Stimulated Brillouin scattering (SBS) is one of the leading nonlinear effects in high-power narrow-linewidth continuous-wave fiber amplifiers, which limits its power scaling capacity.

Recently, the research staff in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has succeeded in establishing a monolithic fiber amplifier configuration to boost a <10 MHz DFB diode laser to 1.27 kW. The linewidth broadening method based on pseudo-random binary sequence (PRBS) phase modulation was demonstrated to suppress the SBS process effectively. The work was published in Scientific Reports.

In this work, using a low-pass filtered PRBS radio frequency signal to broaden the spectral linewidth and with a SBS enhancement factor of 53, the spectral line spacing to maximize the SBS threshold was found theoretically to be 0.5× Brillouin gain spectral bandwidth of the fiber system in order to avoid Brillouin gain-spectrum overlap.

This theory was further experimentally verified by flexibly controlling the spectral line spacing in the spectral envelope. The spectral line spacing of the phase modulation was chosen to be 12.7 MHz, and a 2.2 GHz low-pass radio frequency filter was used to control the total linewidth of the laser signal. The ~50 mW DFB diode seed laser was then boosted to 1.27 kW with a four stage Yb-doped fiber amplifier chain with the 4^th stage backward reflectivity of to be < 1‰ (detected by the 3^rd port of the circulator placed between the 3^rd and the 4^th stage).

To the best of our knowledge, this has hitherto been the highest power level for narrow linewidth fiber amplifier based on phase control under 5 GHz spectral linewidth.

Schematic of the PRBS modulated, low pass filtered, 1.27 kW four-stage monolithic fiber amplifier. (Image by SIOM)