Good Beam Quality Research Articles

Symmetrical Corner-Pumped Planar Waveguide Laser Amplifier

We present a novel structure of planar waveguide for Yb:YAG laser amplifiers as symmetrical corner-pumped planar waveguide, allowing large absorption length. We optimize the parameters of the waveguide using an amplification model. Performance including thermal effects of 10-kW symmetrical corner-pumped planar waveguide laser amplifier is simulated, demonstrating that pump absorption efficiency and uniformity are much better than end-pumped ones. Moreover, a good beam quality causing by less thermal distortion can be expected. The power is scaled from 200W to 7000W with the pump absorption efficiency of 98.2%, leading to an optical-to-optical efficiency of 68%.

Spectral beam combining of discrete quantum cascade lasers

In this paper, we report a spectral beam combining technique based on discrete quantum cascade lasers at λ ~ 4.8 μm. Good beam qualities of M2 < 1.3 for both fast and slow axes are obtained. The entire spectrum span is approximately 29.1 cm−1, which is consistent with the theoretical results of grating equation. Maximum beam combining efficiency of 58.9% with output power exceeding 1 W is demonstrated under continuous wave operation at room temperature. The limit of beam combining efficiency is theoretically investigated. The independent temperature control for the discrete lasers circumvented the issue of thermal crosstalk between the lasers on an array and pave the way to high power and high efficiency laser spectral beam combining.

25 W continuous-wave Yb:LiLuF4 single-crystal-fiber laser oscillator

We present the first demonstration of a solid-state laser oscillator based on a Yb-doped, birefringent single crystal fiber grown by the micro-pulling-down method. Pumping a 42-mm-long, 2%-at. Yb3+:LiLuF4 crystal with a 105-W maximum power fiber coupled laser diode, we obtained a maximum output power of 10.4 W, with diffraction limited beam quality (M2<1.1), in a simple plane-concave resonator. By modifying the resonator layout to increase the laser efficiency, we obtained output power up to 25 W, with still fairly good beam quality (M2≃2). Thermal lensing effect and laser intrinsic slope efficiency and losses were characterized in both resonator configurations.

500 W peak power cavity dumped 2 µm GaSb-based VECSEL.

A single transverse mode high-pulse-energy vertical-external-cavity surface-emitting laser (VECSEL) was developed. The GaSb-based VECSEL emits at a wavelength of 2.04 µm with a peak power exceeding 500 W while maintaining good beam quality. The cavity employs a Pockels cell combined with a low-loss thin film polarizer to selectively dump the intracavity energy into a 10 ns pulse. The laser has promise for incoherent LIDAR, materials processing, gas sensing, and nonlinear optics.

Raman wavelength conversion in a multipass cell.

Positively chirped femtosecond pulses at 1030 nm are wavelength-converted using spontaneous and stimulated Raman scattering in a potassium gadolinium tungstate crystal inserted inside a multipass cell. Recirculation in the cell and the Raman material allows both a high conversion efficiency and good spatial beam quality for the generated Stokes beams. The converted pulses can be compressed to sub-picosecond duration. Multipass cells could be an appealing alternative to other Raman shifter implementations in terms of thermal effects, control of the Raman cascade, and overall output beam quality.

Energy limit for linear-to-nonlinear femtosecond laser pulse focusing in air

Propagation of a tightly focused high-power ultrashort laser pulse in an optical medium is usually substantially influenced by the medium optical nonlinearity that can noticeably affect the laser pulse parameters around the nonlinear focus and lead to unavoidable and often undesirable spatial distortions of the focal waist. We present the results of our experimental study and numerical simulations on a femtosecond Ti:Sapphire laser pulse propagation in air under different spatial focusing. We concentrated our study on spectral-angular and spatial pulse transformations under different focusing regimes - from linear to nonlinear one, when pulse filamentation occurs. For the first time to the best of our knowledge, we found the laser pulse numerical apertures range – namely, from NA = 2·10-3 to 5 10-3(for the laser pulse energy of 1 mJ), where the laser pulse distortions both in frequency-angular spectrum and pulse spatial shape are minimal. By means of the numerical simulations, we found the threshold pulse energy and peak power in a wide range of focusing conditions, within which a transition between the linear and strongly nonlinear laser pulse focusing in air takes place. This energy limit is shown to decrease with pulse numerical aperture enhancement. Our findings identify the laser pulse numerical apertures and energes adequate for getting a maximum laser intensity with a good beam quality around the focal point suitable for various laser micropatterning and micromachining technologies.

Nanosecond pulsed deep-red laser source by intracavity frequency-doubled crystalline Raman laser.

We demonstrated a deep-red laser source by intracavity frequency-doubled crystalline Raman laser for the first time, to the best of our knowledge. The actively Q-switched 1314 nm Nd:LiYF4 laser was first converted to the eye-safe Raman laser using a KGd(WO4)2 (KGW) crystal, which was subsequently frequency-doubled in a bismuth borate crystal. Benefiting from the KGW bi-axial properties, the deep-red laser source was able to lase separately at two different spectral lines at 730 and 745 nm. Under an optimal repetition rate of 4 kHz, the maximum average powers of 1.7 and 2.0 W were attained with good beam quality of M2≈1.7. The corresponding pulse durations were determined to be 3.0 and 2.8 ns with the peak powers up to approximately 140 and 180 kW, respectively.

Surface grating VCSEL-integrated amplifier/beam scanner with high power and single mode operation

We propose and demonstrate a novel surface-grating vertical cavity surface emitting laser (VCSEL)-integrated amplifier/beam scanner. When the surface of the VCSEL section is periodically etched, a single slow-light mode which travels laterally into the amplifier section is selected due to the wavelength selectivity of the grating. The coupled slow light can be amplified with pumping the amplifier above the threshold. The far field angle of the amplified slow light can be continuously tuned through changing the pumping current at the VCSEL. We fabricated the integrated device with a 400 μm long VCSEL section and 1 mm long amplifier section, realizing in a single mode coupling power of over 7 mW and high output power of over 500 mW under pulsed operations of the amplifier. The continuous fan beam steering of 1.5° and a diffraction-limited narrow beam divergence of 0.06° are also achieved. We also fabricated device with extending the amplifier section length to 2 mm. A high single mode power of 1 W under pulsed operations is achieved, which is the record high power operation for single-mode VCSELs. With the performance of both high power and good beam quality, our new device shows great potential to be used as light source for LiDAR and other sensing applications.

Pulsed Nd:YAG laser machining of nitinol: An experimental investigation

Nickel–Titanium (Ni-Ti) shape memory alloy, commonly called nitinol alloys, finds its primary application in the production of biomedical implants, mainly because of itsrare properties such asshape memory, superelasticity and superior biocompatibility. Laser cutting is anon-traditional machining process for the production ofparts with close tolerances andcomplex geometry. Electrical discharge machining (EDM) of nitinol is associated with more heat-affected zone (HAZ) and recast layer thickness. This article aims to study nitinol’s machining characteristics by alaser source with good beam quality to have a less HAZ, recast layer and striations. Experiments were designed and carried out using central composite designs (CCD) by a pulsed Nd:YAG laser. Analysis based on the different parameters chosen was conducted to determine the parameters; effects, including laser power, frequency and cutting speed concerning the surface roughness. From the results, it is observed that the presence of HAZ is measured up to1. 48 mm from the machined surface. The topography analysis reveals that the striation is identified at high speeds, with less pulse overlapping by columnar micro channels, which can be reduced at high pulse frequency.

Mid-infrared optical parametric oscillator pumped by a high-pulse-energy, Q-switched Ho3+:YAG laser.

A high-pulse-energy mid-infrared light source is presented, based on a zinc-germanium-phosphide optical parametric oscillator (ZGP OPO) pumped by an actively $Q$-switched high-pulse-energy ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ laser. The ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ pump laser source is capable of generating a pulse energy of 15 mJ from a single ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ rod at room temperature at a pulse repetition frequency (PRF) of 700 Hz. A maximum power of 20.1 W at a central wavelength of 2090 nm can be obtained in continuous operation, with a slope efficiency of 45.1%. A good beam quality with an ${{\rm M}^2}$ better than 1.3 was achieved in $Q$-switched operation. The presented laser architecture was used as a suitable pump source for a ZGP-based OPO. Operated at a PRF of 2 kHz and pumped with a pulse energy of 8 mJ, a low conversion threshold of 1.5 W and a maximum total output power of 6.3 W could be obtained in a linear ZGP-based OPO. At maximum power, the peak power of the generated mid-infrared radiation exceeded 120 kW, while the beam quality was affected by the strong gain lens building inside the nonlinear material as a consequence of the high-energy pump pulses.

Excellent performance of a cryogenic Nd:YAlO3 laser with low wavefront distortion based on zero thermal expansion.

High-power solid-state lasers with good beam quality are attracting great attention on account of their important applications in industry and military. However, the thermal effects generated in the laser host materials seriously limit power scaling and degrade the beam quality. Thermal lensing and thermally induced wavefront deformation are the main causes of the beam quality deterioration. Here we investigate the performance of a zero thermal expansion (ZTE) solid-state laser gain material. In a proof-of-principle experiment, an ${a}$-cut rod ${\rm Nd}\!:\!{{\rm YAlO}_3}$ (Nd:YAP) perovskite crystal is chosen to be the gain medium for ZTE around 180 K. The laser performance spanning the temperature range from 80 to 290 K is studied. The maximum output power and minimum threshold pump power were obtained at a temperature of 180 K. Moreover, the measured thermal focal power and peak-to-valley value of the wavefront distortion also reach a minimum at this temperature, an additional benefit from the crystal's ZTE coefficient. We envisage that these results will open a new route towards the development of high-power and high-beam-quality lasers through the use of ZTE gain materials.

Thermo-Fluid Design Simulation of Nd3+ POCl3 Transverse Flow Liquid Laser Cavity

Abstract Computational fluid dynamics (CFD) based thermal design of a transverse flow optical cavity is carried out for 1 kW Nd3+ POCl3 liquid laser source to investigate temperature and velocity distribution in the optical pumping region of the cavity. Temperature gradient and turbulence both affect the refractive index of the liquid gain medium, which results in optical path difference (OPD), divergence and hence, poorer quality of the laser beam. The main purpose of this design is to achieve uniform flow and least temperature gradient in the optical pumping region so that the optical path difference can be minimized and a good beam quality can be achieved. CFD model has been developed for carrying out thermo-fluid simulations for this thermal system and based on these simulations, an optimum geometry of inlet ports along with their position from optical pumping region have been proposed. A user defined function (UDF) is incorporated for the input of spatially varying heat source term in each cell of the optical pumping region of the cavity. Variations in refractive index and optical path difference are estimated from the temperature data using another UDF. Simulation reveals that mass flowrate between 1.5 kg/s and 2.0 kg/s maintains the optical homogeneity of gain medium. Preliminary experiments have been carried out to demonstrate the effect of flowrate on the beam divergence and thereby exhibiting the importance of present simulation work.

Optical refrigeration of the Yb3+-doped YAG crystal close to the thermoelectric cooling limit

The Yb3+:YAG crystal has been one of the most widely used active media in the solid-state lasers of high power, mainly thanks to its excellent thermal, mechanical, and optical properties. Thermal effect due to heat deposition in the active medium, however, greatly deteriorates the beam quality of the laser output and sets a limit on its maximum power available. Although the cooling proposal of anti-Stokes fluorescence can help realize the heat-free high-power lasers with good beam quality, so-called radiation-balanced lasers, there is no substantial advancement in the optical cooling of Yb3+:YAG crystals since its latest experimental report with a temperature drop of about 9 K. Here we demonstrate experimentally a remarkable temperature drop of about 80 K in a 3% Yb3+-doped YAG single crystal pumped by a fiber laser at 1030 nm. Further analysis predicts that the cooling limit of the titled crystal can reach as low as 180 K from the room temperature. Our work therefore reveals a key pathway to facilitate the optical refrigeration of the Yb3+:YAG crystal down to the thermoelectric cooling limit, thus offering a unique entry point to practical radiation-balanced lasers.

Thin-disk laser system operating above 10 kW at near fundamental mode beam quality.

We report on a thin-disk laser system with more than 10 kW of output power and a beam quality of M2=1.76 at an overall optical-to-optical efficiency of 51%. The system consists of two thin-disk laser oscillators and a thin-disk multi-pass amplifier system. To reach high output powers while maintaining good beam quality, the output beams of two identical laser oscillators are polarization-combined. Subsequently, the beam is amplified in a multi-pass system. To the best of our knowledge, this is the highest output power achieved for a thin-disk laser system with a beam quality close to fundamental mode.

Efficient 0.75-mJ 100-kHz ultraviolet pulsed laser based on a rod-type photonic crystal fiber.

We have developed a high-power ultraviolet (UV) nanosecond-pulsed laser based on a rod-type photonic crystal fiber. The UV pulse energy and the pulse repetition rate are 0.75 mJ and 100 kHz, respectively, yielding 75-W UV average power. The temporal pulse shape and the linewidth of a 1030-nm seed laser are optimized for efficient third-harmonic generation, and the high conversion efficiency of 50% is achieved with a good beam quality (M2∼1.2). To our knowledge, this is the highest UV pulse energy from fiber lasers.

105 W continuous-wave diode-pumped Tm:YAP slab laser with high beam quality

We demonstrated a high power and good beam quality continuous-wave (CW) Tm:YAP slab laser operating at room temperature. Two fiber-coupled laser diodes were used as pump source, the maximum output power of 105 W at 1936.4 nm was achieved with the total incident pump power of 308 W, corresponding to the slope efficiency and optical-to-optical efficiency were 41.4% and 34.1% with respect to the incident pump power, respectively. At the maximum output power, the beam quality factors M2 were 2.76 and 2.78 in the x and y directions, respectively. Besides, a theoretical model was presented to predict the temperature distribution, thermal induced stress and laser performance of the Tm:YAP slab laser, and the experimental results were consistent with the theoretical simulation results which indicated that this model could simulate hundred-watt-level Tm:YAP lasers well.

Diode-pumped wavelength-switchable visible Pr&lt;sup&gt;3+&lt;/sup&gt;:YLF laser and vortex laser around 670 nm

Here we developed a novel wavelength-switchable visible continuous-wave (CW) Pr³⁺:YLF laser around 670 nm. In single-wavelength laser operations, the maximum output powers of 2.60 W, 1.26 W, and 0.21 W, the maximum slope efficiencies of 34.7%, 27.3%, and 12.3% were achieved with good beam qualities (<italic>M</italic>² &lt; 1.6) at 670.4 nm, 674.2 nm, and 678.9 nm, respectively. Record-high output power (2.6 W) and record-high slope efficiency (34.7%) were achieved for the Pr³⁺:YLF laser operation at 670.4 nm. This is also the first demonstration of longer-wavelength peaks beyond 670 nm in the ³P₁→³F₃ transition of Pr³⁺:YLF. In multi-wavelength laser operations, the dual-wavelength lasings, including 670.1/674.8 nm, 670.1/679.1 nm, and 675.0/679.4 nm, were obtained by fine adjustment of one/two etalons within the cavity. Furthermore, the triple-wavelength lasings, e.g. 672.2/674.2/678.6 nm and 670.4/674.8/679.4 nm, were successfully demonstrated. Moreover, both the first-order vortex lasers (LG₀⁺¹ and LG₀^-1 modes) at 670.4 nm were obtained by off-axis pumping.

Effects of neutron irradiation on optical characteristics of Yb-doped fiber materials

Yb-doped fiber laser is a very appealing technology to implement space communication, laser radar and nuclear facilities due to its reduced weight, size, high efficiency, high peak power combined with good beam quality. However, Yb-doped fiber materials will suffer a harsh ionizing radiation (such as neutron, proton and electron) under the condition of irradiation. The radiation-induced darkening effect can lead the fiber loss to increase obviously, the laser slope efficiency to decrease drastically, and even no laser output to be observed in severe cases. Therefore, it is necessary to conduct in-depth research on the performance changes of Yb-doped fiber materials subjected to the irradiation. In this paper, a series of Yb-doped optical fibers and optical fibers is prepared by the improved chemical vapor deposition method combined with rare-earth chelate-doped. And the optical properties of the optical fiber before and after being irradiated and annealed are tested. We mainly investigate the absorption spectrum of Yb-doped fiber material. The results show that the concentration of Al-related defects in the Yb-doped fiber material increases after neutron irradiation, and the absorption loss in the visible region increases. And the color center defects produced by the irradiation will significantly reduce the Yb ion fluorescence lifetime. The doping of Ce ions can reduce the Al-oxygen hole center (Al-OHC) color center defects, and can suppresse the radiation-induced darkening effect of Yb-doped fiber to a certain extent. Thermal annealing can reduce the absorption of fiber material by reducing the concentration of neutron radiation-induced color center defects, and thus eliminating the darkening effect to a certain extent. Finally, with our previous research, we find that neutron irradiation and ray irradiation have similar effects on the optical properties of Yb-doped fiber materials. The main reason is that the electron ionization effects occur due to both ray irradiation and neutron irradiation to generate free electron and hole pairs, which then combine with the original defects in the material to turn into color center defects. However, the color center defects caused by neutron irradiation are more stable and require thermal annealing to be eliminated. And the results obtained in this study provide theoretical basis and solution for developing the Yb-doped fibers with high laser performance and high radiation resistance.

Optimization and Application of High Power Thulium-Doped Fiber Laser

In recent years, with the continuous progress of laser technology, fiber laser has been developed rapidly. Compared with the traditional laser, fiber laser has the advantages of good beam quality, small volume, easy integration, high luminous efficiency, strong anti-interference ability and good heat dissipation. As a kind of fiber laser, thulium doped fiber laser not only inherits the above advantages, but also has the rich energy level structure and spectral characteristics of thulium ion, which determines that thulium doped fiber laser plays an important role in the research field of long wavelength fiber laser. This paper discusses the influence of the length and concentration of doped fiber on the output of high power laser system. For a given laser system, all parameters have an optimal value. In order to make the high power thulium doped fiber laser work in the best state, we need to carefully select these parameters. Based on the rate equation, the relationship between the dopant concentration and the length of the optical fiber is given in theory. The experimental results show that the optimal fiber length is different for different pump power. The higher the pump power, the larger the optimal fiber length. The experimental results have guiding significance for the manufacturing technology of high power fiber lasers.

Experimental research on the influence of ultrasonic vibrations on the laser cladding process of a disc laser

Disc lasers have a wide range of applications in laser cladding due to a series of advantages such as good beam quality. The cladding layer formed by the disc laser in laser cladding is prone to microcracks, pores and other microscopic defects, which seriously affects the promotion of this technology. Ultrasonic vibration can generate high-frequency and low-amplitude vibrations, which actively affect the formation of the microstructure during the solidification of the cladding layer. They can effectively reduce the formation of microscopic defects. This paper builds an ultrasonic-assisted laser cladding system platform based on a disc laser. We cladded 316L powder onto an ASTM 1045 steel substrate for multiple laser cladding experiments, and performed microscopic characterization and analysis of the cladding layer. The result shows that ultrasonic vibrations can effectively refine the bottom and top microstructures of the cladding layer, especially the morphology of the dendrite arms. The phase structure did not change under different ultrasonic amplitudes. Ultrasonic vibration changes the direction of heat flow inside the molten pool, causing the crystal orientation to change from a (220) crystal orientation to a (111) crystal orientation. This research will provide an important theoretical basis for the precise control and optimization of the cladding process and reduction of the formation of microscopic defects.