Continuous-wave Fiber Research Articles

The effect of laser surface melting on the microstructure, microsegregation and solidification path of service-aged ECY768 cobalt-based gas turbine nozzle

In this study, the laser surface melting of cobalt-based superalloy ECY768, which has been in service for 40,000 h at a temperature of 1000 °C, has been investigated. Subsequently, laser surface welding and post-welding heat treatment is performed on the samples. This study was conducted in order to restore the microstructure of the damaged service-aged gas turbine nozzle to as-cast state. The results showed that by performing laser surface remelting, the microstructure of as-cast state can be approached. Also, the results of this study have investigated the weldability, microsegregation and solidification path of this superalloy, and it can be used in the repairing this superalloy in the industry. Laser surface melting was performed using a continuous wave fiber laser at various speeds of 10, 12, 14, 20, 50, and 70 mm/s. The microstructure and phase characteristics of the samples were examined using optical and scanning electron microscopes. Upon analyzing the macrostructure of the weld metal, it was observed that the depth of the melt zone at the speed of 12 mm/s is 585 ± 40 μm. In the investigating the microstructure of the solidification modes, namely planar, cellular, columnar dendritic, and equiaxed dendritic, it was observed that microsegregation occurs during laser welding of the samples due to the presence of heavy elements such as tantalum and tungsten. The solidification distribution coefficient for tantalum was calculated to be 0.37, which increased to 0.60 after the solution annealing heat treatment, resulting in an improved segregation behavior. Characterizing the surface hardness profiles of laser-melted samples it was observed that the hardness in the melt zone significantly increased compared to the base metal. The results of the current research indicate that the weldability (resistance to various welding defects, especially hot cracks) of the cobalt-based superalloy ECY768 is acceptable, and this process can be used for rejuvenating service-aged ECY768 cobalt-based gas turbine nozzle.

A >20-W, linearly polarized single-frequency continuous-wave all-fiber laser at ∼1540 nm

We have developed both theoretically and experimentally an all polarization-maintaining fiber-based master oscillator power amplifier (MOPA) system, which allows for amplifying the single-frequency continuous-wave (CW) fiber laser to >20 W without compromising the laser intensity noise (−110 dBc/Hz@ relaxation oscillation peak) at the specific wavelength of ∼1540 nm. The experimental results agree well with the simulated results. To the best of our knowledge, this is the first demonstration of a ∼1540-nm single-frequency all fiber laser and reaching such a high-power level. The amplified laser features a narrow linewidth of ∼1.4 kHz and a spectral side-mode-suppression ratio of >38 dB, even as the laser wavelength is tuned from 1538.216 nm to 1540.616 nm. It is believed to be the narrowest laser linewidth realized in the <1550-nm wavelength band of an Erbium-Ytterbium co-doped fiber MOPA system. The all polarization-maintaining configuration of the MOPA ensures the amplified laser linearly polarized with the polarization extinction ratio of >22 dB and power root mean square fluctuations of <0.2 %@1h. This high-performance single-frequency fiber laser is expected to be an ideal pump source in the spin-exchange relaxation-free (SERF)-based atomic sensors after second harmonic generation.

Demonstration of a 0.8 W continuous wave bulk laser around 2 μm in a Tm3+:Ga-rich BGG glass

Efficient lasing of Ga-rich BGG bulk glasses doped with Tm3+ is demonstrated. Two BGG bulk samples with Tm3+ ion concentrations of 1.27×1020 cm−3 and 2.54×1020 cm−3 were tested for their laser performance. In-band pumping was realized using a continuous-wave (CW) homemade Er-doped fiber laser at 1600 nm. A maximum slope efficiency of 27.7% was demonstrated with respect to the absorbed pump power. The emission cross-section was retrieved from the laser efficiency data for three different lasing wavelengths and compared with the theoretical calculations. The power scaling potential of the Tm3+:BGG glass was investigated and a maximum output power of 0.8 W was achieved.

Review of High-Power Continuous Wave Yb-Doped Fiber Lasers near 980 nm

In this paper, the development of a high-power continuous wave (CW) fiber laser near 980 nm is reviewed. This review is focused primary on the power evolution resulting from the designation of Yb-doped fibers, which is important in the suppression of the amplified spontaneous emission (ASE) around 1030 nm. Current studies on the in-band ASE as the power limitation of the Yb-doped fiber lasers near 980 nm are also summarized in this review.

Improving the actuation behavior of nitinol shape memory alloys by nanosecond laser surface texturing

Laser surface texturing of nitinol was performed using a nanosecond fiber laser to modify the surface structure and enhance the laser actuation performance of nitinol. The influence of laser processing parameters on surface morphology and chemical composition was studied in detail. The influence of texturing on the laser actuation behavior was evaluated using a continuous wave fiber laser and a CO2 laser. Compared to the pristine sample, the textured samples demonstrated better displacements and faster actuation. During laser actuation using a fiber laser, the textured sample showed maximum and minimum displacement enhancements of 65% and 15%, respectively. The actuation performance of the textured sample was approximately twice that of the pristine sample during the irradiation with CO2 laser. This paper presents the viability of laser surface texturing to enhance the actuation performance of shape memory alloys, and the results indicate that the power required for actuation can be reduced to avoid the use of bulky power and control units.

Emission Wavelength Limits of a Continuous-Wave Thulium-Doped Fiber Laser Source Operating at 1.94 µm, 2.09 µm or 2.12 µm

We present a thulium-doped single-oscillator monolithic fiber laser emitting successively at three wavelengths, especially at unusual long wavelengths as 2.09 µm and even at 2.12 µm. The 793 nm core absorption of 8.42 dB/m allows for achieving a slope efficiency higher than 43% both at 1.94 µm and 2.09 µm. The operation of the laser at 1.94 µm, 2.09 µm, and 2.12 µm is compared by using different fiber Bragg gratings to push the limit of thulium ions emission above 2.05 µm. This is the first demonstration of emission exceeding wavelengths of 2.1 µm of an only thulium-doped fiber laser, to the best of our knowledge.

Simulation and Experimental Study on Continuous Wave Fiber Laser Removal of Epoxy Resin Paint Film on the Surface of 6061 Aluminum Alloy

Paint removal is an essential process in the industrial field. Laser technology provides an effective method of paint removal to replace traditional mechanical and chemical methods. This paper establishes a continuous wave (CW) laser thermal paint removal model based on heat conduction theory and Arrhenius’ law. The paint stripping process of epoxy paint film on the surface of 6061 aluminum alloy via CW laser was studied through simulation and experiment. We found that the carbonization of the paint film during the CW laser paint removal process will inhibit the laser paint removal process. Therefore, the paint removal efficiency of the CW laser is limited. The depth of CW laser paint removal increases linearly with the CW laser power density. However, during the CW laser paint removal process, due to the pyrolysis of the paint film and the reflection of the laser by the substrate, the surface temperature of the material first increases and then decreases. In addition, after laser paint removal, the surface roughness of the material after paint removal is reduced due to the melting of the base material. The model established in this article can provide a theoretical reference for studying the CW laser paint removal process.

Investigation of aerosol generation through laser cleaning of various surfaces and optimization of mist & spray scavenging

To ensure the safe and effective decommissioning of the Fukushima Daiichi Nuclear Power Plant (NPP), it is crucial to investigate the behavior of radioactive aerosols generated during the laser decontamination of radiation hot spots and laser cutting of fuel debris. Understanding the generation and dispersion patterns of these aerosols is of utmost importance for the proper planning and execution of decommissioning activities. This study focuses on the performance evaluation of high-power continuous wave (CW) fiber lasers for cleaning surfaces composed of carbon steel, stainless steel, and concrete. The effective confinement and scavenging of radioactive aerosols are critical in minimizing the risk of radiation exposure during decommissioning processes. Test samples, made of carbon steel (CS), stainless steel (SS), and concrete, coated with ZrO2, CeO2, and CsI, were subjected to laser decontamination using a class-4 fiber laser within the UTARTS (University of Tokyo Aerosol Removal Test with Sprays) facility. The aerosol particles generated during the laser decontamination process were confined and captured utilizing a spray and mist system. The study proposes the utilization of a combined spray and mist technique, which has demonstrated high efficiency in scavenging aerosols generated through laser irradiation. This research contributes to the broader goal of promoting best practices and innovative solutions in the field of nuclear decommissioning, thereby safeguarding the environment and human health.

Investigations into the interfacial characteristics of Inconel 718/stainless steel 316L fabricated by multi-material Laser Powder Bed Fusion.

The numerous advantages of Additive manufacturing (AM) are being utilized to print multi-material components for different applications. Although the AM method of laser powder bed fusion (LPBF) can print more complex and dimensionally accurate parts than the directed energy deposition (DED) method, printing multi-material components is challenging for LPBF. This study demonstrates an attempt at bimetallic 3D printing of stainless steel 316L and Inconel 718 by multi-material laser powder bed fusion and its characterization. A continuous wave (CW) fiber laser was used for the LPBF process and laser parameters for the bimetallic components were optimized. Microstructural studies were carried out with optical microscopy and scanning electron microscope (SEM) to investigate the interfacial characteristics for different numbers of interlayers. The thickness of the interfacial region was around 50-100 μm. Vickers microhardness (HV) and nanoindentation were performed at various locations around the fusion zone resulting in an average micro and nano hardness of 303 HV and 4.622 GPa respectively.

Laser cutting of CFRP using novel cutting strategies

Laser cutting of carbon fibre reinforced plastics generates a heat affected zone (HAZ) in the vicinity of the cut, affecting the integrity of the material and therefore the uptake of laser cutting for composite applications. This work uses mechanical testing to determine the effect of laser cutting on material properties and to see if they correlate with the observed HAZ. The potential for an extended-HAZ (exHAZ), where polymer remains but the properties have changed due to the proximity to the laser cut, is considered. This more novel concept of the exHAZ is important in understanding the performance of laser cutting CFRP and enabling wider use. A 2 kW multimode 1070 nm continuous wave fibre laser and a range of cutting strategies have been used to determine the influence of different cutting parameters on the HAZ. Cuts are generated using single and multi-pass (pseudo pulsed) cutting strategies with a range of assist gas pressures. The effect of assist gas has been largely overlooked in laser cutting of CFRP to date. The results obtained will provide guidance for future dynamic beam shaping work.

Technics of continuous-wave fiber laser cutting of thin-wall metal materials

Technics of continuous-wave fiber laser cutting of thin-wall metal materials

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.

Application of laser heating to coat ceramic surfaces: An alternative for traditional Raku firing

This study presents a novel approach for coating ceramic surfaces with an organic material using laser heating. To achieve this, white clay ceramics were prepared and subjected to a 150W continuous wave fiber laser irradiation at 1090 nm for various durations. Following the laser treatment, a physical contact was established between an ostrich feather and the non-illuminated surface of the ceramic. As a result, the ostrich feathers underwent distinct stages of chemical changes known as pyrolysis, depending on the duration of laser illumination. Subsequently, the ceramics obtained were analyzed using SEM, EDX, XRD, and optical microscopy. Our experimental results indicate a specific temperature range between low temperatures (below 500 °C) where the pyrolyzed feathers do not adhere properly to the surface and higher temperatures (around and above 800 °C) where excessive pyrolysis occurs, resulting in an aesthetically pleasing pattern on the ceramic surface reminiscent of ostrich feather Raku. EDX and XRD analyses confirmed the penetration of feather residue into the structure of the non-illuminated surface, creating an artistic pattern, while the illuminated side displays a glazed and glassy like surface. Thus, these findings present a promising and innovative method that significantly reduces energy consumption and process time when compared to traditional Raku pottery approaches.

Ytterbium-doped fiber laser as a source of coexisting narrow-band thermal light and optical rogue waves: Experimental demonstration

In this paper, we show that a continuous-wave ytterbium-doped fiber laser implemented in the conventional configuration of a Fabry-Perot cavity with two fiber Bragg gratings as selective reflectors and a moderate output power (tens of watts) generates two groups of photons, each of which is characterized by its own statistics. The statistics of the first group are described by the M−fold degenerate Bose-Einstein distribution, where M is the number of independent states of spontaneous emission, which, in turn, depends on the laser linewidth and the radiofrequency band of the recording equipment. This kind of light is classified as narrow-band thermal light. The second group of photons is described by the extreme-value exponential statistics with a probability of encountering extremely large photon events much greater than expected from the Bose-Einstein distribution, which is classified as optical rogue waves. At a low laser power, the first group of photons strongly dominates over the second. As the laser power increases, the fraction of photons of the second group increases monotonically; above a certain threshold, the fraction of this group increases, reaching tens of percent of the total number of laser photons.

High-efficiency continuous-wave Tm-doped fiber laser with a single fiber Bragg grating at 1942 nm

We report a continuous-wave Tm-doped fiber (TDF) laser with a single fiber Bragg grating (FBG). The Fresnel effect presenting in the passive fiber end-face and high-reflectivity FBG established the resonator. The peak wavelength of the laser was 1942.25 nm with a spectral linewidth of 194 pm. The maximum output power was 37.5 W, corresponding to a slope efficiency of 63.7% with a beam quality factor of M 2 ∼ 1.51. To the best of our knowledge, this is the highest slope efficiency of a TDF laser pumped by 793 nm laser diode.

Preparing metallic bulk samples for transmission electron microscopy analysis via laser ablation in an acidic liquid

The laser ablation process has been presented as a state-of-the-art and robust method for eliminating surface layers of metallic bulk materials on the micro/nanoscale with considerable accuracy and speed. These capabilities make it an ideal instrument for the primary step of metallic bulk sample preparation for transmission electron microscopy (TEM) analysis with minimal damage to the crystal structure of the sample. This work prepared a pure aluminum plate by fiber laser scanning immersed in a 5 % perchloric acid and 95 % ethanol (by volume) solution. The laser ablation in an acidic liquid condition was used in the chemical etching process, to prevent oxidation, minimize the heat-affected region, and induce crystalline defects. For this purpose, a continuous wave fiber laser with a power of 100 W, a wavelength of 1064 nm, and a focusing distance of 305 mm was applied. The TEM analysis for the as-prepared sample by the laser ablation process was compared to the standard jet electro-polishing method in the same solution. The results showed that crystal structure variations such as grain size and lattice defects did not occur in the aluminum sample due to the thermal effects of the laser ablation process. The average size of the grains and zone axis were about 600–800 nm and z = [011], respectively, in as-prepared samples by both methods; In addition, very few dislocations were observed in the grain boundaries and inside the grains. Therefore, based on the obtained results, laser ablation could be considered an eco-friendly, comfortable, fast, and cost-effective method with a small volume of chemicals used, compared to the standard jet electro-polishing method for the TEM preparation of the metallic bulk samples.

Laser cleaning process of high-pressure turbine blade: Characterization and removal of surface contaminants

Laser cleaning has been introduced as an effective method for removing contaminants from the surface of a turbine blade. In this investigation, a continuous wave fiber laser with a power density of 7.96 × 106 W/cm2, a wavelength of 1064 nm, and a focusing distance of 305 mm under an argon gas was applied. The used laser removed CaO-MgO-Al2O3-SiO2 (CMAS) as contaminants from the Y2O3-stabilized ZrO2 layer of a thermal barrier coating (TBC) on a high-pressure turbine blade made of a nickel-based superalloy substrate. At first, the characterization of the CMAS and TBC properties, such as thickness, roughness, and morphology, as well as phase identifications were performed. CMAS on the high-pressure turbine blade (HPTB) surface had a mixture of granular and plate structures. The major imperfections of the surface of the out-of-service HPTB were infiltration of CMAS into the TBC, chemical interaction between CMAS and the TBC, transverse cracks, delamination of the TBC, structural changes of the TBC, separation of YSZ from TGO, and deformation of the TBC on the pressure side and the leading edge of HPTB. The main aim of this work was to remove non-infiltrated CMAS from the TBC coating through the laser cleaning process. Then, various parameters of the laser cleaning process such as power and scanning speed were investigated. After that, various analyses of laser-cleaned samples such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), optical microscopy (OM), X-ray diffraction (XRD), thickness test, and roughness measurement were accomplished for the optimum parameters of the laser cleaning process. Finally, the mechanism involved in the laser cleaning process to remove the contamination was presented in detail. The results demonstrated that the optimum parameters of the laser cleaning process to completely eliminate targeted contaminants were a power density of 7.16 × 106 W/cm2, a scanning speed of 0.01 mm/s, and a focusing distance of 305 mm under an argon gas atmosphere.

High power mid-infrared fiber amplifier at 3.1 µm by acetylene-filled hollow-core fibers.

We characterized high-power continuous-wave (CW) and pulsed mid-infrared (mid-IR) fiber amplifiers at a wavelength of 3.1 µm in acetylene-filled hollow-core fibers (HCFs) with a homemade seed laser. A maximum CW power of 7.9 W was achieved in a 4.2-m HCF filled with 4-mbar acetylene, which was 11% higher than the power without the seed. The maximum average power of the pulsed laser was 8.6 W (pulse energy of 0.86 µJ) at 7-mbar acetylene pressure, a 16% increase over the power without the seed. To the best of our knowledge, backward characteristics are reported for the first time for fiber gas lasers, and the backward power accounted for less than 5% of the forward power. The optimum acetylene pressure and HCF length for the highest mid-IR output are discussed based on theoretical simulations. This study provides significant guidance for high-power mid-infrared (mid-IR) output in gas-filled HCFs.

Laser Surface Remelting in Single-Crystal Nickel-based Superalloy using a Continuous Wave Fiber Laser

Laser Surface Remelting in Single-Crystal Nickel-based Superalloy using a Continuous Wave Fiber Laser

Fiber Laser Alloying of Additively Manufactured 18Ni-300 Maraging Steel Part Surface: Effect of Processing Parameters on the Formation of Alloyed Surface Layer and Its Properties.

The development of new efficient, economical, and safe methods for strengthening the working surfaces of parts is an important task in the field of improving the reliability and resourcefulness of critical equipment and structures. In the present paper, laser boronizing is investigated as an alternative method for improving the wear resistance of maraging steel parts manufactured by laser powder bed fusion (LPBF). After LPBF, the specimens' surface was covered with an amorphous boron paste (0.03-0.6 mm) and laser processed with a continuous-wave fiber laser in melting mode (λ-1070 nm; power-300 W; spot Ø-1.0 mm) at 500-1500 mm/min laser beam scanning speeds. Scanning electron microscopy, X-ray microanalysis, Knoop hardness, and dry sliding wear tests were applied to investigate the geometry, microstructure, hardness and its distribution, heat-affected zones, wear resistance, and wear mechanism of the alloyed layers. The boronized layers of thickness ~280-520 µm with microstructure from hypoeutectic to borides' mixture were obtained, whose hardness varied from ~490 to ~2200 HK0.2. With laser boronizing, the wear resistance was improved up to ~7.5 times as compared with aged LPBF samples. In further method development, the problem of thermal cracking and softening of the heat-affected zone should be solved.