Theoretical and Experimental Investigation of Piecewise Parabolic Phase Modulation for SBS Suppression in Fiber System

We investigate stimulated Brillouin scattering (SBS) suppressing capability of a piecewise parabolic waveform phase modulated fiber system by means of localized nonfluctuating source model. The optical spectra of piecewise parabolic phase modulated laser field are derived analytically. We prove that the parabolic phase modulated laser spectra have a discrete top-hat envelope, and the spectral linewidth can be changed by the product of modulation depth and chirp rate while the spectral line separation remains constant. Theoretically, we calculated the Brillouin gain spectra of piecewise parabolic phase modulation to visually characterize the SBS suppression capability. We define
$L_{ch}$
as the characteristic fiber length. It is found that the Brillouin gain spectra broadened as fiber length
$L$
increases for the case of
$L < L_{ch}$
, while reach the widest linewidth and keep constant for the case of
$L\geq L_{ch}$
. We establish a piecewise parabolic phase modulation experimental setup to verify our simulation results. According to our model and experiments, we also made predictions for SBS suppression of GHz-level fiber system with several kilowatts output power and
$\sim 10$
m fiber length.