High Beam Quality Research Articles

Wide-bandgap GaN-based watt-class photonic-crystal lasers

Short-wavelength (blue-violet-to-green) lasers with high power and high beam quality are required for various applications including the machining of difficult-to-process materials and high-brightness illuminations and displays. Promising light sources for such applications are wide-bandgap GaN-based photonic-crystal surface-emitting lasers (PCSELs), which are based on two-dimensional resonance in the photonic crystal. Developments of these devices have lagged behind those of longer-wavelength GaAs-based PCSELs, because device designs for achieving robust two-dimensional resonance and a nanofabrication process that avoids introducing disorders have remained elusive for wide-bandgap GaN-based materials. Here, we address these issues and successfully realize GaN-based PCSELs with high, watt-class (>1 W) output power and a circular, single-lobed beam with a very narrow (~0.2°) divergence angle at blue wavelengths. In addition, we demonstrate continuous-wave operation with a high output power (~320 mW) and a high beam quality (M2~1). Our results will enable the use of GaN-based PCSELs in the above-mentioned applications.

Recent progress in continuously tunable low-noise all-solid-state single-frequency continuous-wave laser based on intracavity locked etalon

All-solid-state single-frequency continuous-wave (CW) lasers have been applied in many fields of scientific research owing to their intrinsic advantages of high beam quality, low noise, narrow linewidth, and high coherence. In atom-based applications, single-frequency lasers should also be continuously tuned to precisely match their wavelengths with the transition lines of the corresponding atoms. Continuous frequency tuning of the laser is mainly achieved by continuously scanning the laser cavity length after the intracavity tuning element etalon is locked to an oscillating laser mode. However, the modulation signals necessary in current etalon locking systems increase the noise of the continuously tunable lasers and in some respects limit their applications in Frontier scientific research. Moreover, the obtained continuous frequency tuning range with the etalon locking technique is restricted by the free spectrum range of the adopted etalon. In this paper, we systematically summarize recent progress of the continuously tunable single-frequency CW lasers based on intracavity locked etalon, including the advanced etalon locking techniques and the tuning range expansion approach. As a result, the low noise and high stable all-solid-state single-frequency CW tunable lasers are successfully developed.

Experimental observation of the transition between hose and self-modulation instability regimes

Plasma-based acceleration is one of the most promising technologies for the development of compact accelerators providing high-quality beams for research, medical, and industrial applications. The interaction with the plasma, however, can produce detrimental effects on the particle beam, such as the hose-instability, and ultimately limit its implementation. Several methods have been proposed to suppress such a process, for instance, by triggering and bringing to saturation the self-modulation instability. In the framework of plasma acceleration, we present, for the first time, the experimental observation of the transition from hose to self-modulation instability regimes. The measurements are obtained by using an ultra-relativistic electron beam interacting with the plasma confined in a capillary. The results provide a more comprehensive picture of the beam–plasma interaction and are validated with complete particle-in-cell simulations.

Folded-resonator LD side-pumped acousto-optically Q-switched Nd:YAG/SrWO4 solid-state Raman laser

A folded cavity of LD side-pumped acousto-optically Q-switched Nd:YAG/SrWO4 solid-state Raman laser is designed. The effects of the thermal focal length and resonator length on the resonator modes are analyzed. The appropriate mode sizes at laser crystal and Raman crystal are designated to realize low SRS threshold, high conversion efficiency and high beam quality. The folded-resonator LD side-pumped acousto-optically Q-switched Nd:YAG/SrWO4 solid-state Raman laser is realized, and the 1st Stokes laser at 1180 nm with high output power, high single pulse energy and high beam quality is obtained. The maximum output power of the 1st Stokes laser at 1180 nm is 2.89 W, optical–optical efficiency is 4 %, the single pulse energy is 0.96 mJ, and the shortest pulse is 35.5 ns. By analyzing the output spectrum and beam quality, it is shown that the beam spatial profile mode is well distributed, and the experimental results verify the rationality of the cavity mode design.

Compact 15 mJ Fiber–Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper

We report on a millijoule-level fiber–solid hybrid hundred-picosecond laser system with a stable performance and compact structure. The laser system is based on a master oscillator power amplifier structure containing an all-fiber master oscillator, a quasi-continuous-wave side-pumped Nd:YAG regenerative amplifier, and a double-pass amplifier. By using the filtering effect of fiber Bragg grating and the dispersion characteristics of single-mode fiber stretcher, the spectrum broadening caused by self-phase modulation effect is effectively suppressed. Thus, the gain linewidth of the Yb-doped fiber seed source and Nd:YAG laser amplifiers is accurately matched. The reason for thermally induced depolarization in the solid-state laser amplifier is theoretically analyzed, and a more flexible depolarization compensation structure is adopted in amplifier experiment. Furthermore, the pulse energy of 14.58 mJ and pulse width of 228 ps is achieved at 500 Hz repetition rate. The central wavelength is 1064.1 nm with a 3 dB bandwidth of 0.47 nm. The beam quality factors in the horizontal and vertical directions are 1.49 and 1.51, respectively. This laser system has a simple and compact structure and has a power stability of 1.9%. The high pulse energy and beam quality of this hundred-picosecond laser are confirmed by latter theoretical simulation of copper laser ablation. It is a very practical laser system for material processing and laser-induced damage.

High-energy and high-average-power two-channel Yb:YAG amplifier with passive coherent combining

A two-channel laser amplifier with passive coherent combining based on Yb:YAG active elements with bright diode end pumping was developed. The pulse energy limits of a one-channel scheme are investigated, and the ability to double pulse energy in the two-channel scheme is demonstrated. 10 mJ pulse energy at a repetition rate of 1.5 kHz and 5 mJ at 6 kHz simultaneously with high gain and perfect beam quality at coherent combining efficiency over 97% was achieved.

Large dispersion-managed broadband high-energy fiber femtosecond laser system with sub 300 fs pulses and high beam quality output

We experimentally demonstrate a high-energy sub 300 fs polarization maintaining fiber chirped pulse amplification (CPA) system. The preamplifier is a monolithic fiberized system that uses two cascaded temperature-assisted dispersion-tuning broadband chirped fiber Bragg gratings (CFBGs) with a reflected bandwidth of 20 nm as stretchers. To make full use of the stretcher to lower the system’s nonlinearity accumulation, a homemade mode locked fiber laser with a spectral width of 14.8 nm (full width at half maximum) is selected as the seeder to offer a stretched pulse width of 1.69 ns. The main amplifier is based on a one-stage simple Yb:YAG single crystal fiber amplifier with an amplified output power of 40.6 W at a repetition rate of 200 kHz, and the beam quality is conserved in a single mode beam profile with beam quality of 1.246 and 1.142 in the horizontal direction and vertical direction, respectively. During amplification, the spectral gain narrowing effect is observed. To achieve the high-speed switch of the laser, an acoustical optical modulator (AOM) is inserted before the compressor to achieve high-speed turn-on/off control. The compressor is based on a diffraction grating pair with a groove density of 1600 line/mm to offer a dispersion match with the stretcher of the CFBGs. With the CFBG’s fine-tuned capacity of second-order dispersion and higher-order dispersion, the compressed average power of 29.6 W and pulse duration of 278 fs, corresponding to a pulse energy of 148 µJ and a peak power of 532 MW with ignoring the wings of the pulse, is obtained. The beam quality is well conserved after compression, and the beam quality is 1.250 and 1.196 in the horizontal direction and vertical direction, respectively. A power fluctuation of 0.1% (root mean square) and a beam pointing drift of 8.47 µrad/°C over 8 h are realized. This high peak power and high beam quality femtosecond laser is promising in science and industrial applications.

Transient Simulation and Field Measurement of the Trim Quadrupoles and AC Sextupoles for CSNS/RCS Upgrade

In September 2018, China Spallation Neutron Source (CSNS) passed the national acceptance and started stable operation. Many scientific achievements have been made, but more and more experiments require accelerators with higher beam quality. Based on the current lattice layout and further research, an alternative upgrade plan is proposed for the CSNS, i.e., CSNS-II. The trim quadrupole magnet and the AC sextupole magnet are necessary. Both types of magnets with the number of 16 are respectively powered by the AC power supply and they were developed firstly in 2021. This article introduces dynamic magnetic field simulation, fringe field interference analysis, and DC/AC magnetic field measurements of the two types of magnets.

Fast wavefront sensing for X-ray optics with an alternating speckle tracking technique.

Advances in accelerator technologies have enabled the continuous development of synchrotron radiation and X-ray free electron laser (XFEL) sources. At the same time, it has been critical to perform in-situ wavefront sensing to aid delivery of high-quality X-ray beams to the end users of these facilities. The speckle-based scanning technique has obtained popularity due to its high spatial resolution and superior sensitivity compared to other wavefront sensing methods. However, these advantages often come at the expense of longer data acquisition times since multiple images have to be collected to derive the necessary wavefront information. Whereas initial speckle tracking techniques could obtain wavefront information relatively quickly, the installation of additional hardware was routinely required to do so. Here, we propose a novel speckle-based approach, termed Alternating Speckle Tracking (AST), to perform fast wavefront sensing within a conventional beamline setup. The wavefront information derived from the new technique has proven to be valuable for many applications that require temporal resolution. Importantly, both horizontal and vertical wavefront information can be simultaneously derived by moving the speckle generator along the diagonal direction. We expect this method will be widely used by the synchrotron radiation and XFEL community in the future.

Generation of arbitrary vector Bessel beams on higher-order Poincaré spheres with an all-dielectric metasurface

Vector Bessel beams may have a spatially varying polarization state combined with a Bessel-type transversal intensity profile. Such beams can carry both spin angular momentum and orbital angular momentum (OAM) and be characterized by points on higher-order Poincar\'e (HOP) spheres. We report a method to produce vector Bessel beams on any point of a HOP sphere of any topological charge by utilizing only a single all-dielectric planar metasurface. Specifically, by a proper choice of the input polarization and distance from the metasurface, the entire HOP sphere surface can be covered. The proposed method is simple and efficient and applies to any $n\mathrm{th}$-order HOP sphere. Furthermore, it yields an extremely high OAM mode purity. Our paper demonstrates a robust approach for generating arbitrary high-quality vector Bessel beams and will likely inspire future applications ranging from optical manipulation to laser beam engineering.

Efficient Tm:LuYAG laser resonantly pumped by an Er:LuYAG laser

We demonstrate an efficient continuous-wave (CW) and Q-switched Tm:LuYAG laser that was resonantly pumped by an Er:LuYAG laser. The maximum CW output power was 3.12 W, and the corresponding slope efficiency relative to the incident pump power was 51.4%. In the Q-switched mode, a stable laser pulse was achieved from 1 to 10 kHz repetition frequency under the maximum incident pump power of 7 W. The narrowest pulse width of 94 ns with a pulse energy of 2.12 mJ was obtained at 1 kHz, resulting in the highest single pulse peak power of 22.5 kW. The Tm:LuYAG laser was centered at 2018.37 nm with a high-beam quality M2 < 1.2.

First Demonstration of a Bidirectional Tandem-Pumped High-Brightness 8 kW Level Confined-Doped Fiber Amplifier

In this paper, we demonstrated a high-brightness bidirectional tandem-pumped confined-doped fiber amplifier with nearly 8 kW output power. The technical difficulties for realizing the bidirectional tandem-pumping scheme are comprehensively analyzed, according to which, the homemade high signal-isolation side-coupled pump and signal combiner as well as stable 1018 nm pump sources were developed and applied in the experimental study, successfully enabling the first demonstration of the bidirectional tandem pumping scheme. Moreover, benefiting from the confined-doped fiber design and the bidirectional tandem pumping technique, the transverse mode instability was successfully mitigated and the beam quality was well preserved during the power scaling process. Eventually, an output power of 7.88 kW was obtained with the beam quality factor of ∼1.97 and the optical-to-optical efficiency of ∼84.76%. This work provides a good reference for realizing large-mode-area fiber lasers with high output power and good beam quality by utilizing the confined-doped fiber design and bidirectional tandem-pumping technique.

3.3 kW narrow linewidth FBG-based MOPA configuration fiber laser with near-diffraction-limited beam quality

This paper demonstrates a narrow-linewidth fiber laser with near-diffraction-limited beam quality and high Raman suppression ratio by utilizing Master Oscillator Power-Amplifier structure based on narrow-bandwidth fiber Bragg gratings. The output power reaches 3.31 kW with a slope efficiency of 85.0 %. At the maximum output power, the 3 dB, 10 dB, and 20 dB linewidths are 0.40 nm, 1.20 nm, and 3.09 nm, respectively. The beam quality M2 factor is 1.32 and the Raman suppression ratio reaches 26.3 dB. A pair of narrow-bandwidth fiber gratings were used in the oscillator to compress the linewidth of the seed laser. We used backward-pumping configuration and plum-coiled gain fiber in the amplification stage to suppress the Raman effect and select transverse mode by fiber-bending. Finally, a compact all-fiber laser was obtained with narrow-linewidth, high beam quality, and effective suppression of the Raman effect. To the best of our knowledge, this is the highest output power for a sub-nanometer-linewidth laser using narrow-bandwidth fiber gratings and none-wavelength-locked laser diodes, which achieves near-diffraction-limit beam quality. This provides a solution for high-power narrow-linewidth laser sources with high beam quality.

1 kHz, 12 MW, 300 ps microchip oscillator power amplifier system

We report a diode-pumped passively Q-switched (PQS) sub-nanosecond master oscillator power amplifier system. The master oscillator is a PQS Nd:YAG/Cr:YAG monolithic microchip laser, yielding a pulse duration of ∼300 ps and pulse energy of 33 μJ at repetition frequency of 1 kHz. After passing two stages end pumped Nd:YVO4 amplifier, the pulse energy is boosted to 3.6 mJ and pulse peak power up to 12 MW with excellent stability (root mean square ∼ 0.14%) and high beam quality (M 2 < 1.22).

Silicon optical phased array with a 180-degree field of view for 2D optical beam steering

Optical phased arrays (OPAs), the optical counterpart of phased arrays at radio frequencies, can electronically steer an optical beam without any moving parts. To achieve a 180° field of view (FOV), the array emitters should be spaced a half-wavelength apart or less. However, a conventional OPA based on a waveguide grating array suffers from strong cross talk between adjacent waveguides when the pitch is a half-wavelength or less. Here, we theoretically describe and experimentally demonstrate a two-dimensional aliasing-free beam steering regime for an integrated OPA with the entire 180° FOV. We achieve this by using a half-wavelength-pitch waveguide array combined with a trapezoidal slab grating as a single emitter. Our OPA also features a low sidelobe level of &lt; − 19 d B while the beam is steered from − 40 ∘ to + 40 ∘ , breaking the trade-off between FOV and beam quality. The chip-based OPA with a large beam steering range and high beam quality provides a promising route for a compact, solid-state, cost-effective, and high-performance light detection and ranging system, enabling a wide range of classical and quantum applications.

Reflective Tomography Lidar Image Reconstruction for Long Distance Non-Cooperative Target

In the long-distance space target detection, the technique of laser reflection tomography (LRT) shows great power and attracts more attention for further study and real use. However, space targets are often non-cooperative, and normally a 360° complete view of reflection projections cannot be obtained. Therefore, this article firstly introduces an improved LRT system design with more advanced laser equipment for long-distance non-cooperative detection to ensure the high quality of the lidar beam and the lidar projection data. Then, the LRT image reconstruction is proposed and focused on the laser image reconstruction method utilizing the total variation (TV) minimization approach based on the sparse algebraic reconstruction technique (ART) model, in order to reconstruct the laser image in a sparse or incomplete view of projections. At last, comparative experiments with the system are performed to validate the advantages of this method with the LRT system. In both near and far field experiments, the effectiveness and superiority of the proposed method are verified for different types of projection data through comparison to typical methods.

Application of the General Extended-Object Wavefront Sensor into Aberration Correction of Lasers

Thermal aberrations are the main impediments for high-energy transverse multimode lasers to achieve high brightness and beam qualities. To address this issue using adaptive optics, traditional wavefront sensing methods have great limitations for aberration correction in high-energy transverse multimode lasers, because these lasers are no longer point sources. The general extended-object wavefront sensor (GEWFS) invented by the authors can work with extended sources in real time, and it is highly suitable to solve this problem. The application of the GEWFS into the compensation of thermal aberrations in a high-energy laser is preliminarily investigated by means of a simulation experiment. In this simulation experiment, a transverse multimode laser diode simulates a high-energy laser, a and phase plate thermal aberrations, which is compensated by a closed-loop adaptive optics system mainly composed of the GEWFS and a membrane deformable mirror. Simulation experimental results show that the peak intensity of the laser beam image is improved about 2.35 times, and the approximate Strehl ratio of the laser system attained 0.90. The results indicate that the GEWFS is simple and accurate enough for wavefront sensing from extended sources, and it will greatly expand the application fields of adaptive optics.

In Silico Trial of Computed Tomography-Guided Stereotactic Adaptive Radiation Therapy (CT-STAR) for the Treatment of Abdominal Oligometastases

We conducted a prospective, in silico clinical imaging study (NCT04008537) to evaluate the feasibility of cone beam computed tomography-guided stereotactic adaptive radiation therapy (CT-STAR) for the treatment of abdominal oligometastases. We hypothesized that CT-STAR produces improved dosimetry compared with nonadapted CT-stereotactic body radiation therapy (SBRT). Eight patients receiving stereotactic body radiation therapy for abdominal oligometastatic disease received 5 additional kV cone beam CTs on the ETHOS system. These additional cone beam CTs were used for imaging during an emulator treatment session. Initial plans were created based on their simulation (PI) and emulated adaptive plans were based on anatomy-of-the-day. The prescription was 50 Gy out of 5 fractions. Organ-at-risk (OAR) constraints were prioritized over planning target volume coverage under a strict isotoxicity approach. The PI was applied to the patient's anatomy-of-the-day and compared with the reoptimized adaptive plans using dose-volume histogram metrics, with selection of the superior plan. Feasibility was defined as completion of the adaptive workflow and compliance with strict OAR constraints in≥80% of fractions. Fractions were performed under time pressures by a physician and physicist to mimic the adaptive process. CT-STAR was feasible, with successful workflow completion in 38 out of 40 (95%) fractions.PI application to daily anatomy created OAR constraint violations in 30 out of 40 (75%) fractions. There were 8 stomach, 18 duodenum, 16 small bowel, and 11 large bowel PI OAR constraint violations. In contrast, OAR violations occurred in 2 out of 40 (5%) adaptive plans(both small bowel violations, both improved from the PI). CT-STAR also improved gross tumor volume V100 and D95 coverage in 25 out of 40 (63%) and 20 out of 40 (50%) fractions, respectively. Zero out of 40 (0%) fractions were deemed nonfeasible due to poor image quality and/or inability to delineate structures. Adaptation time per fraction was a median of 22.59 minutes (10.97-47.23). CT-STAR resolved OAR hard constraint violations and/or improved target coverage in silico compared with nonadapted CT-guided stereotactic body radiation therapy for the ablation of abdominal oligometastatic disease. Although limitations of this study include its small sample size and in silico design, the consistently high-quality cone beam CT images captured and comparable timing metrics to prior adaptive studies suggest that CT- STAR is a viable treatment paradigm for the ablation of abdominal oligometastatic disease. Clinical trials are in development to further evaluate CT-STAR in the clinic.

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.

Controlled acceleration of GeV electron beams in an all-optical plasma waveguide

Laser-plasma accelerators (LPAs) produce electric fields of the order of 100 GV m−1, more than 1000 times larger than those produced by radio-frequency accelerators. These uniquely strong fields make LPAs a promising path to generate electron beams beyond the TeV, an important goal in high-energy physics. Yet, large electric fields are of little benefit if they are not maintained over a long distance. It is therefore of the utmost importance to guide the ultra-intense laser pulse that drives the accelerator. Reaching very high energies is equally useless if the properties of the electron beam change completely from shot to shot, due to the intrinsic lack of stability of the injection process. State-of-the-art laser-plasma accelerators can already address guiding and control challenges separately by tweaking the plasma structures. However, the production of beams that are simultaneously high quality and high energy has yet to be demonstrated. This paper presents a novel experiment, coupling laser-plasma waveguides and controlled injection techniques, facilitating the reliable and efficient acceleration of high-quality electron beams up to 1.1 GeV, from a 50 TW-class laser.