SRS suppression in multi-kW fiber lasers with a multiplexed CTFBG.

In this paper, we present experimental studies on newly developed multiplexed chirped tilted fiber Bragg gratings (MCTFBGs) for stimulated Raman scattering (SRS) suppression for high-power fiber laser systems. The MCTFBG device is composed of five continuous segments of chirped tilted fiber Bragg gratings (CTFBGs), which are inscribed into the large-mode-area (LMA, 25/400µm) fibers. The SRS suppression capability of the MCTFBG device has been successfully demonstrated with a master oscillator power amplification (MOPA) fiber laser system at the output level of 3.4 KW. The experimental observation thus indicates the MCTFBG's excellent SRS suppression capability at a very high power level (∼15 dB under >3 kW) and high thermal handling capacity (∼1.48℃/kw). Our work thus provides a key development of essential fiber grating components that can effectively suppress the SRS suppression at a very high power level.

Experimental investigation of quasi-static mode degradation in a high power large mode area fiber amplifier.

In this work, quasi-static mode degradation in high power fiber amplifiers has been investigated experimentally. An increase of M2 from 1.3 to 2.6 with distortion of the beam profile is observed, which results in the signal spectra and backward light characterization departing from the traditional phenomena. The amplifier has been operated at the same input pump power of 705 W for nearly 2.2 hours to investigate the relationship between quasi-static mode degradation and photodarkening. The evolution of M2 factor/beam profile, mode correlation coefficient and output laser power at different working times indicate that the quasi-static mode degradation in the high power fiber amplifiers is dependent on photodarkening and evolves on the scale of tens of minutes. A visible green light has been injected to photobleach the gain fiber for 19 hours, which reveals that the quasi-static mode degradation has been suppressed simultaneously. To the best of our knowledge, this is the first detail report of photodarkening-induced quasi-static degradation in high power fiber amplifiers.

High-power cylindrical vector beams generated from an all-fiber linearly polarized laser by metasurface extracavity conversion.

Five-hundred-watt cylindrical vector beams (CVBs) at 1030 nm with the 3 dB linewidth being less than 0.25 nm have been generated from a narrow linewidth all-fiber linearly polarized laser by metasurface extracavity conversion. At maximum output power, the transmission efficiency and polarization extinction ratio of radially polarized cylindrical vector beams (RP-CVBs) are beyond 98% and 95%, respectively. The average power is approximately an order higher than previously reported high-power narrow-linewidth CVBs generated from fiber lasers. The temperature rise of the metasurface is less than 10°C at 500 W output power, which means that the system can be further power-scaled in the near future. The high-power, high-purity, and high-efficiency RP-CVBs generated by the metasurface demonstrate potential application of a metasurface in high-power CVBs lasers.

3 kW high OSNR 1030 nm single-mode monolithic fiber amplifier with a 180 pm linewidth.

In this Letter, the amplified spontaneous emission (ASE) effect of a 1030 nm fiber laser is studied theoretically and, based on the theoretical results, a 3 kW high optical signal-to-noise ratio (OSNR) 1030 nm fiber amplifier with a 180 pm linewidth and near-diffraction-limited beam quality is achieved. A theoretical model, which takes simulate ASE light falling in the range of Raman light as the Raman seed, has been used to optimize the power scaling capability of 1030 nm fiber amplifiers. It shows that the SRS effect seeded by the ASE is the main limiting factor for the fiber amplifiers operating at 1030 nm, and >3kW output power with a high OSNR can be achieved by proper parameter designing of the fiber laser system. A 1030 nm monolithic narrow linewidth fiber amplifier, which delivers 3 kW output power with the OSNR being 37 dB and a 0.18 nm spectrum linewidth, has been demonstrated. At the maximum 3 kW output power, the SRS light peak is obviously higher than ASE light, which agrees with the theoretical predictions. Neither a stimulated Brillouin scattering effect nor a thermal-induced mode instability effect has been observed at ultimate power level, and the beam quality factor M2 is measured to be less than 1.2. To the best of our knowledge, this is the highest average power for a narrow linewidth single-channel fiber laser system reported so far operating at 1030 nm.

kW-level 1030 nm polarization-maintained fiber laser with narrow linewidth and near-diffraction-limited beam quality.

kW-level 1030 nm polarization-maintained fiber laser with narrow linewidth and near-diffraction-limited beam quality is demonstrated. Theoretical simulations based on the power balance equation are first performed to optimize the system parameters of the 1030 nm ytterbium-doped fiber laser for the maximum suppression of amplified spontaneous emission (ASE). With the optimized parameters, both the copumped and counterpumped MOPA lasers are implemented to obtain an output power of >1 kW. In both cases, the ASE suppression ratio reaches 40 dB with a 3 dB linewidth of about 0.14 nm, and the polarization extinction ratio is about 12 dB at 1 kW of output power. The beam quality starts degrading at 900 W of output power in the copumped structure, but maintains nearly single mode (Mx2,My2)=(1.07,1.12) until power is over 1 kW in the counterpumped structure.

Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier.

Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.