A theoretical study of transient stimulated Brillouin scattering in optical fibers seeded with phase-modulated light

Abstract

Beam combining of phase-modulated kilowatt fiber amplifiers has generated considerable interest recently. We describe in the time domain how stimulated Brillouin scattering (SBS) is generated in an optical fiber under phase-modulated laser conditions, and we analyze different phase modulation techniques. The temporal and spatial evolutions of the acoustic phonon, laser, and Stokes fields are determined by solving the coupled three-wave interaction system. Numerical accuracy is verified through agreement with the analytical solution for the un-modulated case and through the standard photon conservation relation for counter-propagating optical fields. As a test for a modulated laser, a sinusoidal phase modulation is examined for a broad range of modulation amplitudes and frequencies. We show that, at high modulation frequencies, our simulations agree with the analytical results obtained from decomposing the optical power into its frequency components. At low modulation frequencies, there is a significant departure due to the appreciable cross talk among the laser and Stokes sidebands. We also examine SBS suppression for a white noise source and show significant departures for short fibers from analytically derived formulas. Finally, SBS suppression through the application of pseudo-random bit sequence modulation is examined for various patterns. It is shown that for a fiber length of 9 m the patterns at or near n=7 provide the best mitigation of SBS with suppression factors approaching 17 dB at a modulation frequency of 5 GHz.

Power scaling of a hybrid microstructured Yb-doped fiber amplifier

“…The resulting tones of the phase modulator were spaced by 66.6 MHz; which is ~1.5x greater than the BG FWHM of the primary response. Zeringue et al argue in [9] that optimal SBS suppression for a given phase-broadened bandwidth is when the spacing of the phase modulator tones are 2-3 times greater than the BG FWHM. The bit pattern in this study is not optimal, but it will be shown that the PRBS scheme sufficiently suppressed SBS.…”

Stimulated Brillouin Scattering Suppression With a Chirped Laser Seed: Comparison of Dynamical Model to Experimental Data

“…Equations (1)-(3) have been used to describe Brillouin scattering in a passive fiber without amplification [6] and with a nominal 10× gain [7]. To simulate an amplifier with realistic gain and gain saturation, (4) and (5) are included.…”

Multi-kilowatt diffractive coherent combining of pseudorandom-modulated fiber amplifiers

“…With the modulation period set at much shorter times than the acoustic phonon lifetime or the light traverse time, large SBS suppression factors can be obtained. 5 Typically, white noise source 3 (WNS) and pseudorandom bit sequence 6,7 (PRBS) have been the prevalent phase modulation techniques for kW-class fiber amplifiers. Recently, in a direct comparison conducted at the kW-level in Yb-doped amplifiers, PRBS phase modulation was shown to provide superior SBS suppression to that provided by WNS.…”