In this paper, an all-fiberized and narrow-linewidth 5 kW power-level fiber amplifier is presented. The laser is achieved based on the master oscillator power amplification configuration, in which the phase-modulated single-frequency laser is applied as the seed laser and a bidirectional pumping configuration is applied in the power amplifier. The stimulated Brillouin scattering, stimulated Raman scattering, and transverse mode instability effects are all effectively suppressed in the experiment. Consequently, the output power is scaled up to 4.92 kW with a slope efficiency of as high as approximately 80%. The 3-dB spectral width is about 0.59 nm, and the beam quality is measured to be M2∼1.22 at maximum output power. Furthermore, we have also conducted a detailed spectral analysis on the spectral width of the signal laser, which reveals that the spectral wing broadening phenomenon could lead to the obvious decrease of the spectral purity at certain output power. Overall, this work could provide a reference for obtaining and optimizing high-power narrow-linewidth fiber lasers.

The fiber laser based on an oscillating-amplifying integrated structure has the potential to benefit from the advantages of a fiber laser oscillator and amplifier with the characteristics of strong anti-back-reflected light ability and high efficiency. Here, we achieved a 3.5-kW near-single-mode (M2 ∼ 1.24) oscillating–amplifying integrated fiber laser with an active fiber length of 8 m in the oscillating section and 17.6 m in the amplifying section. While operating at the maximum power, the optical-to-optical conversion efficiency is 87.0%, and the intensity of stimulated Raman scattering is about 23.61 dB lower than that of the signal light. To the best of the authors’ knowledge, this is the highest output power of an oscillating–amplifying integrated fiber laser, accompanied with the best beam quality and the highest efficiency.

Power scaling based on traditional ytterbium-doped fibers (YDFs) is limited by optical nonlinear effects and transverse mode instability (TMI) in high-power fiber lasers. Here, we propose a novel long tapered fiber with a constant cladding and tapered core (CCTC) along its axis direction. The tapered-core region of the fiber is designed to enhance the stimulated Raman scattering (SRS) threshold and suppress higher-order mode resonance in the laser cavity. The CCTC YDF was fabricated successfully with a modified chemical vapor deposition (MCVD) method combined with solution doping technology, which has a cladding diameter of 400 μm and a varying core with a diameter of ~24 μm at both ends and ~31 μm in the middle. To test the performance of the CCTC fiber during high-power operation, an all-fiber laser oscillator based on a CCTC YDF was investigated experimentally. As a result, a maximum output power of 3.42 kW was achieved with an optical-to-optical efficiency of 55.2%, although the TMI effect was observed at an output power of ~3.12 kW. The measured beam quality (M2 factor) was ~1.7, and no sign of the Raman component was observed in the spectrum. We believe that CCTC YDF has great potential to simultaneously mitigate the SRS and TMI effects, and further power scaling is promising by optimizing the structure of the YDF.

Stimulated Raman scattering (SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers. Different from previous techniques that aim at suppressing SRS, nonlinear fiber amplifier (NFA), which manipulates and employs the SRS for power scaling in rare-earth-doped fiber, is under intensive research in recent years. In this paper, the authors will present an all-round study on this new kind of high-power fiber amplifier. A theoretical model is proposed based on the rate equation and amplified spontaneous emission (ASE), with random noise taken into account. By numerical solving of the theoretical model, the power scaling potential, heat analysis and advantages in suppressing the undesired backscattering light are quantificationally analyzed for the first time. Then two different types of high-power NFAs are demonstrated individually. Firstly, a laser diode pumped NFA has reached kilowatt output power, and the results agree well with theoretical predictions. Secondly, a tandem-pumped NFA is proposed for the first time and validated experimentally, in which 1.5 kW output power has been achieved. The authors also briefly discuss several new issues relating to the complex nonlinear dynamics that occur in high-power NFAs, which might be interesting topics for future endeavors.

In this paper, we experimentally investigated the extreme frequency shift in high-power Raman fiber laser (RFL). The RFL was developed by using a pair of fiber Bragg gratings with fixed and matched central wavelength (1120 nm) combined with a piece of 31-m-long polarization maintaining (PM) passive fiber adopted as Raman gain medium. The pump source was a homemade high-power, linearly polarized (LP) wavelength-tunable master oscillator power amplifier (MOPA) source with ${\sim}25~\text{nm}$ tunable working range (1055–1080 nm). High-power and high-efficiency RFL with extreme frequency shift between the pump and Stokes light was explored. It is found that frequency shift located within 10.6 THz and 15.2 THz can ensure efficient Raman lasing, where the conversion efficiency is more than 95% of the maximal value, 71.3%. In addition, a maximum output power of 147.1 W was obtained with an optical efficiency of 71.3%, which is the highest power ever reported in LP RFLs to the best of our knowledge.

Compared with traditional uniform fibers, tapered fiber has numerous unique advantages, such as larger mode area, higher pump absorption, suppression to nonlinear effects, and maintaining good beam quality. In this manuscript, we have constructed an all-fiberized fiber amplifier which is based on a piece of ytterbium-doped tapered double-clad fiber (T-DCF). The fiber amplifier is operated under continuous wave (CW) regime at 1080 nm wavelength. The $M^{2}$ factor of the amplifier at 1.39 kW output power is ${\sim}1.8$. The maximum output power of the system reached 1.47 kW, which, to the best of our knowledge, is the highest output power of long tapered fiber based fiber laser system. Our result successfully verifies the potential of power scalability and all-fiberized capability of long tapered fiber, and the performance of our system can be further enhanced by fiber design optimization.