A high-power polarization-maintaining fiber laser with a linewidth in the 10 GHz range holds significant potential in beam combining applications such as coherent and spectral beam combining because of its ability to reduce the precision required for optical path control while maintaining high beam quality after spectral combination. However, the low threshold of stimulated Brillouin scattering (SBS) in fibers has become a key limiting factor in further increasing the output power of narrow-linewidth lasers. Recent advancements in phase-modulation techniques for laser linewidth broadening have demonstrated the capability of extending the linewidth of single-frequency lasers to the tens of GHz range, thereby effectively reducing the peak power associated with the SBS effect and substantially increasing the SBS threshold. In this paper, we present a high-power narrow-linewidth fiber laser system that employs pseudo-random binary sequence (PRBS) phase modulation. This method is expected to facilitate power scaling of narrow-linewidth fiber lasers within the 10 GHz linewidth range.

In this study, we carefully selected the pseudo-random binary sequence phase modulation parameters and cutoff frequency of the low-pass filter to achieve optimal suppression of stimulated Brillouin scattering for a given spectral linewidth. Spectral pre-shaping of the modulated signal was achieved through precise control of the modulation depth of the filtered PRBS signal, as well as fine-tuning of the baud rate and code pattern. This process effectively mitigates the degradation in spectral flatness caused by power fluctuations at the top of the modulated spectrum, which arise from the nonlinear response of the radio frequency (RF) amplifier and phase modulator at high frequencies. The discrete flat-top spectrum generated using this method uniformly distributes the backscattered Stokes signal among multiple sidebands, preventing any single mode from prematurely reaching the SBS threshold. Following the PRBS phase modulation, the spectrum exhibited a discrete flat-top profile, providing robust suppression of the SBS in high-power fiber lasers with linewidths below 10 GHz.

The modulated spectrum was characterized by a steep roll-off at the out-of-band edges, with the spectral peak transitioning smoothly from sharp to full. The full width at half maximum (FWHM) of the discrete frequency comb spectrum was 8.97 GHz, and the flat-top section comprised five frequency lines with an in-band flatness of less than 1 dB. The frequency spacing between the newly generated spectral lines was consistent at 66 MHz, which is larger than the Brillouin gain bandwidth (30 MHz) of the main amplifier gain fiber. This effectively prevents overlap and crosstalk between the Brillouin gain regions caused by the longitudinal mode frequencies. The reduced coherent interaction between the optical signal and backward-propagating Stokes wave during the phonon lifetime significantly enhanced the SBS threshold. Additionally, the modulated spectrum was injected into a Yb-doped, double-clad, polarization-maintaining fiber with a core-to-cladding diameter ratio of 20/400 μm. This configuration yielded a maximum output power of 2041 W, polarization extinction ratio of 96.15%, beam quality factor of less than 1.3, and optical-to-optical conversion efficiency of 87.2%. During the power scaling process, no stimulated Brillouin scattering or mode instability was observed, and the linewidth remained stable without broadening.

This paper introduces a novel method for generating a discrete flat-top-modulated spectrum to broaden the linewidth of fiber lasers. By precisely controlling the modulation depth and optimizing multiple parameters to precompensate for the modulated signal, the method effectively minimizes the power fluctuations between the high- and low-frequency components of the spectrum. The discrete flat-top spectrum generated by filtered PRBS signals not only maintains the advantages of SBS suppression observed in previously reported discrete flat-top spectra but also enhances the SBS threshold by reducing phonon accumulation time through rapid π-phase transitions of the modulated signal. Furthermore, by optimizing the modal control of the gain fiber in the main amplifier and employing a counter-pumping scheme to suppress transverse mode instability (TMI), a narrow-linewidth laser output of 2041 W was achieved.