High Beam Quality Research Articles

Watt-scale near-diffraction-limited mid-infrared laser delivery by a chalcogenide anti-resonant fiber

We have developed an effective one-step extrusion method to prepare a nodeless chalcogenide hollow-core anti-resonance fiber, characterized by excellent symmetry and less requirements for drawing pressure in achieving the desired wall thickness. The resulting fiber exhibits excellent uniformity, with an ultra-large effective mode area of 21970 µm2 and a low overlap factor of η = 0.03%. It can withstand an input power exceeding 10 W at 4.5 µm and maintain a stable output power of 1.2 W at an input power of 5.25 W, all while preserving a high beam quality with an M2 value of 1.09. The output single-mode laser remains highly stable when the translation offset of the laser coupling into the fiber is less than 100 µm. The chalcogenide hollow-core fibers with watt-level mid-infrared laser delivery power and near-diffraction-limited beam quality can be used for practical applications in industry, medicine, and defense.

Effect of fiber twist angle and non-uniform symmetric alignment on signal combiner

Signal beam combiners play a pivotal role in enhancing the output power of fiber lasers, which have wide-ranging applications from industrial processing to medical and military uses. This paper explores the influence of fiber twist angle and non-uniform symmetric arrangement on the performance of 19 × 1 fiber signal combiners. A simulation model was developed to analyze the impact of these parameters under adiabatic tapering conditions and the principle of brightness conservation. The model allowed for a systematic investigation of how varying twist angles and non-uniform spacings affect the combiner's performance metrics, such as transmission efficiency and beam quality. The study found that an optimal balance between high transmission efficiency (up to 98.5%) and good beam quality (minimum M2 factor of 1.06) can be achieved when the twist angle is kept below 60° and the non-uniform spacing is maintained within 10–30 μm. These conditions ensure minimal degradation of the beam quality while maximizing the transmission efficiency of the combiner. These findings offer valuable insights into the optimization of signal combiner design, which is critical for advancing high-power fiber laser systems. By carefully controlling the fiber twist angle and non-uniform spacing, designers can achieve superior performance in fiber laser applications, thereby enhancing the overall efficiency and reliability of these systems. This research contributes to the broader field of optical engineering by providing a deeper understanding of the underlying principles that govern the performance of signal combiners.

High-brightness megahertz-rate beam from a direct-current and superconducting radio-frequency combined photocathode gun

High-brightness megahertz-rate electron beams are highly desired for cutting-edge applications in many areas of science. Photocathode electron guns capable of generating such beams with low dark current remain to be a challenging field. In this paper, we report breakthroughs with a hybrid gun combining a direct-current gap and a superconducting radio-frequency (SRF) cavity. The gun, employing K2CsSb photocathodes driven by a green laser, delivers a few MeV electron beam at 1 and 81.25 MHz rates with an average current up to 3 mA and a dark current several orders lower than current normal conducting continuous-wave guns. Emittance compensation and multipole field corrections have been applied to achieve a high-quality beam. The achievement will inspire the development of high-brightness SRF guns and promote megahertz-rate beam applications including coherent linac light sources and ultrafast electron diffraction/microscopy. Published by the American Physical Society 2024

Decoupled tip-tilt detection and steering of beamlets in a fiber laser array using sequential tagging lasers.

The tiled fiber laser array is a promising architecture for power scaling and mitigation of atmospheric turbulence. Precise tip-tilt detection and steering of the beamlets in the array to guarantee effective overlap in the far-field is essential for high beam qualities. However, the beamlets are tightly coupled in the far-field, making it a major challenge to identify each other. Consequently, most researchers turn to indirect methods without tip-tilt detection. We propose what we believe to be a novel approach of decoupled tip-tilt detection and steering of the beamlets for a tiled fiber laser array. A pulsed tagging laser is divided and combined into each beamlet with difference pulse sequences. By synchronizing the tagging laser pulses and exposures of the far-field camera, only one spot of the tagging laser is detected in each image, whose location is coincident with its corresponding beamlet. Thereby, the beamlets are decoupled by different pulses. The pointing of each beamlet is then readily detected and steered individually. The proposed method is validated by experiments with a seven-beamlet adaptive fiber collimator array and proved efficient. This is the first, to the best of the authors' knowledge, demonstration of direct and decoupled beamlet tip-tilt detection and steering in the far-field for a tiled fiber laser array.

Simulation of laser plasma wakefield acceleration with external injection based on Bayesian optimization

Abstract In laser wakefield acceleration (LWFA), injecting an external electron beam at certain energy is a promising approach to achieving high-quality electron beam with low energy spread and low emittance. In this paper, the process of laser wakefield acceleration with an external injection at 10 pC has been studied in simulations. A Bayesian optimization method is used to optimize the key laser and plasma parameters, so that the electron beam is accelerated to the expected energy with a small emittance and energy spread growth. The effect of rising edge of the plasma on the transverse properties of the electron beam is simulated and optimized in order to ensure that the external electron beam is injected into the plasma without significant emittance growth. Finally, a high-quality electron beam with an energy of 1.5 GeV, the normalized transverse emittance of 0.5 mm·mrad and the relative energy spread of 0.5% at 10 pC is obtained.

Twenty kilowatts level high beam quality spectral beam combination by dual gratings with distinct groove densities based on conical diffraction structures

We establish a spectral beam combing structure with two distinct diffraction gratings. A theoretical analysis of the beam quality degradation of this beam combining system is given. A six-channel spectral beam combining experiment is constructed based on conical diffraction structure by transmission and reflection gratings. Over 19 kW combined power is achieved with a combining efficiency of about 92%. The six incident fiber amplifiers, ranging from 1040 nm to 1082 nm, can each deliver a 3.5 kW output power laser with about 60 GHz 3 dB linewidth. Both transmission and reflection diffraction gratings, with different grating periods (1000lines/mm, 1170lines/mm), are employed in this work. The combined beam quality factor is controlled to be M2∼2 by effective dispersion compensation. To the best of our knowledge, this is the first time to demonstrate a high power beam combination by two gratings with different types and periods, which verifies the dispersion capability of this structure and provides an alternative approach for high power beam combining.

Generation of quasi-monoenergetic proton beams from near-critical density plasmas irradiated by picosecond laser pulses

Laser-driven high-quality ion beams hold immense potential for applications in diverse fields such as tumor therapy, fast ignition, and so on. However, current experimental ion beams are often constrained by either a large energy spread or relatively low energy. In this paper, we proposed a novel scheme for generating quasi-monoenergetic proton beams by irradiating near-critical-density plasmas, which have a density gradient with a picosecond laser pulse. This approach leverages two key aspects: first, the sustained interaction between the laser pulse and the plasma enhances the duration of magnetic vortex acceleration, thereby promoting extended ion acceleration. Second, the utilization of a multi-species target facilitates the formation of a dual-peaked electric field, which leads to the accumulation of protons in the negative gradient of the accelerating phase, resulting in a quasi-monoenergetic proton beam. The two-dimensional particle-in-cell simulation reveals that by employing a laser intensity of 1.37 × 1020 W/cm2 with a pulse duration of 0.5 ps, we can achieve a carbon ion beam with an energy of 50 MeV/u, and a quasi-monoenergetic proton beam exhibiting a cutoff energy of 160 MeV/u, a peak energy of 75 MeV/u, an energy spread of 3.1 %, and an angle divergence of ∼ 3.2°. Furthermore, the quasi-monoenergetic property is corroborated in three-dimensional simulation results, underscoring the robustness and effectiveness of our proposed scheme.

Spatiotemporal dissipation dynamics: a route to high-beam-quality and high-peak-power spatiotemporal mode-locked fiber lasers

Spatiotemporal mode-locking (STML) opens a new avenue for implementing high-energy, high-peak-power mode-locked fiber oscillators. However, the compromised beam quality poses a critical limitation to their broader applications. This study presents a method for enhancing the beam quality of STML fiber lasers by employing spatiotemporal dissipation involving the quenching and reabsorption effects of multimode erbium-doped fibers. The proposed technique introduces spatiotemporal saturable absorption, achieving high beam quality without the stringent conditions required for Kerr beam self-cleaning (BSC). Integrating spatiotemporal dissipation with Kerr BSC, we demonstrate an all-anomalous-dispersion Er-doped STML fiber laser, which produces solitons with 6.7 nJ pulse energy (the intracavity solitons with 25.8 nJ pulse energy and >52.8kW peak power), sub-500 fs pulse duration, and beam quality with M x 2/M y 2=1.23/1.20. To our knowledge, it is a record peak power for 1.5 µm band soliton lasers. Additionally, the approach enables the generation of noise-like pulses with M x 2/M y 2=1.04/1.13. This work not only advances our understanding of spatiotemporal dissipation dynamics in STML fiber lasers, but also paves the way toward high-performance STML fiber lasers, rendering them very attractive for applications.

Characterizing post-compression of mJ-level ultrafast pulses via loose focusing in a gas cell.

The ability to generate high-intensity ultrashort laser pulses is a key driver for advancing the strong-field physics and its applications. Post-compression methods aim to increase the peak intensity of amplified laser pulses via spectral broadening through self-phase modulation (SPM), followed by temporal pulse compression. However, other unavoidable nonlinear self-action effects, which typically occur parallel to SPM, can lead to phase distortions and beam quality degradation. Here we study the ability to compress high-energy pulses by loose focusing in a noble gas to induce nonlinear spectral broadening, while limiting unwanted nonlinear effects such as self-focusing. We introduce ptychographic wavefront sensor and FROG measurements to identify the regimes that optimize pulse compression while maintaining high beam quality. Using a 700 mbar argon-filled double-pass-based scheme, we successfully compress 2 mJ, 170 fs, 1030 nm laser pulses to ∼35 fs, achieving 90% overall flux efficiency and excellent stability. This work provides guidelines for optimizing the compressed pulse quality and further energy scaling of double-pass-based post-compression concepts.

High beam quality and high peak power 1.6 µm deuterium Raman laser.

In this work, a 1.67 µm laser was generated in the pressurized deuterium pumped by a pulsed 1064 nm laser via the stimulated vibration + rotation Raman scattering (SV-RRS). By the optimization of the deuterium pressure, focal length, and polarization of the pump laser, a 66.2 mJ Raman laser was achieved with the configuration of 1.7 MPa deuterium pressure, 2245 mm focal length, and 220 mJ circularly polarized pump laser. Correspondingly, the photon conversion efficiency was 47.8%, the energy conversion efficiency was 30.5%, the pulse width was 5.4 ns, and the maximum peak power was 12.3 MW. In addition, SV-RRS was found to improve beam quality significantly: from Mx 2 = 2.67 and My 2 = 3.07 for the pump laser to Mx 2 = 1.42 and My 2 = 1.53 for the 1.67 µm Raman laser.

Generation of High-Quality Cylindrical Vector Beams from All-Few-Mode Fiber Laser

Transverse mode control of laser intracavity oscillation is crucial for generating high-purity cylindrical vector beams (CVBs). We utilized the mode conversion and mode selection properties of two-mode long-period fiber gratings (TM-LPFGs) and two-mode fiber Bragg gratings (TM-FBGs) to achieve intracavity hybrid-mode oscillations of LP01 and LP11 from an all-few-mode fiber laser. A mode-locked pulse output with a repetition rate of 12.46 MHz and a signal-to-noise ratio of 53 dB was achieved with a semiconductor saturable absorber mirror (SESAM) for mode-locking, at a wavelength of 1550.32 nm. The 30 dB spectrum bandwidth of the mode-locked pulse was 0.13 nm. Furthermore, a high-purity CVB containing radially polarized and azimuthally polarized LP11 modes was generated. The purity of the obtained CVB was greater than 99%. The high-purity CVB pulses have great potential for applications in optical tweezers, high-speed mode-division multiplexing communication, and more.

Switchable generation and dynamic evolution of vector vortex beams.

The vector vortex beams (VVBs) are endowed with helical phase and vector polarization. The rich optical properties of VVBs have attracted extensive concern. Here the geometric phase is applied to manipulate both the phase and polarization of light for switchable generation of VVBs by vortex plates. As a natural longitudinally varying optical beam, the dynamic evolution of VVB with the propagation distance is also analyzed and verified based on the Gouy phase. The proposed method features simple structure, high beam quality and flexible switching, which may provide an alternative and flexible way for multi-degree-of-freedom modulation of optical beams and supplies solutions and approaches for applications in classical and quantum fields.

Kilowatt-level supercontinuum generation employing a homemade taper-shaped ytterbium-doped fiber amplifier.

A simple generation method of high-power supercontinuum (SC) based on a homemade long ytterbium-doped taper fiber (T-YDF) amplifier has been demonstrated experimentally and analyzed in this work. The power and spectra of the obtained SC are made adjustable by changing the seed pulse repetition rates. Under a 7.5 MHz seed pulse repetition rate, a SC output is obtained with a spectral range of 1000 nm-1650 nm, maximum output power of 1066 W, and conversion efficiency of 75.7%. The core/clad diameters of the homemade T-YDF are ∼20/400 µm and ∼30/600 µm at the input and output ends, respectively, and the total length is 25 m. Such a long taper fiber can further enhance the properties of the SC while ensuring high power and good beam quality. To the best of the authors' knowledge, this is the first report of directly generating a high average power SC based on the T-YDF amplifier, which provides a proof-of-concept experiment to achieve a high average power SC source and greatly improves the potential application value of the SC source.

All-solid continuous-wave Nd: YVO4-diamond intracavity Raman laser

We demonstrated a continuous intracavity diamond Raman laser operating by linear-cavity and V-cavity design. The diamond crystal as the Raman medium and Nd: YVO4 as the laser gain medium was utilized to achieve high beam quality Stokes output with narrow line width. By employing linear and V-shaped cavity, we achieved Stokes light outputs of 1.68 and 2.26 W, respectively. Compared to the linear cavity, the V-shaped cavity has a higher conversion efficiency of 11.07% and a narrower linewidth of 0.07 nm. Mechanism for linewidth of different cavity design and methods for further increasing power range and high quality are discussed.

Research on mid-infrared quantum cascade lasers spectral beam combining

Quantum cascade lasers (QCLs) in the mid-infrared (MIR) hold significant potential for widespread applications in both military and civilian contexts. However, the utility of present single-chip QCLs is hampered by issues such as low output power and subpar beam quality. This study addresses these limitations by employing spectral beam combining (SBC) based on a diffraction grating, with an aim to enhance both power and beam quality of MIR QCLs. Coaxial power synthesis of three single-chip QCLs (around 4.75 μm) is achieved experimentally, importantly, the beam quality did not decrease after combining, essentially maintaining the same quality as before combining. It is proposed that there are four main factors affecting the combining efficiency, namely operating optical power (or driving current) of the chips, individual differences in QCL chips, diffraction grating, and reflectivity of feedback mirror, and their effects on the combining efficiency are discussed separately. This study confirms that SBC is an effective way to obtain high-power and high-beam quality MIR QCL sources, and lays a research foundation for more beam spectral combining.

Numerical investigation of steering synchronization dynamics of laterally coupled laser array with monolithically-integrated external cold cavity.

This study numerically explores the synchronization dynamics of a one-dimension edge-emitting laser array monolithically-integrated with an external cold cavity comprehensively, aiming to achieve an in-phase mode optical field. By employing the optical feedback rate equation, the impact of mutual feedback coefficient and coupling coefficient on the synchronization process are investigated thoroughly. The proposed external cold cavity, designed according to the Talbot effect, could significantly diminish the reflectivity of front facet through separated electrode structure, therefore facilitating the phase-locking process. Consequently, the study uncovers the effective regime for establishing in-phase mode operation. Additionally, the numerical analysis also reveals the vivid synchronization dynamic from a chaotic state to partially-phase-locked, then completely-phase-locked, and ultimately periodic oscillation. Furthermore, the impact of practical fabrication tolerances on the synchronization process are explored as well. Based on the simulation results, our work could offer valuable insights for steering the on-chip optical field and developing novel laser arrays with high beam quality.

Technical Design Report for the LUXE experiment

AbstractThis Technical Design Report presents a detailed description of all aspects of the LUXE (Laser Und XFEL Experiment), an experiment that will combine the high-quality and high-energy electron beam of the European XFEL with a high-intensity laser, to explore the uncharted terrain of strong-field quantum electrodynamics characterised by both high energy and high intensity, reaching the Schwinger field and beyond. The further implications for the search of physics beyond the Standard Model are also discussed.

Miniaturization of high beam quality 1.543 μm Raman laser with backward stimulated Raman scattering

It is very challenging to make high peak power (big pulse energy) Raman lasers compact, due to laser induce breakdown (LIB) effect; and it is even more difficult to achieve a decent beam quality meanwhile. In this work, a pulsed 1064 nm laser was used as pump source; pressurized methane was used as Raman active medium, which had the largest ratio of backward/forward Raman gain coefficient among all gaseous media; and backward first Stokes (BS1) Raman laser of 1543 nm with good beam quality was realized. When an f=0.5 m lens was used to focus pump beam into a 0.9 m long Raman cell filled with 3.5 MPa methane, 276.1 mJ BS1 was achieved, the corresponding photon conversion efficiency was 70.8% and the peak power was 83.7 MW. BS1 beam quality was measured to be Mx2=2.54,My2=2.28, which was significantly better than that of pump laser. In a setup of f=0.3 m focal lens and 0.5 m long Raman cell, 197.8 mJ BS1 was achieved, with the company of serious LIB. In order to meliorate LIB and improve BS1 conversion efficiency, the focal lens was tilted by 20°, pump laser beam waist and depth of focus increased significantly, BS1 was improved to 225.0 mJ, the corresponding photon conversion efficiency was 58.5%. An even short focal lens and Raman cell with reasonable BS1 energy was possibly achieved by tilting a shorter focal lens by a larger angle. This work also demonstrated that the increment of BS1 conversion may help to reduce the effect of LIB on SRS process.

High beam quality, high brightness dense spectral beam combining of 650 nm laser diode module

Dense spectral beam combining (DSBC) of a laser diode module at a wavelength of 650 nm was realized for what we believe to be the first time. In this paper, the optimal spectral locking interval range for the 650 nm wavelength band was obtained by stimulation. A DSBC device containing 19 single emitters was designed and built according to the optimal spectral interval. The output power of the combining beam reached 17.74 W, and the stable spectral width was 4.75 nm. The beam quality factors (M2) in the fast-axis and slow-axis directions were basically the same as that of the single emitter, and even slightly optimized in the slow-axis direction. The beam brightness of the DSBC beam was about 123.395 MW·cm−2·sr−1, which was about fourteen times higher than that of the single emitter output.

High power spectral beam combining based on four long-wave infrared quantum cascade laser emitters

In this study, we experimentally investigate spectral beam combining based on four long-wave infrared quantum cascade lasers (QCLs) with three dichroic mirrors. The coating curves of the dichroic mirrors were finely designed according to the output spectrum of the four adopted QCLs, whose central wavelength locates in 7.5 μm, 8.3 μm, 9.0 μm and 10.2 μm, respectively. At room temperature the highest combined power reaches as high as 2.39 W, corresponding to a combining efficiency of 83.0 %, and meanwhile, the combined beam keeps a high beam quality with average M2 of 1.78. To the best of our knowledge, this is the highest output power among spectral beam combining for QCLs. Due to the advantage of wide spectral range, high power, high beam quality and compact size, this demonstration may meet the demands of medical diagnosis, remote sensing, free-space optical communication and military application.