High-power Single-mode Fiber Laser Research Articles

Sub-100 fs Kerr-lens mode-locked femtosecond Yb:CaYAlO<sub>4</sub> laser with GHz repetition rate

Femtosecond lasers with GHz repetition rate play an important role in scientific and industrial applications such as spectroscopy, optical frequency combs and GHz-Burst pulse trains for micro-machining in the ablation-cooled regime. Kerr-lens mode-locked (KLM) technique and passively mode-locking based on semiconductor saturable absorber mirror (SESAM) are the primary methods to generate GHz femtosecond all-solid-state lasers (ASSLs). Kerr-lens mode-locked Ti:Sapphire lasers have made significant progress benefited from the high-power green pump lasers, and repetition rate up to 10 GHz has been obtained with the average power of 1.2 W. In the early 21st century, ytterbium ion (Yb<sup>3+</sup>) doped laser crystals and ceramics with emission wavelengths near 1 μm received attention due to their high conversion efficiency and broad gain-bandwidth. Combining the customized SESAM and high-power multimode fiber-coupled laser diodes (LDs), GHz Yb-doped ASSLs with watt-level average power may be easily attained and have made rapid progress. However, GHz KLM lasers have strict requirements for the cavity design and pump sources. For satisfying mode matching and enhancing the soft aperture effect within the gain medium, a high-brightness pump source with excellent beam quality (<i>M</i><sup> 2</sup> ～ 1) is desired, such as the single-mode fiber coupled LD, however, the maximum pump power of which is only ～1 W. As a result, the average power of GHz KLM femtosecond laser is typically limited to few tens of milliwatts, which limits the further applications. In this work, we report the first GHz high-power KLM Yb:CaYAlO<sub>4</sub> laser by using a high-power single-mode fiber laser instead of the low-power single-mode fiber coupled LDs as the pump source. On the basis of <i>ABCD</i> matrix, a simple four-mirror bow-tie ring cavity is built so that the laser mode can match well with the focused pump spot in the crystal. At the pump power of 8 W, stable unidirectional KLM is achieved, the laser has the average power of 2.1 W with a pulse duration of 88 fs and a repetition rate of 1.8 GHz, corresponding to the peak power of 11.57 kW. The high peak power and extremely short pulse duration are crucial for coherent octave-spanning supercontinuum generation. The powerful GHz KLM laser with sub-100 fs pulse duration provides an attractive source for realizing the optical frequency combs and micro-machining applications.

Welding of pure copper wires using a hybrid laser system with a blue diode laser and a single-mode fiber laser

Highly efficient welding of pure copper was achieved using a hybrid laser system, which combined a blue diode laser and a single-mode fiber laser. Pure copper is used as a motor coil for electric vehicles (EVs). However, the processing efficiency of laser welding for pure copper is low since it has a low light absorption rate for the wavelength of the fiber laser. In this study, a hybrid laser system, which combined coaxially with a blue diode laser and a high-power single-mode fiber laser, was developed to realize highly efficient laser welding for pure copper. The beam profile of hybrid laser was alimented to set a stair-shape beam at the processing point. To investigate the effect of the beam profile, the hybrid laser was irradiated onto a 3 × 3 × 20 mm3 copper wire for welding, and the molten volume of copper wire was evaluated. As a result, it was found that the molten volume with a blue diode laser became 1.8 times larger than that without it. Moreover, the welding efficiency of the hybrid laser was 1.45 times higher than that of the fiber laser only.

Limits and solutions in processing pure Cu via selective laser melting using a high-power single-mode fiber laser

The demand for additive manufacturing (AM) of Cu and its alloys shows an increased trend from the energy and heat transfer–related applications. Selective laser melting (SLM) is amongst the key AM processes for metals, providing high geometrical accuracy and design flexibility. The technology is most commonly employed using high-brilliance fibre lasers operating at 1 μm. However, the elevated reflectivity of Cu at this wavelength, combined with its high thermal conductivity, is the cause for a highly unstable process, whereby pore-free products are difficult to obtain. Accordingly, the present work explores the limitations in processing pure Cu powders with a 1-kW single-mode fibre laser providing solutions and different strategies for improving part quality. The process parameters were studied for single and multi-pass melting strategies. The power level requirements, as well as the build plate material, are assessed through an analytical model. The results demonstrate that a correct sequence of multi-pass strategies can improve the part density up to 99.1% ± 0.2% with an industrially acceptable build rate of 12.6 cm3/h.

Single Pass Laser Welding with Multiple Spots to Join Four Sheets in a Butt-joint Configuration

Laser keyhole welding is widely used in industry due to its large welding depth and low heat input. For some industrial cases it is necessary to widen the beam to cover the joint configuration, which instead results in a lower intensity and surface conduction welds. The introduction of the high-power single mode fiber laser makes it possible to deal with this problem, because the beam can be shaped into a pre-defined pattern of multiple spots shaped to the actual joint configuration. The intensity of each spot is sufficient to make a keyhole. A case with four sheets in a butt-joint configuration is used to demonstrate the principle of how to design a spot pattern which ensures weld quality in case of a single pass laser weld.

Joining of multiple sheets in a butt-joint configuration using single pass laser welding with multiple spots

The introduction of high-power single mode fiber lasers has given deeper and narrower welds than previously seen using lasers. In some cases, the weld becomes too narrow and must be expanded to fit the geometrical shape of a given welding task. Instead of using only one beam, it was suggested to split the beam into multiple spots placed in a prespecified pattern. In this way, the dimensions of the weld pool could be controlled. In this work, the spots are placed on a row perpendicular to the welding direction. This provided the ability to control the final weld face by controlling the width and depth of penetration independent of each other with minimum heat input. An examination was conducted on how the weld cross section is influenced by spot distance, number of spots, power, and focus. It is shown that a weld seam can be widened in steps by using multiple spots, and the depth can be controlled independently from the width by adjusting the laser power. It is possible to produce a rectangular like cross section of the weld by choosing correct design parameters. For a pattern with multiple spots on a row, equal depth of penetration in the weld bead is reached if the edge spots have 5%–10% more power than the center spots. Basic rules for design of spot patterns are given for a configuration with a single mode laser and spot diameter of Ø85 μm. The ability to bridge gaps is increased in multispot welding compared to multiple single passes. The technique shows promising results in welding parts which resembles the conditions of a real production regarding robustness and tolerances.

1.5 kW Incoherent Beam Combining of Four Fiber Lasers Using Adaptive Fiber-Optics Collimators

We demonstrate the incoherent beam combining (BC) of four high-power single-mode fiber lasers. Adaptive fiber-optics collimators (AFOC) of 0.5 kW-level are developed to correct the tip/tilt-type phase errors among beamlets. The maximum deflection angle of the AFOC is about 0.29 mrad. A stochastic parallel gradient descent algorithm is employed as the tip/tilt control strategy. The tip/tilt control makes four separate laser beams overlap with each other in the far-field, and the average of normalized power-in-bucket increases from 0.171 in open loop to 0.798 in closed loop when 1.5-kW total output power is achieved. The precise tracking ability and control accuracy of this BC system are investigated. Experimental results expose that thermal-blooming effect disturbs BC seriously when the total output power is up to kW-level, which could be mitigated by inducing air flow near the laser output.

Applications of High Power Single-Mode Fiber Lasers to Novel Microwelding and Micromachning

Single-mode fiber lasers have many advantages over conventional solid-state lasers, which include high beam quality, no need for cooling, high conversion efficiency, compactness, long life time and so on, attracting attention as an industrial tool for materials processing due to rapid increase in the output power. Present article overviews recent status of the applications of high-power single-mode fiber laser to laser materials processing including microwelding and micromachining.

Propagating modes in gain-guided optical fibers.

Optical fibers in which gain-guiding effects are significant or even dominant compared with conventional index guiding may become of practical interest for future high-power single-mode fiber lasers. I derive the propagation characteristics of symmetrical slab waveguides and cylindrical optical fibers having arbitrary amounts of mixed gain and index guiding, assuming a single uniform transverse profile for both the gain and the refractive-index steps. Optical fibers of this type are best characterized by using a complex-valued v-squared parameter in place of the real-valued v parameter commonly used to describe conventional index-guided optical fibers.