examples for typical applications in biological imaging are given. Characteristic beam configurations with the recently achieved values are also described, meeting the different requirements of the applications, such as spot size, coherence or bandwidth. The emphasis of this work is on the different beam shaping, filtering and characterization methods.

Beam shaping design for compact and high-brightness fiber-coupled laser-diode system.

Fiber-coupled laser diodes have become essential sources for fiber laser pumping and direct energy applications. A compact and high-brightness fiber-coupled system has been designed based on a significant beam shaping method. The laser-diode stack consists of eight mini-bars and is effectively coupled into a standard 100μm core diameter and NA=0.22 fiber. The simulative result indicates that the module will have an output power over 440W. Using this technique, compactness and high-brightness production of a fiber-coupled laser-diode module is possible.

Performance analysis of the beam shaping method on optical auditory neural stimulation in vivo.

Previous research has shown that infrared neural stimulation (INS) could be an alternative approach to evoke auditory neural activities. The laser beam property of the fiber output is a considerable aspect of INS, and the corresponding effects on auditory responses in vivo deserve further discussions. The paper presents a beam-shaped infrared laser stimulation method of auditory nerves. Pulsed 980-nm fiber-coupled laser systems were used as the radiant sources. The gradient reflective index (GRIN) lens was added at the port of the optical fiber as a beam shaping structure. The laser spot sizes and properties between the beam-shaped output and the bare fiber output were preliminarily analyzed by a laser beam profiler. And further experiments in vivo with four deafened adult guinea pigs were conducted. Optically evoked auditory brainstem responses (OABRs) of the animal samples were recorded and compared under the two output configurations. The results show a decrease of the beam divergence compared to a bare output fiber, and the INS with a beam shaping design evokes above 13% increase on OABR amplitudes than the bare fiber output, especially when enlarging the distance between the optical fiber and the nerve tissue. The beam shaping design can enhance the effect of INS for evoking auditory nerves, and it could be an optimized design in the future implementation of optical cochlear implants.

High brightness beam shaping and fiber coupling of laser-diode bars.

The strong beam quality mismatch in the fast and slow axes of laser-diode bars requires a significant beam shaping method to reach the parameters needed for fiber coupling. An effective solution to this problem is proposed that is based on a right-angle prism array and a distributed cylinder-lens stack. Coupling 12 mini-bars into a standard 100μm core diameter and 0.15 numerical aperture fiber is achieved, and the output power can reach 400W. Using this technique, production of compact and high brightness fiber-coupled laser-diode modules is possible.

Beam shaping system based on a prism array for improving the throughput of a dispersive spectrometer.

A beam shaping system (BSS) for improving the throughput of a dispersive spectrometer is presented by employing two anamorphic lenses and a prism array to segment the beam. The BSS was designed based on the inverse method of beam shaping for laser diode bars and the means of an optical slicer. In an experiment, a BSS was set up so that the incident light of a neon lamp with a circular spot from an input fiber was transformed into an elliptical spot coupled into a slit of a spectrometer without a change of divergence. Spectral measurement results demonstrate that the throughput of the dispersive spectrometer was doubled without loss of spectral resolution. The BSS can be combined with the existing dispersive spectrometer to improve the luminous flux and signal-to-noise ratio.

Controlling the optical fiber output beam profile by focused ion beam machining of a phase hologram on fiber tip.

A phase hologram was machined on an optical fiber tip using a focused ion beam (FIB) system so that a ring-shaped beam emerges from the fiber tip. The fiber used for this work was a commercial single-mode optical fiber patch cable for a design wavelength of 633nm with a germanosilicate core. The ring-shaped beam was chosen to ensure a simple geometry in the required phase hologram, though the Gerchberg-Saxton algorithm can be used to calculate a hologram for an arbitrary beam shape. The FIB machining took approximately 45 min at 30kV and 200pA. The radius of the resulting ring beam was 0.083m at 1m standoff, as compared to 0.1m as was initially desired. Results suggest that this imaging technique may provide a basis for a beam-shaping method with several advantages over the current commercial solutions, having permanent alignment, compactness, and mechanical robustness. However, it would appear that minimizing the speckle pattern will remain a critical challenge for this technique to become widely implemented.

Femtosecond laser fabrication of monolithically integrated microfluidic sensors in glass.

Femtosecond lasers have revolutionized the processing of materials, since their ultrashort pulse width and extremely high peak intensity allows high-quality micro- and nanofabrication of three-dimensional (3D) structures. This unique capability opens up a new route for fabrication of microfluidic sensors for biochemical applications. The present paper presents a comprehensive review of recent advancements in femtosecond laser processing of glass for a variety of microfluidic sensor applications. These include 3D integration of micro-/nanofluidic, optofluidic, electrofluidic, surface-enhanced Raman-scattering devices, in addition to fabrication of devices for microfluidic bioassays and lab-on-fiber sensors. This paper describes the unique characteristics of femtosecond laser processing and the basic concepts involved in femtosecond laser direct writing. Advanced spatiotemporal beam shaping methods are also discussed. Typical examples of microfluidic sensors fabricated using femtosecond lasers are then highlighted, and their applications in chemical and biological sensing are described. Finally, a summary of the technology is given and the outlook for further developments in this field is considered.

Hollow metallic waveguides integrated with terahertz quantum cascade lasers.

We present the realization of a compact, monolithically integrated arrangement of terahertz quantum cascade lasers with hollow metallic cylindrical waveguides. By directly mounting a copper pipe to the end facet of a double metal waveguide, it was possible to significantly improve the far field emission from such a sub-wavelength plasmonic mode, while preserving the characteristic performance of the laser. Careful alignment of the quantum cascade laser and the hollow waveguide is required in order to prevent the excitation of higher order/mixed modes as predicted with a high degree of accuracy by a theoretical model. Finally, this approach proved to be a superior method of beam shaping when compared to other in situ arrangements, such as a silicon hyper-hemispherical lens glued to the facet, which are presented.

Beam shaping of laser diode stacks for compact and efficient illumination devices at the French-German Research Institute of Saint-Louis

Abstract Laser diode stacks are applied for the realization of compact and efficient laser illumination devices for active imaging (e.g., laser gated viewing). An efficient use of illumination power and sensor arrays has to adapt the laser illumination to the sensor’s field of view (FOV) with a perfectly matched and homogeneous illumination field. In the past decades, ISL has studied different methods for beam shaping and homogenization of laser diode stacks. In this publication, the authors give a review of the development of different beam-shaping techniques for auto-stack, minibar stack in a specific configuration and standard laser diode stacks. The presented methods are based on the application of wedge-type waveguides, fast axis and slow axis collimation, and the rearrangement of the laser beams by polarization overlapping and virtual restacking. All presented methods have very high transmission efficiencies (>80%) and lead to a homogeneous illumination matched to the sensor’s field of view.

Laser diode stack beam shaping for efficient and compact long-range laser illuminator design

Abstract Laser diode stacks are interesting laser sources for active imaging illuminators. They allow the accumulation of large amounts of energy in multi-pulse mode, which is best suited for long-range image recording. Even when the laser diode stacks are equipped with fast-axis collimation (FAC) and slow-axis collimation (SAC) micro-lenses, their beam parameter products BPP are not compatible with direct use in highly efficient and compact illuminators. This is particularly true when narrow divergences are required such as for long-range applications. A solution to overcome these difficulties is to enhance the poor slow-axis BPP by virtually restacking the laser diode stack. We present a beam shaping and homogenization method that is low-cost and efficient and has low alignment sensitivity. After conducting simulations, we have realized and characterized the illuminator. A compact long-range laser illuminator has been set up with a divergence of 3.5×2.6 mrad and a global efficiency of 81%. Here, a projection lens with a clear aperture of 62 mm and a focal length of 571 mm was used.

Precise shaping of laser light by an acousto-optic deflector

We present a laser beam shaping method using acousto-optic deflection of light and discuss its application to dipole trapping of ultracold atoms. By driving the acousto-optic deflector with multiple frequencies, we generate an array of overlapping diffraction-limited beams that combine to form an arbitrary-shaped smooth and continuous trapping potential. Confinement of atoms in a flat-bottomed potential formed by a laser beam with uniform intensity over its central region confers numerous advantages over the harmonic confinement intrinsic to Gaussian beam dipole traps and many other trapping schemes. We demonstrate the versatility of this beam shaping method by generating potentials with large flat-topped regions as well as intensity patterns that compensate for residual external potentials to create a uniform background to which the trapping potential of experimental interest can be added.

Simultaneous compensation for aberration and axial elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective

One of the challenges in laser direct writing with a high numerical-aperture objective is the severe axial focal elongation and the pronounced effect of the refractive-index mismatch aberration. We present the simultaneous compensation for the refractive-index mismatch aberration and the focal elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective. By the use of circularly polarized beam illumination and a spatial light modulator, a complex and dynamic slit pupil aperture can be produced to engineer the focal spot. Such a beam shaping method can result in circularly symmetric fabrication along the lateral directions as well as the dynamic compensation for the refractive-index mismatch aberration even when the laser beam is focused into the material of a refractive index up to 2.35.

A high-efficiency and accuracy holographic tandem method for arbitrary beam shaping

A holographic tandem method is proposed for creating double-holograms to diffract a central symmetric laser beam into a beam with arbitrary intensity distribution. The performance of the method is quantitatively evaluated using diffraction efficiency (DE) and root mean square error (RMSE). Simulation results indicate that more than 92% of the incident laser energy is concentrated into the predicted intensity distribution within the desired region. And the reconstructed intensity distribution agrees well with the target distribution with a fractional error at the percent level. The technique proposed in this paper provides not only a promising convenient beam shaping for laser fusion and laser precision machining but also a potential path to generate arbitrary intensity distribution for atom traps and optical image.

A tutorial on optics for ultrafast laser materials processing: basic microprocessing system to beam shaping and advanced focusing methods

Abstract Ultrafast lasers have excellent characteristics for materials processing in terms of precision and quality. This tutorial paper introduces the basic concepts of ultrafast laser materials processing and the structure and key components of typical ultrafast laser microprocessing systems. The emphasis is on the pulse-shaping devices and systems for controlling and manipulating the spatial, temporal, and polarization properties of focused femtosecond laser pulses.

Self-reconstruction of diffraction-free and accelerating laser beams in scattering media

We experimentally investigate propagation of laser beams with different intensity profiles in highly scattering media. We generate transverse laser amplitude profiles with Gaussian, Bessel and Airy function envelopes. We then propagate these beams through optical phantoms formed with variable density intralipid solutions. At the sample exit, we compare change in maximum intensities, as well as beam profile reconstruction. We show that self-reconstruction properties of Bessel and Airy beams bring about slower decrease in maximum intensity with increasing scatterer density. On the other hand, the beam profiles deteriorate faster, as compared to reference Gaussian beams. Slower decrease in the intensity can be attributed to the wavevector spectra providing a continuous flow of energy to the beam center, while beam deterioration is linked to total beam volume in the scattering medium. These results show that beam shaping methods can significantly enhance delivery of intense light deeper into turbid media, but this enhancement is compromised by stronger speckling of beam profiles.

Focused ion beam fabrication of curved structures using the concept of beam shaping and variable dwell time

This paper introduces a technique for the direct fabrication of curved structures using focused ion beams called beam shaping method (BSM). It is based on the concept of variable dwell time and simulations of sputtering and redeposition to shape overall ion dose profile which is composed of individual focused single pixel beam profile. The pixel dwell times are kept small enough to avoid sputtering yield enhancement due to a locally inclined surface. This provides better control over the fabrication process and allows the combination of the individual overlapped beams (separated by the pixel spacing) into a dose profile for use in the simulation. Varying the dwell time at each pixel we optimize the dose profile for the fabrication of the desired structure using repeated simulation. We demonstrate this technique on a silicon substrate by realizing fundamental structures such as circular and triangular optical gratings. The resultant fabricated structures agree well with the desired geometry within tolerance. Detailed explanation of the technique with experimental verifications is given, and the optimized ion dose profiles are discussed.

Subwavelength plasmonic lens patterned on a composite optical fiber facet for quasi-one-dimensional Bessel beam generation

We demonstrate a beam shaping method by engraving plasmonic lens consisted of subwavelength slit-metallic groove structure on the cleaved end facet of a composite optical fiber. The fiber consisted of a single mode optical fiber serially concatenated with a coreless silica fiber segment to expand the beam diameter. The subwavelength slit-groove structure on a gold film was fabricated using focused ion beam milling process. Plasmonic interaction in the film generated a quasi-one-dimensional Bessel-like beam with a beam width of 0.8 μm at the focal length of 2 μm. Detailed two- and three-dimensional finite-difference time-domain analyses showed beam characteristics consistent with experimental observations.

SU‐GG‐J‐72: Study of Constant Beam Shaping Method in Tumor Tracking Radiotherapy during Respiration

Purpose: To evaluate the tumor tracking radiotherapy without consideration for the deformation of moving tumor. Method: Four dimensional computed tomography (4D CT) corresponding to every 10% phase intervals were acquired for 4 lung and 4 liver cancer patients treated in our institution and the initial treatment planning was established for the end of inhalation phase (phase 1). Results: As a simulation of deformation‐free tumor tracking radiotherapy, the beam center of initial plan was moved to the tumor center in other phases and the constant tumor shape acquired from phase 1 was assumed in all phases. The feasibility of the proposed method was analyzed based on various dosimetric tools such as equivalent uniform dose (EUD), homogeneity index (HI), coverage index (COV), etc. In photon radiation treatment, the movement‐induced dose reduction was not that critical showing 0.5%, 17.3% and 2.8% as the average variation of EUD, HI, COV, respectively. This difference become greater in proton treatment, which shows the change of 0.3%, 40.5%, 2.2% compared to initial EUD, HI, COV, respectively. Conclusion:These results suggest that proton is more sensitive than photon in tumor movement and the deformation of tumor may be disregarded in tumor tracking radiotherapy for photon treatment.

Beam-shaping method for uniform illumination by superposition of tilted Gaussian beams

Efficient beam shaping with uniform illumination is required in laser active imaging and detecting systems. In this paper, a beam-shaping method by superposition of tilted fundamental-mode Gaussian beams to produce nearly uniform illumination is proposed. The analytical formula for the average intensity of propagating multitilted Gaussian beams (MTGBs) is derived and used to study the uniformity of the beam profile with Fourier analysis. It is found that the MTGB maintains its flat-topped shape as it propagates and its uniformity is controlled by the parameters of fundamental-mode beams and superposition. Numerical examples are given. The method is used to reconfigure the beams of a high-power laser diode stack to form a MTGB shape for illumination of a 9×11.6-deg angular field. The simulation results from ASAP software show a shaping efficiency of 75%, and the rms deviation of the irradiance distribution from a flat-top shape is approximately 8%.