Beam Transformation Research Articles

Graded-azimuthal-index fiber as a control element for radial-index and orbital-angular-momentum modes of Laguerre-Gaussian beams

We propose an azimuthally varying graded-index fiber that can efficiently create and control a definite set of radial-index and orbital-angular-momentum (OAM) modes of Laguerre-Gaussian beams and their superpositions. A rigorous coupled-wave analysis formalism is developed, which provides a ready analysis for the azimuthal-refractive-index variation in the Laguerre-Gaussian basis. In particular, we present effective control in the generation and coupling of the various OAM modes, including radial-index modes of the Laguerre-Gaussian beam within the fiber, wherein the propagation length of the fiber and the wavelength act as the control parameters. We further present the possibility of switching every alternate channel in a wavelength division multiplexing protocol to a higher-order spatial mode within the optical fiber to substantially reduce interchannel crosstalk. The graded azimuthal index fiber can be utilized in high-speed fiber-optic communications as a versatile control element for OAM modes and the radial-index mode of the Laguerre-Gaussian beam transformation.

Relativistic space-charge field calculation by interpolation-based treecode

Space-charge effects are of great importance in particle accelerator physics. In the computational modeling, tree-based methods are increasingly used because of their effectiveness in handling non-uniform particle distributions and/or complex geometries. However, they are often formulated using an electrostatic force which is only a good approximation for low energy particle beams. For high energy, i.e., relativistic particle beams, the relativistic interaction kernel may need to be considered and the conventional treecode fails in this scenario. In this work, we formulate a treecode based on Lagrangian interpolation for computing the relativistic space-charge field. Two approaches are introduced to control the interpolation error. In the first approach, a modified admissibility condition is proposed for which the treecode can be used directly in the lab-frame. The second approach is based on the transformation of the particle beam to the rest-frame where the conventional admissibility condition can be used. Numerical simulation results using both methods will be compared and discussed.

Coaxial dual-beam wavefront shaping using nonlocal diffractive metasurfaces in terahertz frequencies.

Metasurfaces for wavefront shaping rely on local phase modulation in subwavelength unit cells, which show limited degree of freedom in dealing with complex and multiple beam transformation. Here, we assign multiple beams into different diffraction orders coaxially located along the same direction, whose wavefronts are tailored by optimizing the diffraction coefficients in two orders and two polarization states of a supercell. By evenly splitting the energy into two orders and adjusting the zeroth-order diffraction phase, a Bessel beam and a vortex beam are simultaneously generated in the near field and far field along a coaxial direction. The effectiveness of the method is validated by the excellent agreement between the simulation and experimental characterization of the two beams.

Analyzing the spreading properties of vortex beam in turbulent biological tissues

Presenting the intensity development of a circular Laguerre-cosh-Gaussian (CLChG) beam in turbulent mouse biological tissues is the major goal of the current work. Using the power spectrum refractive index from Schmitt's model and the extended Huygens-Fresnel integral, the propagation formula of the CLChG beam is produced. In order to determine the spreading properties of the studied beam, analytical expressions of the CLChG beam's effective beam size in turbulent mouse biological tissues are constructed. Some graphical representations have to be carried out in order to discover the impacts of beam and biological turbulence parameters on this sort of beam. The findings show that the transformation of the CLChG beam into a Gaussian-like beam in the far field occurs more quickly when the beam passes through the deep dermis of the mouse. The shape of the CLChG beam can also be changed by choosing a specific value for the parameter linked to the cosh-part. Because the effective beam spot radius along the x- and y-axis are equal, we also see that the beam spot in biological tissues takes on a circular shape.

FORMATION OF MIXED X,n-RADIATION FIELD AT AN ELECTRON ACCELERATOR

For conducting photonuclear programs with the use of a high-intensity photon source the latter is ordinary obtained by transformation of an electron beam into X-ray radiation. Such a process is performed using an intermediate target-converter maid from a high-Z material. The (γ,n) reactions take place in the converter under the action of high-energy bremsstrahlung photons. As a result, a quasi-isotropic neutron flux escapes the converter jointly with the photon beam directed forward. The parameters of the both types of radiation, as well as the ratio of their intensities play the important role depending on a program under way. In work, the spatial radiant characteristics of the mixed X,n-radiation generated with the electron beam in the converter located together with an isotopic target in the middle of a neutron moderator in the electron energy range 40…95 MeV and various size of the moderator are studied by computer simulations with the use of a GEANT4 transport code.

Two-dimensional differential null interferometry of transmission wavefront of axicon lens

Axicon lenses are critical in the beam transforming, such as the Bessel beam and hollow beam generation, and its manufacturing accuracy directly affects the quality of the output beam. Aiming at the versatility and high accuracy, we propose a two-dimensional differential null interferometry for measuring transmission wavefront of axicon lens. The method uses a pair of axicon lens to construct a null interferometric setup, with one as the compensator. The two-dimensional difference separates the wavefront information introduced by the compensator and achieves the orthogonal difference of the measured wavefront. Finally, the measured wavefront is reconstructed from the differential wavefront based on the Hudgin slope method. The experiments proved that this method achieves a measurement accuracy of ±0.125 µm with a repeatability of 0.759 nm.

Ultra-broadband on-chip beam focusing enabled by GRIN metalens on silicon-on-insulator platform

Abstract Metalens has emerged as an important optical block in free-space optical systems, which shows excellent performance. Even the metalens based on gradient index (GRIN) profiles can be implemented for on-chip beam focusing behavior. However, for most previous schemes, the GRIN metalenses can only achieve on-chip beam focusing behavior in one dimension, which limits their applications in low-loss waveguide interconnecting or fiber-to-chip coupling. In this paper, an on-chip half Maxwell’s fisheye lens based on GRIN profiles with subwavelength features, integrated with silicon waveguides, is experimentally demonstrated. Benefitting from the index distribution and beam focusing characteristics of the half Maxwell’s fisheye lens, an on-chip beam transforming can be achieved for transverse electric (TE) fundamental mode in two waveguides with different heights and widths. The simulated 1 dB bandwidth can reach 1100 nm, which exhibits great prospects in integrated photonic circuits. The measured insertion loss of an on-chip 5.4 μm-length lens is less than 1 dB to connect a 220 nm-height, 8 μm-wide waveguide, and a 60 nm-height, 0.5 μm-wide waveguide in the wavelength range of 1280–1620 nm.

Peculiarities of the Gaussian beam transformation in the optical scheme with an axicon and a biaxial crystal

The transformation of a conical-type light beam by a biaxial crystal during propagation along one of its optical axes is herein investigated. The beam is formed by an axicon from the circularly polarized Gaussian input field. Depending on the position of the axicon either a Bessel beam or a combination of Bessel and conical beams falls on the crystal. The conversion coefficient from a zero-order Bessel beam to a first-order beam with phase dislocation is calculated. We show that if the angle of the beam cone and its diameter are large enough, then it is transformed into a field that is a first-order BesselGaussian beam with high accuracy. At the same time the conversion coefficient is close to 1. The case of a small cone angle of an incident Bessel beam is also investigated. In this case the efficiency of transformation significantly depends on the type of the spatial spectrum. At a small cone angle, the shape of the spatial spectrum is determined by the diameter of the incident Gaussian beam. Namely, as the diameter decreases, the beam spectrum changes from annular to close to Gaussian, passing through an intermediate form in the form of a superposition of these two profiles. The influence of the spatial spectrum is the conversion coefficient decreasing with a decrease of the of the ring component contribution to the spectrum. In this case, the conversion coefficient is always higher than for a scheme without an axicon when a Gaussian beam falls on the crystal. Consequently, the introduction of even a small conicity into the beam makes it possible to increase the field transformation coefficient. It can be implemented, for example, using a scheme with two axicons having close angles of a ray deflection. The results obtained are also of practical interest, in particular, for the development of conical-type laser emitters with a small cone angle for long-range reconnaissance and optical communication in free space.

High-power narrow-linewidth 780 nm diode laser based on external cavity feedback technology of volume Bragg grating

A diode-pumped alkali metal vapor laser (DPAL) combines the advantages of a solid laser and a gas laser. DPAL is useful in the industrial, medical, aerospace and military fields and can yield high output power while maintaining excellent beam quality. However, applications in many fields are limited due to the narrow absorption spectrum of the alkali metal vapor laser. In this study, a novel linewidth narrowing structure of the high-power diode laser based on volume Bragg gratings (VBGs) is proposed to match the absorption spectrum of the alkali metal vapor laser. With the combination of fast-axis collimating lens, beam transformation systems, slow-axis collimating lens, VBGs and metal ceramic heaters, the divergence angles of both fast and slow axis are compressed within 10 mrad. A high-power narrow linewidth diode laser source with a central wavelength of 780.044 nm, a spectral width of 0.138 nm, and an output power of 1090 W is achieved at an operating current of 50 A. Experimental results show that the effect of external cavity feedback based on VBG is improved and the multi-channel spectral linewidth is narrowed effectively by such a structure. The diode laser source based on this structure can be applied for pumping the rubidium alkali metal vapor laser.

Terahertz tight-focused Bessel beam generation and point-to-point focusing based on nonlocal diffraction engineering.

Metasurfaces transform the wavefront by spatially varying the amplitude or phase of the incoming beam. Instead of encoding such variation by subwavelength unit cells, it is achievable over diffraction engineering of supercell structures, which outperforms the unit-cell method when the spatial gradient is large. In addition to tight focusing, here we apply this method to achieve plane wave-to-Bessel beam transformation and point-to-point focusing at terahertz frequencies. The Bessel beam has a small beam waist (0.57λ) and long depth of focus (9.1λ) for subwavelength-resolution imaging over a long distance. The point-to-point focusing changes the divergence angle from 16° to 70°. Both devices are validated by numerical simulations and experimental results with good agreement.

A Compact Dual-Mirror Design of Quasi-Optical Mode Converter for Gyrotron Application

The quasi-optical mode converter (QOMC) for high-power fusion gyrotron is a typical beam transformation and shaping device in near-field region. In this letter, the authors report their progress in designing a compact QOMC with only two reflecting mirrors. For achieving the high-efficiency conversion comparable to conventional multimirror converters, the authors exercise modification and optimization on the first mirror based on the classic quasi-elliptical design. It is shown that the optimized first mirror can fulfill multifunctions that have to be achieved by a chain of mirrors in conventional designs. Numerical results show the effectiveness of the optimization, which can be revealed by the achieved 95.97% Gaussian content in the output beam with power transmission efficiency of 99.0%.

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.

Perfect diffractive circular metagrating for Bessel beam transformation.

Bessel beams, with their non-diffractive property, have attracted great interest in recent years. Optical needle shaping of Bessel beams is highly desired in many applications, however, this typically requires low numerical aperture (NA) bulky 4f confocal systems incorporated with spatial light modulators or round filters. Here, we employ a circular dielectric metagrating for perfect Bessel beam transformation at a desired wavelength. The dielectric metagrating exhibits a high transmissive diffraction efficiency (up to 75%) for a broadband (460 nm to 560 nm), wide-angle range, and dual-polarization response, which is capable of a high-performance transformation of Bessel beams with arbitrary NAs. Our results show potential for special-beam-required applications such as light storage, imaging, and optical manipulation.

Reciprocating Side-Firing Fiber for Laser Sealing of Blood Vessels.

Infrared lasers may provide faster and more precise sealing of blood vessels and with lower jaw temperatures than ultrasonic and electrosurgical devices. This study explores an oscillating or reciprocating side-firing optical fiber method for transformation of a circular laser beam into a linear beam, necessary for integration into a standard 5-mm-diameter laparoscopic device, and for uniform irradiation perpendicular to the vessel length. A servo motor connected to a side-firing, 550-μm-core fiber, provided linear translation of a 2.0-mm-diameter circular beam over either 5 mm or 11 mm scan lengths for sealing small or large vessels, respectively. Laser seals were performed, ex vivo, on a total of 20 porcine renal arteries of 1-6 mm diameter (n = 10 samples for each scan length). Each vessel was compressed to a fixed 0.4-mm-thickness, matching the 1470-nm laser optical penetration depth. Vessels were irradiated with fluences ranging from 636 J/cm2 to 716 J/cm2. A standard burst pressure (BP) setup was used to evaluate vessel seal strength. The reciprocating fiber produced mean BP of 554 ± 142 and 524 ± 132 mmHg, respectively, and consistently sealing blood vessels, with all BP above hypertensive (180 mmHg) blood pressures. The reciprocating fiber provides a relatively uniform linear beam profile and aspect ratio, but will require integration of servo motor into a handpiece.

Electrically controlled microstructured liquid-crystal twist elements for phase conversion of light fields

Subject of study. Various topologies are proposed for microstructured photoalignment of the directors in nematic liquid crystals; these topologies can be used in electrically controlled liquid-crystal elements to produce light beams with a specific phase singularity. Main results. Electrically controllable, azimuthally oriented liquid-crystal twist elements were developed to support the transformation of Gaussian beams into beams with a specific number of phase singularities in the wavefront. Polarization microscopy, analysis of the intensity distribution across the beam, and coherent superposition of the transformed light with a plane wave determines the operating voltage range supporting excitation of the beams with optical singularities. It is shown that the developed components perform the wavelength-independent topological phase transformation converting the wavefront associated with a circularly polarized Gaussian beam into optical vortices over a large portion of the optical spectrum without the need for precise adjustment of the control voltage. Diffractive optical elements based on microstructured twist-planar orientation of the liquid-crystal director enable the transformation of linearly polarized wavefronts into optical vortices with different topological charges. A diffractive topological component that produces optical vortices with the topological charge 4 was used as a sample for an experimental study to determine the diffraction efficiency of the componentas a function of the control voltage amplitude. The initial diffraction efficiency (at zero voltage) of the developed diffractive component is about 10% and the peak diffraction efficiency at the control voltage amplitude of a few volts is about 30%. There is no diffractive structure when the external voltage is increased up to 20 V. As revealed by analysis of the interference pattern resulting from coherent superposition of a phase singular beam and a plane wave, the excited optical vortices are stable with respect to the control voltage amplitude. It has been found experimentally that singular beams with high topological charges remain stable during the propagation to a distance approaching 3 m. Practical significance. The developed electrically controlled topological liquid-crystal elements can operate over a wide range of optical wavelengths and may be implemented to produce ultrashort-duration optical vortices and a supercontinuum. Besides, they may be used in the form of multi-trap optical tweezers or in technologies for protection of securities and documents.

Comparison of fiber-optic linear beam shaping designs for laparoscopic laser sealing of vascular tissues.

Infrared lasers may provide faster and more precise sealing of blood vessels and with lower device jaw temperatures than ultrasonic and electrosurgical devices during surgery. Our study explores three beam shaping methods using optical fibers for transformation of a circular laser beam into a linear beam, necessary for integration into a standard 5-mm-diameter laparoscopic device, and for uniform irradiation perpendicular to the vessel length. In the first design, a servo motor connected to a side-firing, 550-μm-core fiber, provided linear translation of a 2.0-mm-diameter circular beam, back, and forth, over either 5 or 11 mm scan lengths for sealing of small or large vessels. The second design used external beam splitters to divide laser power equally into three side-firing fibers, stacked side-by-side, producing a linear beam of 4 × 2 mm. The third design used external beam splitters with three forward-firing fibers and a slanted jaw surface, to produce a linear beam of 5 × 1.5 mm. Laser seals were performed, ex vivo, on 41 porcine renal arteries of 1- to 6-mm diameter (n ≥ 10 samples for each design). Each vessel was compressed to a fixed 0.4-mm-thickness, matching the optical penetration depth at 1470 nm. Vessels were irradiated with fluences of 636 to 800 J/cm2, which, based on previous studies, is sufficient for sealing, but not cutting. A burst pressure setup was used to evaluate vessel seal strength. Reciprocating fiber and fiber bundles produced mean burst pressures of 554 ± 142, 524 ± 132, 429 ± 99, and 390 ± 140 mmHg, respectively. All designs consistently sealed blood vessels, with burst pressures above hypertensive (180 mmHg) blood pressures. The reciprocating fiber produced the most uniform linear beam profile and aspect ratio but will require integration of the servo motor into a handpiece. Fiber bundle designs produced shorter, less uniform beams, but enable optical components to be assembled outside the handpiece.

Intracavity spatial mode conversion by holographic phase masks.

Past beam-shaping techniques, developed to transform a Gaussian beam into other waveforms, rely on a wide selection of available tools ranging from physical apertures, diffractive optical elements, phase masks, free-form optics to spatial light modulators. However, these devices - whether active or passive - do not address the underlying monochromatic nature of their embedded phase profiles, while being hampered by the complex, high-cost manufacturing process and a restrictive laser-induced damage threshold. Recently, a new type of passive phase devices for beam transformation - referred to as holographic phase masks (HPMs), was developed to address these critical shortcomings. In this work, we demonstrated the first integration of HPMs into a laser cavity for the generation of arbitrary spatial modes. Our approach allowed for different phase patterns to be embedded into the outputs of a laser system, while preserving the spatial structure of its intracavity beams. The optical system further possessed a unique ability to simultaneously emit distinct spatial modes into separate beampaths, owning to the multiplexing capability of HPMs. We also confirmed the achromatic nature of these HPMs in a wavelength-tunable cavity, contrary to other known passive or active beam-shaping tools. The achromatism of HPMs, coupled to their ability to withstand up to kW level of average power, makes possible future developments in high-power broadband sources, capable of generating light beams with arbitrary phase distribution covering any desirable spectral regions from near ultraviolet to near infrared.

Computational technique for field-of-view expansion in AOTF-based imagers.

A rather narrow field of view (FOV) has always been considered as an essential limitation of spectral imagers based on acousto-optical tunable filters (AOTFs). We demonstrate a computational technique to overcome this constraint. It is based on preliminary precise spectral-angular characterization of beam transformation caused by light diffraction on an acoustic wave and consequent correction of acquired stack of spectral images. This technique is applicable for any geometry of acousto-optic interaction and opens the way for the development of AOTFs with significantly expanded FOV.

Orbital angular momentum sidebands of Laguerre-Gauss beams reflecting on graphene metasurfaces

In this study, the orbital angular momentum (OAM) sidebands of Laguerre-Gauss beams reflecting on graphene metasurfaces are investigated. Upon reflection, vortex beams carrying orbital angular momentum will acquire sidebands, whose relative intensity varies depending on the Fermi energy, the external magnetic field, and/or the wave frequency. The relative intensity of the sideband OAM modes locally has a small trough for s-polarized beams at the topological transition point between the hyperbolic and elliptic topology. Energy can transfer from the central mode to the neighboring OAM modes increasing the topologic charge l. When the electric field of the incident s-polarized light occurs along the low energy dissipation direction of the graphene metasurfaces, it is helpful for the mode transformation of vortex beams. When the electric field of the incident s-polarized light occurs along the high energy dissipation direction, it is beneficial to suppressing crosstalk of different sidemodes in terahertz communication.

Beam shaping of highly multimode sources with cascaded diffractive optical elements

Multimode laser sources, like multimode fiber lasers and many laser diodes, play a central role in material processing requiring high power and continuous laser light delivery. Their native beam profiles, however, may not be ideal for certain laser applications, therefore creating a demand for beam shaping technologies. For laser applications where beam shaping provides a technical advantage, it is essential that the beam transformation is performed with as little energy loss as possible. The current contribution presents an inverse design algorithm for the creation of beam transformers that meets these expectations by computing optical devices consisting of pure and smooth phase modulations. These devices allow arbitrary intensity shaping and are suitable for free-space or integrated optical realization.