We report a W-type fiber design for improving the beam quality and spectral purity of the Q-switched Yb-doped fiber lasers (YDFLs). Our W-type fiber exhibited higher losses in the high order modes (>100 dB/m) and lower losses in the fundamental modes (<0.1 dB/m). A 20/130 mu m Yb-doped W-type fiber was fabricated by employing a modified chemical vapor deposition (MCVD) process and a solution-doping technique. The Yb-doped W-type fiber was examined in an acousto-optic Q-switched YDFL. A robust single-mode operation with a 1.19 M2 factor was obtained by optimizing the bending radii. The suppression of mixed mode four-wave mixing (FWM) and amplified spontaneous emission (ASE) was observed in the output spectral with an 18 dB signal to noise ratio (SNR) enhancement at 1064 nm and a 5.3 dB ASE inhibition ratio at 1030 nm. The Yb-doped W-type fiber exhibited enhanced beam quality and output spectra than the conventional Yb-doped step-type refractive index fiber. Therefore the W-type fiber suits the Q-switched fiber laser applications.

Silica is one of the most important reinforcing fillers in the rubber industry. As an excellent reinforcing agent, it can endow rubber with higher tensile strength, elongation at break, elasticity, heat resistance, tear strength, etc., and is widely used in rubber products. BIIR has good air tightness, fatigue resistance, aging resistance, chemical resistance, and good processing performance, so it is widely used in tires and other industries. In this paper, three hydrophobic nano-powder materials, silica airgel, hydrophobic fumed silica and hydrophobic precipitated silica, were added to BIIR during rubber mixing and vulcanization. The effects of three nano-powder materials on the physical and mechanical properties of BIIR were compared. The results show that all three hydrophobic silica nano-powder materials can effectively improve the mechanical properties of BIIR, among which silica airgel has the most significant enhancement effect on the mechanical properties of BIIR. The tensile strength of 40% airgel-reinforced vulcanized rubber can be increased from 2.42MPa to 5.84MPa. The 100% constant tensile stress can be increased from 0.34MPa to 1.04MPa, while the tear strength is high (19.07N/mm), the hardness is extremely high (89HA), and the thermal conductivity is as low as 0.13439 (W/m·K).

Fiber lasers are widely used in industrial processing, military defense, and other fields. The energy distribution of the laser output in fiber lasers is Gaussian-like distribution. This nonuniform energy distribution within the spot diameter affects the consistency of the processing effect at different positions along the spot diameter when applied to lithography, welding, etc. Therefore, homogenizing and shaping Gaussian-like beams in practical applications is greatly significant. Compared with the beam shaping method of the traditional spatial structure, using the all-fiber structure in beam shaping provides a simple structure and good compactness for the fiber lasers. By summarizing the research progress of various scholars over the years, the homogenization and shaping technology of the all-fiber structure is classified into two categories: increasing the components of the high-order mode in the output laser and directly changing the energy distribution of the fundamental mode. In this paper, we present the research status of all-fiber structure beam homogenization and shaping technology and discuss prospects of future development in all-fiber structure beam shaping technology.

A low-numerical aperture (NA) confined-doped long-tapered (LCT) Yb-doped fiber is proposed and fabricated by modified chemical vapor deposition combined with solution doping technique. The LCT fiber owns the core NA of ∼0.05 and the gain dopant doping diameter ratio of ∼77%, with a core/cladding diameter of 25/400 µm at both ends and 37.5/600 µm in the middle. The laser performance is demonstrated by a bidirectional pumping all-fiber amplifier, of which a 4.18-kW single-mode (M2 factor ∼1.3) laser output is achieved with a slope efficiency of ∼82.8%. Compared with the conventional fiber, the co-pumped and counter-pumped transverse mode instability thresholds and beam quality of the LCT fiber are remarkably enhanced. Throughout the continuous operation, the LCT fiber amplifier presents high power stability with fluctuation of < 1%. These results indicate that LCT fiber has great potential in power scaling remaining excellent beam quality.

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 M 2 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.

Objective High-power ytterbium-doped fiber lasers (YDFLs) have good beam quality, high conversion efficiency, high reliability, and good compactness, making them widely usable in industrial processing, military, and national defense fields. The output power of YDFLs has been unprecedentedly improved with the development of double-clad fiber, laser diodes, and passive devices. This unprecedented progress is hampered by nonlinear effects, such as stimulated Raman scattering (SRS) and transverse mode instability (TMI). The most effective and fundamental way to suppress these two effects is to optimize the structure design of active fiber. We propose a low-numerical aperture confined-doped long-tapered (LCT) fiber that combines the advantages of low numerical aperture (NA), Yb3+ ions restricted doping, and longitudinal tapered design and can theoretically suppress TMI and SRS effects simultaneously. Methods The LCT fiber is proposed and successfully fabricated using modified chemical vapor deposition (MCVD) in conjunction with a solution doping technique (SDT). The LCT fiber has a core NA of 0.05 and a gain dopant doping diameter ratio of 77%, with a core/cladding diameter of 25/400 tan at both ends and 37.5/600 ttm in the middle. A bidirectional-pumped master oscillator power amplifier (MOPA) system verifies the laser performance of the LCT fiber. Results and Discussions A laser output of 4.188 kW was obtained with a slope efficiency of 82. 8% (Fig. 3). The intensity of the Raman Stocks light was 18 dB lower than that of the signal laser at 4.188 kW output power, and the M-2 factor was about 1.3, maintaining a single-mode output. Further optimization will focus on improving the pump absorption and effective mode area of this fiber to mitigate SRS. Conclusions We present a novel low-NA (0.05) confined-doped long-tapered fiber fabricated using the MCVD process in conjunction with SDT. The Yb-ions doping diameter ratio is similar to 77%, and the middle section core/cladding diameter is 37.5/600 mu m, tapering to 25/400 mu m at both ends. In the bidirectional pumping MOPA configuration, a 4.188 kW laser is obtained with a slope efficiency of 82.8 Yo. The M-2 factor is about 1.3 at 4.188 kW output power, maintaining a single-mode output. The results above reveal that using low-NA confined-doped long-tapered Yb-doped fiber to achieve high power output with high brightness is a promising prospect.

A constant-cladding tapered-core (CCTC) fiber with the core size varying monotonously along the length was fabricated. The core/cladding diameters of narrow end and wide end are 24/400 mu m and 31/400 mu m, respectively. A forward-pumping master oscillator power amplifier (MOPA) was constructed to investigate the laser performance of this fiber under two bending schemes (8-14 cm and 12-16 cm). The maximum output power is scaled up to 2704 W with a slope efficiency of 82.1% in the case of bending diameter ranges from 12 cm to 16 cm, the M2 factor at 2548 W output power is 2.16, and no signs of transverse modal instability (TMI) and stimulated Raman scattering (SRS) were observed in whole experiment. The above result verifies the potential of power scalability of CCTC fiber, and the laser performance can be further enhanced by fiber design optimization and other TMI suppression methods.

We report on an experimental investigation of radiation induced static mode degradation in Yb-Ce co-doped pulsed fiber amplifiers. For investigating the impact of irradiation on beam quality, radiation-darkened fibers were irradiated by gamma rays. The laser properties of three types of fibers were investigated. With the pump power increased, static mode degradation appeared: M-2 factor raised significantly and the output beam profile transformed from LP01 into LP11 shape without high-frequency component. The threshold of static mode degradation decreased with irradiation dose, and related to Yb-Ce co-doped fiber compositions. The doping of Ce benefits to the improvement of the static mode degradation threshold: in the case of the Ce concentration increasing by 35.5%, SMD threshold rose by 16% at most.

A high power short-cavity random fiber laser employing the gain mechanism of the Yb-doped fiber and the half-open cavity structure and the temporal optical rogue waves (RWs) behavior are observed and investigated in the paper. The record output power without the stimulated Raman scattering (SRS) is promoted to 26.6 W in the YDRFL with the GDF length of 120 m. The stochastic pulses and temporal optical RWs are observed and demonstrated in the short cavity YDRFL for the first time. It is found that the proportion of RWs depends on the GDF length which can also affect the stability of output lasing. The research results reveal that achieving the relative stable output power requires the greater pump power for the shorter GDF length, although decreasing the GDF length will promote the maximum output power of the YDRFL without the SRS.

The remarkable evolution of ytterbium-doped fiber (YDF) lasers and amplifiers is interrupted by a limiting thermo-optical effect called transverse mode instability (TMI). Hereon, we propose a Gaussian-shaped gain-dopant distributed (GSGDD) YDF, which is fabricated by a modified chemical vapor deposition (MCVD) process combined with solution doping technique (SDT). By regulating the solution concentrations of soot layers, the content of Yb 3+ ions presents Gaussian-shaped distribution in the transverse direction while the refractive index profile (RIP) exhibits a stepped profile. The laser performance of this fiber is verified by a bidirectional pumped master oscillator power amplifier (MOPA). Over 3 kW near-single-mode laser output is obtained with the slope efficiency of 84.9%. At the highest power output, there are no Stokes light components in the spectrum and the beam quality M 2 factor is ~1.45 These results suggest that the GSGDD fiber owns great potential to achieve high power output with excellent beam quality.

Owing to their excellent optical properties, fiber lasers and fiber amplifiers composed of rare-earth-doped fibers (also known as active fibers) are widely used in space, nuclear power, and high-energy physics facilities. However, in an irradiation environment, rare-earth-doped fibers are vulnerable to radiation-induced loss and their optical performance deteriorates rapidly. Therefore, improving the radiation resistance of active fibers is necessary to prevent such losses. This study first overviews the application background and problems of active fibers in irradiation environments. Second, it introduces the research progress of anti-irradiation active fibers in China and other countries from the following three perspectives: irradiation characteristics of active fibers, the primary factors influencing the irradiation characteristics, and radiation resistant technologies for active fibers. Finally, the future research trend of radiation resistant active fibers is prospected.

Silica aerogels are lightweight, highly porous nanomaterial with extremely large internal surface area but fragile and brittleness. To enhance the applicability of aerogels, fiber reinforced silica aerogel insulation blocks are prepared by frothing method. Polyurethane emulsion acts as binder whereas polyethylene (PE) fiber and polyurethane (PU) fiber is used as reinforcing phase. The effect of polyethylene fiber and polyurethane fiber on the performance of composites were characterized by material's chemical microscopic morphology, compression performance, recovery performance, surface area, thermal conductivity and hydrophobicity. The research results show that the prepared PU fiber reinforced silica aerogel insulation block has excellent comprehensive performance. The water absorption rate of decreased from 19.35% (0 phr, PU fiber) to 6% (20 phr, PU fiber), and comprehensive rebound rate was increased by 97.83% at 20 phr PU fiber compared with pure insulation block (83.88%). Meanwhile, the dimensional stability of the composite aerogel block is above 96%. PU fiber reinforced silica aerogel insulation block has light weight, good hydrophobicity and good thermal and mechanical properties, so it has a wide range of application prospects in the field of insulation.

Compared with traditional uniform fiber, the long tapered fiber has the advantage of suppressing nonlinear effects and maintaining excellent beam quality. Both the cladding size and core size of the long tapered fiber reported so far vary simultaneously in a constant core-to-cladding diameter ratio (CCDR) along the length. In this work, we propose a technique for preparing cladding uniform core tapered fiber (CUCTF) with a varying CCDR, and have successfully fabricated CUCTF. The core diameters of narrow end and wide end are 31.2 mu m and 52.5 mu m, respectively. The cladding diameter is always 400 mu m along the length. The laser performance of CUCTFs with different structures (wide to narrow, narrow to wide) are investigated based on a co-pumped fiber amplifier. Between them, the CUCTF with the wide to narrow structure has a better M2 factor of 1.455 due to mode filtering.

We demonstrate the thermal bleaching effect on a photodarkened thulium-doped fiber (TDF) in detail. The bleaching effect on visible transmission initiates at 250 °C and a complete recovery is achieved at 550 °C. Prior to the recovery, a post-irradiation heat-induced spectral loss is observed. It indicates that an intermediate energy state is generated in the TDF under exposure to near-infrared (NIR) radiation, exhibiting the spectral attenuation in visible (VIS) and NIR region as driven by color center after thermal activation. And, with thermal treatment, the bleached TDF shows a partial photodarkening (PD) resistance when it is subject to photoirradiation again. In addition, the temperature-dependent spectral broadening and red shift that may distort the measured decay curve of excess loss is observed and discussed.

We demonstrate the rapid photodarkening (PD) phenomenon in Tm-doped fiber (TDF) core pumped by a laser at 1080 nm and the bleaching effect of deuterium ( D 2 ) on PD TDF. By D 2 loading for seven days, the PD-induced excess loss (PIEL) in the visible (VIS) and near-infrared (NIR) region have been largely eliminated, and no degradation was observed within 30 days. PD resistance of the D 2 pretreated TDF has been investigated as well. The formation of color centers based on defects and precursors in the silica matrix and the mechanism of D 2 bleaching are discussed.

The radical elimination of photodarkening effect by H2-loading was demonstrated, and the output power was stable during long term operation in high power fiber amplifiers, without mode instability and photodarkening-induced core laser leakage.

Ytterbium-doped fiber (YDF) loaded with deuterium is used herein to mitigate mode instability. Experimental results reveal that this method can increase the mode instability threshold in a laser oscillator. Specifically, when the YDF was loaded with deuterium over two- and four-week periods, the mode instability threshold power increased from ∼459 W to ∼533 W (16%) and to ∼622 W (35%), respectively, but the respective laser efficiencies were almost unaffected (71.5% vs. 72.9% and 75.4%). In conclusion, deuterium loading is effective in the mitigation of mode instability. It is envisaged to be applied in the power scaling of high-power fiber lasers.

The radical suppression of the photodarkening effect and laser performance deterioration via H 2 loading were demonstrated in high-power Yb-doped fiber（YDF） amplifiers. The photodarkening loss at equilibrium was114.4 d B/m at 702 nm in the pristine fiber, while it vanished in the H 2 -loaded fiber. To obtain a deeper understanding of the impact of photodarkening on laser properties, the evolution of the mode instability threshold and output power in fiber amplifiers was investigated. After pumping for 300 min, the mode instability threshold of the pristine fiber dropped from 770 to 612 W, and the periodic fluctuation of the output power became intense,finally reaching 100 W. To address the detrimental effects originating from photodarkening, H 2 loading was applied in contrast experiments. The output power remained stable, and no sign of mode instability was observed in the H 2 -loaded fiber. Moreover, the transmittance at 638 nm confirmed the absence of the photodarkening effect. The results pave the way for the further development of high-power fiber lasers.

A novel method for mitigating photo-darkening and the effective photo-bleaching phenomenon by 532 nm cladding pump in Yb-doped fiber were herein reported. Compared with the pristine fiber, beyond 30% of photo-darkening induced excess loss was suppressed by 532 nm pretreatment. Moreover, the excess loss in the photo-darkened fiber was completely bleached with 532 nm pump. Additionally, the bleached fiber exhibited better photo-darkening resistance. Therefore, for high power application, a 20/400 gamma irradiated fiber was bleached in situ by 532 nm pump and the laser properties were explored. The output power restored to 421W accounting for 82% of the pristine fiber, with the mode instability threshold rising to over 2.6 times and the efficiency increasing from 37% to 63%. The results indicate 532 nm pump has bright prospects for the stable operation of high power fiber lasers.