Continuous Wave Fiber Laser Research Articles

High-power dual-wavelength Ho-doped fiber laser at >2\u2009\u03bcm tandem pumped by a 1.15\u2009\u03bcm fiber laser

We demonstrated a high-power continuous-wave (CW) dual-wavelength Ho-doped fiber laser (HDFL) at 2049 nm and 2153 nm with a simple coupled-cavity configuration. A ~100 W laser diode-pumped fiber laser at 1150 nm served as the pump source. The maximum output power reached ~22.3 W and the slope efficiency was 23%. By altering the incident pump power, the power ratio of two signal wavelengths could be tuned in a large range due to gain competition. As far as we know, this is the first CW dual-wavelength HDFL with the power exceeding ten-watt-level, and the first dual-wavelength HDFL with the central wavelengths exceeding 2.0 μm and 2.15 μm respectively.

Role of Capillary and Thermocapillary Forces in Laser Polishing of Metals

As one of emerging novel surface treatment techniques, laser polishing offers a cost-effective and efficient solution to reduce surface roughness of precision components at micro-/mesoscale. Although it has been applied for industrial and biomedical purposes, the underlying mechanism has not been fully revealed. This paper presents a study to understand the basic fundamentals of continuous wave fiber laser polishing of Ti6Al4V samples. A two-dimensional numerical model that coupled heat transfer and fluid flow is developed to illustrate the molten flow behavior. The roles of capillary and thermocapillary flow in the process of laser polishing are investigated to assist the understanding of the contributions of surface tension (capillary force) and Marangoni effect (thermocapillary force) in the polishing process. Capillary force dominates the molten pool at the initial stage of melting, while thermocapillary force becomes predominant when the molten pool fully develops.

Enhancing the antibacterial performance of orthopaedic implant materials by fibre laser surface engineering

Implant failure caused by bacterial infection is extremely difficult to treat and usually requires the removal of the infected components. Despite the severe consequence of bacterial infection, research into bacterial infection of orthopaedic implants is still at an early stage compared to the effort on enhancing osseointegration, wear and corrosion resistance of implant materials. In this study, the effects of laser surface treatment on enhancing the antibacterial properties of commercially pure (CP) Ti (Grade 2), Ti6Al4V (Grade 5) and CoCrMo alloy implant materials were studied and compared for the first time. Laser surface treatment was performed by a continuous wave (CW) fibre laser with a near-infrared wavelength of 1064nm in a nitrogen-containing environment. Staphylococcus aureus, commonly implicated in infection associated with orthopaedic implants, was used to investigate the antibacterial properties of the laser-treated surfaces. The surface roughness and topography of the laser-treated materials were analysed by a 2D roughness testing and by AFM. The surface morphologies before and after 24h of bacterial cell culture were captured by SEM, and bacterial viability was determined using live/dead staining. Surface chemistry was analysed by XPS and surface wettability was measured using the sessile drop method. The findings of this study indicated that the laser-treated CP Ti and Ti6Al4V surfaces exhibited a noticeable reduction in bacterial adhesion and possessed a bactericidal effect. Such properties were attributable to the combined effects of reduced hydrophobicity, thicker and stable oxide films and presence of laser-induced nano-features. No similar antibacterial effect was observed in the laser-treated CoCrMo.

Optimization of Laser Lap Joining between Stainless Steel 304 and Acrylonitrile Butadiene Styrene (ABS)

In principle, a laser can weld any materials, which can be joined by conventional and other nonconventional processes. Dissimilar joining technologies find applications in many sectors including microelectronics, medical and aerospace. The laser weldability of dissimilar materials depends on many factors such as physical and chemical properties. In the presented study, the laser lap joint of stainless steel 304 and acrylonitrile butadiene styrene (ABS) is investigated using a continuous wave fiber laser. All the important parameters such as laser power and welding speed based on full factorial method have been considered in this paper. The quality criteria considered to determine the optimal parameter settings were the maximization of shearing force. The collected data has been analyzed by analysis of variance (ANOVA) method to find the significance factor that effects on laser welding of stainless steel 304 and ABS. A focused beam with laser power 230W and welding speed 16mm/s were identified as the optimal set of laser welding parameters to obtain stronger and better welds. This research is conducted to reveal the possible of strong dissimilar lap joint by a direct joining laser radiation without using adhesive bonds or mechanical fasteners.

33 W continuous-wave single-frequency green laser by frequency doubling of a single-mode YDFA

A high power continuous-wave single-frequency green fiber laser by second-harmonic generation of a Yb-doped fiber amplifier (YDFA) is developed. A linearly polarized single-mode fiber amplifier produces a 60 W infrared laser at 1064 nm with a 103 W incident diode pump laser at 976 nm, corresponding to an optical conversion efficiency of 58%. An external bow-tie enhancement cavity incorporating a noncritically phase-matched lithium triborate crystal is employed for second-harmonic generation. A 33.2 W laser at 532 nm is obtained with a 45 W incident 1064 nm fundamental laser, corresponding to a conversion efficiency of 74%.

All-fiber radio frequency-modulated pulsed laser based on frequency-shift feedback loop

Lidar-radar by using an radio frequency modulated (RF-modulated) laser transmitter is a powerful technique for applications involving remote sensing. The method is based on the use of an optically carried RF signal in order to acquire the merits of both the directivity of the optical beam (lidar) and the accuracy of RF signal processing (radar). Compared with single-frequency coherent lidars, lidar-radars are less sensitive to atmospheric turbulence and the speckle noise induced by target roughness. For long range detection, pulsed operation is usually required because of the high peak power. In order to meet the requirement for long range detection, an RF-modulated pulse train based on an all-fiber frequency shifted feedback loop is proposed in this paper. A continuous-wave single-frequency fiber laser (seed laser) is coupled into a fiber link and an acousto-optic chopper is used as a frequency shifter and beam chopper. A Yb3+-doped fiber amplifier is used to compensate for the loss of the signal in the fiber loop. The pulse duration is determined by the open time of acousto-optic chopper, which is fixed at 110 ns. A square wave generated by an arbitrary waveform generator is used as a trigger signal of the acousto-optic chopper. The RF within the pulse results from the interference of frequency shifed pulse with the seed laser. By inserting a 10 km fiber in the loop and accurately controlling the trigger cycle of the acousto-optic chopper equal to the roundtrip time of the loop, the pulse train generated by acousto-optic chopper can circulate in the loop, leading to the generation of RF-modulated pulse with about 20 kHz repetition rate and 110 ns width. The gain provided by fiber amplifier in the loop partially compensates for the loss. By adjusting the gain of fiber amplifier, the modulation depth of RF within the pulse can be continuously adjusted and the maximum modulation depth is 0.67. We also present an time-delayed scalar interference model which includes the loop length, trigger cycle, frequency-shift, and the gain. According to the theoretical model, the RF-modulated pulse affected by trigger cycle and fiber amplifier is numerically simulated. The experimental results accord well with theoritical predictions. The RF-modulated pulse has the advantage of high pulse-to-pulse coherence, which provides potential applications in lidar-radar detection. Besides, with an additional frequency doubling stage one can obtain a source for underwater detections and communications. Extension of the scheme to the 1.5 μm telecommunication window is straightforwardfor various radio-over-fiber applications.

High Power Continuous Wave Fiber Laser Technologies

High Power Continuous Wave Fiber Laser Technologies

Functionally graded 3D structures produced by laser cladding

In this study, three-dimensional metallic structures of functionally graded materials (FGMs) were produced using the one step laser cladding technique. AISI 316L with particle size between 63 and 125 µm and a cobalt based super-alloy with 90 µm of average particle size, were used as precursor materials. A Yb:YAG continuous wave fiber laser (λ=1075 nm) was chosen as the energy source of the laser cladding process. By means of a new powder feeder system specifically developed for this work, the precursor materials were mixed and homogenized in the desired percentage. Moreover, the new powder feeder allows the live modification (during the processing time) of the material concentrations, in order to obtain three-dimensional structures with a smooth gradient of the precursor material. The study was completed with the analysis of the microstructure, chemical composition and micro-hardness of the obtained samples.

Fibre laser nitriding of titanium and its alloy in open atmosphere for orthopaedic implant applications: Investigations on surface quality, microstructure and tribological properties

Laser nitriding is known to be an effective method to improve the surface hardness and wear resistance of titanium and its alloys. However, the process requires a gas chamber and this greatly limits the practicability for treating orthopaedic implants which involve complex-shaped parts or curved surfaces, such as the tapered surface in a femoral stem or the ball-shaped surface in a femoral head. To tackle this problem, a direct laser nitriding process in open atmosphere was performed on commercially pure titanium (grade 2, TiG2) and Ti6Al4V alloy (grade 5, TiG5) using a continuous-wave (CW) fibre laser. The effects of varying process parameters, for instance laser power and nitrogen pressure on the surface quality, namely discolouration were quantified using ImageJ analysis. The optimised process parameters to produce the gold-coloured nitride surfaces were also identified: 40W (laser power), 25mm/s (scanning speed), 1.5mm (standoff distance) and 5bar (N2 pressure). Particularly, N2 pressure at 5bar was found to be the threshold above which significant discolouration will occur. The surface morphology, composition, microstructure, micro-hardness, and tribological properties, particularly hydrodynamic size distribution of wear debris, were carefully characterized and compared. The experimental results showed that TiG2 and TiG5 reacted differently with the laser radiation at 1.06μm wavelength in laser nitriding as evidenced by substantial differences in the microstructure, and surface colour and morphology. Furthermore, both friction and wear properties were strongly affected by the hardness and microstructure of titanium samples and direct laser nitriding led to substantial improvements in their wear resistant properties. Between the two types of titanium samples, bare TiG2 showed higher friction forces and wear rates, but this trend was reversed after laser nitriding treatments.

Deep Diffusion of Phosphorus in Silicon using Microsecond-pulsed Laser Doping

Deep diffusion of phosphorus atoms in monocrystalline silicon using laser doping process has been studied in this work. A pulse modulated CW fiber laser of wavelength 1070nm with microsecond pulses has been used to diffuse phosphorus from pre-deposited spin-on-dopant film. The surface and cross-sectional morphology has been studied using SEM and AFM. The concentration-depth profiling was done using PP-TOFMS. Deep junctions of more than 10µm have been obtained under various laser doping conditions while a maximum junction depth of 51.3µm has been obtained through optimization. Diffusion depth enhancement is made possible by increasing the pulse length and reducing laser scan speed. Laser doping led to formation of n+ region with surface concentration varying in the range of 3×1020–5×1020cm−3 for varying scan speed. Cross-sectional TEM and diffraction studies on laser irradiated samples show presence of only monocrystalline silicon phase after laser induced melting and solidification.

In Situ Production of Hard Metal Matrix Composite Coating on Engineered Surfaces Using Laser Cladding Technique

The growing need for high wear-resistant surface with enhanced physical properties has led to extensive researches in the field of surface engineering. Laser cladding emerged to be a promising method to achieve these objectives in a cost-effective way. The present paper studies the viability of cladding of tungsten disulfide (WS2) powder by using 400 W continuous-wave fiber laser. WS2 was used as a coating material, which was decomposed at higher temperature and underwent several chemical reactions. By this process, in situ formation of metal matrix composites and hard face coating on the substrate surface were attained. The characterization of laser cladded surface was done to study its morphological, microstructural, mechanical and tribological properties. It was observed that cladding of WS2 powder on 304 SS resulted in the formation of Cr-W-C-Fe metal matrix composite having improved mechanical and tribological properties. The value of microhardness of the coated surface was found to increase three to four times in comparison with the parent material surface. Wear test results indicated a decrease in wear by 1/9th (maximum) as compared to the parent 304 SS surface. The volume fractions of tungsten particles on the cladded surface were also investigated through EDS analysis.

EVALUATION OF SINGLE TRACKS OF 17-4PH STEEL MANUFACTURED AT DIFFERENT POWER DENSITIES AND SCANNING SPEEDS BY SELECTIVE LASER MELTING

In Selective Laser Melting, the initial units produced are single tracks that overlap to create a single layer; from the sequence of layers, a 3D object is manufactured. The properties of the parts produced by SLM depend heavily on the properties of each single track and each layer formed by these tracks. This study evaluates the effect of processing parameters on the geometrical characteristics of single tracks manufactured from 17-4PH stainless steel powder. A single-mode continuous-wave ytterbium fibre laser was used to manufacture single tracks at laser powers in the range of 100-300 W with a constant spot size of ∼ 80μm. The single tracks produced were subjected to standard metallographic preparation techniques for further analysis with an optical microscope. Deep molten pool shapes were observed at low scan speeds, while shallow molten pool shapes were observed at high scan speeds. At higher laser power densities, under-cutting and humping effects were also observed. The dimensions of single tracks processed without powder generally decrease with increasing scan speed at constant laser power. However, the geometrical features of the single tracks processed with powder revealed pronounced irregularities believed to be caused by non-homogeneity in the deposited powder layer.

Comparison of the polarization dynamics of three different gain fibers in a continuous wave laser oscillator

We experimentally investigate the polarization dynamics of three kinds of gain fiber, the single-polarizing photonic crystal fiber, double clad polarization-maintaining large mode area fiber and double clad large mode area nonpolarizing fiber, in a continuous wave fiber laser oscillator under different initial polarization directions and pumping powers. The comparison results demonstrate that there are some distinguishable differences for the polarization evolutions inside the three kinds of active fibers. The polarization operation status of the fiber oscillator strongly depends on not only the gain fiber’s kinds, but also the length of the fiber, initial polarization directions and pumping power. The results suggest that polarization instability in a fiber laser should be an inherent reason for the higher operation noise.

Laser cooling of 5 mol. % Yb3+ : LuLiF4crystal in air

We report the laser cooling performance of 5 mol. % Yb3+:LuLiF4 crystal in air. Both end faces of the crystal with a size of 3×3×5 mm3 are cut by Brewster angles. A temperature drop of 11 K (from room temperature) of the sample, pumped by a 3-W/1020-nm CW fiber laser, is observed by a thermal camera. The cooling power and efficiency of the sample are estimated to be ∼14.5 mW and ∼1.9%, respectively. Our experiment results show the prospect of this crystal with laser cooling and its potential applications in earth-based optical refrigeration devices.

Glassy behavior in a one-dimensional continuous-wave erbium-doped random fiber laser

The analogue of the paramagnetic to spin-glass phase transition in disordered magnetic systems, leading to the phenomenon of replica symmetry breaking, has been recently demonstrated in a two-dimensional random laser consisting of an organic-based amorphous solid-state thin film. We report here the first demonstration of replica symmetry breaking in a one-dimensional photonic system consisting of an erbium-doped random fiber laser operating in the continuous-wave regime based on a unique random fiber grating system, which plays the role of the random scatterers and operates in the Anderson localization regime. The clear transition from a photonic paramagnetic to a photonic spin glass phase, characterized by the probability distribution function of the Parisi overlap, was verified and characterized. In this unique system, the radiation field interacts only with the gain medium, and the fiber grating, which provides the disordered feedback mechanism, does not interfere with the pump.

The experimental study of a CW 1080 nm multi-point pump fiber laser

In this paper, we report on a CW 1080 nm fiber laser cascaded-pumped by a CW 975 nm diode laser. The fiber used in the experiment has a core diameter of 20 μm (NA = 0.06), inner clad of 400 μm (NA = 0.46), and an absorption coefficient of about 1.26 dB m−1 at 975 nm. An output power of 780 W with an optical conversion efficiency of 71% has been achieved at a pump light of 1.1 kW. To the best of our knowledge, this is the first time that a 1080 nm CW fiber laser has used a cascaded-pump coupler.

Linearly polarized, dual wavelength frequency-modulated continuous-wave fiber laser for simultaneous coherent distance and speed measurements

We have experimentally demonstrated a high power linearly polarized, dual wavelength frequency-modulated continuous-wave (FMCW) fiber laser with master-oscillator power-amplifier (MOPA) configuration, which is specially designed for simultaneous coherent distance and speed measurements. Two single longitudinal mode laser diodes working at 1550.12 and 1554.13 nm are employed as the seeds of the fiber MOPA. The wavelengths of the seeds are externally modulated by two acousto-optic frequency shifters (AOFSes) with a symmetrical sawtooth wave from 330–460 MHz in the frequency domain. The modulation periodicities for the two seeds are 26 and 26.3 μs, respectively, by which the distance ambiguity can be eliminated and therefore the detection range can be extended to a great extent. The seeds are then amplified independently to reduce their power differences during frequency modulation. After being coupled and boosted with three successive fiber amplifiers, an output power of 12.1 W is recorded from the FMCW laser with a power instability <0.14% over 1.5 h. The measured PER and full divergence angle of the laser are >18 dB and <25 μrad, respectively, indicating its excellent performance for field measurements.

Stable Mode-Locked Nanosecond Chirp-Free Pulse Generation With Ultra-Narrow Bandwidth

A stable nanosecond chirp-free pulse generation with ultra-narrow bandwidth is established from a passively mode-locked erbium-doped fiber laser with a homemade fiber Bragg grating as the intra-cavity filter and careful intra-cavity dispersion management introduced. The laser generates nearly chirp-free pulse that is close to the Fourier transform-limited condition. The measured pulsewidth is 2 ns and the spectral width is only 192 MHz. The measured time jitter is below 1%. The pulse repetition rate is 982 kHz, which reveals there are only 195 oscillating longitudinal modes. This is the longest duration of single stable chirp-free pulse in passively continuous-wave mode-locked fiber lasers to our best knowledge.

Hot cracking in laser welding of Hastelloy X with pulsed Nd:YAG and continuous wave fiber lasers

Hastelloy X is a corrosion and heat resistance solution hardened nickel base super alloy. This alloy having an austenitic microstructure and in this regard is expected to be prone to hot cracking in fusion welding. In this research a 400W Nd:YAG pulsed laser and 600W continuous wave fiber laser welding are employed to weld a 2mm thick Hastelloy X plates. Comparison is made with regard to weld penetration and tendency of occurrence of solidification cracking in the weld metal and liquation cracks in the heat affected zone. It is found that with pulsed laser hot cracks are formed, however, with continuous wave laser hot cracking can be avoided. Increasing the laser pulse frequency and pulse duration resulted in a significant reduction in hot cracking tendency in pulsed laser welding. The reduction in cooling rate and thermal stresses during solidification and backfilling of liquated grain boundaries are proposed to be the factors contributing to the prevention or reduction of hot cracks in laser welding of Hastelloy X. With a 0.2mm beam diameter fiber laser at 500W power and 5mm/s travel speed full penetration weld was achieve with no observation of hot cracks in the weld cross sections.

Generation of High Energy Gamma-ray by Laser Compton Scattering of 1.94-μm Fiber Laser in UVSOR-III Electron Storage Ring

Development of a laser Compton scattering (LCS) gamma-ray source in the UVSOR-III storage ring has been proposed to perform basic research on non-destructive three dimensional isotope imaging. As the first step of the development, a numerical simulation was performed to evaluate the performance of the gamma-ray source. The expected total flux is about 1×107 photons/s when a 1.94-μm CW fiber laser with the maximum power of 5W collides with 300-mA electron beam circulating in the storage ring. The maximum gamma-ray energy and the total flux available were measured in experiment and determined as 5403 ± 16 keV with 1×107 photons/s, respectively. The electron beam energy was evaluated as 746 ± 1 MeV from the measured maximum gamma-ray energy. From the numerical calculation and the experimental result, the energy spread of 2.9% in full width at half maximum and the flux of 4×105 photons/s are expected for the LCS gamma-ray beam when a collimator with 2-mm hole is used to limit the scattered angle. The quality of gamma-ray beam generated in UVSOR-III is enough to perform a basic study on the non-destructive three dimensional isotope imaging.