Single-mode Fiber Laser Research Articles

Iterative model for CW fiber lasers

A new iterative numerical model of continuous wave fiber lasers doped with rare-earth ions is proposed. The main advantage of this model is its stability to the choice of initial values of the generated power at the fiber boundaries. The algorithm for solving the system of rate equations is discussed in detail. Simulation results of double-clad fiber lasers obtained using the proposed model are compared with those reported in the literature. The output characteristics of single-mode fiber lasers were modeled and compared with experimentally obtained data.

Characteristics of Solidification Cracking for Ni-P Coated Cu-Al5052 Single-Mode Fiber Laser Welds

This study fundamentally examines the characteristics of solidification cracking in Cu-Al5052 single-mode fiber laser welding by utilizing uncoated and Ni-P coated Cu plates for the high durability busbar welding of pouch-type lithium-ion battery packs. Weld solidification cracking was confirmed regardless of Ni-P coating for both weld materials. However, there were differences in the regions of crack occurrence. Unlike the uncoated Cu-Al5052 welds where cracks occurred randomly within the fusion zone, for the Ni-P coated Cu-Al5052 welds, solidification cracks were concentrated near the faying surface of the upper and lower materials, particularly near the fusion line. The effect of the Ni and P components on the solidification cracking susceptibility, i.e., the welding solidification temperature range, was considered. Diffusion-controlled Scheil’s solidification calculations revealed that the mushy zone range enlarged with the use of a Ni coating layer. It was confirmed that an extremely wide mushy zone range of 1060 K was calculated at the solidification cracking region. Furthermore, P was also identified as an element that expands the solidification temperature range based on the Cu-P binary system. Consequently, during the laser welding of Cu-Al5052 dissimilar materials, the behavior of solidification cracking in the welds is influenced by the composition of the coating material and the incorporated composition within the weld fusion zone. Hence, it is necessary to select the optimal coating material and layer thickness considering weld solidification cracking susceptibility.

Dissimilar soliton molecule formed by dissipative pulses in a single-mode mode-locked fiber laser.

Soliton molecules, or also known as optical bound states, are the most representative example of solitons' particle nature and have given birth to diverse light-matter analogies. Despite detailed research on regular bound states, the soliton molecule synthesis of dissimilar pulses has rarely been reported. Here, soliton molecules formed by dissimilar dissipative solitons are demonstrated in a single-mode mode-locked fiber laser, with an in-depth analysis of their evolution dynamics. This novel bound state features pulse trapping between two ultrafast vector pulses with distinct pulse properties including energy, duration, and chirp, leading to unique temporal and spectral profiles. This laser provides an optimal platform for studying complex interactions between different types of dissipative solitons. The findings here can provide new degrees of freedom for the generation of optical soliton molecules and can fuel applications in optical information processing, metrology, and spectroscopy.

Welding electrical connections between battery sheet metal materials and additively manufactured (AM) aluminum components: The effect of surface roughness

The integration of additive manufacturing's design versatility with a push for electrification can lead to unique innovations. In this context, when discussing welded joints, their quality and reliability play a significant role. The restricting factor for the use of laser powder bed fusion of metal (PBF-LB/M) additive manufactured (AM) parts is often their surface quality in the as-built state. In addition, the surface quality changes when part plane orientation changes. In this study, AlSi10Mg aluminum test samples were additive manufactured in seven different build angles. The AM samples were glass bead blasted after manufacturing, and the surface roughness of the joint surface area was optically measured. After the measurement, the AM samples were welded to nickel-plated copper and aluminum sheet metals. A continuous wave single-mode fiber laser with scanner optics was used to carry out the welding experiments in lap joint configuration. The structural and mechanical properties of the welds were evaluated by the macroscopic images of weld cross sections and tensile testing of the samples. The electric resistivity of the samples was also tested. Based on the measurement results of the samples, the impact of surface quality was discussed. The results indicated that there was no clear effect of building angle on the final weld quality. The worst surface quality in the joint area was on the test pieces in which the joint surface was facing down toward the build plate.

Development workflow based on in situ synchrotron investigations to optimize laser processing of copper pins

To design future laser manufacturing processes for welding of copper materials, more and more high-end analysis methods are required. A fundamental process understanding by analyzing cause-and-effect relations of process dynamics using inline in situ diagnostics allows for an improved description of laser material interaction. Strategies for a reliable and robust welding process are derived from the findings. In this study, a four-step advanced methodical approach is presented and discussed. In the first step, a fundamental process description of the geometry of the vapor capillary and the formation of weld defects is developed. Therefore, welds on electrolytic tough pitch copper (Cu-ETP) and CuSn6 are carried out to analyze the temporal and spatial vapor capillary dynamics depending on laser power, welding speed, and focal diameter. This fundamental process understanding is transferred to the welding of copper pins in the form of I-pins. For this purpose, impurities and imperfections were applied to the pin surface to investigate the effects on the process result. As a third step, strategies by means of laser intensity distributions were adapted to compensate for imperfections in the welding process. Finally, a sensor vision system is adapted for ideal welding results. Investigations are based on in situ synchrotron analysis at Petra III, DESY in Hamburg. For the experiments, a TRUMPF TruDisk laser (100/400 μm fiber diameter), a TRUMPF TruFiber 6000P (50/100 μm fiber diameter), and a single-mode fiber laser (14 μm fiber diameter) were used. The focal diameter was adjusted with the optical system depending on the investigation.

Nonlinear Multimodal Interference as Ultrafast Photonic Device for Dual-Wavelength Domain-Wall Dark Pulse Generation

Abstract In comparison to bright pulses, better stability that is not susceptible to loss makes dark pulses accessible for applications in such fields as signal processing, optics sensing, and quantum communication. Here we investigate the dual-wavelength domain-wall dark pulse generation in a graded-index multimode fiber (GIMF) based anomalous dispersion single-mode fiber (SMF) laser. By optimizing intra-cavity nonlinearity and pulse polarization, the mode-locked states can evolve each other between bright pulses, dark pulses, and bright-dark pulse pairs. The evolution mechanism among them may be relevant to the coherent mode superposition, spectral filtering, and mode selection in SMF-GIMF-SMF hybrid-fiber modulation devices that affect the pulse formation and evolution in temporal, frequency, and space domains. These results provide a valuable reference for promoting further development of nonlinear optics and ultrafast optics, in which ultrafast photonic devices, with low cost, simple manufacture as well as wide adaptability, as novel pulsed generation technique, play a vital role.

Evaluation of Cutting Parameters on Heat-Affected Zone in Wood Plastic Composites by Pulsed Fiber Laser

Biodegradable and environmentally friendly composite materials such as wood plastic composites (WPCs) have gained attention in industrial applications due to environmental concerns and global sustainability goals. However, owing to their unique structures and properties, cutting on WPCs can be challenging. Continuous wave (CW) mode laser may result in heat build-up and easily warp the WPCs during laser cutting. Therefore, minimizing these defects and determining the cutting parameters that influence the heat-affected zone (HAZ) by pulsed mode laser is essential. In this present work, 1 mm thickness of WPC 30 wt.% of wood fiber (WF) filled with recycled high-density polyethylene (rHDPE) has been experimentally cut by single-mode pulsed fiber laser to evaluate the minimum laser energy required to cut the WPC and the influence of cutting parameters on the HAZ. The HAZ was measured using a digital microscope, and the statistical significance of the cutting parameters to HAZ was determined by analysis of variance (ANOVA). The results confirmed that a minimum linear energy of more than 9 J/mm is required to cut the WPC. It has been found that the cutting speed is the major influence on the HAZ, followed by pulse width. Adequate interaction time by cutting speed and duration of the single-laser pulse also significantly affects the cutting process. The higher gas pressure could minimize the HAZ at the surrounding cutting region of the WPCs. Understanding the influence of these parameters could minimize the thermal effect of HAZ and improve the laser cut quality.

A High-Energy, Wide-Spectrum, Spatiotemporal Mode-Locked Fiber Laser.

In this article, we demonstrate a high-energy, wide-spectrum, spatiotemporal mode-locked (STML) fiber laser. Unlike traditional single-mode fiber lasers, STML fiber lasers theoretically enable mode-locking with various combinations of transverse modes. The laser can deliver two different STML pulse sequences with different pulse widths, spectra and beam profiles, due to the different compositions of transverse modes in the output pulses. Moreover, we achieve a wide-spectrum pulsed output with a single-pulse energy of up to 116 nJ, by weakening the spectral filtering and utilizing self-cleaning. Strong spatial and spectral filtering are usually thought to be necessary for achieving STML. Our experiment verifies the necessity of spatial filtering for achieving STML, and we show that weakening unnecessary spectral filtering provides an effective way to increase the pulse energy and spectrum width of mode-locked fiber lasers.

High power and high efficiency yellow laser in Dy3+-doped single-mode double-clad structured waterproof fluoro-aluminate glass fiber

We demonstrated a single-mode yellow fiber laser using a Dy3+-doped single-mode double-clad structured weatherproof fluoro-aluminate glass fiber (Dy3+:SM-DC-WPFGF) and a 450-nm GaN laser diode (LD) as the excitation light. The power of 445 mW and the slope efficiency of 51.2% were achieved for the single-mode yellow fiber laser. The power level was approximately 20 times higher than our previous result due to improved efficiency of the focusing optics and increased Dy3+ doping concentration (10,000 ppm) in the fiber core. This yellow fiber laser shows particular promise for use in the medical field, e.g., for the treatment of melasma and telangiectasia.

Femtosecond pulse generation from a semiconductor saturable absorber mirror mode-locked Yb:LuYAG mixed crystal laser

We report on a femtosecond semiconductor saturable absorber mirror (SESAM) mode-locked Yb:LuYAG laser. Pumped by a 976 nm single-mode fiber laser, a self-starting mode-locked Yb:LuYAG laser was demonstrated, delivering a 618-mW output power with 140-fs pulses and a 95.4-MHz repetition rate. The output spectrum is centered at 1053 nm with a spectral width of 10.4 nm. To the best of our knowledge, this is the shortest pulse ever generated from a mode-locked Yb:LuYAG laser.

Optimally design of amplifier self-similar pulse in thulium-doped multimode fiber laser

High-energy ultrashort pulses fiber lasers could be widely used in scientific research and industrial applications. Researchers often use two methods to boost the pulse energy, one is to achieve self-similar pulse (SSP) output in a single mode fiber (SMF) laser, and the other is to use multimode fiber (MMF) instead of SMF. Here we combine the above two methods to optimally design the output characteristics of SSP in thulium-doped MMF lasers. The influence of different parameters, including spectral filter bandwidth, fiber length, and gain saturation energy, on the output characteristics of SSP have been analyzed, and an optimal SSP with pulse energy of 63.48 nJ, spectrum width of 52.53 nm, dechirped pulse peak power of 283.34 kW, and dechirped pulse width of 219.74 fs has been obtained.

Linewidth compression of a single longitudinal mode ytterbium-doped fiber laser based on femtosecond laser fabricated fiber Bragg gratings.

We demonstrate a single longitudinal mode distributed Bragg reflection (DBR) fiber laser by directly fabricating fiber Bragg gratings (FBGs) on an ytterbium-doped fiber (YDF) using a femtosecond laser. A simple optical self-injection feedback method was used to effectively compress the linewidth and reduce relative intensity noise (RIN) of a single longitudinal mode DBR fiber laser. Further, we investigated the effect of self-injection feedback cavity length and reflectivity on linewidth compression and determined that the linewidth tends to decrease with the increase of the external cavity photon lifetime. By a self-injection feedback, the laser linewidth was compressed from 31.8kHz to 1.4kHz. Meanwhile, the relaxation oscillation peak from -103.2d B/H z at 1.51MHz was suppressed to -122.3d B/H z at 0.16MHz. This low-noise narrow linewidth single longitudinal mode fiber laser is expected to be a promising candidate for applications such as active detection of neutral atmosphere and distributed fiber sensing.

Cnoidal waves and their soliton limits in single mode fiber lasers

Cnoidal waves are a type of nonlinear periodic wave solutions of the nonlinear dynamic equations. They are well known in fluid dynamics, but it is not the case in optics. In this paper we show both experimentally and numerically that cnoidal waves could be formed in a fiber laser either in the net normal or net anomalous cavity dispersion regime, especially because, as the pump power is increased, the formed cnoidal waves could eventually evolve into a train of bright (in the net anomalous cavity dispersion regime) or dark (in the net normal cavity dispersion regime) solitons. Numerical simulations of the laser operation based on the extended nonlinear Schrödinger equation (NLSE) have well reproduced the experimental observations. The result not only explains why solitons can still be formed in a fiber laser even without mode locking but also suggests a new effective way of automatic stable periodic pulse train generation in lasers with a nonlinear cavity.

Assessment of Ttensile-Shear and T-Peel Strengths for Cu-STS 304 Dissimilar Overlap Welded Joints using Single-Mode Fiber Laser with High Welding Speeds

This study focused on the mechanical and metallurgical properties of Ni-coated pure copper (0.2 mm thick) and stainless steel 304 (0.4 mm thick) dissimilar joints fabricated using single-mode laser welding. Overlap and T-peel welded joints were produced using different welding parameters, such as the laser power and speed. The laser-welded specimens were tested by tensile tests of the weldments, optical metallography, and chemical composition examination of the intermediate layer using energy dispersive X-ray spectroscopy (EDS). The welded joint configuration showed the largest influence on the failure strength. The tensile-shear strengths were more than 3.7 times higher than the T-peel strengths. The scanning electron microscopy and EDS analyses confirmed heterogeneity of the elemental composition across the welded joint interface, regardless of the welding parameters, with minimal occurrence of hot cracks within the welds.

High-power polarization-maintaining LP11-mode fiber laser based on long-period fiber grating for precise welding.

An LP11-mode output all-fiber laser was presented, utilizing long-period fiber gratings (LPFGs) and polarization-maintaining optical fiber (PMF). The LPFG was designed and fabricated, achieving a 90.56% efficiency in LP01 to LP11 mode conversion. Furthermore, the transmission stability of LP11-mode in the PMF was also explored, with the spatial mode overlap ratio exceeding 0.95. Ultimately, the high-power polarization-maintaining (PM) fiber laser, capable of the LP11 mode output, was constructed, with the output power of 600 W and the beam quality M2 of 2.84. During the process of welding a thick Al-plate, the LP11 fiber laser exhibits a notable 1.88 times greater depth of fusion compared to the commercial single-mode fiber laser, when operating at the laser welding head speed of 100 mm/s. For applications demanding non-circular symmetric high-order modes, this research holds substantial potential for widespread adoption within the field of industrial processing.

Experimental Investigation of Actively Q-Switched Dy3+ Doped Fluoride Single Mode Fiber Laser Operating Near 3 μm

Experimental Investigation of Actively Q-Switched Dy³⁺ Doped Fluoride Single Mode Fiber Laser Operating Near 3 μm

Characterization and process optimization of remote laser cutting of current collectors for battery electrode production

Aluminum (Al) and copper (Cu) metal foils of thickness 6–20 µm are employed as current collectors for the production of Li-ion battery cathodes and anodes. Greater demand for this product is driving up production rate requirements, especially in the field of car manufacturing. Laser-based cutting processes for trimming and cutting electrodes are considered suitable for meeting this demand as they can achieve very high throughput while maintaining process quality. In order to meet market requirements, laser manufacturers are developing new laser sources, optics, and scanning heads that will improve process productivity and quality. Establishing the relationship between the laser system and cut quality will lead to competitiveness, increased productivity, and sustainability production. This paper presents a thorough analysis of the cutting performance of pulsed and continuous-wave lasers with scanning speeds of up to 28 m/s for processing thin Al and Cu current collectors. Comparisons between process outcomes are made in terms of maximum and minimum cutting speed and power, kerf geometry, cut quality, and presence of defects. Identification of configurations leading to high and low cut quality enables detailed process parameter windows to be defined for both laser system systems employing continuous-wave and pulsed sources. By analyzing correlations between the materials, laser source, and process variables, the main outcome is that continuous-wave single-mode fiber lasers enable highest cut quality in the high-productivity regime, surpassing the current state-of-the-art in laser cutting of metal foils.

Hot Cracking Characteristics During Single-Mode Fiber and Green Laser Welding Processes in Lithium-Ion Battery Pack Manufacturing

Solidification cracking during lithium-ion battery packaging was metallurgically investigated, specifically for Cu-steel dissimilar materials. To this end, single-mode fiber and green lasers were employed under heat input conditions ranging from 1.3 to 8.0 J/mm. For both laser welds, solidification cracking was concentrated in the steel region of the fusion zone, particularly in the locally Cu-depleted region, regardless of the welding condition. Modified self-restraint tests were performed for overlapping dissimilar material combinations to elucidate the mechanism of solidification cracking. Analysis of the solidification cracking surface revealed that approximately 15?30 mass% Cu existed on the surface. Cu was highly enriched with a droplet shape, formed during solidification within the miscibility gap. By calculating the non-equilibrium weld mushy zone range based on the diffusion-controlled Scheil’s model, the solidification cracking in the Cu-depleted region was estimated at 453 K. It was strongly affected by the severe segregation of Cu (95.7 mass%) in the residual liquid at the terminal stage of the solidification path. Therefore, from a welding metallurgical perspective, homogeneous Cu distribution and minimization of Cu segregation within the fusion zone are essential for suppressing or minimizing the solidification cracking susceptibility of Cu?steel dissimilar laser welding.

Ultrafast dissipative soliton generation in anomalous dispersion achieving high peak power beyond the limitation of cubic nonlinearity

The maximum peak power of ultrafast mode-locked lasers has been limited by cubic nonlinearity, which collapses the mode-locked pulses and consequently leads to noisy operation or satellite pulses. In this paper, we propose a concept to achieve mode-locked pulses with high peak power beyond the limitation of cubic nonlinearity with the help of dissipative resonance between quintic nonlinear phase shifts and anomalous group velocity dispersion. We first conducted a numerical study to investigate the existence of high peak power ultrafast dissipative solitons in a fiber cavity with anomalous group velocity dispersion (U-DSAD) and found four unique characteristics. We then built long cavity ultrafast thulium-doped fiber lasers and verified that the properties of the generated mode-locked pulses match well with the U-DSAD characteristics found in the numerical study. The best-performing laser generated a peak power of 330 kW and a maximum pulse energy of 80 nJ with a pulse duration of 249 fs at a repetition rate of 428 kHz. Such a high peak power exceeds that of any previous mode-locked pulses generated from a single-mode fiber laser without post-treatment. We anticipate that the means to overcome cubic nonlinearity presented in this paper can give insight in various optical fields dealing with nonlinearity to find solutions beyond the inherent limitations.

Solidification Crack Free Welding Strategy via Ultra High Precision Laser Scanning at a Single Grain of Directionally Solidified and Single Crystal Superalloys

In this study, possibility of solidification crack-free welding of directionally solidified CM247LC and single crystal CMSX-4 superalloys was metallurgically investigated using single-mode fiber laser scanning. The key metallurgical factors of solidification cracking have been identified as the formation of highly misoriented solidification grain boundaries (low level of epitaxial growth behavior) at the fusion zone that affects the repulsive force between two adjacent dendrites and dendrite coalescence undercooling at the terminal stage of weld solidification. In particular, under extremely low heat input and ultra-high-speed laser beam scan conditions (welding speed: 1,000 mm/s, heat input: 2 J/mm, and energy density: 65 J/mm<sup>2</sup>), an effective fusion zone could be achieved within a single directionally solidified grain of CM247LC alloy owing to the characteristics of single-mode fiber laser. The fusion zone successfully showed a 99.9% of epitaxial growth achievement ratio (that is, low fraction of highly misoriented solidification grain boundaries at the fusion zone) without solidification cracking. Based on these results on CM247LC alloy, solidification crack-free welds could be successfully achieved using quadruple-pass and square-type weld geometries.