In this work, we investigate the SBS mitigation of the pseudo random bit sequences (PRBS) modulation analytically and numerically. The lightwave is phase modulated by a Butterworth lowpass filtered and amplified PRBS signal, and the SBS suppressing capability versus the parameters such as the filter order, the filter cutoff frequency, the modulation depth, and the pattern length are illustrated. Therefore, we redefine the modulation depth by normalized RMS voltage to integrate binary sequences modulation signal and multi-value sequences modulation signal. On this basis, we demonstrate the impact of pattern length, modulation depth and modulation cutting rate on SBS mitigation and found the corresponding optimal component of n=9, 22.72 dBm and 0.54. On this situation, contrast to the unfiltered PRBS phase modulation scheme, a 17.7% enhancement of normalized SBS threshold can be obtained by using normalized power in the barrel to estimates the linewidth. This work may provide a new idea for SBS mitigation by lowpass filtered PRBS phase modulation in narrow linewidth fiber amplifiers.
In this paper, we investigated the association between beam quality and beam characteristic in theory in a spectral beam combining system and built a model to describe the dispersion. We analyzed that how the beam quality varies with different input beam parameters such as the waist radius 𝑤𝑤0 and the spectral width Δλ. There is a surprising result that the increased w0 and Δλ have a coefficient influence on the beam quality degradation, and the degradation is pretty sensitive to the increase of waist radius w0. The result gives us a guidance that besides using the narrow linewidth laser, we also need reduce the waist radius of the input beam in the SBC system appropriately to abate the beam quality degradation caused by dispersion.
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