High-brightness Laser Research Articles

Fiber-solid hybrid end-pumped Nd:YVO4 picosecond amplifier with high beam quality

In recent years, high-brightness picosecond lasers have been widely used in high-end precision processing, spectroscopy and high harmonic generation. To obtain higher power and excellent beam quality, it is an ideal solution to add a high-gain amplifier to the back end of the high-stability picosecond fiber or solid seed. In this article, a fiber-solid hybrid end-pumped Nd:YVO4 picosecond laser system with high beam quality is demonstrated. The influence of the self-developed picosecond fiber seed output characteristics on solid amplifiers has been studied in detail, including the energy, repetition rate and spectrum. After two-stage traveling wave amplifiers, the maximum 155 μJ single pulse energy is obtained with 20 kHz repetition rate, and the corresponding peak power reaches 14.9 MW with 10.4 ps pulse duration. Meanwhile, the beam quality factors under the maximum 18.4 W average power are measured to be M x 2 = 1.27 and M y 2 = 1.25, respectively.

Experimental Demonstration of Central-Lobe Energy Enhancement Based on Amplitude Modulation of Beamlets in 19 Elements Fiber Laser Phased Array

Coherent beam combination of fiber laser phased array is a promising technique to achieve high-brightness laser output. In this paper, we focus on central-lobe energy enhancement of the combined beam in the far-field by theoretically and experimentally investigating the relationship between the power-in-the-bucket (PIB) of the combined beam in the far-field and the beamlets amplitude modulation in the near-field. We employ a 19-elements fiber laser phased array integrating adaptive fiber-optic collimators with a beamlet filling factor of 0.903 and achieve phase-locking and beamlet pointing correction among fiber lasers using the SPGD algorithm. The amplitude modulation applied to the tiled sub-apertures is used to control the shape of the emissive array's amplitude envelope to further change the PIB of the combined beam under the same total output power. According to theoretical analysis, the fiber laser phased array achieves PIB = 0.51 for a zero truncation ratio (Tr), which is increased to PIB = 0.57 for Tr = 1.3. Experimentally, we observe PIB = 0.49 for Tr = 0 and PIB= 0.56 for Tr = 1.3. The experimental results are in good agreement with the theoretical analysis, suggesting that the amplitude modulation is helpful for further improving the energy density of central-lobe.

High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range

A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1% bandwidth (BW)–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 1035 photons s–1 mm–2 mrad–2 0.1%BW–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.

Selective loss tailoring of broad-area diode lasers

High-power high-brightness broad-area diode lasers have poor lateral beam quality, which has limited the application range of this kind of device. To improve the lateral beam quality, a sawtooth microstructure laser was proposed and the microstructures were carefully designed according to the profiles of each lateral mode and their dependence on the self-heating induced thermal lens effect. As a result of selective mode loss adjustment, a 42% improvement in lateral beam quality was achieved under the power-maintained condition. This technology enables us to develop high-brightness direct-diode laser systems.

基于金刚石拉曼转换的光束亮度增强研究进展

High brightness laser sources with different wavelengths play an important role in the fields such as defense, industrial, and life sciences etc. However, due to the intrinsic spectral and thermophysical properties of current available laser gain materials, it is difficult to take into account the wavelength and output power of the traditional inversion lasers, which even leads to the decrease of beam brightness. To overcome this problem, beam cleanup by using nonlinear optical technology has been carried out in recent years, which is directly transferring the low beam quality generated from inversion lasers into the high through the effects such as stimulated Raman or Brillouin scattering. Among them, with excellent properties such as high Raman gain coefficient, high thermal conductivity and wide spectral transmission range, diamond exhibits excellent beam brightness enhancement characteristics while realizing high efficiency Raman conversion, which provides a new technical path to generate high power and high brightness laser beam. Here, the development of brightness enhancement based on first-order and cascaded Raman conversion of diamond was reviewed, and its future applications were discussed.

Рабочие характеристики полупроводниковых лазеров на квантовых ямах в зависимости от ширины волноводной области

Operating characteristics of semiconductor quantum well (QW) lasers are theoretically studied in terms of the thickness of the waveguide region [optical confinement layer (OCL)]. We calculate the maximum modal gain, optical confinement factor (in QW, OCL, and cladding layers), threshold current density, electron and hole densities (in QW and OCL), internal optical loss (in QW, OCL, and cladding layers), internal differential quantum efficiency, stimulated and spontaneous recombination currents, and output optical power of the laser as functions of the OCL thickness. It is shown that up to the pump current density 50 kA/cm2 the output power of the considered lasers depends only slightly on the OCL thickness in the range of thicknesses 1.5–2.8 m. This result is important for designing high brightness lasers as broadened waveguides are used in such lasers to attain low beam divergence. At high pump current densities, the output power is shown to have a maximum as a function of the OCL thickness.

Optical Compensation Method for Pigtail Module in High-Brightness Laser Diodes

Optical Compensation Method for Pigtail Module in High-Brightness Laser Diodes

Highly efficient and thermally robust cyan-green phosphor-in-glass films for high-brightness laser lighting

Highly efficient and thermally robust cyan-green CSS-PiG film is a promising candidate for laser lighting with excellent color rendering.

Simulation and experimental results of a high-gain two-stage and double-pass off-axis Nd:YVO4 picosecond laser amplifier.

A high-gain two-stage double-pass off-axis Nd:YVO4 picosecond laser amplifier has been developed. The comprehensive influence of crystal doping concentration and pump beam quality on the small-signal gain of the Nd:YVO4 amplifier is theoretically analyzed with a model developed by considering energy transfer upconversion and pump light absorption saturation effect. The thermal effect of Nd:YVO4 crystal with different doping concentrations, undoped end cap lengths, and pump beam quality is investigated as well. Based on the theoretical analysis, a high-gain two-stage and double-pass off-axis Nd:YVO4 amplifier based on picosecond fiber seed source is realized by choosing long-composite low-doping Nd:YVO4 crystal and with high brightness laser diode pumping, delivering a gain of 28 dB and an output pulse energy of 67.5 µJ at a repetition rate of 200 kHz.

Probing the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-Ray Scattering.

The development of bright free-electron lasers (FEL) has revolutionized our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the valence electronic structure of solid-density nickel, heated to temperatures of around 10 of eV on femtosecond timescales, can be probed by single-shot resonant inelastic x-ray scattering (RIXS) at the Linac Coherent Light Source FEL. The RIXS spectrum provides a wealth of information on the HED system that goes well beyond what can be extracted from x-ray absorption or emission spectroscopy alone, and is particularly well suited to time-resolved studies of electronic-structure dynamics.

Analytical model for monochromator performance characterizations under thermal load.

Non-uniform thermal load causes performance degradation of crystal X-ray optics. With the development of high-brightness X-ray free-electron lasers, the thermal load on X-ray optics becomes even more severe. To mitigate the thermal load, a quantitative understanding of thermal effects on the optical performance is necessary. We derived an analytical model for monochromator performance under a non-uniform thermal load. This analytical model quantitatively describes the distortion of the rocking curve and attributes different contributions to different factors of thermal load. It provides not only monochromator design insights and considerations, but also a quick estimation of the rocking curve distortion due to thermal load for practical situations such as pump-probe experiments.

High absorption large-mode area step-index fiber for tandem-pumped high-brightness high-power lasers

A short absorption length ytterbium (Yb)-doped large-mode area (LMA) fiber is presented as a step forward to mitigate the stern problem of nonlinear scatterings in a tandem pumping scheme adopted for high-power fiber laser. The short absorption length was realized by incorporating high Yb concentration in the fiber core. Furthermore, by replacing the inherent silica cladding with a Ge-doped cladding, we were able to obtain low core numerical aperture (NA) and negate the detrimental effect of index-raising by high Yb concentrations. This overcomes the long-standing limitation in step-index Yb-doped fibers (YDFs) where high cladding absorption inevitably results in high NA, thus hampering single-mode operation. We report an LMA ( ∼ 575 μm 2 ) YDF with NA of 0.04 and absorption of 27 dB/m at 976 nm—both traits promote power scaling of single-mode tandem pumped fiber lasers. To our knowledge, this is the highest cladding absorption attained in a low-NA step-index fiber to date. An all-fiber tandem-pumped amplifier was built using only ∼ 14 m of the YDF. The amplifier delivered a near-Gaussian beam ( M 2 ∼ 1.27 ) at 836 W output power (pump power limited) with a high slope efficiency of ∼ 83 % . Thanks to the short length and the tandem pumping, no indication of limiting factors such as stimulated Raman scattering, photodarkening, and transverse mode instability was observed.

High-Power 760 nm VECSEL Based on Quantum Dot Gain Mirror

We report high-power second-harmonic generation of 760 nm laser light from optically-pumped vertical-external-cavity surface-emitting laser based on quantum dot active medium. The laser generates ~1.2 W in fundamental transverse mode with fixed linear polarization. The emission wavelength can be continuously tuned from 738 to 778 nm by using an intra cavity birefringent filter for fundamental radiation without readjustment of phase-matching angle of the nonlinear crystal. The result constitutes a viable alternative for applications requiring broadly tunable high brightness lasers in the 700-800 nm range.

High-brightness broad-area diode lasers with enhanced self-aligned lateral structure

Broad-area diode lasers with increased brightness and efficiency are presented, which are fabricated using an enhanced self-aligned lateral structure by means of a two-step epitaxial growth process with an intermediate etching step. In this structure, current-blocking layers in the device edges ensure current confinement under the central stripe, which can limit the detrimental effects of current spreading and lateral carrier accumulation on beam quality. It also minimizes losses at stripe edges, thus lowering the lasing threshold and increasing conversion efficiency, while maintaining high polarization purity. In the first realization of this structure, the current block is integrated within an extreme-triple-asymmetric epitaxial design with a thin p-doped side, meaning that the distance between the current block and the active zone can be minimized without added process complexity. Using this configuration, enhanced self-aligned structure devices with 90 µm stripe width and 4 mm resonator length show up to 20% lower threshold current, 21% narrower beam waist, and slightly higher (1.03×) peak efficiency in comparison to reference devices with the same dimensions, while slope, divergence angle and polarization purity remain almost unchanged. These results correspond to an increase in brightness by up to 25%, and measurement results of devices with varying stripe widths follow the same trend.

Scalable structure of coherent polarization beam combining based on tapered diode laser amplifiers

We develop a scalable multi-stage beam combining system based on the superposition of four tapered diode laser amplifiers using the coherent polarization beam combining. By employing the combination of master oscillator power amplification and active phase control of each amplifier, a stable output power of 6.9 W is obtained with a nearly diffraction-limited beam quality. The combining efficiency is 72%, and the brightness is improved by a factor of 3.3 compared to that of a single amplifier. Our system provides a new solution to obtain a high-coherence and high-brightness diode laser beam.

Morphology-Related Functionality in Nanoarchitectured GaN

Integrating silicon and III-nitride technologies for high-speed and large bandwidth communication demands optically interconnected active components that detect, process, and emit photons and electrons. It is imperative that multifunctional materials can enhance the performance and simplify fabrication of such devices. Spontaneously grown GaN in the nanowall network (NwN) architecture simultaneously displays unprecedented optical and electrical properties. A two-order increase in band-edge emission makes it suitable for high-brightness light-emitting diodes and laser applications. Decorating this NwN with silver nanoparticles further enhances emission through plasmonic interactions and renders it an excellent surface-enhanced Raman spectroscopy substrate for biomolecular detection. The observation of very high electron mobility (approximately 104 cm2/Vs) and large phase-coherence length (60 μm) is a consequence of two-dimensional (2D) electron gas formation applicable for high electron mobility transistors. Detecting ballistic transport in the nanowalls confirms proximity-induced superconductivity (<5 K and 8 T). Charge separation properties render it a device material for UV photodetectors, photoanodes for water splitting, and thermionic field emitters.

Industry-friendly synthesis and high saturation threshold of a LuAG:Ce/glass composite film realizing high-brightness laser lighting

Phosphors that are designed for laser-lighting are essential to possess high luminescence saturation threshold. However, the current phosphors with adequate saturation threshold have been facing several technological limitations including costly, complex and tedious synthetic approaches. Herein, we report the facile fabrication of LuAG:Ce/glass composite films using an industry-friendly blade-coating method. The resultant composite film presents a porous architecture that can facilitate light extraction and confine the laser spots. When pumped by blue laser, the composite film shows a high internal quantum efficiency of 86% and the corresponding white light source shows a high luminous efficacy of 199 lm/W. Furthermore, the composite film shows no sign of saturation even under the irradiation of 30.1 W blue laser; therefore, a high luminous flux of 5496 lm can be achieved. The combination of these outstanding properties and the industry-friendly fabrication method can drive the development of commercially viable phosphors for high-power laser-lighting applications.

High-brightness 808 nm semiconductor laser diode packaged by SiC heat sink

In order to further improve the reliability of high-power output of high-brightness semiconductor lasers, 808 nm semiconductor laser chips with a stripe width of 100 µm were packaged by SiC and AlN heat sinks, and the thermal performance of the composed SiC and AlN devices were investigated. First, the finite element method (FEM) is adopted to simulation and calculation, and then the thermal resistance of the device is measured by wavelength drift method experimentally. Both results show that the thermal resistance of the laser packaged by SiC heat sink is lower and the experimentally measured value is 2.95 K·W−1. In addition, the test results also indicate that the laser packaged by SiC heat sink has higher output power and higher wall-plug efficiency. The output power attains 10.3 W at the injection current of 10 A. and the maximum wall-plug efficiency can reach 53.5% at 25°C.

High-Efficiency Spectral-Beam-Combined 930nm Diode Laser Source**Supported by the National Natural Science Foundation of China under Grant No. 61535013.

Spectral beam combining is an effective way to achieve high-brightness direct diode laser output. We present an experimental study on spectral beam combining with external cavity based on transmission grating. Using a series of cylindrical transform lenses with different focal lengths, spectral beaming combining efficiency is greatly improved, and the results of wavelength intervals are consistent with the theoretical calculations. With the injection current of 90 A, a 75.1 W cw 930 nm output power with wavelength span of 18.6 nm and spectral beam combining efficiency of 92.7% is achieved, the beam parameter product is 5.77 mm⋅mrad.

Ultrafast Laser Inscription and ∼2 μm Laser Operation of Y-Branch Splitters in Monoclinic Crystals

We report on the first active surface Y-branch waveguide in the ∼2 μm spectral range. Depressed-cladding rectangular-cross-section surface waveguides with a splitting ratio of 1 × 2 are fabricated by femtosecond direct laser writing in a thulium (Tm3+) doped monoclinic double tungstate crystal. Confocal laser microscopy and μ-Raman spectroscopy reveal well preserved crystallinity of the waveguide core. Under high-brightness laser pumping at 0.8 μm, a simultaneous continuous-wave laser operation in both arms is achieved resulting in a total output power of 0.46 W at ∼1.84 μm with a slope efficiency of 40.6% and a laser threshold of 0.28 W. The laser output is linearly polarized and spatially multimode (TE12/TE22) with a power splitting ratio between arms of 52.1/47.9%). The waveguide propagation losses at 1.84 μm are ∼1.6 dB/cm and the loss from the Y-junction is 0.1 dB. The fabricated waveguides represent a route towards advanced photonic micro-structures such as a Mach–Zehnder interferometer for bio-sensing at ∼2 μm.