Showing papers on "Flattop published in 2012"

Variable beam shaping with using the same field mapping refractive beam shaper

Abstract: Modern laser scientific techniques and industrial technologies require not only simple homogenizing of a beam but also more freedom in manipulation of intensity profile and generating such profiles like super-Gaussian, inverse-Gaussian, skewed flattop and others. In many cases the task of variable beam shaping can be solved by refractive beam shaping optics of field mapping type which operational principle presumes saving of beam consistency, providing collimated output beam of low divergence, high transmittance and flatness of output beam profile, extended depth of field; another important feature is negligible residual wave aberration. Typically the fields mapping refractive beam shapers, like πShaper, are designed to generate flattop intensity profile for a beam of pre-determined size and input intensity profile. Varying of the input beam diameter lets it possible to realize either super-Gaussian (smaller input) or inverse-Gaussian (bigger input) intensity profiles of output beam that are important in pumping of solid-state lasers, hardening, cladding and other techniques. By lateral shift of a beam with respect to a πShaper the output flattop profile gets a skew in direction of that shift, the skew angle corresponds to the shift value. The skewed profile is important, for example, in some acousto-optical techniques where compensation of acoustic wave attenuation is required. All variety of profiles can be provided by the same beam shaper unit. This paper will describe some design basics of refractive beam shapers of the field mapping type and techniques to vary the output intensity profile, experimental results will be presented as well.

Homogeneous one-dimensional optical lattice generation using a digital micromirror device-based high-precision beam shaper

Abstract: A homogeneous one-dimensional optical lattice is demonstrated by using a high-precision beam shaper based on a digital micromirror device (DMD) with an imaging system containing a pinhole low-pass filter (LPF). This system is capable of producing a high-quality flattop beam profile to form a standing-wave optical lattice with a 50×50 μm2 flattop region. The periodic potential generated by the optical lattice confines ultracold atoms in Bose-Einstein condensate experiments. We conducted beam shaping tests at several wavelengths by implementing various coherent and incoherent light sources in the visible and infrared wavelength ranges. Experiments produced flattop and other well-controlled beam profiles with 0.2% to 0.26% root-mean-square (RMS) error after applying a digital LPF and nearly flat phase. Several concerns for the system design are presented. First, the energy requirement was determined by power conversion analysis and DMD diffraction efficiency simulation. In addition, a LabVIEW program was written to accelerate the speed of the iterative process for beam profile refinement. Finally, various camera calibrations improved the measurement accuracy. We achieved a 1.25% RMS error flattop beam with diameter of 70.4 μm at the atoms' plane. Other beam profile measurements in different diagnostic planes demonstrated a good intensity uniformity of the optical lattice.

Flattop Efficient Cascaded χ $^{\bf {\bm (}2{\bm)}}$ (SFG + DFG)-Based Wideband Wavelength Converters Using Step-Chirped Gratings

Abstract: We numerically evaluate efficient wideband wavelength converters based on quasi-phase-matched cascaded sum and difference frequency generation (SFG + DFG) using step-chirped gratings (SCG) in lossy lithium niobate waveguides, and compare them to the ones using uniform gratings, assuming a large pump wavelength difference considering a full model of depleted pump and sum-frequency waves. For the same length, appropriate critical period shifts are presented for the number of sections and chirp steps to achieve flattop conversion efficiency responses with peak-to-peak ripples less than 0.2 dB. To obtain the maximum efficiency and flat response with a decreasing chirp step of 1 nm, the criteria for the design of optimum four-section single-pass and two-section double-pass SCG-based devices, including the assignment of length (to achieve a desired bandwidth) and pump power, are presented considering a 75-nm pump wavelength difference. Also, the performances of single-pass and double-pass schemes for the two- and four-section SCG-based devices are given, respectively, and compared to those of uniform grating-based devices with and without pump detuning, assuming a 3-cm-long waveguide and 50-mW input pump powers. For the same length and power, using the SCG with fixed pumps instead of the uniform grating with detuned pumps shows a noteworthy increase in the mean efficiency to attain almost the same response flatness and bandwidth.

Refractive beam shaping optics to improve operation of spatial light modulators

Abstract: Inhomogeneity of intensity profile in techniques with illumination of SLM by laser beams leads to not efficient using of the SLM capabilities or complexity of algorithms to control these SLM For example the typical Gaussian intensity distribution has peak intensity in the centre of a beam, and to prevent damaging the SLM it is necessary to reduce power of entire laser beam In laser techniques like Computer Generated Holography (CGH) the not uniform intensity profile leads to essential increasing the complexity of mathematical models or makes some techniques of digital holography unrealizable To overcome these drawbacks it is suggested to apply with SLM the refractive field mapping beam shapers providing high flexibility in building various optical setups due to their unique features: almost lossless intensity profile transformation from Gaussian to flattop, saving of the beam consistency, low output beam divergence and flatness of wavefront, extended depth of field, capability to adapt to real intensity profiles of TEM00 and multimode laser sources Applications include CGH, holographic projection processing applications, holographic lithography, optical trapping and laser illumination in confocal microscopes With a collimated flattop beam provided by refractive field mappers these techniques become easier to use, more effective and reliable in operation This paper will describe some design basics of refractive beam shapers of the field mapping type, with emphasis on the features important for applications with SLMs There will be presented comparative results of applying the refractive beam shapers in systems of holographic lithography and other techniques

Creating round and square flattop laser spots in microprocessing systems with scanning optics

Abstract: Performance of various modern laser based micromachining techniques can be improved by applying square laser spots with uniform intensity distribution. And providing possibility of scanning of such a spot over whole working field with using popular 2- and 3-axis galvo mirror scanners is of great importance for many laser microprocessing technologies, like scribing, drilling vias in PCB, flat panel display repair. These tasks can be successfully solved with using the field mapping refractive beam shaping optics like πShaper and Focal-πShaper. Due to their unique features, such as low output divergence, high transmittance as well as extended depth of field these beam shapers provide a freedom in building an optimum optical system. Depending on the conditions of a particular technique it is possible to apply either a πShaper with imaging optics or a Focal-πShaper with focusing lenses. And important feature of these approaches is in easy adaptability to optical design of already existing material processing systems. There will be considered several optical layouts based on various refractive beam shapers πShaper/Focal-πShaper to generate square shaped laser spots of uniform intensity which sizes span from several tens of microns to millimetres. Examples of real implementations will be presented as well.Performance of various modern laser based micromachining techniques can be improved by applying square laser spots with uniform intensity distribution. And providing possibility of scanning of such a spot over whole working field with using popular 2- and 3-axis galvo mirror scanners is of great importance for many laser microprocessing technologies, like scribing, drilling vias in PCB, flat panel display repair. These tasks can be successfully solved with using the field mapping refractive beam shaping optics like πShaper and Focal-πShaper. Due to their unique features, such as low output divergence, high transmittance as well as extended depth of field these beam shapers provide a freedom in building an optimum optical system. Depending on the conditions of a particular technique it is possible to apply either a πShaper with imaging optics or a Focal-πShaper with focusing lenses. And important feature of these approaches is in easy adaptability to optical design of already existing material processing s...

Bimetallic grayscale photomasks for micro-optics fabrication usingdual wavelength laser writing techniques

Abstract: Microfabrication of high-resolution micro-optic devices requires <1/8λ (~60nm) precision both vertically and horizontally. More critical is the creation of 256-level grayscale masks to create sufficient vertical precision in the photoresist. Grayscale bimetallic photomasks are bi-layer thermal resists of Bismuth-on-Indium or Tin-on-Indium become controllably transparent by varying laser power thermally producing alloy oxide ranging ~3OD (unexposed) to <0.22OD (fully exposed). Previously, a direct-write multi-line CW Argon-ion laser writing system with feedbackcontrolled Gaussian beam achieved 256-level grayscale masks. The feedback system effectively reduced the average gray-level error from ±4.2 gray-levels in an open-loop approach to ±0.3 gray-levels in a closed-loop approach. Remaining gray-level errors were due to the Gaussian beam profile creating variations in gray-levels. Preliminary results show that a beam shaper creating a flattop beam helps reducing gray-level fluctuations. The multi-line Argon laser enables having multiple single beams separated from a single stabilized laser source. The single 514.5nm line used for writing gives better control of beam shape in the modulated laser beam. At the same time a lower power 457.9nm line introduced in the beam path to characterize the grayscale pattern both during and after the writing process. Filtering the writing laser line, sensor below the mask plate measures only the 457.9nm line enabling the high accuracy transparency measurements of the written mask near G-line (435.8nm). One target application is the creation of micro-lens arrays, which are lenses whose optical shape varies from lenslet to lenslet across the entire patterned surface of cm size. Laser direct-written grayscale masks enable relatively low cost, rapid turnaround mask production needed for creating such structures with microfabrication processes.

Multi-wavelength pulse generation using flattop optical frequency comb and arrayed waveguide grating

Abstract: A scheme of multi-wavelength pulse generator using optical frequency comb and arrayed waveguide grating (AWG) is proposed and experimentally demonstrated. A flattop optical frequency comb is shaped into multiple narrowband Gaussian spectra by using an AWG which contains a number of Gaussian channels, and then multi-wavelength optical pulses are achieved. In the experiment, six wavelength pulses with full width at half-maximum (FWHM) of 14.6 ps at 10 GHz are obtained, and two wavelength-interleaved pulse trains at 20 GHz and four wavelength-interleaved pulse trains at 40 GHz are demonstrated by using the multi-wavelength optical pulses. This scheme has flexibility because the pulse width, the repetition rate, and time-interval can be readily controlled.

Applying field mapping refractive beam shapers to improve irradiation of photocathode of FEL

Abstract: Providing the flattop (uniform) or other intensity distributions of short-pulse laser beam by irradiating photocathode of FEL is a technique that is frequently considered as a way to improve performance of FEL. This task can be easily solved with using beam shaping optics, for example, the field mapping refractive beam shapers like piShaper. The operational principle of these devices presumes transformation of laser beam intensity from Gaussian to flattop one with high flatness of output wavefront, saving of beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, negligible residual wave aberration, and achromatic design provides capability to work with ultra-short pulse lasers having broad spectrum. With using the same piShaper it is possible to realize various beam profiles like flattop, inverse Gauss or super Gauss by simple variation of input beam diameter; this feature makes these devices a powerful tool in reaching various effects of generation of an electron beam. This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts of their applying in systems of FEL photocathode irradiation. Examples of real implementations and experimental results will be presented as well.

Si-based tunable flattop photodetector with a stepped Fabry–Perot cavity

Abstract: This paper presents the design and analysis of a Si-based tunable flattop photodetector realized by the introduction of a stepped Fabry-Perot cavity, which can be thermally tuned via applying tuning power on its tuning electrode. By using a transfer matrix method, the spectral response of the photodetector is simulated in detail, indicating a flattop line shape can be achieved with an optimum step height. A trade-off residing in this device between the free spectrum range and the ease of fabrication of step height is also revealed and analyzed. In the final design of the photodetector, 1 dB linewidth of 0.5 nm, 3 dB linewidth of 0.8 nm, 6 dB linewidth of 1.2 nm, peak quantum efficiency of 40%, tuning efficiency of 91 mW/nm are theoretically obtained. We discuss the epitaxial growth and fabrication of the photodetector in the end, exhibiting the mature technique available for this device.

Applying field mapping refractive beam shapers in laser technologies for solar energy

Abstract: The flattop (uniform) and other intensity distributions of a laser beam are frequently considered as techniques to improve performance of laser technologies in manufacturing of solar cells. This task of creating these beam profiles can be easily solved with using beam shaping optics, for example, the field mapping refractive beam shapers like piShaper. The operation principle of these devices presumes transformation of laser beam intensity from Gaussian to flattop one with saving beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, capability to work with galvo-mirror scanning optics. The flattop, inverse Gauss, super Gauss, donut and other intensity distributions can be provided for the laser spots which size spans from microns to millimetres and centimetres; this makes these devices a powerful tool to improve the laser technologies like patterning, scribing, drilling, edge isolation, firing contacts, etc. This paper will describe some design basics of refractive beam shapers π Shaper and optical layouts of their applying in various technologies for solar cell manufacturing. Examples of real implementations and experimental results will be presented as well.

Beam combining with using beam shaping

Abstract: Performance of spectral beam combining depends on thermal effects on the optical components like volume Bragg gratings used for spectral selectivity of the beams combined. These thermal effects are results of absorption of laser radiation and in case of high power lasers can lead to reduction of efficiency of combining and losses. For example the Gaussian intensity distribution of laser beam leads to higher temperature in the central part of a grating and, hence, changing its operating specifications. Homogenizing of the temperature profile over the working field of a volume Bragg grating would increase efficiency of its operation. This can be realized through applying the beam shaping optics, for example refractive field mapping beam shapers providing high flexibility in building various optical setups due to their unique features: almost lossless intensity profile transformation, providing flattop, super Gauss or inverse Gauss profiles with the same beam shaper, saving of the beam consistency, high transmittance and flatness of output beam profile, extended depth of field, capability to adapt to real intensity profiles of TEM00 and multimode laser sources. This paper will describe some design basics of refractive beam shapers of the field mapping type, with emphasis on the features important for building and applications of high-power laser sources. There will be presented results of applying the refractive beam shapers in real research installations.

Tight focusing of cylindrical vector beam with high NA lens axicon

Abstract: Generation of cylindrical vector beam with high NA lens axicon system is investigated theoretically by vector diffraction theory. Results are potential useful such as peak-centered, donut, flattop focal shapes with extended focal depth may occur.

Advanced laser technologies for high-brightness photocathode electron gun

Abstract: A laser-excited photocathode RF gun is one of the most reliable high-brightness electron beam sources for XFELs. Several 3D laser shaping methods have been developed as ideal photocathode illumination sources at SPring-8 since 2001. To suppress the emittance growth caused by nonlinear space-charge forces, the 3D cylindrical UV-pulse was optimized spatially as a flattop and temporally as squarely stacked chirped pulses. This shaping system is a serial combination of a deformable mirror that adaptively shapes the spatial profile with a genetic algorithm and a UV-pulse stacker that consists of four birefringent α-BBO crystal rods for temporal shaping. Using this 3D-shaped pulse, a normalized emittance of 1.4 π mm mrad was obtained in 2006. Utilizing laser's Z-polarization, Schottky-effect-gated photocathode gun was proposed in 2006. The cathode work functions are reduced by a laser-induced Schottky effect. As a result of focusing a radially polarized laser pulse with a hollow lens in vacuum, the Z-field (Z-polarization) is generated at the cathode.

Graded-index design in coherent beam shaping

Abstract: We present a graded index (GRIN) design methodology for application in coherent beam shaping within the confines of geometric optics. In theory, a GRIN structure can be designed to convert a specific but arbitrary incident wave into any general amplitude and phase distribution. The incident and the desired output waves are converted into ray distributions where the local ray density is associated with the field amplitude and the ray direction and its optical path length are associated with the phase front of the waves. A set of ray paths is chosen to achieve the conversion between the initial and final ray distributions. Intermediate wavefronts are extrapolated from the ray paths and used to determine the refractive index profile required to effect the gradual evolution of the wave as it propagates through the structure. Our method is demonstrated by a GRIN structure design that converts a Gaussian beam into a flattop beam. In addition, several techniques used to reduce the dynamic range of the refractive index in the structure are examined and a design recipe is developed for this particular application. A ray trace shows that the resulting structure achieves the desired remapping of rays and produces a flat-top beam from an incident Gaussian beam.

Linear-induction-accelerator beam-energy-spread minimization: Cell models and timing optimization

Abstract: The second axis (Axis II) of the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory (LANL) is a linear induction accelerator (LIA) using 74 cells, each driven by a separate pulsed-power modulator. The summation of the injector and 74 cell voltages is the beam-energy temporal profile. The ability to perform precise multi-pulse radiography is heavily influenced by the temporal beam energy spread, related beam motion, and other focusing and target factors. Beam loading affects both the shape and magnitude of each cell's voltage during the pulse. Ideally, each pulsed-power modulator/cell pair is tuned such that the loaded-cell voltage is flat with minimal amplitude variation during the pulse. However, changes in operating parameters on Axis II (beam current, operating cell voltage) have altered the amount of flattop variation resulting in more energy spread than when commissioned. In this paper, we present an optimization and synthesis method which minimizes the beam's temporal energy spread by adjusting the timing of cell voltages, either advancing or retarding them, such that the injector voltage plus the summed cell voltages in the LIA result in a flatter energy profile. The method accepts as inputs the beam current, injector voltage, cell-voltages, and synthesizes loaded cell voltages as needed. Simulations and experimental data for both unloaded and loaded-cell timing optimizations are presented. For the unloaded cells, the pre-optimization baseline energy spread was reduced by over 30 % as compared to baseline. For the loaded-cell case, the measured energy spread was reduced by 49% compared to baseline.

Beam shaping optics to improve holographic and interferometric nanomanufacturing techniques

Abstract: Performance of various holographic and interferometric nanomanufacturing techniques can be essentially improved by homogenizing the intensity profile of the laser beam with using beam shaping optics like πShaper transforming the laser beam intensity from Gaussian to flattop one with high flatness of output wavefront, saving of beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, negligible residual wave aberration. Applying of these beam shapers brings serious benefits to the Spatial Light Modulator based techniques like Computer Generated Holography, holographic projection processing applications, holographic lithography, interferometric techniques of recording the Volume Bragg Gratings. This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts of their applying in holographic and interferometric systems. Examples of real implementations and comparative experimental results of applying the refractive beam shapers in systems of holographic lithography and other techniques.Performance of various holographic and interferometric nanomanufacturing techniques can be essentially improved by homogenizing the intensity profile of the laser beam with using beam shaping optics like πShaper transforming the laser beam intensity from Gaussian to flattop one with high flatness of output wavefront, saving of beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, negligible residual wave aberration. Applying of these beam shapers brings serious benefits to the Spatial Light Modulator based techniques like Computer Generated Holography, holographic projection processing applications, holographic lithography, interferometric techniques of recording the Volume Bragg Gratings. This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts of their applying in holographic and interferometric systems. Examples of real implementations and comparative experimental results of applying...

Manipulating minibands and achieving polarization-independent flattop band through one-dimensional photonic crystals

Abstract: The minibands and minigaps in the transmission bands of one-dimensional photonic crystals have been studied through Bloch’s theorem, the tight-binding method and calculations based on the transfer matrix method. The minigap depths are dependent on the resonance intensity within the layers of one-dimensional photonic crystals and can be manipulated through the incidence angle. For a special structure and incidence angle, a polarization-independent flattop band can be achieved.