Beam Shaping Optics Research Articles

New Ways to Generate Flat‐Top Profiles

AbstractPreviously, flat‐top beam profiles were usually generated in laser technology through homogenizers. These beam shaping optics are, however, both expensive and diminish the laser output. Optical fibers with a rectangular core geometry offer an alternative, by being able to homogenize the intensity distribution of a laser beam without reducing its performance.

Composite method for precise freeform optical beam shaping.

We present a composite freeform surface construction method for creating a high-accuracy irradiance distribution from a given incident beam under the influence of diffraction. The main idea is that we first determine a fully continuous freeform surface estimate by solving a standard Monge-Ampère equation and then refine it using an iterative Fourier-transform algorithm associated with over-compensation. Although this method can only be implemented in the paraxial approximation, it can significantly simplify the design and is applicable to many examples that fulfill this restriction. The resulting optical surface, unwrapped from the final phase, is an unusual discontinuous freeform surface that can produce very promising performances in terms of surface roughness and irradiance accuracy.

Drop on demand generation from a metal wire by means of an annular laser beam

In the paper a novel system for drop-on-demand (DoD) generation from a metal wire is presented, whose main component is a newly developed laser droplet generation head, consisting of annular laser beam shaping optics and a wire feeding system. In the pendant droplet formation phase of the DoD generation, a laser pulse is used to melt the wire-end, which is fed into the focus of an annular laser beam. The formed pendant droplet is then detached by means of a detachment pulse, which induces Rayleigh–Plateau instability of the molten column of wire above the neck of the pendant droplet. The main process parameters, including the laser pulse and wire feeding parameters as well as the additional parameters which influence particular phases of the DoD generation process, have been identified. The empirical correlations between the influencing process parameters and the droplet characteristics, including droplet diameter and temperature, were determined, based on the analysis of high speed IR records of the process, images being acquired by an optical microscope and temperature data being acquired by pyrometers. As an example, DoD generation from a commercially pure 99.6% Ni wire (Nickel 200) of 0.6mm diameter is considered. It is shown that droplets with diameters ranging from 0.85 to 1.25mm can be generated, with a resolution of 50μm and a standard deviation of 15μm. The temperature of the detached droplet remains above the melting point of the Ni wire, and increases with the droplet diameter within the range from 1650 to 1750°C. Some examples of Ni droplets deposited on a Ti sheet surface are presented, with the aim of demonstrating the capability of the proposed system, and motivating further applications in which drops on demand having a high temperature and a precisely defined diameter need to be generated, while limiting the thermal loading of the surroundings.

Graphene Plasmonic Metasurfaces to Steer Infrared Light.

Metasurfaces utilizing engineered metallic nanostructures have recently emerged as an important means to manipulate the propagation of light waves in a prescribed manner. However, conventional metallic metasurfaces mainly efficiently work in the visible and near-infrared regime, and lack sufficient tunability. In this work, combining the pronounced plasmonic resonance of patterned graphene structures with a subwavelength-thick optical cavity, we propose and demonstrate novel graphene metasurfaces that manifest the potential to dynamically control the phase and amplitude of infrared light with very high efficiency. It is shown that the phase of the infrared light reflected from a simple graphene ribbon metasurface can span over almost the entire 2π range by changing the width of the graphene ribbons, while the amplitude of the reflection can be maintained at high values without significant variations. We successfully realize anomalous reflection, reflective focusing lenses, and non-diffracting Airy beams based on graphene metasurfaces. Our results open up a new paradigm of highly integrated photonic platforms for dynamic beam shaping and adaptive optics in the crucial infrared wavelength range.

Beam Shaping Optics for Process Acceleration

AbstractWithin in the last years the average power of industrial pulsed laser systems is continuously increasing. However one can often not make efficient use of the available power, since the laser process limits the pulse energy which can be applied to the work piece. In particular for ultrashort pulsed laser (USP) processing the maximum pulse energy is further limited due to plasma shielding effects. For those reasons laser processing often is too slow and too expensive (when available laser power cannot fully be used), thus is an uneconomical solution even if process results are excellent. How to overcome such limits?

Beam Shaping for Advanced Laser Materials Processing

AbstractFor recent industrial materials processing with advanced high power lasers, beam shaping optics are utilized as a key technology. Aspheric or diffractive beam shapers transform a Gaussian intensity profile of a single mode (TEM00) laser beam into a flattop one. While an aspheric beam shaper can make a circular flattop, a diffractive one can realize much flexibility in output beam shapes, like square, rectangular, linear, or other complex beam shapes. The beam shaping is often combined with a scanning optical system, consisting of galvanometric mirrors and an f‐theta lens. Those beam shapers are used for either pulsed or CW single mode laser applications, i. e. ablation, cutting, drilling, scribing, annealing, etc. of thin films or bulk materials. This article describes principles, design concepts and specification examples of those optical systems used in practical applications.

Subcellular-level resolution MALDI-MS imaging of maize leaf metabolites by MALDI-linear ion trap-Orbitrap mass spectrometer.

A significant limiting factor in achieving high spatial resolution for matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) imaging is the size of the laser spot at the sample surface. Here, we present modifications to the beam-delivery optics of a commercial MALDI-linear ion trap-Orbitrap instrument, incorporating an external Nd:YAG laser, beam-shaping optics, and an aspheric focusing lens, to reduce the minimum laser spot size from ~50μm for the commercial configuration down to ~9μm for the modified configuration. This improved system was applied for MALDI-MS imaging of cross sections of juvenile maize leaves at 5-μm spatial resolution using an oversampling method. A variety of different metabolites including amino acids, glycerolipids, and defense-related compounds were imaged at a spatial resolution well below the size of a single cell. Such images provide unprecedented insights into the metabolism associated with the different tissue types of the maize leaf, which is known to asymmetrically distribute the reactions of C4 photosynthesis among the mesophyll and bundle sheath cell types. The metabolite ion images correlate with the optical images that reveal the structures of the different tissues, and previously known and newly revealed asymmetric metabolic features are observed.

Formation of optical needle by high NA lens axicon with dedicated complex spiral phase mask

The intensity distribution in the focal region for the tightly focused an azimuthally polarized beam using spiral phase mask and complex phase mask with high NA lens axicon system is studied on the basis of the vector diffraction theory. Here we report a new method that generates optical needle like transversally polarized beam with sub diffraction spot size \((0.39\,\lambda )\) and propagates without divergence over a long distance (of about \(15.8\,\lambda \)) in free space for NA \(=\) 0.95. The authors expect such investigation is worthwhile for optical manipulation, beam shaping and optical trapping applications.

Characterization of Laser Beam Shaping Optics Based on Their Ablation Geometry of Thin Films

Thin film ablation with pulsed nanosecond lasers can benefit from the use of beam shaping optics to transform the Gaussian beam profile with a circular footprint into a Top-Hat beam profile with a rectangular footprint. In general, the quality of the transformed beam profile depends strongly on the beam alignment of the entire laser system. In particular, the adjustment of the beam shaping element is of upmost importance. For an appropriate alignment of the beam shaper, it is generally necessary to observe the intensity distribution near the focal position of the applied focusing optics. Systems with a low numerical aperture (NA) can commonly be qualified by means of laser beam profilers, such as a charge-coupled device (CCD) camera. However, laser systems for micromachining typically employ focus lenses with a high NA, which generate focal spot sizes of only several microns in diameter. This turns out to be a challenge for common beam profiling measurement systems and complicates the adjustment of the beam shaper strongly. In this contribution, we evaluate the quality of a Top-Hat beam profiling element and its alignment in the working area based on the ablated geometry of single pulse ablation of thin transparent conductive oxides. To determine the best achievable adjustment, we develop a quality index for rectangular laser ablation spots and investigate the influences of different alignment parameters, which can affect the intensity distribution of a Top-Hat laser beam profile.

Electrically tunable metasurface perfect absorbers for ultrathin mid-infrared optical modulators.

Dynamically reconfigurable metasurfaces open up unprecedented opportunities in applications such as high capacity communications, dynamic beam shaping, hyperspectral imaging, and adaptive optics. The realization of high performance metasurface-based devices remains a great challenge due to very limited tuning ranges and modulation depths. Here we show that a widely tunable metasurface composed of optical antennas on graphene can be incorporated into a subwavelength-thick optical cavity to create an electrically tunable perfect absorber. By switching the absorber in and out of the critical coupling condition via the gate voltage applied on graphene, a modulation depth of up to 100% can be achieved. In particular, we demonstrated ultrathin (thickness < λ0/10) high speed (up to 20 GHz) optical modulators over a broad wavelength range (5-7 μm). The operating wavelength can be scaled from the near-infrared to the terahertz by simply tailoring the metasurface and cavity dimensions.

Irregular elliptical plasmonic rings for engineering near-field and phase

The near-field and phase distribution patterns of surface plasmon polaritons excited on different types of irregular elliptical rings are investigated with respect to linear and circular polarized lightwave. It is interestingly found that under linear polarization, the rings exhibit the dipole-like field and phase patterns, while under circular polarization they provide the omnidirectional vortex field and phase patterns. The field patterns produced by different rings demonstrated different shapes and amplitudes. The reason is due to different phase-matching conditions, which arise from internal geometrical variations of elliptical rings. These specified phase and near-field manipulations are of high potential for many applications such as optical trapping and beam shaping.

X-Ray Optics and Precision Requirements for Residual Stress Analyses with High Spatial Resolution

In recent years, the demand for high spatial resolution in X-ray residual stress analysis has drastically increased. The locations of interest are frequently small foot radii of teeth of gears. Also the inner surface of holes or hollow cylinders in general with small diameter must be investigated after opening the cylindrical cavities. In resolving such measuring problems, significant progress has been made in reproducibly manufacturing and applying glass capillary X-ray optics. With focusing elliptical polycapillaries and conventional laboratory X-ray sources, spot sizes of few 10 μm can be realized at sufficiently intensities for residual stress analyses. However, glass capillary optics require refined alignment strategies which are completely different from those for conventional beam shaping optics. Moreover, the small spot sizes cannot be aligned and positioned on the sample surface by eye. Microscopy fixtures are required. Finally, measurements in small radii result in high precision requirements for the diffractometers as well as for the sample positioning in axes and directions which are significantly less relevant when measuring on plane surfaces. The specific requirements resulting from residual stress analyses with high spatial resolution using glass capillaries and small spot sizes at curved surfaces are described and discussed.

Optical beam shaping and diffraction free waves: A variational approach

We investigate the problem of shaping radially symmetric annular beams into desired intensity patterns along the optical axis. Within the Fresnel approximation, we show that this problem can be expressed in a variational form equivalent to the one arising in phase retrieval. Using the uncertainty principle we prove various rigorous lower bounds on the functional; these lower bounds estimate the L2 error for the beam shaping problem in terms of the design parameters. We also use the method of stationary phase to construct a natural ansatz for a minimizer in the short wavelength limit. We illustrate the implications of our results by applying the method of stationary phase coupled with the Gerchberg–Saxton algorithm to beam shaping problems arising in the remote delivery of beams and pulses.

Rapid sealing and cutting of porcine blood vessels,ex vivo, using a high-power, 1470-nm diode laser

Suture ligation with subsequent cutting of blood vessels to maintain hemostasis during surgery is time consuming and skill intensive. Energy-based electrosurgical and ultrasonic devices are often used to replace sutures and mechanical clips to provide rapid hemostasis and decrease surgery time. Some of these devices may create undesirably large collateral zones of thermal damage and tissue necrosis, or require separate mechanical blades for cutting. Infrared lasers are currently being explored as alternative energy sources for vessel sealing applications. In a previous study, a 1470-nm laser was used to seal vessels 1 to 6 mm in diameter in 5 s, yielding burst pressures of ∼500 mmHg. The purpose of this study was to provide vessel sealing times comparable with current energy-based devices, incorporate transection of sealed vessels, and demonstrate high vessel burst pressures to provide a safety margin for future clinical use. A 110-W, 1470-nm laser beam was transmitted through a fiber and beam shaping optics, producing a 90-W linear beam 3.0 by 9.5 mm for sealing (400 W/cm2), and 1.1 by 9.6 mm for cutting (1080 W/cm2). A two-step process sealed and then transected ex vivo porcine renal vessels (1.5 to 8.5 mm diameter) in a bench top setup. Seal and cut times were 1.0 s each. A burst pressure system measured seal strength, and histologic measurements of lateral thermal spread were also recorded. All blood vessels tested (n=55 seal samples) were sealed and cut, with total irradiation times of 2.0 s and mean burst pressures of 1305±783 mmHg. Additional unburst vessels were processed for histological analysis, showing a lateral thermal spread of 0.94±0.48 mm (n=14 seal samples). This study demonstrated that an optical-based system is capable of precisely sealing and cutting a wide range of porcine renal vessel sizes and, with further development, may provide an alternative to radiofrequency- and ultrasonic-based vessel sealing devices.

Compact all-fiber plasmonic Airy-like beam generator

A compact all-fiber plasmonic Airy-like beam generator is demonstrated. A single slit and a 1D groove array were fabricated by focused ion beam milling on the gold deposited end facet of a single-mode optical fiber. The single slit excites the surface plasmonic polaritons (SPPs), which are decoupled into free space by the groove array. The phase of decoupling SPPs is adjusted by the grooves position. Experimental generation of the single Airy-like beam has good consistency with theoretical predictions. The transverse acceleration and nondiffraction properties are observed. The interference of double Airy-like beams in the free space is also analyzed. The presented plasmonic Airy-like beam generator is of importance to realize all-fiber optical trapping, beam shaping, and fiber integrated devices.

Theoretical modeling of the divergence of a flat-topped beam from a two-stage beam shaper into a conical intensity profile after propagation in free space

In this article, a two-stage beam shaping device based on a plastic fiber-bundle prism duct coupled waveguide is theoretically and experimentally described. ZEMAX software is used to simulate and investigate the divergence phenomenon on the beam shape outputted and the radiance profiles in both position and angle space. The effect of prism's geometrical structure on the beam divergence is also investigated and the optimum geometric conditions are reported. According to the theoretical results, due to an asymmetrical divergence effect the beam's cross section is increased with distance by a variable aspect ratio. The results also show that propagation in free space transformed a square beam with a flat-top intensity distribution into a rectangular beam with a conical intensity distribution. For experimental study, an imaging technique is applied to investigate the beam's output images and intensity profiles. A source light is used to illuminate the optical beam shaping elements of the reported design. Digital photographs of the beam's output for different axial distances are taken by a camera and the image data is converted into a response curve for comparison with the simulated image profiles. The obtained experimental results are strongly in agreement with the theoretical ones.

Laser cooling of beryllium ions using a frequency-doubled 626 nm diode laser

We demonstrate laser cooling of trapped beryllium ions at 313 nm using a frequency-doubled extended cavity diode laser operated at 626 nm, obtained by cooling a ridge waveguide diode laser chip to -31°C. Up to 32 mW of narrowband 626 nm laser radiation is obtained. After passage through an optical isolator and beam shaping optics, 14 mW of 626 nm power remains of which 70% is coupled into an external enhancement cavity containing a nonlinear crystal for second-harmonic generation. We produce up to 35 μW of 313 nm radiation, which is subsequently used to laser cool and detect 6×10(2) beryllium ions, stored in a linear Paul trap, to a temperature of about 10 mK, as evidenced by the formation of Coulomb crystals. Our setup offers a simple and affordable alternative for Doppler cooling, optical pumping, and detection to presently used laser systems.

Theoretical modeling of generation of hat-top intensity profile from Gaussian beam by means of a two-stage beam shaper

Both theoretical and experimental results of an optical beam shaping system are investigated in this report. The described system is a two-stage beam shaping device including a fiber-bundle and a prism-duct. A source light is used to illuminate the fiber-bundle and the image of output beam is captured by a CCD camera. The fiber-bundle output beam shape shows a linear arrangement of circular spots lights, which are placed in a rectangular cross section of about 21.37mm×2.44mm. In another study, the photograph picture of the prism output beam is taken by a digital camera. The prism output beam cross section is a square shape with a dimension of about 4×4mm2. According to the experimental results, the prism duct converted a Gaussian beam profile with multiple-peak distribution to a hat-top beam profile with the uniform intensity distribution. For theoretical investigations, using ZEMAX software a simulation is performed to analyze the beam shaping design. By proper modeling the output beam shape and radiance profiles in position space and angle space of the fiber-bundle and the prism duct are investigated. Theoretical radiance profiles are obtained by using simulated images and results are in agreement with the experimental results.

Insitu Raman Setup for Deep Ocean Investigations Applying Two 1000 m Optical Fiber Cables and a 785 nm High Power Diode Laser

A laboratory Raman-set-up is used to demonstrate the feasibility of deep ocean measurements using 1000 m long low OH optical fib res for the excitation of the substances under study and the collection of the Raman photons. As excitation laser a 785 n m broad area distributed feedback (DFB-BA) high-power d iode laser with an optical power of 1.25 W and a spectral emission width below 0.6 n m suitable for Raman spectroscopy was used.After passing beam shaping and focusing optics, a 1000 m long fiber, and a pressure-resistant optode275 mW of optical power were available at the samp le. The optode also collects the Raman photons and focuses the light into the second 1000 m long fib re, which transfers the light into a spectrometer. To determine the reachable signal-to-noise ratio, Raman spectra were taken fro m natural minerals (sulfates, carbonates, sulfur). Even for weak Raman-signal and short integration times of 0.5 s per spectrum, the signal-to-noise ratio of 60 would be suitable for the in-situ Raman spectroscopy in deep ocean applications. Even longer optical fibres would increase to integration time up to 60 s. The shown set -up offers the opportunity to used different laser sources and spectrometers on board of a ship, only the fibres and the optode had to be suitable for deep sea conditions.

Efficient Micromachining of Thin Films with Nanosecond Lasers

AbstractEspecially for micromachining, single‐mode solid state lasers are well established nowadays. Due to their ultra short pulse duration and low pulse energies picosecond and especially femtosecond lasers show a quite high precision in micromachining processes and avoid thermal effects. However, these advantages in process quality require higher investment costs for the lasers. As a consequence, in cost sensitive markets such ultra short pulse systems are only rarely used. Hence, the series production of, e.g., solar panels is still mainly based on nanosecond lasers, as these systems are available at significantly lower costs. Besides pulse duration also the wavelength and the beam profile have a strong influence on the quality of the laser ablation process. For many applications the raw Gaussian beam profile does not lead to the best result. Therefore, an adaption of the beam profile to the process is highly desired. It has been shown that especially beam profiles with a homogeneous intensity distribution, so called Top‐Hat profiles can improve the quality of an ablation process and lead to a reduction of the processing costs. This applies in particular to thin film processing. This article shows how a cost efficient micromachining process of thin film layers can be realized by using nanosecond lasers in combination with various beam shaping optics.