High-power Fiber Laser Research Articles

Real-Time Temperature Measurement of High-Power Fiber Laser Core and Its Applications

Real-Time Temperature Measurement of High-Power Fiber Laser Core and Its Applications

Research Progress on Key Passive Devices for High-Power Fiber Lasers

Research Progress on Key Passive Devices for High-Power Fiber Lasers

Direct generation of 5 W all-fiber red laser at 635 nm

We propose and experimentally demonstrate a compact high-power all-fiber Pr3+-doped red laser at 635 nm using highly-resistant dielectric-coated fiber mirror. The fiber laser consists of a 443 nm pump laser diode (LD), a double-clad Pr3+-doped fluoride fiber and a 443/635 nm dichroic fiber mirror. The 635 nm laser is directly generated by down-conversion gain of the Pr3+-doped fiber pumped by the 443 nm LD. The homemade dichroic fiber mirror is specifically designed for anti-reflection at 443 nm pump wavelength and high-reflection at 635 nm laser wavelength, and shows a high damaged density of >15 MW/cm2, which is very favorable to high-power visible laser operation. The red all-fiber laser directly generates a highest average power of ∼5 W at 635 nm which is more than twofold higher than previously reported, and the slope efficiency is as high as 25.7%. Different from the traditional visible laser by frequency doubling, this work offers a promising way for miniaturized, high-power fiber lasers in visible wavelength region.

Highly Integrated Cladding Mode Stripper Array for Compact High-Power Industrial Fiber Laser.

A design integrating multiple cladding mode strippers used in fiber laser architectures into a single device is proposed. This approach can increase the compactness of fiber lasers, thus contributing to industrial laser processing applications. By offset-placing the most intense light-stripping parts, for instance, by inversing the laser injection directions or by displacing the beginning of etched sections, multiple cladding mode strippers bundled together into a single housing can have the hottest regions separated and can operate at full power simultaneously, with no evident cross-influence on each other. Two and three cladding-mode-stripper arrays have been implemented, and validation tests have been conducted with ~500-W cladding power being injected into each input port. For both arrayed devices, compared to the scenario in which only a single cladding mode stripper is working, no greater than a 2.1 °C temperature increment is generated when all components are operating concurrently, which demonstrates the effectiveness of the integration method. In this way, one half and two thirds of space/weight reduction can be realized, respectively, for the two and three cladding-mode-stripper arrays, which is meaningful, since cladding mode strippers are among the most bulky and hottest components in fiber lasers. Moreover, this integration provides a valuable reference for the miniaturization of other components, and thus, could contribute to the development fiber lasers with higher power-to-volume ratios, which would be more economical for industrial applications.

Quantitative evaluation of the heat induced by fusion splices in high-power fiber lasers

As heat sources in the fiber laser system, fusion points are among the most vulnerable parts in high power fiber lasers (HPFLs). A model is built to evaluate the heat induced by fusion splices quantitatively. By introducing the splice thermogenic factor (STF), the temperature of a fusion point is obtained at different output powers. At the same time, a real-time in-situ temperature measurement experiment is carried out, and the actual STF value of 0.449 ppm is obtained by fitting the model parameters with the experimental results. The fitting results indicate that the STF can evaluate the heat characteristics of fusion splices quantitatively and predict the temperature rise. The model can pave the way for further power scalability of HPFLs.

Synthesis and characterization of Yb3+ activated Lu2O3 nanoparticles doped optical fiber preform for high power laser application

Yb3+-activated Lu2O3 is known to exhibit excellent thermal conductance, which can be a potential candidate to develop high-power laser fiber. Here we report the synthesis of Yb3+-activated Lu2O3 nanoparticles following the homogeneous coprecipitation method along with their material and optical characterization. Five different samples were synthesized using different concentrations of Cetyl Trimethyl Ammonium Bromide (CTAB) as a cationic surfactant, Yb–Lu ratio in the presence/absence of aluminium (Al) as an additional dopant, followed by calcination at different temperatures. Differential thermal analysis (DTA) and thermogravimetry (TG) analysis were performed to optimize the thermal annealing condition while the X-ray diffraction (XRD) analysis shows the formation of a pure bixbyite structure with an average particle size of ∼ 15 nm at 800°C that reaches up to ∼ 120 nm with increasing calcination temperature to 1400°C. The field emission scanning electron microscope (FESEM) analysis shows nearly cubic morphology while high-resolution Transmission Electron Microscope (HRTEM) analysis confirms the crystalline nature and the average particle size corroborates with the XRD analysis result. The developed nanoparticle was found to exhibit characteristics of Yb-absorption and emission peaks showing no interference with Lu2O3. The emission intensity and fluorescence lifetime of the Yb3+-activated Lu2O3 nanoparticles were found to depend on the Yb concentration, average particle size, and presence of aluminium as a co-dopant. To evaluate the practical application of the developed nanoparticles, an optical preform is developed using synthesized nanoparticles as Yb-source through modified chemical vapor deposition (MCVD) coupled with solution doping (SD) technique and the initial performance of the preform is also reported.

Self-starting high-order mode oscillation fiber laser.

In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72nm with 3dB of 0.05nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.

Thermally-induced transverse mode instability: Hopf bifurcation in high-power fiber laser

The thermally-induced transverse mode instability (TMI) has become a major challenge for further enhancing the output power of single-mode fiber laser. Even though much work has been done over the past decades, the spontaneous generation process of output spot oscillation has not been well understood yet. Here, we establish a new theoretical model of the thermally-induced TMI in a high-power fiber oscillator and provide a new mechanism explanation from the point of view of nonlinear dynamics. Theoretical analysis shows that the spontaneous oscillation (∼kHz) of the output spot caused by the thermally-induced TMI is a limit-cycle oscillation via Hopf bifurcation, and the oscillation frequency is proportional to square root of the relaxation rate γ1 of thermally-induced refractive index grating (RIG) (νTMI∝γ1) when the thermally-induced TMI is initially triggered. Furthermore, the relationship between the threshold power, together with oscillation frequency and several parameters of fiber laser system was also discussed in detail. This work help us better understand the underlying dynamics of the thermally-induced TMI.

Kilowatt-Level Supercontinuum Generation in Random Raman Fiber Laser Oscillator With Full-Open Cavity

High-power supercontinuum (SC) sources are important for remote sensing, homeland defense, and scientific research. Despite recent advances in power scaling of the SC generation in optical fibers, the power record of SC fiber laser sources is still much lower than conventional high-power fiber lasers. Here we demonstrate a method for kilowatt-level SC generation based on the random Raman fiber laser (RRFL) with a full-open cavity. Theoretically, a model including cascaded Raman effect, modulation instability, and Rayleigh scattering is established to qualitatively highlight the dynamics of the kilowatt-level SC generation. Experimentally, injecting the amplified ytterbium-doped fiber laser and Raman fiber laser into the RRFL oscillator formed by a piece of long passive fiber, 3-kW-level SC spanning from 925 to 1862 nm is achieved. The experimental results suggest that the full-open-cavity RRFL is a promising approach for kilowatt-level SC generation and the theoretical model demonstrates the importance of Rayleigh backscattering during the SC generation.

Pure silica fiber Raman gain enabled high-power low-quantum defect fiber laser

Reducing the quantum defect (QD) is a promising solution to mitigate the thermal load and lower system noise. Besides, owing to many advantages such as wavelength agility and non-photon darkening, Raman fiber laser (RFL) has attracted much attention in recent years. However, to date, there is no report in RFL on QD reduction by silica fiber, which is the most common gain medium with mature fabrication technology of high-power RFL. In this paper, we propose and experimentally demonstrate a ten-watt-level low-QD RFL gained by a piece of commercial silica fiber. The QD can be reduced to 0.56 %, corresponding to a laser power of 6.2 W. After system optimization, the output power can be scaled up to 17.7 W, with a QD of 1.48 % and a slope efficiency of 72.0 %. To the best of our knowledge, this is the lowest QD ever reported in fiber laser based on pure silica gain. This work can offer an effective and low-cost solution for fiber laser system with reduced heat production.

On the initiation of fiber fuse damage in high-power ytterbium-doped fiber lasers

Fiber fuse effect can occur spontaneously and propagate along optical fibers to cause widespread damage; it threatens all applications involving optical fibers. This paper presents two results. First, it establishes that the initiation of fiber fuse (IFF) in silica fibers is caused by virtual-defect-induced absorption. Critical temperatures and critical optical powers for IFF are simulated for the first time using a 3D solid-state heat transfer model with heat source generated by the virtual-defect-induced absorption. In this method, formation energies of the virtual defects can be uniquely determined, which offers critical information on the chemical reasons for fiber fuse. Second, this paper offers a method to evaluate operating temperatures of fiber lasers. General analytical solutions of the operating temperatures along gain fibers are deduced. Results of 976-nm laser-diode-pumped and 1018-nm tandem-pumped ytterbium-doped fiber (YDF) amplifiers using 10/130-μm YDFs are calculated. Potential limits caused by fiber fuse are discussed.

Researching and integrating colour laser marking system on a metal surface using high power fiber laser with MOPA configuration

This paper presents the results of research and fabrication of a colour laser marking device on metal surfaces using a high-power fiber laser with a MOPA (Master oscillator power amplifier) configuration by intergrating semi-finished modules. Using our manufactured colour laser marking device, the effects of laser parameters such as laser power, laser pulse repetition frequency, beam scanning speed, line width on forming colour process have been researched and evaluated explicitly to provide the optimal set of parameters for the colours on each specific stainless steel and titanium metal.

Temporal characteristics of the light output from optical fibers with PRBS-modulated laser injection.

Pseudo-random binary sequence (PRBS) phase modulation is an effective method to promote the stimulated Brillouin scattering (SBS) threshold of high-power narrow linewidth fiber lasers. Most studies focus on the subjects of PRBS pattern selection, filter parameter optimization, threshold promotion, etc. In this paper, the temporal characteristics of the light output from optical fibers with PRBS modulated laser injection are investigated theoretically. We found that serious temporal fluctuations, i.e., high intensity pulses, existed in the output laser and backward Stokes light, even though the time-averaged backward Stokes power is only 1% of the input laser power. Based on the transient three wave coupling equations, through exploring the influence of clock rate and PRBS pattern, we demonstrate that this phenomenon arises from the long dwell times in PRBS patterns. Finally, modified PRBS patterns are proposed to significantly eliminate the intense pulses without decreasing the original SBS threshold.

4 kW narrow-linewidth fiber laser based on a simple one-stage MOPA structure

One-stage master oscillator power amplification (MOPA) structure is a simple and robust way to acquire high-power narrow-linewidth fiber lasers (NLFLs), which have wide applications in spectral/coherent beam combination. In this work, an NLFL system was established based on a home-made fiber oscillator seed. By using this one-stage MOPA system, a 4 kW output power was achieved with 3 dB linewidth of about 0.53 nm and optical-to-optical conversion efficiency of 76.9%. The signal to Raman ratio was ∼22 dB. The measured beam quality (M2 factor) was about 1.6 at the output power of 3780 W then degraded to 2.8 at the maximum power of 4 kW. To the best of our knowledge, the 4 kW is the highest output power for such narrow linewidth one-stage MOPA fiber lasers.

The influence of hole quality on mechanical behavior and strain distribution of open‐hole CFRP laminates subject to different machining processes

AbstractMachining process is generally required for cutting composite material of structural components prior to final assembly in aerospace industry. This study aimed to investigate surface quality and mechanical behavior of open‐hole composite laminates following different machining processes including abrasive waterjet cutting, high power fiber laser machining, conventional drilling and ultraviolet laser machining. Hole quality was quantified for further understanding the correlation between surface damage and mechanical behavior. Experimental results demonstrated that the hole geometrical accuracy and mechanical property were highly dependent on the machining methods. Digital image correlation technique was implemented to capture the strain distribution and strain evolution around open holes during tensile loading. Machining methods exhibited significant impact on , and strain levels around hole edges, while limited influence on strain distribution patterns. and concentrated in the regions consisting of fibers perpendicular to and along the loading direction respectively, and shear strain featured with X shape around hole edge.

First Demonstration and Comparison of 5 kW Monolithic Fiber Laser Oscillator Pumped by 915 nm and 981 nm LDs

Fiber laser performances including transverse mode instability (TMI), stimulated Raman scattering (SRS) and optical-to-optical efficiency are in connection with the pump wavelength. Here we studied the output characteristics of a 5-kW ytterbium-doped fiber laser oscillator pumped with two different pump sources, i.e., 915 nm and 981 nm laser diodes (LDs). The output characteristics of fiber laser oscillators pumped by 915 nm and 981 nm have been compared strictly and directly with the same structure in a high-power situation. Experimental results show that both pump wavelengths can scale the power up to more than 5 kW by suppressing the TMI effect. While in the case of pumping by the 981 nm LDs, the laser oscillator has an optical-to-optical efficiency of 87%, which is 13% higher than that of the 915 nm pumped scheme. In addition, due to the higher backward pumping ratio and lower total pump power, the laser oscillator has a better SRS suppression ratio when pumped at 981 nm. Thus, it reveals a great potential to balance the limitations of TMI and SRS for scaling up to an even higher output while pumping at 981 nm. All the devices of the oscillator are commercial, and it will be helpful for the commercialization of high-power fiber laser oscillators.

Developing a Real-Time Working Method That Improves Process Efficiency in High-Power Fiber Laser Systems.

The need for studies on new simulation and monitoring methods for interactions occurring during material processing in high-power fiber laser systems has increased. In this manuscript, a structure that can intervene in real time and improved solutions that demonstrate the potential of photodiode-based monitoring are presented. By processing the signals instantly received during material processing with InGaAs and Si photodiodes integrated into the cutting head in algorithms, the method that intervenes in the process by detecting the last stage of the piercing process and the problems that may occur during cutting are explained. The stability of the proposed system has been tested on the most used materials in the industry such as St37, stainless steel, and aluminum at laser powers of 6, 8, and 10 kW, respectively. In this article, it is shown that there is a relationship between the signals observed in the infrared (IR) and visible (VIS) spectrum and the characteristics of the cut quality and scenario. Analysis results of photo-diode tracking signals obtained according to material, power, and gas type are presented. Among the innovations added by the method are related application process improvements, material analysis, and cutting and piercing parameter improvements.

Real-Time Definite Sequence Modulation Based Spectral Broadening Scheme for High-Power Narrow-Linewidth Fiber Laser

High-power narrow-linewidth continuous fiber lasers have wide applications in the fields of power synthesis, nonlinear frequency conversion, and industrial processing. The output power of kW or even MW can be achieved by adopting spectral synthesis. However, in the power amplification process, the nonlinear effect represented by the stimulated Brillouin scattering (SBS) effect limits the further increase of the laser power. And the accompanying self-pulsing effect poses a threat to the safety of the laser system. A high-order phase modulation scheme using a definite time sequence has been proposed to realize the controllable spectral broadening of the seed source, so as to suppress the SBS effect and improve the output power in the amplification stage. In this paper, we demonstrate a real-time definite time sequence generation scheme by using an integrated field-programmable gate array (FPGA) and digital-to-analog converter (DAC). Combined with a single-stage phase modulator, the broadened bandwidth up to 30 GHz and controllable spectral shape can be achieved by adjusting the output waveform of the DAC directly. The generation principles and waveform control methods of real-time sequence are introduced in detail. Proven by a master oscillator power amplifier (MOPA) structure, the laser output power has increased by 30% under the extended bandwidth of 10 GHz compared to the cascaded white noise modulation case. This scheme provides a compact real-time control module for spectral broadening, which lays the foundation for implementing low-cost, compact, and standardized high-power narrow-linewidth fiber lasers.

Special issue “optical communications, sensing, and laser applications”

Special issue “optical communications, sensing, and laser applications”

2.5 kW TMI-free co-pump Yb-doped fiber oscillator by 971.5 nm pumping wavelength

Pump wavelength shift from the peak absorption wavelength of 976 nm is one of the strategies for mitigation of transverse mode instability (TMI) in high-power Yb-doped fiber lasers. In the present contribution, 971.5 nm is introduced as an effective pumping wavelength for the increment of TMI threshold. By using this pumping wavelength, a 2580 W co-pumped fiber oscillator is built by 20/400 µm active fiber, with more than a twofold increase in the TMI threshold. Our analytical and experimental investigations indicate that due to the lower heat load of the pumping wavelength of 971.5 nm in the gain medium, it can properly mitigate TMI in fiber oscillators. Power scaling in here presented fiber oscillator is just limited to the occurrence of stimulated Raman scattering (SRS) induced mode instability and to the best of authors’ knowledge, this is the first evidence of SRS-induced mode instability in high-power fiber oscillators and TMI is not the restricting factor in power scaling anymore.