High-power Fiber Laser Systems Research Articles

All-fiber 3 kW LP02 laser output based on long-period fiber grating for precise welding

An innovative all-fiber mode converter specifically designed for high-power fiber lasers is introduced. The mode converter relies on the coupling theory of long-period fiber gratings (LPFGs) and involves the fabrication of a LPFG, where the performance is meticulously studied by examining various fabrication parameters. The LPFG is integrated into a high-power fiber laser system to achieve efficient conversion from the LP01 mode to the LP02 mode with a conversion ratio of 90 % and a low transmission loss of only 0.088 dB. Under a 3.03 kW laser output, the LP02 fiber laser exhibits a remarkable laser-to-raman peak intensity ratio, which is 15.98 dB higher than that of the signal-mode fiber laser. Furthermore, during the welding process of a thick aluminum plate, the LP02 fiber laser demonstrates a significant increase of 1.2 times in the depth of the weld pool morphology compared to the signal-mode fiber laser, particularly evident when operating at a laser welding head speed of 100 mm/s. The mode converter with LPFG exhibits lower insertion loss, enhanced power-handling capacity, making it ideal for diverse high-power fiber laser applications with promising practical prospects.

Tracing the origins of mode coupling for beam quality optimization in high-power fiber laser delivery systems

Abstract Suppressing mode degradation is the key issue for high-power laser delivery; however, diagnosing mode degradation in its entirety, ranging from the contents and origins to locations, has always been a major obstacle. Here, a versatile approach for tracing the origins of mode coupling is demonstrated through addressing the differential intermodal dispersions of fiber modes. Full recognition for modal contents and the origins of mode degradation are experimentally completed in a two-mode fiber laser delivery system, which assists a significant improvement of beam quality M2 from 1.35 to 1.15 at the highest power of over 300 W. This method yields a quantitative characterization for manipulating the individual mode of dual-mode coupling origins or their combinations. This work points toward a promising strategy for the online tracing of mode coupling in cascade fiber links, thus enabling further pursuit of seeking extreme beam quality in high-power fiber laser systems.

An investigation on microstructure and mechanical properties of H62 brass thin-sheet by fiber laser welding: Experiments and multi-scale simulations

H62 brass is widely used as an engineering material in electrical components, automotive parts, aerospace and other fields. Many brass products and structures requiring material forming by means of welding. The reliability of the welding process directly affects the quality of the brass welded joints. In this study, a high-power fiber laser system was used for welding tests on thin sheets of H62 brass. Based on the theory of a single control variable with multiple welding parameters, the laser power, the amount of defocusing and welding speed were investigated in relation to the relationship of joint formation, microstructure and mechanical properties. In addition, two kinds of macroscopic segregations were found in some welded joints due to insufficient heat input and subcooling of the melt pool to the submixing gap. In order to further analyze the forming mechanism during the welding process, the physical model was established by finite element and molecular dynamics research methods. Using finite elements, the influence rule of different process parameters on the temperature field was analyzed; molecular dynamics was used to analyze the formation process of the weld from the atomic scale. Ultimately, this study has resulted in welded joints of H62 brass with excellent mechanical properties and no obvious defects through suitable combinations of different process parameters. Most importantly, a multi-scale simulation model has been established to analyze the rules of temperature influence in the weld formation process.

Optimization and demonstration of a bidirectional output linear-cavity fiber laser with a record high power of 2 × 4 kW

The bidirectional output fiber laser can achieve two laser outputs from a single resonant cavity, which can reduce the volume and cost of high-power fiber laser systems and has good application prospects in the industry. In this work, we demonstrated a bidirectional output fiber laser oscillator structure, and a total output power of 8 kW at the 2 ports × 4 kW level was achieved experimentally for the first time. In order to suppress stimulated Raman scattering (SRS) and transverse mode instability (TMI), the double cladding ytterbium-doped fiber with a core/cladding diameter of 30/400 µm, a pair of matched output coupler fiber Bragg gratings (OCFBG) with a 3 dB bandwidth of 2 nm, and the pump source with wavelength-stabilized 981 nm laser diodes (LDs) were employed. The effect of the coiling method and diameter of the active fiber on the optical-to-optical (O-O) efficiency and TMI threshold were studied intensively. The comparison of laser performance with a unidirectional output oscillator was further exploited, and results indicated that the bidirectional output fiber laser oscillator possessed a higher O-O efficiency and TMI threshold. By optimizing the fiber coiling diameter and coiling method, we finally achieved a total laser output power of 8169 W (3769 W at end A and 4400 W at end B), with an efficiency of 74.6% and beam quality of M2A ∼ 2.13, M2B ∼ 2.36, while the total output power achieved by unidirectional output structure was only 5805 W with signs of TMI. To the best of our knowledge, it is the highest output power of the bidirectional output fiber laser.

All-fiberized linearly polarized superfluorescent fiber source with 5 kW power output.

A superfluorescent fiber source (SFS) is a special fiber source that commonly possesses high temporal stability and a wide spectral linewidth. In this work, an all-fiberized linearly polarized SFS with, to our knowledge, record output power and near-diffraction-limited beam quality is presented. Up to 5.03kW SFS is achieved at a pump power of 6.18kW with a corresponding conversion efficiency of ∼81.1%. At maximum output power, the signal-to-noise ratio to background spectral noise is over 50dB, the polarization extinction ratio is ∼17d B, and the beam quality factor is M x 2=1.49, M y 2=1.44. Further comparisons confirm the power scalability of fiber amplifiers employing SFSs as seed lasers. Overall, this work could provide a good reference for potential exploration of high-power fiber laser systems.

Numerical Investigation of Polarization-Dependent Ultrashort Pulse Amplification in Erbium-Doped Fluoride Fibers

We have theoretically investigated the polarization-dependent ultrashort pulse amplification in erbium-doped fluoride fibers. The numerical model is based on the coupled generalized nonlinear Schrödinger equations and the rate equations. It is found that the Raman effect can be weakened by converting the seed polarization from linear to circular. Compared with the linearly polarized seed pulses, amplification of the circularly polarized seed pulses can generate Raman solitons with a larger pulse energy of the same central wavelength, although more pump power is needed. Furthermore, the amplification dynamics when no pulse splitting occurs have also been investigated. We demonstrate that amplification of the circularly polarized seed pulses allows the main pulse to accumulate more energy while maintaining a single-pulse profile. It is predicted that sub-two-cycle laser pulses at 2.8 μm with megawatt peak power and hundreds of nanojoules pulse energy can be directly generated from the erbium-doped fluoride fiber amplifiers. The investigations conducted in this paper can provide guidelines for the design of Raman-soliton-based tunable fiber laser systems and high-power few-cycle fiber laser systems.

Pulses of 32 mJ and 158 fs at 20-kHz repetition rate from a spatiotemporally combined fiber laser system.

A high-energy, high-power ultrafast fiber laser system based on spatiotemporal coherent combination is presented. Bursts of eight subsequent chirped-pulse amplification (CPA)-stretched pulses are amplified simultaneously in 16 parallel ytterbium-doped rod-type amplifiers. After spatial and temporal coherent combination of the total 128 amplified pulse replicas into a single pulse, it is compressed in a partially protective-gas-filled CPA compressor. Finally, nearly Fourier-transform-limited pulses with an energy of 32 mJ and a duration of 158 fs are emitted with a repetition rate of 20 kHz and a close to diffraction-limited beam quality.

High SBS Threshold Fiber Laser With Spectral Management Using Binarized Multi-Tone Signal Modulation

In the high-power narrow-linewidth continuous fiber laser system based on the master oscillator power amplifier (MOPA) structure, the linewidth and spectral shape of the seed source are directly related to the SBS effect threshold which limits the maximum output power. It is currently under investigation which spectral shape has the highest laser power threshold. In this study, the spectral broadening and shape control of seed sources based on high-order phase modulation of binarized multi-tone signals is proposed. Combined with the heterodyne detected-based high-precision spectral measurement method, the measurement accuracy is improved from GHz to MHz level, which assists in optimizing the spectral shape. The broadened seed sources with rectangular, triangular, concave, and super-Gaussian spectra are experimentally demonstrated. And a maximum power of 2234 W is obtained with a rectangular spectrum with 10-GHz bandwidth. Theoretically, this scheme has multi-dimensional spectra tuning ability and can achieve arbitrary spectral types to explore the optimal seed source spectrum in diverse applications.

Femtosecond laser fabrication of chirped and tilted fiber Bragg gratings for stimulated Raman scattering suppression in kilowatt-level fiber lasers.

Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are ∼25 dB and ∼12 nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7 kW fiber amplifier, and an SRS suppression ratio of ∼4 dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.

Experimental study of chemical-etched high-power cladding mode stripping device toward high attenuation

Fiber cladding mode stripping device with high-power high-attenuation stripping capability and negligible perturbation to signal light has been developed by employing Teflon-capillary segmental-etching fabrication method. By using different etchant solutions, surface roughening and cladding reducing structures can be obtained. The influence of the roughened length on the attenuation of cladding power has been studied experimentally, showing that roughened surface strips cladding light well at length below 5 cm, but becomes less effective beyond that. The impact of the cladding reduction has also been investigated, indicating that reduced cladding can help deplete the cladding light effectively, and the attenuation is able to be improved from 23.0 dB to 43.2 dB by etching the cladding diameter from 400 μm to 50 μm. By combining the roughened surface and reduced cladding structures, cladding mode stripper with 7 °C/kW of temperature rising rate on the etched area has been made, and stripped power up to 2.1 kW at 39 dB of attenuation has been demonstrated, with the device length merely 16 cm. The maximum temperature on the device housing at full operation power for the case water flux being 3.1 L/min and 5.6 L/min is 67 °C and 50 °C, respectively. To the best of our knowledge, this is the first report of >2-kW power cladding mode stripping device with 39-dB attenuation. All results indicate the effectiveness of the proposed fabrication technique to make cladding mode stripping device for high power and superior beam quality fiber laser systems.

Theoretical model and numerical study of effect of target reflected light on high-power fiber laser

<sec>In a high-power fiber laser system, the reflected light generated when the laser hits the target may be recoupled to the laser and amplified by the laser, thereby damaging the laser system. This situation is particularly serious for a high-power laser system, such as spectral beam combining a high power fiber laser, which lacks effective light-return protection. In order to solve the above problems, it is necessary to integrate various physical effects in the whole system link, evaluate and analyze the influence of reflected light on the operating state of the system, so as to optimize the optical path layout and system structure in the beginning of the design of fiber laser to avoid unnecessary losses. Based on the atmospheric transmission theory, fiber rate equation and medium heat conduction equation model, the effect of reflected light on high-power fiber laser is analyzed. In this paper is conducted the numerical simulation of coupling efficiency of reflected light of high-power fiber laser. It is found that under certain atmospheric conditions, the reflected power is related to the transmission distance, the offset angle of optical axis, divergence angle, and the offset of center position of the beam, and will affect the output power, beam quality factor, thermal effect and the signal-to-noise ratio of the stimulated Raman scattering spectrum of the fiber laser. The coupling efficiency of reflected light has a maximum value at a certain transmission distance. For example, the reflected light power up to 140 W is obtained when the transmission distance is 1500 m, which will largely affect the laser system. The reflected power is affected by the offset angle of optical axis far less than by the transmission distance when transmission increases from 500 m to 2000 m. For example, a change of less than 0.1 W can be obtained when offset angle increases from 0.11° to 0.44°. It is also shown that as the divergence angle changes from 0 to 15<i>''</i>, the coupling power decays nearly exponentially with the divergence angle. The coupling efficiency is close to 1% near 12<i>''</i>, which is almost negligible. The output beam quality of the laser system is also affected by the beam quality of reflected light. Deterioration of the beam quality of reflected light will lead to the deterioration of the laser output beam quality. As the reflected light power enters the fiber laser system and increases, the forward output power will decrease and the backward signal power will increase, and the Raman power will increase rapidly near the fiber output end. When the reflected light is present, the thermal effects caused by selecting the gain fiber with different pump absorption coefficients are very important. The stimulated Raman scattering (SRS) rejection ratio decreases with the increase of pump absorption coefficient. For example, the SRS rejection ratio decreases by 5 dB when pump absorption coefficient increases from 1.5 dB/m to 4.5 dB/m, resulting in a decrease in the signal-to-noise ratio of the laser, which will reduce the reliability of the fiber laser system.</sec><sec>In the design and test of spectral beam combining systems for high-power fiber lasers, the attention should be paid to the reflected light in the entire process, which includes the outer optical path and the internal optical path of the laser. The comprehensive constraints of multiple key indicators are analyzed, and the probability of system damage or reliability degradation due to reflected light is evaluated. The research results of this paper are of certain guiding significance in selecting suitable outer optical path layout and system parameters and also in optimizing the design of high energy fiber laser system.</sec>

Spectral manipulation in a high-power ultrafast fiber laser system generating ultrashort pulses with GHz repetition rate

Spectral manipulation in a high-power ultrafast fiber laser system generating ultrashort pulses with GHz repetition rate

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.

An Efficiency Improvement Driver for Master Oscillator Power Amplifier Pulsed Laser Systems

The master oscillator power amplifier (MOPA) pulsed laser, one of the popular topologies for high-power fiber laser systems, is widely applied in industrial machining laser systems. In MOPA, the low-power pulsed laser, stimulated from a seed laser diode, is amplified by the high- power optical energy from pump laser diodes via the gain fiber. Generally, the high-power pump laser diodes are driven by lossy linear current drivers. The switched mode current drivers boost the driver efficiency but suffer from pulse energy consistency due to the current switching ripple. In this paper, a laser driver system that varies the switching frequency of current source to synchronize with pulsed laser repetition rate is analyzed and implemented. Experimental results are demonstrated using a 20 W pulsed fiber laser system.

5-kW-Level Bi-Directional High-Efficiency Pump and Signal Combiner With Negligible Beam Quality Degradation

(6+1)×1 pump and signal combiner with high power handing capability, negligible beam quality degradation and bi-directional signal transmission ability has been developed by employing signal-fiber non-taper fabrication approach and in-line active splicing process. The influences of lateral core offset and angular misalignment between the signal input and output fibers in combiner have been studied theoretically, showing that M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> factor rather than signal efficiency is more suitable to be the evaluation criterion. Three splicing methods have been compared experimentally, revealing that the M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -based active splicing is the most effective to preserve beam quality and obtain good efficiency simultaneously. Signal laser transmitted backward through 25-μm fiber core has shown efficiency of 95.5% and M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> factor deterioration no more than 10% at 1 kW of power. Combined pump from laser diodes into 400-μm fiber clad has reached beyond 5 kW of power at efficiency of 98.2%, with the maximum temperature on combiner being 70.5°C, which was tested in the condition without cooling on fiber pigtails and with flat-cleaved fiber termination aggregating thermal load. Both results indicate the effectiveness of the proposed fabrication technique to make bi-directional (6+1)×1 combiner for high power and superior beam quality fiber laser systems.

Near-single-mode 2-kW fiber amplifier based on M-type ytterbium-doped fiber

High power fiber laser systems have attracted extensive attention due to compactness, good beam quality, efficient heat dissipation and high conversion efficiency. They are widely used in industrial processing, military, medical treatment and other fields. Over the past two decades, owing to the development of double cladding fiber and high-brightness laser diodes, the output power of fiber lasers has been greatly improved. Unfortunately, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), restrict the further enhancement of the output power of fiber lasers. Apparently, increasing the core diameter is the most common way to suppress NLEs in the fiber, but this causes another limiting factor, i.e. mode instability (MI), resulting in the deterioration of the beam quality and in the limitation of the power scaling. Therefore, it is important and urgent to suppress the NLEs and MI simultaneously in fiber lasers. The M-type fiber, by designing refractive index profile, breaks through the stringent trade-off between mode area and numerical aperture (NA), so it possesses a larger mode area than the step index fiber, which helps to avoid NLEs and expand the power range. The M-type ytterbium doped double-clad fiber is fabricated by the modified chemical vapor deposition (MCVD) process with solution doping technology (SDT), the core/cladding diameter is 25/400 μm. The NA of high index ring and index dip in the core are 0.054 and 0.025, respectively. To test the performance of the M-type fiber during high-power operation, a 976 nm bidirectional pumped all-fiber amplifier is constructed. As a result, maximum output power of 2285 W is achieved with an optical-to-optical conversion efficiency of 66.5% under bidirectional pumping scheme, and the measured &lt;i&gt;M&lt;/i&gt;&lt;sup&gt; 2&lt;/sup&gt; factor is 1.42, the central wavelength and 3 dB linewidth of output laser are 1080 nm and 3.01 nm, respectively. To the best of our knowledge, this is the highest output power in a continuous-wave fiber laser employing an M-type fiber at present. However, the MI effect is observed at the output power of 2252 W. The future work will focus on optimizing the structure of the M-type fiber to achieve a stabler higher-power and higher-efficiency laser output.

Design of a 10 W Level Dispersion-Managed High-Power Ultrafast Mid-Infrared Fiber Laser System

Design of a 10 W Level Dispersion-Managed High-Power Ultrafast Mid-Infrared Fiber Laser System

Six kilowatt record all-fiberized and narrow-linewidth fiber amplifier with near-diffraction-limited beam quality

Abstract In this work, an all-fiberized and narrow-linewidth fiber amplifier with record output power and near-diffraction-limited beam quality is presented. Up to 6.12 kW fiber laser with the conversion efficiency of approximately 78.8% is achieved through the fiber amplifier based on a conventional step-index active fiber. At the maximum output power, the 3 dB spectral linewidth is approximately 0.86 nm and the beam quality factor is M x 2 = 1.43, M y 2 = 1.36. We have also measured and compared the output properties of the fiber amplifier employing different pumping schemes. Notably, the practical power limit of the fiber amplifier could be estimated through the maximum output powers of the fiber amplifier employing unidirectional pumping schemes. Overall, this work could provide a good reference for the optimal design and potential exploration of high-power narrow-linewidth fiber laser systems.

Design and fabrication of wideband chirped tilted fiber Bragg gratings

Chirped and tilted fiber Bragg gratings (CTFBGs) have been proved to be effective components for Raman suppression in fiber lasers. However, research shows that the rejection bandwidth of CTFBGs is a great limitation for Raman suppression effect, especially at high power level. In this paper, a simple tandem inscription method for achieving wideband CTFBGs (WB-CTFBGs), to the best of our knowledge, is proposed and experimentally validated for the first time. The influence of several key factors including tilt angle, grating period, and chirp rate of phase mask, on the transmission spectrum is studied in detail by simulation. According to the simulation results, a WB-CTFBG is designed and fabricated. The Raman suppression effect in a 5 kW level master-oscillator-power-amplification (MOPA) fiber laser is compared with that of normal CTFBGs. The validity of the proposed inscription technique has been demonstrated by the better Raman suppression effect. This work provides a simple and effective method for fabrication of WB-CTFBGs, which is significant for Raman suppression in high-power fiber laser systems.

Evolution of Relative Intensity Noise in High-Power Narrow-Linewidth Fiber Laser Systems

In this work, we carry out a systematic study on the evolution of relative intensity noise (RIN) in a high-power narrow-linewidth fiber laser system based on phase-modulated single-frequency seed laser. The experimental results clarify that enhancement of the RIN mainly occurs when the seed laser gets amplified from low power to medium power. In addition, further power scaling could lead to reduction of the RIN until it reaches saturation on the condition that the transverse mode instability (TMI) effect is effectively suppressed. Apart from the power scaling process, the phase-modulation would also lead to enhancement of the RIN at specific frequencies corresponded to the modulation frequency. More importantly, the contrast experiments reveal that the TMI effect could lead to obvious enhancement of the RIN of the signal laser during amplification well below the TMI threshold defined by the traditional method. Therefore, apart from degeneration of the RIN during amplification, potential enhancement of the RIN induced by the phase-modulation or the TMI effect below its traditional threshold should be carefully considered and controlled to achieve high-power low-noise fiber laser. Overall, our experimental results could provide a well reference for design of high-power low-noise fiber lasers and high-power narrow-linewidth fiber lasers.