Continuous Wave Fiber Laser Research Articles

Mechanical Response and Failure Evolution of 304L Stainless Steel under the Combined Action of Mechanical Loading and Laser Heating

Deformation and fracture properties of structural materials are greatly influenced by the factors like applied load, state of stress, and temperature. A precise prediction of the material properties of stainless steel at elevated temperature is necessary for determining the load-carrying capacity of structures under severe conditions. The present work reports the deformation and failure characteristics of 304L stainless steel subjected to combined laser heating and mechanical loading. The effect of main parameters on stress-strain, fracture characteristics, failure time, and temperature profile of specimens have been explored. Specimens were subjected to prescribed loading states, and then irradiated by a continuous wave fiber (1.08 µm) laser. The stress-strain curves indicated that the specimens experienced slight strain hardening in a specific temperature range prior to fracture. The specimen’s ultimate failure time is found to be reduced by increasing either laser power density or preload level. Fracture on a microscopic scale was predominantly ductile, comprising dimples as well as micro-void nucleation, growth, and coalescence. With the increase of laser power density, dimples rupture is the primary fracture mode, while with the increase of preload value, relatively more in-depth and severe deformation effects were observed. The description and characterization of 304L stainless steel failure under the simultaneous action of laser heating and tensile stress have been explored in detail.

High Power, Tunable, Continuous-Wave Fiber Lasers in the L-Band Using Cascaded Raman Amplifiers

We demonstrate a high power, all-fiber, tunable laser source that can operate in the L-band region. A low power, tunable input laser is amplified with a recently proposed, high efficiency, 6th order cascaded Raman amplifier. The proposed system is scalable and overcomes the limitations of Erbium and Erbium-Ytterbium co-doped fiber lasers for power scaling. A tunable Erbium-Ytterbium co-doped fiber ring laser generating ~0.5W of power and tunable in the 1560-1590 nm wavelength range is utilized as the seed source. The output from the seed laser is amplified to ~24 W using 6th order cascaded Raman amplification. A high power Yb laser operating at 1117nm is used as the pump laser for driving the Raman conversions. The operating wavelength of the demonstrated laser in the eye-safe, atmospherically transparent region enables high power free-space applications. In addition, this source enables other interesting applications such as high power supercontinuum generation with conventional silica fibers.

An Adjustable EIT-Based FBG in Yb:silica Master-Oscillator Fiber Power-Amplifier System

An adjustable continuous-wave single-mode double-clad ytterbium-doped silica fiber laser is numerically investigated in the form of a master-oscillator fiber power-amplifier array. An optically-tunable coupler is suggested based on the nonlinear electromagnetically induced transparency (EIT) within fiber Bragg grating. Here, the Si nanocrystals is embedded in SiO2 over the selected regions of the output coupler. The transmittance is adjusted using a low power control field by modulating the refractive index in the EIT material.

Coupled T-H-M processes’ effect on specific energy in continuous wave fiber laser rock perforation

Laser perforation of oil and gas wells is a new method for well completion as an alternative to blasting methods. Under real perforation in oil and gas wells, three main factors have influence on the laser perforation such as specific energy (i.e., energy required to remove the unit weight of rock). These three factors are temperature, the hydraulic pressure of fluid flow, and confining pressure. This paper investigates the role and significance of the coupled T-H-M (thermo-hydraulic-mechanical) processes on specific energy (SE) and rate of penetration (RoP). For physical modeling of T-H-M processes, a new cell and setup is designed and developed. In this setup, a circular part of the front side of the cylindrical sample is open in order to interact with the laser beam while the temperature, hydraulic pressure, and confining pressure are applied to the sample. The rock tested under four temperature steps from 25 to 120 °C (temperature range of major oil reservoir in fractured rocks), the hydraulic pressure was chosen in five steps from 0 to 35 MPa (fluid pressure range of more than 70% of Iranian oil reservoir rocks), and confining pressure was selected from 0 to 40 MPa (ranges of overburden pressures). Test results show rises in specific energy with incremental increases in rock temperature, hydraulic pressure, and confining pressure, while the rate of penetration decreases.Laser perforation of oil and gas wells is a new method for well completion as an alternative to blasting methods. Under real perforation in oil and gas wells, three main factors have influence on the laser perforation such as specific energy (i.e., energy required to remove the unit weight of rock). These three factors are temperature, the hydraulic pressure of fluid flow, and confining pressure. This paper investigates the role and significance of the coupled T-H-M (thermo-hydraulic-mechanical) processes on specific energy (SE) and rate of penetration (RoP). For physical modeling of T-H-M processes, a new cell and setup is designed and developed. In this setup, a circular part of the front side of the cylindrical sample is open in order to interact with the laser beam while the temperature, hydraulic pressure, and confining pressure are applied to the sample. The rock tested under four temperature steps from 25 to 120 °C (temperature range of major oil reservoir in fractured rocks), the hydraulic pressur...

High-order mode Yb-doped fiber lasers based on mode-selective couplers.

We experimentally investigate high-order modes (HOMs) generating at the wavelength of 1.0 μm in all-fiber Yb-doped lasers based on mode-selective couplers (MSCs). Broadband MSCs for HOMs conversion are achieved by optimizing the phase-matching condition at 1.0 μm. Efficient generation of HOMs is demonstrated with the MSCs inserted both into continuous-wave and mode-locked fiber lasers. The slope efficiencies of HOMs (LP11, LP02 and LP21) are presented and discussed. Additionally, cylindrical vector and orbital angular momentum beams are produced out of the MSCs by controlling the polarization states. The results show proof-of-concept implementation of the MSCs, which enables high efficiency of HOMs generation especially in pulsed fiber lasers and holds promising applications in laser material processing.

Fiber laser cutting of CFRP laminates with single- and multi-pass strategy: A feasibility study

Experimental data is presented relating to hole quality and feasibility studies when using continuous wave (CW) fiber laser to cut CFRP laminates (6.0 mm diameter hole) with single- and multi-pass strategy. The effect of typical processing parameters including laser power and cutting speed on thermal defect was also investigated. The statistical significance of individual cutting parameters was determined using main effects plot together with ANOVA analysis. Three methods were proposed to characterize HAZ based on the feature of thermal defect. According to statistical analysis, both cutting speed and laser power were significant with respect to HAZ (recorded at hole exit). The minimal value of HAZ was recorded with laser power of 650 W and cutting speed of 1100 mm/min. Energy per unit length (El) should be set above 40 J/mm to ensure cutting through CFRP laminates using fiber laser in high efficient machining process. Multi-pass strategy was investigated without setting pause/break time between each pass in order to increase cutting efficiency. Results showed limited improvement in terms of the level of HAZ even with cutting speed up to 10,000 mm/min, while machining time was significantly reduced to cut 6.0 mm diameter hole compared with single-pass strategy. For machining small holes with diameter down to 2.0 mm, high power fiber laser was not preferred due to severe thermal defect observed at hole entry and exit.

656 W Er-doped, Yb-free large-core fiber laser.

A continuous-wave erbium-doped ytterbium-free fiber laser generates a record-breaking pump-power-limited output power of 656W at ∼1601 nm when cladding pumped by 0.98μm diode lasers. The slope efficiency was 35.6% with respect to launched pump power, and the beam quality factor (M2) was ∼10.5. This M2 value excludes a fraction ∼25% of the power that emerged from the cladding, which we attribute in part to mode coupling between the 146μm core and 700μm inner cladding. Whereas these parameters are adequate for in-band tandem pumping of Tm-doped fiber lasers, we predict that an output power of over 1kW is possible through pumping with state-of-the-art 0.98μm diode lasers, even with a smaller core that allows for improved beam quality.

Direct metal laser sintering of TiN reinforced Ti6Al4V alloy based metal matrix composite: Fabrication and characterization

In the present research, direct metal laser sintering (DMLS) method was chosen to fabricate titanium nitride (TiN) reinforced Ti6Al4V alloy based metal matrix composites (MMCs) under an argon atmosphere using continuous wave (CW) fiber laser having a capacity of 400 W. Laser sintering process parameters, such as layer thickness (0.4 mm), laser beam spot diameter (0.4 mm), and hatching gap (0.2 mm) were kept constant throughout the experiments. Effects of input variable process parameters, such as laser power (50–65 W), scanning speed (3500–4500 mm/min), and volume % of TiN (5–15% v/v) on density, microhardness, and coefficient of friction of the fabricated MMCs were analyzed. The obtained results show the improvement in the physical properties of the fabricated MMCs and FESEM images evidently confirm the presence of TiN particulates and also revealed the uniform distribution of the TiN reinforcement in Ti6Al4V matrix. It was found that the microhardness measured by Vickers test was improved from 388 to 590 HV0.2 with an increase in the volume percentage of TiN. The results showed the coefficient of friction for fabricated samples were in the range of 0.33–0.42. The density (3.40–4.10 g/cm3) of the MMCs was found to increase with increasing the volume percentage of TiN reinforcement in the powder mixture. X-ray diffraction (XRD) analysis of the fabricated MMC confirmed the presence of different in-situ phases, such as Ti, TiN, TiO2, VN, AlV, Ti3Al2N2, and V6N2.7 as a consequence of a series of a chemical reaction between TiN and different elements of Ti6Al4V in the argon atmosphere.

Dual-wavelength ring-cavity continuous-wave fiber laser based on semiconductor optical amplifier

A dual-wavelength fiber ring-cavity laser based on InP/InGaAsP multi-quantum wells semiconductor optical amplifier is proposed and experimentally demonstrated. The proof of concept device consists of a semiconductor optical amplifier with InP/InGaAsP multi-quantum wells structure as the gain medium and two intracavity cascaded fiber Bragg gratings as wavelength selector by changing their operation temperature. The results demonstrate the new concept of dual-wavelength fiber laser based semiconductor optical amplifier with InP/InGaAsP multi-quantum wells structure and the technical feasibility.

7.5 W blue light generation at 452 nm by internal frequency doubling of a continuous-wave Nd-doped fiber laser.

We present the first frequency-doubled neodymium-doped fiber laser generating multi-watt CW power near 450 nm. A bow-tie resonator incorporating a LBO nonlinear crystal is integrated within a Nd-doped fiber laser emitting near 900 nm. This scheme achieves an IR to blue conversion efficiency close to 55% without any active control of the internal resonant cavity. As a result, up to 7.5 W of linearly-polarized blue power is generated, with beam quality factors Mx2 ~1.0 and My2 ~1.5. A simple numerical model has been developed to optimize and analyse the IR to blue conversion efficiency in the resonant cavity. Performance limitations and prospects for further improvements are discussed.

Laser surface hardening of 11% Cr ferritic stainless steel and its sensitisation behaviour

11% Cr ferritic stainless steel conforming to EN 1.4003 standard was surface hardened by a continuous-wave fibre laser beam. Both single-pass and multi-pass laser hardening was investigated. Different laser parameters were compared and their influence on hardness, microstructure, geometry of the hardened zone and sensitisation was investigated, especially for overlapping passes. The experiments showed that a surface hardness which is double that of the base material hardness was obtainable via martensitic phase transformation and high cooling rate, in spite of the low carbon and nitrogen content. This behaviour could be predicted from the chemical composition using the Kaltenhauser Ferrite Factor. Hardening at higher power levels gives more coarse-grained lath martensite but does not increase the hardness. Sensitisation was not a problem in single-pass hardening. However, the production of overlapping tracks could be detrimental to corrosion resistance in 11% Cr steel due to the formation of chromium carbides and nitrides.

Optimization of CW Fiber Lasers With Strong Nonlinear Cavity Dynamics

In present work the equation for the saturated gain is derived from one-level gain equations describing the energy evolution inside the laser cavity. It is shown how to derive the parameters of the mathematical model from the experimental results. The numerically-estimated energy and spectrum of the signal are in good agreement with the experiment. Also, the optimization of the output energy is performed for a given set of model parameters.

High-efficiency ultrafast Tm-doped fiber amplifier based on resonant pumping.

We demonstrated a high-efficiency ultrafast Tm-doped fiber amplifier based on a resonant pumping technique. A continuous-wave fiber laser at 1940nm was employed as the pump laser. The slope efficiency of the resonantly pumped pulsed Tm-doped fiber amplifier reached 87% with respect to the launched pump power. The maximum average output power reached 40W when the launched pump power was 53W. The repetition rate and the pulse duration of the output pulses from a fiber amplifier were 248MHz and 129ps, respectively. The corresponding peak power was 1.25kW, and the pulse energy was 161.3nJ. To the best of our knowledge, this is the first demonstration of a resonant pumping enabled high-power high-efficiency ultrafast fiber laser operating at a 2μm band.

High pumping-power fiber combiner for double-cladding fiber lasers and amplifiers

A high pumping-power fiber combiner for backward pumping configurations is fabricated and demonstrated by manufacturing process refinement. The pump power handling capability of every pump fiber can extend to 600 W, corresponding to the average pump coupling efficiency of 94.83%. Totally, 2.67-kW output power with the beam quality factor M2 of 1.41 was obtained, using this combiner in the fiber amplifier experimental setup. In addition, the temperature of the splicing region was less than 50.0°C in the designed combiner under the action of circulating cooling water. The experimental results prove that the designed combiner is a promising integrated all-fiber device for multikilowatt continuous-wave fiber laser with excellent beam quality.

Continuous-Wave Mid-Infrared Gas Fiber Lasers

Efficient continuous-wave laser emission of acetylene molecules at 3.1 μ m wavelength has been demonstrated in low-loss hollow-core fiber by optically pumping using a fiber-amplified diode laser around 1.53 μ m. In this paper, we report and compare the continuous-wave laser performance of acetylene molecules in the cavity and single-pass configurations. The use of low-loss antiresonant hollow-core fiber makes the single-pass configuration ideal for high-power laser generation and enables over 1.1 W continuous-wave mid-infrared laser output with over 33% slope efficiency relative to the absorbed pump power. The systematic characterization of power-scaling and pump power absorption properties demonstrates that the molecular kinetics inside hollow-core fiber determines the laser performance, and that low fiber attenuation is key to high-efficiency continuous-wave output.

Highly efficient all-fiber continuous-wave Raman graded-index fiber laser pumped by a fiber laser.

We report for the first time, to the best of our knowledge, an all-fiber Raman graded-index (GRIN) fiber laser pumped by a fiber laser. This configuration points to potential future power and brightness increases. Continuous-wave power of 135W with an M2 value of 2.5 was obtained at a wavelength of 1081nm with an optical-to-optical efficiency of 68%. A commercial GRIN core fiber acts as the Raman fiber in a power oscillator configuration that includes fiber Bragg gratings (FBGs) written onto the GRIN fiber. The efficiency and brightness demonstrated here are, to the best of our knowledge, the highest reported in any Raman GRIN fiber laser. A brightness enhancement of the pump beam by a factor of 5.6 is attained due to the transverse profiles of Raman gain and FBG reflection in the GRIN fiber.

Experimental investigation into metal micro-patterning by laser on polymer-metal hybrid joining

Surface modification pretreatment on laser direct joining of glass fibre reinforced polyamide to steel was studied to assess its effect on the joint's mechanical performance. The steel part was structured by laser radiation to accomplish a proper mechanical interlock when joining with the polymer. In a second step, the opposite side of the micro-structured metal was irradiated by a continuous wave (cw) fibre laser system until reaching the melting temperature of the polymer in both materials interface. The metal micro-structuring was produced by two different laser sources (nanosecond pulses (ns) and cw) in order to study the effect of different groove geometries on the joint's failure force under tensile-shear tests. The impact of structure density and clamping pressure was also assessed. A tight dependence of aspect ratio and recast material height of patterns on joint’s failure force was found for the micro-patterns that were produced by nanosecond pulses. The greatest strength was achieved in the case of patterns produced by ns-pulses. The trend concerning the effect of structure density was validated for the patterns that were produced by ns-pulses and cw-radiation. The changes in clamping pressure did not evidence a significant influence on the joint quality. The morphological features of the detached surfaces showed that the micro-geometric structure aspect ratio has a meaningful effect on the failure mode in the case of structures that were generated by nanosecond pulses.

Validity Analysis of Holmium-Doped Fiber as Saturable Absorber for Passively Q-Switched Thulium-Doped Fiber Laser

We propose and demonstrate a method for measuring nonlinear transmission (NLT) of holmium-doped fiber (HDF) as saturable absorber (SA) by using two calibrated photodiodes. The HDF with a length of 10 cm is under test as reference, and the NLT measurement is carried out from 1930 to 1970 nm by using a tunable continuous-wave (CW) thulium-doped fiber laser (TDFL). The result finds that modulation depth of HDF is 56% at 1950 nm and decreases down to 37% at both 1930 and 1970 nm. Saturation power is about 20 mW for all measured wavelengths. The validity of the HDF SA for a passively Q-switched TDFL is also studied by taking the measured parameters into account. The laser operates in Q-switched mode from 1944 to 1960 nm, whereas in CW mode beyond the wavelength range due to insufficient modulation depth. The pulse temporal characteristics are also discussed.

Intensity statistics in a long random fiber Raman laser.

We study the output emission statistics of a random continuous-wave Raman fiber laser. The signal evolution is governed by a generalized nonlinear Schrödinger equation (NLSE) with inserted gain. The statistics are close to the Rayleigh one, and the deviations are caused by the Kerr nonlinearity. To characterize the deviations, we analytically find the mean of the squared output signal intensity, based on the kinetic theory. We show qualitative agreement with available experimental data and supplement the results with numerical calculations. With the limit of small gain, the kinetic theory gives a finite answer for the mean of squared intensity in the first and the second order with respect to small nonlinearity. The result is consistent with the fact that the NLSE is integrable in the case of zero gain and is applicable to any generalized NLSE if the inserted terms are effectively small.

Metallurgical and tribological investigation of micro-scale fibre laser-based surface hardening

This work presents a comprehensive analysis of the mechanical, metallurgical and tribological responses of the various micro-scale laser hardened patterns of EN-8 steel at different process conditions. A 100 W continuous wave fibre laser with ~200-900 µm beam diameter was used for micro-scale hardening. EBSD was used to investigate the micro-structure and the hardness values were measured using micro hardness tester (Shimadzu-HMV). Additionally, to measure the tribological response, four different hardened patterns were analysed using the 'pin on disk setup' (DUCOM wear and friction monitor TR-20LE). The preliminary results revealed improved surface hardness values with increase in percentage overlap due to the formation of martensite. Furthermore, it was observed that the average wear rate of hardened specimen decreases by ~91% as compared to the unhardened specimen. The results revealed an inverse relationship between friction force and percent-hardened area, with minimum friction force obtained for a fully hardened pattern.