An all-fiberized and narrow-bandwidth master oscillator power amplification (MOPA) system with record output power of 4 kW level and slope efficiency of 78% is demonstrated. Tandem pumping strategy is tentatively introduced into the narrow-bandwidth MOPA system for thermally induced mode instability (TMI) suppression. The stimulated Brillouin scattering (SBS) effect is balanced by simply using one-stage phase modulation technique. With different phase modulation signals, SBS limited output powers of 336 W, 1.2 kW and 3.94 kW are respectively achieved with spectral bandwidths accounting for 90% power of ${\sim}$0.025, 0.17 and ${\sim}$0.89 nm. Compared with our previous 976 nm pumping system, TMI threshold is overall boosted to be ${>}$5 times in which tandem pumping increases the TMI threshold of ${>}$3 times. The beam quality ($M^{2}$ factor) of the output laser is well within 1.5 below the TMI threshold while it is ultimately saturated to be 1.86 with the influence of TMI at maximal output power. Except for SBS and TMI, stimulated Raman scattering (SRS) effect will be another challenge for further power scaling. In such a high power MOPA system, multi-detrimental effects (SBS, SRS and TMI) will coexist and may be mutual-coupled, which could provide a well platform for further comprehensively investigating and optimizing the high power, narrow-bandwidth fiber amplifiers.
An all-fiberized high-average-power narrow linewidth ns pulsed laser with linear polarization is demonstrated. The laser system utilizes a typical master oscillator power amplifier (MOPA) configuration. The stimulated Brillouin scattering (SBS) is effectively suppressed due to the short fiber length and large mode area in the main amplifier, combined with the narrow pulse duration smaller than the phonon lifetime of SBS effect. A maximal output power of 466 W is obtained with a narrow linewidth of ∼203.6 MHz, and the corresponding slope efficiency is ∼80.3%. The pulse duration is condensed to be ∼4 ns after the amplification, corresponding to the peak power of 8.8 kW and the pulse energy of 46.6 µJ. Neardiffraction-limited beam quality with an M 2 factor of 1.32 is obtained at the output power of 442 W and the mode instability (MI) is observed at the maximal output power. To the best of our knowledge, this is the highest average output power of the all-fiberized narrow linewidth ns pulsed fiber laser with linear polarization and high beam quality, which is a promising source for the nonlinear frequency conversion, laser lidar, and so on.
In recent years, the transverse mode instability (TMI) has been a main limiting factor of average power scalability of high-power fiber lasers with near-diffraction-limited beam quality. A number of strategies in experimental or theoretical scopes have been proposed to mitigate the TMI, especially in the bulk-optic systems. In this paper, the enhancement of the TMI threshold is to be systematically investigated in a monolithic high-power large-modearea (LMA) Yb-doped fiber laser system, by all-optical strategies including optimizing the pump distribution as well as adapting the seed power during operation. It is demonstrated that the TMI threshold can be dramatically increased under counter-pump and bi-directional pump regime compared with the co-pump regime. Further enhancement of the TMI threshold is achieved via optimizing the ratio of the forward pump power to the total pump power under bi-directional pump regime. As a result, a ∼3 kW TMI-free high-power output is realized. Moreover, the systematical study on the effect of the seed laser on the TMI threshold reveals a rising trend of the TMI threshold with the seed power increased. At the same time, a semi-analytical model considering both the gain saturation and the photo-darkening effect is set to theoretically analyze the TMI threshold, manifesting good agreements with the experimental data. This paper provides a valuable reference for the construction of monolithic high-power LMA fiber-based laser system in terms of effectively mitigating the TMI threshold.
By focusing on a typical emitting wavelength of 1120 nm as an example, we present the first demonstration of a high-efficiency, narrow-linewidth kilowatt-level all-fiber amplifier based on hybrid ytterbium-Raman (Yb-Raman) gains. Notably, two temporally stable, phase-modulated single-frequency lasers operating at 1064 nm and 1120nm, respectively, were applied in the fiber amplifier, to alleviate the spectral broadening of the 1120 signal laser and suppress the stimulated Brillouin scattering (SBS) effect simultaneously. Over 1 kW narrow-linewidth 1120 nm signal laser was obtained with a slope efficiency of ~ 77% and a beam quality of M2~1.21. The amplified spontaneous emission (ASE) noise in the fiber amplifier was effectively suppressed by incorporating an ASE-filtering system between the seed laser and the main amplifier. Further examination of the influence of power ratios between the two seed lasers on the conversion efficiency had proved that the presented amplifier could work efficiently when the power ratio of 1120 nm seed laser ranged from 31% to 61%. Overall, this setup could provide a well reference for obtaining high power narrow-linewidth fiber lasers operating within 1100-1200 nm.
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