Single-mode Fiber Laser Research Articles

On the Use of Metal Sinter Powder in Laser Powder Bed Fusion Processing (PBF-LB/M).

Metal Laser Powder Bed Fusion (PBF-LB/M Powder Bed Fusion, Laser-Based/Metal) offers decisive advantages over conventional manufacturing processes. Complex geometries can be produced that cannot, or only to a limited extent, be manufactured with conventional manufacturing processes. One of the main disadvantages of the process are high investment and operating costs. In order to make the PBF-LB/M process accessible to new research areas, the costs need to be reduced. Therefore, this work investigates whether laser beam sources and motion systems in currently established PBF-LB/M systems can be replaced by more cost-effective components. To reduce the operating costs for PBF-LB/M, the studies are carried out based on previous work with water-atomized, process-foreign sinter powder instead of gas-atomized, spherical PBF-LB/M powders. A cost-efficient, low-alloyed powder is selected (Höganäs HP1) and processed on two different PBF-LB/M machines with a restricted process window using process parameter values that current low-cost machines can achieve. The results show that a multimode fiber laser leads to a more stable process and wider melt pools compared to a single mode fiber laser. In addition, a lower sensitivity of the process with respect to modified process parameters is observed for the multimode laser, resulting in a wider range of stable process windows. A Cartesian motion system (gantry) is suitable for use in PBF-LB/M despite lower scan speeds compared to galvanometer scanners. Beam guidance in the XY-plane offers new possibilities for machine and process design that are not possible with usual scanner systems.

Spatiotemporal dissipative soliton resonances in multimode fiber lasers

Spatiotemporal mode-locking in multimode fiber lasers is intriguing for the complex nonlinear dynamics and the increase of theoretical energy limit. In this paper, we enrich spatiotemporal mode-locking with dissipative soliton resonances, a kind of solitons which is characterized by large pulse energy in single mode fiber lasers, and demonstrate their emergence in multimode fiber lasers by employing the reverse saturable absorption effect from real saturable absorbers. The spatiotemporal dissipative soliton resonances are expected to raise the energy limit further by the locking of dissipative soliton resonances in different transverse modes, whose energy is about twice the maximum single-mode energy in our results. Moreover, properties of spatiotemporal dissipative soliton resonances are investigated by tailoring parameters of the multimode fiber laser, where evolution and transformation of the proposed pulses including shaping and broadening are disclosed. The spatiotemporal dissipative soliton resonances in multimode fiber lasers may open up new avenue for high-power and spatiotemporally engineered coherent light fields in laser dynamics and nonlinear optics.

Towards a tapered Yb-doped fiber-based narrow linewidth single-mode fiber laser with a high signal to Raman ratio.

We demonstrate an all-fiber high-power narrow-linewidth fiber laser based on a homemade tapered Yb-doped fiber (T-YDF). The laser performance is investigated and systematically compared with different seed powers and pump manners. The experimental results reveal that the injected seed power requires a trade-off designed to take into account the impact of spectral broadening, nonlinear effects, and transverse mode instability (TMI). Compared with the co-pump manner, the counter-pump manner performs well in inhibiting nonlinearities, spectral broadening, and improving the TMI threshold. Under the counter-pump manner, this narrow-linewidth T-YDF amplifier realized a 2.09 kW output power with a 3 dB spectral linewidth of ∼0.34 nm, a beam quality of M2∼1.28 and a high Raman suppression ratio over 53.5 dB, the highest reported power for such a T-YDF-based narrow-linewidth single-mode laser, to the best of our knowledge. This work provides a promising pathway towards implementing monolithic high-power narrow-linewidth single-mode fiber lasers.

A Seawater Salinity Sensor Based on Optimized Long Period Fiber Grating in the Dispersion Turning Point.

Variations of seawater salinity often cause ocean internal waves, water masses and stratification, which affect the stability of the ocean environment. Therefore, the study of seawater salinity is significant for the prediction of changes in the ocean environment. However, existing methods for measuring seawater salinity generally have the disadvantages of low sensitivity and low accuracy. In this work, we proposed a seawater salinity sensor based on long period fiber grating (LPFG) in the dispersion turning point (DTP), which has demonstrated the possibility to fabricate LPFG with a shorter grating period by CO2 laser in a thin single mode fiber (SMF) of 80 μm cladding diameter without etching. For obtaining higher sensitivity that could meet the measurement requirement in practice, the proposed sensor was optimized by combining etching cladding and DTP. After the LPFG working near DTP was fabricated by a CO2 laser, the cladding diameter was reduced to 57.14 μm for making cladding mode LP1,7 work near DTP by hydrofluoric acid (HF) solutions. The experimental results have demonstrated that a sensitivity of 0.571 nm/‱ can be achieved when the salinity increases from 5.001‱ to 39.996‱, and the sensor shows good repeatability and stability. Based on its excellent performance, the optimized LPFG is a prospective sensor to monitor seawater salinity in real time. Meanwhile, a low-cost way was provided to make LPFG work near DTP instead of ultraviolet exposure and femtosecond laser writing.

Method of Suppressing Solidification Cracking by Laser Surface Melting and Epitaxial Growth Behavior for Directionally Solidified 247LC Superalloy

In this study, the relationship between solidification cracking and epitaxial growth behavior with the high-speed laser surface melting of a directionally solidified 247LC superalloys was fundamentally and metallurgically investigated, to develop a successful welding procedure for the next generation of gas turbine blades. Under typical laser surface melting conditions (scan speed: 50 mm/s, heat input: 40 J/mm), severe solidification cracking phenomena occurs. The key metallurgical factors of solidification cracking have been identified as solidification segregation-assisted pipeline diffusion behavior at the solidification grain boundary, and in the randomly formed polycrystalline melting zone microstructure. In addition, under extremely low heat input and high-speed laser beam scan conditions (scan speed: 1000 mm/s, heat input: 2 J/mm), an effective surface melting zone can be obtained within a single directionally solidified grain under a relatively high-energy beam density (65 J/mm2) using the characteristics of single-mode fiber lasers. Results reveal that the laser melting zone successfully shows a 99.9% epitaxial growth achievement ratio. Because of the superior epitaxial growth ratio within the laser surface melting zone, and the rapid solidification phenomena, formation of a solidification grain boundary and solidification segregation-assisted pipeline diffusion behavior can be suppressed. Finally, a solidification crack-free laser melting zone can thus be achieved.

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Kerr-lens mode-locking of an Yb:CLNGG laser.

We report on a Kerr-lens mode-locked laser based on an Yb3+-doped disordered calcium lithium niobium gallium garnet (Yb:CLNGG) crystal. Pumping by a spatially single-mode Yb fiber laser at 976 nm, the Yb:CLNGG laser delivers soliton pulses as short as 31 fs at 1056.8 nm with an average output power of 66 mW and a pulse repetition rate of ∼77.6 MHz via soft-aperture Kerr-lens mode-locking. The maximum output power of the Kerr-lens mode-locked laser amounted to 203 mW for slightly longer pulses of 37 fs at an absorbed pump power of 0.74 W, which corresponds to a peak power of 62.2 kW and an optical efficiency of 20.3%.

Keyhole-in-keyhole formation by adding a coaxially superimposed single-mode laser beam in disk laser deep penetration welding

Laser deep penetration welding is characterized by the formation of a vapor capillary (keyhole) and a high overall absorption (coupling rate) of the laser radiation in comparison to heat conduction welding. Due to multiple reflexions within the highly dynamic keyhole, the absorption mechanisms are very complex. The questions, how a laser beam interacts with the existing keyhole, and, if shadowing effects occur, are a matter of concern for several researchers. In this study, the hypothesis that the beam of a laser source can be transmitted to a significant extent through an existing keyhole and acts predominantly at the keyhole bottom was investigated. Experiments were carried out on mild steel and technical pure aluminum, using a specially designed optical system, which allows to coaxially combine two laser beams and to adjust their settings like the energy and the focal plane individually. A combination of a disk laser with a focal diameter of 390 µm and a single-mode fiber laser with a focal diameter of 30 µm was used. Weld depths and seam shapes were analyzed in cross and longitudinal sections. Based on the results, it is shown that the much smaller focal diameter of the single-mode laser beam acted at the bottom of the keyhole and caused a kind of “keyhole-in-keyhole formation” in the form of a local maximum in weld depth whose expression depended on the focal plane of the single-mode laser beam.

Laser micro-welding of stainless steel foil: Welding mode, microstructure and corrosion properties

Laser micro-welding differs from macro-welding in that at least one dimension of the weld is less than 100 μm. Although process research on laser micro-welding has recently made some progress, the influence of welding mode on microstructure and corrosion resistance has remained unknown, which has been shown to have a significant influence on laser macro-welding. In this work, a single-mode fiber laser equipped with a scanning galvanometer is used to weld AISI304 stainless steel foils with a thickness of 100 μm. Similar to laser macro-welding, keyhole formation is used to describe two welding modes, namely thermal conduction welding and penetration welding. The laser-material interaction experiences a transient phase in which the welding mode alternates between conduction welding and penetration welding as reported by previous work. However, we show that gas protection eliminates the transient phase, proving that the transient phase develops as a result of the unsteadiness of the penetration welding caused by oxidation during the welding process. The crystallographic texture and phase constitution vary between conduction and penetration welds due to variations in heat transfer behavior during welding. The conduction weld has greater Σ3 CSL boundaries and a more uniform microstructure than the penetration weld, resulting in better corrosion resistance.

Sub-100 fs watt-level Kerr-lens mode-locked Yb:CaYAlO4 laser with a gigahertz repetition rate

We report a 1.04 GHz high-power Kerr-lens mode-locked Yb:CaYAlO4 laser pumped by a single-mode fiber laser at 976 nm. Based on a bow-tie cavity, stable unidirectional mode-locked operation is obtained with an output coupler of 1.6%. The oscillator delivers pulses with an average power of 1.46 W and with the pulse duration of 99 fs, which, to the best of our knowledge, is the first gigahertz-level Kerr-lens mode-locked laser based on the Yb:CaYAlO4 gain medium. We believe that the watt-level solid-state femtosecond laser at GHz would be an excellent source for developing time-resolved broadband dual-comb spectroscopy.

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.

Metal spattering in laser scanning welding of T2 copper and welding quality

Considering that copper laser welding tends to produce spattering with poor weld formation, and porosity defects, the single-mode fiber laser was employed to conduct conventional laser linear welding and scanning welding experiments on T2 copper in this study. Using high-speed photography and image processing technology, the metal spattering during welding was studied. Then, the effect of different welding paths on the stability of the laser welding copper process was investigated. Besides, the welding quality of the samples under different welding paths was compared and explored from the perspectives of surface morphology, internal porosity, microstructure, mechanical properties and electrical conductivity of the joints. The results show that: compared with the conventional laser linear welding, laser scanning welding of “O” shape, “8” shape and “∞” shape trajectories can extend the cycle variation of keyhole, reduce the metal vapor and intensity of plasma eruption, improve the welding process stability, and inhibit the production of metal spattering, thus enhancing the welding formation and lowering the welding defects. Among them, the “O” shape scanning trajectory has the best inhibiting effect on metal spattering during welding, which reduces the spattering of the molten metal by 48.8% in the laser linear welding sample. The scanning welding samples under the “O” shape trajectory have flat, continuous, fine and uniform welding seam, with optimal shape, and narrow heat affected zone. The joint has the highest tensile strength of 234 MPa, which can reach 97.5% of the base metal strength. The porosity is the lowest of 1.33%, which is only 38% of the conventional laser linear welding sample. In addition, the highest conductivity is 71.79 mS/m, reaching 91.5% of base metal conductivity.

Thermally-induced transverse mode instability: Hopf bifurcation in high-power fiber laser

The thermally-induced transverse mode instability (TMI) has become a major challenge for further enhancing the output power of single-mode fiber laser. Even though much work has been done over the past decades, the spontaneous generation process of output spot oscillation has not been well understood yet. Here, we establish a new theoretical model of the thermally-induced TMI in a high-power fiber oscillator and provide a new mechanism explanation from the point of view of nonlinear dynamics. Theoretical analysis shows that the spontaneous oscillation (∼kHz) of the output spot caused by the thermally-induced TMI is a limit-cycle oscillation via Hopf bifurcation, and the oscillation frequency is proportional to square root of the relaxation rate γ1 of thermally-induced refractive index grating (RIG) (νTMI∝γ1) when the thermally-induced TMI is initially triggered. Furthermore, the relationship between the threshold power, together with oscillation frequency and several parameters of fiber laser system was also discussed in detail. This work help us better understand the underlying dynamics of the thermally-induced TMI.

Birefringence-Induced Heterogeneous Vector Pulses in Ultrafast Fiber Lasers

Single-mode fiber lasers are capable of supporting trapped vector solitons with two similar orthogonally polarized components, due to the delicate balance between fiber birefringence and chromatic dispersion. Here, we demonstrate that heterogeneous vector pulses (HVPs) universally exist in anomalous-dispersion and near-zero-dispersion regimes, from hybrid-structure fiber lasers composed of low- and high-birefringent fibers. The vector pulses include two distinct orthogonally polarized components, one of which is a robust pulse, while the other is a gradually attenuated wavepacket composed of terahertz- (THz) repetition-rate subpulses. Simulation and analytical results fully reproduce experimental observations and demonstrate that the robust pulse couples a fraction of its energy to the orthogonally polarized component per roundtrip at the high-birefringent fiber, forming the unique HVPs. Apart from the intriguing nonlinear dynamics, the HVP can work as a flexible workhorse for various applications, ranging from optical polarization multiplexing to THz synthesis and optical precision spectroscopy.

Confined-doped active fiber enabled all-fiber high-power single-frequency laser.

In this paper, we investigate the performances of an in-house fabricated confined-doped active fiber in the applications of all-fiber high-power single-frequency amplifiers. A 210-W single-frequency single-mode fiber laser is obtained directly, which confirms the excellent performance of the confined-doped active fiber for high-power single-mode operation. To further demonstrate the power scalability of the fiber amplifier, the strategy of applying a temperature gradient along the active fiber is investigated numerically and experimentally, and an up to ∼75% enhancement of the stimulated Brillouin scattering (SBS) threshold is achieved. As a result, a 368-W single-frequency fiber laser is obtained with the beam quality factor of Mx2 = 1.19, My2 = 1.26. Overall, the technique of the confined-doped active fiber provides a promising approach to scale the output power of single-frequency single-mode fiber lasers.

Liquation cracking susceptibility of remelted and reheated weld metals in CM247LC superalloy using multi-bead Varestraint testing

The liquation cracking temperature range (LCTR) of CM247LC was evaluated, especially in remelted and reheated weld metals produced during multi-pass repair welding. The LCTR of the remelted weld metal was 282 K, and those of the reheated weld metals were 455–805 K depending on the testing position. In the multi-bead within the reheated weld metal, the LCTR increased from 455 K far from the inter-pass boundary to 805 K near this boundary. The mechanism underlying the varying LCTR for the remelted and reheated weld metals was the amount of MC carbide at each weld position. The single-mode fibre laser welding suppressed the carbide within both areas, and lowered the liquation cracking susceptibility.

A highly stable frequency single-mode optical fiber laser based on a Fabry-Perot etalon

In this paper we report the development of a robust highly stable frequency single-longitudinal-mode optical fiber laser based on a Fabry-Perot etalon and an active feedback controlling system. The laser is fundamentally structured on an all-fiber compound-ring resonator which is configurated with two interleaved and optical-path-overlapped sub-ring resonators. By employing a length of erbium-doped fiber as the gain medium and a FBG as a reflector, the effective linewidth of the gain spectrum of the compound-ring resonator is restricted to be as narrow as the reflected spectrum of the FBG, which is about 0.043 nm. Because of the vernier effect between the two interleaved sub-ring resonators, the longitudinal mode spacing of the compound-ring resonator can be expanded to be as wide as the effective linewidth of the gain spectrum of it, which permits only one longitudinal mode oscillating. By employing an external Fabry-Perot etalon as a criterion and using an active feedback controlling system to tune the length of the cavity of the compound-ring resonator, the ambiance disturbances to the cavity can be compensated and the frequency of the emitted laser is locked to the offset frequency of the Fabry-Perot etalon. Therefore the frequency of the emitted laser has been stabilized to be 10−8.

A cladding-pumping based power-scaled noise-like and dissipative soliton pulse fiber laser

We report a high-average-power noise-like pulse (NLP) and dissipative soliton (DS) pulse fiber laser. Average power as high as 4.8 W could be obtained at the fundamental mode-locked repetition rate. The NLP can also be transformed into a more powerful DS mode-locking state by optimizing the polarization and losses of intra-cavity pulses in the nonlinear polarization evolution regime. The operation mode between the NLP and DS can be switched, and the laser output performance in both modes has been studied. The main advantage of this work is switchable high-power operation between the NLP and DS. In comparison with conventional single-mode NLP fiber lasers, the multi-function high-power optical source will greatly push its application in supercontinuum generation, coherence tomography, and industrial processing.

Semiconductor saturable absorber mirror mode-locked Yb:YAP laser.

We report on sub-30 fs pulse generation from a semiconductor saturable absorber mirror mode-locked Yb:YAP laser. Pumping by a spatially single-mode Yb fiber laser at 979 nm, soliton pulses as short as 29 fs were generated at 1091 nm with an average output power of 156 mW and a pulse repetition rate of 85.1 MHz. The maximum output power of the mode-locked Yb:YAP laser amounted to 320 mW for slightly longer pulses (32 fs) at an incident pump power of 1.52 W, corresponding to a peak power of 103 kW and an optical efficiency of 20.5%. To the best of our knowledge, this result represents the shortest pulses ever achieved from any solid-state Yb laser mode-locked by a slow, i.e., physical saturable absorber.

Experimental Study on Transverse Mode Instability Characteristics of Few-Mode Fiber Laser Amplifier Under Different Bending Conditions

We studied the transverse mode instability (TMI) characteristics of few-mode fiber laser amplifier employing ytterbium-doped fiber (YDF) with core/cladding diameter of 30/400 μm under different bending diameters. The gain fiber is coiled in ellipse shape with minimum bending diameter changed from 9 cm, 10 cm, 11 cm to 12 cm in the bending part, and the experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 772 W, 1149 W and 1458 W to 1641 W when the bending diameter of YDF increase from 9 cm, 10 cm, 11 cm to 12 cm, accompanied by the deterioration of beam quality from 1.60, 1.67, 1.87 to 1.95. This is mainly because tightly bending in few-mode fiber will promote the overlap between modes and lead to mode coupling, which may trigger TMI. Besides, the bending losses of HOMs decrease while bending diameter increasing, so that there is higher output efficiency and deteriorated beam quality when bending diameter increase. Although the phenomenon is not agreed with common TMI with near single-mode fiber laser, the result will be helpful for the design and power scaling of few-mode fiber lasers.