CW Fiber 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.

A modelling approach for laser welding of busbars to lithium-ion prismatic cell terminals to enhance failure prediction

Laser welding is increasingly being used to connect automotive battery packs and weld busbars to prismatic cell terminals due to its ability to weld a wide variety of materials at high speed. The choice of material for the connecting busbars is primarily based on weldability, weight, electrical/thermal conductivity and cost. Aluminum (Al) is increasingly being chosen as busbar material to meet these criteria. This paper investigates laser overlap welding of Al for producing busbar-to-terminal interconnects for prismatic cell assembly. To understand the strength of these laser-welded joints in the early stages of design, the joints must be modelled. In addition, the structural modelling of battery module connections is critical to understanding the impact resistance (crash safety) of electric vehicles. Fusion-based modelling of laser welding is computationally inefficient and difficult to integrate into large structural models e.g., full vehicle crash models. Instead, it is more computationally efficient to generate a structural model of the weld, tuned to experimental test data, which can be easily integrated into larger models. LS-DYNA was employed to model the mechanical behavior of 1 mm to 1 mm AA1050 laser welds. Welding was performed using a wobble head integrated with a 1 kW CW fiber laser system. The simulation is based on laser weld with a rectangular pattern and was developed from experimental data from lap shear, T-peel, cross-tension and torsion tests. This modelling approach was extended to obtain joint strength for laser welding a 1.5 mm 1060 Al busbar to a 4 mm 1060 Al prismatic cell terminal. The predicted load–displacement curves closely matched the test data.

Narrow linewidth and wideband tunable continuous-wave terahertz generator based on difference frequency generation with DAST crystal.

A narrow linewidth and wideband tunable continuous-wave terahertz generator with DAST crystal has been demonstrated in this paper. Two narrow-linewidth CW fiber lasers were used as the pump sources for difference frequency generation. The terahertz wave can be continuously tunable in the range of 1.1-3 THz. The maximum output power of 2.79nW was obtained at 2.568 THz. The linewidth of the output THz wave was estimated to be 56.5 MHz by fitting transmission spectrum of CO gas at 450 Pa pressure around 80.52 cm-1 with the Vogit gas model. Furthermore, the output spectra at room temperature and pressure was in good agreement with the air absorption lines in Hitran database. Moreover, the narrower absorption characteristic spectrum of 2-Deoxy-D-Glucose sample has been obtained through the spectrum measurements. Therefore, it could promote the practical prospect of tunable CW-THz source, which will have good potential in THz high-precision spectroscopic detection and multispectral imaging.

Comparison between various dispersion compensation schemes in optical fiber communication system

Abstract Nowadays, the optical fiber communication system has great importance in the communication area. Although the optical fiber communication system has many favorable circumstances, the dispersion was considered one of the main problems. In this paper, the comparison between the design and simulation of different dispersion compensation schemes was displayed. There are different kinds of optical fiber dispersion compensation systems such as fiber Bragg grating (FBG) and dispersion compensation fiber (DCF) are extensively used for optical communication systems. They are usually considered imperative techniques to compensate for the dispersion. The simulation results have been analyzed depending on various parameters by utilizing advanced Opti-System software. The best performance of the system is when the bit error rate (BER) reaches to 1.7 × 10−12 at 1550 nm wavelength and the Q-factor is 6.9 at the 0.000195 eye opening. The modal was simulated using the appropriate parameters which include CW laser wavelength, fiber length (km), and attenuation coefficient (dB/km) at the transmitter side. Three various parameters were tested such as BER, Q factor, and eye-opening.

Novel fiber-tip micro flowmeter based on optofluidic microcavity filled with silver nanoparticles solutions.

A novel fiber-tip micro flowmeter based on optofluidic microcavity filled with silver nanoparticles solutions (SNS) is proposed. CW fiber laser was used to heat SNS that can emit heat obviously due to the excellent optic-thermo effect. The heat generated by the silver nanoparticles would be taken away as the microfluidic flows over the fiber microcavity until thermal balance is established under different velocity. The effective refractive index (RI) of the SNS changed followed by temperature of the thermal balance. The dips of the Fabry-Perot interference spectrum shift and the flow velocity can be demodulated. Moreover, the sensor can measure the flow rate with a high sensitivity due to the superior thermal conductivity and specific heat capacity of sidewalls. The maxflow rate sensitivity can reach 1.5 nm/(μL/s) in the large range of 0-5 μL/s with a detection limitation (DL) of 0.08 μL/s. The Micron scale probe-type flowmeter has strong robustness and can be used to measure flow rate in tiny space. The heating medium also has an excellent biological compatibility and is not contact with the fluidics directly. As such, we believe that the proposed fiber-tip micro flowmeter has great application potentials in haematology, oil prospecting, ocean dynamics and drug research.

Remote laser spot welding of AISI 430 sheets by fiber lasers—A phenomenal effect in refining weld microstructure with nanosecond pulses

In the present work, we have compared the results of microstructural and mechanical property characterization of AISI 430 ferritic stainless steel spot welds made remotely using a repetitive nanosecond pulsed fiber laser and a CW fiber laser. Optical/electron microscopy, microhardness test, and tensile shear tests were performed for evaluation of the welds. Welds made by the pulsed nanosecond laser were found to be superior due to the presence of the very narrow heat-affected zone along with finer grains in the fusion zone. The welds were also found to withstand more load before fracture and were more ductile. Use of short duration laser pulses with lower heat input were found to be responsible for the refinement of grains in the fusion zone and improvement of mechanical properties of nanosecond laser weld specimens.

Micro-dimple and micro-bulge textures:Influence of surface topography types on stick–slip behavior under starved lubrication

Stick-slip behavior, a common problem of contact under severe lubrication, is closely related with surface topography. To investigate the influence of surface topography types on stick–slip behavior, micro-dimple and micro-bulge textures, typical concave and convex topographies, were fabricated by pulse picosecond laser and CW fiber, respectively. A gyroscope sensor was mounted on the pin-on-disc modular of multifunctional tribometer to conduct friction dynamics tests. The intensity of stick–slip behavior is characterized by the amplitude and duration of stick–slip behavior. The results show that, under the same starved lubrication, the micro-bulge textured disc showed the lowest intensity while the untextured one showed the highest intensity. In comparison with micro-dimple and micro-bulge textured disc exhibits higher efficiency in inhibiting stick–slip behavior as well as starting stable sliding. The mechanism of inhibiting stick–slip behavior for micro-dimple textures is generated by trapping wear debris and forming effective oil film. However, the cause of micro-bulge textures is the reduced contact area, which result in the reduction of the propensity of secondary plateaus, plastic flow, and adhesive junctions between asperities.

Direct glass-to-glass bonding obtained via simplified ammonia-based low-temperature procedure resists high shear stress and powerful CW fiber laser irradiation.

Direct glass-to-glass bonding is important for high-technology components in optics, microfluidics, and micro-electromechanical systems applications. We studied direct bonding of 1 mm thick soda-lime float glass substrates. The process is based on the classic RCA-1 cleaning procedure from the semiconductor industry modified with an ammonium hydroxide rinse, followed by a thermal treatment under unidirectional pressure without the need for a dedicated drying step. RCA-1 uses a solution of ammonium hydroxide and hydrogen peroxide to clean contaminants off the surface of silicon and enable subsequent bonding. Bond quality was evaluated using destructive shear testing. Strong bonds (≈7.81 MPa on average) were achieved using unidirectional pressure of approximately 0.88 MPa and bonding temperatures between 160 °C and 300 °C applied for 30 min. Surface roughness and chemistry was characterized before and after cleaning. The optical robustness of the bonds was tested and shown to be capable of surviving high powered continuous wave (CW) fiber laser irradiation of at least 375 W focused for 2 s without delamination. Melting of the substrate was observed at higher powers and longer exposure times.

Influence of temporal pulse-shaping on laser surface texturing of metals using fast modulated cw-fiber laser radiation

Remote ablation cutting with continuous wave laser radiation is a process commonly used for the surface pretreatment of large area metallic parts before joining of metal-polymer hybrid structures, due to their cost-effectiveness and high average powers when compared to short-pulsed lasers. The process requires high power-densities and multiple irradiations at high scanning speeds to achieve the desired kerf depth. In this work a fast modulated cw fiber laser is used to investigate the possibilities of increasing the material removal rate by periodically dropping the laser power and therefore modifying the recoil pressure at the vapor-liquid interface in the interaction zone to assist in melt expulsion. It was found that power modulation with frequencies in excess of 10 kHz can lead to an increased material removal rate when compared to continuous wave processing at comparable average and peak power levels, enabling higher kerf aspect ratios in a single irradiation.

Laser wobble welding of fluid-based cooling channel joining for battery thermal management

Aluminium alloys are increasingly used to fabricate cooling channels for the thermal management of Li-ion batteries. Cooling channel fabrication involves a number of manufacturing operations including material extrusion, forming and joining/welding. In general, welding of aluminium alloys is challenging as they are both highly reflective and thermally conductive. To address the joining challenges, this paper is focused on developing an optimised joining process to connect a thin, flanged cooling channel to the thick module manifold of the battery thermal management system to create a watertight joint with high mechanical strength. As continuous seam welding was required, laser welding was the preferred as it is a non-contact process combining high speed and precision. For this application, 0.4 mm Al cooling channel was welded with 1.5 mm Al endplate/module manifold using a wobble head integrated with 1 kW CW fibre laser system. The effect of process parameters including line energy, incident angle, laser power, welding speed and beam offset were investigated to optimise both the weld geometry and strength. Microstructure, micro-hardness and grain formation analyses were carried out to understand the metallurgical behaviour of the weld. Beam offset had the most significant effect on the responses such as weld strength, throat thickness and modified throat thickness, and laser power had a significant influence on two key geometric features of the fusion zone, i.e. penetration depth and weld width. Weld strength was optimised using a developed surrogate model and a maximum load of 646.89 N was achieved using 0.2 mm beam offset, 331.82 W laser power and 659.10 mm/min welding speed. Using this optimum combination, a leak-proof cooling channel and module manifold joint were produced for battery thermal management.

2.4 kW 1045 nm narrow-spectral-width monolithic single-mode CW fiber laser by using an FBG-based MOPA configuration.

A 24 kW narrow-spectral-width near-diffraction-limited monolithic fiber laser system at ${\sim}{1045.2}\;{\rm{nm}}$ in a fiber Bragg grating (FBG)-based master oscillator power amplifier (MOPA) configuration is demonstrated in this paper. The near-diffraction-limited beam quality (${{\rm{M}}^2}\sim{1.2}$) and a spectral width of 0.35 nm (${\sim}{{96}}\;{\rm{GHz}}$) are achieved. The stimulated Raman scattering (SRS) is theoretically and experimentally investigated. The SRS has been suppressed by carefully optimizing the length of the Yb-doped fiber and the pumping scheme, and a signal-to-noise ratio of ${\sim}{{33}}\;{\rm{dB}}$ between the laser signal and the Raman Stokes component is achieved. The stimulate Brillouin scattering and the transverse mode instability are not observed. To our best knowledge, this is the highest-output power for ${{104}} \times {\rm{nm}}$ single-mode fiber laser with ${\sim}{{96}}\;{\rm{GHz}}$ spectral width by using an FBG-based MOPA configuration.

High-Speed Micro-Grooving of Metal by Angled Irradiation of Single-Mode CW Fiber Laser

Laser micro-grooving has excellent potential to improve the surface properties of metals. In this study, high-speed micro-grooving of the surfaces of mild steel SS400 and aluminum alloy A1050 was investigated using a single-mode continuous-wave fiber laser with an average power of 200 W. Experiments were conducted at laser beam scanning speeds of 1 to 6 m/s. The incident angle of the laser beam in the plane perpendicular to the scanning axis was varied from 0 to 60°. A relatively deep groove (approximately 30 µm in depth) and a high upheaval (10 µm in height) were formed along the scanning line at an incident angle of 45° on the SS400, whereas only shallow grooves were formed on A1050. The micro-groove formation mechanism was discussed on the basis of high-speed camera observation, groove shapes, and the thermophysical properties of the metal. Asymmetric molten metal flow and solidification around the keyhole are thought to play important roles in micro-groove formation. Although the laser irradiation conditions must be optimized according to the thermophysical properties of the material, angled laser irradiation has the potential to effectively form micro-grooves.

보호 가스 종류에 따른 Ti-6Al-4V 합금의 파이버 레이저 스마트 융합 용접 공정 특성

본 연구는 티타늄 합금(Ti-6Al-4V)를 사용하여 CW 파이버 레이저 용접 공정을 수행하였다. 티타늄 합금 용접 공정 시 비교적 고온에서 산소와 탄소 등의 침입형 원소와 반응성이 상대적으로 높아 산화 및 질화에 대해 취약 부분이 형성되거나, 용접부의 내식성 등 기계적 성질을 저하할 수 있다. 따라서 용접 공정에서의 보호 가스 변화 영향에 따른 최적화 공정을 선행 연구와 비교 검증하여, 이를 통해 용접부의 건전성을 평가하였다. 용접부에 대해 현미경을 통해 용융지의 미세 형상을 관찰하였다. 또한, 레이저 유도 플라즈마 분광 분석 기법을 통해 보호 가스 사용 여부 및 변화를 통해 취약한 부분을 추정할 수 있었으며, 용접부 내부의 산화 여부를 검증할 수 있었다. 보호가스 영향에 따라 황색, 청색, 금색 및 은백색 등의 비드를 얻을 수 있었으며, 이는 용접부가 산화 및 질화될 경우 비드 표면뿐만 아니라 용접부 내부까지 산소 및 탄소 등이 증가하여 모재 대비 경도가 증가하였다. 결과적으로 He(헬륨) 과 Ar(아르곤)를 사용한 경우 은백색의 건전한 비드 형상을 얻을 수 있었다. 이를 통해 용융지 내부의 미세 형상 및 용접부의 경도 특성의 차이를 실험적으로 검증하였다.

Short-length CW laser of Nd[formula omitted] heavily doped single-mode silica glass fiber fabricated by zeolite method

We constructed a CW fiber laser using a 40-mm-long Nd3+ heavily doped silica glass single-mode fiber (Nd-SMF). Its core glass was fabricated by the zeolite method, and its Nd2O3 doped density was measured as 1.25 wt%. Nd-SMF also demonstrated laser oscillation. The laser power reached 19.7 mW and its slope efficiency and laser threshold were 14.6% and 22.0 mW, respectively. Such a short single-mode fiber will help us easily make a high repetition rate over GHz and a compact oscillator with adequate saturable absorbers. An Nd-SMF laser has the potential to achieve laser miniaturization and high repetitive pulsed oscillation.

Introducing a novel Green’s function-based method for laser and matter thermal interaction

The thermal interaction of lasers with matter is studied in this work. Using the Green’s function approach, equations governing this interaction are solved semi-analytically. Real conditions, such as the temperature dependence of thermal coefficients, and the surface radiation of matter, were taken into account when solving these equations. A computational code was written in the C programming language to simulate the problem. The proposed method decreases both calculation and simulation times considerably. The proposed method and the developed code are validated by experimental tests, and by means of comparison with another previously-published approach. Since a combination of numerical and analytical methods are used, the developed code is much faster than the numeric software. In addition, the proposed method is more accurate than analytical methods, due to the use of nonlinear boundary conditions. The thermal effects of a CW fiber laser on Al is also investigated. Results show that there is a threshold power density for achieving a melting point due to surface radiation. Higher power densities result in a shorter interaction time to achieve a melting point.

Study of the phase composition and tribological properties of carbon tool steels after laser surface hardening by quasi - CW fiber laser

The paper contains the investigation results on the structure and phase composition of the laser radiation area (LRA) of the U8 and U10 steels over its entire thickness. In the present study, the laser surface hardening of both U8 (ASTM - W1-7) and U10 (ASTM - W1-9) steels in the air was performed by exploiting a quasi - CW fiber laser with a 130 W power and 3 mm/s processing speed. The phase composition of the oxide layer formed as a result of laser treatment (LT) in air, as well as the structure of the oxide-metal interface on the surface of U8 (ASTM–W1-7) and U10 (ASTM–W1-9) carbon tool steels were studied by X-ray photoelectron spectroscopy (XPS). It was established that the thickness of the completely oxidized surface layers for U8 and U10 steels is 38.7 nm and 99 nm, respectively.The composition of the oxides of the steel surface after LT was determined. The presence of a wüstite-based film on U8 steel evidences the low wear properties of the LRA surface, while the thicker oxide layer of the modified U10 steel which contains Fe2O3 and Fe3O4 oxides with better strength properties, on the contrary, provides U10 steel surface with higher wear resistance. It was found that the wear rate of U10 steel modified surface decreases by more than two times, while the given value for U8 steel reduces no more than 17%. The paper reports the metallographic examination of the LRA structure. It was shown that the wear-resistant structural components that appeared after laser treatment lead to an increase in the deformation properties of steels. The maximum microhardness value of the LRA is 710 HV0.1 for U8 steel and 750 HV0.1for U10 steel.

Investigation of output power in ring CW fiber laser using graphene saturable absorber

This paper reported the effect of different coupling ratio in continuous wave fiber laser in a ring cavity configuration. Different coupling ratios of 10/90 and 50/50 were tested. Where the output power may vary depending on the ratio and it can be applied to specific area that requires either high or low output power. In addition, generation of passive Q-switched erbium doped fiber laser (EDFL) using graphene based saturable absorber in ring cavity using different coupling ratio was experimentally investigated. As a result, wavelength centered at 1566.62nm is obtain from EDFL cavity. Moreover, the cavity using coupler of 50/50 is capable to achieve Q-switched pulses as compared to the cavity using coupler of 10/90. Where the maximum output power recorded is 336mW with pulse repetition rate of 23.74 kHz. In addition, the pulse width is 3.84µs, and pulse energy is 14.15nJ.

43 μm fiber laser in CO2-filled hollow-core silica fibers

Rare-earth-doped fiber lasers at 3–5 μm can provide a variety of applications in defense, security, medicine, and so on. Limited by the maximum phonon energy of silicate glass host at room temperature, these lasers are normally based on soft-glass materials, e.g., fluoride or chalcogenide glass. However, due to the limited transparency of these fibers, until now these systems have only achieved coverage up to 3.92 μm. Here, we report a CW fiber laser operating well beyond 4 μm with significant power based on CO2-filled silica hollow-core fibers. By pumping via a homemade 2 μm laser diode, ∼82 mWoptical power at 4.3 μm was achieved at room temperature with a maximum laser efficiency ∼19.3%. Our demonstration represents the longest-wavelength CW fiber laser to date, paving the way towards compact and high-power mid-infrared fiber lasers beyond the 4 μm wavelength.

Investigation of Heating of Optical Elements During Formation of High-Power CW Fiber Laser Radiation

The problem of heating of optical components during the formation of the high-power continuous-wave fiber laser radiation is considered. The results of experimental measurements of heating of optical components made of the domestic quartz glass of the KU-1 brand and yttrium aluminum garnet crystal are presented. The time dependence of the heating temperature of the optical components and their stationary temperature are investigated as functions of the laser radiation power.

Effect of Surface Plasma on Nanosecond Laser Ablation

For the first time, the effect of target surface plasma on nanosecond laser ablation is studied. Surface plasma was generated by a high-power cw fiber laser beam incident on an iron target. A comparison of laser ablation by nanosecond Nd:YAG laser pulses (1064 nm, 5 ns) was performed upon exposure to the cw laser and during its short-term (5 ms) turn off. It was found that the emission intensity twofold increases in the presence of surface plasma. The temperature and electron density of the laser plume induced by the nanosecond laser also increased.