Abstract
We report on pseudo random binary sequence (PRBS) phase modulation for narrow-linewidth, kilowatt-class, monolithic (all-fiber) amplifiers. Stimulated Brillouin scattering (SBS) threshold enhancement factors for different patterns of PRBS modulated fiber amplifiers were experimentally analyzed and agreed well with the theoretical predictions. We also examined seeding of the SBS process by phase modulated signals when the effective linewidth is on the same order as the Brillouin shift frequency. Here ~30% variations in SBS power thresholds were observed from small tunings of the modulation frequency. In addition, a 3 GHz PRBS modulated, 1.17 kW fiber amplifier was demonstrated. Near diffraction-limited beam quality was achieved (M2 = 1.2) with an optical pump efficiency of 83%. Overall, the improved SBS suppression and narrow linewidth achieved through PRBS modulation can have a significant impact on the beam combining of kilowatt class fiber lasers.
Highlights
Fiber lasers offer several advantages over bulk solid state lasers; including compactness, near diffraction-limited beam quality, and high efficiencies
Broadened modulation techniques offer advantages over schemes with a continuum spectrum (e.g. white noise source (WNS)). One such advantage is that they provide, through tuning of the modulation frequency, the ability to reduce the seeding of the stimulated Brillouin scattering (SBS) process
Using a 25-1 pseudo random binary sequence (PRBS) pattern, we demonstrated at a modulation frequency of 3 GHz, a fiber amplifier with an output power of 1.17 kW
Summary
Fiber lasers offer several advantages over bulk solid state lasers; including compactness, near diffraction-limited beam quality, and high efficiencies. Due to small core sizes and long interaction lengths; high-power, single-mode fiber lasers are limited by the onset of detrimental effects, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) The latter phenomenon is the lowest threshold nonlinear effect in singlefrequency and narrow-linewidth continuous-wave (CW) fiber amplifiers. The effective linewidth of the laser would no longer be single-frequency, beam combining of high-power fiber amplifiers modulated at the GHz level has been demonstrated for both spectral and coherent beam combining (CBC) architectures [4,5,6]; further driving interest in kW class narrow-linewidth fiber amplifiers. A 3 GHz PRBS modulated, 1.17 kW near diffraction limited, all-fiber amplifier with 83% opticalto-optical efficiency is demonstrated This represents a significant reduction in effective spectral linewidth for kW class fiber amplifiers
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