Continuous Wave Fiber Laser Research Articles

Diffraction of volume Bragg gratings under high flux laser irradiation.

Diffraction property of transmitting volume Bragg gratings (VBGs) recorded in photo-thermo-refractive glass (PTR) is studied under the irradiation of a continuous-wave fiber laser with flux of 1274 W/cm2. Dependence of temperature characteristics of VBGs prepared by different crystallization temperatures is presented. When temperature of VBGs rises up to 33°C, there are a 2.7% reduction and 1.59% ripple of diffraction efficiency for VBGs. The period variation caused by the thermal expansion of VBGs is used to explain the reduction of diffraction efficiency, and experimental results are in agreement with theoretical analysis.

Kilowatt high average power narrow-linewidth nanosecond all-fiber laser

AbstractA high power narrow-linewidth nanosecond all-fiber laser based on the master oscillator power amplifier (MOPA) configuration is demonstrated. A pulsed seed with high repetition rate of 10 MHz was generated by modulating a continuous-wave (CW) single-frequency fiber laser at $\sim $1064 nm by using an electro-optic intensity modulator (EOIM). After multi-stage cascaded power amplification, the average power was boosted to be kilowatt level. The pulses from the main amplifier had a pulse width of $\sim $3 ns and an average/peak power of 913 W/28.6 kW. Further power scaling of the pulses was limited by stimulated Raman scattering (SRS) for the moment, method for SRS suppression and further power scaling was briefly discussed.

Thermal damages on the surface of a silicon wafer induced by a near-infrared laser

Laser-induced thermal damages of a silicon wafer surface subjected to continuous near-infrared laser irradiation were investigated. Silicon wafer specimens were illuminated by a continuous-wave fiber laser beam (1070-nm wavelength) with irradiances from 93 to 186 W/cm 2 , and the surface morphology of each specimen was analyzed using optical microscopy. With increasing irradiance, straight cracks in the <110> direction appeared first, and partial melting and complete melting were subsequently observed. The mechanism of these laser-induced thermal damages in the silicon wafer surface was discussed with numerical analysis based on the heat transfer and thermoelasticity model. The irradiances initiating the cracking and melting were predicted by determining the irradiances in which the calculated thermal stress and temperature exceeded the corresponding limits of the fracture strength and melting point, respectively. These predictions agreed well with the experimental findings. Laser-induced thermal damages of the silicon wafer surface subjected to a continuous near-infrared laser irradiation were identified based on these investigations.

High-power Yb-doped continuous-wave and pulsed fibre lasers

High-power laser generation using Yb-doped double-clad fibres with conversion efficiencies in excess of 80% have attracted much attention during the last decade due to their inherent advantages in terms of very high efficiency, no misalignment due to in-built intracore fibre Bragg gratings, low thermal problems due to large surface to volume ratio, diffraction-limited beam quality, compactness, reliability and fibre-optic beam delivery. Yb-doped fibres can also provide a wide emission band from ∼1010 nm to ∼1170 nm, which makes it a versatile laser medium to realize continuous-wave (CW), Q-switched short pulse, and mode-locked ultrashort pulse generation for various applications. In this article, a review of Yb-doped CW and pulsed fibre lasers along with our study on self-pulsing dynamics in CW fibre lasers to find its role in high-power fibre laser development and the physical mechanisms involved in its generation has been described. A study on the generation of high-power CW fibre laser of 165 W output power and generation of high peak power nanosecond pulses from acousto-optic Q-switched fibre laser has also been presented.

Numerical Simulation of Non-Ablative Laser Texturing of Silicon Surface with a Continuous Wave Fiber Laser

Numerical Simulation of Non-Ablative Laser Texturing of Silicon Surface with a Continuous Wave Fiber Laser

Electrical Property of Laser-Sintered Nanopastes with Reduced Metal Nanoparticles Prepared by Laser Ablation in Liquids

Reduced iron, aluminum, copper, and magnesium nanoparticles were produced from iron oxide (Fe3O4), aluminum oxide (Al2O3), copper oxide (CuO), and magnesium oxide (MgO) powders by using laser ablation in liquids, and nanopastes were synthesized with the reduced iron, aluminum, copper, and magnesium nanoparticles. The nanopastes were sintered by using a continuous-wave fiber laser in air atmosphere. The laser-sintered nanopastes consist of polycrystalline metal. The structures of the laser-sintered metal nanopastes were analyzed by SEM and EDX, and their resistivities were evaluated by four-terminal method. The metal nanopastes sintered by hot plate have 2.5- to 11-times-higher resistivities than those of common metals fabricated in blast furnaces. Moreover, the laser-sintered metal nanopastes have 9.5- to 45-times-higher resistivities than those of common metals fabricated in blast furnaces.

Modification of electrical properties of TiO&lt;sub&gt;2&lt;/sub&gt; by continuous-wave ytterbium fiber laser irradiation

Modification of electrical properties of TiO&lt;sub&gt;2&lt;/sub&gt; by continuous-wave ytterbium fiber laser irradiation

Laser patterning of thermoelectric iron silicide on alumina substrates by continuous-wave ytterbium fiber laser irradiation

Laser patterning of thermoelectric iron silicide on alumina substrates by continuous-wave ytterbium fiber laser irradiation

Experimental characterization of a micro-hole drilling process with short micro-second pulses by a CW single-mode fiber laser

Laser ablation with pulse durations in a few microseconds is a viable solution for micro-hole drilling applications which require large material removal rate (MRR) with moderate hole quality. However, the body of work regarding short microsecond laser drilling/ablation is small. The objective of this paper is to experimentally characterize this short micro-second laser micro-hole drilling technique using a 300W, CW, single-mode fiber laser. This CW fiber laser is controlled to produce modulated pulses from 1μs to 8μs and these modulated laser pulses have a unique profile which contains an initial spike with a peak power of 1500W for 1μs, followed by the steady state power of 300W. Because of its excellent beam quality, the laser beam produced by this fiber laser can be focused to a small spot size of 10μm to achieve very high power density up to 1.9GW/cm2. With one single laser pulse at approximately 1μs, a blind hole of 167μm in depth and 23μm in opening diameter can be created in a stainless substrate. The experimental characterization of this micro-hole drilling process includes laser control, laser beam characterization, hole formation, photodiode measurements of the vapor intensity, high-speed photography of vapor/plasma formation, and spectroscopic measurements of plasma. The results show that, due to very high irradiance of the fiber laser beam, the absorbed energy not only is sufficient to melt and vaporize the material, but also is able to dissociate vapor into intense plasma at temperatures over 16,000K. The hole drilling mechanism by this short microsecond laser ablation is due to a combination of adiabatic evaporation and ejection of fine droplets.

Monitoring of high-power fiber laser welding based on principal component analysis of a molten pool configuration

There exists plenty of welding quality information on a molten pool during high-power fiber laser welding. An approach for monitoring the high-power fiber laser welding status based on the principal component analysis (PCA) of a molten pool configuration is investigated. An infrared-sensitive high-speed camera was used to capture the molten pool images during laser butt-joint welding of Type 304 austenitic stainless steel plates with a high-power (10 kW) continuous wave fiber laser. In order to study the relationship between the molten pool configuration and the welding status, a new method based on PCA is proposed to analyze the welding stability by comparing the situation when the laser beam spot moves along, and when it deviates from the weld seam. Image processing techniques were applied to process the molten pool images and extract five characteristic parameters. Moreover, the PCA method was used to extract a composite indicator which is the linear combination of the five original characteristics to analyze the different status during welding. Experimental results showed that the extracted composite indicator had a close relationship with the actual welding results and it could be used to evaluate the status of the high-power fiber laser welding, providing a theoretical basis for the monitoring of laser welding quality.

Study on weld bead surface profile and angular distortion in 6 mm thick butt weld joints of SS304 using fiber laser

We deployed a 2kW continuous wave fiber laser integrated with the 5-axis workstation to understand the effect of various processing parameters (laser power, welding speed, beam spot size and chamfer at welded edges) on depth of penetration, angular distortion and welded bead surface profile during autogenous laser welding of 6mm thick austenitic stainless steel type 304 plates. Full penetration with reduced weld bead surface undulation (<100µm) and least angular distortion (<0.8°) was achieved for butt joints having chamfered edges of 0.8mm. The microscopic studies revealed grossly defect-free fusion zone with a few porosities at isolated locations. The microstructure at the fusion zone was largely austenitic with few ferrites and the direction of growth was epitaxial towards the fusion line. The measured values of microhardness at base material and fusion zone were 208±4 HV0.1 and 235±10 HV0.1 respectively. The tensile testing of laser welded samples indicated the ultimate strength >605MPa and these samples could be bent for an angle >170° without noticeable crack during bend test. The study opened the avenues for the deployment of fiber laser welding technology for applications demanding critical values of surface weld bead profile and distortion.

Enhancing the biocompatibility of Ti6Al4V implants by laser surface microtexturing: an in vitro study

The biocompatibility of an implant material depends on the bulk physical properties as well as on the surface properties. Titanium alloy Ti6Al4V is a very popular biological implant material for its bulk physical properties resembling those of natural bone. It is possible to modify the surface properties of Ti6Al4V by laser irradiation. In the current study, using continuous wave fibre laser, grooves of different depths and widths have been created by varying laser power and scan speed. Laser frequency and duty cycle were varied to alter the inside surface topographies of the microgrooves. The surface wettability and the protein adsorption on the surface were found to be dependent on the microtexture parameters. To understand the effect of this type of texturing on the cell behaviour, MG63 cells have been cultured on the sample surfaces, and the result was studied after 3 and 5 days of cell culture. Surface features dimensionally closer to the cell dimensions are able to positively affect the viability and spreading of MG63 cells, as seen from this study.

Oxide films in laser additive manufactured Inconel 718

A continuous-wave 5kW fiber laser welding system was used in conduction mode to deposit Inconel® alloy 718 (IN718) by employing filler wire on as-serviced IN718 parent material (PM) substrates. The direct laser deposited (DLD) coupons and as-serviced IN718 PM were then evaluated through tensile testing. To understand the failure mechanisms, the tensile fracture surfaces of the as-serviced IN718 PM, DLD and DLD-PM samples were analyzed using scanning electron microscopy. The fracture surfaces revealed the presence of both Al2O3 and Cr2O3 films, although the latter was reasoned to be the main oxide in IN718. Both the experimental observations and thermodynamic analysis indicated that oxidation of some alloying elements in IN718 cannot be completely avoided during manufacturing, whether in the liquid state under vacuum (for casting, the electron beam melting, welding and/or deposition) or with inert gas protection (for welding or laser deposition). The exposed surface of the oxide film on the fracture surface has poor wetting with the metal and thus can constitute a lack of bonding or a crack with either the metal and/or another non-wetted side of the oxide film. On the other hand, the wetted face of the oxide film has good atom-to-atom contact with the metal and may nucleate some intermetallic compounds, such as Laves, Ni3Nb-δ, Nb-rich MC and γ′ compounds. The potential of their nucleation on Cr2O3 was assessed using planar disregistry. Coherent planes were found between these intermetallics and Cr2O3.

Experimental study on the all-fiberized continuous-wave ytterbium-doped laser operating near 980 nm

All-fiberized continuous-wave Yb-doped fiber lasers operating near 980 nm are fabricated, and 1.73 W, 980 nm lasing is obtained. Moreover, the output properties of the 980 nm fiber laser are studied by experiment. It is demonstrated, for the first time to the best of our knowledge, that the output power curve versus the active fiber length experiences double-peak values, which are caused by the red shift of the lasing wavelength induced by the longitudinal-mode competition. It is also demonstrated that the pump threshold increases exponentially with the active fiber length. The relationship between the pump threshold and the optimum active fiber length is examined.

20 W continuous-wave cladding-pumped Nd-doped fiber laser at 910 nm

We demonstrate a double-clad fiber laser operating at 910 nm with a record power of 20 W. Laser emission on the three-level scheme is enabled by the combination of a small inner cladding-to-core diameter ratio and a high brightness pump source at 808 nm. A laser conversion efficiency as high as 44% was achieved in CW operating regime by using resonant fiber Bragg reflectors at 910 nm that prevent the lasing at the 1060 nm competing wavelength. Furthermore, in a master oscillator power-amplifier scheme, an amplified power of 14.8 W was achieved at 914 nm in the same fiber.

Fabrication and laser behaviors of Yb3+ doped silica large mode area photonic crystal fiber prepared by sol–gel method

Yb3+, Al3+ co-doped silica large mode area photonic crystal fiber (LMA PCF) has been prepared by sol–gel method combined with stack-capillary-draw technique. The core glass with composition of 0.3Yb2O3–3Al2O3–96.7SiO2 (mol%) has 927μs fluorescence lifetime, 0.82pm2 stimulated emission cross section at ∼1μm and 1.4ppm residual hydroxyl group. 6.8W continuous-wave fiber laser ranging from 1040 to 1070nm has been obtained from a piece of 98-cm-long PCF with the Yb3+ concentration of 6.49×1019ions/cm3 pumped by a commercial 927-nm laser diode. The laser has a threshold of 1.45W launched power with a slope efficiency of 48%. The maximum output power of 6.8W is limited by the available pumping power, and the optical loss is 1.2dB/m at 1053nm. To the best of our knowledge, this is the highest laser output power report on Yb3+, Al3+ co-doped silica LMA PCF prepared by sol–gel method.

Macro- and Microstructural Studies of Laser-Processed WE43 (Mg-Y-Nd) Magnesium Alloy

The macro- and microstructural changes in the fusion zone (FZ) of WE43 magnesium alloy processed by a diode-pumped ytterbium (IPG YLS-3000) continuous wave fiber laser for specified processing conditions (from 4.17 to 12.5 × 107 J/m2) were studied. With the aid of computational heat-transfer model, the temperature history and cooling rate were determined for different laser-processing conditions. The computational heat-transfer results of laser-processed samples were used to correlate with microstructures characterized using optical, scanning, and transmission electron microscopies, and electron backscatter and X-ray diffraction analyses. The microhardness measurement was carried out to establish the structure–property relationship, and the results indicated that the minimal hardness variation (1 pct) within laser FZ was due to a constant heat extraction time (0.1 second), narrow variation in grain size (4 to 7 μm), and the type of precipitate (β-phase) formation.

High power, continuous-wave ytterbium-doped fiber laser tunable from 976 to 1120 nm

We report on a linearly-polarized, continuous-wave Ytterbium-doped fiber laser continuously tunable on the fly over an unprecedented emission bandwidth ranging from 976 nm to 1120 nm and delivering a minimum of 10 W and a maximum of 41 W. Additionally, the bandwidth of the system can be tuned from 100 pm to more than 1 nm at all wavelengths.

Observation of narrow linewidth spikes in the coherent Brillouin random fiber laser

We observed narrow-linewidth lasing spikes in a coherent Brillouin random fiber laser, which uses a continuous-wave fiber laser as the pump source. In this random laser configuration, stimulated Brillouin scattering in a section of conventional single-mode fiber acts as the gain mechanism, while Rayleigh scattering in another section of the nonuniform fiber provides randomly distributed feedback. Above the lasing threshold, coherent random lasing spikes, with a linewidth of ~10 Hz, are measured at the top of the Brillouin gain spectrum. It results from high quality coherent resonance of the Brillouin Stokes light in this open laser cavity.

Gain-switched all-fiber laser with narrow bandwidth

Gain-switching of a CW fiber laser is a simple and cost-effective approach to generate pulses using an all-fiber system. We report on the construction of a narrow bandwidth (below 0.1 nm) gain-switched fiber laser and optimize the pulse energy and pulse duration under this constraint. The extracted pulse energy is 20 μJ in a duration of 135 ns at 7 kHz. The bandwidth increases for a higher pump pulse energy and repetition rate, and this sets the limit of the output pulse energy. A single power amplifier is added to raise the peak power to the kW-level and the pulse energy to 230 μJ while keeping the bandwidth below 0.1 nm. This allows frequency doubling in a periodically poled lithium tantalate crystal with a reasonable conversion efficiency.