Disk Laser Research Articles

High-power 970 nm semiconductor disk laser.

Semiconductor disk lasers (SDLs) have emerged at the frontier of laser technologies. Here, the chip design, packaging process, resonator, pumping strategy, etc. are optimized for the performance improvement of a 970 nm SDL. After optimization, a power of 70.3 W is attained under continuous wave (CW) operation, and the corresponding thermal resistance is around 0.49 K/W. The laser is highly efficient with a maximum slope efficiency of 58.2% and the pump threshold is only around 1.83 kW/cm2. Furthermore, the emission performances under quasi-continuous wave (QCW) pumping are also explored. Setting the duty cycle to about 11%, the chips can output a peak power of 138 W without thermal rollover, and the single pulse energy can reach about 13.6 mJ. As far as we know, they are the best results in terms of power/energy in this wavelength SDL. These explorations may help to understand the thermal characteristics in high-power SDLs and may also be regarded as an extension and enrichment of the earlier works on this topic.

Sapphire-based resonant waveguide-grating mirrors: advancing their intra-cavity power density capability

We report on the design, fabrication, and implementation of a single-layer resonant waveguide-grating (RWG) mirror on a sapphire substrate. Our goal is to enhance these optics capability to withstand high intra-cavity power densities by exploiting the superior thermal properties of sapphire. The RWG was implemented as an intra-cavity folding mirror in an Yb:YAG thin-disk laser to generate linearly polarized and spectrally stabilized radiation. A linearly polarized output power of 191 W with an optical efficiency of 39% was obtained in multi-mode operation. This corresponds to a power density of 52 kW/cm2 on the RWG, for which the increase of its surface temperature was measured to be 12 K, which resulted in a 46-fold reduction of the surface temperature rise dependence on the intra-cavity power density with respect to what has been reported for a RWG on a fused silica substrate. In near fundamental-mode operation, a linearly polarized emission with an output power of 90 W, an optical efficiency of 30%, and a spectral bandwidth of 28 pm FWHM was obtained.

Study of a Semiconductor Disk Laser with a Wavelength of 780 nm Based on a Heterostructure with AlxGa1 – xAs/AlyGa1 – yAs Quantum Wells under Optical Pumping with Different Radiation Wavelengths

Study of a Semiconductor Disk Laser with a Wavelength of 780 nm Based on a Heterostructure with AlxGa1 – xAs/AlyGa1 – yAs Quantum Wells under Optical Pumping with Different Radiation Wavelengths

High-Speed Laser Beam Welding of Magnesium Alloy AZ31 for Enhancing the Mechanical Properties

Laser beam welding is still the focus of research all over the world since new laser sources with more brilliance, higher power, or higher efficiency are being developed. High brilliance leads to thinner fibers when solid-state lasers are used. For welding applications, a thin beam, respective a small focus spot is recommended for low heat input resulting in less deformation. The edge preparation of the welding pieces must be as accurate as possible, and a zero gap is recommended. In earlier research, it was shown, that the gap bridging capacity could be enhanced by the wobbling of small focus spots, as well as refining the grain size in the weld zone by decreasing the focus diameter. Inventions in the optics, like the beam splitting into a core and a ring part, avoid the use of a scanner and can lead to better gap bridging. Nevertheless, the use of a brilliant beam, resulting in a small focus in combination with high power can result in very high welding velocities, just limited by the used machinery. In the present study, a disk laser with 4 kW maximum power and 100 μm focus spot was used to weld 2 mm thick magnesium AZ31 sheets at speeds up to 20 m/min. As expected, the seam width becomes smaller with raising velocity, and some underfill and access material occurred on the surface and the root of the welded sheets. Surprisingly, the texture of the weld seam changed from random at low velocity to a more pronounced texture at high speed with respect to the basal texture of the plate base material. This influences the mechanical behavior, namely the strain to fracture, of the welded joints positively. The high-speed weldments are compared to state-of-the-art weldments of magnesium AZ31, in terms of mechanical strength and elongation to fracture, based on the texture analysis.

Interventional Therapy and Surgical Management of Lumbar Disc Herniation in Spine Surgery: A Narrative Review.

Significant advancements in lumbar disc herniation (LDH) management have been made in interventional pain therapy, operative therapy, peri-operative management, and cost analysis of various procedures. The present review aims to provide a concise narrative of all these topics, current trends, and possible future directions in the management of LDH. Interventional pain management using intradiscal injections often serves as a minimally invasive non-surgical approach. Surgical modalities vary, including traditional open laminectomy, microdiscectomy, endoscopic discectomy, tubular discectomy, percutaneous laser disc decompression, and transforaminal foraminotomy. Prevention of infections during surgery is paramount and is often done via a single-dose preoperative antibiotic prophylaxis. Recurrence of LDH post-surgery is commonly observed and thus mitigative strategies for prevention have been proposed including the use of annular closure devices. Finally, all treatments are well-associated with clear as well as hidden costs to the health system and society as described by billing codes and loss of patients' quality-adjusted life-years. Our summary of recent literature regarding LDH may allow physicians to employ up-to-date evidence-based practice in clinical settings and can help drive future advancements in LDH management. Future longitudinal and comprehensive studies elucidating how each type of treatments fare against different types of herniations are warranted.

Effect of Defocusing Distance on Microstructural, Hardness, and Tribological Behavior of Ni-Cr-Si-B-C Powder Deposit on 316LN Austenitic Stainless Steel Substrate Using Laser Hard-Facing

The Ni-based alloy powder has been melted and bonded to an AISI 316LN austenitic stainless steel (ASS) base material (substrate) using a high-energy disc laser with a maximum power of up to 12 kW. The substrate’s hard-faced reservoir is highly diluted due to the significant difference in melting temperature between the substrate and the commonly used Ni-based alloy. The hardness, macrostructure, microstructure, and wear resistance effects of the defocusing distance were examined. The composition, microstructure, hardness, and wear resistance of phases have been examined with a pin-on-disk wear test, an energy dispersion spectroscopy (EDS), a scanning electron microscope (SEM), and X-ray diffraction (XRD). According to the findings, the height and diameter of the beads rose when the distance was increased from 17 mm to 37 mm. On the other hand, penetration and dilution decreased from 3.7 mm to 2.7 mm and from 9.7% to 3.1%, respectively. When the defocusing length is increased and the penetration profundity and dilution are limited, the density of energy per unit clad is accessible. The laser hard-faced surfaces contain microstructures of Ni-rich solid solution, boride, and carbide. These different factors cause higher hardness and resistance to wear.

Investigation of a 100 W Solar-Pumped Disk Laser with TEM00 Output

Investigation of a 100 W Solar-Pumped Disk Laser with TEM00 Output

Developing a misalignment model for folded optical resonators and designing a kilowatt-class disk laser resonator with a short effective length

In this article, a formulation was developed for designing a stable disk laser resonator and achieving a high power and a beam quality close to the fundamental mode with a short effective length. In this regard, by taking a matrix approach and using Gaussian beam propagation based on the ABCD law for Gaussian beams, the fundamental-mode spot size inside a resonator and its sensitivity to misalignment, as well as dynamic stability in multiple folded resonators, were analyzed in order to achieve a short effective length. In the presented method, the optimal geometrical parameters of the resonator were obtained by applying the length limits, permissible level of sensitivity to misalignment, and range of dynamic stability with the help of MATLAB software coding. Using this design algorithm, a Z-shaped resonator with the arm length of 50 cm and fundamental-mode spot size of 2.4 mm on the active media was designed, while the resonator had a good dynamic stability and low sensitivity to misalignment. Considering these specifications, it was possible to design a compact disk laser with a power of 1 kW, which had a beam quality close to the fundamental mode.To the best of our knowledge, the laser disk resonator described in this paper, which is resistant to misalignment and has a short effective length, has not been previously documented in the literature. The model developed in this research is a general model that can be used in much more complex structures with any numbers of optical elements in other lasers.

Influence of the laser-induced plume on welding behavior in keyhole welding for stainless steel using a 16 kW disk laser

The spatter is one of the defect factors for laser welding. For high-quality laser welding, the elucidation of the spatter reduction mechanism is required. In our previous study, it was elucidated that the molten pool and keyhole fluctuation contribute to spatter generation from the observation of the keyhole and molten pool under different ambient pressure conditions. However, the main cause of the instability of the molten pool and keyhole has not been clarified. It is considered that the interaction between the laser and plume might cause these instabilities. Therefore, in this study, we focused on the plume generated by laser irradiation. The dynamics of the plume during laser welding and the attenuation of the laser were observed under different ambient pressures. According to these observations, the effect of the plume on instability in laser welding was elucidated. The SS304 was fixed in the vacuum chamber, and the disk laser with an output power of 6 kW swept on the sample to form the weld bead. At the same time, the plume behavior was observed by the Schlieren method, and the attenuation of the laser was measured using a probe laser. As a result, the metal vapor jet, which is a periodical plume ejection, was observed. The attenuation of the probe laser increased with increasing atmospheric pressure. These results suggest that the frequent generation of the metal vapor jet under atmospheric pressure caused instability in the heat input of the laser, which caused instability in the keyhole and molten pool.

Determining thermomechanical response of thin disk lasers with physics-informed neural networks

Acquiring structural behavior related to thermal effects is essential to the design of high-power thin disk lasers. However, quantifying the thermomechanical response of a thin disk laser crystal (TDLC) is challenging since these variables of interest are always highly tensorial. Herein, we developed a method to directly quantify the thermo-induced structural behavior of a TDLC from scalar temperature by a physics-informed neural network (PINN). The PINN is established to approximate the unknown states (thermal displacement, strain, and stress fields) using the loss functions encoded with sparse temperature and thermal mechanics equations. Steady-state and transient results of thermo-induced structural responses are precisely obtained from the temperature field by training PINN. In addition, transfer learning is demonstrated to enhance the computation efficiency of PINN for TDLCs with various materials and geometric properties. Implanting PINNs into thin disk lasers paves the way for precise quantification of highly tensorial thermal effects of lasers in-situ using easily accessible scalars.

High-power vortex beams generated from a Yb:YAG thin-disk laser with spot-defect mirrors

We report the generation of high-power and high-order vortex beams based on a Yb:YAG thin-disk laser oscillator with a spot-defect mirror. High-quality vortex beams were directly emitted by the oscillator with an output power of 80 W, which is the highest output power of any vortex laser oscillator. Tunable high-order Hermite-Gaussian modes from HG1,1 to HG5,1 were also generated, and subsequently converted to Laguerre-Gaussian beams by a cylindrical-lens mode converter. The vortex characteristics of the output were confirmed with a Mach-Zehnder interferometer. This work demonstrates an effective way to generate high-quality, high-power and high-order optical vortex beams.

A narrative review of percutaneous laser disc decompression - early experience in Lagos, Nigeria

Overview. Managing chronic pain arising from the degenerative disc disease of the spine has remained a huge challenge while choosing from within a vast array of treatment armamentarium ranging from the use of medications only or in combination with other physical treatment options, minimally invasive heat therapy, and operative interventions. Patients with chronic pain referred to as ‘contained disc’ in cervical, thoracic, and lumbar disc pathology have been successfully managed with percutaneous laser disc ablative therapy. This approach is only for selected patients and advantages derivable from this approach are the preservation of soft tissue anatomy, minimal post-procedure pain, less metabolic responses, and no blood loss when compared to open surgical procedures. Methodology. This is a one-year retrospective narrative study of six adult patients who presented with chronic low back pain related to the spine and had a lumbosacral spine magnetic resonance imaging (MRI) scan with findings of contained disc herniation. Patients’ brief clinical evaluation which involves biodata, history of pain, and pain scores using the visual analogue scale (VAS) pre and post-intervention and Oswestry disability index to assess disability from chronic pain. A narration of these patient’s progression to follow-up six months after the procedure was documented. Results. The results showed significant improvement in the VAS and ODI scores in five of the 6 patients, with one showing marginal improvement. Conclusion. Percutaneous lumbar disc decompression is effective in the management of chronic low back with concordant clinical and radiological evidence of contained disc pathology.

Single-crystal and ceramic Yb:Lu2O3 gain media for thin-disk oscillators

We report on the direct comparison of single-crystal and ceramic Yb3+:Lu203 gain media with respect to emission spectra, fluorescence lifetime, depolarization, and laser performance in a continuous-wave thin-disk laser oscillator. The most efficient laser operation was achieved with a single-crystal disk in multimode operation with a slope efficiency of 72.1% and an average output power of 997 W. At the same temperature level, a ceramic disk delivered 861 W with a slope efficiency of 68.6%. In fundamental-mode operation, the highest average power of 360 W and highest optical efficiency of 41.3% were obtained with a ceramic disk. For the single-crystal disk, the fundamental-mode output power was limited to 113 W at an optical efficiency of 29%, potentially due to stress within the crystal.

Switching of several self-mode-locking states in an optically pumped semiconductor disk laser

Self-mode-locking (SML) of fundamental TEM00 mode and high-order transverse mode were demonstrated in an optically pumped semiconductor disk laser and these states can be switched by simply adjusting the elements of the laser. The repetition rate of the pulse train was 1 GHz, using a straight cavity. Furthermore, we measured the pulse durations and found that the pulse duration of SML pulse with higher-order transverse mode is shorter than that of fundamental mode at the same pump power. To the best of our knowledge, this is the first experiment to obtain two SML states in a single laser. By using the method of changing the cavity length and adjusting the angle of the output coupler, the repetition rate can be continuously tuned from 3 GHz to 3.2 GHz.

Comparison of melting efficiency between blue, green, and IR lasers in pure copper welding

Pure copper parts are commonly used in many industrial products because of their low thermal resistance and high electrical conductivity. However, connecting high-quality and high-efficiency copper materials remains a challenge. This is because pure copper has low absorption of near-infrared light, making it difficult to weld stably with a near-infrared laser. Visible light lasers should realize high-efficiency laser welding of pure copper. However, there are few reports comparing the laser wavelength dependence of welding efficiency for pure copper. In this study, bead-on-plate welding was performed on pure copper plates of 2 mm thickness using a 1.5 kW blue diode laser (445 nm), a 16 kW IR disk laser (1030 nm), and a 3 kW green disk laser (515 nm). Bead-on-plate welding of pure copper was performed in the thermal conduction mode or the keyhole mode by varying the laser spot diameter and power, and the amount of melting was measured from cross-sectional observations. As a result, compared to the IR disk laser, blue and green lasers showed higher melting efficiency in both the thermal conduction and keyhole modes, and the melting behavior was more stable. In thermal conduction mode welding, the melting efficiency was 0.2% with the IR disk laser and 0.7% with the blue diode laser. In keyhole mode welding, the melting efficiency with the blue diode laser or green disk laser was about 7%, which is equivalent to that with the IR disk laser with 2.5 times the output power.

Suppression of instability of output beam of thin-disk lasers caused by the air-wedge effect based on resonator design

Suppression of instability of output beam of thin-disk lasers caused by the air-wedge effect based on resonator design

Generation of a radially polarized beam in a polycrystalline ceramic Yb:Lu2O3 thin-disk laser

We report on the generation of a continuous-wave (CW) radially polarized beam with an Yb:Lu2O3 polycrystalline ceramic disk in a thin-disk laser (TDL) oscillator. A circular grating-waveguide mirror (CGWM) with a high polarization discrimination given by a reflectivity difference between the two orthogonal polarization states of 44.6% was used as a polarization-selective cavity end-mirror. An output power of 175 W was achieved with an optical efficiency of 39.6%. A high degree of radial polarization of 96.2% and a beam propagation factor of M2hor. = 2.05 and M2ver. = 2.32 were measured at the maximum output power.

Compact diode-pumped tunable single- and dual-wavelength single-frequency semiconductor disk lasers

Optically pumped external cavity semiconductor disk laser (SDL) combines the characteristics of semiconductor laser and solid-state laser, and has important research value. In this paper, an InGaAs SDL with different emission peak from others was designed. Under free-running operation and a cooling temperature of 12 °C, a few-longitudinal-mode continuous wave laser at 1037 nm with a maximum output power of 2.53 W and a slope efficiency of 41.3% with respect to an absorbed power of about 8.0 W was achieved. Based on this experiment, stable single-wavelength single frequency SDL with maximum output power of 1.79 W was obtained by inserting an etalon into the laser resonator. The single frequency laser was measured to have near-diffraction-limited beam quality. Flexible and continuous wavelength tuning from 1034.77 nm to 1041.10 nm of the single-wavelength single frequency SDL was also realized by introducing another etalon. Under this situation, we also obtained tunable dual-wavelength single frequency SDLs, for the first time to our knowledge. Compared to complex methods such as particular bandgap engineering design or multi-SDL resonator for generating dual-wavelength lasers, the method proposed in this work by using etalons is a simple, feasible, and low-cost solution.

Injection-seeded high-power Yb:YAG thin-disk laser stabilized by the Pound-Drever-Hall method.

We demonstrate an injection-seeded thin-disk Yb:YAG laser at 1030 nm, stabilized by the Pound-Drever-Hall (PDH) method. We modified the PDH scheme to obtain an error signal free from Trojan locking points, which allowed robust re-locking of the laser and reliable long-term operation. The single-frequency pulses have 50 mJ energy (limited to avoid laser-induced damage) with a beam quality of M2 < 1.1 and an adjustable length of 55-110 ns. Heterodyne measurements confirmed a spectral linewidth of 3.7 MHz. The short pulse build-up time (850 ns) makes this laser suitable for laser spectroscopy of muonic hydrogen, pursued by the CREMA collaboration.

On the Processability and Microstructural Evolution of CuCrZr in Multilayer Laser-Directed Energy Deposition Additive Manufacturing via Statistical and Experimental Methods

Laser-directed energy deposition (LDED) is a promising technology for coating, repairing, and building near-net-shape 3D structures. However, the processing of copper alloys, specifically, has presented a significant challenge due to their low laser absorptivity at the 1060 nm laser wavelength and high thermal conductivity. This study undertook a methodical examination by employing a 2 kW disk laser, operating at a wavelength of 1064 nm, and a coaxial nozzle head to comprehensively examine the processability of the highly conductive CuCrZr alloy for expanding the range of materials that can be successfully processed using LDED. The investigation focuses not only on optimizing the input process parameters that are the laser power, scanning speed, powder feed rate, and overlap ratio, but also on planning the toolpath trajectory, as these factors were found to exert a substantial influence on processability, geometrical accuracy, and the occurrence of defects such as lack of fusion. The optimal toolpath trajectory discovered involved implementing a zigzag strategy combined with a 90° rotation of the scanning direction. Additionally, a start point rotation was considered between each layer to even out the deposition of the layers. Moreover, a contour with a radial path at the corners was introduced to enhance the overall trajectory. Based on the hierarchal experimental study, the appropriate ranges for the key process parameters that leads to 99.99% relative density have been identified. They were found to be from 1100 up to 2000 W for the laser power (P), and from 0.003 up to 0.016 g/mm for the amount of powder that is fed to the melt pool distance (F/V). Regarding the influence of process parameters on the microstructure of the samples with equal deposition height, it was observed that varying combinations of process parameters within the optimal processing window resulted in variations in grain size ranging from 105 to 215 µm.