Improved Beam Quality Research Articles

Mitigating mode instabilities by controllable mode beating excitation with a photonic lantern

In this study, a method of stabilizing the output beam of a large-mode-area fiber amplifier operating above the mode instability threshold is demonstrated. A mode control system based on the photonic lantern is used to excite dynamic mode beatings as the seeding stage of a 42/250 µm Yb-doped fiber amplifier. We propose a theoretical explanation for the validity of this method: provided that the mode interference patterns generated by input mode beatings are antisymmetric to those in the main amplifier caused by thermally induced non-linear effects, the refractive index grating will not be formed to indicate the onset of mode instability. Experimentally, significant mode stability and beam quality improvement have been achieved in this system at power levels of up to nearly four times the mode instabilities threshold.

Improvement of multi-harmonic buncher’s quality factor for RAON

The high-intensity rare-isotope accelerator RAON, a component of the Rare Isotope Science Project, was developed to accelerate heavy ions from elements ranging from proton to uranium. The injector line consists of an Electron Cyclotron Resonance Ion Source, a Low Energy Beam Transport line, a Radio Frequency Quadrupole (RFQ), and a Medium Energy Beam Transport line. To improve beam quality, a multi-harmonic buncher (MHB) was proposed for inclusion before the RFQ. The MHB is needed to efficiently utilize two beams with different mass-to-charge (A/q) ratios. The fundamental frequency of the MHB is half the frequency of the RFQ; as such, a harmonic synthesis is effectively bunching a beam. Electric field simulations of the MHB were conducted using the CST Microwave Studio (MWS). The electrodes were designed as cone- type electrodes so that the electric field can be concentrated between the electrodes through which the beam passes. The electric field was optimized to account for structural changes such as gap length between electrodes, resonator position and other design details. The model optimized in the simulation tool was engineered through the 3D Inventor program.

Carrier density non-pinning at stripe edges and widened lateral far field due to longitudinal temperature variation in broad-area high power diode lasers

Broad area lasers operating at high power with improved beam quality are needed in many applications. In typical high-power diode lasers with asymmetric facet coating, it is observed that the carrier density fails to completely pin above threshold with significant levels of lateral carrier accumulation (LCA) at the front facet stripe edges. Systematic experimental data are presented to quantify such non-pinning and its effect on lateral far field in multiple diode lasers with varying longitudinal temperature variation (LTV) using spontaneous emission imaging. LTV is quantified and correlated with the carrier accumulation and its dependence on facet reflectivity and cavity length. Results of measurements under continuous wave (CW) and quasi-CW conditions are used to regulate the level of heating and, hence, to isolate the contribution of LTV and optical field profile. Conventional long resonator asymmetric-coated devices show a twofold increase in LCA from threshold to 10 A current at the front stripe edge, but LCA varies less than 20% for lower facet asymmetry or short resonators. Similarly, strong temperature driven far field broadening is observed for conventional long-resonator asymmetric-coated devices (3°–4° variation with temperature at 10 A), which is strongly reduced for lower facet asymmetry or short resonators (<1° at 10 A).

Effects of off-axis beam injection to RFQ

It is observed during the RAON injector commissioning that the RFQ (Radio Frequency Quadrupole) output beam properties can change significantly because of an off-axis beam injection to the RFQ. When the beam is injected to the RFQ at an angle or at an offset, beam emittances increase and beam tails are induced along with deviations in output beam parameters α and β, which affects beam profiles in the downstream section of the linac. It is observed also that the X and Y planes of the RFQ are reasonably decoupled. Angle and offset in the X plane affect the X beam properties primarily leaving the Y beam properties almost unaffected, and vice versa. Beam on-axis injection to the RFQ is crucial to improving beam quality and also to the agreement between experiment and simulation.

Wavefront shaping to improve beam quality: converting a speckle pattern into a Gaussian spot

A perfectly collimated beam can be spread out by multiple scattering, creating a speckle pattern and increasing the étendue of the system. Standard optical systems conserve étendue, and thus are unable to reverse the process by transforming a speckle pattern into a collimated beam or, equivalently, into a sharp focus. Wavefront shaping is a technique that is able to manipulate the amplitude and/or phase of a light beam, thus controlling its propagation through such media. Wavefront shaping can thus break the conservation of étendue and, in principle, reduce it. In this work we study how much of the energy contained in a fully developed speckle pattern can be converted into a high quality (low M 2) beam, we support it with a theoretical framework and discuss the advantages and limitations of this approach.

Broadband 2D n × n Vortex Arrays Generated from an Efficient Petal‐Like Raman Laser with an Astigmatic Mode Convertor

Broadband vortex arrays distributed in 2D n × n vortices are extremely needed for multiple microparticle manipulation, high‐capacity optical communication, and quantum information. Vortex arrays are usually generated with spatial light modulators, metamaterials; it is a big challenge to improve beam quality and output power. Herein, crisscross 2D n × n vortex arrays are converted from petal‐like Raman lasers with an astigmatic mode convertor. Efficient petal‐like lasers in the form of the Laguerre–Gaussian mode with zero radial index and l azimuthal index (LG0,l ) are irradiated in a Raman microchip laser (RML) by adjusting optimal beam waist and locating the position of focus spot of the annular pump beam. High‐beam‐quality LG0,9 petal‐like Raman laser oscillating at broadband wavelength (bandwidth: 16.2 nm) is generated with power of 773 mW and an efficiency of 10.6%. The conversion efficiency from petal‐like laser to crisscross vortex array is as high as 90%. The vortex array with 5 × 5 vortices operating around 1078 nm is generated with a power of 693 mW and an optical efficiency of 9.5%. Taking advantages of the broad emission spectrum of ytterbium‐doped laser materials and stimulated Raman scattering effect, RMLs are potential sources for generating flexible vortex arrays with desired 2D n × n vortices oscillating in Raman wavelengths.

3 × 1 fiber signal combiner with high beam quality Gaussian-like beam for a 10kW-level fiber laser.

We present the design and fabrication of a 3 × 1 signal combiner with high beam quality based on supermode theory. For improving beam quality, the fiber with core diameter of 34 µm and numerical aperture of 0.11 is first chosen as the output fiber. An 8.89 kW output laser with a power transmission efficiency of 97.2% and a low temperature rise coefficient of 3.5 °C/ kW at >8 kW is obtained when the combiner launched by three Yb-doped fiber lasers. In addition, the energy density distribution of the output beam is Gaussian-like and M2 factor is 2.32, which is the best beam quality compared with the presented signal combiners for high power laser to the best of our knowledge.

Mitigating thermal blooming effect of seawater using self-rotating beam

When the high-power lasers propagate in seawater, the thermal blooming (TB) effect can cause wavefront distortion, which severely affects the beam quality. Here we proposed a potential approach to reduce the thermally-induced wavefront distortion by using the self-rotating beam in seawater. This approach using the self-rotating beam can directly rotate the intensity distribution of the laser beam and thus break the accumulation path of thermally-induced wavefront distortion, contributing to the improvement of beam quality. Results show that, the self-rotating beam with an appropriate rotation frequency can well mitigate the thermal blooming effect in seawater, exhibiting excellent performance in reducing beam expansion, beam centroid drift and beam quality degradation.

Partitioning local seismogram wavefields using continuous wavelet transform methods for IRIS wavefield experiment arrays

SUMMARY We applied nonlinear thresholding and scale–time gating in the continuous wavelet transform (CWT) domain to denoise, identify and characterize seismic phases contained in gradiometer and phased array waveforms of four seismic events recorded during the 2016 Incorporated Research Institutions of Seismology Wavefields Experiment in northern Oklahoma. A dense, 80-element three component phased array was subset from the linear array deployments to examine background noise, waveform coherence and seismic wave composition for local explosion and earthquake waveforms. CWT techniques were also used to significantly improve gradiometery analyses for data recorded by the geodetic array subexperiment. We observed as much as two orders of magnitude gain in the data signal-to-noise ratio. We also saw improvement in array beam quality after denoising the seismic data. Using the signal partitioning technique, we were able to extract and identify many phases based on their positions on the scale–time plane. CWT denoising and wavefield decomposition techniques also improved gradiometry analysis results from the 112-element geodetic array (also called the gradiometer) since waves could be separated before the computation of wave attributes. The operations of removing noise and gating out signal phases improved signal coherence across array records and provided clear P wave onsets on horizontal records, which can mitigate phase picking error and resulting event location uncertainty.

Design and operation of transfer lines for plasma wakefield accelerators using numerical optimizers

The Advanced Wakefield (AWAKE) Experiment is a proof-of-principle experiment demonstrating the acceleration of electron beams via proton-driven plasma wakefield acceleration. AWAKE Run 2 aims to build on the results of Run 1 by achieving higher energies with an improved beam quality. As part of the upgrade to Run 2, the existing proton and electron beamlines will be adapted and a second plasma cell and new 150-MeV electron beamline will be added. The specification for this new 150-MeV beamline will be challenging as it will be required to inject electron bunches with micron-level beam size and stability into the second plasma cell while being subject to tight spatial constraints. In this paper, we describe the techniques used (e.g., numerical optimizers and genetic algorithms) to produce the design of this electron line. We present a comparison of the methods used in this paper with other optimization algorithms commonly used within accelerator physics. Operational techniques are also studied including steering and alignment methods utilizing numerical optimizers and beam measurement techniques employing neural networks. We compare the performance of algorithms for online optimization and beam-based alignment in terms of their efficiency and effectiveness.

SBS-PCM characteristic of sub-nanosecond laser based on rotating wedge

One major setback in the applications of high-power solid-state lasers in areas like high energy density physics is the beam quality that deteriorates with increasing pump power. To solve this problem, we proposed to develop a high beam quality and stable stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) using a filtration approach and a carefully designed geometry that includes a rotating wedge to operate under a high repetition rate laser. Upon reducing the micro-impurities to 100 nm, the accumulated heat at the focus reduced greatly, resulting in improved beam quality and better phase fidelity. At 6.5 mJ input energy and below 100 Hz repetition rates, there is no observable optical breakdown or obvious thermal effects as opposed to the unfiltered medium that experience optical breakdown at 0.5 mJ and 50 Hz repetition rate. The simulated temperature gradient at the focus shows that the material absorbed the laser within a short distance from the irradiation side, and the temperature probe reveals that the temperature increases with pumping power and time due to heat accumulation at the focus. To disperse the heat accumulation through a large area within the SBS cell, we design and introduce a rotating wedge. The impact of the rotating wedge was much more noticeable when the system works at higher pump power.

Comparison of multimode GRIN-fiber Raman lasers with FBG and random DFB cavity

Abstract Raman lasing in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to wavelength-agile fiber lasers of new type. Here we compare power scaling and brightness enhancement capabilities of Raman laser based on multimode GRIN-fiber of 62.5/125 um core/cladding diameters pumped by ∼700 W multimode source with beam quality M2∼10, performed in two different cavity configurations: 1) linear cavity based on two fiber Bragg gratings and 2) half-open cavity with one FBG and random distributed feedback via Rayleigh backscattering along the GRIN fiber.

High-energy Q-switched 16-core tapered rod-type fiber laser system.

High-energy Q-switched master oscillator power amplifier systems based on rod-type 4 × 4 multicore fibers are demonstrated, achieving energy up to 49 mJ in ns-class pulses. A tapered fiber geometry is tested that maintains low mode order in large multimode output cores, improving beam quality in comparison to a similar fiber with no taper. The tapered fiber design can be scaled both in the number of amplifying cores and in the dimensions of the cores themselves, providing a potential route toward joule-class fiber lasers systems.

Fiber-laser platform for precision brain surgery.

Minimally invasive neurological surgeries are increasingly being sought after for treatment in neurological pathologies and oncology. A critical limitation in these minimally invasive procedures is lack of specialized tools that allow for space-time controlled delivery of sufficient energy for coagulation and cutting of tissue. Advent of fiber-lasers provide high average power with improved beam quality (lower M2), biocompatible silica fiber delivery, reduced cost of manufacturing, and radiant output stability over long operating periods. Despite these advancements, no fiber-laser based surgical tools are currently available for tissue resection in vivo. Here we demonstrate a first to our knowledge, fiber-laser platform for performing precise brain surgery in a murine brain model. In this study, our primary aims were to first demonstrate efficacy of fiber-lasers in performing precise blood-less surgery in a murine brain with limited non-specific thermal damage. Second, fiber-lasers' ability to deliver radiant energy through biocompatible silica fibers was explored in a murine brain model for blood less resection. A bench-top optical coherence tomography (OCT) guided fiber-laser platform was constructed with a stereotactic stage for performing precision brain surgery. A pulsed quasi-continuous wave ytterbium (Yb) fiber-laser (1.07 µm) was used to perform vascular specific coagulation while a pulsed nanosecond thulium fiber-laser (1.94 µm) was used to conduct bloodless cutting, all under the guidance of a swept-source OCT system centered at 1310 +/- 70 nm. Specialty linear and circular cuts were made in an in vivo murine brain for bloodless brain tissue resection. The two fiber-lasers were combined into a single biocompatible silica fiber to conduct brain surgery resection under the bench-top OCT system's imaging microscope. Vascular specific coagulation was demonstrated in all five mice studied. Bloodless linear cuts and point cuts were demonstrated in vivo. Histologically, thermal injury was measured to be less than 100 µm while a removal rate of close to 5 mm3/s was achieved with an average Tm fiber-laser power of 15 W. To the authors' knowledge, this is the first demonstration of a fiber-laser platform for conducting in vivo bloodless brain tissue resection with a pulsed thulium (Tm) fiber-laser and a quasi-continuous wave (QCW) Yb fiber-laser. The demonstrated fiber-laser platform, if successfully configured for use in the operating room (OR), can provide surgeons a tool for rapid removal of tissue while making surgical resections of brain regions more precise, and can be basis for a flexible cutting tool capable of reaching hard-to-operate regions.

Investigations on beam quality improvement of an NCPM-KTA-based high energy optical parametric oscillator using an unstable resonator with a Gaussian reflectivity mirror [Invited

Investigations on beam quality improvement of an NCPM-KTA-based high energy optical parametric oscillator using an unstable resonator with a Gaussian reflectivity mirror [Invited

Optimization of electron bunch quality using a chirped laser pulse in laser wakefield acceleration

In this work, we perform particle-in-cell simulations to investigate the electron bunch quality using a chirped laser pulse in a laser wakefield acceleration mechanism. Particular attention is devoted to the electron beam quality improvement using a chirped laser pulse through group delay dispersion. By adjusting the value of group delay dispersion, the electron beam quality was controlled and improved in the range: charge (31--325 pC), energy (240--485 MeV), energy spread (2.1%--12.5%), and bunch duration (3.3--17 fs). The electron bunch charge highly depends on the chirping of the laser pulse. We anticipate that the ultrashort sub-GeV electron bunch generated in the laser wakefield acceleration mechanism using chirped laser pulses may be crucial in developing free-electron laser-based compact radiation sources.

Beam-quality improvement with a bio-inspired sunflower array for coherent beam combining.

The beam quality of coherent beam combing (CBC) is significantly affected by the beam array schemes, which are regular hexagon arrays in most research. Here, we propose a bio-inspired Fermat spiral array (FSA) for large-array CBC, for the first time to our knowledge. The far-field distribution and beam quality of CBC with various designed FSAs was investigated numerically and experimentally; the simulated and experimental results agreed with each other. The power in the bucket (PIB) increased with the central space density of the FSA, accompanying by the weakening of the far-field sidelobes. In addition, for the FSA with constant space density, the PIB increased, and sidelobes weakened with the increase of the array filling factor. The FSA could effectively improve the PIB and weaken the sidelobe of the CBC far field by the aperiodic and nonuniform space density arrangement, compared to the regular arrays. These results provide a new approach for the beam arrangement of large-array CBC.

High efficiency 355 nm laser by employing a flat-flat cavity with improved harmonic beam quality

A high efficiency 355 nm ultraviolet (UV) laser is obtained by intracavity frequency-tripling of a 1064 nm Nd:YVO4 laser in a flat-flat cavity. The 5.6 W UV maximum average output power is obtained at the incident pump power of 29 W at 808 nm and the repetition rate of 50 kHz, corresponding to the pump-UV conversion efficiency of 19.3%. The corresponding pulse width is 14.2 ns, with the energy of a single pulse and corresponding peak power estimated to be 112 μJ and 7.89 kW, respectively. The frequency doubling and tripling lasers are obtained by employing type-I phase-matching LiB3O5 (LBO) crystal with parallel surfaces and a type-II phase-matching LBO crystal with Brewster angle-cut uncoated surface, respectively. The beam quality of the 355 nm laser is measured to be M x 2 = 1.257 and M y 2 = 1.173, respectively. The reasons for improvement of harmonic beam quality are explained by simulating the laser field intensity distribution changes in two special directions during the nonlinear conversion process.

Recent Progress of the PAL-XFEL after User Service Start

The X-ray Free Electron Laser of the Pohang Accelerator Laboratory (PAL-XFEL) was open to the public in 2017 and started user service. Even after the user service had started, improvement of beam quality was continuously attempted, and several meaningful results were achieved. In this paper, we present recent progress in the PAL-XFEL, including the present status of its operation. The increase in the FEL pulse energy is explained, and the generation of a self-seeding FEL is described. In addition, the generation of a two-color FEL is mentioned.

Improvement of electron beam quality in laser wakefield acceleration by a circularly-polarized laser pulse

Electron acceleration driven by laser wakefield using a laser pulse with circular polarization is studied to generate better-quality electron bunches. In order to inject electrons into the accelerating phase of the plasma wave, a density transition shaped bump is employed. Using particle-in-cell simulations, we demonstrate the influence of laser polarization on the quality of electron bunch. The simulation results (using experimentally achievable parameters) show that the electron bunch with higher peak-energy, narrow energy-spread, and low emittance can be obtained by using a laser pulse with circular polarization as compared to the usually employed linear polarization in a mm-scale length plasma. The results show the improved quality of the electron bunch generated from laser wakefield accelerations. The predicted electron bunch quality may be crucial in developing plasma-based electromagnetic radiation sources.