This paper proposes and verified a modified cavity configuration in oscillating-amplifying integrated fiber laser for stimulated Raman scattering suppression. A short segment of YDF and a long piece of GDF is used in the oscillating section, which can simultaneously suppress the onset of Raman component and avoid the self-pulsing operation caused by the extremely low gain in cavity. Experimental result shows a 31dB Raman suppression ratio at 1080nm when output power reaches 5kW without any other Raman suppression element.
The stimulated Brillouin scattering (SBS) effect in fiber amplifiers using white noise signal (WNS) phase modulated seed is simulated. The influences of cut-off frequencies of WNS and the output fiber structure on SBS threshold are discussed. Basing on simulation results, optimized phase modulation signal and output fiber structure are achieved to suppress SBS. A fiber laser is established according to the simulation results.
The average power scaling of counter pumped Yb-doped monolithic fiber amplifiers emitting a diffraction-limited beam has been investigated. The characteristics of seed laser including linewidth and power are optimized, which are beneficial for the suppression of stimulated Brillouin scattering (SBS), nonlinear-induced spectral broadening and transverse mode instabilities (TMI). The fiber with a mode field area of 700 μm2 is able to amplify a 360 W narrow linewidth seed up to 4.05 kW average output power before the onset of TMI. Up to this power level, a narrow linewidth with (0.372 nm), high optical-to-optical efficiency of 67.6% with diffraction-limited beam quality and without any sign of TMI or stimulated Raman scattering for a spectral dynamic range of higher than -52 dB is obtained.
A direct diode-pumped all-fiber-integrated fiber laser based on backward pumping master oscillator power amplifier configuration at 1080 nm, producing maximum output power of 4.115 kW based on 25/400 μm fiber with corresponding linear fitting optical to optical efficiency of 78.39% was demonstrated. The suppression ratio of stimulated Raman scattering is better than 35dB and it can be further optimized by decreasing the seed input power. Near diffractionlimited beam quality (M2 are 1.7and 1.6 in the x and y directions based on 4-sigma method) is also achieved at the maximum output power. To the best of our knowledge, this is the first report for 4 kW near-diffraction-limited fiber lasers based on 25/400 μm fiber directly pumped by laser diodes.
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